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Influence of different reinforcing particles on the scratch resistance and microstructure of different WC–Ni composites
Wear 352-353 (2016) 130–135
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of different reinforcing particles on the scratch resistance and microstructure of different WC–Ni composites O. Marou Alzouma a,n, M.-A. Azman a, D.-L. Yung b, V. Fridrici a, Ph. Kapsa a a Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513 ECL-ENISE Ecole Centrale de Lyon, Université de Lyon, Bat. H10, 36 avenue Guy de Collongue, 69134 Ecully cedex, France b Tallinn University of Technology, Department of Materials Engineering, Ehitajate tee 5, 19086 Tallinn, Estonia
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2015 Received in revised form 14 February 2016 Accepted 17 February 2016 Available online 23 February 2016
WC composites are largely used in many engineering and industrial fields such as mining, cutting tools, and underground works. This interest in WC materials is related to their high wear resistance due to their convenient mechanical properties and controllable microstructures. However, the abrasive behavior of WC materials is very complicated because they are formed from several phases and their damage mechanism involves numerous parameters and factors that may be related to the composites themselves and the external environment. Reinforcing particles play a critical role in the achievement of wearresistant materials. In this study, the impact of the presence of different types of reinforcing particles in the chemical makeup of WC–Ni composites on their resistance to scratch formation is highlighted through experimental abrasion tests by a single diamond tip. & 2016 Elsevier B.V. All rights reserved.
Keywords: Tungsten carbide Wear mechanism Reinforcement Mean free path Contiguity
1. Introduction Many authors have reported that a balance between hardness and fracture toughness is important to enhance the wear resistance of WC composites [1,2]. Many methods to improve the mechanical properties of composites have been reported in the literature: optimizing WC grain size and microstructure, choosing an adequate binder, etc. [3] A common and usual way to enhance the mechanical properties of WC composites via their microstructure is the addition of reinforcing particles in their chemical makeup [1,4,5]. The reinforcing particles play several roles in the improvement of the wear resistance of WC cermets. Poetschke et al. [6] showed that the simultaneous presence of VC and Cr3C2 particles plays a role in grain growth inhibition of cermets, and therefore, they maintain that the nanoscale size of the WC grains is advantageous for increasing the wear resistance of the cermets. Kimmari et al. [7] reported that the presence of yttria-stabilized ZrO2 in the chemical composition of cermets leads to the refinement of WC grains, which results in increased wear resistance of the cermets from reduced crack propagation. The impact of the size of reinforcing particles has also been discussed in the literature [4,8]. Kim et al. [9] explained that
n
Corresponding author. E-mail address: [email protected] (O. Marou Alzouma).
http://dx.doi.org/10.1016/j.wear.2016.02.011 0043-1648/& 2016 Elsevier B.V. All rights reserved.
smaller reinforcing particles increase the hardness by strengthening the metallic matrix and improving the wear resistance. The works of Yilmaz [10] and Hu [8] mentioned that larger reinforcing particles also present advantages to improve wear resistance, as they withstand most of the wearing forces and protect the metallic matrix. Hu [8] postulated that the presence of both smaller and larger reinforcements in the composition of a metallic composite results in greater enhancement of the wear resistance. Numerical simulations demonstrated that the shape, size distribution, volume fraction of the reinforcing particles and ratio between the reinforcing particle size and the abrasive particle size may have an influence on the wear resistance of WC–Ni composites [11]. In this work, scratch tests using a diamond tip are performed on four grades of WC–Ni cermets with different reinforcing particles; the aim is to observe how these scratches act on the wear behavior and resistance of the cermets. The data obtained are used to determine correlations between the microstructure parameters (mean free path of the binder phase, contiguity of WC phase) and the reinforcing particles in the WC–Ni composites.
2. Experimental details 2.1. Materials and sample preparation procedures Four different grades of WC (A, B, C and D) in the shape of blocks (5 15 20 mm3) are tested in this study.
O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
All samples were placed in a graphite boat on top of sheets of graphite. Sintering of all samples was done in a FCT-system (FPW300-400 Sinter-HIP furnace, Germany). The furnace is designed with the capacity to reach 1600 °C with 100 bars of Argon (Ar) pressure. The final temperature of 1500 °C was chosen. Their chemical compositions and mechanical properties are presented in Tables 1 and 2, respectively. WC grade A represents the control sample (without reinforcing particles). The density of the materials is measured using Archimedes’ method with water as the medium (measured six times); Vickers’ hardness (HV20) is measured using an Indectec HV tester machine (measured six times); fracture toughness is determined via Vickers’ indent method using Omnimet Imaging software (measured six times); the Young’s modulus is measured via Vickers’ indent method (measured six time), and the WC mean grain size is determined by the mean linear intercept method, or Heyn’s technique [12] (measured five times). The mean free path is calculated by two different formulas that give approximately the same values for this parameter. The first formula used by Saito et al. [13] is given by Eq. (1):
λ¼
ð2Þ
where vol% (binder) represents the volume fraction of the binder and N is the average number of intercepts per unit length of the test line in the SEM photographs, as determined using traces of the carbide/cobalt interface. The contiguity between the WC grains is calculated using the formula reported by Jia et al. [15]. It is given by Eq. (3): C¼
2Nc ð2Nc þ N Þ
2.3. Methods of analysis The quantification of wear resistance of the WC specimens is determined by calculating the wear volume, and the topography, determined by interferometry, allows for direct determination of the removed volume. For qualitative evaluation of wear, a digital optical microscope is used. The tests are repeated three times, and the repeatability is confirmed.
