Microstructural, thermal and mechanical properties of HVOF sprayed Ni–Al-based bond coatings on stainless steel substrate

Abstract

This work focuses on the microstructural, thermal and mechanical properties of NiAl coatings fabricated on 316 L stainless steel substrates by using the high-velocity oxygen fuel (HVOF) method. With this regard, NiAl-based coatings were fabricated on 316 L stainless steel substrates by using HVOF technique. The produced coatings were extensively analyzed with respect to X-ray diffraction (XRD), optical microscope (OM), image analyzer, scanning electron microscope (SEM), microhardness and surface roughness testers. Adhesion strength of the coatings was also measured by using a scratch tester. The effect of thermal cycling on the failure behaviour of the Ni–Al-based coating has been carried out without an external load at temperatures between 47 °C and 600 °C for the period of 303 s cycles. XRD study revealed that Ni 3Al phase is present in the coatings. The metallographic observations showed that all coatings had a similar coating microstructure and consisted of high-quality contact to the substrate, highly dense structure, low oxide and porosity contents. In addition to microstructural results, microhardness values of Ni–Al-based coatings and stainless steel were measured to be 450 HV and 220 HV before thermal cycling. These microhardness values drastically decreased after thermal cyclic tests. The adhesion strength value of Ni–Al-based coating was found that 85.3 MPa. In order to determine wear loss, friction coefficient and wear mechanism, wear tests were performed the Ni–Al-coated stainless steel substrates at 49 N and 89 N under dry sliding conditions through a pin-on-plate arrangement against AISI 303 L steel counter body. Regarding as wear mechanism of the coatings, the microstructural analysis of worn surfaces was examined by SEM. It was found that the friction coefficients of NiAl coating for 49 N and 89 N loads were in the range of 0.289 and 0.329 and the extent of the plastic deformation at 49 N was less than that at 89 N under dry sliding conditions. Mechanical properties of coating were examined by Shimadzu Dynamic Ultra-microhardness test machine for estimating Young's modulus due to load–unload sensing analysis and in addition to mechanical investigation of hardness–depth curves of Ni–Al-based coatings was obtained.