The effect of APS parameter on the microstructural, mechanical and corrosion properties of plasma sprayed Ni-Ti-Al intermetallic coatings

Abstract

In the present study, the effect of plasma spraying arc current on the microstructure, Vickers hardness and crack propagation resistance as well as corrosion behavior of Ni-Ti-Al coatings was evaluated. For this purpose, Ni50Ti25Al25 powders mechanically alloyed by attritor ball mill were used as feedstock materials to deposit coatings on AISI 1045 steel substrate using atmospheric plasma spraying (APS) process. Structural characterizations and phase changes of the powders and coatings were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The hardness and crack propagation resistance of coatings were evaluated using indentation tests. Tafel polarization test was employed to study the corrosion behavior of the coatings in 3.5% NaCl solution. XRD analysis indicated that although mechanically alloyed powder showed no intermetallic phases after milling for 10 h, the coatings did. It was found that the APS arc current had remarkable effects on the properties of plasma sprayed Ni-Ti-Al coatings. Among the coatings, the APS600 coating which was deposited with an intermediate arc current (600 A) showed the highest density and the lowest porosity. In addition to microstructural results, microhardness and fracture toughness values of APS600 coating were measured to be 594 HV0.1 and 2.8 MPa m1/2, respectively. The polarization results showed the beneficial effects of the coatings on the corrosion performance of substrate. This study also showed that the corrosion performance of coatings is strongly affected by coating microcracks. The corrosion current density of APS600 coating was 0.85 μA/cm2 which is comparable to that of bulk NiTi alloys.