ð1Þ
where f represents the volume fraction of the particles, Nl is the number of particles per unit length intersected on a random line in the SEM photographs, and Ns is the number of particles per unit area contained in a random area in the SEM photographs. The second formula, used by Jia et al. [14], is given by Eq. (2): 2vol%ðbinderÞ λ¼ N
15 N. For the tests carried out under 10 N, the amount of volume loss is quite low. Furthermore, the ranking of the materials remains the same as in the case of 15 N. The characteristics of the scratches obtained for 15 N are more similar to those observed on real worn carbide inserts used underground. For these reasons, the results shown in this study are obtained with a normal load of 15 N. The tests are performed at room temperature and ambient humidity conditions. The scratch test device is presented in Fig. 2.
3. Results and discussions
2
1f 8 Nl with f ¼ Nl 3π Ns
131
ð3Þ
where Nc is the average number of intercepts per unit length of the test line, with traces of the carbide/carbide grain boundaries. The samples are polished with diamond polishing paper (45 mm and 15 mm) and diamond suspension (6 mm, 3 mm and 1 mm) down to a roughness Ra¼ 0.01 mm. The surfaces are cleaned with acetone before each test. The microstructures of the polished flat samples are shown in Fig. 1. 2.2. Testing device and test conditions A scratch test device is used to perform tests on the WC–Ni composites. In this system, a conical (α ¼120°) diamond indenter with a spherical tip (200 mm of radius) is used as the tip. The scanning speed is 50 mm/min, and the scratch length is 1 mm. Initially two loads are used to perform the experiments: 10 N and
In this work, we will mainly compare the influence of the presence of reinforcing particles on the wear resistance of WC–Ni composites obtained in our experiments to other experiments and computational investigations in the literature. 3.1. Influence of the reinforcing particle size on wear The wear volume and worn surfaces of the WC–Ni are presented in Figs. 3 and 4, respectively. The results show that WC–Ni composites reinforced with larger particles display lower wear volumes. Yilmaz et al. [10] explained that larger reinforcing particles are more beneficial for the wear resistance of metallic matrix composites because they protect the matrix. Larger reinforcing particles reduce the interfacial failure between reinforcements and the matrix because of the smaller interfacial area between them. They also support a great quantity of wearing forces [15]. The numerical simulation performed by Hu et al. [8] is consistent with these results. However, grade B WC, with 55-nm reinforcing particles, shows a wear volume slightly lower than that of grade C WC, which contains larger reinforcing particles (100 nm). The zirconia particles used as reinforcing particles for WC grade B and grade D are known to undergo martensitic transformation with increasing volume during the sintering process [16]. The powder agglomeration could also be a reason of increase of the size of ZrO2 particles. The presence of large reinforcing particles (ZrO2 particles) is known to reduce the probability of interfacial failure between the nickel matrix and these larger reinforcing particles, as explained by Chung et al. [17]. This may be one of the reasons of the promising wear resistance of WC grades B and D. It can be observed on the SEM pictures that the WC grains of grades
Table 1 Designation and chemical composition of the tested WC grades. WC grades Chemical compositions
A B C D
WCnanopowder (90-100 nm) – Ni (8 wt% 13 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 12.68 vol%) – ZrO2 (3 wt% 6.8 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 13.09 vol%) – ZrC (1.3 wt% 2.8 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 12.54 vol%) – ZrO2 (3 wt% 6.7 vol%) – VC/ Cr3C2 (0.5 wt% each 1.19 vol% and 1.03 vol%)
Volume fraction of reinforcements Initial size of additional oxide/ (%) carbide 0 6.8 2.8 9
– ZrO2: 55 nm ZrC: 100 nm ZrO2: 55 nm VC: 1.8 mm Cr3C2: 1.8 mm
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O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
Table 2 Mechanical properties of the tested WC grades. WC grades Density (g/cm3) Hardness HV20 (GPa) A B C D
14.3 70.1 13.9 70.1 14.4 70.04 13.8 70.1
WC grrade A
12.7 70.1 14.3 7 0.1 16.8 7 0.1 17.9 7 0.1
Young’s modulus (GPa)
Fracture toughness (MPa m1/2)
WC grains mean size (mm)
Mean free path (mm)
WC grains contiguity
5977 35 503 743 541 736 538 728
8.4 7.3 9.5 9
0.9 7 0.01 0.53 7 0.03 0.32 7 0.01 0.3 7 0.01
0.36 0.02 0.025 0.013
0.58 0.82 0.61 0.9
WC graade B
WC graade C
Expandedd ZrO2 particlles
WC g rade D
Expandded ZrO2 partticles
Fig. 1. Microstructure of the flat samples of the tested WC grades observed by SEM (the metallic binder phase appears in black, and the WC carbide grains appear in gray).
