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Wear resistance enhancement of bimodal-grained cemented carbide coating
SCT-21735; No of Pages 8 Surface & Coatings Technology xxx (2016) xxx–xxx
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Wear resistance enhancement of bimodal-grained cemented carbide coating Haibin Wang, Tao Yang, Xiaoyan Song ⁎, Xuemei Liu, Xuezheng Wang, Xu Wu College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China
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Article history: Received 31 July 2016 Revised 19 October 2016 Accepted in revised form 29 October 2016 Available online xxxx Keywords: Nanoscale WC-Co composite powder Ultra-coarse WC Bimodal-grained coating Nanoscratch Wear resistance mechanism
a b s t r a c t A new type of bimodal-grained WC-Co cemented carbide coating was fabricated, using the raw materials consisting of the in situ synthesized nanoscale WC-Co composite powder as major component and a certain amount of ultra-coarse WC particles as addition. The effects of coarse WC particles on the microstructure, mechanical properties and wear behaviour of the coating were investigated. It was found that with a suitable addition of coarse WC particles, the coating has a decreased decarburization and significantly increased wear resistance, as compared with the coating without the addition. The coarse WC particles allow a certain degree of plastic deformation by dislocation gliding so that under wearing conditions some stress concentration can be released and their fractures are inhibited. A good combination of coarse WC particles and WC-Co nanocomposite facilitates high properties of the coating, with the former mainly bearing the load and resisting against penetration of the wear debris, and the latter providing high hardness and also plastic accommodation in the wearing process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Studies on the thermal sprayed WC-based cermet coatings have been widely reported in the literature, which have high hardness and wear resistance and are used to prevent engineering components from early surface wear. Up to now, many efforts including design of composition and microstructure of feedstock powders and optimization of thermal spraying processes, have been devoted to improve the wear resistance property of the cermet coatings [1–5]. Particularly, the ultrafine and nanostructured WC-based coatings have attracted increasing attentions due to the improved hardness and acceptable toughness as compared to those of the conventional coarse-grained coatings [6–10]. However, it was found in the previous studies that occurrence of the decomposition and decarburization of nanoscale WC during thermal spraying lead to obvious decrease in the wear resistance of the coating [11,12]. In our previous work [13], the decarburization of the fabricated nanostructured WC-Co coating was effectively inhibited by increasing the density of the feedstock powder particles. In this case, the phase decomposition is not the factor that dominates the wear behavior. The wear resistance may depend on the increase of the volume fraction of the interfaces between WC and amorphous Co due to the decrease of WC particle size and fast solidification of partly melted spraying particles. The negative effect of the increase of the interface fraction in the nanostructured coating is the higher probability of the fracture failure. ⁎ Corresponding author. E-mail address: [email protected] (X. Song).
The propagation of cracks along interfaces leads to removal of coating materials. Due to less bonding area with Co phase, the nanoscale WC particles are more easily worn away from the coating along with the loss of Co phase. In order to minimize the undesirable effects of nanostructuring of WC-Co coatings, the design of bimodal-grained cermet coating was proposed, whose structure consisted of both micron-scale and nanoscale carbides. Skandan et al. found that the multimodal structured coating had significantly improved abrasive wear resistance without an associated increase in hardness [14]. This exceptional wear behavior was attributed to the particular structure that is analogous to the concrete consisting of gravel (coarse WC particles), sand (fine particles of nanoscale WC) and cement (Co phase). Wang et al. stated that the decrease of the wear volume loss of the bimodal-grained WC-Co coating was due to its compact microstructure composed of homogeneous distribution of bimodal WC particles and cobalt binder with small mean free path [15]. Yang et al. reported that high hardness and toughness were simultaneously obtained in the bimodal-grained WC-Co coating [16]. However, the wear behaviors of the bimodal-grained WC-Co coatings have not been sufficiently investigated. Moreover, it's worth noting that in the above reported bimodal-grained coatings, the micronscaled WC had a majority, with a little nanoscale WC particles coexisting. In the present work, we aim to explore the effects of addition of the coarse WC particles to the nanoscale WC-Co composite powder on the properties of the bimodal-grained coating. The microstructures and mechanical properties of the coatings with and without addition of coarse WC particles will be compared. The wear resistance
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mechanisms of the bimodal-grained WC-Co coating will be analyzed based on the systematic characterizations.
2. Experiment 2.1. Materials preparation Nanoscale WC-12wt.%Co composite powder was synthesized by an in situ reactions method using the tungsten oxide, cobalt oxide and carbon black as raw materials. The raw powders were milled for 20 h with a powder-to-ball weight ratio of 3:1. Then the mixture was subjected to in situ reduction and carbonization reactions in the vacuum furnace with a targeted temperature of 1000 °C and holding time of 3 h. The as-synthesized WC-Co powder was milled for 14 h. Then a certain amount of coarse WC powder with a particle size of 5-20 μm was added into the WC-12Co composite powder by a sequent milling of 6 h. The mass ratio of coarse WC powder to the nanoscale WC-12Co was 2%, 5%, 10% and 15% respectively. The bimodal-grained WC-Co powder was agglomerated by spraydrying and a subsequent heat-treatment to prepare the thermal spray feedstock. For comparison, the feedstock powder without an addition of coarse WC particles was also prepared. The JP5000 high velocity oxy-fuel (HVOF) spraying system was used to deposit the feedstock powders onto carbon steel substrate. The same spray parameters were used for each specimen, as listed in Table 1. The thickness of the coatings was controlled at about 150 μm.
2.2. Characterization The phase constitutions of the prepared powders and coatings were analyzed by X-ray diffraction (XRD) using Cu Kα radiation in a Rigaku D/max-3c diffractometer. The cross-sectional microstructures of the feedstock powders and coatings were observed using scanning electron microscopy (SEM, Nova NanoSEM200). Nanoindentation hardness and modulus measurements were conducted on the polished cross-sections of the coating by Agilent Nano Indenter G200 using the fast measurement mode. The maximum depth of the indentation was set as 800 nm. 150 indentations were performed for each specimen. In order to better understand the influence of coarse WC particle on the wear behavior of coating, scratch tests were performed on the polished coating surface under ramp loading conditions. The maximum scratch load was 30 mN. The residual scratch path morphology was observed by nano indenter scanning with a scan data dimension of 200 × 250 (X × Y). The sliding wear test was carried out using a reciprocating sliding tribometer. Before tests, all the coating samples were grinded on the diamond resin polishing discs with the diamond particle sizes of 20 μm and 9 μm and then were subjected to a cloth polishing using the diamond polishing paste of 1.5 μm. The silicon nitride ceramic ball with a radius of 5 mm was used as counter material. The applied normal load, the sliding speed and the total wear time for each test were 80 N, 5 m/min and 30 min, respectively. The wear volume of coating was measured by profilometry. The friction coefficient was recorded automatically during the tests.
