Microstructural control of new generation thermal barrier coatings via a sol-gel route : cyclic oxidation behaviour
Résumé
For several years, turbojet engineers have been searching to increase the efficiency of engines in order to enhance their performances. This can be achieved via the increase of engine service temperature which may dramatically damage the materials used, namely for turbine blades made of single crystal nickel based superalloys. To protect and extend lifetime of those materials, they are generally coated with thermal barriers either deposited using the so-called EB-PVD or APS process, respectively resulting in typically directional columnar and lamellar microstructures. In this study, an alternative process is proposed. It consists to elaborate thermal barriers by sol-gel route, a room temperature method based on soft chemistry, object of several research investigations, showing a real interest for the synthesis of materials with controlled morphology and composition [1,2]. Relative to elaboration step, an initial sequence consists in coating the superalloy with an yttria stabilised zirconia composite sol by dip-coating at controlled speed. This allows to obtain a homogeneous coating with an equi-distributed porosity. Two types of commercial dispersants have been investigated for the implementation in the composite sol, Beycostat C213 and the PVP whose active chemical groups are similar but formulations and physicochemical characteristics are different. After adjustements of the parameters of both synthesis sintering heat treatment, ceramic coating shows a satisfactorily controlled microstructure. According to the nature of the dispersant incorporated, it is shown that the width of micro-cracks of ceramics varies, this controlled micro-cracking being nevertheless favourable for accommodating thermomechanical constraints exerted on the TBC during cyclic oxidation. The next step includes the reinforcement of the micro-crack network. Indeed, filling grooves with additional material has the virtue to strengthen the TBC and ultimately increase the lifetime of the system. The shaping technique used to reinforce the network is the spray-coating. The sols are loaded in powder and the content is adapted to the width of cracks to fill. To evaluate the effect of the reinforcement, several characterisations including cyclic oxidation and microstructure analysis ( SEM, image analysis, surface profilometry) are carried out for the two systems based respectively on C213 and PVP dispersant. The cyclic oxidation experiments (1100°C-1h) allow to compare the behaviour of thermal barrier coatings processed through the sol-gel route to that of standard EB-PVD TBC. Preliminary promising results unambiguously show that lifetimes of both systems are very similar.