Fig. 2. Scratch test device used in the study: (a) whole system view and (b) schematic view of the system.
B and D do not undergo fracture phenomenon. The presence of ZrO2 particles in their composition reduces the size of their WC grains. WC grains of grades A and C undergo important fracture phenomenon. The absence of reinforcing particles in the composition is the consequence for WC grade A. For WC grade C, it can be concluded that the particles of ZrC do not have the same role as grain growth inhibitor (ZrO2). ZrC particles did not allow the reduction of all WC grains, and the largest grains undergo severe fracture phenomena. It is also reported that when small reinforcing particles are homogeneously dispersed, it is possible to observe an increase in the wear volume: the first reason for this observation is that smaller particles can move out of their given sites and participate
in the abrasion [8,10], and second, the resulting total area, which is larger for small reinforcing particles, could lead to interfacial failure. This may be the phenomenon that occurs in the grade C WC, as Figs. 4 and 5 show that the plastic deformation is more important. The smaller ZrC particles are probably removed, and the binder phase is more accessible to the abradant which provokes more material removal. Furthermore, Hu [8] postulated that the presence of both larger and smaller reinforcing particles may be of interest to reducing the wear rate. This is possibly a reason, amongst others, why the grade D WC, which contains smaller (ZrO2) and larger (VC and Cr3C2 reinforcing particles), displays the highest wear resistance.
O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
Computational investigations [8] demonstrated that in cases in which different sizes of reinforcing particles exist, a greater volume fraction of the larger reinforcing particles leads to a lower wear volume. Therefore, increasing the volume fraction of VC and Cr3C2 particles in the chemical makeup of grade D WC may result in an increase in wear resistance. The profiles of the grooves realized on the surface of the composites are presented in Fig. 5. This demonstrates the level of wear depth and plastic deformation observed in the binder phase. The plastic deformation is quantified here, by the presence or not of lateral wedge around the scratches.
Fig. 3. Wear volume for all of the WC grades.
Severe fractu ure of WC grain ns
133
It can be observed that plastic deformation of the binder phase is more important (the presence of lateral wedge) for WC–Ni composites with the lowest volume fraction of reinforcing particles (WC grades A, B and C). A higher volume fraction of reinforcing particles protects and strengthens the binder phase from plastic deformation (case of grade D). 3.2. Impact of the reinforcing particles volume fraction on microstructure and wear In this section, the evolution of the wear volume and some microstructural characteristics (mean free path of the binder phase, WC grains contiguity and WC grains mean size) with the volume fraction of the reinforcing particles is analyzed. Fig. 6 shows that the wear volume decreases with an increase in the reinforcing particle volume fraction. Some works have demonstrated that the wear resistance increases with increases in the reinforcing particle volume fraction combined with the testing parameters, even when different types of reinforcing particles are included, for metallic composites [8,18]. This is consistent with our results. Although it is well known that finer WC grains result in higher wear resistance [7,8], the relationship between the WC grain size and the volume fraction of reinforcement is not clear, as shown in Fig. 6a. According to the previous conclusion, a decreasing WC grain size was expected with increases in the volume fraction of reinforcement. This lack of a clear relationship may be explained by the fact that some of the reinforcing particles act as growth inhibitors and
Shallow pplastic groove
of WC grains No fracture f
Low boonding strength around WC grains 10µ µm
10µm
WC grade A
WC graade B
Fractuure of WC grain ns
Micro-plowingg phenomenon
o fracture of WC No C grains
Shallo ow plastic grooove 10µm
10µm
WC grade C
WC graade D
Fig. 4. SEM observations of worn surfaces of WC–Ni composites.
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O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
Fig. 5. Scratch profile for all of the WC grades, in the direction perpendicular to the scratching.
