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Abstract

Carbon fiber reinforced polyetheretherketone (CF/PEEK) is widely used in the aviation industry due to its high performance and ease of recycling and repair. The PEEK matrix is easy to soften when heated, which causes serious tool wear and affects machining quality and efficiency. However, tool wear mechanisms in drilling CF/PEEK composites are still unclear and there is a lack of research on the relationship between tool wear and drilling performance. This is the first study presents a comprehensive comparative analysis of tool wear mechanisms and their influence on drilling temperature, cutting forces, and hole-making quality between thermoplastic CF/PEEK and thermoset CF/epoxy composites. The results indicate that the primary wear mechanism for drilling thermoset CF/epoxy is abrasive wear, while adhesion wear dominates in thermoplastic CF/PEEK. The drilling thrust force and temperature in CF/PEEK exhibit significantly higher sensitivity to the progression of tool wear. As the number of drilled holes increases, the hole-making quality of CF/PEEK exhibits a more pronounced deterioration compared to that of CF/epoxy. This study provides valuable insights into the distinct wear mechanisms and performance differences between thermoplastic and thermoset composites during drilling processes.

Keywords

CF/PEEK CF/epoxy drilling wear mechanism hole-making quality

Introduction

In recent years, carbon fiber reinforced thermoplastic (CFRTP) has gradually replaced traditional thermosetting carbon fiber reinforced plastic (CFRP) in aerospace and construction fields due to its excellent mechanical properties and good recyclability.^1,2 Carbon fiber reinforced polyetheretherketone (CF/PEEK) is a common high-performance CFRTP, which has an indispensable position in aerospace and construction fields because of its high strength and fatigue resistance. Although CF/PEEK composites can achieve near-net shaping,^3–6 for the positioning and assembly of parts, connection through assembly holes is still indispensable. Due to the anisotropy of the fiber/matrix system of carbon fiber composites, excessive tool wear has always been a key issue in their processing. Due to PEEK is softened by heat, there are significant differences in tool wear mechanism during drilling CF/PEEK and CF/epoxy.

At present, the research on tool wear mainly focuses on drilling thermoset CF/epoxy. Xu et al.⁷ compared the wear performance of candlestick drills and step drills when drilling CFRP. They found that specially designed drill geometries improve wear resistance and tool wear significantly impacts the machining quality of composites. Fernandez-Perez et al.⁸ studied tool wear, delamination, and cutting forces while drilling CF/epoxy. They found consistent tool life with less than 7% variation in surface quality. Gaugel et al.⁹ compared tool wear and material damage in diamond-coated and uncoated tungsten carbide tools when drilling two CF/epoxy materials. They found that coated tools perform better on high-quality laminates with strong layer bonding, extending tool life. Xu and Zang¹⁰ found that tool wear raises cutting forces in both cutting and normal directions. They also observed that when fibers are perpendicular to the cutting direction, friction-induced side surface wear can lead to carbon fiber fractures and reduced surface quality. Another study, Wang et al.¹¹ determined that multi-tooth cutting tools provide advantages in preventing damage to machined surfaces and out-of-plane bending of the fibers. Isbilir et al.¹² studied drilling CF/epoxy with a TiAlN/TiN-coated carbide drill. Flank wear was dominant, with no chipping or deformation. Abrasive wear on flank and crater surfaces caused edge wear due to brittle fibers. Xu et al.¹³ found that drilling CFRP/Ti6Al4V stacks from titanium to carbon fiber reduced forces, improved surface quality, and minimized wear, with diamond-coated bits performing better due to lower temperatures and less wear.

