TWI Industrial Member Report Summary 903/2008
By N Ludford
Advanced ceramics are attractive engineering materials due to their chemical inertness, high hardness and dimensional stability at high temperatures. These properties are exploited industrially and ceramic components are used in gas turbines, internal combustion engines and chemical transformation plants. Whilst the use of ceramic components allows higher operating temperatures to be employed, the low fracture toughness of ceramic materials restricts their use to components that do not experience any significant tensile or shear stresses.
The properties that make ceramic materials attractive for high temperature applications (inertness and stability) also make them difficult to join. Their low reactivity means that there are few routes to generating a chemical bond. The most widely used method is the moly-manganese process. For this process to work, the ceramic must contain an intergranular glass phase. Such a phase is present in alumina of 96% purity and below which, by volume, represents the majority of engineering ceramic consumed, so this route is used 'widely' to metallise and braze alumina components. Other engineering ceramics such as zirconia or silicon carbide (SiC) do not contain an intergranular glass phase and so other methods of metallising them must be employed.
There are a number of metallic elements, termed 'active metals' that, under certain conditions, will react with ceramics; these include Ti, V, Zr and Hf. These may be applied by several methods, including sputter coating the ceramic with the metal, applying a slurry coating of the metal hydride that will decompose to the metal during a braze cycle, or the incorporation of an active metal in a braze alloy that is analogous to a conventional alloy, for example 63Ag-26.7Cu-1.25Ti. These active metals react with the surface of the ceramic to form a non-stoichiometric compound layer which has metallic properties and hence the braze alloy can then wet and chemically bond to it.
In addition to the difficulty of generating a chemical bond to ceramic materials, their dimensional stability and typically low coefficient of thermal expansion (CTE) means that significant stresses can build up between the ceramic and metal during a brazing cycle. In fact, the better the wetting between the braze and the ceramic the greater the stresses developed.
Silicon carbide, and specifically sintered SiC, retains most of its strength to ~1500°C; in addition, it is a very hard material with good wear characteristics. These factors make it a candidate material for use in many aggressive environments; however it has a low CTE, ~4.5 x 10-6/°C.
This project was undertaken in tandem with a modelling activity. The aim of this approach was to try and separate the chemistry from the design and to allow the critical fabrication parameters to be understood. The modelling activity is reported elsewhere but highlights that failure of a ceramic-metal joint is most frequently in the ceramic component. The failure stress primarily arises from thermal expansion mismatches between the ceramic, braze and metal components, but the ceramic fails due to its low fracture toughness and inability to plastically deform and thereby locally reduce stress at points of high stress intensity.
This report focuses on trials undertaken to join SiC to a range of metals for a number of different target application temperatures. It was anticipated at the outset of the project that, whilst some difficulties would be encountered in the preparation of SiC-metal samples, the most significant hurdle would be the generation of low melting point silicides particularly as the brazing temperature increased above 1100°C.
- Determine the feasibility of generating a chemical reaction between SiC and a range of metal braze alloys utilising titanium as an 'active metal'.
- Establish whether simple butt joints between SiC and a stainless steel or nickel alloy can be fabricated.