Fig. 6. Evolution of (a) wear volume and WC grain size, (b) wear volume and mean free path and (c) wear volume and WC grain contiguity with volume fraction of reinforcing particles.
that the shape of the WC grains is not uniform for all of the specimens (see Fig. 1). However, the volume fraction of reinforcing particles is crucial in terms of the wear resistance of metallic composites. Numerical
simulations reported in the literature [8] demonstrated that according to the amount of reinforcing particles, the wear resistance can either increase or decrease. There exists a critical value of the volume fraction of reinforcing particles below and above which the wear resistance decreases. Below this critical value of reinforcing particles, the inclusions are not able to support most wearing forces, and the metallic matrix is weakened. Above this critical value, the composite tends to be sensitive to brittle wear [19]. The maximum volume fraction of reinforcing particles used in our study is lower than the critical value, and this may explain why the wear occurring on the WC–Ni composite is due more to damage in the binder phase than to brittle wear. Therefore, if we continue to increase the percentage of our reinforcement volume fraction, at a certain level, the wear volume will certainly increase, resulting in a decrease of the wear resistance. In Fig. 6b, it can be observed that the wear volume and the mean free path of the binder phase decrease to different degrees with the increase in the volume fraction of reinforcing particles. The mean free path is related to the thickness of the binder phase [14]; in other terms, it represents the distance of binder content between WC grains. It is demonstrated in the literature that the mean free path decreases with increasing hardness because of the reduction in the plastic deformation in the binder phase, and consequently, the wear resistance increases [13]. This is consistent with our results. However, it can be observed in Fig. 6b that the mean free path decreases rapidly between 0% and 2.8% of the volume fraction of reinforcing particles and then tends to stabilize above 2.8%. This suggests that a higher volume fraction of reinforcement can progressively affect the mechanical properties, such as the bonding strength between WC grains or between the binder phase and WC grains, and can consequently hinder the increase in the wear resistance despite the decrease in the binder mean free path. Therefore, a critical value of the reinforcing particle volume fraction should exist at which the mean free path becomes stable and the wear resistance becomes independent of the mean free path. Fig. 6c shows that contiguity of the WC grains of our WC–Ni composites increases with increases in the volume fraction of reinforcing particles and that the wear volume decreases with increasing contiguity of the WC grains. A higher contiguity of WC grains corresponds to a lower thickness of the binder phase between the WC grains. This indicates that for higher contiguity, the plastic deformation of the binder is reduced. This results in an increase in the wear resistance. These results are expected because the contiguity of the WC grains is narrowly linked to the mean free path of the binder phase. It is obvious that lower mean free path corresponds to higher contiguity and, consequently, to higher wear resistance.
O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
4. Conclusions In summary, the following conclusions can be drawn:
Increasing the volume fraction of the reinforcing particles,
especially the larger particles when several sizes are used, enhances the wear resistance. Increasing the volume fraction of reinforcing particles protects the binder phase from plastic deformation. The size distribution of the reinforcing particles protects the metallic matrix by strengthening the interfacial area between the reinforcement, binder and WC grains. The wear resistance of WC–Ni composites may be independent of the mean free path at a certain value of the reinforcing particle volume fraction.
This work shows the importance of the nature and the amount and the reinforcing particles on the nature of the characteristic scratches and on many other microstructural parameters (mean free path, WC grains contiguity, etc.)
Acknowledgments The authors would like to acknowledge the financial support of the European Commission for the project NeTTUN from the Seventh Framework Program for Research, Technological Development and Demonstration (FP7 2007-2013) under Grant Agreement 280712, the colleagues of Tallinn University of Technology (TUT) for their contributions, Emeritus Professor Sture Hogmark for his help and comments and Siegfried Fouvry and his team from LTDS for the provision of the scratch testing device.
References [1] D. Gu, Y. Shen, Influence of reinforcement weight fraction on microstructure and properties of submicron WC-Cop/Cu bulk MMCs prepared by direct laser sintering, J. Alloy. Compd. 431 (2007) 112–120.
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[2] E. Breval, J.P. Cheng, D.K. Agrawal, P. Gigl, M. Dennis, R. Roy, A.J. Papworth, Comparison between microwave and conventional sintering of WC/Co composites, Mater. Sci. Eng. A 391 (2005) 285–295. [3] A. Morales-Rodrıguez, A. Bravo-Leona, A. Domınguez-Rodrıguez, S. LopezEsteban, J.S. Moya, M. Jimenez-Melendo, High-temperature mechanical properties of zirconia/nickel composites, J. Eur. Ceram. Soc. 23 (2003) 2849–2856. [4] A. Özer, W. Kriven, Y. Kemal, Tür, The effect of 3 mol% Y2O3 stabilized ZrO2 produced by a steric entrapment method on the mechanical and sintering properties of Cr3C2 based cermets, Mater. Sci. Eng. A556 (2012) 878–884. [5] B. Mondal, P.K. Das, S.K. Singh, Advanced WC–Co cermet composites with reinforcement of TiCN prepared by extended thermal plasma route, Mater. Sci. Eng. A 498 (2008) 59–64. [6] J. Poetschke, V. Richter, R. Holke, Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide, Int. J. Refractory Met. Hard Mater. 