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Failure mechanisms of TiB2 particle and SiC whisker reinforced Al2O3 ceramic cutting tools when machining nickel-based alloys
International Journal of Machine Tools & Manufacture 45 (2005) 1393–1401 www.elsevier.com/locate/ijmactool
Failure mechanisms of TiB2 particle and SiC whisker reinforced Al2O3 ceramic cutting tools when machining nickel-based alloys Deng Jianxin*, Liu Lili, Liu Jianhua, Zhao Jinlong, Yang Xuefeng Department of Mechanical Engineering, Shandong University of Technology, Jinan, Shandong Province 250061, People’s Republic of China Received 20 November 2004; accepted 28 January 2005 Available online 4 March 2005
Abstract In this paper, Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. The fundamental properties of these composite tool materials were examined. Machining tests with these ceramic tools were carried out on the Inconel718 nickel-based alloys. The tool wear rates and the cutting temperature were measured. The failure mechanisms of these ceramic tools were investigated and correlated to their mechanical properties. Results showed that the fracture toughness and hardness of the composite tool materials continuously increased with increasing SiC whisker content up to 30 vol.%. The relative density decreased with increasing SiC whisker content, the trend of the flexural strength being the same as that of the relative density. Cutting speeds were found to have a profound effect on the wear behaviors of these ceramic tools. The ceramic tools exhibited relative small flank and crater wear at cutting speed lower than 100 m/min, within further increasing of the cutting speed the flank and crater wear increased greatly. Cutting speeds less than 100 m/min were proved to be the best range for this kind of ceramic tool when machining Inconel718 nickel-based alloys. The composite tool materials with higher SiC whisker content showed more wear resistance. Abrasive wear was found to be the predominant flank wear mechanism. While the mechanisms responsible for the crater wear were determined to be adhesion and diffusion due to the high cutting temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: Ceramic cutting tools; Wear mechanisms; Cutting performance; Ceramic composites
1. Introduction Ceramics have intrinsic characteristics, such as high melting point, high hardness, good chemical inertness and high wear resistance, that make them promising candidates for high-temperature structural and wear resistance components, where metallic components achieve only unsatisfactory service lives, owing to inadequate heat, corrosive or wear resistance. Components made of advanced ceramics can survive and perform well at higher operating temperature, and improve the wear resistance. Nowadays the advanced ceramics are widely used in cutting tools, drawing or extrusion, seal rings, valve seats, bearing parts, and a variety of high-temperature engine parts, etc. [1–3].
* Corresponding author. Tel.: C86 531 295 5081x2047; fax: C86 531 295 5999. E-mail address: [email protected] (D. Jianxin).
0890-6955/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2005.01.033
Ceramic cutting tools usually perform better in high speed machining and in the machining of high hardness work piece materials as compared to high-speed steel and carbide tools. However, the use of single-phase ceramic tool materials, even fully densified, has been limited by their properties, such as their low strength and fracture toughness and poor thermal shock resistance. Furthermore, ceramics are very sensitive to microscopic flaws, thus ceramic cutting tools often crack at the tool edge, leading to unpredictable and catastrophic gross fracture of the tool. The low fracture toughness leads to brittle fracture, and the low thermal conductivity and high anisotropy thermal expansion of ceramics lead to large temperature gradients and thermal micro-cracks at the cutting edge and the tool tip. Therefore, fracture toughness and thermal shock resistance are the most limiting parameters in ceramic cutting tool applications, especially for monolithic alumina tool. Improvement in mechanical properties must be achieved before the potential of ceramics can be fully realized. Since, about 1970, ceramic tools have improved remarkably [4,5]. These improvements are mainly due to: (1) microstructures have
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Table 1 Compositions of Al2O3/TiB2/SiCw ceramic tool materials Specimen
Composition (vol.%)
ABW05 ABW10 ABW20 ABW30
Al2O3
TiB2
SiCw
76 72 64 56
19 18 16 14
5 10 20 30
been refined by controlling and improving manufacturing processes; (2) toughening mechanisms have been developed, such as whisker toughening and transformation toughening, thus improving the fracture toughness of ceramic tools while at the same time reducing susceptibility to thermal shock; (3) new ceramic compositions have been developed that are suitable for cutting tool application, particularly in high speed machining; and (4) surfaces have been conditioned by the removal of cracks, irregularities, and residual stresses. These developments have now enabled ceramic tools to be used in the machining of various types of steels, cast iron, and non-ferrous metals such as brass, bronze, and refractory nickel based alloys at high speeds and feeds. Advances in ceramic processing technology have resulted in a new generation of high performance ceramic cutting tools exhibiting improved properties. Considerable improvements has been achieved in tool properties such as flexural strength, fracture toughness, thermal shock resistance, hardness, and wear resistance by incorporating one or more other components into the base material to form ceramic–matrix composite tool materials. The reinforcing component is often in the shape of particles or whiskers. Ceramic tool materials with oxide matrices particularly Al2O3 are of increasing interest. Addition of hard particles or whiskers to the Al2O3 matrix may enhance its mechanical properties considerably. Some of these tool materials, such as Al2O3/TiC, Al2O3/TiB2, Al2O3/ZrO2, Al2O3/Ti(CN), Al2O3/(WTi)C, and Al2O3/SiCw, have been used in various machining applications and offer advantages with respect to friction and wear behaviors [6–12]. The strengthening or the toughening mechanisms of these ceramic tool materials are phase transformation toughening, whisker toughening and precipitate or dispersion strengthening [13,14]. In earlier studies it has already been shown that the additions of TiB2 secondary phases to Al2O3 matrix in amounts higher than 20 vol.% improved fracture toughness, hardness, strength over the monolithic Al2O3 and offered advantages with respect to wear and fracture behavior when used as cutting tool materials [7,11]. Further improvements of
the composite have been made through additions of SiC whiskers (SiCw) by the authors [9,15,16]. Nickel-based alloys are the most widely used superalloy, accounting for about 50 wt% of materials used in an aerospace engine, mainly in the gas turbine compartment. Inconel718 is the most frequently used of nickel-based alloys. Ceramic tools are gaining popularity in the machining of nickel-based alloys because they can withstand higher cutting conditions than carbide tools. In this study, Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. The fundamental properties of these composite tool materials were examined. Machining tests were carried out on the Inconel718 nickel-based alloys. The tool wear rates and the cutting temperature were measured. The failure mechanisms of these ceramic tools were investigated and correlated to their mechanical properties.
