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

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 reporte d. For the first two compounds, in the 1000-1100 degreesC temperature range and for short times (approximate to 20 h) the oxidation kinetics are parab olic. The parabolic rate constants are k(x) (m(2)/s) = 2.68 x 10(5) exp - 4 91.5 (kJ/mol)/RT for Ti2AlC, and 2.55 x 10(5) exp - 458.7 (kJ/mol)/RT for T i2AlC0.5N0.5. At 900 degreesC, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protec tive. For the second pair, at short times (<10 h) the oxidation kinetics ar e parabolic at all temperatures examined (800-1100<degrees>C), but become l inear at longer times. The k(x) values are 3.2 x 10(5) exp - 429 (kJ/mol)/R T, for Ti4AlN2.9 and 1.15 x 10(5) exp - 443 (kJ/mol)/RT for Ti3AlC2. In all cases, the scales that form are comprised mainly of a rutile-based solid s olution, (Ti1-yAly)O2-y/2 where y < 0.05, and some Al2O3. The oxidation occ urs by the inward diffusion of oxygen and the outward diffusion of Al and T i. 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 (Ti1-yAly)O2 -y/2 layer and react with oxygen to fo rm Al2O3 at the high oxygen pressure side. This demixing results in the for mation of pores that concentrate along planes, especially at longer times a nd 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 porou s layer repeat multiple (>10) times. The presence of oxygen also reduces th e decomposition (into TiXx and Al) temperatures of Ti4AlN2.9 and Ti3AlC2 fr om a T > 1400 degreesC, to one less than 1100 degreesC. (C) 2001 The Electr ochemical Society.