31 (2012) 218–223. [7] E. Kimmari, I. Hussainova, A. Smirnov, R. Traksmaa, I. Preis, Processing and microstructural characterization of WC-based cermets doped by ZrO2, Estonian J. Eng. 15 (4) (2009) 275–282. [8] J. Hu, D.Y. Li, R. Llewellyn, Computational investigation of microstructural effects on abrasive wear of composite materials, Wear 259 (2005) 6–17. [9] H.M. Kim, T.S. Kim, C. Suryanarayana, B.S. Chun, Microstructure and wear characteristics of rapidly solidified Al–Pb–Cu alloys, Mater. Sci. Eng. A 287 (2000) 59–65. [10] O. Yilmaz, S. Buytoz, Abrasive wear of Al2O3-reinforced aluminium-based MMCs, Compos. Sci. Technol. 61 (2001) 2381–2392. [11] J. Hu, D.Y. Li, R. Llewellyn, Synergistic effects of microstructure and abrasion condition on abrasive wear of composites–a modeling study, Wear 263 (2007) 218–227. [12] E. Heyn, The Metallographist 5 (1903) 39–64. [13] H. Saito, A. Iwabuchi, T. Shimizu, Effects of Co content and WC grain size on wear of WC cemented carbide, Wear 261 (2006) 126–132. [14] K. Jia, T.E. Fischer, B. Gallois, Microstructure, hardness and toughness of nanostructured and conventional WC–Co composites, Nanostruct. Mater. 10 (5) (1998) 875–891. [15] S. Das, D.P. Mondal, G. Dixit, Correlation of abrasive wear with microstructure and mechanical properties of pressure die-cast aluminum hard-particles composite, Metall. Mater. Trans. A 32A (2001) 633–642. [16] R.H. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in zirconiacontaining ceramics, J. Am. Ceram. Soc. 8 (3) (2000) 461–487. [17] S. Chung, B.H. Hwang, A microstructural study of the wear behavior of SiCp/Al composites, Tribol. Int. 27 (5) (1994) 307–314. [18] R.L. Deuis, C. Subramanian, J.M. Yellup, Abrasive wear of aluminium composites, Wear 201 (1996) 132–144. [19] Q. Chen, D.Y. Li, Computer simulation of solid particle erosion of composite materials, Wear 255 (2003) 78–84.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of different reinforcing particles on the scratch resistance and microstructure of different WC–Ni composites O. Marou Alzouma a,n, M.-A. Azman a, D.-L. Yung b, V. Fridrici a, Ph. Kapsa a a Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513 ECL-ENISE Ecole Centrale de Lyon, Université de Lyon, Bat. H10, 36 avenue Guy de Collongue, 69134 Ecully cedex, France b Tallinn University of Technology, Department of Materials Engineering, Ehitajate tee 5, 19086 Tallinn, Estonia
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2015 Received in revised form 14 February 2016 Accepted 17 February 2016 Available online 23 February 2016
WC composites are largely used in many engineering and industrial fields such as mining, cutting tools, and underground works. This interest in WC materials is related to their high wear resistance due to their convenient mechanical properties and controllable microstructures. However, the abrasive behavior of WC materials is very complicated because they are formed from several phases and their damage mechanism involves numerous parameters and factors that may be related to the composites themselves and the external environment. Reinforcing particles play a critical role in the achievement of wearresistant materials. In this study, the impact of the presence of different types of reinforcing particles in the chemical makeup of WC–Ni composites on their resistance to scratch formation is highlighted through experimental abrasion tests by a single diamond tip. & 2016 Elsevier B.V. All rights reserved.
Keywords: Tungsten carbide Wear mechanism Reinforcement Mean free path Contiguity
1. Introduction Many authors have reported that a balance between hardness and fracture toughness is important to enhance the wear resistance of WC composites [1,2]. Many methods to improve the mechanical properties of composites have been reported in the literature: optimizing WC grain size and microstructure, choosing an adequate binder, etc. [3] A common and usual way to enhance the mechanical properties of WC composites via their microstructure is the addition of reinforcing particles in their chemical makeup [1,4,5]. The reinforcing particles play several roles in the improvement of the wear resistance of WC cermets. Poetschke et al. [6] showed that the simultaneous presence of VC and Cr3C2 particles plays a role in grain growth inhibition of cermets, and therefore, they maintain that the nanoscale size of the WC grains is advantageous for increasing the wear resistance of the cermets. Kimmari et al. [7] reported that the presence of yttria-stabilized ZrO2 in the chemical composition of cermets leads to the refinement of WC grains, which results in increased wear resistance of the cermets from reduced crack propagation. The impact of the size of reinforcing particles has also been discussed in the literature [4,8]. Kim et al. [9] explained that
n
Corresponding author. E-mail address: [email protected] (O. Marou Alzouma).
http://dx.doi.org/10.1016/j.wear.2016.02.011 0043-1648/& 2016 Elsevier B.V. All rights reserved.
smaller reinforcing particles increase the hardness by strengthening the metallic matrix and improving the wear resistance. The works of Yilmaz [10] and Hu [8] mentioned that larger reinforcing particles also present advantages to improve wear resistance, as they withstand most of the wearing forces and protect the metallic matrix. Hu [8] postulated that the presence of both smaller and larger reinforcements in the composition of a metallic composite results in greater enhancement of the wear resistance. Numerical simulations demonstrated that the shape, size distribution, volume fraction of the reinforcing particles and ratio between the reinforcing particle size and the abrasive particle size may have an influence on the wear resistance of WC–Ni composites [11]. In this work, scratch tests using a diamond tip are performed on four grades of WC–Ni cermets with different reinforcing particles; the aim is to observe how these scratches act on the wear behavior and resistance of the cermets. The data obtained are used to determine correlations between the microstructure parameters (mean free path of the binder phase, contiguity of WC phase) and the reinforcing particles in the WC–Ni composites.
