CVD Diamond Tool Performance in Metal Matrix Composite Machining

Surface & Coatings Technology 200 (2005) 1872 – 1878 www.elsevier.com/locate/surfcoat

CVD diamond tool performance in metal matrix composite machining

YR Kevin Chou *, Jie Liu

Mechanical Engineering Department, The University of Alabama, Tuscaloosa, AL 35487, United States Available online 13 September 2005

Abstract

Metal matrix composite (MMC) has found increasing usages in the industry for lightweight high-strength applications. However, because of abrasive nature of the reinforced phase in MMC, machinability is poor, tool wear is rapid, yet only diamond tools are technically suitable to MMC machining. Furthermore, diamond coatings seem to be more economically viable than polycrystalline diamond for MMC machining. In this study, CVD diamond-coated tools, 30 Am thick on a tungsten carbide substrates, were investigated by outside diameter turning of MMC of aluminum-alloy reinforced with silicon-carbide particles. Cutting conditions ranged from 1 m/s to 6 m/s of cutting speed, 0.05 mm/ rev to 0.3 mm/rev feed, and 1 mm to 2 mm depth of cut. Tool wear was measured and compared at different machining conditions. Worn diamond-coated tools were extensively characterized by scanning electron microscopy. Cutting forces, chip thickness, and the chip–tool contact area were also measured for cutting temperature simulation by finite element analysis. The results show that tool wear is sensitive to cutting speed and feed rate, and the dominant wear mechanism is coating failure due to high stresses. The catastrophic coating failure suggests the bonding between the coating and substrate is critical to tool performance. High cutting temperatures will induce greater interfacial stresses at the bonding surface due to different thermal expansions between the coating and substrate, and plausibly result in the coating failure. A thermal management device, heat pipe, has been demonstrated for cutting temperature reductions. D 2005 Elsevier B.V. All rights reserved.

Keywords: Composite; Cutting temperature; CVD; Diamond coating; Machining; Tool wear

1. Introduction The ceramic reinforcement has greater hardness than conventional tool materials, e.g., 2400 Hv of SiC vs. 1800 Metal matrix composite (MMC) is a category of Hv of tungsten carbide (WC) [2], rendering only diamond engineering materials with growing applications in industry tools able to achieve economical tool life in machining such as automotive and aerospace. Examples of MMC ceramic reinforced MMC [2]. Diamond-coated tools, by applications include engine parts, brake system, drive shaft, chemical vapor deposition (CVD), have been investigated as and others such as pump housing and supercharger an economical alternative to brazed polycrystalline diamond compressors [1]. The advantage of MMC is the possibility (PCD) tools for machining advanced materials such as of tailoring material properties by combining matrix alloys MMC [3]. El-Gallab and Sklad reported, in a study of wear and reinforcement phase. The reinforcement phase in MMC mechanism of diamond tools in machining MMC, that can be presented in the form of particulate, short fiber and abrasion/adhesion by reinforced particulates are the domi- whisker, continuous fiber and monofilament. One major nant wear mechanisms and severe machining conditions division of MMC is aluminum-alloy matrix composite with will result in chipping in PCD and peeling of CVD diamond ceramics particulate reinforcement such as silicon carbide coating [4,5]. PCD tool is found to outperform CVD (SiC) and alumina. diamond coated tool and ceramic tools in machining of aluminum matrix composites, however, not economical [6]. Andrewes et al. also studied tool wear in machining Al/SiC * Corresponding author. composites using diamond tools and reported that mild E-mail address: [email protected] (Y.K. Chou). machining conditions are necessary to obtain economical

