For high temperature applications (up to ~1400°C), a number of inorganic adhesives are available which allow bond integrity to be retained to much higher temperatures. Unlike conventional cements, which are based on sand and water, these ceramic adhesives are based on alkali silicates and various metal phosphates, usually with a ceramic filler powder. On heating, dehydration and chemical reactions occur within the adhesive which result in a refractory bond being formed. Thermal expansion mismatch between the adhesive and adherends (materials to be joined) is controlled by correct choice of filler powder.
Such adhesives can be used to join ceramic to ceramic and ceramic to metal with a temperature capability over 1000°C. However, the ceramic adhesive can be very friable and have low resistance to vibration and high stress. Consequently, such bonds are suitable in low stress or corrosive environments.
To achieve a bond between a ceramic and a metal, the metal may first require pre-treatment. Similar to glass-metal sealing, an oxide surface on the metal is usually required.
The adhesive may be applied via syringe, brushing or screen printing, and the bond is usually left under load (where possible) at room temperature to initiate slow curing. This is then accelerated by furnace curing until the adhesive is fully set. The curing temperature is dependent upon the binder solution of the adhesive and may be as low as 250°C, although some adhesives require temperatures above 1000°C before complete curing is achieved.
The bond produced between the substrate and adhesive is chemical and therefore very strong; however, little ductility is achieved and therefore joints between materials having a large coefficient of thermal expansion (CTE) mismatch may be difficult.
- low cost materials
- easy to use and to scale-up for industrial use
- high temperature capability joints
- cannot cope with large CTE mismatch
- are only good for low stress applications
- joints not leak-tight
- surface preparation is critical