Producing solid ceramic components is not always the best approach to solving a wear or corrosion problems. In many cases, taking the original metallic part and applying a coating can be the best solution.

Coatings can vary from a few to several hundred microns and be deposited by different means.

The coating, its thickness and means of deposition will depend on the final use of the components and the environment it has to resist.

There are a wide variety of surface coatings available. Brief details of the most commonly used coatings are given below. If you have a specific coating requirement please contact Dynamic-Ceramic for expert assistance.

Extending Component Life

It is a fact of life that many components are deemed to be worn out when their surfaces have degraded beyond a predetermined limit. This limit may vary from the appearance of minor pitting or scoring marks in bearing surfaces to the removal of several millimetres of material from the bucket of an excavating tool.

However, the useful life of many components, which are unsuitable to be made from advanced ceramics, may be extended by coating with a material tailored to resist the particular environment in which the component is working. These include thermal barrier coatings as used in gas turbines and on piston crowns in large diesel engines; low friction and anti-seizure coatings as used in lubricant free bearings; wear resistant or "hard-facing" coatings as used for the treatment of valves in internal combustion engines and finally corrosion resistant coatings as used in the chemicals industry.

Coatings may be applied by many different techniques with coating thickness' varying from several microns to several millimetres. Thin coatings are usually applied by Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD) and Chemically Formed Processes (CFP) with other techniques e.g. High Velocity Oxy-Fuel (HVOF), plasma and flame spraying together with Plasma Transferred Arc (PTA), weld over-laying and laser cladding, being used to deposit thicker coatings.

Some of these techniques are also capable of building-up worn components to their original tolerances, thus reducing both waste and replacement costs.

Physical Vapour Deposition (PVD)

This technique is used to deposit thin layers of material to reduce friction and wear, or to act as a diffusion barrier (to stop cold welding for example).

Figure 1 shows a schematic of the process. Titanium Nitride (TiN), for example, is deposited in partial vacuum by feeding ionised titanium into a plasma of ionised argon and nitrogen. The operation occurs at a temperature of between 350 and 450°C with the resultant TiN growing on the surface of the work piece.

Figure 1 - Physical Vapour Deposition

		Materials such as Titanium Carbo Nitride, Chromium Nitride and Tungsten Carbide/Carbon can be produced by changing the material in the crucible and the reactive gases. Figure 2 shows a section through a PVD coating, from which it can be seen that the coating is thin, it is well bonded to the substance and that it contours accurately the original surface. Because the process is carried out in a vacuum chamber there are issues of size limitation of the work piece. In addition the process is effectively line of sight so deep holes and bores can not easily be coated.
Figure 2 - Section Through PVD Coating

Chemical Vapour Deposition (CVD)

This is a high temperature process (1000°C). It is carried out in vacuum chamber where the disassociation of gases which then react at the work piece surface to form a solid coating. This is the process by which Diamond and diamond like carbon (DLC) coatings are produced. The greatest problem with the technique is the high temperatures that are required.

Chemically Formed Processes (CFP)

These coatings, which are usually between 10 and 100 microns thick, are typically used for improving the corrosion or wear resistance of a component. The component is covered in the appropriate precursor materials before heating to between 350 and 600°C when the desired chemical reaction takes place and a thin coating is formed. This process is repeated several times until the required thickening is achieved.

Advantages of this process include that it is not a line of sight process and large components (several metres long) can be coated. The coatings are chemically bonded to the substrate and are fully dense (i.e. no porosity) which affords excellent corrosion resistance.

Thermal Barrier Coatings

The efficiency of gas turbine engines is dictated by the maximum temperature that the turbine rotors can sustain during continuous operation.

Such a limitation is usually imposed by the mechanical properties, particularly the creep resistance of the turbine blade material.

Improvements to the composition of the superalloy series, internal air cooling, and in the extreme case, directionally solidified blades and single crystal blades have all been employed to extend the technology of the metal turbine blade to its limit.

Ceramic blades have been manufactured, but are brittle and liable to failure by mechanical and thermal shock arising out of the extreme operating conditions.

However, if a thin coating of ceramic can be applied to a metal blade, the engine temperature may be increased by 50-200°C without the metal temperature increasing; the ceramic acting as a thermal insulating barrier.

