Diffusion, stress and environmental stability issues in oxide ceramic coatings and thin films

Several issues concerning the processing and performance of oxide ceramic coatings where stress, diffusion and environmental stability effects are important are studied using experiments and mathematical modeling. In the first set of problems, oxidation and fracture issues in ceramic composites with porous oxide interphase coatings are addressed using a model SiC/porous Al₂ O₃ interphase laminate system. Experimental investigations show crack deflection with measured interfacial fracture resistances (Γ _i) well in excess of the deflection threshold predicted by the He-Hutchinson criterion. Examination of the fracture surface revealed a tortuous crack path and significant crack-flaw interaction. The effect of oxidation on Γ_i is studied by exposing these laminates to air at 500°C to produce a uniform reaction layer. Γ _i is found to decrease with increasing oxidation times. TEM and X-ray diffraction observations of the fractured interface show a complex multi-phase microstructure. Oxidation behavior of these laminates is explored by mathematical modeling of the reaction/porous diffusion kinetics in the system. Calculations predict that at 500°C, a nearly uniform reaction layer is formed, which is used in the experiments described above. Further, a mixed SiC–Al₂O₃ interphase is found to accelerate pore closure at the surface, which suggests that an optimally mixed interphase can provide good oxidation and fracture resistance. The second part examines the effectiveness of aluminosilicate (in particular, mullite) protective coatings for SiC turbine components through a mathematical model that incorporates among other things: (a) Nonstoichiometry of the mullite, (b) Cation diffusivities, (c) SiO₂ formation after oxygen saturation and (d) Gas velocity effects. Calculations show that mullite coatings are protective at low gas velocities. Cation diffusivities significantly affect coating performance, which suggests that cation doping can produce effective barrier coatings. The third part examines stress-diffusion interactions in nonstoichiometric oxide films. A Gibbsian thermodynamic formalism is used to determine stress dependencies of diffusion potentials and to identify boundary conditions for diffusion-based problems. As an example, the mechanical stability of doped ceria electrolyte-based solid oxide fuel cells (SOFCs) is examined at typical operating temperatures. Calculations show that SOFC is mechanically stable at temperatures below 700°C and at low dopant contents.