15.10 Ceramic Coatings

To recap, enamel is a hard, chemically resistive, thin layer of ceramic that protects metals and also can be used for decoration. Enamel was first used industrially on iron and steel sheets in 1850 in Germany.

Prior to coating, the component is usually prepared by “sand blasting” it with appropriate grades of sharp ceramic grit. The coating can be a single layer fused to the metal or a ground coat and cover coat. The ground coat is often blue as it contains cobalt oxide to help adhesion. Enamels can be applied as a slip and sprayed on; the metal heated and powder applied; or using the method called “electrophoresis”. Electrophoresis uses the phenomenon that when fine particles of ceramic are suspended in water most carry a negative charge, so if two electrodes at different voltages are immersed in the water, the particles are deposited on the positive electrode or anode. So a metal object used as the anode will become coated with the ceramic enamel with good adhesion. Enamels are fired at a relatively low temperature of 550 to 850 ºC for 2 to 15 minutes, but they are always fritted beforehand at temperatures some 50% higher.

An alternative coating method is thermal (flame) spraying that is used to deposit a wide range of ceramic coatings for many industrial applications. The ceramic powder to be applied is passed through an energy source (plasma or electron beam) where it is softened and sprayed in a plastic state on the component where it adheres to its surface. It solidifies on contact to form interlocking coating layers that can be built up to the required thickness. The coatings can be left in this “as sprayed” state or ground to the required finish. Little heat is transferred to the target component so its properties are not affected.

For iron and steel, the enamel compositions are typically: blue ground - feldspar 30%, borax 30%, quartz 30% with 0.5% cobalt oxide; white ground - feldspar 25%, borax 35%, quartz 20% and zirconia 6%; and for white cover – feldspar 35%, borax 25%, silica 20%, antimony oxide 2% and tin oxide 7%. Often seen items that are enamelled are school whiteboards and magnetic boards and external metal building facing, but their use is as diverse as farm storage silos and chemical processing tanks.

Ceramic coating on Sulzer
turbine blades

With the correct choice of ceramic powder and process, coatings can be hard, dense and wear resistant, or softer and able to withstand thermal shock. Less obvious applications include gas turbines, pumps, automobile engines (zirconia and alumina are used as coatings on diesel engine piston crowns, combustion chambers and valves), equipment for textile manufacturing, molten metal handling and temperature measurement, petrochemicals and printing. Titania (titanium dioxide), like alumina, produces a hard, dense coating resistant to corrosion and wear with excellent dielectric properties, while chrome oxide coatings are used in particularly abrasive applications.

Rolls-Royce ceramic coated
turbine blade

A particularly interesting application is in aero engine gas turbines where zirconia-based ceramic coatings are used on various components. To improve fuel efficiency and performance of aero engines, the operating temperature has to be increased, but this makes it difficult to find metals capable of withstanding the higher temperatures. Ceramics tailored for minimum long-term thermal distortion (known as creepage) are used as coatings for aero gas turbine blades, where sufficient temperature can be dropped across the coating to significantly increase the creep life of the underlying metal (a temperature reduction of 15 ºC through the coating doubles the creep life of the blade). One benefit is that it can be more cost effective to use a cheaper metal with a ceramic coating than to use an expensive alloy, but perhaps more importantly the absolute temperature that can be used in the combustion chamber can be increased. For example the latest single crystal nickel alloy turbine blades are able to operate around 1100 ºC, but this can be increased to more than 1200 ºC by using a low thermal conductivity ceramic barrier coating of yttria-doped zirconia deposited using an electron beam.

The market for high-performance ceramic coatings in North America in 2011 was $1.4 billion.