(a) perform better than smaller nose radii, due to increased resistance to plastic deformation [11]. Tool wear, in general, increases with increase of weight percentage and size of the reinforcement phase. However, there seems to exist a critical weight percentage of reinforcement above which abrasive wear would accelerate according to Li and Seah [12]. Cheung et al. studied reinforcement phases in MMC machining [13], and found that machinability is sensitive to the particle size and fraction. A higher amount of SiC will avoid large deformation and lead to better surface finish [13]. Applying cutting fluid in machining of aluminum MMC was reported, by Hung et al., to have no noticeable effects on tool performance [14]. On the other hand, in Cronja¨ger’s study, coolant and lubricant were only found to (b) accelerate tool wear in drilling and milling tests because the cutting fluid mixed with broken reinforcements forms abrasive emery fluid around the cutting zone [15]. MMC machining by CVD diamond tools have been reported with some mixed results. Due to the harder reinforcement phase, machinability of MMC is generally unfavorable and tool performance is sensitive to cutting parameters. Tool wear is complicated by abrasion from the hard ceramic phase, adhesion to the soft matrix material, and the bonding between the coating and substrate. In this study, machining of Al/SiC composite by CVD diamond tools was investigated with cutting force and temperature measured. Tool wear was also evaluated at different 40 µm cutting conditions and characterized by scanning electron microscopy (SEM). Finite element simulation of cutting Fig. 1. Tool and work material characteristics, (a) cutting edge of a CVD temperatures was also conducted to evaluate process diamond insert, and (b) MMC microstructure. parameter effects and to offer insight of CVD tool wear mechanisms. diamond tool life [7]. The authors found that tool wear involves two stages, one being initial flank wear caused by abrasion of hard particles, and the other combined adhe- 2. Experimental procedures sion–abrasion when the work materials start accumulatively adhering to the tool wear-land. Joshi et al. reported that For the machining test, cutting inserts used were CVD diamond tool wear is very sensitive to process commercial diamond coated inserts from sp3 Inc. The insert parameters [8,9]. Davim studied the influence of cutting substrate is 6% cobalt tungsten carbide of triangular shape. conditions on turning of aluminum matrix composites The diamond coating, from chemical vapor deposited (A359/SiCp/20), and concluded that cutting speed shows (CVD), has a nominal thickness of 30 Am, and the cutting more significant influence to tool wear than feed rate in edge radius is about 30 Am with grain size of 15 Am. Fig. 1a turning of A356 matrix composite [10]. El-Gallab and Sklad is a scanning electron micrograph of a CVD diamond further reported that a ‘‘protective’’ work material layer (not cutting edge. One-inch thick steel tool-holder was used and well-defined though) at the tool surface would form under the combination of the tool-holder and cutting insert forms high cutting speeds because of high cutting temperatures 0- and 11- of rake and relief angles, respectively. Work- associated. The authors also found an optimal rake angle, pieces were 47 mm round bars made of A359/SiC/20p bar 0-, to tool performance, and claimed that larger nose radii (cast and T6), which is A359 aluminum alloy reinforced

Table 1 Temperature-dependent thermal conductivity, k (W/mIK), of diamond coating, WC substrate, and steel tool-holder T (-C) 25 100 200 300 400 500 600 700 800 900 1000 CVD diamond 2008 1344 306 93 WC 78.6 70.0 67.1 68.7 68.6 65.6 62.5 59.3 Tool-holder 42.7 42.3 37.7 33.1 1874 Y.K. Chou, J. Liu / Surface & Coatings Technology 200 (2005) 1872–1878

0.4 v = 6 m/s, f = 0.05 mm/rev depth of cut (d). No coolant was used in machining. A v = 6 m/s, f = 0.2 mm/rev triaxial piezoelectric force sensor (Kistler 9257B) with a v = 2 m/s, f = 0.05 mm/rev 0.3 v = 2 m/s, f = 0.2 mm/rev data acquisition system was used to measure three components, i.e., tangential, radial and axial, of cutting forces. Cutting temperature was also monitored using K type 0.2 thermocouples, attached to the tool surfaces, around 3 to 4

VB (mm) mm from the cutting edge. Tool–chip contact length and chip thickness were measured by optical microscopy to 0.1 evaluate the rake face heat-source area and the chip velocity. Tool wear, in flank wear-land width (VB), was periodically 0 measured through the machining test. Worn tools were also 0 5 10 15 20 25 thoroughly examined by scanning electron microscopy t (min) (SEM) to characterize the wear patterns, including material Fig. 2. Tool wear (VB) development at different cutting conditions. deposition, and wear mechanisms of CVD diamond tools in machining MMC. with 20% SiC particles. Fig. 1b shows the mcirostructure of the work material. OD turning of MMC bars by diamond-coated tools was 3. Heat transfer in the cutting tool system carried out on a precision CNC lathe. Machining parameters ranged from 1 m/s to 6 m/s of cutting speed (V), 0.05 mm/ Considering the cutting tool system, heat transfer in the rev to 0.3 mm/rev of feed rate ( f), and 1 mm to 2 mm of cutting insert and tool holder is by conduction. Heat transfer between the cutting tool system and the ambient media is by (a) 250 Fr Ft (a) Fa 200

