Interdiffusion and diffusion structure development in selected refractory metal silicides

Solid-solid diffusion couples set up with disks of Mo, W, Re, Nb, or Ta in contact with disks of a single crystal of MoSi2 were annealed at selected t emperatures between 1300 degrees and 1700 degrees C for diffusion structure , determination of interdiffusion coefficients and energies of activation f or interdiffusion in various silicides developed in the couples. The couple s were analyzed and characterized by SEM and optical microscopy, microprobe analysis, X-ray diffraction and orientation imaging microscopy. From the i nterdiffusion fluxes determined directly from the concentration profiles, i ntegrated and average effective interdiffusion coefficients were calculated for the components in the various binary and ternary silicide layers. The Mo vs. MoSi2 couples developed Mo5Si3 and Mo3Si layers with non-planar inte rface morphologies; the Mo5Si3 layer exhibited oriented growth and microcra cking. The W vs. MoSi2 couples developed W5Si3 and (W,Mo)(5)Si-3 layers wit h little microcracking. The diffusion structure of Re vs. MoSi2 diffusion c ouples consisted of layers of Re2Si, (Re,Mo)Si and (Re,Mo)(5)Si-3 phases an d cracks were blunted in the (Re, Mo)(5)Si-3 layer. The Nb vs. MoSi2 couple s developed the Nb5Si3 and (Nb,Mo)(5)Si-3 phase layers with porosity in the diffusion zone. Layers of Ta2Si, Ta5Si3 and (Ta,Mo)(5)Si-3 phases were obs erved in the Ta vs. MoSi2 couples. The activation energies (Q) for the inte rdiffusion of both Si and W are calculated to be about 360 and 450 kJ mol-l in the W5Si3 and (W,Mo)(5)Si-3 layers, respectively. Values of Q for the i nterdiffusion of both Re and Si are about 190 kJ mol in the Re,Si phase, 32 5 kJ mol(-1) in the (Re,Mo)Si phase, and 270 kJ mol(-1) in the (Re,Mo)(5)Si -3 phase. For the interdiffusion of Si and Nb in the (Nb,Mo)(5)Si-3 phase, Q values are about 300 and 240 kJ mol(-1), respectively; Q is about 265 kJ mol(-1) in the binary Nb5Si3 phase. Zero-flux planes (ZFP) and uphill diffu sion are observed for Mo in the (Re,Mo)Si and (Me,Mo)(5)Si-3 layers where M e = W, Re, Nb or Ta; in these layers, the ternary cross coefficient (D) ove r tilde(MoMe)(Si) is about as large as the main ternary interdiffusion coef ficient (D) over tilde(MoMo)(Si) and the interdiffusion of Mo is enhanced d own the Me concentration gradient. (C) 1999 Elsevier Science S.A. All right s reserved.