News Release

Surface Coating - or how to engineer a silk purse !

18th May 2004

 
Keith Stevens, R&D Director for Poeton Industries, looks at ways that engineers can reduce the cost of components without affecting their performance.

When designing and manufacturing components, cost saving is the engineer's Holy Grail. So if he can start with a cheap, easy to fabricate substrate - a sow's ear - and turn it into a high-performance component that meets all his engineering objectives - a silk purse - he will have accomplished his quest.

However, unlike most quests, this one isn't impossible and need not take a lifetime provided he talks to a surface treatment specialist such as Poeton early in the process to help select the most appropriate coating. But beware! Whilst the choice is huge - as many as fifty different techniques and hundreds of alternatives within the various groups - many are backed by seemingly extravagant claims from their suppliers.

It is true that coating technology can offer increased hardness, greater wear resistance, unsurpassed corrosion protection, low friction, a thermal barrier, electrical insulation or even just pleasing aesthetics - but how exactly does this help our engineer save money?

The answer lies in a number of areas. For example in the cost of the substrate material itself, in the manufacturing processes, and in post-finishing, or the elimination of post-finishing. Unfortunately, too often a surface coating is a last minute add-on, specified in a desperate attempt to stave off disaster and combat some unanticipated problem, and bringing with it unwelcome complications in size, tolerances and finish.

To maximise the cost benefits of coating technology our engineer must look more carefully at his component, and at the earliest stage - designing it and selecting the material with the benefits of coating in mind. He must size it correctly, understand the surface properties he needs, and define the component's mechanical requirements.

So let's look at what can be achieved, review the benefits of some coatings in detail and examine the options available to the engineer.

Making it stainless
One obvious benefit would be to eliminate the need for an expensive stainless steel, perhaps for components in humid or other corrosive situations. Ideally, the component would be fabricated in the cheapest possible material - say cast iron, and coated with a completely impervious layer that provided indefinite protection against everything. Sorry, that's unrealistic!

However, if the engineer isn't concerned about wear or erosion, he has a hundred different options in the form of paints and powder coats to encapsulate the component, many loaded with elements such as zinc to provide the important galvanic protection.

If on the other hand, he wants a bright, attractive finish with hardness to add wear resistance to the component, his options are more limited. The classic solution is electroless nickel, an immersion coating that covers the whole component evenly with, typically, a 25µ hard NI/P alloy. But bear in mind that the protection offered is only as good as the coating and coverage, since nickel will only ever provide a barrier coating to iron-based substrates. Any hole or defect will cause the substrate to corrode and pit, and eventually undermine the coating.

The contract therefore, should be awarded to the supplier who best controls his process, understands the substrate and the optimum pre-treatments, and achieves the best coverage.

Chrome plating is another alternative to stainless steel, but our engineer needs to consider a number of caveats before specifying it. For a start, environmental pressure is growing, making the process less politically acceptable. Then, whilst it might look stainless, the coating is naturally micro-cracked, exposing a steel or cast iron substrate to immediate attack. And finally, to provide it with the properties of stainless steel it will need an undercoat of nickel, and by then the costs may outweigh the savings.

Making it hard
This is easy if cost isn't a consideration.

Our engineer should specify an expensive through-hardening steel, heat it until it's red hot, quench it, then grind it to the final dimensions. Or use a solid ceramic or cermet - either will provide the wear resistance he needs, at a cost.

The sensible and cheaper option is to select the substrate for its core properties, manufacture the component to size or to dimensions suitable for a predictable coating thickness, and then apply the appropriate surface treatment. If only specific areas of a component require wear resistant properties, flame and induction hardening treatments are both ideal. Distortion and growth are minimised, and the core retains its toughness and fatigue resistance.

Alternatively, he could carburise it and raise the bulk temperature above 900oC to achieve his hard surface, but he'll be very lucky to avoid the need to grind out the distortion.

Lower temperatures create less growth and distortion, which makes nitriding and nitrocarburising viable options for hardening the surface of low alloy steels without the need for post-finishing. These processes have become very sophisticated, with novel oxide finishes available to add pleasing aesthetics and corrosion resistance to the finished component.
If an engineer wants a hard coating there are dozens of options, but only a few will allow him the luxury of no post-finishing.

The thinnest coatings are applied by Physical Vapour Deposition (PVD) using ceramics such as TiN and CrC. With temperatures around 300DegC, distortion is controlled. In contrast, Chemical Vapour Deposition (CVD) is carried out at over
900oC and, whilst the resultant carbide and nitride coatings are extremely hard and tough, distortion may be a problem - not for forming tools perhaps, but not recommended for precision parts.

Electroplating offers the option of hard chrome. As a wear resistant coating it takes some beating and it will be a long time before the industry comes up with a ubiquitous replacement. The most experienced processing companies can now manage 'precision' chrome plating, avoiding the worst of the edge build up that usually comes with an electrolytic process. For hardness, abrasion resistance or sliding wear, it is hard to beat, rivalling many of the solid ceramics.

With its even, predictable coverage electroless nickel could be the engineer's dream solution. As well as providing corrosion protection, it can be hardened to nearly 1,000Hv, producing a component that the engineer can cast, won't have to finish, and will have the wear resistance equivalent to a through-hardened steel.

Thicker coatings offer the option of thermal spraying, where a bewildering variety of application techniques (plasma, flame, wire, high-velocity, detonation) bring an even wider portfolio of ceramics, cermets, metals, alloys, thermal barriers and abradables. Whatever the material, someone produces a powder that can coat it, but again, coatings are only as good as their structure and composition. The lower the velocity or temperature in the flame, the more likely it is that the coating will be porous and friable. Whilst it may be a ceramic by composition, it can too easily be a conglomeration of loosely bound particles interlaced with voids. In these circumstances corrosion protection is unlikely and wear resistance will depend on toughness, not hardness.
Thermal spraying is an important technology, and there is no doubt that the best coatings are produced using the highest velocity processes. But before specifying, an engineer needs to shop around and take impartial advice on the various options - and be prepared for some finish grinding.

Making it light
Aluminium alloys are cheap and easy to machine, or even cast, and the light weight is a huge attraction for the engineer - if he can protect the surface of a material is soft, prone to wear and will corrode readily.

To meet his specification the engineer's main option is hard anodising, an electrolytic process that grows an alumina ceramic from the parent metal surface. The layer is porous (with a hardness near 500Hv), but recent developments allow the best processing companies to seal the oxide layer and add further protective polymers to achieve surfaces that can give low friction as well as wear resistance. And, as the layer is even and predictable, this is another process that needs no finishing.

Plasma Electrolytic Oxidation (PEO) is an advanced form of anodising, creating a much harder surface of dense alumina (1,400Hv). The coatings are thicker, extremely wear resistant and provide corrosion protection in their own right. And gradually, the process is developing to provide very effective protection for magnesium alloys, which present the ultimate challenge for wear and corrosion.

To sum up our review of coatings, all the engineer has to do to find his Holy Grail is spend a little time with his Cad/Cam system deciding what he wants before he manufacturers the component. He needs to define his surface requirements, think about his tolerances and select the optimum substrate material. And then he needs to consult a coating specialist that can recommend a surface treatment to give him his cost saving package. Following these simple steps will put him ahead of the game and ensure that his product looks every inch (sorry millimetre!) the silk purse.

The coatings discussed, and more, can be reviewed using ISIS, Poeton's unique coating selection software, which is available on the company's website at www.poeton.co.uk.

Ends
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File: Engineering a Silk Purse