Date of Award
Master of Science in Chemical Engineering (MSChE)
Otto S. Gregory
With the incorporation of more high strength ceramic components into the gas turbine engine, there is a subsequent increase in efficiency and thrust-to- weight ratio since operating temperatures can be be significantly increased. Surface mount technology represents the best alternative to accurately measure the surface temperature of these ceramic components. Therefore, a thin film thermocouple system was developed and tested for hot isostatically pressed (HIP'ed) Si3N4 ceramic substrates.
Oxidation studies were performed on the hot isostatically pressed Si3N4substrates to determine the stability of these ceramics in a variety of oxidizing ambients at high temperature. These studies indicated that the growth kinetics of the oxide scale were strongly dependent on the oxygen partial pressure and the oxidation temperature. Parabolic growth kinetics were observed at temperatures >1300°C and oxygen partial pressures >0.02 atm, while logarithmic kinetics were observed below 1300°C and an oxygen partial pressures of 0.20 atm. An activation energy of 297 kJ/mol was determined for samples oxidized at temperatures in the range 1200°-1400°C in dry oxygen. The oxide scale generally consisted of amorphous SiO2and (101) α-cristobalite with small amounts of yttrium silicate incorporated into the growing scale. Scanning electron microscopy show a cracked oxide scale due to the β to α-cristobalite transformation at 273°C. This as-oxidized surface was not suitable for sputter-deposited sensor elements. Therefore, a surface treatment was employed to improve stability and adhesion, which consisted of thermal and sputtered oxide interlayers to act as a diffusion barrier to high temperature oxidation. Formation of a stable surface prior to sensor element deposition was a critical step in the fabrication of a reliable, accurate sensor.
Thin film thermocouples consisted of 1 μm thick type S thermocouple sensor elements sputter-deposited onto the specially prepared substrates. Lead wires were bonded to the films using parallel gap welding and an Al2O3overcoat was sputtered over the entire sensor pattern to prevent high temperature oxidation of the sensor elements. The thin film sensors were tested in a high temperature furnace where they were exposed to a variety of oxidizing conditions. Scanning electron microscopy of failed sensors revealed that emf drift and sensor failure was due to the oxidation of platinum and rhodium along with thermal sintering of the metal films. Aluminum oxide overcoats minimized the effects of both of these degradation mechanisms.
Aniolek, Gregg E., "Thin Film Sensors for High Performance Silicon Nitride Ceramics" (1994). Open Access Master's Theses. Paper 1351.