Abrasive impact wear of WC-Co and TiC-NiMo cermets

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1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia Abrasive impact wear of WC-Co and TiC-NiMo cermets Juhani, K.; Pirso, J.; Viljus, M.; Letunovitš, S.; Tarraste, M. Abstract: Present paper is focused on abrasive impact wear of tungsten and titanium carbide based cermets with different binder contents. A series of samples of different cermets was tested in experimental impact wear tester DESI using different amounts of granite abrasive to study the wear kinetics. The wear resistance of TiC NiMo and WC-Co cermets was compared. The volume wear of the cermets decreases with the increase of carbide content in the composites, which corresponds to an increase in the bulk hardness. The volume abrasive impact wear of the cermets increases approximately linearly with the increase in abrasive content. The wear tracks of the worn compositions were analysed by scanning electron microscopy (SEM) to study the abrasive impact wear mechanism. Key words: WC-Co, TiC-NiMo, abrasive impact wear, wear mechanism 1. INTRODUCTION WC Co hardmetals are materials well known for their high wear resistance [1 4]. They are used in metal cutting and rock drilling tools and wear parts in various applications. TiC-based cermets also show high abrasive wear resistance [5 8]. They offer an attractive combination of high wear resistance and specific mechanical properties, such as strength/density, because of their relatively low density. They show a great potential as a substitute for the commonly used WC Cobased hardmetals. The impact wear behaviour of different materials and coatings is investigated in [9 11]. The abrasive impact wear of chromium carbide based cermets is investigated in [12]. In impact wear conditions multiphase materials exhibited the optimal wear resistance. Multiphase materials as cermets combined softer matrix dispersed with extremely hard grains. The aim of present paper is to investigate the abrasive impact wear properties of WC-Co and TiC-NiMo cermets with different binder contents and to clarify the abrasive impact wear mechanism. 2. MATERIALS AND EXPERIMENTAL DETAILS All investigated cermets were produced in Tallinn University of Technology with conventional powder metallurgy technique, i.e. form pressing followed by sintering, using optimal sintering parameters. In Fig. 1 is shown the typical microstructures of tested materials. Fig.1. Microstructure of WC-15%Co (a) and TiC-20%NiMo (NiMo 4:1) (b) cermets 621

2 The binder content of tested WC-Co cermets was 6, 10, 15 and 20 wt% Co; TiC-NiMo cermets were with binder contents 20, 30, 40, 50 and 60 wt% NiMo. The calculated (initial before sintering) weight ratio of Ni:Mo in the binder of TiC-NiMo cermets took a value of 1:1, 2:1 and 4:1. Mean carbide grain size for WC Co and TiC NiMo cermets was μm. Additionally coarse grained WC- 20%Co with mean grain size 7.4 μm was tested. The chemical composition, density and hardness of investigated materials is exhibited in table 1. Table 1. Chemical composition, density and hardness of investigated materials WC TiC Co Ni Density Hardness wt% wt% ρ, g/cm 3 HV , , , , K * 13, , , , , , , , , , , , , , , , K* coarse grained WC-20%Co Impact wear tests were carried out in experimental impact wear tester DESI, described in [13]. Principal scheme of impact wear tester is exhibited in Fig kg of granite abrasive was used in tests; the granite abrasive fraction was 4 5 mm. the hardness of granite abrasive is approximately 1075HV. To investigate the wear kinetics, tests with abrasive contents 3, 6, 9, 12 and 15 kg were carried out. The tests were carried out at the velocity of about 60 m/s and the estimated impact angles were about 90 o. The blocks were finished to a surface roughness of about 1 m prior to each test. Each specimen was weighed before and after testing with an accuracy of 0.1 mg. Weight loss was converted into the volumetric wear rate. The worn surfaces were observed with scanning electron microscope JEOL JSM 840A to investigate the impact wear mechanism. Fig.2. Principal scheme of impact wear test device [13] 3. RESULTS AND DISCUSSION 3.1. Wear Rate In Fig.3 is exhibited the volume loss of WC- Co cermets depending on binder content. Volume loss increases in increase of binder content, due to the decrease of the bulk hardness of materials. The coarse grained WC-20%Co material exhibits higher wear rate compared to fine grained material, coarse grained WC-20%Co has lower hardness compared with fine grained material and although in case of coarse grained materials 622

