Adhesion strength of cordierite glass-ceramic coatings on molybdenum substrates
- PDF / 398,703 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 199 Views
Cordierite glass-ceramic coatings do not bond to molybdenum directly because of the absence of a suitable transition layer. Dual metallic interlayers of copper/chromium and nickel/chromium were electroplated on the substrates to enhance adhesion. The adhesion of the glass-ceramic coatings was evaluated by indentation testing and a modified analytical method based on interfacial fracture toughness. Quantitative chemical analyses of the interfaces were used to explain differences in fracture toughness among samples with different interlayers. Of those tested, the best interface for adhering glass-ceramic coatings to molybdenum was found to be a 4 m chromium over 2 m copper on molybdenum.
I. INTRODUCTION
Research work on developing an electrostatic wafer chuck for high temperature processes has led to the consideration of cordierite-base glass-ceramics as dielectric coatings on molybdenum substrates.1 The mismatch between the coefficients of thermal expansion (CTE) of the two materials (approximately 1 × 10−6/C) is small such that residual stresses do not pose a problem. One issue that has to be overcome is the poor adhesion of the glass on molybdenum. Adhesion can be accomplished either by mechanical interlocking or by a chemical bond between the coating and the substrate, the latter being more desirable because of the higher bonding strength. Since the coating will preferentially bond to an oxide surface, it is therefore desirable to have an oxide intermediate layer, though not necessarily a discrete oxide layer. However, unlike some metals that form strongly adherent and stable oxides, e.g., chromic oxide on chromium and stainless steel, molybdic oxide is volatile at temperatures well below the sintering temperature range of 950 to 1000 °C. To overcome this obstacle, an interlayer can be deposited by appropriate means on the molybdenum substrate prior to coating with the dielectric material. The interlayer structure must have a good adhesion to the substrate; i.e., some interdiffusion may be desirable to promote bonding. The top surface of the interlayer should also provide an interface whose chemical structure and composition promotes bonding with the glass-ceramic. This ensures a chemical and electronic structural continuity across the interface and results in a stronger bond. For this reason, metallic interlayers consisting of electrodeposited copper or nickel and chromium were used in this study. Although chromium can J. Mater. Res., Vol. 15, No. 12, Dec 2000
http://journals.cambridge.org
Downloaded: 04 Apr 2015
be deposited directly on molybdenum, the process of depositing an initial layer of either nickel or copper proved to be more easily accomplished. A method to quantitatively measure adhesion strength and characterize the interface was also required to determine which interface would be best suited for this application. Currently there is little agreement about what the fundamental comparison parameter for adhesion strength should be. In many cases, little more than rank comparisons can be made bet
Data Loading...
