Engineering coatings - not just icing on the cake

Keith Stevens discusses the surface coatings available to engineers in their search to reduce component costs and improve performance, and reviews applications and the selection process.

In many respects the role of an engineering coating is to "ice the cake" by enhancing the appearance and performance of a relatively mundane substrate to make it more attractive to the end user.

But there the analogy ends, as an engineer requires a great deal more from a coating than a sweet taste and an attractive finish.

He or she might, for example, demand hardness, wear resistance, corrosion protection, low friction, a thermal barrier, electrical insulation or even pleasing aesthetics.

And the objective is usually to minimise cost by using a cheaper, easy to fabricate substrate material and achieve his special properties and additional performance by applying a surface treatment.

So how does he or she begin to choose a surface treatment?.

A good start would be to understand that they fall into three broad categories.

Category 1 involves modifying the surface without altering the substrate's chemical constitution.

In this case, the existing metallurgy of the component is changed within the surface regions, either by thermal or mechanical means, to increase its hardness.

Category 2 involves changing the surface layers by altering the alloy chemistry.

New elements are diffused into the surface, usually at elevated temperatures, so that the outer layers are changed in composition and properties compared with those of the bulk.

Category 3 involves adding layers of material to the surface.

This group includes a wide variety of coating processes where a material different from the bulk is laid on the surface.

Unlike the first two categories there will be a clear boundary at the coating/substrate interface and the adhesion of the coating is a primary issue.

Category 1 is dominated by the traditional flame and induction hardening processes where the prime objective is to increase the wear resistance of an engineering steel (typically 0.4% C content) by heating and then quenching the outer layers to form martensite.

It is an alternative to through-hardening the whole part with the advantage of less distortion.

Also, only those selected areas that need wear protection can be treated, with less compromise to overall toughness and fatigue resistance.

More refined surface hardening can be achieved with lasers or electron beams.

A surface can also be cold worked to increase its hardness, albeit by a very modest amount, and shot peening is now considered a very positive way of creating a compressive stress in the surface and thus increasing fatigue life.

Category 2 includes a wide variety of treatments, mainly for steels and aluminium alloys.

Processes such as nitriding and nitro-carburising operate at near 550C, forming nitrides with alloying elements like Cr, Mo and V which are contained within the steel.

Again, hardness and wear resistance is the main objective, but without the complications of distortion or the need to post finish.

Higher temperature processes include carburising (case-hardening), where carbon is diffused to a level of 1% into the outer layers at over 900C before the part is quenched, thus hardening the outer surface but retaining the core toughness.

With oxygen as the diffused element the most common process is anodising, where the surface of an aluminium alloy is transformed to alumina ceramic by an electrolytic action.

With hardness and inertness, such layers can provide both wear and corrosion protection to an otherwise very vulnerable substrate.

Category 3 includes by far the largest number of options, with a wide range of processes able to deposit a tailored coating material on to the surface of nearly any metal or alloy.

The thinnest coatings are applied by physical vapour deposition, with materials like TiN and diamond like carbon (DLC), providing high hardness and wear resistance for components such as cutting and forming tools.

Chemical vapour deposition (CVD) is used to produce hard carbide and nitride coatings, mainly for hot working tools.

Traditional electroplating covers the application of hard chrome for hardness and wear resistance, nickel for corrosion resistance, and metals like cadmium and zinc for galvanic corrosion protection.

Thicker coatings are applied by thermal spraying, with a huge range of coating options from ceramics and cermets for wear, metals and alloys for reclamation, zirconia for thermal barriers and metal/solid lubricant composites for abradables.

Finally, for the most arduous conditions, weld overlays and spray-fused coatings can provide the most extreme toughness and wear resistance.

So, with such a bewildering array of options, how does the engineer actually make his or her selection?.

The answer lies with the experts, and more specifically in an "expert system".

The coatings discussed, and more, can be reviewed using useful software packages such as Isis, Poeton's own unique coating selection software, which is available on the company's website.

The engineer will benefit from systematic routines, addressing all aspects of his component, its material, design, required performance, size and weight, and the environment and all the other elements that make up the design.

Such an approach allows the engineer to distil his or her options to only the most appropriate and promising solutions, enabling him or her to move more quickly to prototyping or field trials.

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