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Influence of Drilling Parameters on the Delamination and Surface Roughness of Insulative-Coated Glass/Carbon-Hybrid Composite

Influence of Drilling Parameters on the Delamination and Surface Roughness of Insulative-Coated... Hindawi Advances in Polymer Technology Volume 2023, Article ID 6271115, 15 pages https://doi.org/10.1155/2023/6271115 Research Article Influence of Drilling Parameters on the Delamination and Surface Roughness of Insulative-Coated Glass/Carbon- Hybrid Composite 1 2 2 3 Sarower Kabir , Faiz Ahmad, Chowdhury Ahmed Shahed, and Ebru Gunister Department of Mechanical and Production Engineering, Ahsanullah University of Science and Technology, Dhaka, Bangladesh Department of Mechanical Engineering, Universiti Teknologi PETRONAS (UTP), Perak, Malaysia Department of Mechanical Engineering, Istanbul Health and Technology University, TR-34015 Istanbul, Turkey Correspondence should be addressed to Sarower Kabir; sarower.mpe@aust.edu Received 4 October 2022; Revised 4 May 2023; Accepted 21 May 2023; Published 2 June 2023 Academic Editor: Ngoc A. Nguyen Copyright © 2023 Sarower Kabir et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Drilling in synthetic fiber-reinforced polymer composites is facing challenges due to their anisotropic, inhomogeneity, and abrasive machining behavior. The joining of composite parts using fasteners is commonly done by the drilling, and the generated heat is one of the main causes to damage the drilled hole in the composite. Moreover, the quality of drilled hole is crucial for joining parts effectively. The paper presents the design, fabrication, and drilling of a hybrid fiber-reinforced polymer (HFRP) based on insulative coating. These composites were fabricated using vacuum infusion molding (VIM) and coated with different thicknesses to investigate the influence of drilling parameters and associated damages. Cutting speed, feed rate, and coating thicknesses were varied, and a full factorial design of the experiment was formulated. High-speed steel (HSS) twist drill bit was used to drill the coated composite and test samples, and delamination factor and surface roughness were measured. ANOVA and full factorial response optimizer were used to evaluate the influence and optimum drilling parameters. The delamination factor (DF) at the entry and surface roughness were found to decrease with the increasing cutting speed. However, the DF at the exit showed the opposite. Coating thickness influenced the delamination at the entry whereas delamination at the exit has been found insignificant. For drilling HFRP composite with 1 mm coating thickness, 3000 RPM spindle speed and 0.08 mm/rev feed rate were found optimum parameters in minimizing surface roughness and delamination damage. However, 6000 RPM and 0.02 mm/rev were found optimum parameters for drilling HFRP composite with 1.5 mm coating thickness. 1. Introduction chemical engineering, and foremost civil engineering appli- cation due to their superior advantages over traditional The fastest growth in the industrial revolution has driven the materials [2]. Moreover, the FRP composites are widely used need for better materials in terms of strength, stiffness, resis- in the vital dessign and structure for well-known companies tance to fatigue, and corrosion with improved sustainability. such as Boeing and Airbus passenger plane's landing gears, Conventional materials are now being replaced by advanced and body parts of the racing cars [3–5]. Fibers are bounded composite materials due to their wide range of advantages in by the polymer matrix, thus transferring the load to the high-performance applications. It is not only found in racing fibers and also protecting fibers from environmental attack cars, sporting goods, and airplanes but also in the low-cost [6]. There are a wide variety of fibers and resin systems that high-volume industry like automotive [1]. Over the years, can be used to fabricate FRP composite, each of these having fiber-reinforced polymer (FRP) composites are getting huge its advantages and disadvantages. Besides that, the cost of attention due to their wide variety of applications in the field the materials, their strength to mass ratio, stiffness, fatigue of aerospace constructions, transportation, sporting goods, limit, and corrosion resistance are some other important 2 Advances in Polymer Technology requirements that must be met which is practically not possible delamination has a proportional relation with feed rate and to achieve in a single-type FRP composite. For example, having inversely associated with speed. The geometry of the drill higher strength, stiffness, and lower density than glass fiber, bit played an important role in forming delamination when using carbon fiber alone is not still recommended in the auto- drilling CFRP composite, and an investigation reported that mobile industry as it incurs higher costs. Therefore, hybridiza- a 5 mm diameter is optimum in minimizing the delamina- tion is an ideal concept that has been developed to attain tion [21]. Feed rate has a proportional relationship with desired properties in one single type of composite. The ultimate delamination while cutting speed is inversely related to advantage of using hybrid composite in advanced applications delamination [22]. Kumar and Singh [23] studied and lies in the synergistic effects of the constituent’smaterials. reported that increased feed rate also increases delamination High-strength carbon fiber and high elongation glass fiber are on both sides of the composite. Fiber push-out delamination popular choices in the manufacturing of composites [7, 8]. can occur when the drill bit is in contact with the workpiece Manders and Bader [9] demonstrated and proved that using due to higher applied forces generated [24]. Babu and Philip glass fiber and carbon fiber combination has better advantages [25] observed the feed rate as the most significant; thus, the due to higher specific strength, higher stiffness, higher elonga- delamination was stepped high with the increase of cutting tion, and higher strain to failure. Moorthy et al. [10] found that speed. Krishnamoorthy et al. [26] found feed rate as the the conglomeration of glass and carbon can significantly most significant parameter that affects the delamination improve the mechanical properties compared to the single glass when drilling CFRP composite. Kılıçkap [27] concluded in or carbon-based composites. Sayer et al. [11] reported that his research that delamination was higher at the exit side combining glass fiber reinforcement with carbon fiber in the in comparison to the entrance at a 13-30% rate and it can automobile industry not only reduces the weight of the part be minimized by setting a low cutting speed and feed rate. but also maintains the overall cost. They also added that glass Wang and Feng [28] studied and reported that spindle speed fiber-reinforced polymer (GFRP) and carbon fiber-reinforced plays a significant role in inducing roughness over the sur- polymer (CFRP) composites can be an ideal option in the face. Surface roughness is high when the feed rate is construction of wind turbine rotor blades. Drilling of FRP 0.010 mm/rev at a lower cutting speed, but roughness also composites comes with a challenge due to the damaging ten- increases when the cutting speed goes up [29]. Palanikumar dency of the materials under various cutting parameters. How- et al. [30] investigated the effect of drilling parameters on ever, to solve the drilling-generated damage incurred, many surface roughness and concluded that feed rate is the most researchers have studied the optimization of the drilling pro- significant factor; using a small drill diameter reduces sur- cess of single-type FRP composite [12–14]. Inappropriate selec- face roughness. When cutting speed is high, surface rough- tion of cutting parameters can lead to unacceptable damage in ness is low as higher cutting speed generates temperature materials such as fiber pull out, matrix cratering, thermal dam- which softens the work materials [31]. Feed rate and drill age, and delamination [15]. Drilling-generated damage such as diameter were found to significantly affect the surface fiber pull-out/push-out and delamination reduces the strength roughness followed by drilling speed [32]. Feed rate was against failure hence degrading the longevity of produced parts found to influence the surface roughness followed by cutting [16]. Delamination is the most occurred damage when drilling velocity and was investigated by Shunmugesh and Panneer- FRP composites and reduces the mechanical strengths. Delam- selvam [33]. Eneyew and Ramulu [5] reported that average ination occurs on both sides of the sample (entrance and exit), surface roughness was affected by point angle and better and investigation shows that push-out delamination (exit) is hole quality can be achieved with higher cutting speed and more drastic than peel up (entrance) [6]. Figures 1(a) and 1(b) lower feed rate. Feed rate was found the most prominent fac- shows a schematic diagram of geometrical damage and peel- tor with 39% and speed with 24% that affects the delamina- up and push-out delamination, respectively. Surface finish is tion in order to achieve a 5% level of significance when also a significant factor and focus study of many researchers drilled pultruded glass fiber polymer composite [34]. Marga- [17]; quality surface finish is one of the main determinant fac- bandu and Subramaniam [35] found that the drilling speed tors when selecting or rejecting an engineered part. During the was the most influencing factor that affects jute-/carbon- drilling, the cutting edges of the drill bit contact alternatively reinforced hybrid composite and suggested to drill at a speed with the separate oriented reinforced fibers; thus, dynamic of 1750 RPM and 0.03 mm/rev of feed. change of fiber cutting angle, distinct delamination profile, Shafi et