Close bonds in the new stone age

Tom Shelley reports on leading edge developments in joining ceramics

Combinations of ceramic and ceramic-to-metal can now be bonded together to produce fabrications that a few years ago would have been thought impossible to make. Uses are mainly in research, aerospace and defence, medical implants and machine tools, all applications requiring maximum precision and reliability. Morgan Technical Ceramics, at its new facility in Rugby, tackles design and production problems involving the fabrication of fairly complex parts involving ceramics that most other companies wouldn’t even contemplate. As general manager Roy Mason explains, the majority of projects relate to customer designs where there is often only a conceptual drawing or just a brochure from which to work. Even when the customer has a fully toleranced drawing, says sales engineer Martin Davidson, “we very often find that not many engineers are familiar with ceramic assemblies, so, if possible, we invite them here”. Fortunately, the company has a number of technologies for bonding ceramics and ceramics to metals, with their own particular tweaks and techniques to get them to work better. Notable among past achievements are an electrical brake for a neutron generator for the Isis facility at the Rutherford Appleton Laboratory, a short length of wide, but thin, walled hexagonal tube with a metallised edge for a space telescope for NASA; and porous, but highly reflecting, alumina bonded to dense alumina, with joints of imperceptible width, for very high powered lasers. The part for the Isis, chosen because she was an ancient Egyptian goddess reputed to be able to bring the dead back to life, consists of two metal rings, with an intervening alumina ring to provide high voltage insulation. The joints are brazed, something the company manages in a number of ways, including a method patented for producing implantable feed-throughs. The usual process starts with screen printing or painting on a proprietary molybdenum ink, which is followed by sintering in a reducing atmosphere, nickel plating - because the brazes used do not wet molybdenum – further sintering in a reducing atmosphere and then brazing using a metal pre-form. A typical braze to bond to ‘Kovar’ would be a silver copper eutectic foil. For the implantable feed thrus, which are made by the company’s Alberox products division in New Bedford, Massachusetts, the process starts by sputtering a biocompatible thin film on to the ceramic, followed by bonding metal to it using a gold braze. The products are used in various circulatory assist devices, including one produced for HeartWare, for patients with congestive heart failure. If ceramics are to be bonded to ceramics or metals, provided service temperatures are not too high and there are no out-gassing avoidance requirements for use in a hard vacuum, the company bonds its assemblies with Henkel Loctite epoxy adhesives. The edge of the hexagonal tube section for NASA was metallised using Morgan’s process but all general manager Roy Mason would say about its manufacture was that it was done “with difficulty”. Round rings are made in the factory all the time – by forming green pot shapes, machining them to dimension, slicing them up and firing the rings. However, firing a hexagonal ring section was problematic, it seems, because of uneven shrinkage during firing. The company is also able to bond ceramics by glaze bonding and thermal diffusion bonding. Design work is undertaken using Autodesk Inventor and Ansys FEA software. Pointers * Ceramic to metal bonds are made either by a thick film metallising, nickel plating and brazing process; by a patented biocompatible sputtering and brazing process; or, if service conditions permit, by adhesive bonding using epoxy * Ceramic to ceramic bonds can be made using brazing, glaze bonding, adhesive bonding or by using epoxy adhesive