Unlike traditional thermoset CF/epoxy composites, the matrix of CFRTP is significantly influenced by temperature variations. When the drilling temperature surpasses the glass transition temperature (Tg) of the matrix, thermal softening occurs, which substantially impacts the cutting process. The research summary on the wear of CFRTP drilling tools is shown in Table 1. Batista et al.¹⁴ examined cryogenic cooling’s impact on drilling CF/epoxy and carbon fiber reinforced polyphenylene sulfide (CF/PPS). The results show that cryogenic cooling enhanced hole accuracy in CF/PPS but increased exit delamination. Mudhukrishnan et al.¹⁵ studied feed rate effects on delamination in CF/polypropylene (CF/PP) drilling using solid carbide, tipped carbide, and high-speed steel tools. Higher feed rates increased the delamination factor. Varatharajan et al.¹⁶ compared GF/polypropylene (GF/PP) and GF/polyester composites. They found that thermoplastics generate lower thrust forces, and the heat in the cutting zone creates a lubricating layer, reducing tool wear. Ahmad et al.¹⁷ found that the elongated CFRTP chips adhered to the drill bit main cutting edge, which led to severe tool clogging, deteriorated tool cutting performance and accelerated tool wear. Hocheng and Puw¹⁸ obtained a similar conclusion in their study of thermoplastic CF/acryloitrile butadiene styrene (CF/ABS). Meinhard et al.¹⁹ investigated the drilling of carbon fiber-reinforced polyamide 12 (CF/PA12) and observed that burrs were not the only type of damage occurring during the drilling process. Xu et al.²⁰ compared carbon/PI and carbon/PEEK composites. Carbon/PEEK had poor machinability, with high drilling force, temperature, delamination, and tool wear. Its toughness caused continuous chips, while carbon/PI mainly showed abrasive wear, and carbon/PEEK suffered chip adhesion, coating peeling, and edge damage. Pathak et al.²¹ compared three drill bits for carbon fiber-reinforced polypropylene drilling. They found that the flat drilsignificantly outperformed the conventional twist drill at lower feed rates (60 mm/min) and medium rotational speeds (900 rpm), reducing temperature, thrust force, delamination, and surface roughness. Ilhan et al.²² studied drilling parameters for thermoplastic composites and investigated the effects on force, momentum, and delamination. Zhang et al.²³ delved into the mechanisms of damage formation and evolution during the drilling of CF/PEEK using twist drills equipped with chip-split grooves and two-step step drills. The results show that the chip-split grooves can mitigate peel-up delamination by enhancing chip evacuation, while a short chisel edge can decrease push-out delamination by reducing thrust forces.

Table 1.

A summary of various studies addressing the tool wear issues in drilling CFRTP laminates.

References	Tool details	Composite types	Cutting conditions	Key topics addressed
Varatharajan et al.¹⁶	Carbide tipped drill	GF/PP	S:2500 rpm	Chip
Varatharajan et al.¹⁶	Carbide tipped drill	GF/Polyester	f:0.1 mm/rev	Tool wear fracture mode
Hocheng and Puw¹⁸	HSS drill	CF/epoxy	S:124-4000 rpm	Chip
Hocheng and Puw¹⁸	HSS drill	CF/ABS	f:50-300 mm/min	Cutting energy hole quality
Meinhard et al.¹⁹	Step drill	CF/polyamide	S:1000 rpm	Burr
Meinhard et al.¹⁹	Step drill	CF/polyamide	f:0.6 mm/rev	Chip
Xu et al.²⁰	TiAlN-coated drill	CF/PI CF/PEEK	Vc:75 m/min	Hole accuracy
Xu et al.²⁰	TiAlN-coated drill	CF/PI CF/PEEK	f:0.08 mm/rev	Delamination tool wear
Pathak et al.²¹	Split point drill, brad spur drill	CF/PP	S:900rpm	Delamination
Pathak et al.²¹	Split point drill, brad spur drill	CF/PP	f:60-180 mm/min	Roughness
Zhang et al.²²	Twist drill, step drill	CF/PEEK	Vc:50-100 m/min	Chip
Zhang et al.²²	Twist drill, step drill	CF/PEEK	f:0.02-0.1 mm/rev	Burr

Obviously, due to the differences in material properties, the research conclusions on thermosetting CF/epoxy are not applicable to thermoplastic CF/PEEK, and the relevant research on the tool wear mechanism of CF/PEEK is rather limited. So we report the first study comparing the tool wear mechanisms and effect of tool wear on drilling temperature, drilling force and hole-making quality in thermoplastic CF/PEEK and thermoset CF/epoxy. The influence of drilling parameters on tool wear evolution characteristics was investigated. In order to ensure a fair comparison, two composites with the same fiber volume fraction, stacking sequence and similar fiber properties were used in this study. This study is the first to investigate the tool wear progression, microscopic wear mechanisms and its influence on machining quality for both materials.

Experimental set-up

Workpiece material

CF/PEEK laminates were prepared using a hot-pressing method. The prepreg is stacked into the mold via the hot pressing forming method. The temperature of hot pressing is 380°C, the pressure is 6 MPa, and the holding time is 150 min. CF/PEEK composite laminates are obtained after cooling. CF/epoxy laminates were fabricated from T700 unidirectional prepregs supplied by Changzhou Baifeng Carbon Fiber Technology Co., Ltd, China. Both CF/epoxy and CF/PEEK laminates were fabricated from 20 plies of unidirectional prepregs with 60 vol% fiber content and a stacking sequence of [0/90]₁₀, resulting in a total thickness of 3 mm. Table 2 lists the mechanical and physical properties of two types composite laminates provided by the manufacturers. The coupons for the drilling experiment measure 250 mm × 250 mm × 3 mm. The drilling tool selected for the experiments was a solid carbide twist drill with a diameter of 5 mm, a point angle of 118°, and a helix angle of 30°. The morphologies of the drill bit are shown in Figure 1.

