Zirconia thin-films for toughened ceramics

Partially-stabilized zirconia (PSZ) has been studied extensively, due to its high temperature stability and stress-induced tetragonal to monoclinic phase transformation, which can elicit enhanced fracture resistance. Applications include thermal barriers, high-k dielectric gate oxides, biomedical components, and solid oxide fuel cells, where not only temperature is a factor, but aggressive environments can compromise function. Research has advanced from using PSZ in bulk form to creating thin films that utilize the same material properties. PSZ, where the high temperature tetragonal phase is stabilized at room temperature, offers the ability to create a thin film that takes advantage of the unique properties of zirconia by improving the fracture behavior of brittle substrates. Yttria (3 mol%) stabilized zirconia (YSZ) can be deposited by radio frequency (rf) magnetron sputtering under varying deposition parameters to produce thin films with unique microstructures and properties. Most YSZ films are characterized by a columnar grain structure, and it has been found that inter-crystalline porosity and overall film density can be controlled by applying differing substrate bias during film deposition. Film stress can subsequently be manipulated over a broad range. Initial film stresses ranging from approximately 100 MPa tensile to 200 MPa compressive have been reproducibly produced. It has also been found that exposure of YSZ thin films containing measurable inter-granular porosity (10 - 100Å) to ambient conditions (25°C, 75% relative humidity) leads to a substantial increase in compressive stress of films (as much as 100 MPa). Thermal reversibility of this environmental aging effect suggests a water vapor absorption mechanism that might be tailored to specific applications.