al. [36] investigated the effects of silica gel as a and mode of chip removal (Figure 1(c)) can be observed [18]. heat insulation layer on silica aerogel/glass fiber composites Most researchers observed mainly four types of cutting models and concluded that the compressive strain was improved. for four relative fiber orientation angles with the cutting edge. C-bonded C-fiber-reinforced composites were fabricated Figure 1(d) illustrates the cutting mechanism where the and coated with novel carbon aerogel to improve the heat bending-induced fractured type, compression and interlaminar insulation properties [37]. Flammability was reduced for a shear type, crushing dominated type, and macro fracture type sustainable composite made of wool, canola oil, and sulphur, ° ° ° ° can be observed at 0 ,45 ,90 , and 135 angles, respectively. and the composite was considered favorable for future Kilickap [19] investigated the effect of cutting parame- energy conservation [38]. Khalili et al. [39] approached the ters such as cutting speed, feed rate, and drill point angle investigation of improving flame retardancy of Elium rein- on the delamination when drilling GFRP composite and forced natural fiber composite made of intumescent mats concluded that the cutting speed is the main influential fac- with consisting expandable graphite. They concluded that tor followed by feed rate. Köklü et al. [20] stated that the the flame retardancy was significantly enhanced due to the Advances in Polymer Technology 3 (a) (b) (d) Image Crack generation Micro-cracks Debris 0° 90° Matrix smearing Fibre pull-out Delamination Peel up Burrs 45° 135° Push-out (c) 𝜃 = 180° (0°) 𝜃 = 150° 𝜃 = 30° Direction 𝜃 = 120° 𝜃 = 60° 𝜃 = 90° 𝜃 = 90° 𝜃 Tearing V 𝜃 = 60° 𝜃 = 120° 𝜃 = 30° 𝜃 = 150° 𝜃 = 0° (180°) Figure 1: Schematic drilling challenges in CFRP: (a) macro and micro geometrical damages, (b) peel-up and push-out delamination, (c) burr characteristics [18], and (d) material removal and delamination for different fiber orientation. expansion of expandable graphite flakes. However, lack of and carbon fiber was 0.31 mm and 0.16 mm, respectively. study was noticed on the machining performance of Carbon fiber was taken first to achieve a higher flexural hybrid/composites when coated with insulative materials. modulus [42], and the experimental setup is given in In this study, we are aimed to investigate the effects of Figure 4. The vacuum pressure was kept at 80 kPa to pro- drilling parameters on the insulative-coated HFRP compos- duce a bubble-free sample with a fiber resin ratio of 46 : 54. ite and expected to obtain an impact on the delamination behavior of the drilled materials. In addition, it is also 2.2. Preparation of Insulative Coating. The constituent mate- important to look at the scope of composite materials in rials required to formulate the coating were purchased from heat-resistant applications in various sensitive industries. different suppliers. Zirconium phosphate (ZrP) was pur- chased from Sichuan HongChang Plastics Indus. Co. Ltd., China. Expandable graphite (EG), boric acid (BA), ammo- 2. Experimental nium polyphosphate (APP), melamine (MEL), and halloy- 2.1. Fabrication of Hybrid Fiber-Reinforced Polymer (HFRP) site nanotube (HNT) were purchased from Sigma-Aldrich Composite. Carbon and glass fibers in the woven form were (M) Sdn Bhd., Malaysia. Epoxy resin BE-188 (BPA) and hardener H-2310 polyamide amine were brought from Mc- used as the reinforcement materials (Figure 2) and epoxy as the matrix material in this investigation and purchased from Growth Chemical Sdn Bhd., Malaysia. The detailed formula- RP Products Sdn. Bhd. and Advance Altimas Sdn. Bhd., tion ingredients are given in Table 2. Malaysia, respectively. The mechanical properties of glass All the ingredients such as APP, BA, MEL, HNT, and and carbon fibers are given in Table 1 according to the ZrP were mixed according to formulations given in Table 2 spreadsheet provided by the supplier. Other accessories such and ground in a shear mixer for about 90 seconds with as peel plies, mesh, spiral tubes, hose pipes, and vacuum bags 21000 RPM to make it homogenous. EG was later added to required to manufacture the composite laminate were also the mixture and stirred a little with a spoon to prevent the purchased from UK Composite, UK. EG from grinding, as bigger EG flakes are responsible for The hybrid composite was fabricated using the vacuum better expansion. After that, epoxy (BE-188) was added to infusion molding method in the lab. Orientation of fiber the mixture followed by the hardener (H-2310) and stirred materials was maintained at 0/90 , and the lamina stacking at about 40 RPM for 15 min and then 5 min at 60 RPM, sequence (C-G-G-G-C-G-G-G-C) (Figure 3) was selected respectively, by using an automatic shear mixer CAFRAMO to measure delamination and average roughness on the sur- (BDC 6015-220). A total of 24 samples of 70 mm × 25 mm face of the drilled component. Stacking carbon fiber at the was used as substrate, and coating was applied (Figure 5) exterior helps the bending deformation, and stacking in core using a hand lay-up technique with the help of a specially reduces delamination during drilling [40]. Placing glass fiber designed mold with a 1 mm pitch screw where every half right after carbon fiber helps reducing the propagation of turn of the screw makes the 0.5 mm vertical displacement microcracks at the interface [41]. The thickness of glass fiber (Figure 6) which ensures the desired thickness of 0.5, 1.0, 4 Advances in Polymer Technology (a) (b) Figure 2: Woven fibrous materials used in this investigation: (a) E-glass and (b) carbon. Table 1: Mechanical properties of woven glass and carbon fibers. 3.1. Measurement of Delamination Factor (DF). Delamina- Properties Glass fiber Carbon fiber tion factor (DF) is considered the major and well-known tool to determine the drill hole quality at the entrance and Tensile strength (ksi) 360 512 exit. DF of the drilled hole was identified according to equa- Tensile modulus (msi) 11.4 33.4 tion (1) and Figure 9(a). Strain to failure (%) 3.0 1.5 max DF = , ð1Þ hole Carbon fiber where D is the maximum diameter after drilling and D Glass fiber max hole is the nominal diameter of the hole. Leica LX 00971A optical microscope was used to identify the delamination on the drilled samples (Figure 9(b)). The Glass fiber microscope magnified 5 times of 1 mm resolution. Maxi- Glass fiber mum diameter and nominal diameter were evaluated through the images by using smart dimensioning tool of SOLIDWORKS software. Figure 3: Fiber’s orientation. 3.2. Measurement of Surface Roughness. The surface topol- 1.5, and 2.0 mm and the curing time was a day at the ambi- ogy of the drilled HFRP composites was obtained by ent room temperature. employing scanning electronic microscopy (SEM, Phenom, Pro-X, Netherlands). Samples were first cut at a dimension 2.3. Drilling of HFRP Composite. A drill bit with a 5 mm of 1mm × 1mm using an abrasive cutter. The average diameter of high-speed steel (HSS) was selected in this study roughness (R ) and roughness height (R ) were assessed a z to investigate the effect of drilling parameters on the delam- from 3D images based on “shape from shading” technology. ination and surface roughness of coated HFRP composite The device was operated at 15 kV, and the field of view was with various coating thicknesses. To perform the drilling 964 μm. The images were captured at different magnifica- operation, Mazak variaxis 630 CNC machines were used tions to obtain clear surface roughness information. without pouring out coolant to avoid moisture absorption which may affect the microstructure, dimensional accuracy, 4. Results and Discussion and mechanical properties of the composite [43]. The sam- ple size selected was 70 mm in length and 25 mm in width. The procedure discussed above was repeated corresponding The samples were clamped by the fixture. Each sample had to the experimental sequence provided in Table 3 according undergone drilling operation on it using a 5 mm HSS drill to the ANOVA full factorial analysis, and the measured bit shown in Figure 7. values are shown in the table. 4.1. Analyzing the Effects of Drilling Parameters on the 3. Characterizations Delamination at the Entrance and Exit. Effects of cutting The quality of drilled hole was evaluated based on the parameters on the delamination at the entry and exit were delamination factor and surface roughness of the HFRP observed and analyzed after drilling. DF at the entry was composite. Figures 8(a) and 8(b) show drilled holes obtained found to decrease with the increasing cutting speeds shown for uncoated and coated HFRP composites, respectively. in Figures 10(a) and 10(b). This scenario can be elucidated Advances in Polymer Technology 5 Vacuum infusion molding (VIM) setup Vacuum chamber Vacuum pump Inlet Outlet Resin Mold Figure 4: Vacuum infusion molding (VIM) setup used in the lab. Table 2: Formulation of insulative coating used in this study (gm). APP MEL BA EG HNT ZrP Epoxy resin Hardener 11.36 5.5 11 5.5 0.5 0.5 41.94 19.72 from 1.076 to 1.220. The lowest DF value at the exit was obtained at 4500 RPM, 0.08 mm/rev, and 0.5 mm thick coat- ing. The highest delamination damage at the exit was obtained at the drilling condition of 6000 RPM, 0.08 mm/ rev, and 2.0 mm thick coating. This could be happened due to the different states of the top and bottom surfaces of the composites. At the entry, the composites were experiencing compression, and at the exit, those were in tension. Coating thicknesses have played a vital role in the damage factor at the entry. It has been observed that the delamination was Figure 5: Coated HFRP composite samples. found to increase with the increased cutting speeds on exit sides for a 2.0 mm thickly coated sample. This is because the coating agitated the fiber push out at the exit; however, by relating to the temperature generated during the drilling. the coating thickness had no significant impact