2. Materials and experimental procedures 2.1. Materials and processing A monolithic Al2O3 (average particle size 0.8 mm) was used as the baseline material. Additions of TiB2 particles (average particle size 1 mm) and SiC whiskers (diameter 1– 3 mm, length 20–80 mm) were added to Al2O3 matrix according to the combinations listed in Table 1. The material was fabricated using colloidal and ultrasonic processing techniques. Filter pressing was used to consolidate the multicomponent slurries into green bodies approximately 60 mm in diameter and 15 mm thick. Following drying, the powdered material was formed and compacted in a metal dye with a pressure of 60 MPa. Following the forming stage, the compacted powder was then filled in a graphite die, and the final densification was accomplished by hot pressing with a pressure of 35 MPa in nitrogen atmosphere for 40–60 min to produce a disk. The required sintering temperature was in the range of 1750– 1800 8C. Details of these procedures and specific processing parameters employed are described elsewhere [9,16]. 2.2. Material characterization Densities of the hot-pressed materials were measured by the Archimedes’s method. Test pieces of 3 mm!4 mm! 36 mm were prepared from the hot-pressed disks by cutting and grinding using a diamond wheel and were used for
Table 2 Composition of Inconel718 work piece material by weight percentage Ni
Mo
Si
Co
Ti
Nb
Al
Cr
P
Mn
Ca
Cu
Me
Fe
32.4
2.98
0.04
0.12
1.01
5.15
0.49
18.7
0.003
0.02
0.01
0.02
0.01
Balance
D. Jianxin et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1393–1401 Table 3 Experimental conditions Cutting speed (m/min) Feed rate (mm/rev) Depth of cut (mm) Environment
50–180 0.15 0.3 Dry
Table 4 Mechanical properties of Al2O3/TiB2/SiCw ceramic tool materials with different volume fraction of TiB2 particles and SiC whiskers Specimen
Relative density (g/cm3)
Hardness (GPa)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
ABW05 ABW10 ABW20 ABW30
99.90 99.84 99.42 98.93
21.4 21.6 21.7 22.0
778 750 726 670
5.90 7.60 7.97 8.42
the measurement of flexural strength, Vickers hardness and fracture toughness. A three-point bending mode was used to measure the flexural strength over a 30 mm span at a crosshead speed of 0.5 mm/min. Fracture toughness measurement was performed using indentation method in a hardness tester (ZWICK3212) using the formula proposed by Cook and Lawn [17]. On the same apparatus the Vickers hardness was measured on the polished surface with a load of 98 N. Data for flexural strength, hardness and fracture toughness were gathered on five specimens. 2.3. Cutting tests Cutting tests were carried out on a CA6140 lathe equipped with a commercial tool holder having the following geometry: rake angle g0ZK58, clearance angle a0Z58, inclination angle lsZK58, side cutting edge angle KrZ758, br0!r01Z0.2!(K208). The geometry of the Al2O3/TiB2/SiCw tool inserts was of ISO SNGN150608 with a 0.2 mm at 208 edge chamfer. The work piece material used was Inconel718 nickel-based alloy with a hardness of
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HRC46 in the form of round bar with an external diameter of 150 mm. The compositions of the material are listed in Table 2. No cutting fluid was used in the machining processes. The experimental conditions are shown in Table 3. Tool flank wear was measured using a !20 optional microscope system linked via transducers to a digital read out. The average cutting temperature of the tool rake face was measured by means of nature thermocouple technique [18]. The worn rake and flank regions on the ceramic tools were examined using scanning electron microscopy (HITACH S-570). Electron microprobe analysis was used to analyze the adhesion and the element diffusion.
3. Results and discussions 3.1. Mechanical properties and microstructural characterization Results of the fracture toughness, flexural strength, hardness and relative densities of the composite tool materials with different TiB2 and SiCw content are presented in Table 4. It was shown that the fracture toughness and hardness continuously increased with increasing SiCw content up to 30 vol.%. The relative density of the composites decreased with increasing SiC w content, the trend of the flexural strength being the same as that of the relative density. The decrease of flexural strength with increasing SiCw content is likely due to the decrease in the relative density associated with SiCw agglomerates [9,16]. Fig. 1 shows SEM micrograph of the fracture surface of ABW20 ceramic tool material. As can be seen that the composite exhibited a rough fracture surface, and the fracture mode was mixed transgranular and intergranular. Protruding whiskers and holes where whiskers were lodged prior to fracture were observed (Fig. 1(b)), and these are evidences of whisker pullout and bridging. The SEM micrograph of the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool
Fig. 1. SEM micrographs of the fracture surface of ABW20 ceramic tool material.
