Ceramics at TWI

Metallisation of ceramics

Metallising is the surfacing process whereby a metallic coating is applied to the surface of a substrate (plastic, ceramic, glass or metal) through the evaporation of a resistance heated metallic source. The process usually takes place under vacuum.

Typical coating materials include aluminium, copper, gold and silver, and the coatings may be applied to provide corrosion protection, electric shielding, or just for decorative purposes.

Metallisation of ceramics improves the ability to braze them, by providing a surface which wets more easily than a non-metallised ceramic surface.

Engineering ceramics are inherently difficult to wet with conventional brazing filler metals. Another problem is the difference in coefficient of thermal expansion (CTE) between the ceramic and the brazing filler metal. Due to the poor thermal conductivity and tensile strength of ceramics, care has to be taken to prevent cracking in the joint.

There are a number of established processes that promote wetting of the ceramic by the braze alloy, these are:

molybdenum-manganese process
active filler metal brazes
vapour sputter coating

Comparison of active metal brazing with Mo-Mn process

Molybdenum-Manganese Process

The most widely used approach in industry for brazing alumina involves the molybdenum-manganese (Mo-Mn) process. This process metallises the alumina surfaces by coating it with a mixture of metal and oxide powders, followed by a sintering treatment at high temperature, and then a plating operation. The alumina surfaces can then be brazed with a number of standard braze filler metals.

ASTM Leak Test Samples

Active metal braze

Engineering ceramics can be directly brazed to other ceramics and to metals without prior surface coating, using active filler metals. To promote wetting on the ceramic, elements such as titanium, hafnium and zirconium are added to standard braze alloys. The most well established active metal braze is based on the Ag-28%Cu eutectic, with titanium as the active element, such as Ag-27.5%Cu-2%Ti and Ag-35%Cu-2%Ti. Other active metal braze alloys include elements such as Au, Ni, V and In. These braze alloys have different properties, such as brazing temperature, flow characteristics, CTE and corrosion resistance.

Active filler metals can be used to braze all of the engineering ceramics, including: alumina, silicon carbide, silicon nitride, boron nitride, aluminium nitride, zirconia, diamond, tungsten carbide, and sialons.

Silicon Nitride turbocharger rotors
(Courtesy of NGK/NTK)

Vapour sputter coating

Another approach to metallising the ceramic surface is the use of a vapour deposited metal coating. The process involves depositing a thin (several microns) coating of a reactive metal, such as titanium, onto the ceramic prior to brazing. This promotes the wetting of non-active braze alloys onto the ceramic.