3 the brittle cracking of coarser carbide grains takes place during wear process. Volume loss, mm 3 /kg 0, coarse Binder content, wt% Fig.3. Volume loss of WC-Co cermets vs. binder content Volume loss, mm 3 /kg 0,5 0,4 0,0 Ni:Mo 4:1 Ni:Mo 2:1 Ni:Mo 1: Binder content, wt% Fig.4. Volume loss of TiC-NiMo cermets vs. binder content and Ni:Mo ratio In Fig.4 is shown the volume loss of TiC- NiMo cermets depending on binder content and Ni:Mo ratio. The volume loss depends on binder content, when binder content increase the volume wear although increase, materials with higher amount of molybdenum in microstructure exhibit higher wear resistance due to the higher bulk hardness. Volume loss, mm % Co 10% Co 15% Co 20% Co 20% Co coarse Abrasive content, kg Fig.5. Volume loss of WC-Co cermets depending on binder content and the amount of abrasive used in test The volume loss of WC-Co cermets in case of higher binder content increases approximately linearly depending on increase in the amount of abrasive (Fig.5). In case of lower binder contents the volume loss at first stays stabile and increase rapidly after 12 kg of abrasive used in test. In case of TiC-NiMo cermets the volume loss increase approximately linearly for all tested carbide grades (Fig.6). Volume loss, mm % NiMo 30% NiMo 40% NiMo 50% NiMo 60% NiMo Abrasive content, kg Fig.6. Volume loss of TiC-NiMo (NiMo: 2:1) cermets depending on binder content and the amount of abrasive used in test Volume loss, mm3/kg 5 5 0,05 WC-Co TiC-NiMo Binder content, vol% Fig.7. Volume loss of WC-Co and TiC-NiMo (Ni:Mo 2:1) cermets depending on binder content (vol%) In Fig.7 is compared the volume loss of WC- Co and TiC-NiMo (Ni:Mo 2:1) cermets; the binder content is calculated into volume percentage to compare materials with similar binder contents. In case of lower binder contents the volume loss of different type of cermets is comparable, when binder content increases WC-Co cermets exhibited higher 623

4 wear resistance compared to TiC-NiMo cermets, due to their higher hardness. As seen in Fig.8 the volume loss of investigated materials depends on the bulk hardness of the composites, the volume loss decreases with the increase of the bulk hardness of tested cermets. and removing of carbide network and carbide grains. Volume loss, mm 3 /kg 0,4 WC-Co Ni:Mo 4:1 Ni:Mo 2:1 Ni:Mo 1:1 0, Hardness, HV 10 Fig.8. Volume loss vs. bulk hardness of the tested carbide grades 3.2. Wear Mechanism The wear mechanism of investigated materials was studied by analysing SEM images of worn surfaces. Worn surface of investigated materials are exhibited in Fig.9 and Fig.10. Surface of the material is covered with tracks, where the abrasive particles have hit the material, abrasive material penetrate into the material, deform plastically the binder phase and at the same time crush carbide grains and carbide network. Carbide network is crushed; some of the carbide grains are fractured or removed from the surface (Fig.9 b,d; Fig.10 b,d). In case of WC-Co cermets with lower binder content the plastic deformation of binder content is not obvious, the dominant wear mechanism is the crushing of carbide grains and carbide network (Fig.9 a,b). Some of the granite abrasive is crushed during wear process and the granite dust is formed, it damages the material surface locally. The wear mechanism of TiC-NiMo cermets is similar for low and high binder content materials (figure 10), consisting of the plastic deformation of the binder phase and crushing Fig.9. Abrasive impact wear mechanism of WC-Co cermets a, b WC-5%Co; c.d WC-15%Co 624

5 CONCLUSIONS 1. The abrasive impact wear properties of WC-Co and TiC-NiMo were investigated; 2. Abrasive impact wear resistance of investigated cermets depend on the bulk hardness of material for both types of cermets; materials with lower binder content with higher hardness exhibited higher wear resistance compared to materials with higher binder content; 3. Coarse grained WC-Co composites exhibited lower wear resistance compared to fine grained materials; 4. Abrasive impact wear resistance of TiC- NiMo cermets depends also on NiMo ratio; higher wear resistance exhibited materials with higher molybdenum content due to their higher hardness; 5. The wear mechanism of investigated materials consisted of plastic deformation of binder phase and brittle cracking of carbide grains and carbide network; in case of WC-Co materials with lower binder content the brittle cracking of carbide grains are the dominant wear reason. ACKNOWLEDGEMNT The authors are grateful to the technician Peeter De Bakker. This research was supported by the Estonian Ministry of Education and Science and the Estonian Science Foundation (T062 and ETF8817). REFERENCES Fig.10. Abrasive impact wear mechanism of TiC-NiMo cermets a,b TiC-20%NiMo; c,d TiC60%NiMo 1. Upadhyaya G.S. Cemented Tungsten Carbides: Production, Properties, and Testing, Noves Publications, Larsen-Basse, J. Friction in two-body abrasive wear of a WC Co composite by SiC. Wear, 1997, 205, Wayne, S.F., Baldoni, J.G., Buljan, S.T. Abrasion and erosion of WC Co with controlled microstructures Tribol. Trans. 1990, 33,

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