Abstract
Cavitation erosion is a sever wear mechanism that takes place in hydrodynamic systems. Examples are turbine vanes of hydropower plants or components of valves and pumps in hydraulic systems. Nickel-titanium shape memory alloys (NiTi) are attractive materials for cavitation-resistant coatings because of their pronounced intrinsic damping mitigating cavitation-induced erosion. In this work, NiTi coatings were produced by cold gas spraying. The phase transformation behaviors of the powder feedstock and the as-sprayed coatings were investigated. Regarding the obtained transformation temperatures, the measured substrate temperatures during spraying rule out that either the shape memory effect or the pseudoelasticity of NiTi could affect the deposition efficiency under the applied conditions of cold gas spraying. Another potential effect is stress-induced amorphization which could occur at the particle–substrate interfaces and impair particle bonding by stress relaxation. Moreover, also oxide formation can be significant. Thus, the presence of amorphous phases and oxides in the near-surface zone of particles bounced off after impact was investigated. Oxidation could be confirmed, but no indication of amorphous phase was found. Besides, also the evolution of local microstrains implies that the substrate temperatures affect the deposition efficiency. These temperatures were significantly influenced by the spray gun travel speed.
Highlights
Cavitation is a mechanism in technical applications that degrades parts in contact with fast-flowing liquids showing rapid changes of pressure (Ref 1)
The differential scanning calorimetry (DSC) analysis of an as-sprayed coating revealed virtually no phase transformation. These results demonstrate the importance of protective measures against oxidation if Nickel-titanium shape memory alloys (NiTi) is thermally sprayed under atmospheric conditions
cold gas spraying (CGS) was tried as an alternative manufacturing process for cavitation-resistant NiTi coatings
Summary
Cavitation is a mechanism in technical applications that degrades parts in contact with fast-flowing liquids showing rapid changes of pressure (Ref 1). The dissipatHed energy per unit initial volume is given by Edis 1⁄4 rde, where r is the engineering stress, e is the engineering strain, and the integration is carried out along the closed loop of loading and relief (Ref 6) This phenomenon of pseudoelasticity ( referred to as superelasticity) is obtained only below a maximum temperature denoted by Md (martensite desist t., denoted as martensite deformation t.). The differential scanning calorimetry (DSC) analysis of an as-sprayed coating revealed virtually no phase transformation These results demonstrate the importance of protective measures against oxidation if NiTi is thermally sprayed under atmospheric conditions. In (Ref 28) it is reported that this phenomenon occurred in cold gas spraying of mechanically alloyed NiTi particles (working gas helium, 2.0-2.5 MPa, 420-580 °C) leading to a partial melting which significantly decreased the deposition efficiency as many particles obviously splashed off the substrate on impact. CGS could be an alternative to LPPS if the phase composition and the transformation behavior could be still improved by limiting the oxygen uptake and enhancing the homogeneity of the material
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