In this work, a mechanically clad NiCr powder feedstock was deposited on alumina substrates by atmospheric plasma arc spraying. The resultant splats were analyzed for features such as interfacial bonding, splat classification and, critically, Cr distribution. Using a slice-and-view sectioning technique in a dual-beam FIB-SEM system, a representative splat exhibiting discrete Ni and Cr regions was physically deconstructed then reconstructed with visualization software to analyze individual layers with the splat. Although the powder feedstock contained Ni particles clad with clusters of Cr, the splats solidified into distinct layers of Ni and Cr with no signs of interaction between them. A model formulated based on this observation shows that the distribution of Cr cladding on the Ni particulates influences the amount and location of Cr around the solidified Ni splats.

In the present work, we have examined the role of active elements in controlling the extent of splashing of plasma sprayed splats. Splats of Copper (Cu) and Copper alloyed with Al, Zr were deposited by plasma spray or as free-falling droplet. The splat formation and splat-substrate interfaces were characterised using SEM and FIB. It was found that the presence of Zr, either in the splat material or the substrate, enhanced disk splat formation in free falling droplets. Similarly, the presence of aluminium in plasma sprayed Cu- 10%Al increased the portion of favourable disk-shaped splats compared to pure copper. It was also found that the disk-shaped splat proportion of Cu-10%Al at room temperature is nearly identical to that of Cu splat at 300°C, indicating that the addition of Al in the alloy leads to the improvement of splat formation, correlating with the improvement in the interface bonding with the substrate.

Considerable effort has been made to translate the beneficial properties of bulk Ti(C,N)-based hardmetals to wear resistant thermal spray coatings. Such efforts have focused primarily on as-sprayed coatings. However, past work has shown that hardmetal coatings can undergo significant changes when operated at elevated temperature for extended periods. This work characterised the microstructural changes in a HVOF sprayed (Ti,Mo)(C,N)-Ni coating treated in air at 700°C for up to 30 days. The microstructural development of the carbonitride phase was very subtle. Image analysis indicated that the Mo-rich rim phase underwent the greatest degree of dissolution during spraying and precipitation with heat treatment. Dissolution of the carbonitride phases during spraying led to significant alloying of the Ni binder. Rapid recovery of the Ni binder composition occurred after one day of treatment, but it retained a higher steady state degree of alloying relative to the starting powder.

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.

In this study, Ni-chrome alloy particles were thermally sprayed onto a variety of substrate materials using the high velocity air fuel (HVAF) technique. Although the various substrate materials were sprayed using identical powder material and thermal spray conditions, the type and variation of splat morphologies was strongly dependent on the substrate material. Predominantly solid particles are observed penetrating deeply into softer substrates, such as aluminium, whereas molten splats were observed in harder substrates, which resisted particle penetration. While the observed correlation between molten splats and substrate hardness could be due a dependency of deposition efficiencies of solid particles and molten splats on the substrate material, it was found that conversion of particle kinetic energy into plastic deformation and heat, dependent on substrate hardness, can make a significant contribution towards explaining the observed behaviour.

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.

The tribological behaviour of WC-Co-Cr coatings deposited by HVOF and HVAF and WC-Co coating deposited by HVAF was investigated in pin-on-disc tests. Wear rates were determined and wear tracks on the coatings and counterbodies were investigated in SEM. The HVAF sprayed coatings showed greater wear resistance compared to the HVOF coatings. The main wear mechanism in the WC-Co coatings was adhesive wear. The cobalt matrix is lubricious, resulting in very low wear rates and low debris generation. The main wear mechanisms in the WC-Co-Cr coatings were adhesive and abrasive wear. Adhesive wear results in pull-outs that are trapped in the contact zone and act as a third-body abrasive. Particle pull-out of the coating significantly increases the wear rate of the coated specimen. The HVAF WC-Co-Cr coatings proved to have a better resistance to particle pull-out which reflected in a considerably lower wear rate than the HVOF WC-Co-Cr coatings.