Table 2.

Mechanical and physical properties of the CF/PEEK and CF/epoxy.

Properties	CF/PEEK	CF/epoxy
Tensile strength (MPa)	2367	2450
Young^,s modulus (GPa)	137	132
Poisson^,s ratio	0.3	0.3
Density (g/cm³)	1.59	1.58
Fiber volume content (%)	60	60
Glass transition temperature of matrix (°C)	143	130

Figure 1.

Drill bit geometry.

Drilling experiments

All the drilling tests were conducted using Hanchuan CNC XK714D machine depicted in Figure 2. Drilling parameter deployed is in accordance to the parameter range recommended by previous studies,^24–27 five settings of spindle speed (N) and a fixed feed rate (F) were chosen in this study, as listed in Table 3. The positions marked with yellow dots on the sample are used for temperature measurement, and the positions marked with green dots are used for drilling thrust force and surface topography measurement. The distance between hole centers was maintained at 10 mm. The time interval between consecutive drills was 30 seconds. The composites laminates were not supported on the back surface. The cutting force measurement system consists of a Kistler 9257B dynamometer, 5070A charge amplifier, and a 5697A1 data acquisition card. The measured cutting force data were analyzed and processed by the Dynoware software. An infrared thermal imager (VariOC-880) was employed to measure the temperature of the hole wall during drilling. The imager was fixed at a distance of 120 mm from the specimen, with a distance of 0.15 mm from the hole wall to the specimen edge.

Figure 2.

Experimental device for drilling: (a) Physical picture. (b) Localized enlarged area. (c) Experimental system diagram.

Table 3.

Details of drilling experiment parameters.

Material	Spindle speed N (rpm)	Feed rate F (mm/rev)
CF/PEEK	1592,3185,4777,6369,7961	0.05
CF/epoxy	(25,50,75,100,125 m/min)	0.05

Measurement methods of tool wear and machining quality

The 2D profile of the cutting edge was obtained by 3D laser scanning confocal microscope (KC-X1000, KathMatic, China). The feature parameters were extracted with MATLAB software. The tool wear degree was examined via scanning electron microscopy (SEM) (Sigma300, Zeiss, Germany) and 3D digital microscope (KC-X1000, KathMatic, China). In this paper, the delamination factor proposed by Chen²⁸ was used to evaluate the degree of delamination damage. The delamination factor is equal to the quotient of the maximum circumferential diameter of tear damage and the nominal diameter:

F_{d} = \frac{D_{\max}}{D_{n o m}}

(1)

where

F_{d}

is the delamination factor,

D_{\max}

is the maximum diameter of the damaged area, and

D_{n o m}

is the nominal diameter, as shown in Figure 3.

Figure 3.

The schematic illustration of delamination factor.

Results and discussion

Effect of tool wear on chip morphology

Table 4 shows the chip morphology of CF/PEEK and CF/epoxy with increasing number of holes drilled. The chip morphology of the two composites was obviously different under different processing conditions. Powdery or short ribbon-like chips are produced in CF/epoxy drilling. In contrast, CF/PEEK generates continuous and crimped. A similar phenomenon were also founded during the drilling of CF/PEKK,²⁹ CF/poly-cyclic butylene terephthalate (CF/pCBT)³⁰ and CF/ABS.^21,31 The difference of chip morphology between the two materials is mainly related to chip formation mechanism. The microscopic SEM images of chips are shown in Figure 4. It can be clearly seen that CF/epoxy chips exhibit a flake structure. The surface of the chip exposes a large number of broken fibers. This is because the fibers undergo brittle fracture under the extrusion of the tool. Due to the plastic deformation of the matrix, CF/PEEK produces continuous and curly chips. The chip surface is smooth and coated with matrix. Due to the excellent ductility of PEEK, with the increase of the number of holes, the cutting temperature gradually increases, and the higher cutting temperature promotes the sliding between the chains and improves the continuity of the chips.³¹ However, no significant change was observed in the chip morphology of CF/epoxy.

