作者：Bai Cui, William Edward Lee

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刊名：refractories worldforum

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出版年：2013-03-01

卷：5

期：1

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语种：英文

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内容简介

To understand the fundamental oxidation mechanisms in MAX phases the microstructural evolution of the oxide scales formed during oxidation over a range of temperatures was investigated. Two typical MAX phases, Ti2AlC and Ti2AiN, have been examined. In Ti2AlC between 1100-1300°C an outer rutile TiO2 layer and a thicker, predominant and protective α-Al2O3 layer grow. Above 1400°C the α-Al2O3 layer becomes cracked leading to loss of oxidation protection. In Ti2AIN the oxidation is more complex involving formation of mixed rutile, anatase TiO2 and Al2O3 layers which become dense from 900-1100°C and under which a void layer forms possibly via the Kirkendall effect and gaseous NOx release. With increasing temperature Al2TiO5 and a series of void layers additionally form. High-temperature oxidation of MAX phases generally obeys a parabolic rate law, which can be explained by the diffusion-controlled mass transport mechanism during oxidation. The oxidation mechanism of Al-containing MAX phases involves selective oxidation of Al, leading to formation of continuous and protective Al2O3-rich scales on the substrates. Cracks may arise from stress generation in the oxide scale. Thermal stresses formed during cooling may result from thermal expansion mismatch of phases in the oxide scale, the high anisotropy of thermal expansion in Al2TiO5 and thermal expansion mismatch between the oxide scale and substrate. Growth stresses formed during isothermal oxidation treatment may arise from volume changes associated with oxidation reactions. Ti2AIC and TiAIN could survive without oxidation damage and will potentially be tough below 1400 and 1200°C, respectively. Improved knowledge of oxidation will lead to further use of these MAX phases in high temperature applications such as bearings, hot pressing dies, heating elements, corrosion resistant coatings and cladding materials for nuclear reactors.