Chemically Derived Yttria-Stabilized Zirconia for Plasma-Spraying
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CHEMICALLY DERIVED YTTRIA-STABILIZED ZIRCONIA FOR PLASMA-SPRAYING
FAWZY G. SHERIF* AND H. HERMAN** *Akzo Chemicals Inc., Research Department, Dobbs Ferry, NY 10522 **State University of New York at Stony Brook, Department of Materials Science and Engineering, Stony Brook, NY 11794 ABSTRACT Yttria-stabilized zirconia powders were prepared by three chemical routes utilizing emulsion hydrolysis techniques. Metal alkoxides, acetates and mixtures of alkoxides and acetates were used as starting materials. Under controlled hydrolysis conditions, particles with different morphology and size were obtained. Spherical or granular particles in the range of 20-60u in diameter were formed. The grain size varied from 0.1-0.2u. The powders were applied successfully as plasma spray coatings. The chemical uniformity was demonstrated by the presence of 100% non-transformable tetragonal zirconia in the powder and in the coating. INTRODUCTION Although extensive studies were carried out on the chemical synthesis of ceramic oxide powders[l], only moderate efforts have been directed towards preparing chemically uniform oxide powders for plasma spray coating[2). A number of chemical methods developed in the past decade were utilized to achieve high purity and phase uniformity. Phase uniformity is essential for durable coatings in thermal barrier applications. One example is the yttria partially stabilized zirconia. The goal of this work is to demonstrate that chemically produced yttria-stabilized zirconia powder, under proper synthesis condition, mainly contains the non-transformable tetragonal zirconia crystalline phase needed for the plasma spray coating applications. Most commercial plasma-spraying powders are usually made by blending the individual oxides, melting, thermal spraying or rapid solidification. These processes often result in molecularly non-uniform compositions, with variable mixtures of cubic, tetragonal and monoclinic zirconia. EXPERIMENTAL Three chemical processes were carried process depended upon the raw materials used:
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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From Yttrium Acetate and Zirconium n-Butoxide-A Commercial yttrium acetate tetrahydrate was dehydrated at 110°C/4hrs. Anhydrous yttrium acetate (69.6g) was mixed with 1215g of zirconium n-butoxide butanol complex in a five-liter flask under nitrogen. The mixture was refluxed at 130°C until the yttrium salt went into solution. A liquid (250g) was then distilled at 114°C from the solution. The residue was cooled down to room temperature, 2200g of toluene was added, and the resulting solution was stirred vigorously. Water (213cc) was then added and the stirring of the resulting emulsion was continued until gelation occurred. The gel was filtered and dried in air at 125°C for four hours, then milled gently until all powder passed through a 140-mesh screen. The powder was calcined at 200°C, 400°C, 800°C, and 1100°C, each for two hours. The weight of powder was 356.8g. The powder was classified by sieving through a 500
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