Table 4.

The chip morphology under different number of holes drilled for CF/PEEK and CF/epoxy.

Figure 4.

The microscopic SEM images of the chips. (a) CF/epoxy, (b) CF/PEEK.

Tool wear mechanisms and progression

The resin matrix of thermoplastic CF/PEEK is easy to soften and produce high viscosity when heated.³² Consequently, tool wear emerges as a critical factor in drilling CF/PEEK composites, profoundly influencing machining performance and demanding considerable attention in the manufacturing process. A comprehensive understanding of the mechanisms and patterns of tool wear is essential to develop effective strategies to mitigate tool degradation. After drilling 32 and 64 holes in the two composite materials, the morphology of the tool flank face and chisel edge is illustrated in Table 5.

Table 5.

Tool wear morphology at different numbers of holes drilled for CF/PEEK and CF/epoxy.

During the drilling process, the chisel edge operates at the lowest linear cutting speed, approaching zero in its central region. As a result, wear on the chisel edge is predominantly driven by impact loads, which are strongly correlated with drilling forces.³³ As shown in Table 5, the chisel edge wear of the tool used for machining CF/PEEK is significantly more severe than that of the tool used for CF/epoxy, regardless of the number of holes drilled. CF/PEEK demonstrates ductility and plastic deformation, generating higher thrust force than those observed when machining thermoset CF/epoxy.

Similarly, the peripheral cutting edges near the tool periphery exhibited the most severe wear due to their highest instantaneous cutting speeds.³⁴ As shown in Table 5, the wear on the flank face near the periphery is more severe for tools machining CF/PEEK compared to those machining CF/epoxy. Moreover, the flank faces of tools machining the two materials exhibit different wear mechanisms near the periphery. For tools machining CF/epoxy, the damage primarily manifests as edge chipping and minor scratches, which are mainly influenced by drilling forces. Quantitative analysis indicated that the scratched area (HN = 64) occupied 8.0% of the total flank face area. In contrast, tools machining CF/PEEK experience more severe degradation, characterized by edge rounding and adhesion of matrix, primarily driven by drilling temperatures. Quantitative analysis indicated that the matrix adhesion area (HN = 64) occupied 11.0% of the total flank face area.

To further observe and analyze the wear mechanisms of the two types of tools, the SEM morphology of the tool flank face after drilling 32 and 64 holes in the two composite materials is presented in Figure 5. For CF/epoxy, more frequent and extensive edge chipping occurs during intensified tool-workpiece interactions, accompanied by pronounced scratching on the flank face and microcrack. This abrasive wear mechanism is attributed to the higher hardness of the carbon fiber reinforcement in CF/epoxy. For CF/PEEK, catastrophic tool degradation is observed near the periphery, characterized by severe edge blunting, fracture-induced spalling, and extensive chip adhesion. This phenomenon occurs because progressive tool wear increases temperatures to a level sufficient to melt the PEEK matrix and chips, which subsequently adhere to the cutting edge. Under these conditions, adhesive wear becomes the dominant failure mechanism. Compared to thermoset CF/epoxy, thermoplastic CF/PEEK leads to accelerated and more severe tool wear, indicating that processing conditions, particularly temperature control and tool integrity, require significantly stricter management.

Figure 5.

SEM morphology of the tool’s flank face at different hole numbers. (a) CF/epoxy HN = 32, (b) CF/epoxy HN = 64, (c) CF/PEEK HN = 32, (d) CF/PEEK HN = 64.

Figure 6 summarizes the different tool wear evolution mechanisms of the two composites. During the drilling of thermoset CF/epoxy, carbon fibers fracture due to compressive bending, generating powdery or short ribbon-like chips. In contrast, thermoplastic CF/PEEK exhibits resin matrix softening under elevated temperatures, which facilitates matrix smearing on machined surfaces under the action of the cutting edge. The thermoplastic matrix accommodates significant plastic deformation, resulting in the formation of continuous long chips. A similar phenomenon has been reported in the literature.^35,36 This difference in chip morphology directly affects the duration of tool contact with chips, and the long-term interaction between continuous chips and cutting edges and flanks accelerates the progress of wear. In CF/epoxy, the elastic rebound of rigid fibers continuously impacts the cutting edge, promoting edge chipping, while fiber abrasion causes scratches on the flank face, ultimately leading to particle detachment from the tool surface. Conversely, CF/PEEK undergoes plastic deformation owing to its high ductility. As temperatures increase, the softened PEEK matrix forms adhesive interactions with the cutting edge and flank face, increasing cutting resistance and leading to catastrophic edge fracture and delamination.

