Evidence for a Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature environment

摘要

Cr deposits are widely used as protective coatings but multifunctional performances are required in harsh environments motivating research on new processes. MOCVD of Cr metal coatings was carried out by direct liquid injection (DLI) of a unique solution containing bis(ethylbenzene) chromium as metal source and thiophenol as inhibitor of carbide formation. A low amount (<6%) of the metastable delta-Cr phase was found embedded in the stable alpha-Cr phase. The formation of this metastable phase originates from both the low deposition temperature (<723 K) and the use of thiophenol. It was not reported under other CVD conditions. Dense coatings were deposited by implementing a multilayer growth mode. Such coatings exhibit a high nanohardness of about 17 GPa. The delta-Cr metastable phase undergoes an irreversible structural transformation to bcc-Cr above 723 K. The mechanical properties of coatings are not affected by the structural transformation because of the similarity of their crystallographic structures (both cubic), their density very close (a volume contraction of only 0.4% during the transformation) and its low content. This metastable phase is a signature of the DLI-MOCVD process and it can be used as a tracer for Cr coatings operating in high temperature environment without loss of the basic properties. (C) 2017 Elsevier B.V. All rights reserved.