Crystallographical analysis of surface layers of refractory ceramics formed using combined flame spray and simultaneous laser treatment

The crystalline characteristics of the surface layers of an alumina-based refractory ceramic, prepared using a novel combined flame spraying and laser melting method, have been investigated using X-ray diffraction and the Rietveld method. The results have indicated that 20% Cr2O3 by weight added to Al2O3 powder retards the metastable γ-Al2O3 to α-Al2O3 phase transition during flame spraying. The mullite crystals grew preferentially along the [111] direction for surface layers prepared by laser melting only or by the combined process using pure Al2O3 powder as the feedstock for flame spraying. In the latter case, the surface layer contained more alumina-rich mullite and more crystalline cristobalite than the surface layer prepared by laser melting. However, using a composite powder of 20% Cr2O3 and 80% Al2O3 by weight as feedstock in the combined process, not only did the preferred orientation of the mullite grains change to the [220] direction, but, also, the amount of SiO2 incorporated in the mullite crystals was closer to that for stoichiometric mullite.