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wear resistance of the tool materials when with higher SiCw content corresponds to its higher fracture toughness and hardness. The flank wear rate at various cutting speeds of ABW20 ceramic tool is shown in Fig. 5. The ceramic tool showed higher flank wear at cutting speed higher than 100 m/min. Cutting speeds less than 100 m/min seems to be the best range for ABW20 ceramic tool when turning Inconel718 nickel-based alloys. Fig. 6 shows the crater wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickelbased alloys. It was shown that the ABW20 ceramic tool exhibited relative small crater wear at cutting speed lower than 100 m/min, and within further increasing of the cutting speed the crater wear increased greatly. Fig. 2. SEM micrograph of the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool material.
material is shown in Fig. 2. In this structure, the white needle-like phases with clear contrast are SiC whiskers, and the grey phases are of Al2O3 and TiB2. It is indicated that porosity is virtually absent, the SiC whiskers are uniformly distributed within the matrix, and there were few whisker agglomerates. Fig. 3(a) shows the imprint of Vickers indentation test on the polished surface perpendicular to the hot pressing direction of ABW20 ceramic tool material. Crack path produced by Vickers indentation at higher magnification is shown in Fig. 3(b). It is noted that the cracks were deflected considerably. 3.2. Flank wear and crater wear rates Fig. 4 shows the flank wear of Al2O3/TiB2/SiCw ceramic tools with different TiB2 and SiCw content when machining Inconel718 nickel-based alloys. It can be seen that the ceramic tools with higher SiCw content showed more flank wear resistance under the same test conditions. The higher
3.3. Tool wear surfaces Different modes of tool failure including rake face wear, flank wear, and breakage were observed when machining of Inconel718 nickel-based alloys with Al2O3/TiB2/SiCw ceramic tools in this study. Among these tool wear patterns, abrasive wear was found to be the main mode of flank wear, while adhesive and diffusion wear were the main rake face wear types, and also reported by researchers [9,11,19–22] with ceramic tools such as Al2O3/TiC, Al2O3/TiB2, and Al2O3/SiCw. Breakage was found to be the main tool failure type during high-speed cutting. It is common that several tool wear patterns appear simultaneously at the same time and have an effect on each other. Abrasive wear is usually a dominant wear mechanism on the flank face, it may also be observed on the rake face. Abrasion is characterized by development of grooves and ridges in the direction of tool sliding against a newly machined surface of the work piece or chip sliding against the rake face. The severity of abrasion can be increased in cases where the work piece materials contain hard inclusions, such as Inconel718 nickel-based alloy.
Fig. 3. Imprint of Vickers indentation test on the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool material.
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Fig. 4. Flank wear of Al2O3/TiB2/SiCw ceramic tools with different TiB2 and SiCw content when machining Inconel718 nickel-based alloys (test conditions: VZ100 m/min, apZ0.3 mm, fZ0.15 mm/r).
The wear by abrasion is usually due to crack development and intersection caused by wear particles acting as small indenters on the tool face. Fig. 7(a) shows the SEM micrograph of the tool wear profile of ABW20 ceramic tool at a cutting speed of 80 m/min. It appears that both the rake face and flank face were severely worn under these test conditions. The SEM micrograph of the flank wear track at higher magnification is illustrated in Fig. 7(d). Ridges and mechanical plowing grooves are clearly evident on the flank wear surfaces, and it is indicative of typical abrasive wear. The probability of finding such features on the flank wear surface is significantly greater. In many cases, the abrasive action may also be attributed to special features of the flowing chip, which is characterized by a serrated profile along its edges (see Fig. 8). This type of serrated chips abrades the tool rake face and creates scars in the rake wear surface. High stresses generated at the tool–chip interface during machining may also cause plastically deformed grooves and ridges on rake faces. The SEM micrograph of the crater wear track at higher magnification is shown in Fig. 7(b). Abrasion
Fig. 5. Flank wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickel-based alloys.
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Fig. 6. Crater wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickel-based alloys.
plastically deformed and marks are clearly evident on the rake wear surfaces. Fig. 9 shows the cutting temperatures of ABW20 ceramic tool as a function of cutting speeds when machining Inconel718 nickel-based alloys. It was found that the cutting temperature increased exponentially with increasing cutting speeds. The cutting temperature is higher than 800 8C when the cutting speed is above 80 m/min, and 1109 8C up to cutting speed of 120 m/min. The increases in cutting temperature at higher speeds may cause work piece materials pressure weld onto the tool rake face [21]. Subsequent random plucking of the welded materials removes aggregates of the tool particles, which accelerate the tool wear. Pressure welding of the work piece material is possible because the interfacial temperature between the tool and work piece at high-speed conditions falls in range of the melting point of the Inconel718 nickel-based alloys. The SEM micrographs in Fig. 10 were taken from the wear track of rake face of ABW20 ceramic cutting tool at cutting speed of 100 and 120 m/min. The wear track clearly shows lots of adhering materials smeared on the rake face. The adhered work piece particles often remain attached to the tool surface. Adhesive wear of cutting tools involves the mechanism in which individual grains or their small aggregates are pulled out of the tool surface and are carried away at the underside of the chip or torn away by the adherent work piece. Weaker interface bonding between different ceramic phases can increase the severity of adhesion wear. Diffusion wear involves element diffusion and chemical reaction between the work piece and the ceramic tool, and the process is activated by high-temperatures and is observed mainly at the tool–chip interface. This type of wear is more pronounced at high cutting speeds or when there is a high-temperature at the tool–chip interface, and is accelerated by a high chemical affinity between the work piece and the tool. At high cutting speed, the temperature at the tool–chip interface increases and the transfer of material between the work piece material and the tool occurs.
