Effect of lanthanum on the structure, properties and high-temperature oxidation patterns of the (Hf,Ta)B2–SiC ceramics

This paper focuses on the kinetics and the mechanism of oxidation (at 1650°C) of the ceramics based on the (Hf_0.8Ta_0.2)B₂, β-SiC and LaF₃ (0 and 3.5 wt%) phases fabricated by a combination of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The physicomechanical and thermophysical properties of the ceramics were determined. The effect of doping with lanthanum on the kinetics and mechanism of oxidation as well as the structural and morphological features of the resulting oxide layers after efficiency of their protective action was analyzed. The evolutionary changes in the structure were studied: the formation of a three-layered heterogeneous oxide film, modification of borosilicate glass with Hf⁴⁺, Ta⁵⁺ and La³⁺ cations, formation of Hf⁴⁺-doped La_1/3TaO₃ globules followed by their growth via the Ostwald ripening mechanism, generation and oxidation of the secondary TaB₂ phase, significant reduction of the content of the HfSiO₄ phase, etc. Doping the ceramics with lanthanum was shown to exhibit a negative effect on high-temperature oxidation resistance, and the main reasons for that were analyzed.