CERAMIC THERMAL BARRIER COATINGS DEPOSITED WITH THE ELECTRON BEAM-PHYSICAL VAPOR-DEPOSITION TECHNIQUE

The deposition of ceramic thermal barrier coatings for high-temperatur e applications is of great interest. Particularly when the thermal str esses are superimposed by mechanical stresses, the metallic base mater ial will expand much more than the ceramic top layer, so that the coat ing is not able to withstand the interfacial stresses and will spall o ff. Therefore such ceramic coatings should have a special type of micr ostructure to ensure that the complete system can fulfil the desired p roperties, e.g. thermal shock resistance. By physical vapour depositio n (PVD) techniques the microstructure of coatings can be adapted with respect to the several applications. The electron beam (EB)-PVD techni que is particularly suitable for the deposition of thermal barrier coa tings (TBC), owing to the relatively high deposition rate and the poss ibility of influencing the microstructure of the coating. The energy o f the condensing film varies depending on the temperature of the subst rate. It is known that the mobility of the adatom increases within inc reasing temperature, so long as there is no adatom reflection at a too hot surface. In this paper the connection between the method of heati ng the substrates during deposition and the resulting microstructure o f the coatings is investigated. The aim is to achieve coatings of a co lumnar microstructure with a thickness of about 200 mu m, using the EB -PVD technique to deposit partially stabilized zirconia. Two different types of heat source are investigated, an indirect one using a resist ance heater and a direct one using an electron beam. The other deposit ion parameters such as apparatus geometric, deposition rate, substrate temperature, gas pressure, etc., were kept constant. For both substra te heating methodes the temperature is measured by thermocouples. The type of heating source appears to play an important role, because diff erent coating microstructures and thus various surface temperatures ar e obtained while measuring the same temperature at the back of the sam ples. Determination of different surface excitation, which obviously d epends on the type of heating source, is also discussed. Some addition al investigations are in progress to confirm the observations. (C) 199 8 Elsevier Science S.A.