Table 1 HVOF process parameters. Spraying parameters
WC-12Co coatings
O2 flow rate (L/min) Kerosene flow rate (L/h) N2 flow rate (L/min) Stand off distance (mm) Powder feed rate (g/min)
944 22.7 10.8 380 90
3. Results Fig. 1(a) shows the morphologies of the in situ synthesized nanoscale WC-Co composite powder. The particle size of the powder is mostly in the range of 70–200 nm. The morphology of the coarse WC powder is shown in Fig. 1(b). The coarse particles with smooth surface are partly connected, which is probably caused by local superficial melting. Fig. 1(c) and (d) show the cross-sectional microstructures of the feedstock particles without and with addition of the coarse WC powder, respectively. Both of the agglomerated particles have a relatively high density. The feedstock powder prepared by the nanoscale composite particles has a homogeneous WC size distribution, as shown in Fig. 1(e). Fig. 1(d) and (f) confirm the co-existence of nanoscale and micron-scale WC particles (which are marked by the dashed curves) in the bimodalgrained feedstock particle. Fig. 2 shows the phase constitutions of the in situ synthesized nanoscale WC-Co composite powder and both types of HVOF sprayed WC-Co coatings. The XRD pattern confirms that the composite powder has only WC and Co phases, as shown in Fig. 2(a). However, a small amount of W2C formed in the prepared coatings due to the decomposition and decarburization of WC during spraying. As compared to the coating fabricated by the nanoscale WC-Co powder, the bimodal-grained coating has a slightly decreased amount of W2C phase, as shown in Fig. 2(b) and (c). It is attributed to the decreased specific surface area of the bimodal-grained feedstock powder. In addition, the Co phase in the initial WC-Co composite powder has transformed into amorphous state in the coatings due to melting and rapid solidification in the thermal spraying process [17]. Fig. 3 shows the cross-sectional microstructures and WC particle size distribution of coatings. The nanostructured coating has a narrower WC particle size distribution, mainly in the range of 200– 400 nm (see Fig. 3(a) and (c)), which is slightly larger than that of the starting nanoscale powder. In the bimodal-grained coating, most of the WC particles are less than 2 μm while a few of them are in a range of 2–6 μm, as shown in Fig. 3(b) and (d). It is noted that even smaller WC particles in the bimodal-grained coating are obviously larger than the WC particles in the nanostructured coating. This is attributed to the following processes. Firstly, the initially added coarse WC powder was partially crashed into finer particles during the milling process. However, the crushed particles are still much larger than the nanoscale particles. Moreover, in the procedures of heat-treatment of the agglomerated powder and subsequent thermal spraying, the coarsening of WC particles occurred through solid-state coalescence and/or solutionprecipitation processes [18]. As shown in Fig. 3(c) and (d), the mean WC particle sizes in the nanostructured and bimodal-grained coatings are 0.77 μm and 0.34 μm, respectively. The bimodalgrained coating has a stronger grain coarsening tendency than the nanostructured coating at the same processing conditions. This difference is due to that there is a greater driving force for grain growth in the bimodal-grained WC-Co powder than the nanoscale composite powder, because the coarse WC particles may act as seeds for accelerating coarsening by consumption of finer particles [19]. Fig. 4 shows the distribution of hardness and elastic modulus of the coatings measured by nanoindentation. It is clearly seen that both the hardness and modulus values of the coatings have a broad distribution. This is attributed to the multiple microstructures of the coatings consisting of carbides (i.e. WC and W 2 C) of different grain sizes and Co binder with various amounts of dissolved W and C, as well as defects including pores, micro-cracks and residual stress within the coating. Different mechanical behavior may be induced when nanoindentations are applied on different regions in the coating.
Please cite this article as: H. Wang, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.090
H. Wang et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
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Fig. 1. Morphologies of in situ synthesized nanoscale WC-Co composite powder (a) and commercially available ultra-coarse WC powder (b). Cross-sectional microstructures of thermal spray feedstock particles prepared by the nanoscale powder (c and e) and the nanoscale powder mixed with 5 wt.% coarse WC particles (d and f), as marked with the dashed curves.
Fig. 5(a) and (b) show examples of the nanoindentations applied on a single carbide particle and the multiphase zone in the bimodalgrained coating, respectively. The side lengths of the triangle indentations are estimated as about 6–7 μm. Clearly, the regions covered by the indentations have different compositions and microstructures. Thus, it is not surprising that the measured hardness and modulus of the bimodal-grained coating have values in a range of 2–22 GPa and 100–500 GPa, respectively. The relatively lower hardness and modulus correspond to the Co-rich zone or defects of the coating, and higher values correspond to the carbides-rich zones. Also, it's found that the fraction of regions with higher hardness (e.g. 18–22 GPa) and higher modulus (e.g. 400–500 GPa) increases in the bimodal-grained coating as compared with those in the nanostructured coating. The reason is that the addition of coarse WC particles increases the possibility that the indentations applies on a single coarse WC particle. In contrast, in the nanostructured coating the indentation zone usually contains carbides and soft Co binder. Due to the higher hardness and modulus of WC as compared to those of Co, the probability of high hardness and modulus measured by nanoindentation was increased for the
bimodal-grained coating. This also implies that the bimodal-grained coating would have higher ability to resist against the penetration of abrasives during wearing process. Fig. 6 shows the comparison of the dynamic friction coefficient, wear rate and cross-sectional profile of wear traces between the nanostructured coating and the bimodal-grained coating. The friction coefficients of both coatings are similar and remain at about 0.5 during the wearing process (see Fig. 6(a)). However, the wear rate of the bimodal-grained coating is decreased by more than half as compared to that of the nanostructured coating (see Fig. 6(b)). Correspondingly, the wear depth of the bimodal-grained coating is obviously decreased (see Fig. 6(c)). Fig. 7 shows the microstructures of worn surfaces of both coatings. The nanostructured coating suffered from much higher wear as compared with the bimodal-grained coating (see Fig. 7(a) and (b)). The main wear mechanisms of the nanostructured WC-Co coating are plastic deformation and micro-ploughing. From the enlarged microstructure of the nanostructured coating (see Fig. 7(c)), it's found that the upper Co binder is almost removed and the carbides are exposed. In this case, the ultrafine WC particles would be easily peeled away from
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Further, to better understand the enhancement effect of coarse WC particles, scratch tests were performed on the coatings under ramp loading conditions by nanoindentation. The scratch profiles and corresponding residual scratch path morphology of both coatings are shown in Fig. 8. The penetration depth increases following a linear tendency with the progress of scratching in the nanostructured coating, in which the maximum residual depth reaches about 350 nm, as shown in Fig. 8(a). However, the penetration process is retarded by the coarse WC particle existing in the bimodal-grained coating, as indicated by the decrease in the slope of the vertical displacement curve as a function of scratch distance in Fig. 8(b). As a result, the maximum residual displacement in the bimodal-grained coating is decreased to less than 300 nm. Therefore, it is understandable that a few coarse WC particles can reduce the penetration depth of wear debris into neighbouring coating surface during wearing process. The wear induced by micro-cutting and resultant fractures of materials is greatly inhibited in the bimodal-grained coating (see Fig. 7(b)).
