Thick film and thin film techniques are differentiated in the manufacture of ceramic coatings.
Thick films (> 20 µm) are applied, for example, by thermal spraying. Plasma spraying has shown excellent results in this application. During plasma spraying, oxide ceramic powder or powder mixtures such as aluminium oxide, aluminium oxide / titanium dioxide, zirconium oxide and chromium oxide are melted in a plasma flame at approx. 10,000 °C and sprayed on a previously sandblasted metal surface.
Figure 73: The principle of plasma spraying
However, it is necessaryto ensure that before plasma spraying a fissured surface is created through sandblasting to which the ceramic particles can cling as they arrive.
Several layers are applied, so that a lamellar layer structure with good adhesion results.
Figure 74: Section through a coating showing the intermediate layer
Figure 75: Ground surface of a chromium oxide coating
An advantage of this process is that any metallic material (with the exception of magnesium) can be coated with ceramics. Oxide ceramic powders such as pure white aluminium oxide may be applied for electrical insulation and as a protection against wear, or zirconium oxide for thermal insulation. For increased wear resistance, mixtures of aluminium oxide and titanium dioxide in various proportions are used. Titanium dioxide stabilises the alpha phase of aluminium oxide and makes this ceramic coating tough, and resistant to impact and fracture. Its colour varies between grey, blue grey and black, depending on the mixture. Chromium oxide is preferred over metallic materials for bearings and pumps because of its very good tribological properties. An intermediate layer of nickel-chromium (the ‘bond coat’) improves adhesion.
The ceramic surface is brought to a reproducible roughness through subsequent machining. In most applications, a standard surface with an Ra = 1.5 to 2 µm has been found to be reliable. A shiny surface with a Ra 0.5 µm is achieved through hard machining.
For aluminium oxide, the hardness of the ceramic layers is around HV 1,800. In comparison, the hardness of hardened steel is around HV 700, and that of hard-chromed surfaces around HV 1,200.
Figure 76: Components coated for wear resistance
The advantage of an insulation layer 0.1 to 0.5 mm thick is the saving of space. It is, moreover, insensitive to heat, shock, and, to a certain degree, to aggressive media and wear. Due to the porous lamellar structure, stainless steel is recommended as the substrate in corrosive environments. An important application of ceramic coatings is their electrical insulation (up to approximately 500 V) at operating temperatures over 200° C, for example of heating elements. The ceramic layer adheres reliably up to 600 °C.
The metal parts should be free from visible blow-holes, scratches and pores, since the growth of the ceramic layer reproduces any surface irregularities. A finely turned or polished metal surface with Rz = 8 to 16 µm is recommended. The thickness of the metal parts should be a minimum of 1 mm in order to avoid heat build-up, which can deform the parts. Thin sheets or wires cannot therefore be plasma sprayed. The maximum length of the metal parts to be coated is dependent on the size of the chamber and extract ducting available, and is typically limited to 2 m.
Figures 77 and 78: Components with coatings for electrical and thermal insulation
Adhesion to edges is limited. For this reason, edges should be rounded to have a radius > 0.7 mm. Consideration of the clamping region (thread or pin) is necessary if a design appropriate for coating is to be achieved. Products can also be partially coated. In this case, the metallic surfaces are covered where necessary.
These coating procedures using ceramics are suitable for experimental batches of 1 to 10 as well as for production series of 10,000 or more.
The procedure offers a facility for rapid production at short notice. It allows metal parts to be created on CNC machines, coated and further processed within a few days.
Repair coatings are also of interest. Used ceramic coatings that have been damaged by shocks and impacts during heavy use can simply be sandblasted away. Expensive metal parts can be re-ground and coated with ceramic again.
Plasma sprayed coatings have proved themselves, for example, in textile and wire drawing machines.
Thin films (< 20 µm) are applied by PVD (physical vapour deposition) or CVD (chemical vapour deposition), by galvanic processes or by sol-gel processes. Here, for example, tools with the familiar gold coloured TiN coatings have found wide acceptance.