Thin film temperature sensors for gas turbine engines: Problems and prospects

Abstract

The increasing trend towards high-temperature, fuel efficient jet engines has led to the development of complex cooling schemes for the turbine blades. The measurement of temperature of the blade during operation, which is accomplished in conventional blade design by embedding wire thermocouples in the blade wall, causes serious structural and aerodynamic problems in the case of cooled turbines. In order to meet the requirement of temperature measurement in cooled turbines, it is desirable to develop surface-mounted thin-film thermocouples or a resistance thermometer. In the current state of the art of thin-film thermocouples, the sensing element consists of 2-μm-thick Pt and Pt 10% Rh thin-film elements deposited on the insulating surface of the blades and vanes. The insulator is developed by thermal oxidation of a MCrAlY coating which is deposited on the blade and vane surface in the current state of turbine technology. The understanding of the structural and thermoelectric stability of the sensor elements and of the insulating layer of Al₂O₃ in the hostile environment of a gas turbine requires an in-depth study of the metallurgical reactions occurring at the thin-film Al₂O₃ and Al₂O₃-MCrAlY interfaces and of the corrosive reactions on the surface of the metal film. The work presented in this review addresses the problems associated with obtaining highly adherent and insulating Al₂O₃ on the MCrAlY surfaces, adhesion of the sensor elements, thermoelectric stability of the sensors on contamination, and finally the development of a corrosion protection coating. The desired quality Al₂O₃ has been grown on NiCoCrAlY-coated nickel-based superalloy substrates by a combination of oxidation treatments. The interface-modified Pt and Pt/Rh films are deposite- - d on the oxide by a dc magnetron sputtering technique. The corrosion protection requirements involve deposition of Si-O-N and Si₃N₄ graded structures on the sensors by the plasma-assisted chemical vapor deposition process. Details of the electrical and metallurgical characteristics of the device at each stage of the coating/film growth have been analyzed by a number of surface sensitive and bulk analytical techniques.