Ceramics at TWI

Coating techniques

Ceramic coating technology encompasses a bewildering array of coating types and deposition processes. Coatings that are frequently used include many oxide, carbide, and nitride ceramic types, deposited by commercial processes that are tailored to a specific ceramic material and a desired set of coating properties. They can, however, be divided into three very broad categories: vapour deposition, thermal spraying and wet processes. In turn these can be sub-divided into a number of generic techniques, as shown by Fig.1.

Fig.1. Deposition techniques for ceramic coatings
Deposition techniques for ceramic coatings Vapour Deposition

Sol-gel

Metallisation

Thermal Spraying

The vapour phase processes, such as those based on the various PVD and CVD techniques, are regarded as thin coating processes. They rely on atom by atom deposition from a gas phase, which is necessarily slow, and as a consequence economically attainable thicknesses rarely exceed 100µm. On the other hand, thermal spraying techniques deposit a heated powder, at high velocity, and build up a thick coating as individual particles deform and adhere on impact. This is regarded as a thick coating technique, producing coatings from a few tens of microns to several millimetres.

Numerous other techniques fall into the category of wet processing, and one of particular interest is sol-gel. This 'wet' technique offers a route to thin ceramic coatings using colloidal or complex organic solutions as ceramic precursors.

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