on DF. This happened because of the brittle nature of the coating as brit- Higher cutting speed produces heat at the drilling zone which soften the composites, and thus, reduced delamina- tle materials have a lack of ductility in tension and they will tion factors can be observed [44]. On the other hand, this fail prematurely [46]. Therefore, the changes in the thickness of the coating failed to show any significant improvements. scenario can be illustrated as the high spindle speed produc- ing high shear force; therefore, the composites undergo a On the other hand, delamination at the entry was reduced with the increase of the coating thickness. DF at the entry shear deformation, thus reducing delamination. Again, delamination had shown a proportional relationship with was 5.5% and 2.7% less in comparison to the 0.5 mm thick the feed rate as it increased with the increase of the feed rate. coated samples while drilling at 3000 and 4500 RPM, respec- tively. This was presumable that the entry point is in com- Thrust force increased when feed rate is increased due to expanding cross-sectional area thus producing more delam- pression and brittle materials under compressive load tend to close up the transverse crack; therefore, the delamination ination [45]. DF at the entry ranges from 1.020 to 1.178, as presented in Table 3. The lowest DF value at the entry was was lesser [47]. obtained at the drilling condition of 3000 RPM, 0.02 mm/ rev, and 2.0 mm thick coating; meanwhile, the highest 4.2. Optimized Drilling Parameters for Delamination. The delamination onset was obtained at 6000 RPM, 0.08 mm/ influence of speed, feed rate, and coating thickness on the delamination at the entry of coated HFRP composite was rev, and 2.0 mm thick coating. This is because when an overly brittle coating of 2.0 mm thick drilled at a high speed calculated using ANOVA and presented in Table 4. ANOVA and feed, it could not take much compressive force that leads table suggested that the interactions between speeds and to the delay in damping effect and fracture and finally coating thicknesses are the most important factor that affects resulted higher delamination at the entrance. However, the delamination at the entry followed by coating thickness alone and the interactions between feed rate and coating delamination at the exit showed the opposite behavior. DF was found to increase with the cutting speeds shown in thicknesses. F value is > F for both the interactions 0:05 Figures 10(c) and 10(d). DF at the exit has higher delamina- obtained, and the corresponding P value is less than 0.05. tion damages than at the entrance. DF at the exit ranges Feed rate alone is found insignificant since the F value and 6 Advances in Polymer Technology Screw Composite substrate placed inside mold Mold Figure 6: Especially designed adjustable mold used in this study to maintain coating thickness. Flute length 25 mm values. Table 5 shows the response table of S/N ratio for the delamination at the entry and exit, and Figure 12 illustrates the main effects obtained from the S/N plots and highlights that coating thickness and cutting speed played the vital role in the delamination factor at the entrance and exit. Feed rate No. of flute of two was found to be less significant compared to speed and coat- ing thickness. Figure 7: HSS twist drill bit geometry used in this study. A full factorial response optimizer was used to determine the optimum cutting condition in drilling coated HFRP com- P value are showing opposite results to the others. Table 4 posite and presented in Figure 13. 3000 RPM, 0.08 mm/rev illustrates that the 98.90% variations in the response are feed rate, and 1.0 mm coating thickness are considered the explained by the model and it is considered significant. optimum cutting condition. Moreover, the difference between the R-square value and R -square (predicted) value is acceptable which gives us the 4.3. Analyzing the Effects of Drilling Parameters on the idea that all the parameters involved are significant. Surface Roughness. Understanding the precision of the Similarly, the influence of speed, feed rate, and coating drilled hole part and measurement of roughness is impor- thickness on the delamination at the exit of coated HFRP tant, and it is occurred due to the inappropriate fracture of composite was calculated using ANOVA and presented in fiber leading to the sharp end in the inner surface, failure Table 4. ANOVA table suggested that the cutting speed is under fatigue load, high friction, and generation of heat at the drilled wall [35]. The surface roughness (R )ofthe the major factor that is affecting the delamination at the exit followed by coating thickness. Feed rate has been found drilled hole wall has been found to decrease with increasing nonsignificant, and no interaction effects were present. This cutting speeds shown in Figure 14(a). Surface roughness could be happened due to the notch sensitivity of the brittle values range from 1.30 μm to 1.82 μm, as highlighted in materials. Table 4 illustrates that 82.72% of variations in the Table 3. The lowest roughness value was obtained at the dril- response are explained by the model and the difference ling condition of 6000 RPM, 0.02 mm/rev, and 1.5 mm thick between the R-square value and R-square (predicted) value coating. This might be happened due to the smearing effect is quite high enough (17.16%) to give us the idea that some in the fiber-matrix composite at the elevated temperature. of the important factors like temperature, the bonding The highest roughness value was noticed at the drilling con- strength between layers, axial thrust force, and drill geome- dition of 3000 RPM, 0.08 mm/rev, and 0.5 mm thick coating. It has been observed that 0.5 mm thick coating samples try are missing. Figure 11 presents the residual plots for both delamina- showed a 3.8% increment in roughness value when the feed tion at the entry and exit for coated HFRP composite. In rate increased from 0.02 mm/rev to 0.08 mm/rev at both cases, the normal probability plot shows that all the 3000 RPM. However, the roughness value was reduced by points are close to the straight line meaning no unusual approximately 20% and 16% when the cutting speed increased from 3000 RPM to 6000 RPM for the similar con- observations. The residual vs. fit graph shows that all the data are nearly distributed evenly and randomly below and dition of 0.02 and 0.08 mm/rev feed rate and 0.5 mm thick above the straight line. Bell shape curve is obtained for coating, respectively. Comparably, measured roughness delamination at the exit shown in the histogram. No specific value (R ), while drilling at 3000 RPM and 0.02 mm/rev feed pattern is shown in the residual vs. observation graph mean- with 1.0 mm thick coating, was obtained as 1.69 μm, but it ing no biasness involved in the data set. has decreased to 1.33 μm when speed increased to S/N ratio, a systematic approach to analyze the response 6000 RPM. In all the cases, the surface roughness of the parameters, has been used in this study. The greater is the drilled holes shows an inverse relationship with cutting value of S/N ratio, the lesser is the variance in the optimum speeds. This scenario is common in other machining 85° Advances in Polymer Technology 7 (a) (b) Figure 8: Drilled HFRP composite: (a) uncoated; (b) coated. max (a) (b) Figure 9: (a) Microscopic view of damaged HFRP composite surface. (b) Optical microscope to identify delamination. Table 3: Experimental sequence followed to drill the HFRP composite samples using full factorial design of experiment. Feed rate Surface roughness Sample no. Spindle speed (RPM) IC thickness (mm) Delamination (entry) Delamination (exit) (mm/rev) (μm) 1 0.5 1.080 1.086 1.75 2 1.0 1.070 1.090 1.69 0.02 3 1.5 1.060 1.100 1.56 4 2.0 1.020 1.160 1.47 5 0.5 1.080 1.080 1.82 6 1.0 1.050 1.108 1.71 0.08 7 1.5 1.068 1.078 1.52 8 2.0 1.038 1.080 1.61 9 0.5 1.080 1.100 1.63 10 1.0 1.068 1.110 1.58 0.02 11 1.5 1.060 1.120 1.61 12 2.0 1.050 1.158 1.42 13 0.5 1.064 1.076 1.70 14 1.0 1.052 1.110 1.63 0.08 15 1.5 1.062 1.102 1.62 16 2.0 1.080 1.128 1.53 17 0.5 1.052 1.116 1.41 18 1.0 1.040 1.150 1.33 0.02 19 1.5 1.026 1.166 1.30 20 2.0 1.140 1.206 1.35 21 0.5 1.034 1.116 1.52 22 1.0 1.040 1.150 1.47 0.08 23 1.5 1.026 1.160 1.46 24 2.0 1.178 1.220 1.49 8 Advances in Polymer Technology 1.20 1.20 1.18 1.18 1.16 1.16 1.14 1.14 1.12 1.12 1.10 1.10 1.08 1.08 1.06 1.06 1.04 1.04 1.02 1.00 1.02 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (a) (b) 1.24 1.24 1.22 1.22 1.20 1.20 1.18 1.18 1.16 1.16 1.14 1.14 1.12 1.12 1.10 1.10 1.08 1.08 1.06 1.06 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (c) (d) Figure 10: DF: at the entry (a) 0.02 mm/rev and (b) 0.08 mm/rev and at the exit (c) 0.02 mm/rev and (d) 0.08 mm/rev. methods as well. According to Ghani et al., the cutting pro- effects mentioned earlier because of the large vertical dis- cess becomes stable more at the high cutting speed [48]. placement component due to the high feed rate. Exactly sim- Also, researchers showed that the interactions and the ilar behavior was noticed when coating thickness varied adherence between the composites and cutters are more at between 1.5 mm and 2.0 mm. In Figure 15(b), the 3 K, the lower cutting speeds, thus creating a built-up edge that 4.5 K, and 6 K indicate the spindle speed in RPM and the may lead to a rough surface [49]. This scenario is more likely 0.02 and 0.08 represent the feed rate in millimeters. It is clear here. However, keeping the speed (3000 and 6000 RPM) and that the coating thickness has been slightly significant to coating thickness (1.0 mm) constant, the roughness value reduce the surface roughness at a lower cutting speed. How- was found higher for the 0.08 mm/rev feed rate. Composite ever, it was not found significant at a relatively higher cut- laminates when drilling at higher spindle speeds and feed ting speed. Among all, 6000 RPM has given the U-shaped rate, increases the temperature