Title

Ceramics with graded surfaces for contact damage resistance

Date of Completion

January 1999

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Two types of Functionally Graded Materials (FGMs) with gradations in the elastic modulus were fabricated and were shown to suppress Hertzian cone-cracks under spherical indentation. Both FGMs were based on silicon nitride , a highly successful structural ceramic material. Spherical indentation simulates contact damage, and as cone-cracks in these structures have been suppressed, their contact damage resistance has been enhanced. This research has been extended from work involving similar structures in alumina-based ceramics, and further validates a recently developed theory concerning elastic modulus-graded structures. ^ The first graded system investigated was glass-based, and the second was non-glass based. The first system was a perfectly elastic, model system, and was produced by infiltrating a low modulus oxynitride glass into a dense, high modulus, silicon nitride material to produce the gradient. The second, non-glass-based structure, graded from bulk silicon nitride (lower modulus) at the surface, to bulk silicon carbide (higher modulus) in the interior. This system was more complicated than the first, with processing defects as well as inherent residual stresses present. It also exhibited a quasi-plastic response to indentation compared to the perfectly elastic response of the model glass-based material. ^ This thesis describes the fabrication of these novel structures and their subsequent mechanical testing. Hertzian indentation has been used to determine the type and degree of contact damage exhibited by these structures under a variety of loading conditions and shows their enhanced contact-damage resistance. Vickers and Knoop indentation have been used to determine the hardness and toughness of these structures, and sliding pin-on-disc tests have been performed to determine their wear resistance. In addition, finite element modeling (FEM) of the stresses associated with the indentation of these graded structures has provided insight into the mechanism for their enhanced contact damage resistance. Modeling also enabled predictions for further gradient improvements, which form the basis for future work in this area. ^