Influence of the non-metal species on the oxidation kinetics of Hf, HfN, HfC, and HfB2 coatings

The influence of the non-metal species on the oxidation resistance of transition metal ceramic based thin films is still unclear. For this purpose, we thoroughly investigated the oxide scale formation of a metal (Hf), carbide (HFC0.96), nitride (HfB1.5), and boride (HfB2.3) coating grown by physical vapor deposition. The non-metal species decisively affect the onset temperature of oxidation, ranging between 550 degrees C for HfC0.96 to 840 degrees C for HfN1.5. HfB2.3 and HfN1.5 obtain the slowest oxide scale kinetic following a parabolic law with k(p) values of 4.97.10(-10) and 5.66.10(-11) kg(2) m(-4) s(-1) at 840 degrees C, respectively. A characteristic feature for the oxide scale on Hf coatings, is a columnar morphology and a substantial oxygen inward diffusion. HfC0.96 reveals an ineffective oxycarbide based scale, whereas HfN(1.5 )features a scale with globular HfO2 grains. HfB(2.3 )exhibits a layered scale with a porous boron rich region on top, followed by a highly dense and crystalline HfO2 beneath. Furthermore, HfB(2.3 )presents a hardness of 47.7 +/- 2.7 GPa next to an exceptional low inward diffusion of oxygen during oxidation. This study showcases the strong influence of the non-metallic bonding partner despite the same metallic basis, as well as the huge potential for HfB2 based coatings also for oxidative environments. (C) 2021 Published by Elsevier Ltd.