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Abrasive wear of WC–FeAl composites
Wear 258 (2005) 1337–1341
Abrasive wear of WC–FeAl composites A.Y. Mosbah, D. Wexler∗ , A. Calka1 Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia Received 19 November 2002; received in revised form 21 September 2004; accepted 23 September 2004 Available online 13 November 2004
Abstract The abrasive wear behavior of tungsten-carbide iron-aluminide composite materials was investigated using a pin-on-drum wear-testing machine. Samples were prepared by uniaxially hot pressing blended powders. The wear rates of specimens containing 40 vol.% matrix of atomic composition, Fe60 Al40 , were measured and results compared with those of conventional WC–10 vol.% Co hardmetal. They were found to be comparable to those of WC–10% Co hardmetal, when abraded by 120 m SiC papers under identical conditions. The wear resistance of WC–Fe60 Al40 composites increased with reduction in WC-grain size and associated with increase in composite hardness. Scanning electron microscopy revealed that the wear surfaces of WC–40% Fe60 Al40 composites and WC–Co hardmetal were similar in appearance. The higher hardness and work hardening ability of Fe60 Al40 binder, as compared to Co metal, are believed to be responsible for the excellent abrasive wear resistance of WC composites containing iron aluminide binder. © 2004 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Wear rate; Composites; WC–FeAl
1. Introduction Iron aluminide intermetallic based on the ordered B2 structure has been investigated over the last several decades for its potential use as a high-temperature structural material [1,2]. However, this material is also very attractive as a potential binder phase for hard ceramic particles such as WC, TiC, TiB2 and ZrB2 . Iron aluminides in general have high oxidation and sulfidation resistance, high strength, low density, high work hardening and they are comprised of inexpensive raw materials [3]. The combination of high oxidation and corrosion resistance and high strength and work hardening of B2–FeAl intermetallics is specifically important for excellent wear resistance [4,5]. The shortcomings of B2–FeAl intermetallics of low tensile ductility and fracture toughness are not as essential for wear applications as they are for structural applications [2,4]. In wear applications, loads are compressive in nature, and properties such as hardness, strength, and work hardening ability are more critical [4]. ∗
Corresponding author. Tel.: +61 2 42 214 739. E-mail addresses: david [email protected] (D. Wexler), andrzej [email protected] (A. Calka). 1 Tel.: +61 2 42 214 945. 0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.061
As a result, intermetallics and intermetallic composites with hard particle reinforcements are potential candidates for wear applications, especially, in severe environments. In general, the excellent wear resistance of cemented carbides is related to their high hardness combined with a significant level of ductility imparted by the Co binder. Cobalt in the WC–Co system plays an important role in the material resistance to the repeated localised deformation, which takes place during wear [5]. Cobalt has a strong bond to WC as a result of a good level of alloying. In comparison, B2–iron aluminide with a composition of 40 at% Al has been shown to possess a significant level of ductility when it exists as a matrix in a composite with hard ceramic phases such as WC [3]. The fracture toughness values of WC–40% Fe60 Al40 composites were reported to be in the range of those of conventional cemented carbides [3,6]. The comparison of the abrasive wear behaviours of WC–FeAl composites and WC–Co hardmetal can help in understanding the role of the binder material in the abrasive wear resistance of the composite. In this study, an attempt was made to understand the effects of the binder material on the wear rate and wear mechanism of the composite utilising the different binders of Co metal and Fe60 Al40 intermetallic.