2. Experimental details 2.1. Materials and sample preparation procedures Four different grades of WC (A, B, C and D) in the shape of blocks (5 15 20 mm3) are tested in this study.
O. Marou Alzouma et al. / Wear 352-353 (2016) 130–135
All samples were placed in a graphite boat on top of sheets of graphite. Sintering of all samples was done in a FCT-system (FPW300-400 Sinter-HIP furnace, Germany). The furnace is designed with the capacity to reach 1600 °C with 100 bars of Argon (Ar) pressure. The final temperature of 1500 °C was chosen. Their chemical compositions and mechanical properties are presented in Tables 1 and 2, respectively. WC grade A represents the control sample (without reinforcing particles). The density of the materials is measured using Archimedes’ method with water as the medium (measured six times); Vickers’ hardness (HV20) is measured using an Indectec HV tester machine (measured six times); fracture toughness is determined via Vickers’ indent method using Omnimet Imaging software (measured six times); the Young’s modulus is measured via Vickers’ indent method (measured six time), and the WC mean grain size is determined by the mean linear intercept method, or Heyn’s technique [12] (measured five times). The mean free path is calculated by two different formulas that give approximately the same values for this parameter. The first formula used by Saito et al. [13] is given by Eq. (1):
λ¼
ð2Þ
where vol% (binder) represents the volume fraction of the binder and N is the average number of intercepts per unit length of the test line in the SEM photographs, as determined using traces of the carbide/cobalt interface. The contiguity between the WC grains is calculated using the formula reported by Jia et al. [15]. It is given by Eq. (3): C¼
2Nc ð2Nc þ N Þ
2.3. Methods of analysis The quantification of wear resistance of the WC specimens is determined by calculating the wear volume, and the topography, determined by interferometry, allows for direct determination of the removed volume. For qualitative evaluation of wear, a digital optical microscope is used. The tests are repeated three times, and the repeatability is confirmed.
ð1Þ
where f represents the volume fraction of the particles, Nl is the number of particles per unit length intersected on a random line in the SEM photographs, and Ns is the number of particles per unit area contained in a random area in the SEM photographs. The second formula, used by Jia et al. [14], is given by Eq. (2): 2vol%ðbinderÞ λ¼ N
15 N. For the tests carried out under 10 N, the amount of volume loss is quite low. Furthermore, the ranking of the materials remains the same as in the case of 15 N. The characteristics of the scratches obtained for 15 N are more similar to those observed on real worn carbide inserts used underground. For these reasons, the results shown in this study are obtained with a normal load of 15 N. The tests are performed at room temperature and ambient humidity conditions. The scratch test device is presented in Fig. 2.
3. Results and discussions
2
1f 8 Nl with f ¼ Nl 3π Ns
131
ð3Þ
where Nc is the average number of intercepts per unit length of the test line, with traces of the carbide/carbide grain boundaries. The samples are polished with diamond polishing paper (45 mm and 15 mm) and diamond suspension (6 mm, 3 mm and 1 mm) down to a roughness Ra¼ 0.01 mm. The surfaces are cleaned with acetone before each test. The microstructures of the polished flat samples are shown in Fig. 1. 2.2. Testing device and test conditions A scratch test device is used to perform tests on the WC–Ni composites. In this system, a conical (α ¼120°) diamond indenter with a spherical tip (200 mm of radius) is used as the tip. The scanning speed is 50 mm/min, and the scratch length is 1 mm. Initially two loads are used to perform the experiments: 10 N and
In this work, we will mainly compare the influence of the presence of reinforcing particles on the wear resistance of WC–Ni composites obtained in our experiments to other experiments and computational investigations in the literature. 