Zr soft-coatings and nano-textures in dry cutting

Wear 318 (2014) 12–26

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

Cutting performance and wear characteristics of Al2O3/TiC ceramic cutting tools with WS2/Zr soft-coatings and nano-textures in dry cutting Youqiang Xing, Jianxin Deng n, Shipeng Li, Hongzhi Yue, Rong Meng, Peng Gao Key Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan 250061, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 17 March 2014 Received in revised form 28 May 2014 Accepted 1 June 2014 Available online 19 June 2014

To improve the cutting performance and reduce the tool wear, novel Al2O3/TiC cutting tools were developed with coating and laser technologies: first, the WS2/Zr coated Al2O3/TiC ceramic cutting tool; second, nano-textured Al2O3/TiC ceramic cutting tools deposited with WS2/Zr composite soft-coatings. Dry cutting tests were carried out on hardened steel with the conventional and developed tools. The cutting force, cutting temperature, friction coefficient, and tool wear were measured. Results show that the WS2/Zr coated Al2O3/TiC cutting tools with and without nano-textures significantly improve the lubricity at the tool-chip interface; the cutting force, cutting temperature, friction coefficient and tool wear are reduced compared with the conventional tools; the nano-textured tools deposited with WS2/Zr composite soft-coatings are the most effective. In addition, the geometry of nano-textures has a profound effect on the lubricity, the WS2/Zr coated cutting tool with areal nano-textures is the most effective in improving the cutting performance and reducing the tool wear. The abrasive wear, chipping and adhesions are the predominant wear characteristics of conventional tools, the abrasive wear and coating flaking is for coated tools, and the adhesions at the tool tip is mainly for the coated tools with nano-textures. & 2014 Elsevier B.V. All rights reserved.

Keywords: Cutting tools Ceramic-matrix composite Laser processing PVD coatings WS2/Zr

1. Introduction Al2O3 based ceramic cutting tools are applied widely for dry cutting and high-speed machining of high hardness workpiece materials in the industry due to their unique intrinsic properties: high melting point, high hardness, good chemical inertness and high wear resistance [1,2]. However, the friction coefficient of Al2O3 based ceramic cutting tool under dry cutting condition of hard materials is relatively high [3,4], and this will result in increased tool wear and reduced tool life. Therefore, considerable efforts had been made to reduce friction and wear and extend the tool life. Deng et al. [5] reported a ceramic cutting tool with the additions of CaF2 solid lubricants. The results showed that the friction coefficient at the tool–chip interface in dry cutting with this ceramic tool can be reduced compared with the tool without solid lubricants. Broniszewski [6] developed the Al2O3 ceramic tool with the additions of Mo to improve its tribological properties by forming MoO3 and MoO2 oxides. Zhao [7,8] developed the Al2O3 n Corresponding author at: Department of Mechanical Engineering, Shandong University, No. 17923 Jingshi Road, 250061 Jinan, Shandong Province, PR China. Tel.: þ 86 531 88399769. E-mail address: [email protected] (J. Deng).

http://dx.doi.org/10.1016/j.wear.2014.06.001 0043-1648/& 2014 Elsevier B.V. All rights reserved.

based functionally gradient ceramic tool and it exhibited high thermal shock resistance and long tool life. Surface coating is also an effective way to improve the tribological performance and wear resistance of materials, including hard-coatings (TiN, TiCN, TiAlN, etc.) and soft-coatings (MoS2, WS2, CaF2, etc.), and the coating technology had been used in cutting tools [9–15]. For ceramics, a few researches about coating technology were reported. Soković et al. [16,17] deposited different hard-coatings (TiN, Ti(C, N), (Ti, Al)N, etc.) on the Al2O3 ceramic tools surface by PVD and CVD technologies. Results showed that the Al2O3 ceramic tools with hard-coatings reduced the tool wear and increased the tool life. Dobrzański and Mikuła [18] investigated the mechanical and functional properties of Al2O3 þZrO2 ceramic deposited with multi-layer hard-coatings (TiAlSiNþ TiN, TiN þTiAlSiN þ AlSiTiN, TiCNþ TiN, etc.). Results showed that the Al2O3 þ ZrO2 ceramic deposited with hard-coatings resulted in an increasing microhardness, a high wear resistance and a significant increase of the tool life in cutting of grey cast iron. Aslantas et al. [19] deposited TiN coating on the surface of Al2O3 based ceramic tool to improve its wear resistance and increased tool life. However, hard-coatings may be not the most suitable for the ceramic substrates due to the high hardness of ceramic itself; soft-coatings may be expected to be more suitable for ceramic and a few