In this manner the efficiency of an engine may be increased by ~ 6-12% thereby saving $250,000 per year in fuel costs on a large aircraft engine. The economic inducement to find a successful coating is therefore high.

Figure 3. Schematic of the structure of a two layer thermal barrier coating on a turbine blade surface together with a temperature profile

At present, coatings are applied as a duplex structure, shown schematically in the diagram, figure 3.

The thermal barrier is made up of plasma spayed ceramic layer, up to 0.6mm thick, over an intermediate layer, up to 0.7mm thick, which usually consists of a metallic bonding coat, or of a graded composition ceramic layer designed to minimise thermal mismatch of the adjacent layers.

Desirable properties of the ceramic barrier coating include a high thermal expansion coefficient, low thermal conductivity, chemical stability in the gas turbine environment and thermal shock resistance.

Plasma sprayed zirconia compositions have been investigated and the most suitable composition was found to be ZrO2-6 to 8wt% Y2O3, which formed an adherent layer with the Ni, Cr, Al, Y, bond coat.

The most durable coatings were found to be formed from a partially stabilised zirconia composition.

Investigation of the structure of the plasma sprayed coating has been undertaken. As would be expected from the rapid cooling rates, the structure is non-equilibrium and extremely fine.

Furthermore the structure varies considerably over short distances, indicating considerable fine scale inhomogeneity within the thermal barrier coating.

X-ray diffraction studies indicate that both the partially stabilised composition (ZrO2-8 wt % Y2O3) and the fully stabilised but inhomogeneous ceramic coating (ZrO2-20 wt % Y2O3) consist of tetragonal and cubic phases with minor quantities of monoclinic present in the lower Y2O3 material.

Thermal Spraying

Thermal spraying of which flame spraying, plasma spraying and high velocity oxy fuel (HVOF) are all examples work on the same fundamental principle. Figure 4 is a schematic of the process, which consists of a heat source, a means of introducing powder particles into the heat source and a way of accelerating the now semi solid particles toward the target.

Upon hitting the target the particles deform and "key-into" mechanically to defects in the substrate. Other particles follow and a layer is built up.

Figure 4 - Schematic of Thermal Spraying

Considerable heat is imparted to the substrate and after deposition the coating can crack as the component cools. This is related to the different coefficients of thermal expansion between the metallic substrate (high) and the surfacing ceramic (low).

To avoid this cracking a bond coat is sometimes used. Figure 5 is a cross section through a ceramic coating showing this bond layer together with other features and defects. As the process is often performed in air, impurities (or oxidation products) can be introduced into the layer; it is also possible to see the deformations caused as the semi solid particles impact upon the previously deposited material. One can think of the process as throwing cowpats against a wall.

Figure 5. Coating Features and Defects

Flame Spraying

This is the most basic form of thermal spraying and often involves an oxy-acetylene burner as the heat source.

Advantages of such a system are that it is cheap, moderate deposition rates can be achieved (0.5-0.6 kg/hour) and manual operation of the spraying unit can be employed.

Disadvantages of the system include low particle velocities (40-200 m/s) leading to low bond strengths, high porosity (10-15%) and a high impurity level.

Plasma Spraying

This system, as the name implies, uses an electric arc as the heat source which is much hotter than the temperature produced by an oxy-acetylene flame. This means that higher melting point materials can be deposited at higher velocities (200-400 m/s) leading to moderate bond strength in Air Plasma Spraying and high bonded strength in vacuum plasma spraying.

In addition, porosity contents are lower at 2 to 5% as are impurity levels, especially in the case of vacuum plasma spraying.

Disadvantages are that the process requires more expensive equipment and that it is not suitable for manual operation, i.e. some form of manipulation or robotic system is required.

High Velocity Oxy Fuel (HVOF)

This system is a refined oxy-fuel burner which uses advanced nozzle design technology to accelerate the gas/particle stream to achieve particle velocities in excess of 600 m/s.

Coating with very high densities and bond strengths can be achieved. Unfortunately the noise levels are very high (>130db) and only some systems are suitable for depositing ceramics. In addition deposition rates are moderate – on a par with air plasma spraying.