1-WC 2-Co 3-W2C
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Fig. 2. XRD patterns of the in situ synthesized WC-Co composite powder (a) and HVOF sprayed WC-based coatings prepared from the nanoscale composite powder (b) and the nanoscale powder mixed with 5 wt.% coarse WC particles (c).
4. Discussion 4.1. Influence of amount of coarse WC particles on wear
the coating during wearing process due to loss of supporting from the surrounded Co binder. In the bimodal-grained coating, the plastic deformation and ploughing are significantly reduced. Moreover, a number of micron-scale WC particles (as indicated by the arrows) are still firmly embedded in the Co binder in the lower layer, as shown in Fig. 7(d). The coarse WC particles protects the Co binder from rapid abrasion by bearing the load and resisting against penetration of the wear debris. Meanwhile, the WC-Co nanocomposite provides high hardness and plastic accommodation to enhance the wear-resistant property of the coating. As a consequence, the loss of coating materials is delayed and a high wear resistance is obtained for the bimodal-grained coating.
The above experimental results demonstrate that the addition of coarse WC particles is favorable to improve the wear resistance of the nanostructured WC-Co coating. Then it is proposed that whether a better wear property will be obtained for the bimodal-grained coating by increasing the amount of coarse WC particles. In order to verify this idea, a series of experiments were further performed. As shown in Fig. 9, the wear rate of the coatings is reduced firstly then is increased while the amount of coarse WC particles changes from 2% to 15%. Obviously, the wear resistance of the bimodalgrained coating is severely degraded due to excess addition of coarse
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Fig. 3. Cross-sectional microstructure and corresponding WC particle size distribution of the nanostructured coating (a, c) and the bimodal-grained coating (b, d). The coarse WC particles in the bimodal-grained coating were marked in green. The inset of (d) shows the enlargement of the dashed rectangle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Nanoindentation hardness (a) and modulus (b) of the nanostructured coating and the bimodal-grained coating.
WC particles. This can be explained by the insufficient melting of coarse WC particles during the deposition of agglomerated WC-Co feedstock particles. The limited deformation of coarse WC particles does not fit the homogenous spreading of the sprayed particles on the substrate. As a result, the loose microstructure and regions with stress concentration may form in the fabricated coating. Thus, the wear resistance of the bimodal-grained coating is affected due to an excess addition of coarse WC particles. Further, what will happen if the WC particle size is reduced to 1– 2 μm, namely the size range of most conventional WC-Co coatings? Fig. 10 shows the worn surfaces of such coating. It is observed that a large amount of transgranular fractures and fragmentations occurred in the individual WC particles in the coating. Interestingly, similar wear behavior is not found in the coarser WC particles (2–6 μm) of the bimodal-grained coating. This observation is attributed to that the ultra-coarse WC grain allows a certain degree of plastic deformation by dislocations gliding [20]. The stress induced by loading and wearing can be effectively coordinated by the mobile dislocations in the coarse WC particles [21]. However, in the WC particles with smaller sizes (e. g. 1–2 μm), the stress concentration can hardly be relaxed due to the limited dislocation nucleation and motion. Severe fractures of WC particles may occur in the conventional WC-Co coatings. Thus, the configuration of the coating consisting of conventional sizes of WC particles and the nanoscale ones is not favorable to improve the wear property of the coating.
(a)
4.2. Plastic coordination of WC-Co nanocomposite As compared with the nanostructured WC-Co coating (see Fig. 7a), the degree of plastic deformation in the bimodal-grained coating (see Fig. 7b) is significantly decreased under the wearing condition. It is understandable that the dislocation-induced deformation of coarse WC particles is very limited in the bimodal-grained WC-Co coating. Moreover, the WC-Co nanocomposite, as a major component of the bimodal-grained coating, has restricted deformation due to the existence of coarse WC particles. It is proposed by the authors that the WC-Co nanocomposite of the bimodal-grained coating plays a significant role in accommodating the stress generated in the wearing process. The plastic accommodation of the WC-Co nanocomposite mainly originates from the interfacial sliding at the interfaces between crystalline WC and amorphous Co. As reported [22], such sliding began heterogeneously at some weaker sites induced by the formation of shear deformation zones in amorphous layer and at partial dislocations rather than homogeneously along the whole interface. The decrease of WC particle size results in the reduction of the mean free path of Co phase, i.e. the reduction in the mean thickness of amorphous Co layer. For the nanostructured coating (with a mean WC particle size of 0.34 μm) and the bimodal-grained coating (with a mean WC particle size of 0.77 μm), the mean free paths of the Co binder are estimated as about 180 nm and 80 nm according to the reference [23], respectively. The increase of volume fraction of WC/Co interfaces
(b)
Coarse WC
2µm m
2µm m
Fig. 5. Images of the nanoindentations applied on a single WC particle (a) and the multiphase zone (b) in the bimodal-grained coating.
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Fig. 6. Comparisons of dynamic friction coefficient (a), wear rate (b) and wear profile (c) between the nanostructured coating and the bimodal-grained coating.
facilitates the interfacial sliding in the nanostructured coating. Moreover, as the thickness of amorphous layer is reduced to nanoscale, instead of shear-band propagation, the deformation induced by homogeneous flow that generally occurred at high temperatures is probably to be triggered, giving rise to increased deformability of the nanostructured coating [24]. However, the severe deformation may result in the decrease of the contacting area between the coating and the
(a)
wear counterpart. Then the protruded coating materials, which have to bear a higher load than the sunk part, are easily removed by microcutting. In the bimodal-grained coating, the deformation of WC-Co nanocomposite is restricted by the homogeneously distributed coarse WC particles to some extent. However, the wear-induced stress can still be coordinated by the nanocomposite without severe deformation. This
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Fig. 7. SEM images of worn surfaces of the nanostructured coating (a, c) and the bimodal-grained coating (b, d).
