Heat Treatment of Zirconia Powders with Different Morphologies Under Thermal Plasma Conditions
- PDF / 2,296,291 Bytes
- 9 Pages / 420.48 x 639 pts Page_size
- 111 Downloads / 198 Views
HEAT TREATMENT OF ZIRCONIA POWDERS WITH DIFFERENT MORPHOLOGIES UNDER THERMAL PLASMA CONDITIONS.
M. VARDELLE, A. VARDELLE, A. DENOIRJEAN AND P. FAUCHAIS Laboratoire "Cdramiques Nouvelles" URA CNRS 320 123, rue Albert Thomas, 87060 Limoges, France
ABSTRACT Plasma sprayed ceramic coatings are mainly used for the protection of metal components in diesel engines, gas turbines and aeroengines. Increasing demands for better thermomechanical properties at higher temperatures require well defined and reproducible spraying conditions. The way the powder is manufactured plays an important role in the quality of the coatings. This paper presents a study carried out on three grades of zirconia powders manufactured by different processes. The heat treatment of the powders has been monitored within the plasma jet by in flight pyrometry and laser velocimetry. The particulates melting is controlled by observation of individual particulates splats collected on glass slides. A mathematical model of plasma particulates heat transfer allows to explain the obtained results.
INTRODUCTION Ceramic thermal barrier coatings (TBCs) deposited by plasma spraying at ambient atmosphere are now widely used in diesel engines, gas turbines and aeroengines. If the conditions of adiabatic engines is by far not yet reached TBCs have extended the service lifetime of the engines, allowed operation with lower quality fuels and reduced the cooling requirements [I to 3]. In most cases, TBCs consist of a duplex structure composed of a superalloy bondcoat (usually sprayed with the LPPS process) and a 7% wt yttria stabilized zirconia (partially stabilized zirconia : PSZ) coating. However, many works are still under developement to improve the mechanical properties and thermal shock resistance of TBCs. These properties, among other parameters, depend on the molten state of the particulates upon impact onto the substrate or the previously deposited layers[4] and this paper is focused on the study of the molten state of the PSZ particulates. The used plasma torch was optimized at the lab [4 to 7] and for an Ar-H 2 mixture and given working parameters,the melting of three powders, differing by their morphologies (fused and crushed : FC, agglomerated :A, agglomerated and sintered : AS), was studied. For these three powders, three size distributions were also examined. The melting conditions were followed on one hand in flight by measuring the flux, velocity and surface temperature distributions of the powders, and on the other, by examination of the cross sections of the powders collected in water after their flight in plasma jet, or of the splats resulting from the interception of the particulates by glass slides. EXPERIMENTAL PROCEDURE The heat treatment undergone by particulates within the plasma jet was investigated by measuring in flight particulates velocity and temperature distributions. A schematic diagram of the experimental set-up is shown in figure 1.
Mat. Res. Soc. Symp. Proc. Vol. 190. ©1991 Materials Research Society
176
Figure 1 : Schematic of
Data Loading...
