Wear coefficient

Some dimensionless wear coefficients /k are given in Table 1 (3). 

Table 1. Wear Coefficients for Various Sliding Combinations ...

The wear, W, of friction materials can best be described by the wear equation (32,33) W = KP° P where K is the wear coefficient, P the normal load, D the sliding speed, t the sliding time, and a,b, and c ate a set of parameters for a given friction matenal—rotor pair at a given temperature. 

Classical, macroscopic devices to measure friction forces under well-defined loads are called tribometers. To determine the dynamic friction coefficient, the most direct experiment is to slide one surface over the other using a defined load and measure the required drag force. Static friction coefficients can be measured by inclined plane tribometers, where the inclination angle of a plane is increased until a block on top of it starts to slide. There are numerous types of tribometers. One of the most common configurations is the pin-on-disk tribometer (Fig. 11.6). In the pin-on-disk tribometer, friction is measured between a pin and a rotating disk. The end of the pin can be flat or spherical. The load on the pin is controlled. The pin is mounted on a stiff lever and the friction force is determined by measuring the deflection of the lever. Wear coefficients can be calculated from the volume of material lost from the pin during the experiment. 

Here, H the hardness of the material and kw the wear coefficient (in m3/s). The practical usefulness of such equations, however, is limited. Ludema [452] comments on the results of an extensive literature scan [513], that yielded more that 300 different equations describing wear under different conditions Many of the equations appeared to contradict each other and very few equations incorporated the same array of variables. It is common to find, for example,... 

Wear volume = (wear coefficient load - distance of sliding)/hardness (5.2)... 

Figure 5.7. Variation of wear volume and wear coefficient of UHMWPE and composites against sliding distance, Kanagaraj et al. (75).

The mechanisms of film formation previously described involve both physical and chemical processes. It follows that factors favourable to film formation can influence friction and wear. Coefficients of friction were shown to vary from 0.04 for a reactive metal such as zinc, lubricated with 1% lauric acid, to 0.55 for an inert metal, silver, with the same lubricant [10], These factors include strong dipole interactions or strong hydrogen bonding which aid physical adsorption and the ease of chemical reaction from this adsorbed layer. Both interactions favour the formation of low-shear-strength films and similar influences have been reported by many workers including wear for mixtures of dilinoleic and linoleic acids [11], and for ZDDPs [6]. 

The wear properties of PEEK-based composites filled with 5% nanometer or micron AI2O3 against the medium carbon steel are improved by the addition of AI2O3. In contrast, the friction properties are not improved. However, the filling of 10% poly(tetrafluoroethylene) (PTFE) into pure PEEK results in a simultaneous decrease of the friction coefficient and the wear coefficient of the filled composite. ... 

Figure 24. Friction coefficient, f, and wear coefficient, k, of SiC/SiC sliding pairs as a function of sliding velocity with 10 N for various temperatures (sintered SiC EKasic D , after Habig and Woydt, BAM Berlin/Germany [220]).

L.J. Yang, Wear coefficient of tungsten carbide against hot work steel disc with two different pin settings. Wear 257 (2004) 481-495. 

Wad = rate (worn volume per unit sliding distance) K = wear coefficient V = volume of wear L = sliding distance Fn = normal load H = hardness of the softer material Implies that wear rate is proportional to real contact area in plastic contacts and may not be applicable for cases involving elastic contacts... 

K = wear coefficient V = volume of wear L = sliding distance Fn = normal load... 

Fatigue Wear (Hailing, 1975) Hailing model Wf, = K 4 Fn Wf3 = wear rate j] = line distribution of asperities 7 = constant defining particle size ej = strain to failure in one loading cycle H = hardness of the softer material K = wear coefficient Incorporates the concept of fatigue failure as well as simple plastic deformation failure. 

Material Contact Wear rate Wear Coefficient Tribological Conditions Wear Mechanisms References... 

This equation relates the volumetric material loss per unit sliding distance (Q) to the normal load (W) and hardness of the soft surface (H). The dimensionless constant K given above is an important property that provides a measure of the severity of the interaction between the asperities of two interacting surfaces and the likelihood of this interaction generating wear. However, in engineering applications, it is often more useful to use a dimensional wear coefficient, k (mm m ), i.e. the volume of material lost to wear per unit distance slid, per unit normal load on the contact. The use of this coefficient... 

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