Pulsed Laser Mixing of Metal Overlayers on Ceramics
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PULSED LASER MIXING OF METAL OVERLAYERS ON CERAMICS R.K. Singh, N. Biunno and J. Narayan Department of Materials Science and Engineering
North Carolina State University Raleigh, N.C.27695-7916 ABSTRACT Pulsed laser mixing has been used as surface modification technique for the improvement in the mechanical properties of ceramics. Thin metallic layers of nickel were deposited on structural silicon nitride and were irradiated with Xenon Chloride (XeCl) laser pulses. The laser parameters were optimized to lead to the formation of mixed layers. The mixed interfacial layers were analyzed using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Rutherford Backscattering (RBS) techniques. Detailed heat flow calculations were performed to simulate the effects of intense laser irradiation on metal coated ceramic structures. The melt lifetimes and the interfacial temperatures obtained using these calculations, were applied to understand the laser mixing phenomena occuring in these layered structures. Thermodynamics of chemical reactions between the metal overlayers and the substrate were done to predict the formation of mixed interfacial layers during laser irradiation. INTRODUCTION Pulsed laser irradiation provides a controlled source of heat by which thin layers of metal on ceramic substrates can be melted, held in this molten state for short times, and then be rapidly quenched rapidly to obtain desired microstructures[I-2]. The subsequent interfacial mixing between the molten overlayer and the substrate provides good adhesion of the film which have been used as ohmic contacts for wide band gap semiconductors[3]. Another application of laser induced surface mixing lies in the improvement of fracture toughness of ceramic materials such as silicon nitride and silicon carbide[41. These materials have potential uses for high temperature applications but are restricted in use due to presence of surface flaws which greatly decrease the fracture strength of the material. The fracture toughness of these materials can be improved by subjecting the ceramic to a compressive surface stress which decreases the crack opening, thus requiring a higher level of stress for the crack to propagate from the surface. This is achieved by coating the ceramic with softer phase like nickel and subsequently laser irradiating the ceramic. Laser
irradiation causes thin overlayers to melt and mix with the substrate and filling of the voids on the surface. The formation of mixed interfacial layers leads to good adhesion with the substrate, thus producing a compressive stress on the ceramic substrate. It has beem seen that such treatments have led to improvements of 35 -50% in the fracture strength of silicon carbide[ 1]. In this paper, the interfacial mixing of nickel layers with silicon nitride by excimer pulsed laser radiation is investigated. The mechanism leading to an improvement in fracture properties of the metal coated ceramics after pulsed laser radiation is discussed. The themodynamics of the mixing phenomenon is
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