High-temperature oxidation of AlN–SiC–TiB 2 ceramics in air

Abstract

The oxidation behaviour of AlN–SiC–TiB 2 composite materials with 2, 5 and 10 mass% TiB 2 and 3 mass% Fe additive obtained using powder metallurgy methods was studied in air up to 1500 °C by thermogravimetry (TG) and differential thermal analysis (DTA) techniques. The phase composition and structure of the oxide films formed were investigated using metallography, X-ray diffraction (XRD) and electron probe microanalysis (EPMA) methods. The two-stage character of non-isothermal oxidation kinetics (heating rate of 15 grade/min) of composites was established. During the first oxidation stage (up to 1350 °C), the formation of α-Al 2O 3, TiO 2 (rutile), B 2O 3 and β-cristobalite as well as different aluminium borates was found. They formed as a result of interaction between Al 2O 3 and melted B 2O 3. During the second stage (above 1350–1400 °C), the mullite 3Al 2O 3·2SiO 2 proved to be a main oxidation product in the scale; besides, some amounts of β-Al 2TiO 5 were formed as well. The iron additive dissolved in the mullite and aluminium titanate phases that led to the stabilization of a scale formed. It was established that for the three different TiB 2 contents, oxidation isotherms follow the parabolic or paralinear rate law. The slope change on the Arrhenius plot given by the dependence of the parabolic rate constants on the reciprocal temperature, suggests a change of the oxidation mechanism in the temperature range of 1300–1350 °C. For example, for the (AlN–SiC)–5% TiB 2 composite specimen, the calculated values of apparent activation energy are equal to 285 kJ/mol (1100–1300 °C) and 500 kJ/mol (1350–1550 °C), respectively. The AlN–SiC–TiB 2 ceramics developed here can be recommended as high-performance materials for a use in oxidizing medium up to 1450 °C.