Si alloyed transition metal diborides – A novel class of oxidation resistant coating materials

Abstract

Surface protection of highly stressed components used in aviation or energy production is of great interest, especially to extend the operation ranges in oxidative high-temperature environments. Here, transition metal (TM) ceramics are a suitable class to resist such conditions. However, studies on a drastic improvement of the oxidation resistance of TM-diborides based coatings are relatively rare, as up to now many studies focus on the synthesis and mechanical properties of such films. Therefore, within this study we suggest the combination of TM-boride based coating materials – as a high temperature stable ceramic – with the excellent oxide former Si, resulting in coatings with outstanding oxidation resistance, tested up to 1500 °C. Various TM-Si-B2±z (TM=Ti, Cr, Hf, Ta, W) films have been sputter deposited and subsequently oxidized in a DTA/TG setup to study their oxidation kinetics. Above certain Si contents, Ti-Si-B2±z, Cr-Si-B2±z, and Hf-Si-B2±z coatings exhibit strongly retarded oxidation kinetics obtaining kp values below 10-11 kg2m-4s-1 at 1100 °C. Long term oxidation tests at 1200 °C (up to 60 h in ambient air on hard substrates) confirmed the superior protective capability of these coatings, with i.e. Hf0.20Si0.23B0.57 possessing an oxide scale thickness of only 1.5 µm after 60 h. Along with a broad set of high-resolution characterization techniques (i.e. HR-TEM, APT) the study introduces this new class of hard coatings for oxidation protection at ultra-high temperatures.