of the accumulated heat curve with the lowest roughness values with different coating around the drill cutting edges due to the low thermal coeffi- thicknesses. And overall, 1.5 mm coating thickness has been cient and destroys the matrix stability, and produces rough found optimum for R . cuts around the wall which leads to the surface roughness Figures 15(a) and 15(b) present the SEM texture [50]. This can be distinguished differently from the smearing obtained for drilled hole wall of insulative-coated HFRP DF (exit) DF (entry) DF (exit) DF (entry) Advances in Polymer Technology 9 Table 4: ANOVA table for delamination factor. At the entry At the exit Source Degree of freedom F value P value Degree of freedom F value P value Model 15 47.75 0.001 6 13.56 0.001 Linear 6 20.90 0.001 6 13.56 0.001 Spindle speed (S) 2 4.70 0.045 2 23.18 0.001 Feed rate (F) 1 0.35 0.573 1 2.46 0.135 Coating thickness (C) 3 38.55 0.001 3 10.86 0.001 2-way interactions 9 65.66 0.001 —— — S C 6 91.53 0.001 —— — F C 3 13.92 0.002 —— — Error 8 —— 17 —— Total 23 —— 23 —— Standard deviation 0.0062452 0.0189956 R-square 98.90% 82.72% R-square (adjusted) 96.82% 76.62% R-square (predicted) 90.06% 65.56% Residual plots for DF entry Residual plots for DF exit Normal probability plot Versus fits Normal probability plot Versus fits 99 99 0.010 0.010 90 90 0.005 0.005 50 0.000 50 0.000 –0.005 –0.005 10 10 –0.010 –0.010 1 1 –0.02 –0.01 0.00 0.01 0.02 1.05 1.10 1.15 1.20 –0.02 –0.01 0.00 0.01 0.02 1.1 1.2 1.3 1.4 Residual Fitted value Residual Fitted value Versus order Versus order Histogram Histogram 0.010 2.0 2.0 0.010 0.005 1.5 0.005 1.5 0.000 0.000 1.0 1.0 –0.005 –0.005 0.5 0.5 –0.010 –0.010 0.0 0.0 –0.010–0.005 0.000 0.005 0.010 –0.010 –0.005 0.000 0.005 0.010 1 2 3 4 5 6 1 2 3 4 5 6 Residual Observation order Residual Observation order Figure 11: Residual plots obtained for delamination at the entry and exit. fibers at the exit wall which occurred due to the higher speed Table 5: Response table for delamination at the entry and exit (S/N ratio); smaller is better. and feed exerted on the overly brittle coating that also delayed the damping effect. Level Cutting speed Feed rate Coating thickness 1 -0.6552 -0.8014 -0.6728 4.4. Optimized Drilling Parameters for Surface Roughness. 2 -0.7409 -0.7549 -0.7254 The influence of cutting speeds and feed rates on the surface 3 -0.9384 -0.7204 roughness of coated HFRP composite was calculated using 4 -0.9941 ANOVA and presented in Table 6. ANOVA table suggested Delta 0.2832 0.0465 0.3213 that cutting speed has the most impact on the surface rough- Rank 2 3 1 ness followed by feed rate, coating thickness, and speed interaction with coating thicknesses. The combined effect of speed and feed rate has nonsignificant interaction since composite when drilling at 6000 RPM at varied feed and their P value is slightly over 0.05. Table 6 illustrates that thickness of coating due to the lower roughness value the 97.32% variations in the response are explained by obtained for these two drilling conditions. Morphology is the model and it is considered significant. Moreover, the shown in Figure 15(a) which illustrates that good fiber-matrix difference between the R-square value and R-square (pre- interfacial bonding is present and no matrix cracking and dicted) value is quite high (16.4%) which gives us the idea push-/pull-out damage is observed. However, Figure 15(b) that some of the important factors such as temperature shows fiber pull out and matrix crack in the SEM image indi- and machine vibration are missing that are not considered cating surface damage. This damage resulted from the loose in this research. Frequency Percent Residual Residual Frequency Percent Residual Residual 10 Advances in Polymer Technology Main effects plot for SN ratios Data means Cutting speed Feed rate Coating thickness –0.65 –0.70 –0.75 –0.80 –0.85 –0.90 –0.95 –1.00 3000 4500 6000 0.02 0.08 0.5 1.0 1.5 2.0 Signal-to-noise: Smaller is better Figure 12: S/N ratio plots for delamination at the entry and exit. Spindle Feed rate Coating Optimal High 6000 0.08 2 D: 0.8474 Cur 3000 0.08 1 Predict Low 3000 0.02 0.5 Composite desirability D: 0.8474 DF exit minimum y = 1.0882 d = 0.91493 DF entry minimum y = 1.0540 d = 0.78481 Figure 13: Optimized cutting speed, feed rate, and coating thickness achieved to minimize delamination for coated HFRP composite. Figure 16 presents the residual plots for surface rough- obtained from the S/N plots and highlights that the cutting ness of coated HFRP composite. Normal probability plot speed contributed highest in the roughness occurred in the shows that all the points are close to the straight line mean- surface of the composites followed by the coating thickness ing no unusual observations. The residual vs. fit graph shows and feed rate. that all the data are distributed evenly below and above the A full factorial response optimizer was used to deter- straight line. Bell shape curve is obtained in the histogram. mine the optimum cutting condition in drilling coated No specific pattern is shown in the residual vs. observation HFRP composite and presented in Figure 18. 6000 RPM, graph meaning no biasness involved in the data set. 0.02 mm/rev feed rate, and 1.5 mm coating thickness are Table 7 shows the response table of S/N ratio for the sur- considered the optimum cutting condition for minimizing face roughness, and Figure 17 illustrates the main effects surface roughness. Mean of SN ratios Advances in Polymer Technology 11 1.8 2.0 1.9 1.7 𝜇 𝜇 1.8 1.6 1.7 1.5 1.6 1.4 1.5 1.3 1.4 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (a) 1.8 1.7 1.6 1.5 1.4 1.3 0.0 0.5 1.0 1.5 2.0 2.5 Coat. Thickness (mm) 3 K, 0.02 4.5 K, 0.08 3 K, 0.08 6 K, 0.02 4.5 K, 0.02 6 K, 0.08 (b) Figure 14: (a) Surface roughness (R ) for varied drilling parameters and (b) roughness and coating thickness comparison plot. Matrix crack Fiber pull-out (a) (b) Figure 15: Microscopic image of drilled hole wall: (a) 6000 RPM, 0.02 mm/rev, and 1.5 mm thick coating; (b) 6000 RPM, 0.08 mm/rev, and 2.0 mm thick coating. Good fiber orientation and interfacial bonding with matrix Surface roughness ( m) Roughness ( m) Surface roughness ( m) 12 Advances in Polymer Technology Table 6: ANOVA table for surface roughness. Source Degree of freedom F value P value Model 14 23.38 0.001 Linear 6 46.95 0.001 2 90.68 0.001 1 31.07 0.001 3 23.10 0.001 2-way interactions 8 5.71 0.009 S F 2 4.03 0.056 S C 6 6.26 0.008 Error 9 —— Total 23 —— Standard deviation 0.0351584 R-square 97.32% R-square (adjusted) 93.16% R-square (predicted) 80.97% Residual plots for surface roughness Normal probability plot Versus fits 0.050 0.025 50 0.000 –0.025 –0.050 –0.050 –0.025 0.000 0.025 0.050 1.4 1.6 1.8 Residual Fitted value Histogram Versus order 0.050 6.0 0.025 4.5 0.000 3.0 –0.025 1.5 –0.050 0.0 –0.04 –0.02 0.00 0.02 0.04 2 4 6 8 10 12 14 16 18 20 22 24 Residual Observation order Figure 16: Residual plots obtained for surface roughness. Table 7: Response table for surface roughness (S/N ratio); smaller is better. Level Cutting speed Feed rate Coating thickness 1 -4.283 -3.531 -4.256 2 -4.017 -4.009 -3.876 3 -3.010 -3.565 4 -3.382 Delta 1.273 0.478 0.874 Rank 1 3 2 Frequency Percent Residual Residual Advances in Polymer Technology 13 Main effects plot for SN ratios Data means Cutting speed Feed rate Coating thickness –3.0 –3.2 –3.4 –3.6 –3.8 –4.0 –4.2 –4.4 3000 4500 6000 0.02 0.08 0.5 1.0 1.5 2.0 Signal-to-noise: smaller is better Figure 17: S/N ratio plots for surface roughness. Spindle Feed rate Coating Optimal High 6000 0.08 2 D: 0.9784 Cur 6000 0.02 1.5 Predict Low 3000 0.02 0.5 Surface minimum y = 1.3112 d = 0.97837 Figure 18: Optimized cutting speed, feed rate, and coating thickness achieved to minimize surface roughness for coated HFRP composite. 5. Conclusions to analyze the response parameters, and it is clearly visible that coating thickness and speed have the dominant effect on This study has successfully fabricated HFRP composite, and delamination. For coated HFRP composite, a coating thick- the influence of drilling parameters such as cutting speed ness of 1 mm, cutting speed of 3000 RPM, and feed rate of and feed rate on delamination and surface roughness on 0.08 mm/rev are the optimized parameters for minimizing insulative-coated HFRP composite has been investigated and delamination based on the full factorial response optimizer. well presented. In general, it can be concluded from the discus- Similarly, surface roughness was found to have decreased with sion that the delamination factor (DF) at the entrance the increasing cutting speeds. Lower cutting speed generates decreases with the increasing cutting speeds. On the other higher roughness whereas higher cutting speed incurs lower hand, the delamination factor at the exit showed the opposite roughness. S/N plots also showed that cutting speed is pivotal manner. However, coating thickness played an important role in occurring roughness on the wall of drilled composites. in delamination at the entrance. The DF was found higher However, based on the full factorial response optimizer, cut- with increasing cutting speeds especially when the coating ting speed of 6000 RPM, feed rate of 0.02 mm/rev, and coating thickness was higher. But the exit side of the hole showed no thickness of 1.5 mm are considered optimum in minimizing significant difference in this manner. S/N plot has been used surface roughness when drilling coated HFRP composites. Mean of SN ratios 14 Advances in Polymer Technology Data Availability [12] P. Benardos and G.-C. Vosniakos, “Predicting surface rough- ness in machining: a review,” International Journal of Machine All data including figures, tables, and experimental results Tools and Manufacture, vol. 43, no. 8, pp. 833–844, 2003. are available. [13] A. M. Abrão, J. C. C. Rubio, P. E. Faria, and J. P. 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Influence of Drilling Parameters on the Delamination and Surface Roughness of Insulative-Coated Glass/Carbon-Hybrid Composite

