Development of Embedded Thermocouple Sensors for Thermal Barrier Coatings (TBCs) by a Laser Cladding Process

摘要

Thermal barrier coatings (TBCs) are now being widely used on gas turbine engines to lower thesurface temperatures of metallic substrate from extreme hot gas stream in combustor and turbinecomponents. The thermally grown oxide (TGO) growth rate plays an important role in the lifetime of TBC systems. The accurate real-time monitoring of bond-coat/ 8YSZ interfacetemperature in thermal barrier coatings (TBCs) in hostile environments opens large benefits toefficient and safe operation of gas turbines. A new method for fabricating high temperaturethermocouple sensors which can be placed close to this interface using laser cladding technologyhas been developed.K-type thermocouple powders consisting of alumel (Ni2Al2Mn1Si) and chromel (Ni10Cr) werestudied as candidate feedstock materials. A thermocouple sensor using these materials was firstproduced by coaxial continuous wave (CW) or pulsed laser cladding process onto the standardyttria partially stabilized zirconia (7~8 wt.% YSZ) coated substrate and afterwards embeddedwith a second YSZ layer deposited by the atmospheric plasma spray (APS) process. The processparameters of the laser cladding were optimized with respect to the degradation of the substrate,dimensions, topography, thermosensitivity and embeddability, respectively. Infrared cameraswere used to monitor the surface temperature of clads during this process.The manufacture of the cladded thermocouple sensors provides minimal intrusive features to thesubstrate. The dimensions were in the range of two hundred microns in thickness and width forCW laser cladding and less than 100 microns for pulsed laser cladding. Additionally, continuousthermocouple sensors with reliable performance were produced. It is possible to embed sensorsmanufactured by CW laser cladding rather than by pulsed laser cladding due to the limitedbonding strength between the clads and the substrate. Periodically droplets were formed alongthe clads under improper parameters, the mechanism to this is discussed in terms of particle sizedistribution after interaction with the laser beam, melts duration and Rayleigh’s theory.To sum up, laser cladding is a prospective technology for manufacturing microsensors on thesurface of or even embedded into functional coatings that can survive in operation environmentsfor in-situ monitoring. Production of sensors within thermal barrier coatings (TBCs) increasesthe application field of the laser cladding technique