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Fig. 7. SEM micrographs of the wear profile of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (test conditions: cutting speed vZ80 m/min, depth of cut apZ0.3 mm, feed rates fZ0.15 mm/r).
There was experimental evidence of diffusion of Ni and Co element of Inconel718 nickel-based alloys to the tool materials. EDX analysis of the cross-section of ABW20 is shown in Fig. 11. The dashed line represents the EDX line of Ni, Co, Cr and Mo elements. It can be seen that Cr and
Mo did not greatly penetrate into the ceramic tool surfaces, while the Ni and Co of the Inconel718 nickel-based alloys diffused a long way into the rake face of ABW20 ceramic tool. Ni and Co has a low melting point and may lower the hardness of the ceramic tool surface.
Fig. 8. SEM micrographs of the serrated chips when machining Inconel718 nickel-based alloys with ABW20 ceramic cutting tool (test conditions: cutting speed vZ80 m/min, depth of cut apZ0.3 mm, feed rates fZ0.15 mm/r).
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be attributed to the mechanical impact, transient thermal stresses, and excessive crater and flank wear. Thus, the ceramic tools are not suitable for machining of Inconel718 nickel-based alloys with a cutting speed higher than 150 m/min.
4. Conclusions Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. Machining tests with these ceramic tools were carried out on the Inconel718 nickelbased alloys. Results showed that Fig. 9. Effect of cutting speed on the cutting temperatures of ABW20 ceramic tool when machining Inconel718 nickel-based alloys.
Fig. 12 shows the SEM micrograph of the tool wear profile of ABW20 ceramic tool at a cutting speed of 180 m/min. It can be seen that both the tool tip and the cutting edges were broken down completely under these test conditions. This may
1. The fracture toughness and hardness of the composite tool materials continuously increased with increasing SiCw content up to 30 vol.%. The relative density decreased with increasing SiC whisker content, the trend of the flexural strength being the same as that of the relative density.
Fig. 10. SEM micrographs of the worn rake face of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (a) vZ100 m/min, (b) vZ 120 m/min, (c) enlarged SEM micrograph corresponding to (a).
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Fig. 11. Cross-sectional view SEM micrographs of the worn rake face of ABW20 ceramic tool when machining Inconel718 nickel-based alloys. The dashed line represented the EDX line of scanning analysis results of Ni, Co, Cr and Co elements (test conditions: cutting speed vZ120 m/min, depth of cut apZ 0.3 mm, feed rates fZ0.15 mm/r).
2. Cutting speeds were found to have a profound effect on the wear behaviors of these ceramic tools. The ceramic tools exhibited relative small flank and crater wear at cutting speed lower than 80 m/min, within further increasing of the cutting speed the flank and crater wear increased greatly. Cutting speeds less than 80 m/min were proved to be the best range for this kind of ceramic tool when machining Inconel718 nickelbased alloys. The composite tool materials with higher SiCw content showed more wear resistance. 3. Abrasive wear was found to be the predominant flank wear mechanism when machining Inconel718 nickelbased alloys. While the mechanisms responsible for the crater wear were determined to be adhesion and
Fig. 12. SEM micrograph of the wear profile of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (test conditions: cutting speed vZ180 m/min, depth of cut apZ0.3 mm, feed rates fZ 0.15 mm/r).
chemically activated diffusion due to the high cutting temperature.
Acknowledgements This work was supported by ‘the National Natural Science Foundation of China (50275088, 50475133)’, ‘the Excellent Young Teachers Program of MOE (2055)’, and ‘the Scientific Research Foundation for the Excellent Young Scientists of Shandong Province (02BS064)’.
References [1] B. John, J.R. Wachtman, Structural Ceramics, Academic Press, London, 1989. [2] D.W. Richerson, Modern Ceramic Engineering, Marcel Dekker, New York, 1992. [3] E.D. Whitney, Microstructural engineering of ceramic cutting tools, Ceramic Bulletin 67 (6) (1988) 1010–1015. [4] G. Brandt, Ceramic cutting tools, state of the art and development trends, Materials Technology 14 (1) (1999) 17–22. [5] A. Xing, L. Zhaoqian, D. Jianxin, Development and perspective of advanced ceramic cutting tool materials, Key Engineering Materials 108 (1995) 53–66. [6] G. Brandt, A. Gerendas, M. Mikus, Wear mechanisms of ceramic cutting tools when machining ferrous and non-ferrous alloys, Journal of the European Ceramic Society 6 (5) (1990) 273–290. [7] D. Jianxin, A. Xing, Friction and wear behavior of Al2O3/TiB2 ceramic composite against cemented carbide in various atmosphere at elevated temperature, Wear 195 (1996) 128–132. [8] J. Barry, G. Byrne, Cutting tool wear in the machining of hardened steels Part I: alumina/TiC cutting tool wear, Wear 247 (2001) 139–151. [9] D. Jianxin, A. Xing, Effect of whisker oritenation on the friction and wear behaviour of Al2O3/TiB2/SiCw ceramic composite both in sliding wear and in cutting processes, Wear 201 (1996) 178–185. [10] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, Machinability of hardened steel using alumina based ceramic cutting tools, International Journal of Refractory Metals and Hard Materials 21 (3/4) (2003) 109–117.