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Hindawi Advances in Polymer Technology Volume 2023, Article ID 6271115, 15 pages https://doi.org/10.1155/2023/6271115 Research Article Influence of Drilling Parameters on the Delamination and Surface Roughness of Insulative-Coated Glass/Carbon- Hybrid Composite 1 2 2 3 Sarower Kabir , Faiz Ahmad, Chowdhury Ahmed Shahed, and Ebru Gunister Department of Mechanical and Production Engineering, Ahsanullah University of Science and Technology, Dhaka, Bangladesh Department of Mechanical Engineering, Universiti Teknologi PETRONAS (UTP), Perak, Malaysia Department of Mechanical Engineering, Istanbul Health and Technology University, TR-34015 Istanbul, Turkey Correspondence should be addressed to Sarower Kabir; sarower.mpe@aust.edu Received 4 October 2022; Revised 4 May 2023; Accepted 21 May 2023; Published 2 June 2023 Academic Editor: Ngoc A. Nguyen Copyright © 2023 Sarower Kabir et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Drilling in synthetic fiber-reinforced polymer composites is facing challenges due to their anisotropic, inhomogeneity, and abrasive machining behavior. The joining of composite parts using fasteners is commonly done by the drilling, and the generated heat is one of the main causes to damage the drilled hole in the composite. Moreover, the quality of drilled hole is crucial for joining parts effectively. The paper presents the design, fabrication, and drilling of a hybrid fiber-reinforced polymer (HFRP) based on insulative coating. These composites were fabricated using vacuum infusion molding (VIM) and coated with different thicknesses to investigate the influence of drilling parameters and associated damages. Cutting speed, feed rate, and coating thicknesses were varied, and a full factorial design of the experiment was formulated. High-speed steel (HSS) twist drill bit was used to drill the coated composite and test samples, and delamination factor and surface roughness were measured. ANOVA and full factorial response optimizer were used to evaluate the influence and optimum drilling parameters. The delamination factor (DF) at the entry and surface roughness were found to decrease with the increasing cutting speed. However, the DF at the exit showed the opposite. Coating thickness influenced the delamination at the entry whereas delamination at the exit has been found insignificant. For drilling HFRP composite with 1 mm coating thickness, 3000 RPM spindle speed and 0.08 mm/rev feed rate were found optimum parameters in minimizing surface roughness and delamination damage. However, 6000 RPM and 0.02 mm/rev were found optimum parameters for drilling HFRP composite with 1.5 mm coating thickness. 1. Introduction chemical engineering, and foremost civil engineering appli- cation due to their superior advantages over traditional The fastest growth in the industrial revolution has driven the materials [2]. Moreover, the FRP composites are widely used need for better materials in terms of strength, stiffness, resis- in the vital dessign and structure for well-known companies tance to fatigue, and corrosion with improved sustainability. such as Boeing and Airbus passenger plane's landing gears, Conventional materials are now being replaced by advanced and body parts of the racing cars [3–5]. Fibers are bounded composite materials due to their wide range of advantages in by the polymer matrix, thus transferring the load to the high-performance applications. It is not only found in racing fibers and also protecting fibers from environmental attack cars, sporting goods, and airplanes but also in the low-cost [6]. There are a wide variety of fibers and resin systems that high-volume industry like automotive [1]. Over the years, can be used to fabricate FRP composite, each of these having fiber-reinforced polymer (FRP) composites are getting huge its advantages and disadvantages. Besides that, the cost of attention due to their wide variety of applications in the field the materials, their strength to mass ratio, stiffness, fatigue of aerospace constructions, transportation, sporting goods, limit, and corrosion resistance are some other important 2 Advances in Polymer Technology requirements that must be met which is practically not possible delamination has a proportional relation with feed rate and to achieve in a single-type FRP composite. For example, having inversely associated with speed. The geometry of the drill higher strength, stiffness, and lower density than glass fiber, bit played an important role in forming delamination when using carbon fiber alone is not still recommended in the auto- drilling CFRP composite, and an investigation reported that mobile industry as it incurs higher costs. Therefore, hybridiza- a 5 mm diameter is optimum in minimizing the delamina- tion is an ideal concept that has been developed to attain tion [21]. Feed rate has a proportional relationship with desired properties in one single type of composite. The ultimate delamination while cutting speed is inversely related to advantage of using hybrid composite in advanced applications delamination [22]. Kumar and Singh [23] studied and lies in the synergistic effects of the constituent’smaterials. reported that increased feed rate also increases delamination High-strength carbon fiber and high elongation glass fiber are on both sides of the composite. Fiber push-out delamination popular choices in the manufacturing of composites [7, 8]. can occur when the drill bit is in contact with the workpiece Manders and Bader [9] demonstrated and proved that using due to higher applied forces generated [24]. Babu and Philip glass fiber and carbon fiber combination has better advantages [25] observed the feed rate as the most significant; thus, the due to higher specific strength, higher stiffness, higher elonga- delamination was stepped high with the increase of cutting tion, and higher strain to failure. Moorthy et al. [10] found that speed. Krishnamoorthy et al. [26] found feed rate as the the conglomeration of glass and carbon can significantly most significant parameter that affects the delamination improve the mechanical properties compared to the single glass when drilling CFRP composite. Kılıçkap [27] concluded in or carbon-based composites. Sayer et al. [11] reported that his research that delamination was higher at the exit side combining glass fiber reinforcement with carbon fiber in the in comparison to the entrance at a 13-30% rate and it can automobile industry not only reduces the weight of the part be minimized by setting a low cutting speed and feed rate. but also maintains the overall cost. They also added that glass Wang and Feng [28] studied and reported that spindle speed fiber-reinforced polymer (GFRP) and carbon fiber-reinforced plays a significant role in inducing roughness over the sur- polymer (CFRP) composites can be an ideal option in the face. Surface roughness is high when the feed rate is construction of wind turbine rotor blades. Drilling of FRP 0.010 mm/rev at a lower cutting speed, but roughness also composites comes with a challenge due to the damaging ten- increases when the cutting speed goes up [29]. Palanikumar dency of the materials under various cutting parameters. How- et al. [30] investigated the effect of drilling parameters on ever, to solve the drilling-generated damage incurred, many surface roughness and concluded that feed rate is the most researchers have studied the optimization of the drilling pro- significant factor; using a small drill diameter reduces sur- cess of single-type FRP composite [12–14]. Inappropriate selec- face roughness. When cutting speed is high, surface rough- tion of cutting parameters can lead to unacceptable damage in ness is low as higher cutting speed generates temperature materials such as fiber pull out, matrix cratering, thermal dam- which softens the work materials [31]. Feed rate and drill age, and delamination [15]. Drilling-generated damage such as diameter were found to significantly affect the surface fiber pull-out/push-out and delamination reduces the strength roughness followed by drilling speed [32]. Feed rate was against failure hence degrading the longevity of produced parts found to influence the surface roughness followed by cutting [16]. Delamination is the most occurred damage when drilling velocity and was investigated by Shunmugesh and Panneer- FRP composites and reduces the mechanical strengths. Delam- selvam [33]. Eneyew and Ramulu [5] reported that average ination occurs on both sides of the sample (entrance and exit), surface roughness was affected by point angle and better and investigation shows that push-out delamination (exit) is hole quality can be achieved with higher cutting speed and more drastic than peel up (entrance) [6]. Figures 1(a) and 1(b) lower feed rate. Feed rate was found the most prominent fac- shows a schematic diagram of geometrical damage and peel- tor with 39% and speed with 24% that affects the delamina- up and push-out delamination, respectively. Surface finish is tion in order to achieve a 5% level of significance when also a significant factor and focus study of many researchers drilled pultruded glass fiber polymer composite [34]. Marga- [17]; quality surface finish is one of the main determinant fac- bandu and Subramaniam [35] found that the drilling speed tors when selecting or rejecting an engineered part. During the was the most influencing factor that affects jute-/carbon- drilling, the cutting edges of the drill bit contact alternatively reinforced hybrid composite and suggested to drill at a speed with the separate oriented reinforced fibers; thus, dynamic of 1750 RPM and 0.03 mm/rev of feed. change of fiber cutting angle, distinct delamination profile, Shafi et al. [36] investigated the effects of silica gel as a and mode of chip removal (Figure 1(c)) can be observed [18]. heat insulation layer on silica aerogel/glass fiber composites Most researchers observed