Chemical Strengthening of Polycrystalline Alumina

Low-expansion solid solution surface layers formed by reaction of Cr2O3 and Al2O3 were used to chemically strengthen alumina bodies. The surface layers were formed by packing the samples in Cr2O3 powder and heating to high temperatures. Fluorides or chlorides were added to the packing material, leading to substantial additional improvement. The flexural strengths of the control samples were increased by refiring in the presence of the products of decomposition of the fluorides. At high concentrations of fluorides in the packing material, the outside dimensions and the weights of the samples decreased during the treatments.