Thermal spraying of silicon nitride has been considered impossible because the high temperatures of the spray processes lead to its decomposition and oxidation. To suppress the degradation oxide bonded silicon nitride spray powders (OBSN) have been developed. In this paper spray experiments with these powders by detonation gun spraying (DGS) and atmospheric plasma spraying (APS) with axial powder injection are described. All coatings were characterized using optical microscopy and X-ray diffraction. In addition, DGS coatings were also investigated using SEM, microhardness testing and a rubber wheel abrasion wear test. For the first time, relatively dense Si3N4-rich coatings with an oxide binder phase were produced. The abrasion wear resistance of some DGS coatings was found to be sufficiently high to envisage industrial application. Further progress is expected from optimized spray experiments with variations of spray powder composition and particle size.
In cold spraying, various issues affect the mechanical properties of the coatings. Adhesion strength is usually considered to be the most crucial mechanical property. The aim of this study was to investigate the adhesion strength of cold sprayed copper and nickel chromium alloy coatings. The focus was especially in studying the influence of powders, substrates and heat treatments on adhesion strength. Three different copper and three nickel-20%chromium powders were tested. The substrate materials were carbon steel and copper. Furthermore, effect of substrate pre-treatment to the adhesion strength of the cold sprayed copper coatings was investigated. The influences of the powders and the substrates on the cold sprayed coatings were dependent on the materials. Adhesion strength of the cold sprayed Cu coatings was significantly higher on the Cu substrate than on the steel substrate. Moreover, heat treatment improved the adhesion strengths with increasing annealing temperature.
In this paper the oxidation behaviour of WC, Cr3C2, and TiC and their composites with binder metals is compared on the basis of literature data and some oxidation experiments. It was found that the oxidation mechanisms in air, which are more or less equal to service conditions of coatings, seem to be totally different from those in the spray process in the case of WC, but similar in the case of TiC and Cr3C2. Oxide layer scale adhesion strengths and, possibly, the high volatility of WO3 seem to be responsible for this difference. It can also be assumed, that the problem of oxidation of TiC-based materials and Cr3C2-NiCr is that of local oxidation on the surface of the spray powder particles during the spray process. Cr3C2-NiCr and TiC-based materials are characterized by a high oxidation resistance in air. Plain WC-Co oxidizes readily in the temperature range 400-600°C, but alloying with Cr, as for instance in WC-CoCr and WC-(W,Cr)2-Ni spray powders increases the oxidation resistance.
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