Y. Xing et al. / Wear 318 (2014) 12–26

researches were reported. For example, Wang et al. [20] reported an Al2O3 ceramic with CaF2/Al2O3 composite soft-coatings by laser cladding technology. Results showed that the developed ceramic had much superior wear resistance and noticeable lower friction coefficient under dry sliding wear test conditions. Liu [21] deposited MoS2 soft-coatings on the ceramic tools by magnetron sputtering. Results showed that the flank wear of the coated cutting tools was reduced, and their wear life was extended markedly in dry cutting 1045 and 302 steels. Recently, surface texturing as a way for improving tribological properties of contact surfaces had received a great deal of attention and it had already been used in some fields such as bearings, engine cylinder liners and seal rings [22–24]. For cutting tools, the experimental observations and theoretical calculations showed that the tool surface topography had a significant impact on the tribological performance during machining [25–27]; meanwhile, due to the advantages of surface texturing, it applied to the cutting tools for decreasing the friction and wear was studied for many years. For example, Xie et al. [28,29] reported that the microgrooves patterned on the tool rake face by micro-grinding method contributed to reducing friction and cutting force, excluding cutting heat and then reducing tool wear in dry turning of titanium alloy. Xu et al. [30] fabricated the textures on forming tools using an Nd:YVO4 picosecond laser system. Results showed that the textured tools reduced the friction at the tool–workpiece interface, forming forces and temperatures when machined aluminum alloy. Ling et al. [31] studied the effect of surface microtextures on drills fabricated using a diode-pumped Nd:YVO4 picosecond laser in machining of titanium plate. Results revealed that the textured drills reduced adhesion of titanium chips on the drills and the significantly improved the lifetime of drills. Previous studies also demonstrated that the effect of textures had a significant correlation with the geometrical characteristic of textures. Deng et al. [32] fabricated microscale textures with different geometrical characteristics on the tool rake face, and MoS2 solid lubricants were filled into the textures. Results showed the elliptical grooves were more effective than parallel or perpendicular grooves. Koshy and Tovey [33] used sink electrical discharge machining (EDM) to generate areal and linear textures on the rake face of cutting tools. Results demonstrated a significant reduction of feed and cutting forces in cutting of steel and aluminum, and the areal textures showed more effective compared with the linear textures. Kawasegi et al. [34] reported that textures parallel to the cutting edge on the rake face were more effective in reducing friction and wear compared with other textures, which was in line with the results [35,36], but contradicted to the results obtained by Chang et al. [37]; they also reported that nanoscale textures on the tool rake face were more effective than microscale textures in decreasing friction and adhesion. Researchers also found that the combination of surface textures, lubricants and coatings on the cutting tools seemed to take a synergic effect on the cutting performance. Lei et al. [38,39] reported that the textured tools filled with liquid and soild lubricants, results showed that the micropool lubricated cutting tools improve the cutting performance and reduce the tool wear. Obikawa et al. [35] reported that the textured cemented carbide cutting tools coated with DLC or TiN coatings can improve the lubrication and cutting performance in machining aluminum alloy. Sugihara and Enomoto [36,40] developed DLC coated cutting tools with nano-/micro-textured surfaces. Face-milling experiments on aluminum alloys showed that DLC-coated cutting tool with textured surface significantly promoted the lubricity and anti-adhesive properties at the tool– chip interface. Deng et al. [41] used a femtosecond laser to fabricate nano-textures on cemented carbide cutting tools and then deposited with WS2 lubricant coatings. Results showed that the deposition of WS2 lubricant coatings on the textured rake face

13

reduced the friction and wear in dry cutting, and it was an effective way to improve the cutting performance. WS2 is well known intrinsic low-friction materials that have been thoroughly investigated in the forms of solid lubricant powders and burnished coatings, as well as in the form of thin coatings deposited by PVD [42–44]. They have a lamellar structure of stacked S–W–S planes with strong bonding within the planes and weak interactions between the planes. Due to its extreme degree of anisotropy of the layered crystal structures, it exhibits a small positive net charge outside of the lamellas, resulting in very low shear strength [45,46]. In this paper, the authors fabricated Al2O3/TiC ceramic cutting tools deposited with WS2/Zr composite soft-coatings by medium-frequency magnetron sputtering together with multi-arc ion plating, and the Al2O3/TiC ceramic cutting tools with different types of nano-textures on their rake face generated by femtosecond laser and then deposited with WS2/Zr composite soft-coatings by medium-frequency magnetron sputtering together with multi-arc ion plating. Microstructural and fundamental properties of the coatings were examined. Dry cutting tests on hardened steel were carried out with untextured and nano-textured tools deposited with WS2/Zr composite softcoatings. The cutting force, friction coefficient, cutting temperature and tool wear were measured. The effects of WS2/Zr composite soft-coatings and nano-textures on the cutting performance and wear characteristics of Al2O3/TiC ceramic tools were investigated.

2. Experimental details 2.1. Preparation of WS2/Zr coated Al2O3/TiC ceramic tools The substrate material utilized for the study was hot-pressed Al2O3/TiC ceramic (Zibo Dongtai Co., Ltd., China). Composition, physical, and mechanical properties of this tool material are listed in Table 1. The dimensions of cutting tools were 12  12  7.94 mm3 with a 0.1 mm at 51 edge chamfer and nose radius of 0.1 mm. The rake face of these tools was finished by grinding and polishing to the roughness Ra less than 0.02 μm, and then was cleaned with 30 min ultrasonic bath in alcohol and acetone, respectively. After that, they were dried for approximately 10 min in a pre-vacuum dryer. The physical vapor deposition (PVD) method was used to deposit the coatings with the PVD coating equipments (AS-585, China). For WS2/Zr composite soft-coatings, two WS2 targets (medium-frequency magnetron sputtering) and one Zr target (multi-arc ion plating) were used. Before deposition, the coating chamber was heated up to 180 1C and the vacuum in the chamber was pumped to 1.0  10  3 Pa. Then, the substrate was cleaned by argon ion bombardment for 10 min with a bias voltage of  600 V. Prior to synthesize WS2/Zr coatings, a thin adhesion interlayer of Zr was deposited first with multi-arc ion plating process for 15 min to increase adhesion strength; then the substrates were rotated to pass in front of each of the WS2 targets and Zr target in turn so that the WS2/Zr coatings with the uniform thickness were deposited, and the deposition time was 100 min. All the PVD coating conditions are listed in Table 2.

Table 1 Properties of Al2O3/TiC ceramic. Composition (wt%)

Density (g cm  3)

Al2O3/55%TiC 4.76

Hardness (GPa)

Flexural strength (MPa)

Fracture toughness (MPa m1/2)