mainly four types of cutting models and concluded that the compressive strain was improved. for four relative fiber orientation angles with the cutting edge. C-bonded C-fiber-reinforced composites were fabricated Figure 1(d) illustrates the cutting mechanism where the and coated with novel carbon aerogel to improve the heat bending-induced fractured type, compression and interlaminar insulation properties [37]. Flammability was reduced for a shear type, crushing dominated type, and macro fracture type sustainable composite made of wool, canola oil, and sulphur, ° ° ° ° can be observed at 0 ,45 ,90 , and 135 angles, respectively. and the composite was considered favorable for future Kilickap [19] investigated the effect of cutting parame- energy conservation [38]. Khalili et al. [39] approached the ters such as cutting speed, feed rate, and drill point angle investigation of improving flame retardancy of Elium rein- on the delamination when drilling GFRP composite and forced natural fiber composite made of intumescent mats concluded that the cutting speed is the main influential fac- with consisting expandable graphite. They concluded that tor followed by feed rate. Köklü et al. [20] stated that the the flame retardancy was significantly enhanced due to the Advances in Polymer Technology 3 (a) (b) (d) Image Crack generation Micro-cracks Debris 0° 90° Matrix smearing Fibre pull-out Delamination Peel up Burrs 45° 135° Push-out (c) 𝜃 = 180° (0°) 𝜃 = 150° 𝜃 = 30° Direction 𝜃 = 120° 𝜃 = 60° 𝜃 = 90° 𝜃 = 90° 𝜃 Tearing V 𝜃 = 60° 𝜃 = 120° 𝜃 = 30° 𝜃 = 150° 𝜃 = 0° (180°) Figure 1: Schematic drilling challenges in CFRP: (a) macro and micro geometrical damages, (b) peel-up and push-out delamination, (c) burr characteristics [18], and (d) material removal and delamination for different fiber orientation. expansion of expandable graphite flakes. However, lack of and carbon fiber was 0.31 mm and 0.16 mm, respectively. study was noticed on the machining performance of Carbon fiber was taken first to achieve a higher flexural hybrid/composites when coated with insulative materials. modulus [42], and the experimental setup is given in In this study, we are aimed to investigate the effects of Figure 4. The vacuum pressure was kept at 80 kPa to pro- drilling parameters on the insulative-coated HFRP compos- duce a bubble-free sample with a fiber resin ratio of 46 : 54. ite and expected to obtain an impact on the delamination behavior of the drilled materials. In addition, it is also 2.2. Preparation of Insulative Coating. The constituent mate- important to look at the scope of composite materials in rials required to formulate the coating were purchased from heat-resistant applications in various sensitive industries. different suppliers. Zirconium phosphate (ZrP) was pur- chased from Sichuan HongChang Plastics Indus. Co. Ltd., China. Expandable graphite (EG), boric acid (BA), ammo- 2. Experimental nium polyphosphate (APP), melamine (MEL), and halloy- 2.1. Fabrication of Hybrid Fiber-Reinforced Polymer (HFRP) site nanotube (HNT) were purchased from Sigma-Aldrich Composite. Carbon and glass fibers in the woven form were (M) Sdn Bhd., Malaysia. Epoxy resin BE-188 (BPA) and hardener H-2310 polyamide amine were brought from Mc- used as the reinforcement materials (Figure 2) and epoxy as the matrix material in this investigation and purchased from Growth Chemical Sdn Bhd., Malaysia. The detailed formula- RP Products Sdn. Bhd. and Advance Altimas Sdn. Bhd., tion ingredients are given in Table 2. Malaysia, respectively. The mechanical properties of glass All the ingredients such as APP, BA, MEL, HNT, and and carbon fibers are given in Table 1 according to the ZrP were mixed according to formulations given in Table 2 spreadsheet provided by the supplier. Other accessories such and ground in a shear mixer for about 90 seconds with as peel plies, mesh, spiral tubes, hose pipes, and vacuum bags 21000 RPM to make it homogenous. EG was later added to required to manufacture the composite laminate were also the mixture and stirred a little with a spoon to prevent the purchased from UK Composite, UK. EG from grinding, as bigger EG flakes are responsible for The hybrid composite was fabricated using the vacuum better expansion. After that, epoxy (BE-188) was added to infusion molding method in the lab. Orientation of fiber the mixture followed by the hardener (H-2310) and stirred materials was maintained at 0/90 , and the lamina stacking at about 40 RPM for 15 min and then 5 min at 60 RPM, sequence (C-G-G-G-C-G-G-G-C) (Figure 3) was selected respectively, by using an automatic shear mixer CAFRAMO to measure delamination and average roughness on the sur- (BDC 6015-220). A total of 24 samples of 70 mm × 25 mm face of the drilled component. Stacking carbon fiber at the was used as substrate, and coating was applied (Figure 5) exterior helps the bending deformation, and stacking in core using a hand lay-up technique with the help of a specially reduces delamination during drilling [40]. Placing glass fiber designed mold with a 1 mm pitch screw where every half right after carbon fiber helps reducing the propagation of turn of the screw makes the 0.5 mm vertical displacement microcracks at the interface [41]. The thickness of glass fiber (Figure 6) which ensures the desired thickness of 0.5, 1.0, 4 Advances in Polymer Technology (a) (b) Figure 2: Woven fibrous materials used in this investigation: (a) E-glass and (b) carbon. Table 1: Mechanical properties of woven glass and carbon fibers. 3.1. Measurement of Delamination Factor (DF). Delamina- Properties Glass fiber Carbon fiber tion factor (DF) is considered the major and well-known tool to determine the drill hole quality at the entrance and Tensile strength (ksi) 360 512 exit. DF of the drilled hole was identified according to equa- Tensile modulus (msi) 11.4 33.4 tion (1) and Figure 9(a). Strain to failure (%) 3.0 1.5 max DF = , ð1Þ hole Carbon fiber where D is the maximum diameter after drilling and D Glass fiber max hole is the nominal diameter of the hole. Leica LX 00971A optical microscope was used to identify the delamination on the drilled samples (Figure 9(b)). The Glass fiber microscope magnified 5 times of 1 mm resolution. Maxi- Glass fiber mum diameter and nominal diameter were evaluated through the images by using smart dimensioning tool of SOLIDWORKS software. Figure 3: Fiber’s orientation. 3.2. Measurement of Surface Roughness. The surface topol- 1.5, and 2.0 mm and the curing time was a day at the ambi- ogy of the drilled HFRP composites was obtained by ent room temperature. employing scanning electronic microscopy (SEM, Phenom, Pro-X, Netherlands). Samples were first cut at a dimension 2.3. Drilling of HFRP Composite. A drill bit with a 5 mm of 1mm × 1mm using an abrasive cutter. The average diameter of high-speed steel (HSS) was selected in this study roughness (R ) and roughness height (R ) were assessed a z to investigate the effect of drilling parameters on the delam- from 3D images based on “shape from shading” technology. ination and surface roughness of coated HFRP composite The device was operated at 15 kV, and the field of view was with various coating thicknesses. To perform the drilling 964 μm. The images were captured at different magnifica- operation, Mazak variaxis 630 CNC machines were used tions to obtain clear surface roughness information. without pouring out coolant to avoid moisture absorption which may affect the microstructure, dimensional accuracy, 4. Results and Discussion and mechanical properties of the composite [43]. The sam- ple size selected was 70 mm in length and 25 mm in width. The procedure discussed above was repeated corresponding The samples were clamped by the fixture. Each sample had to the experimental sequence provided in Table 3 according undergone drilling operation on it using a 5 mm HSS drill to the ANOVA full factorial analysis, and the measured bit shown in Figure 7. values are shown in the table. 4.1. Analyzing the Effects of Drilling Parameters on the 3. Characterizations Delamination at the Entrance and Exit. Effects of cutting The quality of drilled hole was evaluated based on the parameters on the delamination at the entry and exit were delamination factor and surface roughness of the HFRP observed and analyzed after drilling. DF at the entry was composite. Figures 8(a) and 8(b) show drilled holes obtained found to decrease with the increasing cutting speeds shown for uncoated and coated HFRP composites, respectively. in Figures 10(a) and 10(b). This scenario can be elucidated Advances in Polymer Technology 5 Vacuum infusion molding (VIM) setup Vacuum chamber Vacuum pump Inlet Outlet Resin Mold Figure 4: Vacuum infusion molding (VIM) setup used in the lab. Table 2: Formulation of insulative coating used in this study (gm). APP MEL BA EG HNT ZrP Epoxy resin Hardener 11.36 5.5 11 5.5 0.5 0.5 41.94 19.72 from 1.076 to 1.220. The lowest DF value at the exit was obtained at 4500 RPM, 0.08 mm/rev, and 0.5 mm thick coat- ing. The highest delamination damage at the exit was obtained at the drilling condition of 6000 RPM, 0.08 mm/ rev, and 2.0 mm thick coating. This could be happened due to the different states of the top and bottom surfaces of the composites. At the entry, the composites were experiencing compression, and at the exit, those were in tension. Coating thicknesses have played a vital role in the damage factor at the entry. It has been observed that the delamination was Figure 5: Coated HFRP composite samples. found to increase with the increased cutting speeds on exit sides for a 2.0 mm thickly coated sample. This is because the coating agitated the fiber push out at the exit; however, by relating to the temperature generated during the drilling. the coating thickness had no significant impact on DF. This happened because of the brittle nature of the coating as brit- Higher cutting speed produces heat at the drilling zone which soften the composites, and thus, reduced delamina- tle