Surface engineering: Combating friction

Surface engineering is often overlooked, but is vitally important. Gary Hughes, product engineering manager at bearing manufacturer, The Barden Corporation, outlines the main techniques the latest developments that can be used.

Most surfaces are not perfectly flat, smooth or clean. So when two surfaces come into contact, the interaction that is supporting the load is actually only a small percentage of the apparent surface area. This results in higher than expected contact stresses, increased friction and wear on components.

Surface engineering is about giving cost effective performance improvements by modifying a surface or substrate. The properties of a surface are contained within a relatively thin 'skin' and it's these characteristics of this surface layers, not the bulk material, which determine and control system performance.

The scope of surface engineering technology encompasses a whole range of coatings and surface treatments that can be applied to combat friction, prevent corrosion and reduce wear. The resulting benefits are improved performance, lower running costs and longer service intervals.

As the wear rate of a material is proportional to the load applied to it, and inversely proportional to its hardness, one obvious way of reducing wear on bearing components is to increase the hardness of the surface. This can be achieved using hard coatings such as electroless nickel plating, hard-anodising, thin dense chrome, plasma nitriding, arc evaporated titanium nitride, carburising and carbo-nitriding.

Other hard coatings, such as titanium carbide or galvanised zinc, can also be used to prevent corrosion and delay lubricant degradation. However, processes offering good wear resistance do not always give anti-corrosion properties. Some hard coatings can actually render steel more susceptible to corrosion. Conversely, materials offering corrosion protection may not provide good wear resistance. This is evidenced by the use of soft metal films, which have negligible wear resistant capability, but are nevertheless effective in combating corrosion.

Hard coatings can also be used to prevent fretting, small amplitude oscillations or vibrations. A fretting motion disrupts naturally present surface oxide films and exposes highly reactive metal, which rapidly oxidises and is, in turn, disrupted by motion. These particles are usually harder than the original material and can cause a system to degrade through three-body abrasion where a hard particle is in contact with two opposite surfaces. The oxidised particles naturally occupy greater volume than the original metal and can cause seizure on close-tolerance mating parts.

In contrast to hard coatings, soft films are used to provide solid lubrication for bearings in extreme applications where traditional fluid lubricants would be rendered ineffective. Soft films are independent of temperature so they do not evaporate or creep in a vacuum environment and can be used in both cryogenic and extreme high temperature applications.

The solid soft film lubricant can either be applied directly to the surface or transferred from a sacrificial source such as a self-lubricating bearing cage. Examples of these two processes include the application of physical vapour deposited MoST and WS2 and Barden's PTFE-based BarTemp polymeric cage material, Vespel or Torlon. The processes are complementary and have been used successfully in a variety of extreme aerospace applications.

Diffusion processes can be effective in reducing the amount of wear on engineering components and therefore extending useful life. The process itself is a function of time and temperature and is limited only by the natural saturation limit of the substrate.

Traditional diffusion processes such as case-hardening rely on the diffusion of elements such as nitrogen and carbon into the surface. Examples include nitriding, boronising and carburising. In contrast, high-energy processes such as ion-implantation can be used to increase the relative atomic amount of carbon and nitrogen into the surface beyond the limits of traditional diffusion techniques.

For applications requiring good anti-corrosion performance, Barden also uses advanced material technologies such as its unique X-Life Ultra high nitrogen steel bearings. In controlled salt-spray tests, these bearings offer superior corrosion protection to those manufactured from industry standard steels such as AISI 440C.

Specialised processes is a term that describes ways in which surface engineering techniques and processes can be combined to further enhance the properties of a bearing, or other tribological system.
Multi-layer coatings can be employed to enhance the physical and tribological characteristics of the surface. The success of such techniques relies on the avoidance of distinct layers by generating a graduated or diffused interface between different materials. Similarly, keying layers such as nickel or copper are frequently used to improve the adhesion of soft films to hard or passivated substrates.

Specialised coatings can also be applied to increase thermal conductance, reduce reactivity to the atmosphere and improve optical transmission or reflectance characteristics. The properties of ceramics and metals can be combined in the form of 'cermets' such as NiSiC and NiAl2O3 in order to realise outstanding mechanical and chemical performance.

The role of surface engineering will become more pivotal in the future, particularly for rolling bearing, as moving and interacting parts gets progressively smaller, run faster, carry higher loads and operate reliably for longer with marginal lubrication.

The five basic categories: surface engineering processes
• Transformation processes (thermal and mechanical)
• Hard coatings
• Soft films
• Diffused layers
• Specialised treatments

Design checklist
Which process is best for my application?
1. Identify the limiting factor(s) on bearing life – friction, wear and corrosion.
2. Prepare a list of candidate coatings and surface treatments, eliminating those considered unsuitable on grounds of thickness and/or processing requirements (e.g. high temperature).
3. Where possible, consult previous case histories of similar applications for verification of process suitability and produce a shortlist of preferred candidates.
4. Refer to detailed surface engineering specifications to select the optimum process.

Author
Gary Hughes, The Barden Corporation

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