23.5

900

5.04

14

Y. Xing et al. / Wear 318 (2014) 12–26

The surface morphologies and composition analyses of the coatings were examined by scanning electron microscope (SEM, QUANTA FEG 250, USA), energy dispersive X-ray spectroscope (EDX, X-MAX50, UK) analysis and X-ray diffraction (XRD, D8 ADVANCE, German) analysis. Scratch tests were used to measure the adhesion evaluation of the coatings. The measurements were performed using MFT-3000 device (multi-functional tester for material surface properties) by moving the diamond stylus with 200 μm radius along the examined specimen's surface with the gradually increasing load. The tests were made with the following parameters: load range 0–70 N, load increase rate 100 N/ min, and scratch travel 4 mm. The hardness of coatings was measured on an MH-6 microhardness tester. The microhardness tests were performed in five different positions under a normal load of 0.2 N for eliminating the influence of the substrate on the measurement results. The thickness of the coatings was characterized using the cross-section and measured using scanning electron microscope (SEM). To simplify the nomenclature, the prepared WS2/Zr coated Al2O3/TiC ceramic cutting tool was named AS-W. For comparison, a conventional Al2O3/TiC ceramic tool with the same composition and geometry was also used, and it was named AS. 2.2. Preparation of WS2/Zr coated textured Al2O3/TiC ceramic tools A regenerative amplified Ti: sapphire femtosecond laser system (Legend Elite-USP, Coherent Inc., USA) with a wavelength of 800 nm, pulse duration of 120 fs and the repetition rate of 500 Hz was used to generate the nano-textures on the rake face cutting tools. Three different types of nano-textures: (a) perpendicular to the main cutting edge (AN-PE); (b) parallel to the main cutting edge (AN-PA), (c) areal textures (AN-A) were designed and fabricated in this study, see Fig. 1. The optimal processing parameters of regular nano-textures used in the experiments were obtained in the authors’ previous studies [47]: pulse energy was 1.75 μJ, frequency was 500 Hz and scanning speed was 500 μm/s. After laser texturing, the cutting tools were deposited with WS2/Zr composite soft-coatings as the preparation methods in Section 2.1, and the AN-PE, AN-PA and AN-A tools deposited with WS2/Zr composite soft-coatings were named AN-PEW, AN-PAW and AN-AW, respectively. The surface morphologies of samples were examined by scanning electron microscope (SEM) and atomic force microscope (AFM, Nanoscope IIIa, USA).

2.3. Cutting tests Dry cutting tests were carried out on a CA6140 lathe equipped with a commercial tool holder having the following geometry: rake angle γo ¼  51, clearance angle αo ¼ 51, inclination angle λs ¼ 51 and side cutting edge angle Kr ¼451. The workpiece material used was AISI 1045 hardened steel with a hardness of HRC 40–50 in the form of a round bar with an external diameter of 120 mm. All tests were carried out with the following parameters: cutting speed v ¼80–260 m/min, depth of cut ap ¼0.2 mm, feed rate f ¼0.2 mm/r. Cutting force was obtained with a KISTLER 9275A piezoelectric quartz dynamometer linked via change amplifiers to a chart recorder. The highest cutting temperature of the tool rake face was measured by an infrared thermography (TH5104R, Japan). The worn morphology of the cutting tool was examined by scanning electron microscopy (SEM), and the chemical composition on the wear track was identified by energy dispersive X-ray spectroscopy (EDX).

3. Results 3.1. WS2/Zr coated cutting tools 3.1.1. Fundamental structures and mechanical properties of the WS2/Zr composite soft-coatings Fig. 2 illustrates the XRD analysis of the cutting tools coated with and without WS2/Zr composite soft-coatings. It revealed that only a very broad band pattern on the left (arrow A region) indicating a structure consisting of quasi-amorphous WS2/Zr.

Table 2 The PVD coating conditions. Deposition temperature (1C)

Ar pressure (Pa)

Bias voltage (V)

Zr current (A)

WS2 current (A)

Deposition time (min)

180

0.5

 100

80

1.2

100

0.7mm

Fig. 2. X-ray diffraction patterns of the samples coated with and without WS2/Zr composite soft-coatings.

0.1mm

0.7mm

0.05mm

0.1mm

0.05mm

0.6mm

0.6mm

Fig. 1. Schematic diagram of the nano-textures with different geometrical characteristics on the rake face of the cutting tools: (a) AN-PE, (b) AN-PA, (c) AN-A.

Y. Xing et al. / Wear 318 (2014) 12–26

This is in agreement with the similar results of MoS2 and WS2 coatings obtained in literatures [13,46,48]. Fig. 3 shows the SEM micrograph and the corresponding EDX maps of the distribution of W, S and Zr elements on the surface of WS2/Zr composite soft-coatings. The results showed that the surface of the coatings exhibited smooth, dense and uniform

15

structures, and no cracks or delamination can be observed on the surface. The EDX results showed that the W, S and Zr elements uniformly distributed on the surface of the WS2/Zr coated samples. The structures of the WS2/Zr composite soft-coatings were further characterized by the cross-sectional profile and the Al, Ti, W, S and Zr elements diffusion along the cross-section using SEM and EDX

Fig. 3. SEM micrograph and the corresponding EDX maps of the distribution of S, W and Zr elements on the coated surface.

Fig. 4. SEM micrograph of the cross-sectional view of the coated sample, and the corresponding energy dispersive X-ray (EDX) line of scanning analysis results of Al, Ti, S, W and Zr elements along the cross-section.

16

Y. Xing et al. / Wear 318 (2014) 12–26

analysis, the results are shown in Fig. 4. From the figure, the substrate, Zr interlayer and WS2/Zr coatings can be clearly seen. The thickness of the Zr interlayer was about 0.13 μm and thickness of the WS2/Zr coatings was about 0.79 μm. The corresponding distribution qualitative compositional profile of Al, Ti, S, W and Zr elements along the cross-section measured by energy dispersive X-ray (EDX) analysis confirmed that the WS2/Zr coatings were coated on the substrate surface, and the Zr interlayer existed between the substrate and WS2/Zr coatings. The hardness, thickness and critical load of the WS2/Zr composite soft-coatings are presented in Table 3. It revealed that the WS2/Zr composite soft-coatings deposited on the surface caused the surface layer hardness decrease. The coating/substrate critical load characterizing the adherence of the coating to the substrate was 47 N determined as the friction coefficient increased significantly in scratch test.

average cutting forces (Fx, Fy and Fz) in the stable cutting process of tools with WS2/Zr composite soft-coatings (AS-W) were slightly lower than those of the tools without coatings (AS). The average cutting forces in the stable cutting process of tools with and without coatings at different cutting speeds were calculated and plotted in Fig. 6. As shown in this figure, cutting speed seemed to have a profound effect on the cutting forces of the tools, and the cutting forces decreased with the increasing cutting speeds. The average cutting forces of Fx, Fy and Fz of WS2/Zr coated tools (AS-W) were lower compared with uncoated tools (AS) under the same cutting conditions.

