Thin film thermocouples for internal combustion engines

The feasibility of fabricating thin film thermocouples on internal combustion engine hardware was investigated. The goal was to find a procedure that would be useful for the measurement of the surface metal temperature of valves, valve seats, combustion chamber surfaces, cylinder walls, and piston heads during engine operation. The approach pursued was to coat the engine hardware material with an aluminum‐containing, oxidation‐resistant ferrous alloy (FeCrAlY) which forms a thermal oxide layer with good electrical resistance. This thermal oxide was coated with a thin layer of reactively sputtered aluminum oxide and sputtered thin film type S thermocouple legs of platinum and platinum plus rhodium. This project was used to investigate the materials problems related to obtaining good adhesion in the metal/metal–oxide/oxide/metal laminate and the electrical insulating properties of the oxide. Thermal oxidation, reactive sputtering of Al₂O₃, and platinum alloy sputtering were investigated using optical microscopy, x‐ray photoemission spectroscopy (XPS), laminar adhesion testing, and the evaluation of high temperature electrical properties. A successful method for fabricating thin film thermocouples on internal combustion engine materials was developed. Adherent, electrically insulating aluminum oxide with a type S thermocouple was produced on a 10 cm long stainless steel bar. The fabrication techniques and results of tests run on that bar as well as on numerous small test coupons are presented.