What are ceramic coatings?
Ceramic coatings are thin films made of ceramic materials. These materials represent the class of materials which usually consist of a metal and an oxygen, carbon or nitrogen atom. Examples are TiN (titanium nitride), SiC (silicon carbide), Al2O3 (aluminium oxide).
[MAG x2,200, ACCV 20kV] SEM micrograph of a dense SiC coatings on a porous substrate.
The characteristics of ceramic materials are usually very different to those of metals or organic components (plastics). They are extremely hard, have an extremely high melting point, over 1500°C, and are chemically very stable. Furthermore most ceramics are highly insulating.
There is however a disadvantage, they are very difficult to machine, and have generally very poor mechanical properties. This makes these class of materials not suitable for many applications.
By applying a ceramic coating on a for instance a steel surface it is possible to give a product best of both characteristics: The good mechanical properties of the steel, and the high chemical stability and hardness of the ceramic. Some characteristics of ceramic materials are given in table 3.
Table 3: Some characteristics of ceramic materials (bulk)
Figure 5 shows the hardness of some ceramic (bulk) materials compared to some metals. The hardness of a coated metal is not equivalent to that of the coating material due to the so called “egg-shell“ effect.
Although the egg shell is many times harder than the inside, if enough force is applied on the surface the egg shell will break due to the weak material inside.
The tickness of the coating that can be applied depends mainly on the thermal expansion coefficient difference between substrate and coating material. The larger the difference, the thinner a dense coating can be applied.
Figure 5: Vickers hardness of some ceramics and steel.
The Vickers hardness is expressed in kg/mm2.