An investigation on synthesis and characterization of superhard Cr-Zr-O coatings

Abstract

Coatings with superior hardness, high temperature thermal stability, and oxidation resistance are required to protect cutting tools in machining applications. Hence, the synthesis of such coatings was investigated in a ternary Cr–Zr–O system by reactive radio-frequency magnetron sputtering technique. For this purpose, the Cr–Zr–O coatings with chemical compositions up to 9 at. % Zr were deposited on Si (100) substrates. The coatings were then characterized by energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, and Nanoindentation and a correlation between their chemical composition, crystal structure, phase composition, and mechanical properties was established. The structural stability of the coatings was also evaluated after annealing treatments. The results indicated that adding zirconium to Cr2O3 coatings shifted the onset of crystallization for the coatings to higher temperatures. The hardness measurements showed that Cr-Zr-O coatings have the potential to reach a hardness value over 40 GPa. Based on the XRD and Raman spectroscopy analysis, it was assumed that grain size refinement and solid solution hardening were the most responsible mechanisms for the enhanced hardness. Annealing treatments at elevated temperatures showed that the thermal stability of the superhard Cr-Zr-O coatings was higher than pure chromium oxide coatings. Moreover, the metastable (Cr, Zr)2O3 structure in the superhard coatings decomposed to Cr2O3 and ZrO2 phases at 1000 °C, indicating that Zr has <2 at. % solubility in Cr2O3 in an equilibrium condition.