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[12]. Actually, a large number of WC/Co interfaces of the nanostructured coating can improve the resistance against plastic deformation (which is related with the hardness property) and crack propagation (which is related with the toughness property). However, the bonding strength of the individual WC/Co interface in the nanostructured coating is decreased. As a result, the micro-cutting of the wear debris, generally accompanied by the interfacial fracture and massive loss of coating materials, is detrimental to the nanostructured WC-Co coating. In the present study, the combination of the nanostructured WC-Co and ultra-coarse WC particles takes advantages of both favorable properties. The coarse WC particles with a larger load-bearing surface effectively resist against penetration of the wear debris (see Fig. 8(b)). The wear induced by plastic deformation and resulting micro-cutting on the WC-Co nanocomposite is thus reduced. Simultaneously, local stress concentration can be released through plastic accommodation of the ultra-coarse WC grains and nanostructured WC-Co composite, with the dislocations gliding in the former and interfacial sliding occurring in the latter. For this reason, the transgranular fracture of coarse WC particles is inhibited in the bimodal-grained coating (see comparison between Fig. 7(d) and Fig. 10(b)). Then, the high hardness and good toughness of the nanostructured WC-Co enable to enhance the wear resistance of the coating. However, considering the fast coarsening of the nanoscale WC particles during fabrication of the coating, addition of grain growth inhibitor in the starting powder may be useful to further improve the wear resistance of the bimodal-grained coating.
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5. Conclusions
Fig. 8. Nanoscratch profiles of nanostructured coating (a) and bimodal-grained coating (b) under ramp loading conditions and scanned images of the corresponding residual scratch path by nanoindenter.
wear behavior enables a steady contact between the bimodal-grained coating and the wear counterpart.
The bimodal-grained WC-Co coating was fabricated using the in situ synthesized nanoscale powder having a particle size of 70–200 nm and a small amount of ultra-coarse WC particles in the size range of 5–20 μm as raw materials. The addition of the coarse WC powder results in decreased decarburization and broadened distribution of the hardness and modulus of the coating. The mechanisms for the high wear resistance of the bimodal-grained WC-Co coating are proposed as follows: (i) the coarse WC particles with a larger load-bearing surface reduce wear of the WC-Co nanocomposite which in turn slows down the pullout of the particles, (ii) a certain degree of deformability of ultra-coarse WC particles coordinates the stress concentration and their fractures are thus inhibited and (iii) in case of the reduced micro-cutting, the WC-Co nanocomposite, as a major component of the bimodal-grained coating, provides high hardness and also plastic accommodation in the wearing process.
4.3. Wear resistance mechanisms of the bimodal-grained coating
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Let's return to where we started. Which one has better wear resistance between the nano- and micron-structured WC-Co composites? Actually, there is no doubt that the wear resistance of the sintered nanocrystalline WC-Co bulk materials is much better than that of the coarsegrained cemented carbides, since the former has excellent combination of hardness and toughness [25–27]. However, it becomes complex for the thermal sprayed WC-Co coatings. Firstly, Co, as a ductile phase, mainly transforms into the amorphous phase after spraying. Moreover, a part of WC phase transforms into brittle W2C due to decarburization. As the WC particle size decreases to nanoscale, both the melting degree and decarburization of the feedstock powder are increased. In consideration of the heat and phase-transformation stresses generated in the coating, a weak bonding may be caused between nanoscale WC particles and Co binder. Thus, the intergranular fractures along WC/Co interfaces are more likely to occur in the nanostructured coating. Once the surface Co binder is removed, the nanoscale WC particles are easily pulled out of coating due to the loss of supporting from the binder. This is why the nanostructured WC-Co coating does not always have improved wear resistance though it normally has higher hardness and good toughness as compared with the micron-structured counterparts
3.0
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Fig. 9. Wear rate of coating as a function of the adding amount of coarse WC particles in the starting nanoscale WC-Co composite powder.
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(b)
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Fig. 10. SEM images of worn surfaces of HVOF-sprayed conventional WC-Co coating with a WC particle size of 1–2 μm. (b) Local enlargement of (a).
Acknowledgments This work was supported by the National High-tech R&D Program of China (2013AA032001), the Beijing Key Program of Natural Science Foundation (2131001) and the Beijing Natural Science Foundation (2154045). References [1] T.M. Gong, P.P. Yao, X.T. Zuo, Z.Y. Zhang, Y.L. Xiao, L. Zhao, H.B. Zhou, M.W. Deng, Q. Wang, A.W. Zhong, Influence of WC carbide particle size on the microstructure and abrasive wear behavior of WC-10Co-4Cr coatings for aircraft landing gear, Wear 362-363 (2016) 135–145. [2] J.H. Yuan, C.W. Ma, S.L. Yang, Z.S. Yu, H. Li, Improving the wear resistance of HVOF sprayed WC-Co coatings by adding submicron-sized WC particles at the splats' interfaces, Surf. Coat. Technol. 285 (2016) 17–23. [3] R. Ahmed, O. Ali, N.H. Faisal, N.M. Al-Anazi, S. Al-Mutairi, F.-L. Toma, L.-M. Berger, A. Potthoff, M.F.A. Goosen, Sliding wear investigation of suspension sprayed WC-Co nanocomposite coatings, Wear 322-323 (2015) 133–150. [4] N. Ma, L. Guo, Z.X. Cheng, H.T. Wu, F.X. Ye, K.K. Zhang, Appl. Surf. Sci. 320 (2014) 364–371. [5] S. Hong, Y.P. Wu, B. Wang, Y.G. Zheng, W.W. Gao, G.Y. Li, High-velocity oxygen-fuel spray parameter optimization of nanostructured WC-10Co-4Cr coatings and sliding wear behavior of the optimized coating, Mater. Des. 55 (2014) 286–291. [6] L. Benea, S.-B. Başa, E. Dănăilă, N. Caron, O. Raquet, P. Ponthiaux, J.-P. Celis, Fretting and wear behaviors of Ni/nano-WC composite coatings in dry and wet conditions, Mater. Des. 65 (2015) 550–558. [7] H.B. Wang, X.Z. Wang, X.Y. Song, X.M. Liu, X.W. Liu, Sliding wear behavior of nanostructured WC-Co-Cr coatings, Appl. Surf. Sci. 355 (2015) 453–460. [8] H.-J. Kim, C.-H. Lee, S.-Y. Hwang, Superhard nano WC-12%Co coating by cold spray deposition, Mater. Sci. Eng. A 391 (2005) 243–248. [9] S.L. Liu, X.P. Zheng, G.Q. Geng, Influence of nano-WC-12Co powder addition in WC10Co-4Cr AC-HVAF sprayed coatings on wear and erosion behaviour, Wear 269 (2010) 362–367. [10] A.K. Basak, J.-P. Celis, M. Vardavoulias, P. Matteazzi, Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC-Co coatings, Surf. Coat. Technol. 206 (2012) 3508–3516. [11] D.A. Stewart, P.H. Shipway, D.G. McCartney, Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC-Co coatings, Wear 225–229 (1999) 789–798.
