HVOF- and HVAF-sprayed Cr3C2-NiCr coatings deposited from feedstock powders of spherical morphology: Microstructure formation and high-stress abrasive wear resistance up to 800 °C

Chromium carbide based coatings are commonly applied to protect surfaces against wear at high temperatures. This work discusses the influence of spray torch and feedstock powder selection on the microstructure and high-stress abrasion resistance of thermally sprayed Cr3C2-NiCr coatings. Four commercial feedstock powders with spherical morphology were deposited by different high-velocity spray processes, namely third generation gas- and liquid fuelled HVOF-torches and by the latest generation HVAF-torch. The microstructures of the coatings were studied in the as-sprayed state and after various heat treatments. The high-stress abrasion resistance of as-sprayed and heat treated coatings was tested at room temperature and at 800 °C. The study reveals that the selection of the spray torch mainly affects the room temperature abrasion resistance of the as-sprayed coatings, which is due to differences in the embrittlement of the binder phase generated by carbide dissolution. At elevated temperatures, precipitation and growth of secondary carbides yields a fast equalization of the various coatings microstructures and wear properties

Entwicklung wirtschaftlich effizienter Hartmetallbeschichtungslösungen für Hochtemperaturanwendungen. Teil 2: Einfluss von Wärmebehandlungen und tribologische Untersuchungen

The aim of the present work was to investigate the effect of feedstock powder properties and the spraying process on the performance capabilities and cost-effectiveness of preparation of Cr3C2-NiCr coatings by high velocity oxy-fuel (HVOF) and high velocity air-fuel (HVAF) spray processes. Based on the results specific improvements were targeted. In part two of this study the effect of heat treatment on coating properties and the resistance of the coatings against abrasion and erosion are discussed. To improve the tribological properties it is necessary to minimize the content of dissolved carbides in the binder matrix to avoid embrittlement and corresponding cracking susceptibility. This can be achieved either by decreasing the process temperature and increasing the particle velocity during spraying or by heat-treating the sprayed coatings. The WC-containing compositions were found to exhibit improvements in both performance characteristics and cost-effectiveness of coating preparation and offered considerable potential for further improvement

Improving the high temperature abrasion resistance of thermally sprayed Cr3C2-NiCr coatings by WC addition

Two experimental agglomerated and sintered (a&s) feedstock powders were prepared, in order to reveal the role of WC addition on the microstructure, hardness, and the abrasion resistance of HVOF-sprayed Cr3C2-NiCr coatings. These powders contained 10 wt.% of sub-micron WC, 20 or 10 wt.% of nickel binder, and Cr3C2 as balance. Experimental coatings were deposited by a liquid fueled high velocity oxygen-fuel (HVOF) spray process and subsequently heat treated at 800 C for 8h to simulate elevated temperature service conditions. The microstructures of powders and coatings were studied by SEM and X-ray diraction, and the hardnesses of coatings were probed by means of micro and nanoindentation. In addition, high stress abrasion resistance was tested in a temperature range from room temperature up to 800 C. The microstructural characterization of coatings Displays the presence of WC and core-rim structured Cr3C2 grains, wherein the rim contains tungsten. The coating hardness increases after the heat treatment, which stems from precipitation of secondary carbides and solid solution strengthening of binder by tungsten. In addition, the study reveals that both experimental coatings have high wear resistance at room and elevated temperatures