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[17] R.F. Cook, B.R. Lawn, A modified indentation toughness technique, Journal of American Ceramic Society 66 (11) (1983) C200–C201. [18] C. Riyao, Principle of Metal Cutting, China Machine Press, Beijing, 1992. [19] N. Narutaki, Y. Yamane, K. Hayashi, T. Kitagawa, High-speed machining of Inconel 718 with ceramic tools, Annual of CIRP 42 (1) (1993) 103–106. [20] M.A. El-Bestawi, T.I. El-Wardany, D. Yan, M. Tan, Performance of whisker-reinforced ceramic tools in milling nickel base superalloy, Annual of CIRP 42 (1) (1993) 99–102. [21] E.O. Ezugwu, Z.M. Wang, R. Machado, The machinability of nickelbase alloys: a review, Journal of Materials Processing Technology 86 (1998) 1–16. [22] E.O. Ezugwu, J. Bonney, Y. Yamane, An overview of the machinability of aeroengine alloys, Journal of Materials Processing Technology 134 (2) (2003) 233–253.
Failure mechanisms of TiB2 particle and SiC whisker reinforced Al2O3 ceramic cutting tools when machining nickel-based alloys Deng Jianxin*, Liu Lili, Liu Jianhua, Zhao Jinlong, Yang Xuefeng Department of Mechanical Engineering, Shandong University of Technology, Jinan, Shandong Province 250061, People’s Republic of China Received 20 November 2004; accepted 28 January 2005 Available online 4 March 2005
Abstract In this paper, Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. The fundamental properties of these composite tool materials were examined. Machining tests with these ceramic tools were carried out on the Inconel718 nickel-based alloys. The tool wear rates and the cutting temperature were measured. The failure mechanisms of these ceramic tools were investigated and correlated to their mechanical properties. Results showed that the fracture toughness and hardness of the composite tool materials continuously increased with increasing SiC whisker content up to 30 vol.%. The relative density decreased with increasing SiC whisker content, the trend of the flexural strength being the same as that of the relative density. Cutting speeds were found to have a profound effect on the wear behaviors of these ceramic tools. The ceramic tools exhibited relative small flank and crater wear at cutting speed lower than 100 m/min, within further increasing of the cutting speed the flank and crater wear increased greatly. Cutting speeds less than 100 m/min were proved to be the best range for this kind of ceramic tool when machining Inconel718 nickel-based alloys. The composite tool materials with higher SiC whisker content showed more wear resistance. Abrasive wear was found to be the predominant flank wear mechanism. While the mechanisms responsible for the crater wear were determined to be adhesion and diffusion due to the high cutting temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: Ceramic cutting tools; Wear mechanisms; Cutting performance; Ceramic composites
1. Introduction Ceramics have intrinsic characteristics, such as high melting point, high hardness, good chemical inertness and high wear resistance, that make them promising candidates for high-temperature structural and wear resistance components, where metallic components achieve only unsatisfactory service lives, owing to inadequate heat, corrosive or wear resistance. Components made of advanced ceramics can survive and perform well at higher operating temperature, and improve the wear resistance. Nowadays the advanced ceramics are widely used in cutting tools, drawing or extrusion, seal rings, valve seats, bearing parts, and a variety of high-temperature engine parts, etc. [1–3].
* Corresponding author. Tel.: C86 531 295 5081x2047; fax: C86 531 295 5999. E-mail address: [email protected] (D. Jianxin).
0890-6955/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2005.01.033
Ceramic cutting tools usually perform better in high speed machining and in the machining of high hardness work piece materials as compared to high-speed steel and carbide tools. However, the use of single-phase ceramic tool materials, even fully densified, has been limited by their properties, such as their low strength and fracture toughness and poor thermal shock resistance. Furthermore, ceramics are very sensitive to microscopic flaws, thus ceramic cutting tools often crack at the tool edge, leading to unpredictable and catastrophic gross fracture of the tool. The low fracture toughness leads to brittle fracture, and the low thermal conductivity and high anisotropy thermal expansion of ceramics lead to large temperature gradients and thermal micro-cracks at the cutting edge and the tool tip. Therefore, fracture toughness and thermal shock resistance are the most limiting parameters in ceramic cutting tool applications, especially for monolithic alumina tool. Improvement in mechanical properties must be achieved before the potential of ceramics can be fully realized. Since, about 1970, ceramic tools have improved remarkably [4,5]. These improvements are mainly due to: (1) microstructures have
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Table 1 Compositions of Al2O3/TiB2/SiCw ceramic tool materials Specimen
Composition (vol.%)
ABW05 ABW10 ABW20 ABW30
Al2O3
TiB2
SiCw
76 72 64 56
19 18 16 14
5 10 20 30
