Wear studies

For tests performed in simulated situations such as the snag tests, the results are meaningful only if an estabUshed correlation exists between performance in the tests and in wear situations. Thus, the acceptable level of performance in the test should not be selected arbitrarily but should be estabhshed in actual wear studies (122). 

The physiological effect of a particular lens can be determined by measuring the increased thickness of the cornea after lens wear studies have shown the relationship between the DkjD of a lens and the subsequent swelling of the underlying cornea (20,21). 

Nistorica, C. Liu, J.-F. Gory, I. Skidmore, G. D. Mantiziba, F. M. Gnade, B. E. Kim, J. 2005. Tribological and wear studies of coatings fabricated by atomic layer deposition and by successive ionic layer adsorption and reaction for micro-electromechanical devices. J. Vacuum Sci. Technol. A. 23 836-840. 

Dr. A. Shaw at the University of Maryland Eastern Shore in Princess Anne, Maryland, is evaluating textile substrate for pesticide barrier effectiveness and comfort. Tests will be conducted to assess effectiveness of decontamination processes for these personal protection devices. Diazinon emulsifiable concentrates will be used to contaminate fabrics. Simulated wear studies will be conducted in the laboratory to assess the efficacy of these fabrics in protecting human health. 

The surfaces of the mouth are coated with a layer of salivary proteins known as the acquired pellicle, which provides a protective effect from both chemical and mechanical attacks to the tooth surface. The pellicle layer both moderates diffusion of ions away from the tooth surface, thus inhibiting the dissolution of enamel by erosion, and provides a lubricating layer to protect from mechanical attacks [10, 11], It is known that dental prophylaxis and the use of regular oral hygiene procedures are able to reduce or remove the pellicle layer. However, recent evidence has shown that the pellicle is able to re-form very rapidly and thus, maintain a protective layer over the tooth surface [10]. For this reason, tooth wear studies performed in vivo or in situ, where the mediating effects of pellicle are present, will give a much more realistic assessment of tooth wear than in vitro studies. For a more detailed discussion on the effects of pellicle, see chapter 2. 

Rice SL, Bailey WF, Wayne SF, Burns JA Comparative in vitro sliding-wear study of conventional, microfilled and light cured composite resin and glass ionomer cement. J Dent Res 1984 63 1173-1175. 

Table 1 The average values of the water content, and the material properties concerning the PAMPS/PAAm DN gel, PAMPS/PDMAAm DN gel. BC/PDMAAm DN gel, and BC/Gelatin DN gel, which were used in the wearing study. (Reproduced, with permission, from the literature [106])...

A knee joint simulator was designed and built in the bioengineering laboratory at Leeds in the late 1970 s to enable the mechanical and tribological characteristics of current and projected knee joint replacements tp.he assessed. The simulator has been described by Dowson et al and it is shown in Figure 1. Realistic load and motion cycles are applied to the knee joints to simulate any desired activity, but usually walking, and the penetration of the metallic femoral components into the polymeric tibial components after a large number of cycles can then be used to assess the life of the joints. Furthermore, the penetrations recorded, which include both wear and creep, can be related to results of laboratory wear studies of the behaviour of UHMWPE and to the in-vivo performance of total replacement knee joints. The simulator is therefore an Important machine in both the pragmatic and fundamental aspects of total replacement knee join development. 

Wear factors based upon at least 10° from the overall penetrations recorded in the simulator and compared with laboratory wear studies and a single result from in-vivo conditions. 

An example of how visual observation can assist in wear studies has been demonstrated in the erosion behavior of thermoplastics ( ). In the erosion of polymers the shape of the impinging particle on the polymer surface determines the mechanism by which wear occurs. 

FIGURE 50.6 Device for in vitro cartilage-on-cartilage wear studies. 

TABLE 50.2 Key Features of Test Device Designed for Cartilage Wear Studies [33]... 

Burkhardt, B.M. Development and Design of a Test Device for Cartilage Wear Studies, M.S. thesis. Mechanical Engineering, Virginia Polytechnic Institute State University, Blacksburg, VA, December 1988. 

Hellier W.G., T.P. Hedman, and J.P Kostuik. 1992. Wear studies for development of an intervertebral disc prosthesis. Spine 17 586-96. 

The adsorption of deposited monolayer films can be very tenacious, especially when the solid substrate can react with the deposited material. Carbox-yhc acid monolayers deposited on metal or metal oxide surfaces, for example, almost certainly form metal soaps that are extremely difficult to remove by any means other than direct chemical etching. For example, such strongly adsorbed films are very useful in friction and wear studies of boundary lubrication (see Chapter 18). 