Figure 6.

Schematic diagram of tool wear evolution mechanism.

This conclusion is supported by a comparative analysis of the tip profiles of the worn tool and the original cutting tool, along with the measured data of the flank wear width. The cutting edge profiles of the tools used for the two materials under different machining hole numbers are shown in Figure 7. The material removal mechanism in thermoset CF/epoxy is predominantly characterized by brittle fracture, which induces repetitive impact loading on the cutting edge, resulting in edge chipping. This is evidenced by the significant fluctuations in the edge profile observed in Figure 7(a) and (c). In contrast, thermoplastic CF/PEEK exhibits a more complex material removal mechanism. At lower processing temperatures, despite partial plastic deformation, the dominant wear mechanism arises from direct abrasive contact between the tool and the workpiece.^37,38 As the tool wear progresses and the temperature increases, thermal softening will promote the melting of the PEEK matrix, resulting in the adhesion of the material to the tool surface. This transition causes adhesive wear to become the main failure mechanism. The relatively smooth edge profiles in Figure 7(b) and (d) reflect the combined effect of the plastic deformation of the matrix and the interaction of the adhesive.

Figure 7.

Cutting edge profile at different hole numbers: (a) CF/epoxy HN = 32, (b) CF/PEEK HN = 32, (c) CF/epoxy HN = 64, (d) CF/PEEK HN = 64, (e) New tool, (f) Variation in the average flank wear width with hole numbers.

Since the tool has two flank faces, the flank wear width of two face is extracted and averaged, as shown in Figure 7(f). The results showed that the average wear tool wear width of CF/PEEK was 24.3% higher than CF/epoxy when HN was 32, and 32.8% when HN was 64. The reason for this increase is the different tool wear mechanisms of the two materials.

The tool wear morphology under different drilling parameters are shown in Figure 8. The tool wear becomes significantly more severe with increasing spindle speed, particularly on the flank face. This phenomenon can be attributed to the higher contact frequency between the tool and carbon fiber reinforcement per unit time at elevated spindle speeds, resulting in intensified abrasive wear caused by fiber scratching.³³ Notably, under the same spindle speed, tool wear is more pronounced when machining CF/PEEK composites compared to CF/epoxy composites. This discrepancy primarily stems from the superior ductility and enhanced interlaminar fracture toughness of CF/PEEK, which impose greater mechanical and thermal loads on the cutting tool during the machining process.

Figure 8.

Tool wear morphology at different drilling parameters. (a) CF/epoxy 1592 r/min, (b) CF/epoxy 7961 r/min, (c) CF/PEEK 1592 r/min, (d) CF/PEEK 7961 r/min.

The SEM morphological characteristics of the tool flank face under different drilling parameters are shown in Figure 9. The results demonstrate that significant melting and adhesion of the PEEK matrix were observed at the peripheral edge of the flank face when drilling CF/PEEK composites at high spindle speeds. This phenomenon occurs because the peripheral cutting edge attains the highest linear velocity during drilling, leading to intensified tool-material interfacial friction and a consequent sharp rise in localized temperature. These conditions induce thermal softening and eventual melting of the PEEK matrix, ultimately resulting in the formation of an adhesive layer.

Figure 9.

SEM morphology of the tool’s flank face at different drilling parameters. (a) CF/epoxy 1592 r/min, (b) CF/epoxy 7961 r/min, (c) CF/PEEK 1592 r/min, (d) CF/PEEK 7961 r/min.

Effect of tool wear on drilling temperature and drilling force

When the machining temperature exceeds the glass transition temperature (Tg) of the resin, the thermal softening of the matrix occurs. Therefore, the monitoring and control of the cutting temperature becomes the key to the processing of composite materials.³⁹ The drilling temperature at a close proximity (∼0.15 mm) to the hole wall was captured and the temperature field evolution during the entire drilling process was recorded and analysed. According to Ge et al,⁴⁰ based on the tool‐workpiece interaction status, cutting stages can be divided into three different stages depending on positions of the drill bit, see Figure 10(a) (P-I: tool cutting edge fully enters the workpiece; P-II: drill bit tip reaches the workpiece exit surface; P-III: tool cutting edge fully emerges from the hole exit).

Figure 10.

(a) Schematics showing typical drill bit positions during drilling. The representative temperature field and the entrance and exit temperature change curve: (b) CF/epoxy HN = 64, (c) CF/PEEK HN = 64.