been refined by controlling and improving manufacturing processes; (2) toughening mechanisms have been developed, such as whisker toughening and transformation toughening, thus improving the fracture toughness of ceramic tools while at the same time reducing susceptibility to thermal shock; (3) new ceramic compositions have been developed that are suitable for cutting tool application, particularly in high speed machining; and (4) surfaces have been conditioned by the removal of cracks, irregularities, and residual stresses. These developments have now enabled ceramic tools to be used in the machining of various types of steels, cast iron, and non-ferrous metals such as brass, bronze, and refractory nickel based alloys at high speeds and feeds. Advances in ceramic processing technology have resulted in a new generation of high performance ceramic cutting tools exhibiting improved properties. Considerable improvements has been achieved in tool properties such as flexural strength, fracture toughness, thermal shock resistance, hardness, and wear resistance by incorporating one or more other components into the base material to form ceramic–matrix composite tool materials. The reinforcing component is often in the shape of particles or whiskers. Ceramic tool materials with oxide matrices particularly Al2O3 are of increasing interest. Addition of hard particles or whiskers to the Al2O3 matrix may enhance its mechanical properties considerably. Some of these tool materials, such as Al2O3/TiC, Al2O3/TiB2, Al2O3/ZrO2, Al2O3/Ti(CN), Al2O3/(WTi)C, and Al2O3/SiCw, have been used in various machining applications and offer advantages with respect to friction and wear behaviors [6–12]. The strengthening or the toughening mechanisms of these ceramic tool materials are phase transformation toughening, whisker toughening and precipitate or dispersion strengthening [13,14]. In earlier studies it has already been shown that the additions of TiB2 secondary phases to Al2O3 matrix in amounts higher than 20 vol.% improved fracture toughness, hardness, strength over the monolithic Al2O3 and offered advantages with respect to wear and fracture behavior when used as cutting tool materials [7,11]. Further improvements of
the composite have been made through additions of SiC whiskers (SiCw) by the authors [9,15,16]. Nickel-based alloys are the most widely used superalloy, accounting for about 50 wt% of materials used in an aerospace engine, mainly in the gas turbine compartment. Inconel718 is the most frequently used of nickel-based alloys. Ceramic tools are gaining popularity in the machining of nickel-based alloys because they can withstand higher cutting conditions than carbide tools. In this study, Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. The fundamental properties of these composite tool materials were examined. Machining tests were carried out on the Inconel718 nickel-based alloys. The tool wear rates and the cutting temperature were measured. The failure mechanisms of these ceramic tools were investigated and correlated to their mechanical properties.
2. Materials and experimental procedures 2.1. Materials and processing A monolithic Al2O3 (average particle size 0.8 mm) was used as the baseline material. Additions of TiB2 particles (average particle size 1 mm) and SiC whiskers (diameter 1– 3 mm, length 20–80 mm) were added to Al2O3 matrix according to the combinations listed in Table 1. The material was fabricated using colloidal and ultrasonic processing techniques. Filter pressing was used to consolidate the multicomponent slurries into green bodies approximately 60 mm in diameter and 15 mm thick. Following drying, the powdered material was formed and compacted in a metal dye with a pressure of 60 MPa. Following the forming stage, the compacted powder was then filled in a graphite die, and the final densification was accomplished by hot pressing with a pressure of 35 MPa in nitrogen atmosphere for 40–60 min to produce a disk. The required sintering temperature was in the range of 1750– 1800 8C. Details of these procedures and specific processing parameters employed are described elsewhere [9,16]. 2.2. Material characterization Densities of the hot-pressed materials were measured by the Archimedes’s method. Test pieces of 3 mm!4 mm! 36 mm were prepared from the hot-pressed disks by cutting and grinding using a diamond wheel and were used for
Table 2 Composition of Inconel718 work piece material by weight percentage Ni
Mo
Si
Co
Ti
Nb
Al
Cr
P
Mn
Ca
Cu
Me
Fe
32.4
2.98
0.04
0.12
1.01
5.15
0.49
18.7
0.003
0.02
0.01
0.02
0.01
Balance
D. Jianxin et al. / International Journal of Machine Tools & Manufacture 45 (2005) 1393–1401 Table 3 Experimental conditions Cutting speed (m/min) Feed rate (mm/rev) Depth of cut (mm) Environment
50–180 0.15 0.3 Dry
Table 4 Mechanical properties of Al2O3/TiB2/SiCw ceramic tool materials with different volume fraction of TiB2 particles and SiC whiskers Specimen
Relative density (g/cm3)
Hardness (GPa)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
ABW05 ABW10 ABW20 ABW30
99.90 99.84 99.42 98.93
21.4 21.6 21.7 22.0
778 750 726 670
5.90 7.60 7.97 8.42
the measurement of flexural strength, Vickers hardness and fracture toughness. A three-point bending mode was used to measure the flexural strength over a 30 mm span at a crosshead speed of 0.5 mm/min. Fracture toughness measurement was performed using indentation method in a hardness tester (ZWICK3212) using the formula proposed by Cook and Lawn [17]. On the same apparatus the Vickers hardness was measured on the polished surface with a load of 98 N. Data for flexural strength, hardness and fracture toughness were gathered on five specimens. 2.3. Cutting tests Cutting tests were carried out on a CA6140 lathe equipped with a commercial tool holder having the following geometry: rake angle g0ZK58, clearance angle a0Z58, inclination angle lsZK58, side cutting edge angle KrZ758, br0!r01Z0.2!(K208). The geometry of the Al2O3/TiB2/SiCw tool inserts was of ISO SNGN150608 with a 0.2 mm at 208 edge chamfer. The work piece material used was Inconel718 nickel-based alloy with a hardness of
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HRC46 in the form of round bar with an external diameter of 150 mm. The compositions of the material are listed in Table 2. No cutting fluid was used in the machining processes. The experimental conditions are shown in Table 3. Tool flank wear was measured using a !20 optional microscope system linked via transducers to a digital read out. The average cutting temperature of the tool rake face was measured by means of nature thermocouple technique [18]. The worn rake and flank regions on the ceramic tools were examined using scanning electron microscopy (HITACH S-570). Electron microprobe analysis was used to analyze the adhesion and the element diffusion.
