O. J. Gregory, X. Chen, C. Cummiskey, University of Rhode Island, Kingston, RI
Ceramic strain gages based on reactively sputtered indium tin oxide (ITO) are being developed to measure both static and dynamic strain of rotating components in the hot sections of gas turbine engines. Such strain measurements are necessary to validate structural models and monitor the performance of newly developed materials. ITO strain gages exhibit excellent oxidation resistance and high temperature electrical stability. They can survive tens of hours of strain testing at temperatures as high as 1550oC in air. However, due to the limitations associated with the alumina constant strain beams used to evaluate the piezoresistive response of these ceramic gages at elevated temperatures, little or no work has been done to determine the response of these gages at strain levels beyond 400 με. With specially prepared ceramic substrates (laminates based on scandium doped zirconia) capable of being strained in excess of 1000 με without fracture, the piezoresistive response of the ITO strain gages can now be evaluated at strain levels approaching 1000 με. Therefore, the effect of strain on gage factor was systematically studied from room temperature to 1000oC, and the range of conditions expanded over which the difference in piezoresistive response (between tension and compression) could be studied. The results indicated that gage factor was somewhat dependent on strain at these higher strain levels, regardless of being loaded in tension or compression. However, when the ITO strain gages were sputter-coated with approximately 2 μm of alumina and tested in tension, the gage factor became independent of strain. As might be expected, this effect was not observed when the same alumina coatings were applied to ITO strain gages and subsequently tested in compression.
Summary: Ceramic strain gages based on reactively sputtered indium tin oxide (ITO) are being developed to measure both static and dynamic strain of rotating components in the hot sections of gas turbine engines. Such strain measurements are necessary to validate structural models and monitor the performance of newly developed materials. ITO strain gages exhibit excellent oxidation resistance and high temperature electrical stability. They can survive tens of hours of strain testing at temperatures as high as 1550oC in air. However, due to the limitations associated with the alumina constant strain beams used to evaluate the piezoresistive response of these ceramic gages at elevated temperatures, little or no work has been done to determine the response of these gages at strain levels beyond 400 ìå. With specially prepared ceramic substrates (laminates based on scandium doped zirconia) capable of being strained in excess of 1000 ìå without fracture, the piezoresistive response of the ITO strain gages can now be evaluated at strain levels approaching 1000 ìå. Therefore, the effect of strain on gage factor was systematically studied from room temperature to 1000oC, and the range of conditions expanded over which the difference in piezoresistive response (between tension and compression) could be studied. The results indicated that gage factor was somewhat dependent on strain at these higher strain levels, regardless of being loaded in tension or compression. However, when the ITO strain gages were sputter-coated with approximately 2 ìm of alumina and tested in tension, the gage factor became independent of strain. As might be expected, this effect was not observed when the same alumina coatings were applied to ITO strain gages and subsequently tested in compression.