3.1.2. Cutting performance of the WS2/Zr coated tools The measured three cutting forces of tools with and without WS2/Zr soft-coatings in cutting process at cutting speed of 200 m/min are shown in Fig. 5. It can be seen that the cutting forces increased dramatically when the cutting began, and they oscillated around an average value in the stable cutting process. The corresponding

Table 3 Properties of WS2/Zr coatings. Substrate Coating Hardness (GPa) Critical load (N) Coating thickness (μm) Al2O3/TiC WS2/Zr

6.2

47

0.8

Fig. 7. Friction coefficient at the tool–chip interface of tools with and without WS2/Zr composite soft-coatings at different cutting speeds.

Fig. 5. Variations of cutting forces of tools with and without WS2/Zr composite soft-coatings with cutting time at cutting speed of 200 m/min: (a) axial thrust force Fx, (b) radial thrust force Fy, and (c) main force Fz.

Fig. 6. Cutting forces of tools with and without WS2/Zr composite soft-coatings at different cutting speeds: (a) axial thrust force Fx, (b) radial thrust force Fy, and (c) main force Fz.

Y. Xing et al. / Wear 318 (2014) 12–26

The friction coefficient at the cutting tool rake face was calculated using the measured cutting forces and the results were plotted in Fig. 7. It was indicated that the friction coefficient of the AS-W tool was lower than that of the AS tool and the coated tool significantly improved the lubricity of tool rake face. The highest temperatures at the tool–chip interface in the stable cutting condition were plotted and shown in Fig. 8. It was evident that the cutting temperatures increased with the increasing cutting speeds; the coated tools (AS-W) showed the lower

Fig. 8. Cutting temperatures at the tool–chip interface of tools with and without WS2/Zr composite soft-coatings at different cutting speeds.

17

cutting temperature and it was reduced by 9–13% compared with the uncoated tools (AS).

3.1.3. Wear properties at the rake face of the WS2/Zr coated tools Fig. 9 shows the SEM micrographs and the corresponding EDX map of the surface distribution of Fe element on the worn rake face of the AS tool after 800 m dry cutting at the speed of 200 m/min. It revealed that a large amount of ploughs can be seen on the worn rake face, and the surface damage was in the form of abrasive wear (Fig. 9(b)); meanwhile, a large area of chipping occurred on the worn surface and the tool tip was broken down (Fig. 9(c)). The corresponding EDX map of the surface distribution of Fe element (Fig. 9(d)) showed that a large amount of Fe elements can be seen on the worn surface. Fig. 10 shows SEM micrographs, the corresponding SEM/EDX composition analyses and EDX maps of the surface distribution of S, W, Zr, and Fe elements on the worn rake face of the AS-W tool after 800 m dry cutting at the speed of 200 m/min. The results showed that no chipping was seen on the worn surface and the tool tip was intact. Most of the coatings were taken away from the substrate, and the substrate was exposed. The corresponding EDX composition analyses of points A and B showed that part of Zr interlayer was appeared on the worn surface and a small amount of S and W was still identified on the worn area (Fig. 10(d) and (e)). The enlarged SEM micrograph of the worn rake face showed that a few ploughs can be seen on the worn rake face (Fig. 10(f)), and the abrasive wear was milder compared with the uncoated tools (Fig. 9(b)). The corresponding EDX maps of the surface distribution of W, S, Zr and Fe elements on the worn rake face (Fig. 10(g)–(j)) confirmed that a large amount of WS2/Zr coatings and the Zr

Fig. 9. SEM micrographs ((a)–(c)) and the corresponding EDX map of the surface distribution of Fe element (d) on the worn rake face of the AS tool after 800 m dry cutting at the speed of 200 m/min.

18

Y. Xing et al. / Wear 318 (2014) 12–26

S

W

Zr

Fe

Fig. 10. SEM micrographs ((a)–(c) and (f)), the corresponding SEM/EDX composition analyses of points A and B ((d) and (e)) and EDX maps of the surface distribution of S, W, Zr, and Fe elements ((g)–(j)) on the worn rake face of the AS-W tool after 800 m dry cutting at the speed of 200 m/min.

interlayer were taken away at the tool–chip contact interface and a small amount of adhesions of Fe can be seen on the worn surface.

3.2. WS2/Zr coated textured cutting tools 3.2.1. Characteristics of the surface textured tools with and without WS2/Zr composite soft-coatings Fig. 11 shows the SEM micrographs of three types of nanotextures on the rake face of the cutting tools. As shown in the figure, the regular nano-textures were successfully fabricated on the tool rake face. For AN-PA and AN-A cutting tools, the grooves of nano-textures were parallel to the main cutting edge; and for AN-PE cutting tool, the grooves of nano-textures were perpendicular to the main cutting edge. Fig. 12 shows the AFM topography images and profiles of the nano-textures obtained by our previous research [47]. It was noted that the width of the nano-grooves was about 350–400 nm, the depth was about 120–150 nm, and the period was about 700–800 nm.

Fig. 13 shows the SEM images of the three types of nano-textures deposited with WS2/Zr composite soft-coatings on the tool rake face. It can be seen that the nano-textures were fully covered by the coatings, and the nano-grooves cannot be seen on the coated surface. In addition, it indicated that the dense and smooth coatings were observed on the untextured surfaces, while the loose and rough coatings with some gaps were observed on the textured surfaces. It was due to the fact that the roughness of the textured surface was increased by the nano-textures and which resulted in loose coatings with a thin WS2/Zr composite film.

3.2.2. Cutting performance of the WS2/Zr coated textured tools Fig. 14 shows the cutting forces of three kinds of nano-textured tools deposited with WS2/Zr composite soft-coatings at different cutting speeds. As shown in this figure, the average cutting forces of the three kinds of coated nano-textured tools (AN-PEW, ANPAW, AN-AW) were lower compared with the conventional tool (AS) and the coated tool without nano-textures (AS-W), see Fig. 6.