3. Results and discussions 3.1. Mechanical properties and microstructural characterization Results of the fracture toughness, flexural strength, hardness and relative densities of the composite tool materials with different TiB2 and SiCw content are presented in Table 4. It was shown that the fracture toughness and hardness continuously increased with increasing SiCw content up to 30 vol.%. The relative density of the composites decreased with increasing SiC w content, the trend of the flexural strength being the same as that of the relative density. The decrease of flexural strength with increasing SiCw content is likely due to the decrease in the relative density associated with SiCw agglomerates [9,16]. Fig. 1 shows SEM micrograph of the fracture surface of ABW20 ceramic tool material. As can be seen that the composite exhibited a rough fracture surface, and the fracture mode was mixed transgranular and intergranular. Protruding whiskers and holes where whiskers were lodged prior to fracture were observed (Fig. 1(b)), and these are evidences of whisker pullout and bridging. The SEM micrograph of the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool
Fig. 1. SEM micrographs of the fracture surface of ABW20 ceramic tool material.
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wear resistance of the tool materials when with higher SiCw content corresponds to its higher fracture toughness and hardness. The flank wear rate at various cutting speeds of ABW20 ceramic tool is shown in Fig. 5. The ceramic tool showed higher flank wear at cutting speed higher than 100 m/min. Cutting speeds less than 100 m/min seems to be the best range for ABW20 ceramic tool when turning Inconel718 nickel-based alloys. Fig. 6 shows the crater wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickelbased alloys. It was shown that the ABW20 ceramic tool exhibited relative small crater wear at cutting speed lower than 100 m/min, and within further increasing of the cutting speed the crater wear increased greatly. Fig. 2. SEM micrograph of the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool material.
material is shown in Fig. 2. In this structure, the white needle-like phases with clear contrast are SiC whiskers, and the grey phases are of Al2O3 and TiB2. It is indicated that porosity is virtually absent, the SiC whiskers are uniformly distributed within the matrix, and there were few whisker agglomerates. Fig. 3(a) shows the imprint of Vickers indentation test on the polished surface perpendicular to the hot pressing direction of ABW20 ceramic tool material. Crack path produced by Vickers indentation at higher magnification is shown in Fig. 3(b). It is noted that the cracks were deflected considerably. 3.2. Flank wear and crater wear rates Fig. 4 shows the flank wear of Al2O3/TiB2/SiCw ceramic tools with different TiB2 and SiCw content when machining Inconel718 nickel-based alloys. It can be seen that the ceramic tools with higher SiCw content showed more flank wear resistance under the same test conditions. The higher
3.3. Tool wear surfaces Different modes of tool failure including rake face wear, flank wear, and breakage were observed when machining of Inconel718 nickel-based alloys with Al2O3/TiB2/SiCw ceramic tools in this study. Among these tool wear patterns, abrasive wear was found to be the main mode of flank wear, while adhesive and diffusion wear were the main rake face wear types, and also reported by researchers [9,11,19–22] with ceramic tools such as Al2O3/TiC, Al2O3/TiB2, and Al2O3/SiCw. Breakage was found to be the main tool failure type during high-speed cutting. It is common that several tool wear patterns appear simultaneously at the same time and have an effect on each other. Abrasive wear is usually a dominant wear mechanism on the flank face, it may also be observed on the rake face. Abrasion is characterized by development of grooves and ridges in the direction of tool sliding against a newly machined surface of the work piece or chip sliding against the rake face. The severity of abrasion can be increased in cases where the work piece materials contain hard inclusions, such as Inconel718 nickel-based alloy.
Fig. 3. Imprint of Vickers indentation test on the polished surfaces perpendicular to the hot pressing direction of ABW20 ceramic tool material.
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Fig. 4. Flank wear of Al2O3/TiB2/SiCw ceramic tools with different TiB2 and SiCw content when machining Inconel718 nickel-based alloys (test conditions: VZ100 m/min, apZ0.3 mm, fZ0.15 mm/r).
The wear by abrasion is usually due to crack development and intersection caused by wear particles acting as small indenters on the tool face. Fig. 7(a) shows the SEM micrograph of the tool wear profile of ABW20 ceramic tool at a cutting speed of 80 m/min. It appears that both the rake face and flank face were severely worn under these test conditions. The SEM micrograph of the flank wear track at higher magnification is illustrated in Fig. 7(d). Ridges and mechanical plowing grooves are clearly evident on the flank wear surfaces, and it is indicative of typical abrasive wear. The probability of finding such features on the flank wear surface is significantly greater. In many cases, the abrasive action may also be attributed to special features of the flowing chip, which is characterized by a serrated profile along its edges (see Fig. 8). This type of serrated chips abrades the tool rake face and creates scars in the rake wear surface. High stresses generated at the tool–chip interface during machining may also cause plastically deformed grooves and ridges on rake faces. The SEM micrograph of the crater wear track at higher magnification is shown in Fig. 7(b). Abrasion
Fig. 5. Flank wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickel-based alloys.
