Process Parameter Impact on Suspension-HVOF-Sprayed Cr 2 O 3 Coatings
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Process Parameter Impact on Suspension‑HVOF‑Sprayed Cr2O3 Coatings Jarkko Kiilakoski1 · Richard Trache2 · Stefan Björklund3 · Shrikant Joshi3 · Petri Vuoristo1
Submitted: 9 July 2019 / in revised form: 6 September 2019 © The Author(s) 2019
Abstract Chromium oxide (Cr2O3) is commonly used as an atmospheric plasma-sprayed (APS) coating from powder feedstock in applications requiring resistance to sliding wear and corrosion, as well as amenability to texturing, e.g., in anilox rolls. Recently, high-velocity oxy-fuel spray methods involving suspension feedstock have been considered an extremely promising alternative to produce denser and more homogeneous chromium oxide coatings with lower as-sprayed surface roughness, higher hardness and potentially superior wear performance compared to conventional APS-sprayed coatings. In this study, the impact of process parameters namely auxiliary air cleaning nozzles and a transverse air curtain on suspension high-velocity oxy-fuelsprayed Cr2O3 suspensions is presented. The produced coatings are characterized for their microstructure, mechanical properties and wear resistance by cavitation erosion. The results reveal the importance of optimized air nozzles and air curtain to achieve a vastly improved coating structure and performance. Keywords Cr2O3 · influence of spray parameters · mechanical properties · SHVOF · suspension spraying
* Jarkko Kiilakoski jarkko.kiilakoski@saint‑gobain.com 1
Materials Science and Environmental Engineering, Tampere University, Korkeakoulunkatu 6, 33720 Tampere, Finland
2
Treibacher Industrie AG, Althofen, Austria
3
Production Technology, University West, Trollhättan, Sweden
Introduction Thermally sprayed chromium oxide coatings are known to exhibit a very high hardness of up to 1900-2000 H V5N (Ref 1) and possess good adhesive and abrasion wear resistance (Ref 2) in conjunction with excellent corrosion properties (Ref 3). Chromium oxide coatings are commonly used in applications such as anilox rolls and doctor blades in the paper making industry (Ref 4), pump sleeves and feeding screws (Ref 3, 5, 6) where their wear and corrosion resistance is beneficial. Chromium oxide has traditionally been processed by atmospheric plasma spray (APS) (Ref 6, 7), but other methods like high-power atmospheric plasma spray (Ref 7), detonation gun spraying (Ref 8) or HVOF spraying (Ref 9, 10) have also been explored. One of the greatest challenges in spraying C r2O3 is its high volatility at high temperatures encountered during spraying, leading to formation of gaseous species according to (Ref 11-13) (1)
Cr2 O3 (s) + 3∕2O2 (g) = 2CrO3 (g) in a dry atmosphere or
(2) in a moist atmosphere. Although the above gaseous species cannot be verified from samples of thermally sprayed C r 2O 3 coatings, Cr(VI)has been detected therein (Ref 14). The vaporization also lowers significantly the deposition efficiency (DE) and can adversely influence coating quality. To hinder vaporization, among other benefits, specific additives, such as T
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