150 F (N) 100

0 (2,0.05) (2,0.2) (6,0.05) (6,0.2) (b) 120 (b) T_rake T_flank 100

80 C)

° 60 T (

0 (2,0.05) (2,0.2) (6,0.05) (6,0.2) Fig. 4. SEM photographs of a CVD diamond insert after the first cutting Fig. 3. Measured cutting forces and temperatures from the machining test, pass (3 m/s, 0.1 mm/rev, 2 mm, 50 s), (a) low magnification, and (b) high (a,b)=(speed, feed). magnification showing work material deposition. Y.K. Chou, J. Liu / Surface & Coatings Technology 200 (2005) 1872–1878 1875

(a) (a)

(b) (b)

Fig. 5. SEM photographs of a worn CVD diamond insert (6 m/s, 0.05 mm/ rev, 1 mm, 8 min), (a) low magnification, and (b) high magnification Fig. 6. SEM photographs of a worn CVD diamond insert (6 m/s, 0.05 mm/ showing edge deformation and material deposition. rev, 1 mm, 8 min) after leaching, (a) low magnification, and (b) high magnification showing coating removed (during cutting) and severe substrate wear. convection, however, negligible. The governing equation is of transient, three-dimensional heat conduction. At chip– with the minimum element size of about 10 Am in this tool contact, the thermal boundary condition is specified as study. a heat flux, brqr, across the contact area, presumably br, the heat partition coefficient, represents the portion of uniform; where qr is the overall heat flux at the rake face the overall frictional heat that conducts into the tool, the and br heat partition coefficient. The initial condition is remaining, (1Àbr)qr, being carried away by the moving room temperature.

The overall heat flux, qr due to friction, at the chip–tool 500 contact is determined by ( FfVc)/Ac,whereFf is the T_max frictional force at the tool rake face, Vc is the chip sliding T_ave 400 speed and Ac is the tool–chip contact area. Both Ff and Vc can be analyzed from cutting mechanics using measured cutting forces and chip thickness, and Ac is experimentally 300 C) measured by optical microscopy. ° T ( Because of three-dimensional geometry involved, finite 200 element analysis, using ANSYS R8.0, was employed to build the actual coating, insert, and tool-holder geometry, then to simulate temperature distributions in a cutting tool 100 system during cutting, with a given heat flux, heat source area, and boundary conditions. Element type used was 0 Solid70, an 8 nodes thermal element, applicable to a 3D, (1,0.1) (1,0.3) (3,0.1) (3,0.3) transient thermal analysis. Element size in the geometric Fig. 7. Maximum and average tool temperature (at chip–tool contact), from model meshing is dependent on the object dimensions, simulation, at different cutting conditions, (a,b)=(speed, feed). 1876 Y.K. Chou, J. Liu / Surface & Coatings Technology 200 (2005) 1872–1878

chip. To determine br, which varies with cutting conditions, the shear plane, the initial temperature of the chip model is a separate 2-D chip model (also by ANSYS) was the shear-plane temperature calculated from cutting mechan- established. The 2-D chip model consists of a long strip ics. With the tool and chip models and by driving the with chip thickness as the width dimension, and the other average temperature at the chip–tool contact the same dimension relatively long to simulate an infinite domain. between the two models, br can be numerically determined The model also has a heat source, heat flux (1Àbr)qr and through iterations. size of chip–tool contact length, moving with the chip Temperature-dependent thermal conductivity was used in velocity on the boundary of the chip. Because the material the thermal model, Table 1 for CVD diamond [16],WC entering the chip zone, from the uncut area, passes through substrate, and the tool-holder (AISI 4140 steel). The specific

(a)

Tool-holder (Low temperature occurring)

Cutting insert

Tool tip (Maximum temperature)

(b)

Tool-holder (Low temperature occurring)

Cutting insert

Heat-pipe

Tool tip (Maximum temperature)