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2. Experimental A pin-on-drum wear-testing machine designed for laboratory use was utilised to evaluate the abrasive wear of the composites. The two-body abrasive wear-testing machine was chosen because it could deliver conditions of high stress and low sliding speed with minimum variables that can easily be controlled for all tests [5]. The pin specimen was 6 mm in diameter with allowable length of 20–35 mm. The rotating drum was 86 mm in diameter with length of 300 mm and driven by a variable speed electrical motor. The specimen was moved horizontally in relation to the rotating drum by an electrical motor as shown in Fig. 1. Load was applied to the specimen by adding weights at the end of the arm. Wear testing was carried out on three WC–40 vol.% Fe60 Al40 and one WC–10 vol.% Co specimens under laboratory conditions of temperature and humidity. The WC–10% Co reference sample was supplied by Sandvik Hard Materials. The WC–40% Fe60 Al40 samples were prepared by uniaxial hot pressing of milled powder samples in cylindrical graphite dies under induction heating and controlled atmosphere. Initial preparation of the powder blends involved controlled milling of arc-melted alloy ingot in Ar, followed by controlled atmosphere milling with WC powder. The WC–Fe60 Al40 specimens consisted of the same WC–Fe60 Al40 volume ratio of 60/40 but with different WCgrain sizes; 8.7, 2.41 and 0.7 m. The WC–Co specimen had an average WC-grain size around 2 m. All samples had densities higher than 99% of theoretical density. Prior to wear testing, the specimen was run in on fresh abrasive paper to produce a wear pattern over the abraded end of the specimen, concentric with the curve of the drum. The specimen was then removed, cleaned with alcohol, dried and weighed; this procedure was repeated after each wear test. The wear rate was expressed in terms of volume loss, which was estimated from the weight loss of the specimen. WC–10% Co hardmetal with hardness of 17.65 GPa was selected as a reference material. Mass loss of the reference
material was used to estimate the relative wear rates of the composites tested. The relative wear rate of a material was expressed as the mass loss of the test material to the mass loss of the reference material tested under identical conditions. The wear rate was calculated using the following equation: ˙ s = m W ρLFN
(1)
˙ s is the wear rate, mm3 /mN, m the mass loss of test where, W specimen during abrasion test of N revolutions, ρ the density of test material, g cm−3 . L is the total sliding distance, m and FN the normal force on the pin, N. Wear testing was performed 10 times on each specimen and the results were averaged. The coefficient of variation of the 10 tests was selected to be less than 5%. Any result with coefficient of variation value of higher than this was discarded. The error in the wear testing of the materials was estimated using the formula proposed by Pollard [7]: n 1 σ= (mi − m) ¯ 2 (2) n−1 n=1
where, σ is the standard deviation from the arithmetic mean, n m ¯ = n1 mi . n=1
The coefficient of variation is obtained by dividing the standard deviation by the mean. The reproducibility of the mass loss from the wear test was examined using the reference material. Ten tests were carried out on WC–10% Co under the same conditions used for testing the WC–Fe60 Al40 composites. A normal force of 100 N, corresponding to 3.24 MPa pressure on the wearing surface of the pin, was applied to the specimen at a sliding speed of 1.5 mm/s. Fresh 120# -grit silicon carbide paper (nominal particle size of 120 m; hardness 24.0 GPa; fracture toughness 2.5 MPa m1/2 was used to abrade the material in each test. The worn surfaces of the materials tested were examined by scanning electron microscopy (SEM), using a Leica Stereoscan 440 scanning electron microscope and secondary electron imaging.
3. Results and discussion
Fig. 1. Schematic of pin-on-drum wear testing device.
Fig. 2 shows the mass loss from 10 repeated tests of WC–10 vol.% Co. The standard deviation for the 10 tests was 0.055 and the coefficient of variation was 2.7%. The value of the coefficient of variation of less than 5% indicated that the results were reliable for use in such a wear study of materials. WC–40% Fe60 Al40 composites were found to exhibit comparable rates of mass loss per unit sliding distance, and wear rates to that of the reference material (Tables 1 and 2). The average mass loss and mass loss per unit sliding distance (mg/m) of WC–40% Fe60 Al40 composites are shown in Table 1, in addition to those of the WC–10 vol.% Co reference material. The hardest composite in the group of materi-
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Fig. 3. Specific wear rate of composites plotted as a function of hardness. Fig. 2. Mass losses from 10 tests of WC–10% Co hardmetal.
als tested, WC–40% Fe60 Al40 with WC grain size of 0.7 m, had the lowest mass loss per unit sliding distance. The mass loss per unit sliding distance of WC–40% Fe60 Al40 composite with an average WC particle size of 2.41 m was equal to that of WC–10 vol.% Co. Unsurprisingly, the harder composites showed greater wear resistances (Table 2). Materials with similar WC particle sizes and comparable hardness values exhibited fairly similar specific wear rates, even though the binder materials were different (Table 2). The two following figures show specific wear rate plotted as functions of composite hardness (Fig. 3) and WC grain size (Fig. 4), for the four composites investigated. These results show that wear resistance of WC–40% Fe60 Al40 increases with the increases in composite hardness and decreases with decreasing WC-grain size. WC–40% Fe60 Al40 composites with WC grain size of 0.7 m had an average wear rate 8% lower than that of the reference material while WC–40% Fe60 Al40 composite with the coarser WC grain size of 8.7 m had a wear rate about 15% higher than that of WC–10% Co material. From these results, it is evident that the abrasive wear rate of WC–FeAl is proportional to the hardness of the composite and that a reverse dependence of abrasive wear on
Fig. 4. Specific wear rate of composites plotted as a function of WC grain size.
