Improving the tribological properties of plasma sprayed NiAl–Bi2O3–Ag–Cr2O3 composite coatings by hot isostatic pressing

Abstract

In this study, the hot isostatic pressing (HIP) process was adopted to enhance the tribological response of plasma-sprayed NiAl–Bi2O3–Ag–Cr2O3 coatings under different temperature conditions. The HIP process was performed at a temperature of 800 °C, under a pressure of 100 MPa using argon gas. When compared with as-sprayed NiAl–Bi2O3–Ag–Cr2O3 composite coatings, the results revealed that the post-HIP process greatly reduced the porosity to a sufficiently low level of 2.7%, and led to a significant transformation from the splat lamellar to composition homogeneity across the entire coating. As highlighted in the hot isostatically pressed (HIPed) coating, more NiBi intermetallic compounds emerged. The mechanical hardness and adhesive strength increased considerably by 15.9% and 22.7%, respectively. The HIPed coating exhibited improved running stability in friction when exposed to different temperatures. In particular, the wear resistance increased significantly by one level of magnitude at the temperature range of room temperature (25 °C) to 400 °C, compared to the as-sprayed composite coating. This was attributed to the presence of the NiBi intermetallic compound and structural restoration after the HIP process. A protective tribo-layer was always present under alternating temperature conditions, and this allowed for continuous inhibition of wear. The mechanical evolution of the tribo-layer was further determined to clarify its effect on the resulting tribological behavior of the HIPed NiAl–Bi2O3–Ag–Cr2O3 coatings.