Made to Measure…Seals

Who are you?: Paul Yates
What is your role?: Ph.D. Researcher
What?: Characterisation of glass-to-metal seals
I beg your pardon?: Glass-to-metal seals are useful when an electrical feed-through (generally feeding a wire into/out of a component) is required and the system needs to withstand higher pressures or temperatures than could be dealt with by a rubber or silicone seal. They are used in a wide range of components, ranging from spark plugs to light bulbs and from medical devices, like pacemakers, to aerospace applications.
OK, Why?: In order to make a glass to metal seal, the parts are placed in a jig to hold them in place and heated so that the glass will soften and can flow over the metal pieces and form a good contact. This generally means heating to between 500 and 1000 °C, to allow the glass to flow over metal components and fill all the tiny gaps. However, heating also gives the components enough energy to trigger reactions between the glass and metal. These reactions can limit the integrity and life of the seal.
And?: Glass-to-metal seals are routinely made using steel sealed with conventional silicate glasses (most normal everyday glasses are predominantly silicate-based; they are made from melting sand in many cases). Despite steel’s widespread popularity, however, it can sometimes be useful to use metals with a different property balance. The work at The University of Surrey, supported by AWE, has focussed on making seals with titanium (specifically the alloy Ti-6Al-4V) which is useful for its high strength to weight ratio. This means that lighter components can be made than if steel was used.
So what?: When titanium is joined to silicate glasses they react together to form titanium silicides which have a different coefficient of thermal expansion to the titanium and the glass. They are also brittle and poorly adherent, so when the seal cools back down to room temperature and the glass hardens, thermal stresses cause the seal to crack. This can be avoided by using borate-based glasses which only react very slowly with titanium so there isn’t time for too many products to form at the interface. However, the reactions between boron-based glasses and titanium are not well understood and have not been widely studied. My research programme is studying the interface between a boron-based glass and Ti-6Al-4V and that between the same glass and the ferrous alloy known as Kovar. The possibility of having both of these interfaces increases the range of complex applications for which these seals can be used. The research has required chemical and microstructural characterisation at length scales from millimetres down to nanometres.
Final Thought: A good macroscopic glass-to-metal seal requires interfaces that are made-to-measure in terms of the thermal properties, chemistry and microstructure of the glass, metal and reaction layers.