An improved test fixture for biaxial-tension strength testing of ceramics featuring uniform pressure loading of disks was developed and qualified. Biaxial data were obtained for an alumina ceramic, along with comparable uniaxial data from three-and four-point flexure tests. Weibull statistics provided a good description of the size effect on data from the two uniaxial tests, but underestimated the effect of stress biaxiality. The biaxial strengthening observed in the alumina ceramic is consistent with that observed previously in a glass-ceramic.

No abstract

An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-tometal bonds between the atoms in the base metal and 'the proper metallic ions in the enamel. To accomplish this type of bond, the enamel must meet certain chemical and thermodynamic requirements: (1) The enamel at the inteuface must be saturated with an oxide of the metal and (2) this oxide must be one which, when in solution in the glass, will not be reduced by the metal. In the case of iron, the oxide is FeO. Many of the phenomena observed in commercial enameling were investigated and found to be related to adherence, but not essential for its development. An example is the precipitation of metallic particles in the enamel. Much of the complexity in commercial enameling arises from the limitations imposed by practical considerations. For example, because enamels usually are &ed in air, the heavy scale developed during the early stages of firing must be removed before adherence can be developed.Likewise, as the conditions of the enamel-metal interface change rapidly during flring, "adherence-promoting oxides" are used to help maintain the necessary conditions for the time required in commercial enameling. Surface roughness, although not necfor excellent adherence, was found to improve the apparent adherence when the bond between the enamel and the metal was relatively weak.

Surface cracks produced by indentation with a Vickers indenter were modeled as half-penny cracks extended by symmetric disk-shaped wedges. The wedge diameter was assumed to be equal to the hemispherical plastic zone size, and the wedge thickness was calculated by equating the wedge volume to the hardness-impression volume. The resulting equilibrium relation between the indentation load and the radius of the half-penny crack is in good quantitative agreement with similar relations obtained from fracture toughness calibration.