Fig. 8. Simulated cutting temperature distributions: (a) without heat pipe, and (b) with heat pipe. Y.K. Chou, J. Liu / Surface & Coatings Technology 200 (2005) 1872–1878 1877 heat and density of diamond coating, carbide substrate, and Machining temperatures were numerically simulated tool-holder are 502 J/kgIK and 3510 kg/m3, 210 J/kgIKand using the approach described earlier. Fig. 7 compares 14,900 kg/m3, 520 J/kgIK and 7800 kg/m3, respectively. simulated tool temperatures, maximum and average (over the chip–tool contact area), at different machining conditions. It indicates that cutting speed dominates temper- 4. Results and discussion ature rises of the cutting tool. The results qualitatively agree with the temperature measurements from machining, Fig. 2 shows tool wear development (in VB) along showing the trend of temperature changes by different cutting time at different machining conditions. It is noted process parameters. that both speed and feed rate affect tool performance, with High cutting temperatures result in aggressive adhesion feed rate dominant. In addition, all four cutting conditions wear at the tool surface. Additionally, for coated tools, due have the same trend that tool wear drastically increases, to different thermal expansion between the coating and after a slow steady wear, in one single pass after certain substrate, e.g., 1.5Â10À 6 1/K of CVD diamond [17] and cutting period. Low feed tends to delay the onset of the 5.5Â10À 6 1/K of WC [18], higher tool temperatures will drastic increase of tool wear. It is speculated that induce a high level of stresses at the bonding interface, catastrophic failure of diamond coating, peeling off, occurs which may exceed the bonding strength, and eventually lead during the cut and results in rapid wear of the carbide to coating failure. Thus, reducing cutting temperatures, not substrate. using coolant though, may delay the onset of coating failure. Fig. 3a shows measured cutting forces (during the first A thermal management device, heat pipe, was employed to pass) vs. cutting conditions, Fr, Ft, and Fa being the radial, attempt cutting temperature reductions. Heat pipe is a tangential, and axial components, respectively. It is noted passive device with a high thermal conductance used to that feed rate has a dominant effect on cutting forces, transport heat by means of evaporating and subsequent while cutting forces decrease slightly with increasing condensation of an appropriate fluid [19]. A flat-plate heat cutting speed. Moreover, the rapid increase of tool wear pipe with 135- angle (from Thermacore Inc.) is attached on (one particular pass in Fig. 2) was also detectable from a the tool rake face for machining. The cutting temperature sudden escalation of the cutting forces during that simulation with a heat pipe incorporated has been con- particular cutting pass. This further supports catastrophic ducted; thermal properties of the heat pipe used are 11960 failure of the coating due to high stresses combined with W/mIK thermal conductivity, 1985 J/kgIK specific heat, and high temperatures. Fig. 3b compares measured cutting 3395 kg/m3 density [20]. Fig. 8a and b compare the temperatures at different machining conditions and it temperature contours in a cutting tool during machining (1 seems that cutting speed is dominant to the cutting m/s, 0.3 mm/rev, 2 mm), showing temperature reductions by temperature. heat pipe cooling, roughly 15 -C of the maximum temper- Fig. 4 shows scanning electron micrographs of a CVD ature. The heat pipe was positioned, as shown in Fig. 8b, at diamond insert cut at 3 m/s, 0.1 mm/rev, and 2 mm depth about 10 mm away from the cutting tip. Further, actual of cut for about 50 s. The diamond tip (edge) is partially machining employing a heat pipe has also been tested with covered by the residual work material from chip formation temperature measurements. Fig. 9 compares temperature (sliding at the rake face and flank face). At a higher response, at measurement location, for machining MMC magnification, material deposition, as thin film, can be without and with heat pipe. Temperature reduction due to clearly observed and was identified as mainly Al and Si heat pipe is noted. (by EDAX). Note, however, that no SiC particles, intact or broken, have been identified. In addition, diamond grains show little, if any, trace of wear. Fig. 5a and b present, 80 from SEM, a worn CVD diamond tip, cut at 6 m/s, 0.05 70 mm/rev, and 1 mm for about 8 min. The adhered work material deposition, severely deformed, can be seen at the 60 already large flank wear-land area. To reveal the under- 50 neath wear structure, the worn tip was leached by a 10% T (C) HCl solution, in ultrasonic bath, for about 20 minutes to 40 With heat pipe remove the deposited material. Fig. 6a and b are SEM 30 pictures of the same CVD diamond tip from Fig. 5, but Without heat pipe 20 after leaching. The coating peeling-off is clearly evidenced with noticeable boundary. In addition, the substrate has 10 experienced severe plastic deformation, resulting in rapid 0 20406080 wear. All worn diamond coating tools from the machining t (s) test exhibit similar features, i.e., material deposition and Fig. 9. Temperature comparison, from experimental measurements, for coating failure. CVD diamond machining without and with heat pipe. 1878 Y.K. Chou, J. Liu / Surface & Coatings Technology 200 (2005) 1872–1878