materials have a lack of ductility in tension and they will tion factors can be observed [44]. On the other hand, this fail prematurely [46]. Therefore, the changes in the thickness of the coating failed to show any significant improvements. scenario can be illustrated as the high spindle speed produc- ing high shear force; therefore, the composites undergo a On the other hand, delamination at the entry was reduced with the increase of the coating thickness. DF at the entry shear deformation, thus reducing delamination. Again, delamination had shown a proportional relationship with was 5.5% and 2.7% less in comparison to the 0.5 mm thick the feed rate as it increased with the increase of the feed rate. coated samples while drilling at 3000 and 4500 RPM, respec- tively. This was presumable that the entry point is in com- Thrust force increased when feed rate is increased due to expanding cross-sectional area thus producing more delam- pression and brittle materials under compressive load tend to close up the transverse crack; therefore, the delamination ination [45]. DF at the entry ranges from 1.020 to 1.178, as presented in Table 3. The lowest DF value at the entry was was lesser [47]. obtained at the drilling condition of 3000 RPM, 0.02 mm/ rev, and 2.0 mm thick coating; meanwhile, the highest 4.2. Optimized Drilling Parameters for Delamination. The delamination onset was obtained at 6000 RPM, 0.08 mm/ influence of speed, feed rate, and coating thickness on the delamination at the entry of coated HFRP composite was rev, and 2.0 mm thick coating. This is because when an overly brittle coating of 2.0 mm thick drilled at a high speed calculated using ANOVA and presented in Table 4. ANOVA and feed, it could not take much compressive force that leads table suggested that the interactions between speeds and to the delay in damping effect and fracture and finally coating thicknesses are the most important factor that affects resulted higher delamination at the entrance. However, the delamination at the entry followed by coating thickness alone and the interactions between feed rate and coating delamination at the exit showed the opposite behavior. DF was found to increase with the cutting speeds shown in thicknesses. F value is > F for both the interactions 0:05 Figures 10(c) and 10(d). DF at the exit has higher delamina- obtained, and the corresponding P value is less than 0.05. tion damages than at the entrance. DF at the exit ranges Feed rate alone is found insignificant since the F value and 6 Advances in Polymer Technology Screw Composite substrate placed inside mold Mold Figure 6: Especially designed adjustable mold used in this study to maintain coating thickness. Flute length 25 mm values. Table 5 shows the response table of S/N ratio for the delamination at the entry and exit, and Figure 12 illustrates the main effects obtained from the S/N plots and highlights that coating thickness and cutting speed played the vital role in the delamination factor at the entrance and exit. Feed rate No. of flute of two was found to be less significant compared to speed and coat- ing thickness. Figure 7: HSS twist drill bit geometry used in this study. A full factorial response optimizer was used to determine the optimum cutting condition in drilling coated HFRP com- P value are showing opposite results to the others. Table 4 posite and presented in Figure 13. 3000 RPM, 0.08 mm/rev illustrates that the 98.90% variations in the response are feed rate, and 1.0 mm coating thickness are considered the explained by the model and it is considered significant. optimum cutting condition. Moreover, the difference between the R-square value and R -square (predicted) value is acceptable which gives us the 4.3. Analyzing the Effects of Drilling Parameters on the idea that all the parameters involved are significant. Surface Roughness. Understanding the precision of the Similarly, the influence of speed, feed rate, and coating drilled hole part and measurement of roughness is impor- thickness on the delamination at the exit of coated HFRP tant, and it is occurred due to the inappropriate fracture of composite was calculated using ANOVA and presented in fiber leading to the sharp end in the inner surface, failure Table 4. ANOVA table suggested that the cutting speed is under fatigue load, high friction, and generation of heat at the drilled wall [35]. The surface roughness (R )ofthe the major factor that is affecting the delamination at the exit followed by coating thickness. Feed rate has been found drilled hole wall has been found to decrease with increasing nonsignificant, and no interaction effects were present. This cutting speeds shown in Figure 14(a). Surface roughness could be happened due to the notch sensitivity of the brittle values range from 1.30 μm to 1.82 μm, as highlighted in materials. Table 4 illustrates that 82.72% of variations in the Table 3. The lowest roughness value was obtained at the dril- response are explained by the model and the difference ling condition of 6000 RPM, 0.02 mm/rev, and 1.5 mm thick between the R-square value and R-square (predicted) value coating. This might be happened due to the smearing effect is quite high enough (17.16%) to give us the idea that some in the fiber-matrix composite at the elevated temperature. of the important factors like temperature, the bonding The highest roughness value was noticed at the drilling con- strength between layers, axial thrust force, and drill geome- dition of 3000 RPM, 0.08 mm/rev, and 0.5 mm thick coating. It has been observed that 0.5 mm thick coating samples try are missing. Figure 11 presents the residual plots for both delamina- showed a 3.8% increment in roughness value when the feed tion at the entry and exit for coated HFRP composite. In rate increased from 0.02 mm/rev to 0.08 mm/rev at both cases, the normal probability plot shows that all the 3000 RPM. However, the roughness value was reduced by points are close to the straight line meaning no unusual approximately 20% and 16% when the cutting speed increased from 3000 RPM to 6000 RPM for the similar con- observations. The residual vs. fit graph shows that all the data are nearly distributed evenly and randomly below and dition of 0.02 and 0.08 mm/rev feed rate and 0.5 mm thick above the straight line. Bell shape curve is obtained for coating, respectively. Comparably, measured roughness delamination at the exit shown in the histogram. No specific value (R ), while drilling at 3000 RPM and 0.02 mm/rev feed pattern is shown in the residual vs. observation graph mean- with 1.0 mm thick coating, was obtained as 1.69 μm, but it ing no biasness involved in the data set. has decreased to 1.33 μm when speed increased to S/N ratio, a systematic approach to analyze the response 6000 RPM. In all the cases, the surface roughness of the parameters, has been used in this study. The greater is the drilled holes shows an inverse relationship with cutting value of S/N ratio, the lesser is the variance in the optimum speeds. This scenario is common in other machining 85° Advances in Polymer Technology 7 (a) (b) Figure 8: Drilled HFRP composite: (a) uncoated; (b) coated. max (a) (b) Figure 9: (a) Microscopic view of damaged HFRP composite surface. (b) Optical microscope to identify delamination. Table 3: Experimental sequence followed to drill the HFRP composite samples using full factorial design of experiment. Feed rate Surface roughness Sample no. Spindle speed (RPM) IC thickness (mm) Delamination (entry) Delamination (exit) (mm/rev) (μm) 1 0.5 1.080 1.086 1.75 2 1.0 1.070 1.090 1.69 0.02 3 1.5 1.060 1.100 1.56 4 2.0 1.020 1.160 1.47 5 0.5 1.080 1.080 1.82 6 1.0 1.050 1.108 1.71 0.08 7 1.5 1.068 1.078 1.52 8 2.0 1.038 1.080 1.61 9 0.5 1.080 1.100 1.63 10 1.0 1.068 1.110 1.58 0.02 11 1.5 1.060 1.120 1.61 12 2.0 1.050 1.158 1.42 13 0.5 1.064 1.076 1.70 14 1.0 1.052 1.110 1.63 0.08 15 1.5 1.062 1.102 1.62 16 2.0 1.080 1.128 1.53 17 0.5 1.052 1.116 1.41 18 1.0 1.040 1.150 1.33 0.02 19 1.5 1.026 1.166 1.30 20 2.0 1.140 1.206 1.35 21 0.5 1.034 1.116 1.52 22 1.0 1.040 1.150 1.47 0.08 23 1.5 1.026 1.160 1.46 24 2.0 1.178 1.220 1.49 8 Advances in Polymer Technology 1.20 1.20 1.18 1.18 1.16 1.16 1.14 1.14 1.12 1.12 1.10 1.10 1.08 1.08 1.06 1.06 1.04 1.04 1.02 1.00 1.02 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (a) (b) 1.24 1.24 1.22 1.22 1.20 1.20 1.18 1.18 1.16 1.16 1.14 1.14 1.12 1.12 1.10 1.10 1.08 1.08 1.06 1.06 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (c) (d) Figure 10: DF: at the entry (a) 0.02 mm/rev and (b) 0.08 mm/rev and at the exit (c) 0.02 mm/rev and (d) 0.08 mm/rev. methods as well. According to Ghani et al., the cutting pro- effects mentioned earlier because of the large vertical dis- cess becomes stable more at the high cutting speed [48]. placement component due to the high feed rate. Exactly sim- Also, researchers showed that the interactions and the ilar behavior was noticed when coating thickness varied adherence between the composites and cutters are more at between 1.5 mm and 2.0 mm. In Figure 15(b), the 3 K, the lower cutting speeds, thus creating a built-up edge that 4.5 K, and 6 K indicate the spindle speed in RPM and the may lead to a rough surface [49]. This scenario is more likely 0.02 and 0.08 represent the feed rate in millimeters. It is clear here. However, keeping the speed (3000 and 6000 RPM) and that the coating thickness has been slightly significant to coating thickness (1.0 mm) constant, the roughness value reduce the surface roughness at a lower cutting speed. How- was found higher for the 0.08 mm/rev feed rate. Composite ever, it was not found significant at a relatively higher cut- laminates when drilling at higher spindle speeds and feed ting speed. Among all, 6000 RPM has given the U-shaped rate, increases the temperature of the accumulated heat curve with the lowest roughness values with different coating around the drill cutting edges due to the low thermal coeffi- thicknesses. And overall, 1.5 mm coating