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Surface engineering combats friction and wear

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Surfaceengineering combats friction and wear

Working together with recognized leadersin advanced coatings and surface treatments has enabled Barden to providespecialized Surface Engineering Technology in support of the most demandingapplications for precision bearings. Gary Hughes, Product Engineering Managerat The Barden Corporation, outlines the latest developments.

Engineering surfaces are neitherperfectly flat, smooth nor clean. therefore, in terms of rolling bearings, whentwo surfaces come into contact, only a very small percentage of the apparentsurface area is actually supporting the load. This can often result in highcontact stresses, which lead to increased friction and wear of the component.

Surface engineering is the design andmodification of a surface and substrate in combination to give cost effectiveperformance improvements that would not otherwise be achieved. Surfaceengineering recognises that the properties and characteristics of a surface arecontained within a relatively thin ‘skin’. It is therefore the properties ofthe surface layers, not the bulk material, which determine and control systemperformance.

In all types of environments, fromaerospace to offshore – where precision bearing systems are challenged byharsh, difficult operating conditions such as marginal lubrication, aggressivemedia and hostile environments – surface engineering processes can provideimproved tribological performance for protection against potential friction andwear problems.

The scope of surface engineeringtechnology encompasses a whole range of coatings and surface treatments thatcan be applied to engineering surfaces in order to combat friction, preventcorrosion and reduce wear. The resulting benefits are improved performance,lower running costs and longer service intervals. Surface engineering processesgenerally fall into one of five basic categories:

Transformation processes (thermal andmechanical)

Hard coatings

Soft films

Diffused layers

Specialised treatments

For this article, transformationprocesses (i.e. the metallurgy of steels and the effects of processes and heattreatments) about which much is already documented, will be left aside,enabling a focus on the equally important, but (arguably) more dynamic, areasof coatings and other surface 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 onbearing components is to increase the hardness at their surface. This isprimarily achieved using hard coatings such as electro less nickel plating,hard-anodizing, thin dense chrome, plasma nit riding, arc evaporated titaniumnitride, carburizing and carbo-nitriding.

Other hard coatings, such as titaniumcarbide or galvanized zinc, can also be used to prevent corrosion and delaylubricant degradation. however, it is incorrect to assume that all processesoffering good wear resistance also confer anti-corrosion properties. some hardcoatings can render the substrate steel more susceptible to corrosion.Conversely, materials offering corrosion protection may not necessarily providegood 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 toprevent fretting (i.e. small amplitude oscillations or vibrations). Thefretting motion disrupts the naturally present surface oxide films and exposeshighly reactive metal, which then rapidly oxidizes and is, in turn, disruptedby the motion. Metal oxide wear particles are usually harder than the originalmaterial and can cause the system to degrade through three-body abrasion.Furthermore, the oxide particles naturally occupy greater volume than theoriginal metal and hence there is a risk of seizure on close-tolerance matingparts. Hard surface engineering coatings, by being very effective at preventingfretting in the first instance, can prevent this from happening.

Soft Films
In contrast to hard coatings, soft films are primarily used to provide solidlubrication for bearings in extreme applications where traditional fluidlubricants would be rendered ineffective. These offer advantages in that theirfriction is independent of temperature (from cryogenic to extreme hightemperature applications), and they do not evaporate or creep in terrestrialvacuum or space environments.

The solid soft film lubricant can eitherbe applied directly to the surface or transferred by rubbing contact from asacrificial source such as a self-lubricating bearing cage. Examples of thesetwo processes include the application of physical vapor deposited MoST and WS2and Barden’s PTFE-based BarTemp polymeric cage material, Vespel or Torlon. Theprocesses are complementary and have been used successfully in a variety ofextreme aerospace application.

Diffusion Layers
The value of diffusion processes is that they can effectively reduce the amountof wear on engineering components, thereby extending their useful life. Theprocess itself is a function of time and temperature and is limited only by thenatural saturation limit of the substrate.

Traditional diffusion processes such ascase-hardening rely on the diffusion of elements such as nitrogen and carboninto the surface. Examples include nitriding, boronising and carburising. Incontrast, high-energy processes such as ion-implantation can be used toincrease the relative atomic per cent of carbon and nitrogen into the surfacebeyond the limits of traditional diffusion techniques.

for applications requiring goodanti-corrosion performance, Barden also uses advanced material technologiessuch as its unique X-Life Ultra high nitrogen steel bearings. In controlledsalt-spray tests, these bearings offer superior corrosion protection to thosemanufactured from industry standard steels such as AISI 440C.

Specialized Processes
Specialized processes is a term that describes the way in which surfaceengineering techniques and processes can be combined to further enhance theproperties of the bearing system.

For example, multi-layer coatings can beemployed to enhance the physical and tribological characteristics of thesurface. The success of such techniques relies on the avoidance of distinctlayers by generating a graduated o diffused interface between differentmaterials. Similarly, keying layers such as nickel or copper are frequentlyused to improve the adhesion of soft films to hard or passivized substrates.

Specialized coatings can also be appliedto increase thermal conductance, reduce reactivity to the atmosphere and toimprove optical transmission or reflectance characteristics. The properties ofceramics and metals can be combined in the form of ‘cermets’ such as NiSiC andNiAI2O3 in order to realize outstanding mechanical andchemical performance.

Which process is best for theapplication?
Because of the large number of coatings and surface treatments that areavailable to combat friction, corrosion and wear, it is often difficult fordesigners to select the optimum process for a particular application. To help,Barden has identified four steps to approach the problem:

1. Identify thelimiting factor(s) on bearing life – friction, wear and corrosion

2. Prepare a listof candidate coatings and surface treatments, eliminating those consideredunsuitable on grounds of thickness and/or processing requirements (e.g. hightemperature)

3. Where possible,consult previous case histories of similar applications for verification ofprocess suitability and produce a shortlist of preferred candidates

4. Refer todetailed surface engineering specifications to select the optimum process

In addition, in all cases, particularly where there islittle or no proven heritage of a process for the application, it isrecommended that suitable qualification trials be carried out before arespective process is selected, in order to verify its suitability. Cost and availabilitywill also need careful consideration here.

The Future
The role of surface engineering in rolling bearing technology will become morepivotal in the future as new bearing designs become progressively smaller, butare still required to run faster, carry higher loads and operate reliably forlonger periods, even under conditions of marginal lubrication. Whilst surfaceengineering technologies have been pivotal to the success of deep spaceapplications such as spacecraft engines, similar performance demands are nowbeing regularly encountered in terrestrial application. What this illustratesis the rapid pace of development of bearing technology, driven by marketdemands, and the equally important role that surface engineering is set to playin helping to achieve these demands.