grain size of WC hard phase exists. Composites with coarser WC grains had lower abrasion resistance. However, there is no one function that could describe the dependence of wear on the properties of the material. Hardness, grain size and binder content are not independent variables. The hardness of a composite is generally determined by the mean free path of dislocations in the binder phase [8]. The mean free path is proportional to the product of binder content and average grain size.
Table 1 Wear rates of WC–40% Fe60 Al40 composites as compared to WC–10% Co Material
Path length (m)
Mass loss (mg)
WC–10% Co (WC; 2.0 m) WC–40% FeAl (WC; 8.7 m) WC–40% FeAl (WC; 2.4 m) WC–40% FeAl (WC; 0.7 m)
6 6 6 6
10 11.75 10 9.40
Mass loss/unit sliding distance (mg/m)
Coefficient of variation (%)
1.67 1.96 1.67 1.57
2.7 3.6 3.8 4.3
Table 2 Change in wear rates of WC–40% Fe60 Al40 composites with hardness Material
Measured density (mg/mm3 )
Hardness (GPa)
WC–10% Co (WC; 2.0 m) WC–40% Fe60 Al40 (WC; 8.70 m) WC–40% Fe60 Al40 (WC; 2.41 m) WC–40% Fe60 Al40 (WC; 0.7 m)
15.13 13.55 13.56 13.56
17.65 15.69 18.63 20.59
Specific wear rate (×103 mm3 /N m) 1.10 1.44 1.23 1.15
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Fig. 5. SEM images of worn surfaces of WC–40 vol.% FeAl (a)–(c) and WC–10 vol.% Co reference material; (a) 8.7 m WC, (b) 2.4 m WC, (c) 0.7 m WC, (d) ∼2 m WC.
Fig. 6. Evidence of microfracturing of carbide grains and gradual binder extrusion in worn surfaces. (a)–(c) WC–40 vol.% FeAl and (d) WC–10 vol.% Co; (a) 8.7 m WC, (b) 2.4 m WC, (c) 0.7 m WC, (d) ∼2 m WC.
A.Y. Mosbah et al. / Wear 258 (2005) 1337–1341
The role of the binder on the wear behaviour of composite materials becomes evident when the binder content of WC–Co hardmetal and WC–Fe60 Al40 composites with similar WC-grain sizes are compared. WC–40% Fe60 Al40 composites had 33% lower volume fraction of WC hard phase than WC–10% Co. However, both materials showed similar wear rates. In general, all the WC–Fe60 Al40 composites tested had wear rates comparable to that of WC–Co hardmetal despite containing considerably lower volume fractions of WC hard phase. Cobalt metal and FeAl intermetallic are two different materials with different properties. The wetting characteristics of WC in each material vary, solubilites of W and C in each vary, and the mechanical properties of each material are completely different. Compared with Co, iron aluminide (FeAl) has lower tensile ductility at room temperature, and fracture toughness and higher hardness, and work hardening ability. The higher hardness and work hardening ability of Fe60 Al40 binder, as compared to Co metal, are believed to be responsible for the significant gains in abrasive wear resistance shown by the WC–Fe60 Al40 composites. The worn surfaces of three composites and WC–Co hardmetal had similar appearances. Fig. 5 shows low magnification SEM images of the wear surfaces with indication of formation of grooves and craters at the contact areas between the composite and abrasive particles. At higher magnifications, (Fig. 6) there is evidence of particle debonding and pullout (Fig. 6(a)), binder extrusion, microfracturing of carbide grains and band slipping on the surfaces of the carbide grains (Fig. 6(b)–(d)). Evidence of binder extrusion was also confirmed by SEM examination of fracture surfaces of tensile specimens (Fig. 7), which indicated ductile failure of FeAl regions less than ∼2 m across (the fracture properties of WC–FeAl composites is the subject of a separate paper, in preparation). No cracks were observed on the surfaces of the composites or WC–Co hardmetal tested in this work in contrast to reports by other investigators [3,9]. The selective binder removal from the surface layers of composites seems to have a major role in the abrasive wear of WC–FeAl com-
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posites. Other investigators have reported that local preferential removal of Co-binder in WC–Co system was an important step in the wear process of these materials [8].
