Advances in Thin Film Thermocouple Durability Under High Temperature and Pressure Testing Conditions

Lisa C. Martin and Gustave C. FralickNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135andKeith F. TaylorCortez III Service CorporationLewis Research CenterCleveland, Ohio 44135SUMMARYThin film thermocouples for measuring material surface temperature have been previously demonstrated onseveral material systems and in various hostile test environments. A well-developed thin film fabrication procedureutilizing shadow masking for patterning the sensors elements had produced thin films with sufficient durability forapplications in high temperature and pressure environments that exist in air-breathing and hydrogen-fueled burnerrig and engine test facilities. However, while shadow masking had been a reliable method for specimens with flatand gently curved surfaces, it had not been consistently reliable for use on test components with sharp contours. Thiswork reports on the feasibility of utilizing photolithography processing for patterning thin film thermocouples.Because this patterning process required changes in the thin film deposition process from that developed for shadowmasking, the effect of these changes on thin film adherence during burner rig testing was evaluated. In addition tothe results of changing the patterning method, the effects on thin film adherence of other processes used in the thinfilm fabrication procedure is also presented.INTRODUCTIONThe continuous development and evaluation of both space and aeronautics propulsion systems requires surfacetemperature measurement techniques that are reliable and durable in the hostile test conditions typically encounteredin burner rig and test engine environments. Reliable surface temperature data enables experimental verification ofanalytical techniques that have been created to determine surface temperature distribution. Such information is rel-evant to several research areas, including materials and structures evaluation, and fluid mechanics.Ideally, a surface temperature measurement device that is used in burner rig or engine tests should not disturbthe conditions at the surface during testing. Wire sensors mounted on the test surface may significantly disrupt theflow conditions. This can be overcome by installing the sensors in machined grooves in the test structure; however,the heat transfer and temperature distribution profiles may be compromised. In addition, this procedure can alter thestructural integrity of the test component which is often unacceptable in burner rig and test engine situations.Thin film thermocouples are intended to minimize the negative effects of placing a sensor on the surface of atest structure. These sensors do not require that the surface be machined and are many orders of magnitude thinnerthan wire sensors. As a result, thin film thermocouples add negligible mass to the surface and create minimal distur-bance of the gas flow over the surface.At NASA Lewis Research Center, thin film thermocouples were initially developed for superalloy turbine bladeapplications for air-breathing propulsion systems (ref. 1). The technology has since been extended to other high tem-perature material systems, including ceramic, intermetallic and ceramic matrix composite systems (refs. 2 to 3).Additionally, the technology has been demonstrated in hydrogen-fueled test environments (refs. 2 and 4). The testconfigurations have ranged from testing sensors on coupons in static furnace conditions to sensors on flat specimensand complex shaped components in burner rig and engine test environments.NASA/TM--1999-208812 1