[12] P.H. Shipway, D.G. McCartney, T. Sudaprasert, Sliding wear behaviour of conventional and nanostructured HVOF sprayed WC-Co coatings, Wear 259 (2005) 820–827. [13] H.B. Wang, X.Y. Song, X.Z. Wang, X.M. Liu, X.L. Wang, Int. J. Refract. Met. Hard Mater. 53 (2015) 92–97. [14] G. Skandan, R. Yao, B.H. Kear, Y.F. Qiao, L. Liu, T.E. Fischer, Multimodal powders: a new class of feedstock material for thermal spraying of hard coatings, Scr. Mater. 44 (2001) 1699–1702. [15] Q. Wang, Z.H. Chen, L.X. Li, G.B. Yang, The parameters optimization and abrasion wear mechanism of liquid fuel HVOF sprayed bimodal WC-12Co coating, Surf. Coat. Technol. 206 (2012) 2233–2241. [16] G.J. Yang, P.H. Gao, C.X. Li, C.J. Li, Mechanical property and wear performance dependence on processing condition for cold-sprayed WC-(nanoWC-Co), Appl. Surf. Sci. 332 (2015) 80–88. [17] D.A. Stewart, P.H. Shipway, D.G. McCartney, Microstructural evolution in thermally sprayed WC-Co coatings: comparison between nanocomposite and conventional starting powders, Acta Mater. 48 (2000) 1593–1604. [18] Z.Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—a review, Int. J. Refract. Met. Hard Mater. 27 (2009) 288–299. [19] K. Mannesson, I. Borgh, A. Borgenstam, J. Ågren, Abnormal grain growth in cemented carbides—experiments and simulations, Int. J. Refract. Met. Hard Mater. 29 (2011) 488–494. [20] T. Csanádia, M. Nováka, A. Naughton-Duszová, J. Dusza, Anisotropic nanoscratch resistance of WC grains in WC-Co composite, Int. J. Refract. Met. Hard Mater. 51 (2015) 188–191. [21] X.Y. Song, Y. Gao, X.M. Liu, C.B. Wei, H.B. Wang, W.W. Xu, Effect of interfacial characteristics on toughness of nanocrystalline cemented carbides, Acta Mater. 61 (2013) 2154–2162. [22] K.G. Chen, S.Q. Shi, W.J. Zhu, X.J. Peng, Plastic deformation due to interfacial sliding in amorphous/crystalline nanolaminates, Comput. Mater. Sci. 109 (2015) 266–276. [23] S. Luyckx, A. Love, The dependence of the contiguity of WC on Co content and its independence from WC grain size in WC-Co alloys, Int. J. Refract. Met. Hard Mater. 24 (2006) 75–79. [24] J.Y. Kim, D.C. Jang, J.R. Greer, Nanolaminates utilizing size-dependent homogeneous plasticity of metallic glasses, Adv. Funct. Mater. 21 (2011) 4550–4554. [25] K. Jia, T.E. Fischer, Abrasion resistance of nanostructured and conventional cemented carbides, Wear 200 (1996) 206–214. [26] K. Jia, T.E. Fischer, Sliding wear of conventional and nanostructured cemented carbides, Wear 203-204 (1997) 310–318. [27] I. Konyashin, B. Ries, Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part II: laboratory performance tests on rock cutting and drilling, Int. J. Refract. Met. Hard Mater. 45 (2014) 230–237.
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Wear resistance enhancement of bimodal-grained cemented carbide coating Haibin Wang, Tao Yang, Xiaoyan Song ⁎, Xuemei Liu, Xuezheng Wang, Xu Wu College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing University of Technology, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 31 July 2016 Revised 19 October 2016 Accepted in revised form 29 October 2016 Available online xxxx Keywords: Nanoscale WC-Co composite powder Ultra-coarse WC Bimodal-grained coating Nanoscratch Wear resistance mechanism
a b s t r a c t A new type of bimodal-grained WC-Co cemented carbide coating was fabricated, using the raw materials consisting of the in situ synthesized nanoscale WC-Co composite powder as major component and a certain amount of ultra-coarse WC particles as addition. The effects of coarse WC particles on the microstructure, mechanical properties and wear behaviour of the coating were investigated. It was found that with a suitable addition of coarse WC particles, the coating has a decreased decarburization and significantly increased wear resistance, as compared with the coating without the addition. The coarse WC particles allow a certain degree of plastic deformation by dislocation gliding so that under wearing conditions some stress concentration can be released and their fractures are inhibited. A good combination of coarse WC particles and WC-Co nanocomposite facilitates high properties of the coating, with the former mainly bearing the load and resisting against penetration of the wear debris, and the latter providing high hardness and also plastic accommodation in the wearing process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Studies on the thermal sprayed WC-based cermet coatings have been widely reported in the literature, which have high hardness and wear resistance and are used to prevent engineering components from early surface wear. Up to now, many efforts including design of composition and microstructure of feedstock powders and optimization of thermal spraying processes, have been devoted to improve the wear resistance property of the cermet coatings [1–5]. Particularly, the ultrafine and nanostructured WC-based coatings have attracted increasing attentions due to the improved hardness and acceptable toughness as compared to those of the conventional coarse-grained coatings [6–10]. However, it was found in the previous studies that occurrence of the decomposition and decarburization of nanoscale WC during thermal spraying lead to obvious decrease in the wear resistance of the coating [11,12]. In our previous work [13], the decarburization of the fabricated nanostructured WC-Co coating was effectively inhibited by increasing the density of the feedstock powder particles. In this case, the phase decomposition is not the factor that dominates the wear behavior. The wear resistance may depend on the increase of the volume fraction of the interfaces between WC and amorphous Co due to the decrease of WC particle size and fast solidification of partly melted spraying particles. The negative effect of the increase of the interface fraction in the nanostructured coating is the higher probability of the fracture failure. ⁎ Corresponding author. E-mail address: [email protected] (X. Song).
