Ceramic thermal barrier coatings deposited with the electron beam-physical vapour deposition technique

Abstract

The deposition of ceramic thermal barrier coatings for high-temperature applications is of great interest. Particularly when the thermal stresses are superimposed by mechanical stresses, the metallic base material will expand much more than the ceramic top layer, so that the coating is not able to withstand the interfacial stresses and will spall off. Therefore such ceramic coatings should have a special type of microstructure to ensure that the complete system can fulfil the desired properties, e.g. thermal shock resistance. By physical vapour deposition (PVD) techniques the microstructure of coatings can be adapted with respect to the several applications. The electron beam (EB)-PVD technique is particularly suitable for the deposition of thermal barrier coatings (TBC), owing to the relatively high deposition rate and the possibility of influencing the microstructure of the coating. The energy of the condensing film varies depending on the temperature of the substrate. It is known that the mobility of the adatom increases within increasing temperature, so long as there is no adatom reflection at a too hot surface. In this paper the connection between the method of heating the substrates during deposition and the resulting microstructure of the coatings is investigated. The aim is to achieve coatings of a columnar microstructure with a thickness of about 200 μm, using the EB-PVD technique to deposit partially stabilized zirconia. Two different types of heat source are investigated, an indirect one using a resistance heater and a direct one using an electron beam. The other deposition parameters such as apparatus geometric, deposition rate, substrate temperature, gas pressure, etc., were kept constant. For both substrate heating methodes the temperature is measured by thermocouples. The type of heating source appears to play an important role, because different coating microstructures and thus various surface temperatures are obtained while measuring the same temperature at the back of the samples. Determination of different surface excitation, which obviously depends on the type of heating source, is also discussed. Some additional investigations are in progress to confirm the observations.