3.1. Influence of the reinforcing particle size on wear The wear volume and worn surfaces of the WC–Ni are presented in Figs. 3 and 4, respectively. The results show that WC–Ni composites reinforced with larger particles display lower wear volumes. Yilmaz et al. [10] explained that larger reinforcing particles are more beneficial for the wear resistance of metallic matrix composites because they protect the matrix. Larger reinforcing particles reduce the interfacial failure between reinforcements and the matrix because of the smaller interfacial area between them. They also support a great quantity of wearing forces [15]. The numerical simulation performed by Hu et al. [8] is consistent with these results. However, grade B WC, with 55-nm reinforcing particles, shows a wear volume slightly lower than that of grade C WC, which contains larger reinforcing particles (100 nm). The zirconia particles used as reinforcing particles for WC grade B and grade D are known to undergo martensitic transformation with increasing volume during the sintering process [16]. The powder agglomeration could also be a reason of increase of the size of ZrO2 particles. The presence of large reinforcing particles (ZrO2 particles) is known to reduce the probability of interfacial failure between the nickel matrix and these larger reinforcing particles, as explained by Chung et al. [17]. This may be one of the reasons of the promising wear resistance of WC grades B and D. It can be observed on the SEM pictures that the WC grains of grades
Table 1 Designation and chemical composition of the tested WC grades. WC grades Chemical compositions
A B C D
WCnanopowder (90-100 nm) – Ni (8 wt% 13 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 12.68 vol%) – ZrO2 (3 wt% 6.8 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 13.09 vol%) – ZrC (1.3 wt% 2.8 vol%) WCnanopowder (90-100 nm) – Ni (8 wt% 12.54 vol%) – ZrO2 (3 wt% 6.7 vol%) – VC/ Cr3C2 (0.5 wt% each 1.19 vol% and 1.03 vol%)
Volume fraction of reinforcements Initial size of additional oxide/ (%) carbide 0 6.8 2.8 9
– ZrO2: 55 nm ZrC: 100 nm ZrO2: 55 nm VC: 1.8 mm Cr3C2: 1.8 mm
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Table 2 Mechanical properties of the tested WC grades. WC grades Density (g/cm3) Hardness HV20 (GPa) A B C D
14.3 70.1 13.9 70.1 14.4 70.04 13.8 70.1
WC grrade A
12.7 70.1 14.3 7 0.1 16.8 7 0.1 17.9 7 0.1
Young’s modulus (GPa)
Fracture toughness (MPa m1/2)
WC grains mean size (mm)
Mean free path (mm)
WC grains contiguity
5977 35 503 743 541 736 538 728
8.4 7.3 9.5 9
0.9 7 0.01 0.53 7 0.03 0.32 7 0.01 0.3 7 0.01
0.36 0.02 0.025 0.013
0.58 0.82 0.61 0.9
WC graade B
WC graade C
Expandedd ZrO2 particlles
WC g rade D
Expandded ZrO2 partticles
Fig. 1. Microstructure of the flat samples of the tested WC grades observed by SEM (the metallic binder phase appears in black, and the WC carbide grains appear in gray).
Fig. 2. Scratch test device used in the study: (a) whole system view and (b) schematic view of the system.
B and D do not undergo fracture phenomenon. The presence of ZrO2 particles in their composition reduces the size of their WC grains. WC grains of grades A and C undergo important fracture phenomenon. The absence of reinforcing particles in the composition is the consequence for WC grade A. For WC grade C, it can be concluded that the particles of ZrC do not have the same role as grain growth inhibitor (ZrO2). ZrC particles did not allow the reduction of all WC grains, and the largest grains undergo severe fracture phenomena. It is also reported that when small reinforcing particles are homogeneously dispersed, it is possible to observe an increase in the wear volume: the first reason for this observation is that smaller particles can move out of their given sites and participate
in the abrasion [8,10], and second, the resulting total area, which is larger for small reinforcing particles, could lead to interfacial failure. This may be the phenomenon that occurs in the grade C WC, as Figs. 4 and 5 show that the plastic deformation is more important. The smaller ZrC particles are probably removed, and the binder phase is more accessible to the abradant which provokes more material removal. Furthermore, Hu [8] postulated that the presence of both larger and smaller reinforcing particles may be of interest to reducing the wear rate. This is possibly a reason, amongst others, why the grade D WC, which contains smaller (ZrO2) and larger (VC and Cr3C2 reinforcing particles), displays the highest wear resistance.
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Computational investigations [8] demonstrated that in cases in which different sizes of reinforcing particles exist, a greater volume fraction of the larger reinforcing particles leads to a lower wear volume. Therefore, increasing the volume fraction of VC and Cr3C2 particles in the chemical makeup of grade D WC may result in an increase in wear resistance. The profiles of the grooves realized on the surface of the composites are presented in Fig. 5. This demonstrates the level of wear depth and plastic deformation observed in the binder phase. The plastic deformation is quantified here, by the presence or not of lateral wedge around the scratches.
Fig. 3. Wear volume for all of the WC grades.