4. Conclusions • The WC–40% Fe60 Al40 composites and WC–10% Co with similar hardness values and WC grain sizes exhibited comparable abrasive wear rates, when tested under identical conditions. • The change in binder type and binder content of materials seems to have little influence on wear rate as long as the materials compared have equal hardness values. Bulk hardness of composite was found to be an efficient indicator of the flow properties of the particular composites under abrasive wear conditions. • The Intermetallic iron aluminide with a composition of 40 at% Al was found to be a superior matrix material to Co metal, on the basis of the binder volume fraction in the composite, under the conditions of abrasive wear used in this work. Significantly, lower volume fraction of WC hard phase in WC–FeAl composite was required to achieve the same wear resistance of WC–Co system. • Binder extrusion appears to play a major role in the abrasive wear of WC–40% Fe60 Al40 composites. The wear processes were dominated by the formation of grooves and craters and they included binder extrusion and microfracturing of WC grains. Further work is needed to definitely establish the sequence of processes in the wear of these materials.
Acknowledgment Financial support from the Australian Research Council, under ARC-Large Grant No. A00103022, is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6]
Fig. 7. Tensile fracture surface of WC–40 vol.% FeAl composite. Arrows indicate narrow regions of binder extrusion.
P.R. Munroe, Mater. Forum 21 (1997) 79. N.S. Stoloff, R.G. Davis, Prog. Mater. Sci. 13 (1996) 1. J.H. Schneibel, et al., Intermetallics 5 (1997) 61. D.L. Anton, et al., J. Met. 41 (1989) 12. R.L. Fleischer, et al., Ann. Rev. Mater. Sci. 19 (1989) 231. A.Y. Mosbah, Ph.D. Thesis, University of Wollongong, Wollongong (2001). [7] J.H. Pollard, IA Handbook of Numerical and Statistical Techniques, Cambridge University Press, Cambridge, 1997, p. 80. [8] K. Jia, T.E. Fischer, Wear 203–204 (1997) 310. [9] J. Larsen-Basse, J. Met. 35 (1983) 35.
Abrasive wear of WC–FeAl composites A.Y. Mosbah, D. Wexler∗ , A. Calka1 Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia Received 19 November 2002; received in revised form 21 September 2004; accepted 23 September 2004 Available online 13 November 2004
Abstract The abrasive wear behavior of tungsten-carbide iron-aluminide composite materials was investigated using a pin-on-drum wear-testing machine. Samples were prepared by uniaxially hot pressing blended powders. The wear rates of specimens containing 40 vol.% matrix of atomic composition, Fe60 Al40 , were measured and results compared with those of conventional WC–10 vol.% Co hardmetal. They were found to be comparable to those of WC–10% Co hardmetal, when abraded by 120 m SiC papers under identical conditions. The wear resistance of WC–Fe60 Al40 composites increased with reduction in WC-grain size and associated with increase in composite hardness. Scanning electron microscopy revealed that the wear surfaces of WC–40% Fe60 Al40 composites and WC–Co hardmetal were similar in appearance. The higher hardness and work hardening ability of Fe60 Al40 binder, as compared to Co metal, are believed to be responsible for the excellent abrasive wear resistance of WC composites containing iron aluminide binder. © 2004 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Wear rate; Composites; WC–FeAl
1. Introduction Iron aluminide intermetallic based on the ordered B2 structure has been investigated over the last several decades for its potential use as a high-temperature structural material [1,2]. However, this material is also very attractive as a potential binder phase for hard ceramic particles such as WC, TiC, TiB2 and ZrB2 . Iron aluminides in general have high oxidation and sulfidation resistance, high strength, low density, high work hardening and they are comprised of inexpensive raw materials [3]. The combination of high oxidation and corrosion resistance and high strength and work hardening of B2–FeAl intermetallics is specifically important for excellent wear resistance [4,5]. The shortcomings of B2–FeAl intermetallics of low tensile ductility and fracture toughness are not as essential for wear applications as they are for structural applications [2,4]. In wear applications, loads are compressive in nature, and properties such as hardness, strength, and work hardening ability are more critical [4]. ∗
Corresponding author. Tel.: +61 2 42 214 739. E-mail addresses: david [email protected] (D. Wexler), andrzej [email protected] (A. Calka). 1 Tel.: +61 2 42 214 945. 0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.061
