Oxidation of Nb2AlC and (Ti,Nb)2AlC in air

In this paper, we report on the oxidation kinetics in air of the ternary compounds Nb2AlC (actual Nb:Al:C atomic ratios 1.95:1.05:0.95) and (Ti,Nb)(2)AlC (actual Ti:Nb:Al: C ratios 0.94:1.06:0.93:0.94). In the 650-800degreesC temperature range, the oxidation reaction for the former occurs in two steps. First is 2Nb(2)AlC + (( 7 + 4x)/2)O-2 = 4NbO(x) + Al2O3 + 2CO(2) At longer times and higher temperatures, NbOx and Al2O3 react to form NbAlO4 for an overall reaction of 2Nb(2)AlC + 8.5O(2) = Nb2O5 + 2NbAlO(4) + 2CO(2) In the 900-1200degreesC temperature range, oxidation of (Ti,Nb)(2)AlC also occurs in two steps. First is (Ti-0.5,Nb-0.5)(2)AlC + 4O(2) + 0.5Nb(2)O(5) + 0.5Al(2)O(3) + TiO2 + CO2. At longer times and higher temperatures, Nb2O5 and Al2O3 react to form NbAlO4 for an overall reaction of (Ti-0.5,Nb-0.5)(2)AlC + 4O(2) = NbAlO4 + TiO2 + CO2. In all cases, the oxidation occurs by the inward diffusion of oxygen. The formation of TiO 2 substantially enhances the oxidation resistance of (Ti, Nb)(2)AlC relative to that of Nb2AlC. At 650 and 900degreesC the oxidation kinetics are subparabolic, with time exponents of approximate to0.4 +/- 0.02 and 0.34 +/- 0.02 for Nb2AlC and (Ti,Nb)(2)AlC, respectively. At 700 and 750degreesC, the oxidation kinetics of Nb2AlC are linear, with rates comparable to those for the oxidation of pure Nb. The oxidation kinetics for (Ti,Nb)(2)AlC at temperatures between 1000 and 1100degreesC start parabolic but become linear at longer times. At 1200degreesC, the oxidation is parabolic up to about 16 h; beyond that, the oxide layers tend to spall off. At 1200degreesC, a Nb-aluminide phase is observed at the oxide/material interface after 4 h of oxidation. Further oxidation leads to the formation of an Al2O3-rich oxide phase at the oxide/substrate. Based on this work, the maximum extended-use temperature for Nb2AlC in air is 650degreesC; for (Ti-0.5,Nb-0.5)(2)AlC it is 900degreesC. (C) 2003 The Electrochemical Society.