This work assesses the effect of heat treatment on the oxidation resistance of blended-powder CrC-NiCr coatings produced by HVOF spraying. The as-sprayed coating phases oxidized independently with NiCr rapidly forming nickel oxide, which was subsequently consumed by a solid-state reaction with Cr 2 O 3 . The NiO formed two distinct morphologies: large bulbous oxides on thin regions of exposed alloy and thin, flat oxide layers on large alloy regions. Heat treatment led to sintering of the splats and diffusion of Cr from the carbide phase into the NiCr alloy, increasing Cr to the point where Cr 2 O 3 dominated the oxide composition from the earliest exposure time. The implications of the change in oxidation mechanism and oxide morphology are discussed.

Cr 3 C 2 -NiCr thermal spray coatings have been extensively applied to mitigate erosion in high temperature applications such as aircraft and power generation turbines. Much laboratory based erosion research has been conducted under ambient temperature and mild erosion conditions. However, little has been presented about the coating response under the high temperature, high velocity erosion conditions typically experienced in industrial applications. This work presents the mechanisms of high velocity erosion based on experiments conducted under realistic service conditions. Single impact studies were carried out on a range of Cr 3 C 2 -NiCr coatings to assess the variation in erosion mechanism with phase degradation and starting powder morphology. Comparisons were made between the coating response in the as-sprayed state and after long-term heat treatment to determine how the erosion response changes as a function of exposure time in-service. Erosion of the as-sprayed coatings was heavily influenced by splat boundary related mass loss mechanisms. This was accentuated by in-flight carbide dissolution in the coatings based on agglomerated/sintered powders. Heat treatment led to splat sintering and a transition in the erosion response towards more microstructural based erosion mechanisms. The variation in erosion response as a function of microstructural development with heat treatment and starting powder type is presented.

This paper investigates the oxidation behavior of HVAF and HVOF sprayed Cr 3 C 2 -NiCr coatings under isothermal conditions. Over the range of 700 to 900 °C, all HVAF coating-only samples showed initial rapid mass gain before levelling off in a manner indicative of the formation of a protective oxide layer. The kinetics of the process were complex and did not conform to linear, parabolic, or logarithmic rate laws. The HVOF coatings exhibited a similar response. Where variations and anomalies are observed, they are attributed to differences in microstructure, composition, and spraying equipment. Paper includes a German-language abstract.