| Hydro-turbine components are known to be affected by cavitation and/or sand slurry erosion. Energy is more and more important today, the hydro-turbine cavitation and sand slurry erosion research has attracted wide attention from many researchers in many countries. In order to improve the cavitation resistance performance and sand slurry erosion resistance performance of materials, provide reference for surface protection of hydro-turbine components, in this paper, two kinds of WC-10Co4 Cr coating were deposited on ASTM316 stainless steel by high velocity oxy-fuel and high velocity air-fuel spray processes respectively. The coating structure, mechanical properties, cavitation resistance and sand slurry erosion performance were investigated and the corresponding results are as follow.The cavitation resistance property of the WC-10Co4 Cr coating(AK coating) in the preparation of HVAF spray process and the WC-10Co4 Cr coating(DJ coating) in the preparation of HVOF spray process are both better than that of ASTM316 stainless steel. Specifically, the AK coating shows a twice lifetime than ASTM316 substrate,while the DJ coating shows a little bit better lifetime performance than the substrate.In addition, the WC-10Co4 Cr coating deposited by the HVAF spraying process exhibited lower porosity, higher hardness, fracture toughness, cavitation resistance than one produced by the HVOF spraying process.The sand slurry erosion property of the WC-10Co4 Cr coating in the preparation of high velocity(air)oxy-fuel thermal spray process is much better than the ASTM316 stainless steel, and the lifetime of WC-10Co4 Cr coating is four times as long as the substrate.Under laboratory ultrasonic cavitation experiment conditions, sealing with epoxy can slightly improve the coating's cavitation resistance performance, although the effect is not obvious,because it can hardly change the microstructure of WC-10Co4 Cr coating. The cavitation mechanism of ASTM 316 stainless steel is generation and propagation of fatigue cracks induced by the cyclic mirco-impact loads, which led to a larger number of particles being continuously pulled off the surface of the tested materials and the further materials fatigue. There are two ways of crack propagation,one way is along the crystal interface, and the other way is penetrate the crystal interface.The cavitation mechanism of WC-10Co4 Cr coating is generation and propagation of Co-Cr binder fatigue crack, which resulting in spalling of coating.After a certain time of the sand slurry erosion testing, the coatings and the substrate material's surface roughness decreased at first then entered a stable state.The performance of AK coating's sand slurry erosion resistance is better than that of DJ coating, but the two coating's sand slurry erosion resistance are much better than that of the ASTM 316 stainless steel. The micro-cutting and micro-plowing were the main mechanisms for the sand slurry erosion of both the ASTM 316 stainless steel and the WC-10Co4 Cr coatings at the low impact angle, while the mirco-cutting and micro-chiseling of the Co-Cr binder phase accompanied by the breaking and pulling off the WC particles were the main mechanism for the sand slurry erosion of the WC-10Co4 Cr coatings at the high impact angle.In summary,WC-10Co4 Cr coating produced by HVAF spraying process can reduce the degree of decarburization of WC due to the use of air instead of pure oxygen. Consequently, AK coating's cavitation and sand slurry erosion resistances performance are better than DJ coating, which was sprayed by HVOF spray process,and the two coatings have better performance than the substrate. It shows that the WC-10Co4 Cr coatings which prepared by the HVAF spray process have a great potential for being used for the surface protection of hydro-turbine components against cavitation and slurry erosion. |
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