The Development of Dense Plasma Sprayed yttria Coatings on Graphite
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THE DEVELOPMENT OF DENSE PLASMA SPRAYED YTTRIA COATINGS ON GRAPHITE P.DIEZO*, R.SMITH0 , D.APELIAN0 , R.FARON*, G.REGAZZONI # Drexel university, Philadelphia, PA, USA. 'Uranium Pechiney. Paris-La DWfense, France. Centre de recherches de Voreppe, Voreppe, France.
ABSTRACT Plasma spraying of two yttria powders with different morphology (agglomerated and porous vs fused and dense) was investigated. Coatings were produced on steel and TZM (Ti-Zr-Mo alloy) substrates. Coatings microstructure, both degree of melting of particles and coating density, were measured by metallographic techniques and successfully related to plasma processing parameters. Microstructure were explained by particles behavior in the plasma jet. Yttria coatings were deposited on graphite using molybdenum as an interlayer to match thermal expansion differences and interfaces have been characterized by SEM observation.
INTRODUCTION Graphite has excellent mechanical properties at high temperature; however its use as crucible for melting and casting reactive metals or alloys (hafnium, titanium, uranium, zirconium, and alloys....) is limited by its high reactivity. Previous investigations (1,2] have stated the stability of refractory ceramics with molten reactive metals and graphite at temperatures up to 1400'C. Among them, yttria (Y20 3) has been recognized as the most promising material to protect graphite crucibles against molten metal corrosion. Yttria coatings on graphite were reported to be chemically stable in contact with molten metal, provided: - the temperature of use was below the potential reactions temperature. - the coating was dense (no interconnected porosity) and without limiting microstructural defects. Diffusion in the coating (carbon -> melt, metal -> coating) limits the potential application as well as mechanical stability of the materials system during thermal cycling. Thermal expansion mismatch (graphite = 4.5 gtn/m.°C, yttria = 9 ptn/m.0 C) has been shown to create interfacial stresses, leading to the need for interfacial layers which might accommodate the mismatch. An interlayer between graphite and yttria has been reported useful to enhance the chemical and mechanical stabilities of the material systems. Molybdenum was chosen as a first candidate as it forms a carbide layer (Mo 2C) which will slow carbon diffusion through the coating. Molybdenum also matches the thermal expansion differences between yttria and graphite ( CTE Mo = 6.5 im/m.*C). EXPERIMENTAL PROCEDURE DC Plasma spraying was selected as deposition technique for the development of yttria barrier coatings on graphite. Our objectives were to develop dense yttria coatings and a sufficient bond between them and the graphite substrate.
Mat. Res. Soc. Symp. Proc. Vol. 190. v1991 Materials Research Society
56
Two powders, processed by different methods were obtained : an agglomerated powder (= powder A) and a calcined powder (= powder B). Their morphology was characterized by metallography and flowability tests were carried out. Powder A was sprayed on glass slides
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