Figure 10(b) and (c) shows the representative temperature fields of CF/PEEK and CF/epoxy at different stages of the drilling process. It is observed that as the drilling depth increases, the drilling temperature gradually rises, with the highest temperature occurring in the P-III stage. This is attributed to the poor thermal conductivity of carbon fiber-reinforced composite materials, which hinders the dissipation of cutting heat. As the drilling depth increases, heat accumulation leads to a continuous rise in temperature. The temperature field distribution corresponds to the tool geometry. Heat is initially generated at the contact point between the tool and the material, and as the machining process progresses, it gradually propagates along the cutting edge, reaching the highest temperature during the P-III stage. During the drilling process of CF/epoxy composites, both the entrance and exit temperatures remained below the glass transition temperature (Tg = 130°C) of the epoxy matrix. In contrast, the drilling temperature of CF/PEEK composites significantly exceeded the Tg (143°C) of the PEEK matrix. This pronounced thermal effect not only accelerated tool wear progression but also markedly deteriorated hole quality.

The progression of tool wear inevitably leads to a significant increase in cutting forces and temperatures due to enhanced friction between the tool and the composite specimen. When the drilling temperature exceeds the glass transition temperature (Tg) of the polymer matrix, the mechanical properties of the composite can be significantly degraded, and thermal damage may be induced. Figure 11(a) shows the maximum hole wall temperature as a function of the number of drilled holes.

Figure 11

(a) Effect of tool wear progression (i.e.,the HN). on the maximum hole wall temperature (b) Effect of tool wear progression (i.e.,the HN), on the maximum thrust force.

The maximum hole wall temperature of CF/PEEK exhibited a clear upward trend, approaching and exceeding the glass transition temperature (Tg = 143°C) of the matrix at 32 holes. In contrast, the maximum hole wall temperature of CF/epoxy showed no significant increase and remained below its Tg (130°C). According to the research by Ge and Xu et al^20,29 in drilling of CF/epoxy, the chip is powdered and can be efficiently discharged during processing. The limited interaction between the chips and the tool slows down the tool wear process, resulting in a minimal impact on temperature. In contrast, the resin matrix properties of thermoplastic composites are highly temperature-dependent and undergo significant changes, particularly when the temperature exceeds the glass transition temperature (Tg) of the matrix. Under such conditions, PEEK resin matrix is softened by heat to produce plastic deformation, continuous chip can not be excluded in time. This allows the heat that should have been carried away by the chips (approximately 28% of the total heat generated when the CFRTP holes are drilled⁴¹) to be transferred to the tool.⁴² These chips adhere to the tool, increasing frictional heat during processing, accelerating tool wear.

The drilling force is a primary factor contributing to delamination damage and structural damage of the hole surface during the drilling of composite materials.⁴³ As tool wear progresses, the drilling force inevitably increases. Figure 11(b) shows the maximum thrust force based on the number of holes drilled. It is evident that the drilling thrust force for both materials increases with the number of holes (HN), but the rate of increase differs significantly. The thrust force for CF/PEEK increases by approximately 158% (from 59.02 N to 152.3 N) with the number of holes (HN). In contrast, the thrust force for CF/epoxy increases by about 51% (from 46.8 N to 70.59 N). The drilling thrust force for CF/PEEK is approximately 15-20 N higher than that for CF/epoxy at the same number of holes (HN). This difference is attributed to the distinct material removal mechanisms of the two materials. CF/epoxy is generally regarded as a brittle material that undergoes brittle fracture during drilling, generating powdery chips that are easily removed. CF/PEEK exhibits excellent ductility, forming continuous chips during drilling that tend to adhere to the tool. This adhesion generates additional friction, thereby increasing the drilling thrust force.

Figure 12(a) shows a comparison of maximum hole wall temperature under different drilling parameters. The increase in the spindle speed raises the maximum hole wall temperature of the two materials. The maximum hole wall temperature of CF/epoxy is always lower than the Tg (130°C). The increase in the maximum hole wall temperature of CF/PEEK was significantly greater and ultimately exceeded the Tg (143°C). This phenomenon occurs because the poorer thermal conductivity of the PEEK matrix than the epoxy resin matrix.^44,45 Figure 12(b) shows a comparison of maximum thrust force under different drilling parameters. Increasing spindle speed reduced the maximum thrust force in both materials. Unlike CF/epoxy composites which exhibited a steady decline in thrust force, CF/PEEK showed a slight increase in thrust force at higher spindle speeds. This behavior may be attributed to the drilling temperature exceeding the glass transition temperature (Tg) of PEEK, causing matrix softening and subsequent adhesion to the cutting tool, thereby increasing cutting resistance during material removal.

