Alumina ceramics are widely used as ceramic components in modern electronic and machine industries, for example IC boards, bearings, cutting tools, etc. However, the applications of the ceramic materials including alumina have been restricted because of their susceptibility to brittle fracture. Brittle fracture may catastrophically occur from the surface flaws, bringing about the poor reliability of the components. The Diffusion Induced Grain-Boundary Migration (DIGM) can be a method for the modification of surface microstructure of alumina ceramics. In the present thesis, we studied the diffusion-induced microstructure changes and then their effects on mechanical properties. The evaluation of mechanical properties of surface modified or layer-structured materials has been investigated.
In Chap. Ⅱ, the diffusion induced surface undulation was studied in polycrystalline and single crystal alumina using $Fe_2O_3$ and $Cr_2O_3$. When the alumina sample are annealing in a chemical inequilibrium condition, undulation of solid-vapor interface as well as the DIGM occurs. The undulated solid-vapor interfaces were investigated using scanning electron microscopy and cross-sectional transmission electron microscopy. Much more misfit dislocations were observed beneath the protrusion regions than at the concave regions. The driving force for such a material transfer, which resulted in an undulation of the surface, was explained in terms of the diffusional coherency strain.
In Chap. Ⅲ, surface modified alumina has been designed by using the DIGM technique in 1000 ppm Fe-doped alumina. To minimize the chemical effect on mechanical properties, only 1000 ppm Fe (Fe/Al) was doped. 1000 ppm Fe-doped alumina samples were sintered at 1500℃ for 3 h in $95N_2-5H_2$. Sintered sample had a typical polycrystalline microstructure with average grain size of about 6 mm. Transmission electron microscopy (TEM) observation showed that excess $Fe_2O_3$ remained as a-Fe precipitates at triple grain-junctions. On annealing the sintered sample in air, however, a-Fe precipitates dissolved into $Al_2O_3$ grains at surface region and induced grain-boundary migration. To prepare reference samples with a typically polycrystalline microstructure, some sintered samples were annealed in $95N_2-5H_2$, the same atmosphere as the sintering atmosphere. Both samples were indented by a spherical WC indenter at various loads. Hertzian cone crack initiated at 700 N in the sample without surface DIGM. However, the crack was formed at 900 N in the sample with surface DIGM. The increase in the critical load for cone crack initiation was attributed to the curved grain-boundary shape and the increase in grain-boundary area by diffusion induced grain-boundary migration.
In Chap. Ⅳ. surface modified alumina has been designed by using DIGM technique in 1 $wt%-Fe_2O_3-added$ alumina. When 1 $wt%-Fe_2O_3-added$ $Al_2O_3$ samples were sintered at 1600℃ in a reducing atmosphere, the excess $Fe_2O_3$ remained as Fe precipitates at the triple grain-junctions. On annealing the sintered samples in an oxidizing atmosphere, however, the Fe precipitates dissolved into $Al_2O_3$ grains at the surface region and induced grain-boundary migration. This diffusion induced grain-boundary migration (DIGM) resulted in the corrugation of grain boundaries and the formation of misfit dislocations in the migration region. The mechanical properties of the samples were evaluated by the Hertizian indentation technique under static and cyclic loading. The sample with DIGM showed better mechanical properties than that without DIGM: improvement in the critical load for cone crack initiation under static loading and in the number of cycles for crack initiation under cyclic loading. The surface chipping at the contact area under cyclic loading was also much reduced in the sample with DIGM. Stress calculation suggested that the mechanical property improvement was due to the microstructural changes, the misfit dislocations and the grain boundary corrugation, rather than compressive stresses introduced in the migration layer. DIGM appears to be a possible means of improving the mechanical properties of $Al_2O_3$.
In Chap. Ⅴ, simple relations for the onset of competing brittle and quasiplastic damage modes in Hertzian contact are derived. The formulations are expressed in terms of well-documented material parameters, elastic modulus, toughness and hardness, enabling a priori predictions for given ceramics and indenter radii. Data from a range of selected ceramic (and other) materials are used to demonstrate the applicability of the critical load relations, and to evaluate coefficients in these relations. The results confirm that quasiplasticity is highly competitive with fracture in ceramics, over a sphere radius range 1-10 mm.
In Chap. Ⅵ, simple explicit relations are presented for the onset of competing fracture modes in ceramic coatings on compliant substrates from Hertzian-like contacts. Special attention is given to a deleterious mode of radial cracking that initiates at the lower coating surface beneath the contact, in addition to traditional cone cracking and quasiplasticity in the near-contact area. The critical load relations are expressed in terms of well-documented material parameters-elastic modulus, toughness and hardness, strength; and geometrical parameters-coating thickness and sphere radius. Data from selected glass, alumina and zirconia coating materials on polycarbonate substrates are used to demonstrate the validity of the relations. The formulation provides a basis for designing ceramic coatings with optimum damage resistance.