Y. Xing et al. / Wear 318 (2014) 12–26

19

Fig. 11. SEM images of the three types of nano-textures on the tool rake face: (a)–(c) AN-PE; (d)–(f) AN-PA; (g)–(i) AN-A.

It also can be seen that the type of nano-textures had an effect on the cutting forces, the direction of textures parallel to the cutting edge was better than perpendicular type, and the AT-AW tool showed the smallest cutting force. Fig. 15 shows the friction coefficient on the cutting tool rake face of different tools calculated using the cutting forces. It was indicated that the friction coefficient of coated nano-textured tools was lower compared with AS and AS-W tools (see Fig. 7) and the AN-AW tool showed the lowest friction coefficient. Fig. 16 shows the cutting temperatures at the tool–chip interface of three kinds of coated nano-textured tools. It can be seen that the cutting temperatures of coated nano-textured tools were lower compared with the AS and AS-W tools (see Fig. 8). The AN-AW tool showed the lowest cutting temperature, and it was reduced by 12–18% compared with the AS tool.

3.2.3. Wear properties at the rake face of the WS2/Zr coated textured tools Figs. 17–19 show the SEM micrographs and the corresponding SEM/EDX analyses on the worn rake face of AN-PEW, AN-PAW and AN-AW tools after 800 m dry cutting at the speed of 200 m/min.

As shown in Fig. 17, there was no chipping on the worn surface, while large adhesions of Fe confirmed by EDX analysis of point A (Fig. 17(c)) can be seen at the tool tip. The enlarged micrographs of nano-textured surface showed that the nano-textures were still visible on the worn rake face; the EDX analysis of point B (Fig. 17 (h)) indicated that the W, S and adhesions of Fe existed on the textured surface. The enlarged micrographs of untextured surface showed that there was no obvious abrasive wear on the worn surface (Fig. 17(g)); the EDX analysis of point C (Fig. 17(i)) confirmed that W and S elements still existed and no Fe was observed on the surface. The corresponding EDX maps of the distribution of S, W, Zr and Fe elements at the rake face (Fig. 17(j)– (m)) indicated most of WS2/Zr soft-coatings also had been taken away from the untextured and textured surface on the tool–chip interface, the Zr interlayer appeared on the worn rake face. Large amounts of workpiece materials were adhered to the tool tip, while a small amount of adhesions were adhered to the worn rake face compared with the AS and AS-W tools (Figs. 9 and 10), and they mainly existed on the nano-textured surface. Similar results can also be observed at the worn surface of AN-PAW and AN-AW tools (Figs. 18 and 19). Large amounts of adhesions of workpiece materials were adhered to the tool tip, and there was no chipping

20

Y. Xing et al. / Wear 318 (2014) 12–26

Fig. 12. AFM topography images and profiles of the nano-textures.

on the worn surface. For AN-PAW tool (Fig. 18), it can be seen that a large amount of Zr interlayer appeared on the nano-textured surface (Fig. 18(f) and (h)) and part of W, S and Zr can be observed on the untextured surfaces between the nano-textured surfaces (Fig. 18(e) and (g)). The corresponding EDX maps of the distribution of S, W, Zr and Fe elements at the rake face (Fig. 18(i)–(l)) confirmed that part of WS2/Zr coatings still existed on the worn surface, and a large amount of Zr interlayer on the nano-textured surface was still visible, while they were little on the untextured surfaces between the nano-textured surfaces; and a small amount of Fe can be seen on the worn rake face. For AN-AW tool (Fig. 19), the nano-textures can be seen clearly, and the WS2/Zr coatings existed on the textured surface identified by EDX composition

analysis of point A (Fig. 19(e)). The EDX maps of the distribution of S, W, Zr and Fe elements on the wear area (Fig. 19(f)–(i)) indicated that large amounts of WS2/Zr coatings and a small amount of Fe can be seen on the worn surface among all the experimental tools, and a small amount of WS2/Zr coatings were taken away by the chips compared with the AS-W, AN-PEW and AN-PAW tools.

4. Discussion In dry cutting, the friction force Ff, friction angle β between the chip and rake face, and the three cutting forces (axial thrust force Fx,

Y. Xing et al. / Wear 318 (2014) 12–26

21

Fig. 13. SEM images of the three types of nano-textures deposited with WS2/Zr composite soft-coatings on the tool rake face: (a)–(c) AN-PEW; (d)–(f) AN-PAW; (g)–(i) AN-AW.

Fig. 14. Cutting forces of three kinds of nano-textured tools deposited with WS2/Zr composite soft-coatings at different cutting speeds: (a) axial thrust force Fx, (b) radial thrust force Fy, and (c) main force Fz.

radial thrust force Fy, and main force Fz) can be calculated as [49]: F f ¼ Ar τc ¼ aw lf τc

ð1Þ

β ¼ arctanðμÞ

ð2Þ

Fx ¼

Ff sin β

sin ðβ  γ 0 Þ cos ðψ r þ ψ λ Þ

  sin γ 0 cos ðψ r þ ψ λ Þ ¼ aw lf τc cos γ 0  tan β

ð3Þ

22

Y. Xing et al. / Wear 318 (2014) 12–26

Fig. 15. Friction coefficient at the tool–chip interface of three kinds of nanotextured tools deposited with WS2/Zr composite soft-coatings at different cutting speeds.

Fig. 16. Cutting temperatures at the tool–chip interface of three kinds of nanotextured tools deposited with WS2/Zr composite soft-coatings at different cutting speeds.