Figure 12.

(a) Effect of drilling parameters on the maximum hole wall temperature, (b) Effect of drilling parameters on the maximum thrust force.

Effect of tool wear on surface morphology and delamination damage

Delamination and burr damage are regarded as the most critical forms of damage in the drilling of composite materials. These defects are primarily concentrated at the hole exit and significantly compromise hole quality.^46,47 Figure 13 shows the hole exit surface morphology of the two materials at different hole numbers. The hole surface damage in CF/epoxy composites was primarily characterized by fiber tear and uncut fiber burrs. This is attributed to the low interlaminar fracture toughness of CF/epoxy composites, which makes them susceptible to exit tear damage under cutting forces. The hole surface damage in CF/PEEK composites is predominantly characterized by fiber agglomeration and fiber crimp. This phenomenon occurs because the PEEK resin matrix softens under thermal effects, and the uncut fiber burrs undergo agglomeration and curling. As the number of holes increases, the exit surface damage for both materials become more severe, with CF/PEEK exhibiting particularly pronounced damage. This is due to the thermal softening of the PEEK resin matrix, which adheres to the tool, significantly elevating drilling temperatures and forces. These conditions accelerate tool wear, leading to more severe hole exit damage.

Figure 13.

Surface morphology of hole exit at different hole numbers: (a) CF/epoxy HN = 1, (b) CF/PEEK HN = 1, (c) CF/epoxy HN = 64, (d) CF/PEEK HN = 64.

Figure 14(a) shows the variation curve of the delamination factor with the tool wear (i.e., HN). It is evident that the delamination factor at the hole exit for both materials increases with the tool wear. For HN ≤32, the delamination factor of CF/PEEK tends to be smaller than that of CF/epoxy. This phenomenon can be attributed to the excellent fracture toughness of CF/PEEK. Its mode I fracture toughness is approximately three times greater than that of CF/epoxy, and type II fracture toughness is about two times that of CF/epoxy,^48,49 which helps to inhibit interlayer fracture.^39,45 Thermoplastic matrix absorbs energy through plastic deformation at crack tips,^22,50 suppressing delamination initiation. Therefore, although the CF/PEEK composite was subjected to higher thrust force, the delamination factor at the hole exit remained relatively low. For HN ≥32, the delamination factor of CF/PEEK is significantly higher than that of CF/epoxy. This is due to the drilling temperature of CF/PEEK exceeds the glass transition temperature (T = 143°C) of the matrix. The thermal softening of the PEEK resin matrix leads to adhesion, accelerating tool wear and causing a sharp increase in the delamination factor. In contrast, the drilling temperature of the CF/epoxy consistently remained below its glass transition temperature (Tg = 130°C). Consequently, the progression of tool wear only induced gradual evolution of the delamination factor. When the delamination factor exceeds 1.3,^33,46,51 the hole quality did not meet acceptable standards. It is recommended to replace the tool after drilling 44 holes in CF/PEEK composite to maintain machining quality.

Figure 14

(a) Effect of tool wear progression (i.e.,the HN). on the delamination factor. (b) Effect of drilling parameters on the delamination factor.

Figure 14(b) shows a comparison of delamination factor under different drilling parameters. For CF/epoxy, the delaminating factor decreases with increasing spindle speed, mainly related to the reduction of the drilling thrust force. For CF/PEEK, the delaminating factor first decreases and then increases with the increase of spindle speed. Under the higher spindle speed, PEEK matrix was softened and its support for the carbon fiber was weakened, making the composite more prone to delamination under drilling thrust force.

Effect of tool wear on morpholog of hole wall

Evaluating the surface morphology of the hole wall after the drilling process is particularly critical, as the surface quality directly influences the assembly performance of the components.⁵² Figure 15 shows the hole wall SEM morphology of the two materials at different hole numbers. A comparison of Figure 15(a) and (b) reveals that at HN = 1 (i.e., without the influence of tool wear), SEM images showed that the machined surface of CF/PEEK had matrix smearing, while for the thermosetting CF/epoxy, fragmented debris adhered to the hole wall. The reason for this difference is mainly related to the properties of the material. For CF/epoxy, brittle fracture is the main mechanism of removal. The stiffness of the epoxy matrix can be maintained at the processing temperature.⁵³ In contrast, CF/PEEK exhibits superior hole wall quality at HN = 1. The surface is relatively smooth. This phenomenon is due to the fact that compared with brittle fracture, plastic removal of materials during machining can obtain better surface quality than brittle removal.^54,55

Figure 15.

