Stabilization of Ceramic Strain Gages to Temperatures beyond 1500°C

摘要

Thin film strain gages based on indium-tin-oxide (ITO) are being developed to measure static and dynamic strain at temperatures approaching 150°C. These ceramic strain gages exhibit excellent oxidation resistance and high temperature stability, surviving tens of hours of strain testing at temperatures as high as 1530°C in air. A thickness dependence of the active ITO strain elements on the high temperature stability and piezoresistive response of the ceramic strain gages was observed. Thick ITO strain elements exhibited a relatively large piezoresistive response at temperatures approaching 1500°C and beyond. In addition to the ITO thickness, it was determined that aluminum doping of the ITO strain elements had a profound effect on the high temperature stability of these ceramic strain sensors. Electron spectroscopy for chemical analysis (ESCA) depth files showed that interfacial reactions between ITO and aluminum were responsible for its enhanced stability via the inter-diffusion of aluminum and indium at these temperatures, which produced a very stable ITO/Al{{sub}2O{sub}3 solid solution. To determine the nature of the interfacial reaction product, ITO films were deposited onto high purity Al{sub}2O{sub}3 and AlN surfaces and were thermally cycled to 1500°C. The reason for including AlN as part of this study was to eliminate sources of oxygen transport to the interface, so that aluminum-indium interactions alone could be studied. ITO films were deposited on Al{sub}2O{sub}3 and AlN surfaces by rf sputtering. In some instances, platinum was deposited on the surface prior to the ITO deposition. ESCA depth profiles showed that an interfacial reaction had occurred between the ITO and the Al{sub}2O{sub}3 and/or AlN. This observation was based on the presence of two new indium-indium peaks at 448.85 and 456.40eV. These peaks correspond to the indium 3d5 and 3d3 binding energies, the values of which are significantly higher than these associated with stoichiometric indium-tin-oxide films. The results of our efforts to improve the high temperature stability of ITO strain gages are presented as well as the piezoresitive response of selected ITO strain gages in the temperature range 25- 1550°C.