When a complex geometry is rotated in front of the thermal spray gun, the following kinematic parameters vary in a coupled fashion dictated by the geometry: Stand-off distance, spray angle and gun traverse speed. These fluctuations affect the conditions of particle impact with major implications on the coating’s properties. This work aims to probe into the interplay and isolated effect of these parameters on vital coating characteristics in applications requiring variable stand-off distance and spray angles. WC-17Co powders are sprayed via HVOF on steel substrates in a set of experiments that simulates the spray process of a non-circular cross section, while it allows for individual control of the kinematic parameters. Comprehensive investigation of their influence is made on deposition rate, residual stresses, porosity and microhardness of the final coating. It was determined that oblique spray angles and long stand-off distances compromise the coating properties but in some cases, the interplay of the kinematic parameters produced non-linear behaviours. Microhardness is related negatively with oblique spray angles at short distances while a positive correlation emerges as the stand-off distance is increased. Porosity and residual stresses are sensitive to the spray angle only in relatively short stand-off distances

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The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W 2 C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape.

The state of particle impact (velocity and temperature) significantly affects the coating's microstructural, mechanical properties and wear performance. The spray stand-off distance and spray angle dictate particle velocity components and temperature at impact. This work aims in advancing the understanding of their effect and interplay in regard to the sliding wear performance as well as the mechanisms of wear themselves of HVOF-sprayed WC-17Co coatings. Dry sliding wear resistance was evaluated by a pin-on-disk test, and significant interplay between the spray parameters was observed. A large number of experiments in a full-factorial manner provided insights into the progressive tribofilm build-up and wear damage modes and allowed for proposing mechanisms regarding their occurrence. Small deviations from the normal spray angle at long stand-off distances proved to be beneficial for the wear performance of the coatings. The highest friction coefficient and most aggressive wear damage were observed in coatings with a rich coverage of the wear track with a tribofilm. The phase composition of the coatings proved to be the principal contributor in the wear performance.

This work outlines the development of an analytical software tool that enables the prediction of various coating properties on any given sprayable geometry. The prediction is achieved by analyzing the input computeraided design geometry and by correlating the resulting kinematic conditions with experimental measurements. The results of the developed tool have been validated experimentally using HVOF-sprayed WC-17Co coatings. Specifically, coating thickness, microhardness, WC vol.% and specific sliding wear rate are examined and their values are predicted for the case of the external spray of a rotorlike model.