Technical SupportSurface engineering combats friction and wear
Working together with recognized leaders in advanced coatings and surface treatments has e€nabled Barden to provide specialized Surface Engineering Technology in support of the most dema"nding applications for precision bearings. Gary Hughes, Product Engineering Man ager at The Barden Corporation, outlines the latest developments.
Engineering surfaces are neither perfectly flat, smooth nor clean. therefo÷re, 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. εThis can often result in high contact stresses, which lead to increased frictiαon and wear of the component.
Surface engineering is the design and modification of ←a surface and substrate in combination to give cost effective performance improveme£nts that would not otherwise be achieved. Surface engineering recognises that the propert÷ies and characteristics of a surface are contained within a ≈relatively thin ‘skin’. It is therefore the properties of the surface layers, not the bul k material, which determine and control system performance.
In all types of environments, from aerospace to of★fshore – where precision bearing systems are challenged by harsh, difficult operating co≠nditions such as marginal lubrication, aggressive media and hostile environments – ≥surface engineering processes can provide improved tribological perfo rmance for protection against potential friction and wear problems.
The scope of surface engineering technology encompasses a wh∏ole 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 andα 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 effecΩts of processes and heat treatments) about which mu✔ch is already documented, will be left aside, enabling a focus on the equally importa∑nt, but (arguably) more dynamic, areas of coatings and other surface treatment technologie"s.
Hard Coatings
Because the wear rate of a material is proportional to the load applied to it, αand inversely proportional to its hardness, one obvious way of reducing wear on bearing components is to increas₩e the hardness at their surface. This is primarily achieved using ✔hard coatings such as electro less nickel plating, hard-an≈odizing, thin dense chrome, plasma nit riding, arc evaporated titanium nitride, carburiz<ing and carbo-nitriding.
Other hard coatings, such as titanium carbide or galvanized 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 conf★er anti-corrosion properties. some hard coatings can render the substrate steel more susceptiblαe to corrosion. Conversely, materials offering corrosion protection may not necessarily provid e good resistance to wear. This is evidenced by the use of soft metal f☆ilms, which have negligible wear resistant capability, but are, nevertheless, effec¥tive in combating corrosion.
Hard coatings can also be used to prevent fretting (i.e↓. small amplitude oscillations or vibrations). The fretting motion disrupts the naturallλy present surface oxide films and exposes highly reactive metal, which then rapidly oxidizes and is, in turn, disrupted by the motion. Metal oxide wear particles are u≈sually harder than the original material and can cause the system to degrad•e through three-body abrasion. Furthermore, the oxide p✘articles naturally occupy greater volume than the origina₽l metal and hence there is a risk of seizure on close-tolerance mating parts. Hard surface "engineering coatings, by being very effective at preventing fretβting in the first instance, can prevent this from happening.
Soft Films
In contrast to hard coatings, soft films are primarily used to provide solid lub rication for bearings in extreme applications where traditional fluid lubr icants would be rendered ineffective. These offer advanta&ges in that their friction is independent of temperature (from cryogenic tαo extreme high temperature applications), and they do not evaporate or creeεp in terrestrial vacuum or space environments.
The solid soft film lubricant can either be applied directly to the s>urface or transferred by rubbing contact from a sacrificial source such as a self-lubricating bearεing cage. Examples of these two processes include the application of physicaβl vapor 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 aero♦space application.
Diffusion Layers
The value of diffusion processes is that they can δeffectively 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 byα the natural saturation limit of the substrate.
Traditional diffusion processes such as case-hardening rely on the diffusion of element✘s such as nitrogen and carbon into the surface. Examples ∑include nitriding, boronising and carburising. In contrast, high-energy processes αsuch as ion-implantation can be used to increase the relative atomic per cent of carbon a♠nd nitrogen into the surface beyond the limits of traditional diffusion techniques.
for applications requiring good anti-corrosion performance, Barden also uses advanced mΩaterial technologies such as its unique X-Life Ultra high nitrogen steel bearings. InΩ controlled salt-spray tests, these bearings offer superior↓ corrosion protection to those manufactured from industry standar¥d steels such as AISI 440C.
Specialized Processes
Specialized processes is a term that describes the way in which surfa∑ce 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 tribolog™ical characteristics of the surface. The success of such techn iques relies on the avoidance of distinct layers by generating a graduated o diffus£ed interface between different materials. Similarly, keying layers such as nicke®l or copper are frequently used to improve the adhesion of soft films to hard or pas¶sivized substrates.
Specialized coatings can also be applied to increase thermal conductance, reduce reactivity t₹o the atmosphere and to improve optical transmission or r$eflectance characteristics. The properties of ceramics and metals can be combineλd in the form of ‘cermets’ such as NiSiC and NiAI2O3 in order to realize outstanding mechanical and chemical performance.
Which process is best for the application?
Because of the large number of coatings and surface treatments that are avail↓able to combat friction, corrosion and wear, it is often difficult for designers to selπect the optimum process for a particular application. To help, Barden has identified four stσeps 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 thoφse considered unsuitable on grounds of thickness and/or processing requirem×ents (e.g. high temperature)
3. Where possible, consult previous case histories of similar applications for verif™ication 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 foεr 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 alλso need careful consideration here.
The Future
The role of surface engineering in rolling bearing te♦chnology will become more pivotal in the future as new bearing designs become p♣rogressively smaller, but are still required to run faster, carry higher loads and operate reliably↕ for longer periods, even under conditions of marginal lubrication. Whilst su☆rface engineering technologies have been pivotal to the success of deep spaceε applications such as spacecraft engines, similar performance de₩mands are now being regularly encountered in terrestrial application. What this illustrates is the rapid pace of development of bearing technology, driveΩn by market demands, and the equally important role that surface eπngineering is set to play in helping to achieve these dem ands.