SEM morphologies of hole wall at different hole numbers: (a) CF/epoxy HN = 1, (b) CF/PEEK HN = 1, (c) CF/epoxy HN = 64, (d) CF/PEEK HN = 64.

A comparison of Figure 15(c) and (d) reveals that tool wear significantly degrades the hole wall morphology of both composite materials. At HN = 64, a significant amount of chips adheres to the surface of CF/epoxy composites, leading to increased surface roughness, more frequent cracks and voids, and extensive fiber damage. This deterioration can be attributed to the degradation of hole wall quality caused by progressive tool wear. The hole wall damage in CF/PEEK composites is more severe, primarily characterized by a coating effect due to matrix melting and exposed fiber pull-out. This phenomenon occurs because, as tool wear progresses, the drilling temperature exceeds the glass transition temperature (Tg) of the PEEK matrix. Softened matrix is either deposited on the hole wall surface or adheres to the tool under mechanical action. This further increases drilling temperatures and thrust forces, accelerating tool wear and exacerbating hole wall damage.

Conclusion

In this paper, the drilling experiments of thermoset CF/epoxy and thermoplastic CF/PEEK composites were carried out. The wear process and micro wear mechanism of the tool were analyzed. The relationship between tool wear and machining quality was established. The effects of tool wear on drilling temperature, drilling force, surface roughness and drilling damage were analyzed. The following conclusions are obtained:

(1) CF/epoxy primarily experiences tool wear as localized edge chipping, scratching, and particle detachment, accelerated by the high hardness and inherent brittleness of carbon fibers.

(2) CF/PEEK primarily experiences tool wear characterized by chip adhesion, cutting edge blunting, and delamination, exacerbated by PEEK’s high ductility forming continuous, adhesive long chips.

(3) CF/epoxy exhibits minimal temperature variation during machining, consistently staying below its glass transition temperature (Tg), powdery chips facilitate efficient heat dissipation, resulting in stable drilling forces.

(4) CF/PEEK exhibits rapid temperature escalation due to PEEK’s thermal softening behavior, chip adhesion causes a significant increase in drilling forces (up to 158% higher), exacerbating tool wear and machining difficulty.

(5) CF/epoxy frequently suffers from fiber tearing and exit burrs due to its poor interlaminar toughness, resulting in degraded surface quality and increased roughness.

(6) CF/PEEK can achieve smoother machined surfaces under optimal conditions, but excessive cutting temperatures induce fiber curling and elevated cutting forces trigger fiber agglomeration, compromising quality.

(7) CF/PEEK significantly reduces tool life under high-temperature/high-wear conditions, requiring optimized coatings or advanced cooling methods, with temperature control being crucial to prevent thermal damage.

(8) CF/epoxy shows milder tool wear (though prone to edge chipping) and is suitable for shorter tasks, benefiting from lower cutting speeds to extend tool life.

(9) Higher spindle speed can reduce the exit delamination factor of CF/epoxy. However, when the drilling temperature exceeds the glass transition temperature of PEEK and the matrix adhesion, the delamination damage of CF/PEEK will be aggravated.

Footnotes

Author contributions

The authors confirm their contribution to the paper as follows:

Conception and design, data collection, experimentation, draft manuscript preparation: Yu Du

Experimentation, data collection, analysis of results, draft manuscript preparation: Peichao Li

Design, resources, supervision: Sinan Liu

All authors reviewed the results and approved the final version of the manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/ or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Tianjin 131 Research Team of Innovative Talents (No. 201916), and the Tianjin Diversified Investment Project in Applied Basic Research (23JCYBJC00100). The authors would like to acknowledge the editors and the anonymous referees for their insightful comments.

ORCID iD

Yu Du

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Comparative study of tool wear and machining quality in drilling of thermoplastic CF/PEEK and thermoset CF/epoxy composites

Abstract

Keywords

Introduction

Experimental set-up

Workpiece material

Drilling experiments

Measurement methods of tool wear and machining quality

Results and discussion

Effect of tool wear on chip morphology

Tool wear mechanisms and progression

Effect of tool wear on drilling temperature and drilling force

Effect of tool wear on surface morphology and delamination damage

Effect of tool wear on morpholog of hole wall

Conclusion

Footnotes

Author contributions

Declaration of conflicting interests

Funding

ORCID iD

References