Fy ¼

Ff sin β

sin ðβ  γ 0 Þ sin ðψ r þ ψ λ Þ   sin γ 0 sin ðψ r þ ψ λ Þ ¼ aw lf τc cos γ 0  tan β

Fz ¼

Ff cos ðβ  γ 0 Þ ¼ aw lf τc sin β

 sin γ 0 þ

cos γ 0 tan β

ð4Þ  ð5Þ

where aw is the cut width, lf is the tool–chip contact length, τc is the average shear strength at the tool–chip interface, γ0 is the rake angle, β is the friction angle, ψr is the approach angle and ψλ is the chip flow angle. According to Eq. (1), the friction force at the tool–chip interface varies linearly with the tool–chip contact length lf and the average shear strength τc under the same cutting conditions. Meanwhile, under the same cutting conditions, the rake angle γ0, approach angle ψr and chip flow angle ψλ remain basically unchanged values. Eqs. (3)–(5) show that the cutting forces of Fx, Fy, and Fz are influenced by the shear strength τc, the tool–chip contact length lf, and the friction angle β. For WS2/Zr coated tool (AS-W), the coatings had a lower critical shear strength τc and friction angle β than the substrate, and thus resulted in lower friction force, friction coefficient, cutting force and cutting temperature

compared with the conventional tool (AS); for the nano-textured tools with WS2/Zr coatings (AN-PEW, AN-PAW and AN-AW), the loose coatings with some gaps further decreased tool–chip contact length lf, which resulted in a larger reduction of friction force, friction coefficient, cutting force and the cutting temperature compared with the untextured tool. On the other hand, the previous researches showed that the nano-textures had a selflubrication without lubricants in dry cutting with different tools [34,41]. From the results, we also found that the cutting performance had a strong correlation with the geometry of the nanotextures. Results showed that the direction of nano-textures parallel to the cutting edge (AN-PAW) was better than that perpendicular to the cutting edge (AN-PEW), the areal nanotextures (AN-AW) were more effective compared with the banded nano-textures, which was in line with the results obtained by Koshy and Tovey [33], but contradicted to the results [50]. It can be explained that the nano-textures increased the adhesion strength between the coatings and substrate [51,52]; while, for the ANPEW tool, the lubricant coatings may be easily taken away by the flowing chips compared with the AN-PAW tool due to the fact that the direction of nano-grooves was parallel to the chip flowing direction; for the AN-AW tool, the direction of nano-grooves was perpendicular to the chip flowing direction and large area of nanotextures with loose coatings resulted in the largest reduction of tool–chip contact area. Different modes of tool failure including abrasive wear, adhesive wear, coating flaking and chipping were observed by the wear morphologies of different tools (Figs. 9, 10 and 17–19). Results showed that the whole tool tip of conventional tool without WS2/ Zr coatings (AS) was broken down, and the significant abrasive wear occurred on the worn rake face (Fig. 9(a)–(c)); meanwhile, the EDX map indicated that large amounts of Fe were adhered to the worn rake face (Fig. 9(d)). For the AS-W tool, there was no chipping at the tool tip, and the abrasive wear was milder compared with the AS tool (see Fig. 10). It can be explained that the thin film at the tool–chip interface can be acted as lubricants, which may contribute to the decrease of the tool wear. From the EDX maps, we can see that the coated film was flaked and the substrate was exposed (Fig. 10(g)–(j)), which meant that the lubricating film was not expected to be effective for the flat tool for a long time because the lubricating film WS2/Zr coatings were taken away immediately by the flowing chips after a long time dry cutting. For the AN-PEW, AN-PAW and AN-AW coated nanotextured tools (Figs. 17–19), it was found that there was no chipping at the worn tool tip, the worn rake face was milder compared with the AS and AS-W tools, the nano-textures still existed. However, it was noted that some adhesions of Fe can be seen at the tool tip of nano-textured tools (AN-PEW, AN-PAW and AN-AW), which consisted with the previous results [50,53]. It can be explained when the coatings were worn off at the tool tip, the chips were squeezed into the nano-textures under the high pressure and then led to large adhesions at the tool tip; however, it was precisely because of the adhesions smeared at the tool tip that the tool tip was protected and free from the chipping.

5. Conclusions This paper reports a research on utilization of WS2/Zr composite soft-coatings and surface nano-textures on Al2O3/TiC ceramic cutting tools to improve the cutting performance and reduce the tool wear. Dry cutting tests on hardened steel were carried out with the conventional tools, WS2/Zr coated tools and nanotextured tools deposited with WS2/Zr coatings. In this study, the effects of WS2/Zr composite soft-coatings and three types of

Y. Xing et al. / Wear 318 (2014) 12–26

23

Fig. 17. SEM micrographs ((a), (b) and (d)–(g)), the corresponding SEM/EDX composition analyses of points A, B and C ((c), (h) and (i)) and EDX maps of the surface distribution of S, W, Zr, and Fe elements ((j)–(m)) on the worn rake face of the AN-PEW tool after 800 m dry cutting at the speed of 200 m/min.

nano-textures on Al2O3/TiC ceramic cutting tools were investigated. The following conclusions were obtained: (1) WS2/Zr composite soft-coatings and three types of nanotextures are successful made on Al2O3/TiC ceramic cutting tools,

the cutting performance of the WS2/Zr coated tools and WS2/Zr coated nano-textured tools is significantly improved compared with the conventional Al2O3/TiC ceramic cutting tools. (2) The nano-textured Al2O3/TiC ceramic cutting tools deposited with WS2/Zr composite soft-coatings are more effective in

24

Y. Xing et al. / Wear 318 (2014) 12–26

Fig. 18. SEM micrographs ((a)–(f)), the corresponding SEM/EDX composition analyses of points A and B ((g) and (h)) and EDX maps of the surface distribution of S, W, Zr, and Fe elements ((i)–(l)) on the worn rake face of the AN-PAW tool after 800 m dry cutting at the speed of 200 m/min.

reducing the cutting force, cutting temperature, friction coefficient and tool wear compared with the WS2/Zr coated tool without nano-textures on its rake face. The geometry of nanotexture has a profound effect on the cutting performance, and the WS2/Zr coated cutting tool with areal nano-textures is the

most effective in improving the cutting performance and reducing the tool wear. (3) The abrasive wear, chipping and adhesions are the predominant wear characteristics of conventional tools, the abrasive wear and coating flaking is for coated tools, and the adhesions

Y. Xing et al. / Wear 318 (2014) 12–26

25

Fig. 19. SEM micrographs ((a)–(d)), the corresponding SEM/EDX composition analysis of point A (e) and EDX maps of the surface distribution of S, W, Zr, and Fe elements ((f)–(i)) on the worn rake face of the AN-AW tool after 800 m dry cutting at the speed of 200 m/min.

at the tool tip is mainly for the coated tools with nanotextures. The formation of a WS2 lubricating film with low shear strength between the tool–chip interface and the reduction of tool–chip contact length may be responsible for the improvement of the cutting performance and tool wear.

