Research On Cavitation Erosion Resistance Of NiTi Coating

The purpose of this paper is to research the cavitation erosion resistance of NiTi coating. The NiTi coating was fabricated on lCr18Ni9Ti martensitic stainless steel substrate by arcspraying protected by argon atmosphere and cold gas dynamic spray, then, heat treatment and laser surface remelting process was conducted. The cavitation erosion resistance of NiTi coating, after heat treatment and laser surface remelting, was evaluated by ultrasonic vibration cavitation erosion experiment machine. The microstructure, phase composition and the microhard-ness of the coating was analyzed. Combined with the result of cavitation erosion experiment, the cavitation erosion resistance of NiTi coating was studied after the two kinds of surface treatment technology process.The results indicate that:the phase composition of NiTi coating fabricated by arc spraying was formed by austenitic NiTi (B2) phase, TiO and NiO phase. Internal coating, with small number of holes and cracks, layer structure is obvious. The microhardness fluctuation range is smaller because the homogeneity of structure. After heat treatment, the NiTi coating phase composition was consisted of austenitic NiTi(B2)phase, Ti2Ni and N13T1phase, while the layer structure disappeared gradually. Under high temperature heat treatment, the number of holes and the micro cracks was increased, due to eutectic reaction happened at β-Ti and Ti2Ni phase boundary. The microhardness was improved significantly, but microstructure inhomogeneity leads to its volatility. After laser surface remelting process, layer structure and holes can’t be observed, large numbers of fine dendritic grain distribute within the coating dipersely. The phase composition was formed by austenitic NiTi (B2) phase, martensitic NiTi (B19’) phase, TiO and NiO phase, γ-Fe. The microhardness of NiTi coating remelted by laser was higher than heat treatment coating.Cavitation experiment results indicate that:the cavitation erosion resistance of NiTi coating, fabricated by arc spraying, can be improved by heat treatment. This phenomenon can be deciphered by layer structure disappeared, grain refinement and higher interlayer bondingforce. But more defects (pores, cracks) and brittle phase appeared significantly after hightemperature heat treatment (1050℃), the cavitation erosion resistance of NiTi coatingdegraded. While the coating remelted by laser, cavitation damage area can’t be found onmolten pool surface, cavitation erosion morphology only emerged on unmelted zone, and thecavitation erosion morphology is similar to heat treatment coating. Comparing the cavitationerosion results of coatings conducted by two kinds of surface treatment process we can inferthat: the NiTi coating remelted by laser has more excellent cavitation erosion resistance thanheat treatment coating.Phase composition analyses demonstrate that: no intermetallic compounds occurred incoatings manufactured by cold gas dynamic spray. Phase structure is still maintained nickeland titanium elemental state that prepared by mechanical alloying. Metal particles in randomcollision combination during coating formation process, thus internal coating particles appearcluttered. After heat treatment, stick structure of metal particles reduced and microhardnessof coating was enhanced, but some cracks appeared between substrate and coating. This leadto coating appeared a large area of spalling in a short time during the process of cavitationerosion. Phase composition included NiTi（B2）,Ti2Ni and Ni3Ti,while, NiTi (B2) phaseshowed high temperature stability. Processed by laser remelting, the way of combiningbetween substrate and coating was metallurgical bonding; stick structure of metal particleswas disappeared completely. The phase structure was formed by austenitic NiTi（B2）,martensitic NiTi（B19′）and γ-Fe. During cavitation experiment, the cavitation erosionmorphology occurred on infusion area. Research has shown that laser cladding can improvethe cavitation erosion resistance of NiTi coatings than heat treatment coatings.