High-temperature oxidation of AlN-SiC-TiB2 ceramics in air

The oxidation behaviour of AlN-SiC-TiB2 composite materials with 2, 5 and 10 mass% TiB2 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-Al2O3, TiO2 (rutile), B2O3 and β-cristobalite as well as different aluminium borates was found. They formed as a result of interaction between Al2O3 and melted B2O3. During the second stage (above 1350-1400 °C), the mullite 3Al2O3·2SiO2 proved to be a main oxidation product in the scale; besides, some amounts of β-Al2TiO5 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 TiB2 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% TiB2 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-TiB2 ceramics developed here can be recommended as high-performance materials for a use in oxidizing medium up to 1450 °C.