Acknowledgments This work is supported by the National Natural Science Foundation of China (51375271), the Fundamental Research Funds of Shandong University (2014JC039) and Independent Innovation Foundation of Universities in Jinan (201401226). References [1] X. Ai, H. Xiao, Ceramic Cutting Tool Machining, China Machine Press, Beijing, 1988. [2] S. Kim, Material properties of ceramic cutting tools, Key Eng. Mater 96 (1994) 33–80.

[3] B. Prakash, J. Mukerji, S. Kalia, Tribological properties of Al2O3/TiN composites, Am. Ceram. Soc. Bull 77 (1998) 68–72. [4] Q.Y. Qiu, New Ceramic Cutting Tool, Defense Industry Press, Beijing, 1987. [5] J.X. Deng, T.K. Cao, X.F. Yang, Self lubrication of sintered ceramic tools with CaF2 additions in dry cutting, Int. J. Mach. Tools Manuf 46 (2006) 957–963. [6] K. Broniszewski, J. Wozniak, K. Czechowski, L. Jaworska, A. Olszyna, Al2O3–Mo cutting tools for machining hardened stainless steel, Wear 303 (2013) 87–91. [7] J. Zhao, X. Ai, X.P. Huang, Relationship between the thermal shock behavior and the cutting performance of a functionally gradient ceramic tool, J. Mater. Process. Technol. 129 (2002) 161–166. [8] J. Zhao, X. Ai, Fabrication and cutting performance of an Al2O3–(W,Ti)C functionally gradient ceramic tool, Int. J. Mach. Mach. Mater 1 (2006) 277–286. [9] J.X. Deng, J.H. Liu, J.L. Zhao, W.L. Song, M. Niu, Friction and wear behaviors of the PVD ZrN coated carbide in sliding wear tests and in machining processes, Wear 264 (2008) 298–307. [10] P.C. Jindal, A.T. Santhanam, U. Schleinkofer, A.F. Shuster, Performance of PVD TiN, TiCN, and TiAlN coated cemented carbide tools in turning, Int. J. Refract. Met. Hard Mater. 17 (1999) 163–170. [11] Y.J. Lin, A. Agrawal, Y. Fang, Wear progressions and tool life enhancement with AlCrN coated inserts in high-speed dry and wet steel lathing, Wear 264 (2008) 226–234. [12] M. Fallqvist, Microstructural, Mechanical and Tribological Characterisation of CVD and PVD Coatings for Metal Cutting Applications, Doctoral Thesis, ISBN: 978-91-554-8371-5, 2012. [13] S. Watanabe, J. Noshiro, S. Miyake, Tribological characteristics of WS2/MoS2 solid lubricating multilayer films, Surf. Coat. Technol 183 (2004) 347–351.

26

Y. Xing et al. / Wear 318 (2014) 12–26

[14] J.X. Deng, W.L. Song, H. Zhang, J.L. Zhao, Performance of PVD MoS2/Zr-coated carbide in cutting processes, Int. J. Mach. Tools Manuf 48 (2008) 1546–1552. [15] M.C. Jeng, Y.L. Soong, Wear behaviour of solid lubricants Ag and BaF2–CaF2 obtained by laser surface cladding, Surf. Coat. Technol 57 (1993) 145–150. [16] M. Soković, B. Barišić, S. Sladić, Model of quality management of hard-coatings on ceramic cutting tools, J. Mater. Process. Technol. 209 (2009) 4207–4216. [17] M. Soković, J. Mikuła, L.A. Dobrzański, J. Kopača, L. Koseč, P. Panjan, J. Madejski, A. Piech, Cutting properties of the Al2O3 þ SiC(w) based tool ceramic reinforced with the PVD and CVD wear resistant coatings, J. Mater. Process. Technol 164165 (2005) 924–929. [18] L.A. Dobrzański, J. Mikuła, The structure and functional properties of PVD and CVD coated Al2O3 þZrO2 oxide tool ceramics, J. Mater. Process. Technol. 167 (2005) 438–446. [19] K. Aslantas, I. Ucun, A. Cicek, Tool life and wear mechanism of coated and uncoated Al2O3/TiCN mixed ceramic tools in turning hardened alloy steel, Wear 274-275 (2012) 442–451. [20] H.M. Wang, Y.L. Yu, S.Q. Li, Microstructure and tribological properties of laser clad CaF2/Al2O3 self-lubrication wear-resistant ceramic matrix composite softcoatings, Scr. Mater. 47 (2002) 57–61. [21] Y.R. Liu, J.J. Liu, Z. Du, The cutting performance and wear mechanism of ceramic cutting tools with MoS2 coating deposited by magnetron sputtering, Wear 231 (1999) 285–292. [22] V. Brizmer, Y. Kligerman, I. Etsion, A laser textured parallel thrust bearing, Tribol. Trans 46 (2003) 397–403. [23] A. Borghi, E. Gualtieri, D. Marchetto, L. Moretti, S. Valeri, Tribological effects of surface texturing on nitriding steel for high-performance engine applications, Wear 265 (2008) 1046–1051. [24] I. Etsion, E. Sher, Improving fuel efficiency with laser surface textured piston rings, Tribol. Int 42 (2009) 542–547. [25] M. Fallqvist, F. Schultheiss, R. M’Saoubi, M. Olsson, J.-E. Ståhl, Influence of the tool surface micro topography on the tribological characteristics in metal cutting: Part I Experimental observations of contact conditions, Wear 298-299 (2013) 87–98. [26] F. Schultheiss, M. Fallqvist, R. M’Saoubi, M. Olsson, J.-E. Ståhl, Influence of the tool surface micro topography on the tribological characteristics in metal cutting—Part II Theoretical calculations of contact conditions, Wear 298-299 (2013) 23–31. [27] T.H.C. Childs, Friction modeling in metal cutting, Wear 260 (2006) 310–318. [28] J. Xie, M.J. Luo, K.K. Wu, L.F. Yang, D.H. Li, Experimental study on cutting temperature and cutting force in dry turning of titanium alloy using a non-coated micro-grooved tool, Int. J. Mach. Tools Manuf 73 (2013) 25–36. [29] J. Xie, M.J. Luo, J.L. He, X.R. Liu, T.W. Tan, Micro-grinding of micro-groove array on tool rake face for dry cutting of titanium alloy, Int. J. Precis. Eng. Manuf. 13 (2012) 1845–1852. [30] D.K. Xu, W.C. Wu, R. Malhotra, J. Chen, B. Lu, J. Cao, Mechanism investigation for the influence of tool rotation and laser surface texturing (LST) on formability in single point incremental forming, Int. J. Mach. Tools Manuf 73 (2013) 37–46. [31] T.D. Ling, P.Z. Liu, S.W. Xiong, D. Grzina, J. Cao, Q.J. Wang, Z.C. Xia, R. Talwar, Surface texturing of drill bits for adhesion reduction and tool life enhancement, Tribol. Lett. 52 (2013) 113–122. [32] J.X. Deng, Z. Wu, Y.S. Lian, T. Qi, J. Cheng, Performance of carbide tools with textured rake-face filled with solid lubricants in dry cutting processes, Int. J. Refract. Met. Hard Mater 30 (2012) 164–172. [33] P. Koshy, J. Tovey, Performance of electrical discharge textured cutting tools, CIRP Ann. 60 (2011) 153–156.

