Title

Contact-damage resistance in alumina-based ceramics with elastic-modulus-graded surfaces

Date of Completion

January 1999

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Hertzian (spherical) indentation experiments were carried out in a graded alumina-glass composite whose Young's modulus (E) increased with depth (z) beneath the indented surface. An in-situ processing method involving impregnation of a dense, fine-grained alumina by an aluminosilicate glass was employed to fabricate such a composite. With this technique, a monotonic, unidirectional variation in Young's modulus of as much as 50% was introduced over a distance of approximately 2 mm, while keeping the coefficient of thermal expansion (CTE) and the Poisson ratio (ν) for the glass and the alumina nearly the same. The macroscopically graded, elastic composites so produced with nearly full density have no macroscopic, long-range residual stresses following processing. The unidirectional variation in Young's modulus under the indenter is shown to fully suppress the formation of Hertzian one cracks. Without these elastic-modulus gradients, cone-crack formation was observed in bulk glass and alumina. Finite element analyses of spherical indentation on elastically graded substrates were also performed to develop a quantitative understanding of the experiment trends. ^ The wear properties of those composites were also measured, and it was found that the wear resistance is high for shorter sliding time, but low for longer sliding time. It is reasoned that the present innovations, involving functionally-graded surfaces and their in-situ processing, provide new possibilities for enhancing certain contact-damage and wear resistance characteristics in various ceramic materials for a broad range of engineering applications. ^