Surface engineering the key to longer bearing life and resistance to wear, corrosion and seizure

Citation

Williams, B. (2002), "Surface engineering the key to longer bearing life and resistance to wear, corrosion and seizure", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 1. https://doi.org/10.1108/aeat.2002.12774aaf.001

Surface engineering the key to longer bearing life and resistance to wear, corrosion and seizure

Keywords: Surface engineering, Corrosion, Barden Corporation (UK) Limited

Working together with recognised leaders in advanced coatings and surface treatments has enabled Barden to provide specialised Surface Engineering Technology in support of the most demanding applications for precision bearings. In this article Brian Williams outlines the latest developments.

In all types of environments from deep space to offshore, where precision bearing systems are challenged by harsh 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.

Surface Engineering Technology is a synergistic process, which has developed from the recognition that the properties and characteristics of a surface are contained within a relatively thin "skin". Translated, this means that it is the properties of the surface layers not the bulk material that determines and controls system performance.

Surface Engineering is designed to enhance and improve these properties as its definition states:... "the design and modification of a surface and substrate in combination to give cost effective performance enhancement that would not otherwise be achieved". The scope of the 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.

In general terms the spectrum of surface engineering processes can be considered to fall into one of five basic categories, these are:

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  Transformation processes (thermal and mechanical)

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  Hard coatings

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  Soft films

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  Diffused layers

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  Specialised treatments

For the purposes of 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 surfaces treatment technologies.

Hard coatings

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 commonly accomplished using hard coatings such as electroless nickel, hard anodised aluminium and thin dense chrome.

Other hard coatings, such as titanium carbide, can also be used to prevent corrosion and delay lubricant degradation. However, it would be wrong to assume that all processes offering good wear resistance also confer anti- corrosion properties. Indeed, some hard coatings can render the substrate steel more susceptible to corrosion. Conversely, materials offering corrosion protection may not necessarily have a good resistance to wear. This is evidenced by the use of soft metal films, such as ion-plated lead, on specialised spacecraft components. These have negligible wear resistant capability, but are, nevertheless, effective in combating corrosion.

Hard coatings can also be used to combat fretting – small amplitude oscillations or vibrations. In simple terms, 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.

Soft films

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. They offer the advantages 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 MoS₂ and Barden's PTFE-based "BARTEMP" polymeric cage material. The processes are complementary and have been used successfully in a variety of extreme aerospace applications.

Diffusion layers

The value of diffusion processes is that they can effectively reduce the amount of wear on engineering components, thereby extending the useful life of the product. 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. Nitriding, boronising and carburising are three such examples. In contrast, high- energy processes such as ion-implantation can be used to increase the relative atomic percent of carbon and nitrogen into the surface beyond the limits of traditional diffusion techniques. This process is successfully used to harden bearing components for critical aerospace and Formula 1 applications.

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

Specialised processes

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 a technique 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 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 NiAI₂O₃ in order to realise outstanding mechanical and chemical performance.

Surface engineering processes - four key steps in determining which one to use

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 designers in this respect Barden has produced a four-step guide, which gives a simplified approach to the problem. The four steps are:

(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 short-list of preferred candidates.

(4) Refer to detailed surface engineering specifications to select the optimum process.

As an addition to these guidelines, 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 before a respective process is selected, in order to verify its suitability.

The future

The role of surface engineering in rolling bearing technology can only become more pivotal in the future as bearings get progressively smaller, but are still required to run faster, carry higher loads and operate reliably for longer periods, even under conditions of marginal lubrication. (Plate 1)

Plate 1 Barden - surface engineering the key to longer bearing life and resistance to wear, corrosion and seizure

In one sense the future is today, because Barden is already meeting these demands with materials such as the unique Cronidur 30, a new type of steel that has been used successfully in the engines of the space shuttle. The unique demands of space mean that Cronidur 30 has always been at the leading edge of performance and technology. However, similar performance demands are now being routinely encountered in terrestrial applications. What this ably illustrates is the rapid pace of development of bearing technology, generally; driven by market demands, and the (commensurately) important role that surface engineering is set to play in helping industry to achieve these demands.

Details available from: The Barden Corporation, Tel: +44 (0) 1752 735555; Fax: +44 (0) 1752 733481

Brian Williams,The Barden Corporation