[34] N. Kawasegi, H. Sugimori, H. Morimoto, N. Morita, I. Hori, Development of cutting tools with microscale and nanoscale textures to improve frictional behavior, Precis. Eng. 33 (2009) 248–254. [35] T. Obikawa, A. Kamio, H. Takaoka, A. Osada, Micro-texture at the coated tool face for high performance cutting, Int. J. Mach. Tools Manuf 51 (2011) 966–972. [36] T. Enomoto, T. Sugihara, Improving anti-adhesive properties of cutting tool surfaces by nano-/micro-textures, CIRP Ann. 59 (2010) 597–600. [37] W. Chang, J. Sun, X. Luo, J.M. Ritchie, C. Mack, Investigation of microstructured milling tool for deferring tool wear, Wear 271 (2011) 2433–2437. [38] S. Lei, S. Devarajan, Z.H. Chang, A study of micropool lubricated cutting tool in machining of mild steel, J. Mater. Process. Technol. 209 (2009) 1612–1620. [39] S. Lei, S. Devarajan, Z.H. Chang, A comparative study on the machining performance of textured cutting tools with lubrication, Int. J. Mech. Manuf. Syst 2 (2009) 401–413. [40] T. Sugihara, T. Enomoto, Improving anti-adhesion in aluminum alloy cutting by micro stripe texture, Precis. Eng. 36 (2012) 229–237. [41] J.X. Deng, Y.S. Lian, Z. Wu, Y.Q. Xing, Performance of femtosecond lasertextured cutting tools deposited with WS2 solid lubricant coatings, Surf. Coat. Technol 222 (2013) 135–143. [42] J. Oksanen, T.J. Hakala, S. Tervakangas, P. Laakso, L. Kilpi, H. Ronkainen, J. Koskinen, Tribological properties of laser-textured and ta-C coated surfaces with burnished WS2 at elevated temperatures, Tribol. Int. 70 (2014) 94–103. [43] M. Hiranoand, S. Miyake, Sliding life enhancement of a WS2 sputtered film by ion beam mixing, Appl. Phys. Lett. 47 (1985) 683–685. [44] B. Deepthi, H.C. Barshilia, K.S. Rajam, M.S. Konchady, D.M. Pai, J. Sankar, Structural, mechanical and tribological investigations of sputter deposited CrN–WS2 nanocomposite solid lubricant coatings, Tribol. Int. 44 (2011) 1844–1851. [45] F. Gustavsson, F. Svahn, U. Bexell, S. Jacobson, Nanoparticle based and sputtered WS2 low-friction coatings-differences and similarities with respect to friction mechanisms and tribo film formation, Surf. Coat. Technol 232 (2013) 616–626. [46] G.Y. Du, D.C. Ba, Z. Tan, K. Liu, Tribological behavior of radio-frequency sputtering WS2 thin films with vacuum annealing, Thin Solid Films 520 (2011) 849–852. [47] Y.Q. Xing, J.X. Deng, Y.S. Lian, X.T. Feng, Femtosecond pulsed laser nanotexturing of Al2O3/TiC ceramic, Laser Phys. 23 (2013) 066002. [48] N.M. Renevier, N. Lobiondo, V.C. Fox, D.G. Teer, J. Hampshire, Performance of MoS2/metal composite soft-coatings used for dry machining and other industrial applications, Surf. Coat. Technol 123 (2000) 84–91. [49] R.Y. Chen, Principle of Metal Cutting, China Machine Press, Beijing, 1985. [50] T. Sugihara, T. Enomoto, Development of a cutting tool with nano/microtextured surface: improvement of anti-adhesive effect by considering the texture patterns, Precis. Eng. 33 (2009) 425–429. [51] D. Neves, A.E. Diniz, M.S.F. de Lima, Efficiency of the laser texturing on the adhesion of the coated twist drills, J. Mater. Process. Technol. 179 (2006) 139–145. [52] D. Neves, A.E. Diniz, M.S.F. Lima, Microstructural analyses and wear behavior of the cemented carbide tools after laser surface treatment and PVD coating, Appl. Surf. Sci. 282 (2013) 680–688. [53] Y.Q. Xing, J.X. Deng, K.D. Zhang, G.D. Zhang, H.H. Gao, Effect of femtosecond laser pretreatment on wear resistance of Al2O3/TiC ceramic tools in dry cutting, Int. J. Refract. Met. Hard Mater 43 (2014) 291–301.