This invention relates to methods for improving surface mechanical properties such as hardness and wear resistance of materials incorporating partially stabilized zirconia.
Partially stabilized zirconia ("PSZ") is a ceramic with unusual properties that make it attractive for a number of commercial applications. The review article "An Introduction to Zirconia" by Dr. R. Stevens (Magnesium Elektron, Ltd., Publication No. 113, June, 1983) provides an overview of PSZ and its properties.
Briefly, zirconia exhibits three well-defined crystalline phases or lattice modifications: the monoclinic, tetragonal, and cubic phases. The monoclinic phase is stable up to about 1170.degree. C., at which point it transforms to the tetragonal phase. The tetragonal phase is stable up to 2370.degree. C., at which point it transforms to the cubic phase. The cubic phase is stable up to 2680.degree. C., the melting point. The monoclinic phase has a lower density than either the tetragonal or cubic phase, and the transition from the tetragonal to the cubic phase results in a volume increase of three to nine percent. As explained below, this volume expansion has been used to improve the mechanical properties of zirconia.
PSZ is obtained by mixing zirconium dioxide with a stabilizing additives such as MgO, CaO, Y.sub.2 O.sub.3, or other rare metal oxides, which increase the stability of the cubic phase at room temperature. PSZ is usually made up of two or more intimately mixed phases, and can include a mixture of all three phases.
Several approaches to toughening PSZ are described at Section 5 of the above-identified Stephens article, including stress induced transformation toughening (Sections 5.2 and 5.3). In this approach, particles of metastable tetragonal phase PSZ are introduced into a ceramic body, either during initial fabrication or by heat treatment during or after sintering. Surface grinding is then used to create stresses in a surface layer, which cause the tetragonal phase particles to transform to the monoclinic phase. This transformation generates large compressive surface layer stresses which increase the toughness and strength of the surface layer. In many applications, such surface grinding can bring disadvantages, because it alters the exterior dimensions of the article being processed. For this reason, molding techniques cannot be used to form articles to finished dimensions when surface grinding is required to obtain desired surface mechanical properties of the article. Also, surface grinding is not suitable for all potentially useful surface contours.
A second approach is to anneal or age PSZ at elevated temperatures in order to obtain a desired crystalline structure. Garvie U.S. Pat. Nos. 4,067,745, 4,279,655, and 3,620,781, disclose a number of such aging processes, in which PSZ is aged at elevated temperatures for time periods ranging between 0.5 hour and 40 hours. Similarly, Yamada U.S. Pat. No. 4,344,904 discloses a process in which PSZ is annealed at an elevated temperature for 1-3 hours. Claussen U.S. Pat. No. 4,525,464 discloses a process in which PSZ is annealed for two hours to eliminate the monoclinic phase in a surface layer.
The processes described in these patents are directed to bulk heating steps in which heating is not limited to a surface region. Furthermore, the object of the Garvie and Yamada processes is to alter the crystalline form of the bulk of the article. The Claussen process attempts to eliminate the monoclinic phase in the surface layer. In all of these respects, the Garvie, Yamada and Claussen processes differ significantly from the present invention.