As a result, intermetallics and intermetallic composites with hard particle reinforcements are potential candidates for wear applications, especially, in severe environments. In general, the excellent wear resistance of cemented carbides is related to their high hardness combined with a significant level of ductility imparted by the Co binder. Cobalt in the WC–Co system plays an important role in the material resistance to the repeated localised deformation, which takes place during wear [5]. Cobalt has a strong bond to WC as a result of a good level of alloying. In comparison, B2–iron aluminide with a composition of 40 at% Al has been shown to possess a significant level of ductility when it exists as a matrix in a composite with hard ceramic phases such as WC [3]. The fracture toughness values of WC–40% Fe60 Al40 composites were reported to be in the range of those of conventional cemented carbides [3,6]. The comparison of the abrasive wear behaviours of WC–FeAl composites and WC–Co hardmetal can help in understanding the role of the binder material in the abrasive wear resistance of the composite. In this study, an attempt was made to understand the effects of the binder material on the wear rate and wear mechanism of the composite utilising the different binders of Co metal and Fe60 Al40 intermetallic.
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2. Experimental A pin-on-drum wear-testing machine designed for laboratory use was utilised to evaluate the abrasive wear of the composites. The two-body abrasive wear-testing machine was chosen because it could deliver conditions of high stress and low sliding speed with minimum variables that can easily be controlled for all tests [5]. The pin specimen was 6 mm in diameter with allowable length of 20–35 mm. The rotating drum was 86 mm in diameter with length of 300 mm and driven by a variable speed electrical motor. The specimen was moved horizontally in relation to the rotating drum by an electrical motor as shown in Fig. 1. Load was applied to the specimen by adding weights at the end of the arm. Wear testing was carried out on three WC–40 vol.% Fe60 Al40 and one WC–10 vol.% Co specimens under laboratory conditions of temperature and humidity. The WC–10% Co reference sample was supplied by Sandvik Hard Materials. The WC–40% Fe60 Al40 samples were prepared by uniaxial hot pressing of milled powder samples in cylindrical graphite dies under induction heating and controlled atmosphere. Initial preparation of the powder blends involved controlled milling of arc-melted alloy ingot in Ar, followed by controlled atmosphere milling with WC powder. The WC–Fe60 Al40 specimens consisted of the same WC–Fe60 Al40 volume ratio of 60/40 but with different WCgrain sizes; 8.7, 2.41 and 0.7 m. The WC–Co specimen had an average WC-grain size around 2 m. All samples had densities higher than 99% of theoretical density. Prior to wear testing, the specimen was run in on fresh abrasive paper to produce a wear pattern over the abraded end of the specimen, concentric with the curve of the drum. The specimen was then removed, cleaned with alcohol, dried and weighed; this procedure was repeated after each wear test. The wear rate was expressed in terms of volume loss, which was estimated from the weight loss of the specimen. WC–10% Co hardmetal with hardness of 17.65 GPa was selected as a reference material. Mass loss of the reference
material was used to estimate the relative wear rates of the composites tested. The relative wear rate of a material was expressed as the mass loss of the test material to the mass loss of the reference material tested under identical conditions. The wear rate was calculated using the following equation: ˙ s = m W ρLFN
(1)
˙ s is the wear rate, mm3 /mN, m the mass loss of test where, W specimen during abrasion test of N revolutions, ρ the density of test material, g cm−3 . L is the total sliding distance, m and FN the normal force on the pin, N. Wear testing was performed 10 times on each specimen and the results were averaged. The coefficient of variation of the 10 tests was selected to be less than 5%. Any result with coefficient of variation value of higher than this was discarded. The error in the wear testing of the materials was estimated using the formula proposed by Pollard [7]: n 1 σ= (mi − m) ¯ 2 (2) n−1 n=1
where, σ is the standard deviation from the arithmetic mean, n m ¯ = n1 mi . n=1
The coefficient of variation is obtained by dividing the standard deviation by the mean. The reproducibility of the mass loss from the wear test was examined using the reference material. Ten tests were carried out on WC–10% Co under the same conditions used for testing the WC–Fe60 Al40 composites. A normal force of 100 N, corresponding to 3.24 MPa pressure on the wearing surface of the pin, was applied to the specimen at a sliding speed of 1.5 mm/s. Fresh 120# -grit silicon carbide paper (nominal particle size of 120 m; hardness 24.0 GPa; fracture toughness 2.5 MPa m1/2 was used to abrade the material in each test. The worn surfaces of the materials tested were examined by scanning electron microscopy (SEM), using a Leica Stereoscan 440 scanning electron microscope and secondary electron imaging.