thickness has been cient and destroys the matrix stability, and produces rough found optimum for R . cuts around the wall which leads to the surface roughness Figures 15(a) and 15(b) present the SEM texture [50]. This can be distinguished differently from the smearing obtained for drilled hole wall of insulative-coated HFRP DF (exit) DF (entry) DF (exit) DF (entry) Advances in Polymer Technology 9 Table 4: ANOVA table for delamination factor. At the entry At the exit Source Degree of freedom F value P value Degree of freedom F value P value Model 15 47.75 0.001 6 13.56 0.001 Linear 6 20.90 0.001 6 13.56 0.001 Spindle speed (S) 2 4.70 0.045 2 23.18 0.001 Feed rate (F) 1 0.35 0.573 1 2.46 0.135 Coating thickness (C) 3 38.55 0.001 3 10.86 0.001 2-way interactions 9 65.66 0.001 —— — S C 6 91.53 0.001 —— — F C 3 13.92 0.002 —— — Error 8 —— 17 —— Total 23 —— 23 —— Standard deviation 0.0062452 0.0189956 R-square 98.90% 82.72% R-square (adjusted) 96.82% 76.62% R-square (predicted) 90.06% 65.56% Residual plots for DF entry Residual plots for DF exit Normal probability plot Versus fits Normal probability plot Versus fits 99 99 0.010 0.010 90 90 0.005 0.005 50 0.000 50 0.000 –0.005 –0.005 10 10 –0.010 –0.010 1 1 –0.02 –0.01 0.00 0.01 0.02 1.05 1.10 1.15 1.20 –0.02 –0.01 0.00 0.01 0.02 1.1 1.2 1.3 1.4 Residual Fitted value Residual Fitted value Versus order Versus order Histogram Histogram 0.010 2.0 2.0 0.010 0.005 1.5 0.005 1.5 0.000 0.000 1.0 1.0 –0.005 –0.005 0.5 0.5 –0.010 –0.010 0.0 0.0 –0.010–0.005 0.000 0.005 0.010 –0.010 –0.005 0.000 0.005 0.010 1 2 3 4 5 6 1 2 3 4 5 6 Residual Observation order Residual Observation order Figure 11: Residual plots obtained for delamination at the entry and exit. fibers at the exit wall which occurred due to the higher speed Table 5: Response table for delamination at the entry and exit (S/N ratio); smaller is better. and feed exerted on the overly brittle coating that also delayed the damping effect. Level Cutting speed Feed rate Coating thickness 1 -0.6552 -0.8014 -0.6728 4.4. Optimized Drilling Parameters for Surface Roughness. 2 -0.7409 -0.7549 -0.7254 The influence of cutting speeds and feed rates on the surface 3 -0.9384 -0.7204 roughness of coated HFRP composite was calculated using 4 -0.9941 ANOVA and presented in Table 6. ANOVA table suggested Delta 0.2832 0.0465 0.3213 that cutting speed has the most impact on the surface rough- Rank 2 3 1 ness followed by feed rate, coating thickness, and speed interaction with coating thicknesses. The combined effect of speed and feed rate has nonsignificant interaction since composite when drilling at 6000 RPM at varied feed and their P value is slightly over 0.05. Table 6 illustrates that thickness of coating due to the lower roughness value the 97.32% variations in the response are explained by obtained for these two drilling conditions. Morphology is the model and it is considered significant. Moreover, the shown in Figure 15(a) which illustrates that good fiber-matrix difference between the R-square value and R-square (pre- interfacial bonding is present and no matrix cracking and dicted) value is quite high (16.4%) which gives us the idea push-/pull-out damage is observed. However, Figure 15(b) that some of the important factors such as temperature shows fiber pull out and matrix crack in the SEM image indi- and machine vibration are missing that are not considered cating surface damage. This damage resulted from the loose in this research. Frequency Percent Residual Residual Frequency Percent Residual Residual 10 Advances in Polymer Technology Main effects plot for SN ratios Data means Cutting speed Feed rate Coating thickness –0.65 –0.70 –0.75 –0.80 –0.85 –0.90 –0.95 –1.00 3000 4500 6000 0.02 0.08 0.5 1.0 1.5 2.0 Signal-to-noise: Smaller is better Figure 12: S/N ratio plots for delamination at the entry and exit. Spindle Feed rate Coating Optimal High 6000 0.08 2 D: 0.8474 Cur 3000 0.08 1 Predict Low 3000 0.02 0.5 Composite desirability D: 0.8474 DF exit minimum y = 1.0882 d = 0.91493 DF entry minimum y = 1.0540 d = 0.78481 Figure 13: Optimized cutting speed, feed rate, and coating thickness achieved to minimize delamination for coated HFRP composite. Figure 16 presents the residual plots for surface rough- obtained from the S/N plots and highlights that the cutting ness of coated HFRP composite. Normal probability plot speed contributed highest in the roughness occurred in the shows that all the points are close to the straight line mean- surface of the composites followed by the coating thickness ing no unusual observations. The residual vs. fit graph shows and feed rate. that all the data are distributed evenly below and above the A full factorial response optimizer was used to deter- straight line. Bell shape curve is obtained in the histogram. mine the optimum cutting condition in drilling coated No specific pattern is shown in the residual vs. observation HFRP composite and presented in Figure 18. 6000 RPM, graph meaning no biasness involved in the data set. 0.02 mm/rev feed rate, and 1.5 mm coating thickness are Table 7 shows the response table of S/N ratio for the sur- considered the optimum cutting condition for minimizing face roughness, and Figure 17 illustrates the main effects surface roughness. Mean of SN ratios Advances in Polymer Technology 11 1.8 2.0 1.9 1.7 𝜇 𝜇 1.8 1.6 1.7 1.5 1.6 1.4 1.5 1.3 1.4 3000 4500 6000 3000 4500 6000 Spindle speed (RPM) Spindle speed (RPM) 0.5 mm thick coating, 0.02 mm/rev 0.5 mm thick coating, 0.08 mm/rev 1.0 mm thick coating, 0.02 mm/rev 1.0 mm thick coating, 0.08 mm/rev 1.5 mm thick coating, 0.02 mm/rev 1.5 mm thick coating, 0.08 mm/rev 2.0 mm thick coating, 0.02 mm/rev 2.0 mm thick coating, 0.08 mm/rev (a) 1.8 1.7 1.6 1.5 1.4 1.3 0.0 0.5 1.0 1.5 2.0 2.5 Coat. Thickness (mm) 3 K, 0.02 4.5 K, 0.08 3 K, 0.08 6 K, 0.02 4.5 K, 0.02 6 K, 0.08 (b) Figure 14: (a) Surface roughness (R ) for varied drilling parameters and (b) roughness and coating thickness comparison plot. Matrix crack Fiber pull-out (a) (b) Figure 15: Microscopic image of drilled hole wall: (a) 6000 RPM, 0.02 mm/rev, and 1.5 mm thick coating; (b) 6000 RPM, 0.08 mm/rev, and 2.0 mm thick coating. Good fiber orientation and interfacial bonding with matrix Surface roughness ( m) Roughness ( m) Surface roughness ( m) 12 Advances in Polymer Technology Table 6: ANOVA table for surface roughness. Source Degree of freedom F value P value Model 14 23.38 0.001 Linear 6 46.95 0.001 2 90.68 0.001 1 31.07 0.001 3 23.10 0.001 2-way interactions 8 5.71 0.009 S F 2 4.03 0.056 S C 6 6.26 0.008 Error 9 —— Total 23 —— Standard deviation 0.0351584 R-square 97.32% R-square (adjusted) 93.16% R-square (predicted) 80.97% Residual plots for surface roughness Normal probability plot Versus fits 0.050 0.025 50 0.000 –0.025 –0.050 –0.050 –0.025 0.000 0.025 0.050 1.4 1.6 1.8 Residual Fitted value Histogram Versus order 0.050 6.0 0.025 4.5 0.000 3.0 –0.025 1.5 –0.050 0.0 –0.04 –0.02 0.00 0.02 0.04 2 4 6 8 10 12 14 16 18 20 22 24 Residual Observation order Figure 16: Residual plots obtained for surface roughness. Table 7: Response table for surface roughness (S/N ratio); smaller is better. Level Cutting speed Feed rate Coating thickness 1 -4.283 -3.531 -4.256 2 -4.017 -4.009 -3.876 3 -3.010 -3.565 4 -3.382 Delta 1.273 0.478 0.874 Rank 1 3 2 Frequency Percent Residual Residual Advances in Polymer Technology 13 Main effects plot for SN ratios Data means Cutting speed Feed rate Coating thickness –3.0 –3.2 –3.4 –3.6 –3.8 –4.0 –4.2 –4.4 3000 4500 6000 0.02 0.08 0.5 1.0 1.5 2.0 Signal-to-noise: smaller is better Figure 17: S/N ratio plots for surface roughness. Spindle Feed rate Coating Optimal High 6000 0.08 2 D: 0.9784 Cur 6000 0.02 1.5 Predict Low 3000 0.02 0.5 Surface minimum y = 1.3112 d = 0.97837 Figure 18: Optimized cutting speed, feed rate, and coating thickness achieved to minimize surface roughness for coated HFRP composite. 5. Conclusions to analyze the response parameters, and it is clearly visible that coating thickness and speed have the dominant effect on This study has successfully fabricated HFRP composite, and delamination. For coated HFRP composite, a coating thick- the influence of drilling parameters such as cutting speed ness of 1 mm, cutting speed of 3000 RPM, and feed rate of and feed rate on delamination and surface roughness on 0.08 mm/rev are the optimized parameters for minimizing insulative-coated HFRP composite has been investigated and delamination based on the full factorial response optimizer. well presented. In general, it can be concluded from the discus- Similarly, surface roughness was found to have decreased with sion that the delamination factor (DF) at the entrance the increasing cutting speeds. Lower cutting speed generates decreases with the increasing cutting speeds. On the other higher roughness whereas higher cutting speed incurs lower hand, the delamination factor at the exit showed the opposite roughness. S/N plots also showed that cutting speed is pivotal manner. However, coating thickness played an important role in occurring roughness on the wall of drilled composites. in delamination at the entrance. The DF was found higher However, based on the full factorial response optimizer, cut- with increasing cutting speeds especially when the coating ting speed of 6000 RPM, feed rate of 0.02 mm/rev, and coating thickness was higher. But the exit side of the hole showed no thickness of 1.5 mm are considered optimum in minimizing significant difference in this manner. S/N plot has been used surface roughness when drilling coated HFRP composites. Mean of SN ratios 14 Advances in Polymer Technology Data Availability [12] P. Benardos and G.-C. Vosniakos, “Predicting surface rough- ness in machining: a review,” International Journal of Machine All data including figures, tables, and experimental results Tools and Manufacture, vol. 43, no. 8, pp. 833–844, 2003. are available. [13] A. M. Abrão, J. C. C. Rubio, P. E. Faria, and J. P. 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