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Fig. 6. Crater wear of ABW20 ceramic tool at various cutting speeds when machining Inconel718 nickel-based alloys.
plastically deformed and marks are clearly evident on the rake wear surfaces. Fig. 9 shows the cutting temperatures of ABW20 ceramic tool as a function of cutting speeds when machining Inconel718 nickel-based alloys. It was found that the cutting temperature increased exponentially with increasing cutting speeds. The cutting temperature is higher than 800 8C when the cutting speed is above 80 m/min, and 1109 8C up to cutting speed of 120 m/min. The increases in cutting temperature at higher speeds may cause work piece materials pressure weld onto the tool rake face [21]. Subsequent random plucking of the welded materials removes aggregates of the tool particles, which accelerate the tool wear. Pressure welding of the work piece material is possible because the interfacial temperature between the tool and work piece at high-speed conditions falls in range of the melting point of the Inconel718 nickel-based alloys. The SEM micrographs in Fig. 10 were taken from the wear track of rake face of ABW20 ceramic cutting tool at cutting speed of 100 and 120 m/min. The wear track clearly shows lots of adhering materials smeared on the rake face. The adhered work piece particles often remain attached to the tool surface. Adhesive wear of cutting tools involves the mechanism in which individual grains or their small aggregates are pulled out of the tool surface and are carried away at the underside of the chip or torn away by the adherent work piece. Weaker interface bonding between different ceramic phases can increase the severity of adhesion wear. Diffusion wear involves element diffusion and chemical reaction between the work piece and the ceramic tool, and the process is activated by high-temperatures and is observed mainly at the tool–chip interface. This type of wear is more pronounced at high cutting speeds or when there is a high-temperature at the tool–chip interface, and is accelerated by a high chemical affinity between the work piece and the tool. At high cutting speed, the temperature at the tool–chip interface increases and the transfer of material between the work piece material and the tool occurs.
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Fig. 7. SEM micrographs of the wear profile of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (test conditions: cutting speed vZ80 m/min, depth of cut apZ0.3 mm, feed rates fZ0.15 mm/r).
There was experimental evidence of diffusion of Ni and Co element of Inconel718 nickel-based alloys to the tool materials. EDX analysis of the cross-section of ABW20 is shown in Fig. 11. The dashed line represents the EDX line of Ni, Co, Cr and Mo elements. It can be seen that Cr and
Mo did not greatly penetrate into the ceramic tool surfaces, while the Ni and Co of the Inconel718 nickel-based alloys diffused a long way into the rake face of ABW20 ceramic tool. Ni and Co has a low melting point and may lower the hardness of the ceramic tool surface.
Fig. 8. SEM micrographs of the serrated chips when machining Inconel718 nickel-based alloys with ABW20 ceramic cutting tool (test conditions: cutting speed vZ80 m/min, depth of cut apZ0.3 mm, feed rates fZ0.15 mm/r).
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be attributed to the mechanical impact, transient thermal stresses, and excessive crater and flank wear. Thus, the ceramic tools are not suitable for machining of Inconel718 nickel-based alloys with a cutting speed higher than 150 m/min.
4. Conclusions Al2O3/TiB2/SiCw ceramic cutting tools with different volume fraction of TiB2 particles and SiC whiskers were produced by hot pressing. Machining tests with these ceramic tools were carried out on the Inconel718 nickelbased alloys. Results showed that Fig. 9. Effect of cutting speed on the cutting temperatures of ABW20 ceramic tool when machining Inconel718 nickel-based alloys.
Fig. 12 shows the SEM micrograph of the tool wear profile of ABW20 ceramic tool at a cutting speed of 180 m/min. It can be seen that both the tool tip and the cutting edges were broken down completely under these test conditions. This may
1. The fracture toughness and hardness of the composite tool materials continuously increased with increasing SiCw content up to 30 vol.%. The relative density decreased with increasing SiC whisker content, the trend of the flexural strength being the same as that of the relative density.
Fig. 10. SEM micrographs of the worn rake face of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (a) vZ100 m/min, (b) vZ 120 m/min, (c) enlarged SEM micrograph corresponding to (a).
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Fig. 11. Cross-sectional view SEM micrographs of the worn rake face of ABW20 ceramic tool when machining Inconel718 nickel-based alloys. The dashed line represented the EDX line of scanning analysis results of Ni, Co, Cr and Co elements (test conditions: cutting speed vZ120 m/min, depth of cut apZ 0.3 mm, feed rates fZ0.15 mm/r).
2. Cutting speeds were found to have a profound effect on the wear behaviors of these ceramic tools. The ceramic tools exhibited relative small flank and crater wear at cutting speed lower than 80 m/min, within further increasing of the cutting speed the flank and crater wear increased greatly. Cutting speeds less than 80 m/min were proved to be the best range for this kind of ceramic tool when machining Inconel718 nickelbased alloys. The composite tool materials with higher SiCw content showed more wear resistance. 3. Abrasive wear was found to be the predominant flank wear mechanism when machining Inconel718 nickelbased alloys. While the mechanisms responsible for the crater wear were determined to be adhesion and
Fig. 12. SEM micrograph of the wear profile of ABW20 ceramic cutting tool when machining Inconel718 nickel-based alloys (test conditions: cutting speed vZ180 m/min, depth of cut apZ0.3 mm, feed rates fZ 0.15 mm/r).
chemically activated diffusion due to the high cutting temperature.
Acknowledgements This work was supported by ‘the National Natural Science Foundation of China (50275088, 50475133)’, ‘the Excellent Young Teachers Program of MOE (2055)’, and ‘the Scientific Research Foundation for the Excellent Young Scientists of Shandong Province (02BS064)’.
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