5. Conclusions machining tests. M. North of Thermacore provided heat pipes for the machining tests too. Machining of MMC (Al/SiC) by CVD diamond tools has been conducted to investigate tool performance, process parameter effects, and tool wear mechanisms. From wear References measurements, force data, and worn surface character- izations, catastrophic coating failure (peeling-off) caused [1] W.H. Hunt Jr., D.B. Miracle, Composite, ASM Metals Handbook, vol. by strong work material adhesion and high cutting temper- 21, p. 1029. [2] J.A. Vaccari, C.T. Lane, Am. Mach. 11 (1993) 56. atures limits the tool life. Difference in thermal expansions [3] C.H. Shen, Surf. Coat. Technol. 86 (1996) 672. between the coating and substrate results in high stresses, [4] M. El-Gallab, M. Sklad, J. Mater. Process. Technol. 83 (1998) 151. due to elevated cutting temperatures, and subsequent coat- [5] M. El-Gallab, M. Sklad, J. Mater. Process. Technol. 101 (2000) 10. ing removal once the internal built-up stress by high [6] S. Durante, G. Rutelli, F. Rabezzana, Surf. Coat. Technol. 94–95 temperatures overcomes the bonding strength. (2000) 632. [7] C.J.E. Andrewes, H.Y. Feng, W.M. Lau, J. Mater. Process. Technol. Cutting tool temperatures in machining have also been 102 (2000) 25. simulated by ANSYS to study process parameter effects and [8] S.S. Joshi, N. Ramakrishnan, H.E. Nagarwalla, P. Ramakrishnan, the analysis qualitatively agrees with the trend observed Wear 230 (1999) 124. from the machining experiments. A heat pipe was included [9] S.S. Joshi, N. Ramakrishnan, P. Ramakrishnan, J. Mater. Process. in MMC machining and the results show, from both Technol. 88 (1999) 90. [10] J.P. Davim, J. Mater. Process. Technol. 132 (2003) 340. experiment and simulation, cutting temperature reductions [11] M. El-Gallab, M. Sklad, J. Mater. Process. Technol. 83 (1998) 277. which may delay the onset of coating failure. [12] X. Li, W.K.H. Seah, Wear 247 (2001) 161. [13] C.F. Cheung, K.C. Chan, S. To, W.B. Lee, Scr. Mater. 47 (2002) 77. [14] N.P. Hung, S.H. Yeo, B.E. Oon, J. Mater. Process. Technol. 67 (1997) Acknowledgments 157. [15] L. Cronja¨ger, D. Meister, Ann. CIRP 41 (1992) 63. [16] J. Nedelik, W. Kremser, B. Lux, Diam. Films Technol. 8 (1998) 37. This research has been supported by Center for [17] P.W. May, Endeavour 19 (1995) 101. Advanced Vehicle Technologies. Graduate council fellow- [18] P.J. Heath, Ind. Diamond Rev. 3 (1986) 120. ship, from UA Graduate School, supporting JL to conduct [19] P.D. Dunn, D.A. Reay, Heat Pipes, 3rd edR, Pergamon Press, 1982. this work is also gratefully acknowledged. sp3 Inc., [20] R.Y. Chiou, J.S.J. Chen, L. Lin, M.T. North, Proc. of ASME Intl. Mech. Eng. Cong. and Expo., 2003, IMECE2003-55464, in CD-ROM. particularly, J. Zimmer and K. Bennett, provided CVD diamond inserts, along with technical information, for the