Ceramics out-perform metals and plastics in a number of areas since they are, in general, harder and more wear resistant, more durable in aggressive chemical environments and capable of higher temperature operation. However, although the use of engineering ceramics is increasing (as they become less expensive and new processing methods are developed), it is true to say that frequently it is not economic to produce a full component from ceramic. It is better to either join a ceramic component to a substrate or in some cases coat the substrate.
Coatings are generally used to confer a desired functionality to a substrate that fails to exhibit the function unaided. Coatings are usually applied to make the component: chemically more inert (corrosion resistant); impervious to liquids and gases; harder; more wear resistant; insulating (electrically and thermally); and decorative.
In a most common case this may be to provide an aesthetic quality, ie enamel on cookers. Ceramic coatings are primarily used to enhance metallic components; however, this article will also provide examples for plastics and other ceramics.
The type of coating and the method of deposition depend on a number of factors including: the desired thickness; the substrate material and its properties; the function of the coating; the operating parameters of the component; the geometry of the area to be coated; and economic considerations.
There are two primary methods for depositing ceramic coatings onto components for engineering applications, Table 1; these are Thermal Spraying and Vapour Deposition (Chemical and Physical Vapour deposition; CVD and PVD respectively). In addition, there is a wide range of other coating methods mostly based on the deposition and sintering of slurries. However, these `wet’ methods are `small’ on the industrial scene compared to the two main families.
The estimated market size in North America for the various ceramic coating types is given in Table 1.
Ceramic coatings can be categorised in terms of thickness, deposition process or materials type.
As indicated in Fig 1, thick coatings are deposited via thermal spraying and enamelling respectively.
Thermal spraying is the most commonly used method for depositing ceramic coatings onto metal substrates. Thermal spraying is a generic name for a family of thick overlay processes. The coating material, usually in the form of a powder, is rapidly heated in a gas and projected toward the substrate using a gun attachment that can be handheld or mechanically manoeuvred relative to a complex shaped part.
THERMAL SPRAYING METHODS
There are a number of different thermal spraying methods Table 2, the main difference being their heat sources and propellants. Process selection is dependent on what is required of the final coating in terms of its mechanical/chemical properties and the economic benefits the coating will confer on the component. In general, the spraying processes become more expensive as they produce higher quality (lower porosity) coatings. Table 2 illustrates the primary differences between the processes. The ceramic materials that can be deposited by thermal spraying are primarily cemented carbides (such as tungsten carbide – cobalt) for cutting tool applications, or oxides such as alumina (for wear protection) and zirconia (thermal protection).
The final coating thickness can be up to 0.5mm for most methods (built up by depositing thin (10 micro m) coatings in each pass). As the coating thickness increases, so does the strain in the coating and eventually it will spall off.
Significant effort over recent years has been put into depositing ceramic `thermal barriers’ onto the interior of combustion chambers of turbine engines (usually zirconia). The driving force behind this activity has been the desire to increase the operational temperature of the engine. As this temperature increases so do the efficiency and the power output. Current commercial engines operate at 1300 degrees C and have an efficiency of 31%. Every 100 degrees C increase in operational temperature will increase the efficiency by 1%, and yield a 3% reduction in energy consumption. When viewed in global terms a 3% reduction in energy consumption by turbine engines not only represents a massive economic benefit, but also a considerable environmental benefit (lower emissions). Fig 3 shows a thermal barrier, zirconia, deposited onto a fuel injector assembly.
RANGE OF PROPERTIES
A familiar example of a thick ceramic coating is glazing (or glass enamel). Glass is a particular type of ceramic (it is amorphous rather than crystalline) and can be modified to have a range of properties. Within the home there are numerous examples of enamelled products, such as cooking ranges, crockery and sanitary ware. The purpose of enamelling is to prevent the underlying metal from getting scratched and also to protect the metal from corroding and discolouring. The glaze has the advantage that it can be coloured and so also serves a decorative function.
The CVD and PVD (both vapour phase processes) begin by placing the substrate in a vacuum chamber. Both deposit material fairly slowly, and it is only economic to produce relatively thin coatings. In addition the size of component is limited by the size of vacuum chamber.
In CVD a solid coating is deposited onto the substrate as a result of chemical reactions in the gas phase. The coating material (usually a metal halide such as titanium chloride) is introduced as a vapour into the vacuum chamber and reacted to give the required coating material.
There are a number of advantages associated with the CVD process: high deposition rates (tens of microns per hour); tailored coating compositions; tailored coating microstructure, and; non-line of sight coating.
Conventional CVD has the drawback of requiring the substrate to be heated to high temperatures (600 degrees C-1200 degrees C) which limits the substrates that can be used. However, recent developments have taken place to overcome this to allow low temperature materials such as plastics to have a thin ceramic coating deposited on them.
In a similar fashion to the methods used in CVD, the component to be coated via PVD is placed in a vacuum chamber. Instead of injecting the vapour into the chamber the coating material is present in bulk form called the target. The target is then vaporised; the vapour fills the chamber and coats all the surfaces in the chamber.
There are a number of ways of vaporising the target; it can be heated (either directly or indirectly) to cause evaporation at the surface; or it can be bombarded with energetic electrons, this technique is called sputtering.
PVD methods have been used to deposit hard/wear resistant coatings onto industrial cutting tools (drill bits and such like), wristwatch components and home cutting tools (knives for example). Fig 2 shows a selection of cutting tools.
A wet chemical process receiving increasing attention is sol-gel. This is a method of fabricating ceramics from solution at room temperature. The solution is deposited on a substrate by dipping or spin-coating. The coating (1 micro m-2 micro m) is cured at temperatures typically between 200 degrees C and 300 degrees C. Sol-gel methods are used to deposit ceramic coatings onto: metals for corrosion protection; glass for water repellence; plastics to improve scratch resistance.
There are a range of coating techniques that can be used to deposit ceramic materials onto metals and other materials. The choice of which ceramic coating to use and which deposition process can only be made on a case by case basis.
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