3. Results and discussion
Fig. 1. Schematic of pin-on-drum wear testing device.
Fig. 2 shows the mass loss from 10 repeated tests of WC–10 vol.% Co. The standard deviation for the 10 tests was 0.055 and the coefficient of variation was 2.7%. The value of the coefficient of variation of less than 5% indicated that the results were reliable for use in such a wear study of materials. WC–40% Fe60 Al40 composites were found to exhibit comparable rates of mass loss per unit sliding distance, and wear rates to that of the reference material (Tables 1 and 2). The average mass loss and mass loss per unit sliding distance (mg/m) of WC–40% Fe60 Al40 composites are shown in Table 1, in addition to those of the WC–10 vol.% Co reference material. The hardest composite in the group of materi-
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Fig. 3. Specific wear rate of composites plotted as a function of hardness. Fig. 2. Mass losses from 10 tests of WC–10% Co hardmetal.
als tested, WC–40% Fe60 Al40 with WC grain size of 0.7 m, had the lowest mass loss per unit sliding distance. The mass loss per unit sliding distance of WC–40% Fe60 Al40 composite with an average WC particle size of 2.41 m was equal to that of WC–10 vol.% Co. Unsurprisingly, the harder composites showed greater wear resistances (Table 2). Materials with similar WC particle sizes and comparable hardness values exhibited fairly similar specific wear rates, even though the binder materials were different (Table 2). The two following figures show specific wear rate plotted as functions of composite hardness (Fig. 3) and WC grain size (Fig. 4), for the four composites investigated. These results show that wear resistance of WC–40% Fe60 Al40 increases with the increases in composite hardness and decreases with decreasing WC-grain size. WC–40% Fe60 Al40 composites with WC grain size of 0.7 m had an average wear rate 8% lower than that of the reference material while WC–40% Fe60 Al40 composite with the coarser WC grain size of 8.7 m had a wear rate about 15% higher than that of WC–10% Co material. From these results, it is evident that the abrasive wear rate of WC–FeAl is proportional to the hardness of the composite and that a reverse dependence of abrasive wear on
Fig. 4. Specific wear rate of composites plotted as a function of WC grain size.
grain size of WC hard phase exists. Composites with coarser WC grains had lower abrasion resistance. However, there is no one function that could describe the dependence of wear on the properties of the material. Hardness, grain size and binder content are not independent variables. The hardness of a composite is generally determined by the mean free path of dislocations in the binder phase [8]. The mean free path is proportional to the product of binder content and average grain size.
Table 1 Wear rates of WC–40% Fe60 Al40 composites as compared to WC–10% Co Material
Path length (m)
Mass loss (mg)
WC–10% Co (WC; 2.0 m) WC–40% FeAl (WC; 8.7 m) WC–40% FeAl (WC; 2.4 m) WC–40% FeAl (WC; 0.7 m)
6 6 6 6
10 11.75 10 9.40
Mass loss/unit sliding distance (mg/m)
Coefficient of variation (%)
1.67 1.96 1.67 1.57
2.7 3.6 3.8 4.3
Table 2 Change in wear rates of WC–40% Fe60 Al40 composites with hardness Material
Measured density (mg/mm3 )
Hardness (GPa)
WC–10% Co (WC; 2.0 m) WC–40% Fe60 Al40 (WC; 8.70 m) WC–40% Fe60 Al40 (WC; 2.41 m) WC–40% Fe60 Al40 (WC; 0.7 m)
15.13 13.55 13.56 13.56
17.65 15.69 18.63 20.59
Specific wear rate (×103 mm3 /N m) 1.10 1.44 1.23 1.15
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Fig. 5. SEM images of worn surfaces of WC–40 vol.% FeAl (a)–(c) and WC–10 vol.% Co reference material; (a) 8.7 m WC, (b) 2.4 m WC, (c) 0.7 m WC, (d) ∼2 m WC.
