Enhancement of the Adhesion of Cordierite Glass-Ceramic Coatings on Molybdenum by the Deposition of Metallic Interlayers
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bond well to the Ni-Cr alloy layer that formed after firing. INTRODUCTION The formation of dense, stable glass or glass-ceramic coatings on substrates by cofiring is possible under certain conditions. Parameters that determine the success of the process include coefficient of thermal expansion (CTE) mismatch and adhesion between the coating and the substrate. A cordierite-base glass-ceramic was found to adhere well to stainless steel. However, the huge CTE mismatch produced a very large residual stress that caused the coating to spall quickly.[1] Molybdenum presented less of a problem because of the small CTE mismatch of around lx106/C.[2I One problem that had to be overcome is the poor adhesion of the glass on molybdenum. A stable oxide strongly adhering to the substrate is highly desirable as an interlayer to enhance adhesion with the glass-ceramic coating. While this was the case with chromic oxide on stainless steel, molybdenum oxide is volatile below the sintering temperature and good adhesion is not possible. To overcome this problem, an interlayer can be deposited on the molybdenum substrate before it is coated with a green layer of the dielectric material. The interlayer structure must have a good adhesion to the substrate. Interdiffusion during sintering would be preferable to further enhance bonding. The surface of 243 Mat. Res. Soc. Symp. Proc. Vol. 586 ©2000 Materials Research Society
the interlayer should provide an interface whose chemical structure and composition promotes bonding with the glass-ceramic. This will ensure a structural continuity across the interface resulting in a very high bond strength.[3] Metallic interlayers consisting of electrodeposited copper or nickel and chromium were selected as interface materials. Chromium was plated as the top layer. Although chromium can be plated directly on molybdenum, plating an initial layer of either nickel or copper made chromium plating easier to accomplish. Chromium readily forms an oxide layer that the glass-ceramic coating can adhere to. While the focus of the discussion is on the chemical composition across the interface and how it affected the adhesion, a quantitative measurement of the adhesion strength was also necessary to compare the different interlayers formed. There are different types of adhesion tests available[4,5] and there is little agreement on what fundamental parameter for adhesion strength should be used for comparison. It was decided upon that indentation testing was simple enough to perform and that sufficient work had been completed to quantitatively compare samples using interfacial fracture toughness, G., as a relative measurement of adhesion strength.[6,7] The details of the indentation experiments and fracture toughness calculations are discussed in more detail elsewhere. [8] Finally, quantitative chemical analysis of the interface can provide clues to why one interface may be stronger than the other. EXPERIMENTAL Molybdenum plate, 0.635 cm (1/4 in.) thick, was cut into various sized pieces, ground flat, and then light
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