The propagation of cracks along interfaces leads to removal of coating materials. Due to less bonding area with Co phase, the nanoscale WC particles are more easily worn away from the coating along with the loss of Co phase. In order to minimize the undesirable effects of nanostructuring of WC-Co coatings, the design of bimodal-grained cermet coating was proposed, whose structure consisted of both micron-scale and nanoscale carbides. Skandan et al. found that the multimodal structured coating had significantly improved abrasive wear resistance without an associated increase in hardness [14]. This exceptional wear behavior was attributed to the particular structure that is analogous to the concrete consisting of gravel (coarse WC particles), sand (fine particles of nanoscale WC) and cement (Co phase). Wang et al. stated that the decrease of the wear volume loss of the bimodal-grained WC-Co coating was due to its compact microstructure composed of homogeneous distribution of bimodal WC particles and cobalt binder with small mean free path [15]. Yang et al. reported that high hardness and toughness were simultaneously obtained in the bimodal-grained WC-Co coating [16]. However, the wear behaviors of the bimodal-grained WC-Co coatings have not been sufficiently investigated. Moreover, it's worth noting that in the above reported bimodal-grained coatings, the micronscaled WC had a majority, with a little nanoscale WC particles coexisting. In the present work, we aim to explore the effects of addition of the coarse WC particles to the nanoscale WC-Co composite powder on the properties of the bimodal-grained coating. The microstructures and mechanical properties of the coatings with and without addition of coarse WC particles will be compared. The wear resistance
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mechanisms of the bimodal-grained WC-Co coating will be analyzed based on the systematic characterizations.
2. Experiment 2.1. Materials preparation Nanoscale WC-12wt.%Co composite powder was synthesized by an in situ reactions method using the tungsten oxide, cobalt oxide and carbon black as raw materials. The raw powders were milled for 20 h with a powder-to-ball weight ratio of 3:1. Then the mixture was subjected to in situ reduction and carbonization reactions in the vacuum furnace with a targeted temperature of 1000 °C and holding time of 3 h. The as-synthesized WC-Co powder was milled for 14 h. Then a certain amount of coarse WC powder with a particle size of 5-20 μm was added into the WC-12Co composite powder by a sequent milling of 6 h. The mass ratio of coarse WC powder to the nanoscale WC-12Co was 2%, 5%, 10% and 15% respectively. The bimodal-grained WC-Co powder was agglomerated by spraydrying and a subsequent heat-treatment to prepare the thermal spray feedstock. For comparison, the feedstock powder without an addition of coarse WC particles was also prepared. The JP5000 high velocity oxy-fuel (HVOF) spraying system was used to deposit the feedstock powders onto carbon steel substrate. The same spray parameters were used for each specimen, as listed in Table 1. The thickness of the coatings was controlled at about 150 μm.
2.2. Characterization The phase constitutions of the prepared powders and coatings were analyzed by X-ray diffraction (XRD) using Cu Kα radiation in a Rigaku D/max-3c diffractometer. The cross-sectional microstructures of the feedstock powders and coatings were observed using scanning electron microscopy (SEM, Nova NanoSEM200). Nanoindentation hardness and modulus measurements were conducted on the polished cross-sections of the coating by Agilent Nano Indenter G200 using the fast measurement mode. The maximum depth of the indentation was set as 800 nm. 150 indentations were performed for each specimen. In order to better understand the influence of coarse WC particle on the wear behavior of coating, scratch tests were performed on the polished coating surface under ramp loading conditions. The maximum scratch load was 30 mN. The residual scratch path morphology was observed by nano indenter scanning with a scan data dimension of 200 × 250 (X × Y). The sliding wear test was carried out using a reciprocating sliding tribometer. Before tests, all the coating samples were grinded on the diamond resin polishing discs with the diamond particle sizes of 20 μm and 9 μm and then were subjected to a cloth polishing using the diamond polishing paste of 1.5 μm. The silicon nitride ceramic ball with a radius of 5 mm was used as counter material. The applied normal load, the sliding speed and the total wear time for each test were 80 N, 5 m/min and 30 min, respectively. The wear volume of coating was measured by profilometry. The friction coefficient was recorded automatically during the tests.
Table 1 HVOF process parameters. Spraying parameters
WC-12Co coatings
O2 flow rate (L/min) Kerosene flow rate (L/h) N2 flow rate (L/min) Stand off distance (mm) Powder feed rate (g/min)
944 22.7 10.8 380 90
3. Results Fig. 1(a) shows the morphologies of the in situ synthesized nanoscale WC-Co composite powder. The particle size of the powder is mostly in the range of 70–200 nm. The morphology of the coarse WC powder is shown in Fig. 1(b). The coarse particles with smooth surface are partly connected, which is probably caused by local superficial melting. Fig. 1(c) and (d) show the cross-sectional microstructures of the feedstock particles without and with addition of the coarse WC powder, respectively. Both of the agglomerated particles have a relatively high density. The feedstock powder prepared by the nanoscale composite particles has a homogeneous WC size distribution, as shown in Fig. 1(e). Fig. 1(d) and (f) confirm the co-existence of nanoscale and micron-scale WC particles (which are marked by the dashed curves) in the bimodalgrained feedstock particle. Fig. 2 shows the phase constitutions of the in situ synthesized nanoscale WC-Co composite powder and both types of HVOF sprayed WC-Co coatings. The XRD pattern confirms that the composite powder has only WC and Co phases, as shown in Fig. 2(a). However, a small amount of W2C formed in the prepared coatings due to the decomposition and decarburization of WC during spraying. As compared to the coating fabricated by the nanoscale WC-Co powder, the bimodal-grained coating has a slightly decreased amount of W2C phase, as shown in Fig. 2(b) and (c). It is attributed to the decreased specific surface area of the bimodal-grained feedstock powder. In addition, the Co phase in the initial WC-Co composite powder has transformed into amorphous state in the coatings due to melting and rapid solidification in the thermal spraying process [17]. Fig. 3 shows the cross-sectional microstructures and WC particle size distribution of coatings. The nanostructured coating has a narrower WC particle size distribution, mainly in the range of 200– 400 nm (see Fig. 3(a) and (c)), which is slightly larger than that of the starting nanoscale powder. In the bimodal-grained coating, most of the WC particles are less than 2 μm while a few of them are in a range of 2–6 μm, as shown in Fig. 3(b) and (d). It is noted that even smaller WC particles in the bimodal-grained coating are obviously larger than the WC particles in the nanostructured coating. This is attributed to the following processes. Firstly, the initially added coarse WC powder was partially crashed into finer particles during the milling process. However, the crushed particles are still much larger than the nanoscale particles. Moreover, in the procedures of heat-treatment of the agglomerated powder and subsequent thermal spraying, the coarsening of WC particles occurred through solid-state coalescence and/or solutionprecipitation processes [18]. As shown in Fig. 3(c) and (d), the mean WC particle sizes in the nanostructured and bimodal-grained coatings are 0.77 μm and 0.34 μm, respectively. The bimodalgrained coating has a stronger grain coarsening tendency than the nanostructured coating at the same processing conditions. This difference is due to that there is a greater driving force for grain growth in the bimodal-grained WC-Co powder than the nanoscale composite powder, because the coarse WC particles may act as seeds for accelerating coarsening by consumption of finer particles [19]. Fig. 4 shows the distribution of hardness and elastic modulus of the coatings measured by nanoindentation. It is clearly seen that both the hardness and modulus values of the coatings have a broad distribution. This is attributed to the multiple microstructures of the coatings consisting of carbides (i.e. WC and W 2 C) of different grain sizes and Co binder with various amounts of dissolved W and C, as well as defects including pores, micro-cracks and residual stress within the coating. Different mechanical behavior may be induced when nanoindentations are applied on different regions in the coating.