Fig. 6. Evidence of microfracturing of carbide grains and gradual binder extrusion in worn surfaces. (a)–(c) WC–40 vol.% FeAl and (d) WC–10 vol.% Co; (a) 8.7 m WC, (b) 2.4 m WC, (c) 0.7 m WC, (d) ∼2 m WC.
A.Y. Mosbah et al. / Wear 258 (2005) 1337–1341
The role of the binder on the wear behaviour of composite materials becomes evident when the binder content of WC–Co hardmetal and WC–Fe60 Al40 composites with similar WC-grain sizes are compared. WC–40% Fe60 Al40 composites had 33% lower volume fraction of WC hard phase than WC–10% Co. However, both materials showed similar wear rates. In general, all the WC–Fe60 Al40 composites tested had wear rates comparable to that of WC–Co hardmetal despite containing considerably lower volume fractions of WC hard phase. Cobalt metal and FeAl intermetallic are two different materials with different properties. The wetting characteristics of WC in each material vary, solubilites of W and C in each vary, and the mechanical properties of each material are completely different. Compared with Co, iron aluminide (FeAl) has lower tensile ductility at room temperature, and fracture toughness and higher hardness, and work hardening ability. The higher hardness and work hardening ability of Fe60 Al40 binder, as compared to Co metal, are believed to be responsible for the significant gains in abrasive wear resistance shown by the WC–Fe60 Al40 composites. The worn surfaces of three composites and WC–Co hardmetal had similar appearances. Fig. 5 shows low magnification SEM images of the wear surfaces with indication of formation of grooves and craters at the contact areas between the composite and abrasive particles. At higher magnifications, (Fig. 6) there is evidence of particle debonding and pullout (Fig. 6(a)), binder extrusion, microfracturing of carbide grains and band slipping on the surfaces of the carbide grains (Fig. 6(b)–(d)). Evidence of binder extrusion was also confirmed by SEM examination of fracture surfaces of tensile specimens (Fig. 7), which indicated ductile failure of FeAl regions less than ∼2 m across (the fracture properties of WC–FeAl composites is the subject of a separate paper, in preparation). No cracks were observed on the surfaces of the composites or WC–Co hardmetal tested in this work in contrast to reports by other investigators [3,9]. The selective binder removal from the surface layers of composites seems to have a major role in the abrasive wear of WC–FeAl com-
1341
posites. Other investigators have reported that local preferential removal of Co-binder in WC–Co system was an important step in the wear process of these materials [8].
4. Conclusions • The WC–40% Fe60 Al40 composites and WC–10% Co with similar hardness values and WC grain sizes exhibited comparable abrasive wear rates, when tested under identical conditions. • The change in binder type and binder content of materials seems to have little influence on wear rate as long as the materials compared have equal hardness values. Bulk hardness of composite was found to be an efficient indicator of the flow properties of the particular composites under abrasive wear conditions. • The Intermetallic iron aluminide with a composition of 40 at% Al was found to be a superior matrix material to Co metal, on the basis of the binder volume fraction in the composite, under the conditions of abrasive wear used in this work. Significantly, lower volume fraction of WC hard phase in WC–FeAl composite was required to achieve the same wear resistance of WC–Co system. • Binder extrusion appears to play a major role in the abrasive wear of WC–40% Fe60 Al40 composites. The wear processes were dominated by the formation of grooves and craters and they included binder extrusion and microfracturing of WC grains. Further work is needed to definitely establish the sequence of processes in the wear of these materials.
Acknowledgment Financial support from the Australian Research Council, under ARC-Large Grant No. A00103022, is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6]
Fig. 7. Tensile fracture surface of WC–40 vol.% FeAl composite. Arrows indicate narrow regions of binder extrusion.
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