Thermally Sprayed Cr3C2-NiCr Coatings: Improving the Abrasion Resistance
|Kustantaja||Tampere University of Technology|
|Tila||Julkaistu - 30 marraskuuta 2018|
|Nimi||Tampere University of Technology. Publication|
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.