Severe fractu ure of WC grain ns
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It can be observed that plastic deformation of the binder phase is more important (the presence of lateral wedge) for WC–Ni composites with the lowest volume fraction of reinforcing particles (WC grades A, B and C). A higher volume fraction of reinforcing particles protects and strengthens the binder phase from plastic deformation (case of grade D). 3.2. Impact of the reinforcing particles volume fraction on microstructure and wear In this section, the evolution of the wear volume and some microstructural characteristics (mean free path of the binder phase, WC grains contiguity and WC grains mean size) with the volume fraction of the reinforcing particles is analyzed. Fig. 6 shows that the wear volume decreases with an increase in the reinforcing particle volume fraction. Some works have demonstrated that the wear resistance increases with increases in the reinforcing particle volume fraction combined with the testing parameters, even when different types of reinforcing particles are included, for metallic composites [8,18]. This is consistent with our results. Although it is well known that finer WC grains result in higher wear resistance [7,8], the relationship between the WC grain size and the volume fraction of reinforcement is not clear, as shown in Fig. 6a. According to the previous conclusion, a decreasing WC grain size was expected with increases in the volume fraction of reinforcement. This lack of a clear relationship may be explained by the fact that some of the reinforcing particles act as growth inhibitors and
Shallow pplastic groove
of WC grains No fracture f
Low boonding strength around WC grains 10µ µm
10µm
WC grade A
WC graade B
Fractuure of WC grain ns
Micro-plowingg phenomenon
o fracture of WC No C grains
Shallo ow plastic grooove 10µm
10µm
WC grade C
WC graade D
Fig. 4. SEM observations of worn surfaces of WC–Ni composites.
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Fig. 5. Scratch profile for all of the WC grades, in the direction perpendicular to the scratching.
Fig. 6. Evolution of (a) wear volume and WC grain size, (b) wear volume and mean free path and (c) wear volume and WC grain contiguity with volume fraction of reinforcing particles.
that the shape of the WC grains is not uniform for all of the specimens (see Fig. 1). However, the volume fraction of reinforcing particles is crucial in terms of the wear resistance of metallic composites. Numerical
simulations reported in the literature [8] demonstrated that according to the amount of reinforcing particles, the wear resistance can either increase or decrease. There exists a critical value of the volume fraction of reinforcing particles below and above which the wear resistance decreases. Below this critical value of reinforcing particles, the inclusions are not able to support most wearing forces, and the metallic matrix is weakened. Above this critical value, the composite tends to be sensitive to brittle wear [19]. The maximum volume fraction of reinforcing particles used in our study is lower than the critical value, and this may explain why the wear occurring on the WC–Ni composite is due more to damage in the binder phase than to brittle wear. Therefore, if we continue to increase the percentage of our reinforcement volume fraction, at a certain level, the wear volume will certainly increase, resulting in a decrease of the wear resistance. In Fig. 6b, it can be observed that the wear volume and the mean free path of the binder phase decrease to different degrees with the increase in the volume fraction of reinforcing particles. The mean free path is related to the thickness of the binder phase [14]; in other terms, it represents the distance of binder content between WC grains. It is demonstrated in the literature that the mean free path decreases with increasing hardness because of the reduction in the plastic deformation in the binder phase, and consequently, the wear resistance increases [13]. This is consistent with our results. However, it can be observed in Fig. 6b that the mean free path decreases rapidly between 0% and 2.8% of the volume fraction of reinforcing particles and then tends to stabilize above 2.8%. This suggests that a higher volume fraction of reinforcement can progressively affect the mechanical properties, such as the bonding strength between WC grains or between the binder phase and WC grains, and can consequently hinder the increase in the wear resistance despite the decrease in the binder mean free path. Therefore, a critical value of the reinforcing particle volume fraction should exist at which the mean free path becomes stable and the wear resistance becomes independent of the mean free path. Fig. 6c shows that contiguity of the WC grains of our WC–Ni composites increases with increases in the volume fraction of reinforcing particles and that the wear volume decreases with increasing contiguity of the WC grains. A higher contiguity of WC grains corresponds to a lower thickness of the binder phase between the WC grains. This indicates that for higher contiguity, the plastic deformation of the binder is reduced. This results in an increase in the wear resistance. These results are expected because the contiguity of the WC grains is narrowly linked to the mean free path of the binder phase. It is obvious that lower mean free path corresponds to higher contiguity and, consequently, to higher wear resistance.
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4. Conclusions In summary, the following conclusions can be drawn:
Increasing the volume fraction of the reinforcing particles,
especially the larger particles when several sizes are used, enhances the wear resistance. Increasing the volume fraction of reinforcing particles protects the binder phase from plastic deformation. The size distribution of the reinforcing particles protects the metallic matrix by strengthening the interfacial area between the reinforcement, binder and WC grains. The wear resistance of WC–Ni composites may be independent of the mean free path at a certain value of the reinforcing particle volume fraction.
This work shows the importance of the nature and the amount and the reinforcing particles on the nature of the characteristic scratches and on many other microstructural parameters (mean free path, WC grains contiguity, etc.)
Acknowledgments The authors would like to acknowledge the financial support of the European Commission for the project NeTTUN from the Seventh Framework Program for Research, Technological Development and Demonstration (FP7 2007-2013) under Grant Agreement 280712, the colleagues of Tallinn University of Technology (TUT) for their contributions, Emeritus Professor Sture Hogmark for his help and comments and Siegfried Fouvry and his team from LTDS for the provision of the scratch testing device.
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