Effects of Heat Treatment on the Structures and Wear Behaviors of HVOF-Sprayed Ni–MoS<sub>2</sub> Coatings

Abstract

Numerous studies have addressed the augmentation of the hardness and tribological properties of material surfaces with the aim of diminishing the wear. However, there is significant scope for further enhancing the performance of self-fluxing nickel alloys utilized as abrasion-resistant coatings. In this work, Ni–MoS2 powders were prepared by electroless plating, and high-velocity oxy-fuel (HVOF) spraying was used to prepare low-coefficient-of-friction coatings with these powders. The coatings were then subjected to heat treatment. Various surface analysis techniques including hardness testing, scanning electron microscopy, and X-ray fluorescence were subsequently used to characterize the compositions and mechanical properties of the composite HVOF coatings. In addition, ball-on-disc tests were performed under dry abrasive conditions on specimens that were heat treated at different temperatures according to the ASTM G99 standard. The wear of each specimen was evaluated, and the measurements were used to provide a comprehensive assessment of the coating’s wear resistance. When the Ni–MoS2 composite coating was heat treated at 500°C, the growth of MoO2 crystals evaporates from the surface, and condenses on the protruding oxide crystals, which led to an increase in porosity and structural looseness. Consequently, the hardness and structural strength of the coating decreased significantly, dramatically increasing its wear loss. The HVOF Ni–MoS2 composite coatings are suitable for high-temperature abrasion environments with temperatures below 500°C, and an operating temperature of 500°C or higher should be avoided to maintain its hardness, structural strength, and wear resistance.