Wear debris

Because of the very small bearing clearances in gas bearings, dust particles, moisture, and wear debris (from starting and stopping) should be kept to a minimum. Gas bearings have been used in precision spindles, gyroscopes, motor and turbine-driven circulators, compressors, fans, Brayton cycle turbomachinery, environmental simulation tables, and memory dmms. 

Brakes and clutches operate both dry and wet. In dry friction couples, the heat is removed by conduction to the surrounding air and stmctural members. Wet friction couples operate within a fluid, usually an oil, which absorbs the heat and maintains the couple at relatively low (below 200°C) temperatures. The fluid also traps the wear debris. 

In order to define the extent of emissions from automotive brakes and clutches, a study was carried out in which specially designed wear debris collectors were built for the dmm brake, the disk brake, and the clutch of a popular U.S. vehicle (1). The vehicle was driven through various test cycles to determine the extent and type of brake emissions generated under all driving conditions. Typical original equipment and aftermarket friction materials were evaluated. Brake relines were made to simulate consumer practices. The wear debris was analyzed by a combination of optical and electron microscopy to ascertain the asbestos content and its particle size distribution. It was found that more than 99.7% of the asbestos was converted to a nonfibrous form and... 

Mechanical. Premature wearout or loss of contact metal during engagement and separation can result in loss of tolerances, reduced spring forces, formation of loose metallic wear debris, which may short-circuit contacts, and development of porosity in noble metal contacts. Underplatings, contact lubricants, and hard materials reduce mechanical wear. 

Fretting corrosion (36,37) can lead to high contact resistance of base metal contacts, such as tin plate in electronic connectors. Small cycHcal displacements of the connector halves occur because of external vibration or differential thermal expansion and contraction of the mating contacts. The wear debris that is formed remains in the contact zone. The accumulation of oxide debris in the contact region leads to increased contact resistance. Solutions to this problem are stmctures that do not permit movement of contact surfaces with respect to one another, the use of gold as a contact finish, and the appHcation of thick coatings of contact lubricants and greases, which reduce the rate of wear and restrict access of air to the contact surfaces. 

Condition monitoring is an established technique which has been used by capital-intensive or high-risk industries to protect their investment. The concept has developed radically in recent years largely due to advances in computerization, which offer greater scope for sophisticated techniques. These fall into three types of monitoring vibration, performance and wear debris. The last monitors particulate debris in a fluid such as lubricating oil, caused by the deterioration of a component. 

Oil-related analysis encompasses a variety of physical and chemical tests such as viscosity, total acid number and particulate contamination. This is often extended to include the identification of wear debris, as an early warning of component failure, by either spectrographic... 

In both procedures, a concern is the extent to which the poly(ethylene) undergoes wear within the artificial joint. The constant rubbing of metal on polymer is capable of generating particles of wear debris, and these can collect around the joint. In extreme cases, this debris can interfere with the metabolic processes in the remaining bone, leading to bone resorption and... 

If the surfaces are damaged during sliding so that wear debris and multi-asperity contacts are involved in the process, the mechanism of friction will be substantially different from what we discussed for wearless friction. 

One distinction between test types is that some use a loose abradant and others use a solid abradant. A loose abrasive powder can be used rather in the manner of a shotblasting machine as a logical way to simulate the action of sand or similar abradants impinging on the plastic in service. Conveyor belts or tank linings are examples of products subject to abrasion by loose materials. A loose abradant can also be used between two sliding surfaces to simulate a contaminant or wear debris. The majority of wear situations involve the plastic moving in contact with another solid material and solid abradants can consist of almost anything. 

Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris would extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance. In particular, modifications of UHMWPE include (12) ... 

Once the wear track was formed, the coefficient of friction increased two to three times due to two factors. One was the increase in contact area between the ball and the conforming wear track. A second was the development of wear debris which was... 

It is fairly obvious that other factors such as lubricants, wear debris, ageing of the surfaces and humidity can also affect friction and, once again, test conditions must be chosen that resemble those found in service. 

In terms of metals, avoidance of wear debris and corrosion resistance is important to achieve biocompatibility and to minimize cytotoxicity. For example, Koike et al. found that corrosion-resistant titanium alloys are biocompatible and were similar to the excellent results obtained from PTFE in terms of cytotoxicity.70 Titanium-gold alloys are also corrosion resistant and relatively noncytotoxic.71... 

Bobyn J, Jacobs J, Tanzer M, Urban R, Aribini R, Sumner D, Turner T, Brooks C. The susceptibility of smooth implant surfaces to periimplant fibrosis and migration of polyethylene wear debris. Clinical Orthopaedics and Related Research 1995 311, 21-39. 

Some fundamental aspects of tribochemistry of ZDDP have been investigated using an ultrahigh vacuum UHV analytical tribometer on chemisorbed films previously formed on a steel surface (Martin et al., 1996). The steel surface was immersed in a solution of ZDDP at a concentration of 2 wt.% in PAO synthetic lubricant base at 130°C for 24 hours. The chemistry of the treated steel surfaces was investigated by XPS and AES, and friction tests were carried out in UHV just after the analysis. At the end of the friction tests, AES microanalysis was performed both inside and outside the wear scar. The AES analysis showed that sulfide was present inside the wear scar and phosphorus was eliminated from the surface, presumably as wear debris. Summary data of the tribofilm composition after ZDDP degradation are presented in Table 4.1. 

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