Improvement of the Adhesion of a Ceramic Coating on a Ceramic Substrate
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Improvement of the Adhesion of a Ceramic Coating on a Ceramic Substrate K. Kishitake, H. Era, F. Otsubo, and T. Sonoda (Submitted 24 March 1997; in revised form 14 October 1997) The structure and adhesion of an alumina coating on a ceramic substrate with NiCrAlY alloy bond coating was investigated by heating at 1573 and 1673 K in the air. Phases of NiO, NiCrO3, NiAl2O4, αAl2O3, and Ni were revealed in a 100 µm thick bond coating on heating at 1573 and 1673 K. A veined structure was also detected in the coating heated at 1573 K. The adhesion strength of the coating was improved and reached approximately 20 MPa on heating at 1573 and 1673 K for 14.4 ks in air although the strength of the as-sprayed coating was only 2 MPa. The improvement of adhesion strength may arise from the formation of NiAl2O4 with a spinel structure at the interfaces of the top coating/bond coating/substrate coating system. The adhesion strength of the coating improved on decreasing the bond coating thickness and reached approximately 45 MPa for a 20 µm thick bond coating which was heated at 1673 K. Only NiAl2O4 oxide was formed in the bond coating.
Keywords
adhesion strength, alumina coating, bond coat, post treating, spinel
1. Introduction Engineering ceramics of high strength as well as corrosion resistance have been required recently in some industries. Cladding of different ceramics by brazing is one of the technologies (Ref 1-4) to meet this requirement. Thermal spraying is not often used due to the adhesion strength of the ceramic coating being generally low when sprayed directly on ceramics or metallic bond coatings. The adhesion of a ceramic coating on ceramic substrates with a nickel-base alloy bond coating is able to be improved by heat treatment in air (Ref 5). The strength of the ceramic coating increased by approximately 10 times (~15 MPa) from the assprayed condition when heat treated at 1273 K for 7.2 ks in air. This improvement is due to the formation of oxides which have a veined structure and grow into cracks or pores of the ceramic coating substrates. Thus, the improvement of adhesion is attributed to chemical and physical interactions at the interfaces. Therefore, it is anticipated that the adhesion of a ceramic coating will improve with further chemical bonding on both interfaces between the metallic bond coating and the ceramic materials on both sides by heat treatment above 1273 K in air. The change in strength of the ceramic coating when heat treated above 1273 K was investigated relative to the interface structures through the ceramic and bond coating, as well as the ceramic substrate.
K. Kishitake, H. Era, and F. Otsubo, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Institute of Technology, Kita-Kyushu 804, Japan; T. Sonoda, Graduate student of Kyushu Institute of Technology (currently: Kyoritu Alloy Co. Ltd., Japan).
64Volume 7(1) March 1998
2. Experimental Procedure A substrate of sintered alumina was plasma sprayed (Table 1) with gray alumina powder (SHOWA DENKO SHOCOAT K13, 45
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