Abstract
ABSTRACTThe influence of yttrium, as well as the thickness of chromium layers sputter-deposited on valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) on their oxidation resistance, was studied under isothermal (900°C) and thermal shock conditions (25–900°C). We found out that chromium layers with higher thickness increase oxidation resistance only in the case of valve steels with lower chromium content. On the other hand, a modification made by deposition of small yttrium additions on the surface of coatings significantly increases oxidation resistance of protected steels. This is due to the formation of highly protective chromia scale or chromium-containing spinel oxide phases during oxidation, instead of non-protective iron oxides. Additionally, 100-h corrosion tests were carried out on valves covered by chromium coatings in real working conditions of an internal combustion engine. The obtained positive results strongly support the idea of tailoring a new generation of inexpensive, thin coatings for engine valve protection.
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