Thermally Sprayed Cr3C2-NiCr Coatings: Improving the Abrasion Resistance

There are many industrial applications in which the surfaces of components are exposed to abrasive wear. Protecting such surfaces against harsh abrasive conditions sets high technical requirements for the materials; typically these materials must combine extreme hardness with adequate toughness. These requirements can usually be satisfied with alloys that consist of hard carbides in the ductile metal matrix, e.g. liquid-phase sintered bulk hardmetals. Large components can often be protected against wear by applying a coating to their surface. The processing of hardmetals into a dense wear-resistant surface layer is achieved solely by thermal spraying technology. In thermal spraying, the hardmetal particles are heated and projected towards the component’s surface by high-pressure combustion. Nowadays for most industrial applications, this thermal spraying is done with a High Velocity Oxy-Fuel (HVOF) torch or, more recently, a High Velocity Air-Fuel (HVAF) torch.Even though tungsten carbide (WC) based hardmetals, e.g. WC-Co and WC-CoCr, serve for the vast majority of abrasion resistant applications, these compositions have technical restrictions. These limitations include rapid oxidation above 500 ◦C and an incommensurate coefficient of thermal expansion with steels. Nevertheless, oxidationresistant compositions that consist of up to 80 wt.% of chromium carbides in a nickelchromium binder, commercially designed as Cr3C2-NiCr, are regularly utilized at high service temperatures. The major disadvantage of Cr3C2-NiCr is its inferior abrasion resistance when compared against WC-Co.This work focuses on the characteristics of the Cr3C2-NiCr composition that influence its abrasion resistance at room temperature and above. A unique high-stress abrasion testing procedure for thermally sprayed coatings was established, wherein the sample was heated-up to the testing temperature by induction heating. Moreover, the coated samples underwent various heat-treatments, aimed at simulating high-temperature service.The commercially available Cr3C2-NiCr coatings under study were varied by selecting different feedstock powders and spray technologies. However, there were only minor variations in their wear resistance after long heat-treatments. This was attributed to the dissolution of the carbides during spraying and the re-precipitation of the excess C and Cr as chromium carbides during the subsequent heat-treatment. The prolonged heat-treatment resulted in coarse carbides with a high degree of coalescence and thus equalized the variations in the as-sprayed microstructures.In order to provide enhanced abrasion resistance at room and elevated temperatures, novel compositions were developed with only 10 wt.% of WC. Both experimental compositions, designated as 70Cr3C2-10WC-20Ni and 80Cr3C2-10WC-10Ni, demonstrated attractive abrasion properties in their as-sprayed state and after heat-treatment. The technical performance of the experimental coatings was attributed to the role of dissolved W as a substitutional solid solution strengthener. Moreover, the high carbide content in 80Cr3C210WC-10Ni was considered essential to provide abrasion resistance at high temperatures.Another major challenge in thermally sprayed coating is the spraying-induced dissolution of the carbides. The dissolved carbides supersaturate the binder of the as-sprayed coating with residual carbide elements, and thus make it brittle. Here, laser post-treatment was used to re-precipitate the residual carbide-forming elements from the brittle as-sprayed binder. This improved the room temperature abrasion resistance of the commercially available 75Cr3C2-25NiCr and 45Cr3C2-37WC-18NiCrCo coatings. In 75Cr3C2-25NiCr the most promising improvement to its abrasion resistance was achieved when relatively low laser fluences were applied, which precipitated small nano-sized particles in the binder. In 45Cr3C2-37WC-18NiCrCo, however, relatively high laser fluences had to be used. This resulted in the formation of hard and wear-resistant (Cr,W)2C grains.