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Fig. 1. Morphologies of in situ synthesized nanoscale WC-Co composite powder (a) and commercially available ultra-coarse WC powder (b). Cross-sectional microstructures of thermal spray feedstock particles prepared by the nanoscale powder (c and e) and the nanoscale powder mixed with 5 wt.% coarse WC particles (d and f), as marked with the dashed curves.
Fig. 5(a) and (b) show examples of the nanoindentations applied on a single carbide particle and the multiphase zone in the bimodalgrained coating, respectively. The side lengths of the triangle indentations are estimated as about 6–7 μm. Clearly, the regions covered by the indentations have different compositions and microstructures. Thus, it is not surprising that the measured hardness and modulus of the bimodal-grained coating have values in a range of 2–22 GPa and 100–500 GPa, respectively. The relatively lower hardness and modulus correspond to the Co-rich zone or defects of the coating, and higher values correspond to the carbides-rich zones. Also, it's found that the fraction of regions with higher hardness (e.g. 18–22 GPa) and higher modulus (e.g. 400–500 GPa) increases in the bimodal-grained coating as compared with those in the nanostructured coating. The reason is that the addition of coarse WC particles increases the possibility that the indentations applies on a single coarse WC particle. In contrast, in the nanostructured coating the indentation zone usually contains carbides and soft Co binder. Due to the higher hardness and modulus of WC as compared to those of Co, the probability of high hardness and modulus measured by nanoindentation was increased for the
bimodal-grained coating. This also implies that the bimodal-grained coating would have higher ability to resist against the penetration of abrasives during wearing process. Fig. 6 shows the comparison of the dynamic friction coefficient, wear rate and cross-sectional profile of wear traces between the nanostructured coating and the bimodal-grained coating. The friction coefficients of both coatings are similar and remain at about 0.5 during the wearing process (see Fig. 6(a)). However, the wear rate of the bimodal-grained coating is decreased by more than half as compared to that of the nanostructured coating (see Fig. 6(b)). Correspondingly, the wear depth of the bimodal-grained coating is obviously decreased (see Fig. 6(c)). Fig. 7 shows the microstructures of worn surfaces of both coatings. The nanostructured coating suffered from much higher wear as compared with the bimodal-grained coating (see Fig. 7(a) and (b)). The main wear mechanisms of the nanostructured WC-Co coating are plastic deformation and micro-ploughing. From the enlarged microstructure of the nanostructured coating (see Fig. 7(c)), it's found that the upper Co binder is almost removed and the carbides are exposed. In this case, the ultrafine WC particles would be easily peeled away from
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Further, to better understand the enhancement effect of coarse WC particles, scratch tests were performed on the coatings under ramp loading conditions by nanoindentation. The scratch profiles and corresponding residual scratch path morphology of both coatings are shown in Fig. 8. The penetration depth increases following a linear tendency with the progress of scratching in the nanostructured coating, in which the maximum residual depth reaches about 350 nm, as shown in Fig. 8(a). However, the penetration process is retarded by the coarse WC particle existing in the bimodal-grained coating, as indicated by the decrease in the slope of the vertical displacement curve as a function of scratch distance in Fig. 8(b). As a result, the maximum residual displacement in the bimodal-grained coating is decreased to less than 300 nm. Therefore, it is understandable that a few coarse WC particles can reduce the penetration depth of wear debris into neighbouring coating surface during wearing process. The wear induced by micro-cutting and resultant fractures of materials is greatly inhibited in the bimodal-grained coating (see Fig. 7(b)).
1-WC 2-Co 3-W2C
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4. Discussion 4.1. Influence of amount of coarse WC particles on wear
the coating during wearing process due to loss of supporting from the surrounded Co binder. In the bimodal-grained coating, the plastic deformation and ploughing are significantly reduced. Moreover, a number of micron-scale WC particles (as indicated by the arrows) are still firmly embedded in the Co binder in the lower layer, as shown in Fig. 7(d). The coarse WC particles protects the Co binder from rapid abrasion by bearing the load and resisting against penetration of the wear debris. Meanwhile, the WC-Co nanocomposite provides high hardness and plastic accommodation to enhance the wear-resistant property of the coating. As a consequence, the loss of coating materials is delayed and a high wear resistance is obtained for the bimodal-grained coating.
The above experimental results demonstrate that the addition of coarse WC particles is favorable to improve the wear resistance of the nanostructured WC-Co coating. Then it is proposed that whether a better wear property will be obtained for the bimodal-grained coating by increasing the amount of coarse WC particles. In order to verify this idea, a series of experiments were further performed. As shown in Fig. 9, the wear rate of the coatings is reduced firstly then is increased while the amount of coarse WC particles changes from 2% to 15%. Obviously, the wear resistance of the bimodalgrained coating is severely degraded due to excess addition of coarse
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WC particles. This can be explained by the insufficient melting of coarse WC particles during the deposition of agglomerated WC-Co feedstock particles. The limited deformation of coarse WC particles does not fit the homogenous spreading of the sprayed particles on the substrate. As a result, the loose microstructure and regions with stress concentration may form in the fabricated coating. Thus, the wear resistance of the bimodal-grained coating is affected due to an excess addition of coarse WC particles. Further, what will happen if the WC particle size is reduced to 1– 2 μm, namely the size range of most conventional WC-Co coatings? Fig. 10 shows the worn surfaces of such coating. It is observed that a large amount of transgranular fractures and fragmentations occurred in the individual WC particles in the coating. Interestingly, similar wear behavior is not found in the coarser WC particles (2–6 μm) of the bimodal-grained coating. This observation is attributed to that the ultra-coarse WC grain allows a certain degree of plastic deformation by dislocations gliding [20]. The stress induced by loading and wearing can be effectively coordinated by the mobile dislocations in the coarse WC particles [21]. However, in the WC particles with smaller sizes (e. g. 1–2 μm), the stress concentration can hardly be relaxed due to the limited dislocation nucleation and motion. Severe fractures of WC particles may occur in the conventional WC-Co coatings. Thus, the configuration of the coating consisting of conventional sizes of WC particles and the nanoscale ones is not favorable to improve the wear property of the coating.
