Oxidation protective coatings for ultra high temperature ceramics

Kurzfassung

Ultra-high temperature ceramics (UHTCs) are the class of materials that offer great potential for aerospace applications such as wing leading edges and nose cones of hypersonic vehicles, scramjet propulsion, combustion chambers, thruster components and nozzles for rocket engines as well as for long-range missiles. They offer unique properties at extremely high temperatures that no other material can offer. They are also suitable for space applications as thermal protection systems for space vehicles (Including nose and leading edges). While a “Sharp” configuration for the leading edges and nose re-entry vehicles increases aerodynamic efficiency and vehicle maneuverability, and it causes higher thermal loads to applied materials compared to a blunt configuration. This makes the thermal and chemical stability of UHTC compounds of great importance, causing them to be candidates for use in extreme environments. Among the family of UHTCs, ZrB2 and HfB2 are reported as the most promising Candidates to be used as thermal protection systems (TPS). However, the issue of using monolithic ZrB2 and HfB2 is the phase transformation of the ZrO2 and HfO2 columnar oxide scale as a by-product at elevated temperature (> 1400 °C), leading to a volume change that results in cracking of the formed oxide scale. Oxide scale growth is driven by the oxidation kinetics, which depends on the removal rate of boria (B2O3) from the oxide scale. In this work, the systematic oxidation kinetics study of ZrB2 is carried out in three different temperature regimes 1200 °C, 1400 °C and 1500 °C and the dwell times of 1 h, 4 h, and 10 h. Previous studies have revealed that Oxidizing environments play a vital role in oxidation kinetics of ZrB2 This study gives insights to the role of the oxidizing environment such as Synthetic air, ambient air, Argon and moisture on the oxidation behavior of pure ZrB2. Besides, this study presents the oxidation behavior of ZrB2 under various flow rate at 1500 °C after 1 h. Researchers have tried to improve the oxidation of pure ZrB2 by adding SiC, transition metals, etc. but the proposed solutions were either limited till 1800 °C or they hindered the required properties by forming the other phases. In this work, the new approach has been proposed to improve the oxidation of ZrB2 by depositing the oxidation protective coatings on ZrB2. High temperature stabilized oxides such as HfO2 and ZrO2 were deposited in thin films (~8-10 µm) by using Magnetron sputtering. The oxidation mechanism of coated ZrB2, such as oxidation kinetics, diffusion of the deposited material to ZrO2 at elevated temperature is studied. Later on, this oxidation kinetics will be compared with oxidation kinetics of pure ZrB2