The principle of isotopic wear studies is to label the material to be worn and monitor the material loss by radiation measurement. A typical example is studying car engine piston rings labeled, e.g., by neutron activation and measuring the accumulation of radioactivity in the lubricant oil. The measured count rates can be converted to mass by the appropriate calibration. Another example from the vehicle industry is to study the wearing of car tyres. 

Unfortunately surface wear of an implant results from its use, and therefore, cannot be avoided or eliminated. Because wear is a limiting factor in the successful outcome and lifetime of an implant, it is of the utmost importance to characterize the wear resistance of materials used in implant design, and the effect of the design on wear. The volume of material removed from surfaces in specific tribosystems as a result of wear processes has been described phenomenologically and estimated by different models (Table 7.2). Several experimental wear studies have been... 

Wright, K., Dobbs, H. and Scales, J. T. (1982) Wear studies on prosthetic materials using pin on disc-machines. Biomaterials, 3, 41-48. 

M. Garc d, M. de Rooij, L. Winnubst, W.E. van Zyl, H. Verweij, Friction and wear studies on nylon-6/Si02 nanocomposites. Journal of Apphed Polymer Science 2004 (1855) 92. 

Poddevin N, Kin MW and Guidoin RG. Failure mechanisms of anterior cruciate ligament prostheses in vitro wear study. / Biomed Mater Res. 1997 38 370-831. 

An excellent way of measuring the material loss at the surface is use of thin-layer activation. A thin layer of radioactive tracer is produced on the surface by irradiation with charged particles. This technique can be applied to all types of iron, steel, nonferrous metals and alloys. It is particularly suitable for metal wear studies, because, if done carefully, the charged-particle activation little alters the mechanical properties of the surfaces of specimens. As the charged particles for irradiation, protons, deuterons, a particles, and occasionally He-particles accelerated by a cyclotron or other accelerators are used. Choosing the kind and energy of the charged particles, a radioactive layer of 10-1,000 pm thickness is produced. Typical nuclear reactions used are as follows. 

Scholes SC and Unsworth A (2009), Wear studies on the likely performance of CFR-PEEK/CoCrMo for use as artificial joint bearing materials , J Mater Sci Mater Med, 20, 163-170, DOl 10.1007/sl0856-008-3558-3. 

G. W. Hastings. In vivo wear studies of polyethylene used... 

A major problem of wear studies is reproducibility. To get reproducible results, one needs first of all a mechanically well designed equipment that is sufficiently rigid to permit precise control of the mechanical variables. In addition, one needs to rigorously control the operational and environmental parameters, including humidity and temperature. 

Several workers have proposed new combinations of materials in an attempt to overcome wear. Studies involving polyimides, polyamide-imides, and poly-tetrafluoroethylene-filled polyoxymethylene demonstrated that although wear characteristics were good in dry conditions, the presence of lubricants (blood plasma, water) decreased the wear resistance. Results obtained with reinforcing materials such as carbon fibre and with an aluminium oxide ceramic ball used in conjunction with a polyethylene socket have been presented, Examples of other types of reconstructive surgery involving hard tissue replacement are the use of poly(methyl methacrylate) in chest wall reconstruction and repair of depressed skull fractures, the repair of major crano-orbital defects with the aid of a polyurethane-coated poly(ethylene terephthalate) mesh, and the use of silicone rubber in total finger joint and carpal bone replacement. 

The primary reason to use lubricants is to reduce friction and wear between two interacting surfaces. Hydrocarbon oils have the proper friction properties to meet these requirements but their low viscosity may cause them to be forced out of the contact region between interfaces. Powders of low MW PIEE may be added to liquid lubricants to provide reserve lubrication in case the liquid phase is forced out. Low-MW PTFE that is used this way is sometimes called an extreme pressure or boundary additive. The type of PTFE used in lubricants may be from either suspension or dispersion polymerization but the small particle size of dispersion-type PTFE is usually preferred to help maintain dispersion in the oil. Many Journal articles and patents have been published that report the performance of lubricating oils with and without the addition of PTFE. For example, Rico et al. [44] provided the results of an extreme pressure wear study of steel balls (Shell four-ball test) with several mineral oils containing four different percentages (1-10%) of PTFE. 

Cobalt-60 Reactor Cancer therapy sterilization food irradiation waste treatment industrial radiography gauging devices mechanical wear studies... 

---