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4.2. Plastic coordination of WC-Co nanocomposite As compared with the nanostructured WC-Co coating (see Fig. 7a), the degree of plastic deformation in the bimodal-grained coating (see Fig. 7b) is significantly decreased under the wearing condition. It is understandable that the dislocation-induced deformation of coarse WC particles is very limited in the bimodal-grained WC-Co coating. Moreover, the WC-Co nanocomposite, as a major component of the bimodal-grained coating, has restricted deformation due to the existence of coarse WC particles. It is proposed by the authors that the WC-Co nanocomposite of the bimodal-grained coating plays a significant role in accommodating the stress generated in the wearing process. The plastic accommodation of the WC-Co nanocomposite mainly originates from the interfacial sliding at the interfaces between crystalline WC and amorphous Co. As reported [22], such sliding began heterogeneously at some weaker sites induced by the formation of shear deformation zones in amorphous layer and at partial dislocations rather than homogeneously along the whole interface. The decrease of WC particle size results in the reduction of the mean free path of Co phase, i.e. the reduction in the mean thickness of amorphous Co layer. For the nanostructured coating (with a mean WC particle size of 0.34 μm) and the bimodal-grained coating (with a mean WC particle size of 0.77 μm), the mean free paths of the Co binder are estimated as about 180 nm and 80 nm according to the reference [23], respectively. The increase of volume fraction of WC/Co interfaces
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facilitates the interfacial sliding in the nanostructured coating. Moreover, as the thickness of amorphous layer is reduced to nanoscale, instead of shear-band propagation, the deformation induced by homogeneous flow that generally occurred at high temperatures is probably to be triggered, giving rise to increased deformability of the nanostructured coating [24]. However, the severe deformation may result in the decrease of the contacting area between the coating and the
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wear counterpart. Then the protruded coating materials, which have to bear a higher load than the sunk part, are easily removed by microcutting. In the bimodal-grained coating, the deformation of WC-Co nanocomposite is restricted by the homogeneously distributed coarse WC particles to some extent. However, the wear-induced stress can still be coordinated by the nanocomposite without severe deformation. This
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Fig. 7. SEM images of worn surfaces of the nanostructured coating (a, c) and the bimodal-grained coating (b, d).
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[12]. Actually, a large number of WC/Co interfaces of the nanostructured coating can improve the resistance against plastic deformation (which is related with the hardness property) and crack propagation (which is related with the toughness property). However, the bonding strength of the individual WC/Co interface in the nanostructured coating is decreased. As a result, the micro-cutting of the wear debris, generally accompanied by the interfacial fracture and massive loss of coating materials, is detrimental to the nanostructured WC-Co coating. In the present study, the combination of the nanostructured WC-Co and ultra-coarse WC particles takes advantages of both favorable properties. The coarse WC particles with a larger load-bearing surface effectively resist against penetration of the wear debris (see Fig. 8(b)). The wear induced by plastic deformation and resulting micro-cutting on the WC-Co nanocomposite is thus reduced. Simultaneously, local stress concentration can be released through plastic accommodation of the ultra-coarse WC grains and nanostructured WC-Co composite, with the dislocations gliding in the former and interfacial sliding occurring in the latter. For this reason, the transgranular fracture of coarse WC particles is inhibited in the bimodal-grained coating (see comparison between Fig. 7(d) and Fig. 10(b)). Then, the high hardness and good toughness of the nanostructured WC-Co enable to enhance the wear resistance of the coating. However, considering the fast coarsening of the nanoscale WC particles during fabrication of the coating, addition of grain growth inhibitor in the starting powder may be useful to further improve the wear resistance of the bimodal-grained coating.
(a)
(b)
Coarse WC
5. Conclusions
Fig. 8. Nanoscratch profiles of nanostructured coating (a) and bimodal-grained coating (b) under ramp loading conditions and scanned images of the corresponding residual scratch path by nanoindenter.
wear behavior enables a steady contact between the bimodal-grained coating and the wear counterpart.
The bimodal-grained WC-Co coating was fabricated using the in situ synthesized nanoscale powder having a particle size of 70–200 nm and a small amount of ultra-coarse WC particles in the size range of 5–20 μm as raw materials. The addition of the coarse WC powder results in decreased decarburization and broadened distribution of the hardness and modulus of the coating. The mechanisms for the high wear resistance of the bimodal-grained WC-Co coating are proposed as follows: (i) the coarse WC particles with a larger load-bearing surface reduce wear of the WC-Co nanocomposite which in turn slows down the pullout of the particles, (ii) a certain degree of deformability of ultra-coarse WC particles coordinates the stress concentration and their fractures are thus inhibited and (iii) in case of the reduced micro-cutting, the WC-Co nanocomposite, as a major component of the bimodal-grained coating, provides high hardness and also plastic accommodation in the wearing process.
4.3. Wear resistance mechanisms of the bimodal-grained coating
2.5
3
Wear rate (x10 mm /Nm)
2.0
-5
Let's return to where we started. Which one has better wear resistance between the nano- and micron-structured WC-Co composites? Actually, there is no doubt that the wear resistance of the sintered nanocrystalline WC-Co bulk materials is much better than that of the coarsegrained cemented carbides, since the former has excellent combination of hardness and toughness [25–27]. However, it becomes complex for the thermal sprayed WC-Co coatings. Firstly, Co, as a ductile phase, mainly transforms into the amorphous phase after spraying. Moreover, a part of WC phase transforms into brittle W2C due to decarburization. As the WC particle size decreases to nanoscale, both the melting degree and decarburization of the feedstock powder are increased. In consideration of the heat and phase-transformation stresses generated in the coating, a weak bonding may be caused between nanoscale WC particles and Co binder. Thus, the intergranular fractures along WC/Co interfaces are more likely to occur in the nanostructured coating. Once the surface Co binder is removed, the nanoscale WC particles are easily pulled out of coating due to the loss of supporting from the binder. This is why the nanostructured WC-Co coating does not always have improved wear resistance though it normally has higher hardness and good toughness as compared with the micron-structured counterparts
3.0
1.5 1.0 0.5 0.0
0 5 10 15 Amount of coarse WC particle (wt.%)
Fig. 9. Wear rate of coating as a function of the adding amount of coarse WC particles in the starting nanoscale WC-Co composite powder.
Please cite this article as: H. Wang, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.090
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H. Wang et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
(b)
(a)
Transgranular fracture of WC
10µm
2µm
Fig. 10. SEM images of worn surfaces of HVOF-sprayed conventional WC-Co coating with a WC particle size of 1–2 μm. (b) Local enlargement of (a).
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Please cite this article as: H. Wang, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.090