Oxidation of Tin+1AlXn (n = 1-3 and X = C, N) II. Experimental Results

Abstract

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds Ti2AlC, Ti2AlC0.5N0.5. Ti4AlN2.9. and Ti3AlC2 are reported. For the first two compounds. in the 1000-1100°C temperature range and for short times (≈20 h) the oxidation kinetics are parabolic. The parabolic rate constants are ka (m2/s) = 2.68 × 105 exp - 491.5 (kJ/mol)/RT for Ti2AlC. and 2.55 × 105 exp - 458.7 (kJ/mol)/RT for Ti2AlC0.5N05.. At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective For the second pair, at short times (< 10 h) the oxidation kinetics are parabolic at all temperatures examined (800-1100°C), but become linear at longer times. The k1 values are 3.2 × 105 exp - 429 (kJ/mol)/RT, for Ti4AlN2.9 and 1.15 × 105 exp - 443 (kJ/mol)/RT for Ti3AlC2. In all cases, the scales that form are comprised mainly of a rutile-based solid solution. (Ti1yAly)O2y/2 where y < 0.05, and some Al2O3. The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al and Ti. The C and N atoms are presumed to also diffuse outward through the oxide layer. At the low oxygen partial pressure side, the Al3+ ions dissolve in and diffuse through the (Ti1yAly)O2y/2 layer and react with oxygen to form Al2O3 at the high oxygen pressure side. This demixing results in the formation of pores that concentrate along planes, especially at longer times and higher temperatures. These layers of porosity impede the diffusion of Al. but not those of Ti and oxygen, which results in the formation of highly striated scales where three layers, an Al2O3-rich, a TiO2-rich, and a porous layer repeat multiple (>10) times. The presence of oxygen also reduces the decomposition (into TiX3 and Al) temperatures of Ti4AlN2.9 and Ti3AlC2 from a T > 1400°C, to one less than 1100°C. © 2001 The Electrochemical Society. [DOI: 10.1149/1.1380256] All rights reserved.

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Journal of the Electrochemical Society

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