Surface engineering combats friction and wear
17 January 2013
Engineering surfaces are neither perfectly flat, smooth nor clean. Therefore, in terms of rolling bearings, when two surfaces come into contact, only a very small percentage of the apparent surface area is actually supporting the load. This can often result in high contact stresses, which lead to increased friction and wear of the component.
Surface engineering is the design and modification of a surface and substrate in combination to give cost effective performance improvements that would not otherwise be achieved. Surface engineering recognises that the properties and characteristics of a surface are contained within a relatively thin ‘skin’. It is therefore the properties of the surface layers, not the bulk material, which determine and control system performance.
In all types of environments from aerospace to offshore – where precision bearing systems are challenged by harsh, difficult operating conditions such as marginal lubrication, aggressive media and hostile environments – surface engineering processes can provide improved tribological performance for protection against potential friction and wear problems.
The scope of surface engineering technology encompasses a whole range of coatings and surface treatments that can be applied to engineering surfaces in order to combat friction, prevent corrosion and reduce wear. The resulting benefits are improved performance, lower running costs and longer service intervals. Surface engineering processes generally fall into one of five basic categories:
* Transformation processes (thermal and mechanical)
* Hard coatings
* Soft films
* Diffused layers
* Specialised treatments
For this article, transformation processes (i.e. the metallurgy of steels and the effects of processes and heat treatments) about which much is already documented, will be left aside, enabling a focus on the equally important, but (arguably) more dynamic, areas of coatings and other surface treatment technologies.
Because 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 at their surface. This is primarily 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, it is incorrect to assume that all processes offering good wear resistance also confer anti-corrosion properties. Some hard coatings can render the substrate steel more susceptible to corrosion. Conversely, materials offering corrosion protection may not necessarily provide good resistance to wear. 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 (i.e. small amplitude oscillations or vibrations). The fretting motion disrupts the naturally present surface oxide films and exposes highly reactive metal, which then rapidly oxidises and is, in turn, disrupted by the motion. Metal oxide wear particles are usually harder than the original material and can cause the system to degrade through three-body abrasion. Furthermore, the oxide particles naturally occupy greater volume than the original metal and hence there is a risk of seizure on close-tolerance mating parts. Hard surface engineering coatings, by being very effective at preventing fretting in the first instance, can prevent this from happening.
In contrast to hard coatings, soft films are primarily used to provide solid lubrication for bearings in extreme applications where traditional fluid lubricants would be rendered ineffective. These offer advantages in that their friction is independent of temperature (from cryogenic to extreme high temperature applications), and they do not evaporate or creep in terrestrial vacuum or space environments.
The solid soft film lubricant can either be applied directly to the surface or transferred by rubbing contact 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.
The value of diffusion processes is that they can effectively reduce the amount of wear on engineering components, thereby extending their 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 per cent 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 the way in which surface engineering techniques and processes can be combined to further enhance the properties of the bearing system.
For example, 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 to 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.
Which process is best for the application?
Because of the large number of coatings and surface treatments that are available to combat friction, corrosion and wear, it is often difficult for designers to select the optimum process for a particular application. To help, Barden has identified four steps to approach the problem:
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.
In addition, in all cases, particularly where there is little or no proven heritage of a process for the application, it is recommended that suitable qualification trials be carried out before a respective process is selected, in order to verify its suitability. Cost and availability will also need careful consideration here.
The role of surface engineering in rolling bearing technology will become more pivotal in the future as new bearing designs become progressively smaller, but are still required to run faster, carry higher loads and operate reliably for longer periods, even under conditions of marginal lubrication. Whilst surface engineering technologies have been pivotal to the success of deep space applications such as spacecraft engines, similar performance demands are now being regularly encountered in terrestrial applications. What this illustrates is the rapid pace of development of bearing technology, driven by market demands, and the equally important role that surface engineering is set to play in helping industry to achieve these demands.
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