Glass-metal bonding is generally believed to occur via two mechanisms: mechanical and/or chemical.
Mechanical bonding occurs because of the nature of the roughened surfaces. In shear, surface roughness provides a frictional force which must be overcome to separate the parts. The glass can penetrate surfaces, pores or cavities thus providing further surface area for interlocking between the structures.
Chemical bonding refers to atomic or molecular bonds between materials. Metal surfaces are not readily compatible with melted glass. To produce an intimate bond between the materials commercially, the metal must be pre-oxidised before application of the glass.
A typical procedure for production of a bond is as follows:
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The metal piece is heated in an oxidising atmosphere in order to evaporate gas-producing species and remove hydrocarbons.
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The metal piece is then heated in a wet hydrogen atmosphere in order to eliminate the oxide layer formed on the surface since its composition and thickness are not controlled. The work piece thus presents a pure metal surface which can be properly oxidised.
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Controlled oxidation in air atmosphere is required to produce an adherent oxide for the glass to dissolve and to which it can strongly bond. A precise control of the atmosphere, temperature and time for the oxidation step is required to optimise the oxide layer thickness, uniformity and the oxide species which define the glass-to-metal bond reliability.
The most common glasses used are boro-silicate, although frequently lead-silicate is used on copper alloys and stainless steels. Increasingly though, lead is being replaced with barium because of the potential health hazards. The glasses are applied as frits, paste or solid preform and a range of heating methods, such as electrical, RF or flame, may be used to soften the glass to the correct viscosity for joining (usually under a slight pressure to provide intimate contact with the metal). Graphite and stop-offs are used for alignment, since glasses do not readily bond to these materials and flow is inhibited. The atmosphere for joining is usually a forming or protective gas to promote wetting and adhesion, with an annealing stage generally included during cooling to inhibit development of residual stress.
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