Wear rate constant

For dry sliding friction, the wear rate constant, k, is given by... 

Figure 8.13 shows a material selection chart of the wear rate constant for dry sliding friction in Eq. (8.21) plotted as a function of hardness, FI, in MPa. The most favorable materials will have a high hardness and a low wear rate constant. Diamond would be the best choice from this diagram, but may be prohibitive from a cost standpoint, even when used only as a liner, if it is even possible to produce such components. 

Figure 8.13 Materials selection chart of dry sliding wear rate constant [Eq. (8.21)] versus hardness. Reprinted, by permission, from M. F. Ashby, Materials Selection in Mechanical Design, 2nd ed., p. 60. Copyright 1999 by Michael F. Ashby.

Wear. Eor a fixed amount of braking the amount of wear of automotive friction materials tends to remain fairly constant or increase slightly with respect to brake temperature, but once the brake rotor temperature reaches >200° C, the wear of resin-bonded materials increases exponentially with increasing temperature (26—29). This exponential wear is because of thermal degradation of organic components and other chemical changes. At low temperatures the practically constant wear rate is primarily controlled by abrasion, adhesion, and fatigue (30,31). 

The importance of amplitude of slip is emphasised above. In recent years, the area of very low amplitudes and very carefully controlled amplitudes has been investigated. These researches confirm that the specific wear rate (volume removed, per unit distance of sliding per unit applied load) increases dramatically in the region 30-70 /xm and then becomes constant, as would be expected in reciprocating or unidirectional sliding . Damage is produced at amplitudes of 1 /xm or less", but it tends to be characteristic of surface fatigue rather than wear. 

Conditioner aggressiveness The pad wear rate during conditioning indicates the abrasiveness, or aggressiveness, of a pad conditioner. The wear increases linearly with time at a constant wear (cut) rate. 

Figure 15.8 shows the effect of aramid fibers on the friction coefficient and the specific wear rate of brake pads. Additions of up to 15 vol% aramid fiber are economical to reduce the coefficient of friction decrease which remains constant up to 40 vol%. At the same time, the specific wear rate decreases steadily as fiber concentration increases. This suggests that wear rate is improved by the material reinforce-ment.55... 

Hence for a fixed load the volumetric wear rate dV/dL is constant. A physical model of wear has been derived from the purely geometric... 

Figure 13-18. Influence of contact pressure on wear rate under constant load. Hardened steel lubricated by white oil at 23.2 N load, 50.8 cm/s rubbing speed, (a) Course of wear. (b) Wear rate as a function of contact pressure. Data by Dorinson and Broman [4].

By collecting constants the effect of additive concentration on the wear rate can be expressed as... 

The lubricated wear described above is squarely at odds with the behavior illustrated in Fig. 14-6 and with the wear-reducing action of 22% di-t-octyl disulfide in white oil reported by Dorinson and Broman [10] and shown in Table 11-6 (Chapter 11, Section 11.2.1). If Eqn 14-49 is a correct representation of additive action, it should be valid for both the reduction and the increase of wear by such action. To reduce wear, the first term on the right-hand side of the equation must control the overall rate and one way to do so is for the lump removal factor wear rate. But there is no physical necessity that q remains constant for all conditions of load, pressure, speed or state of lubrication. Since in physical terms the predominant effect of the lubricant is to inhibit the asperity adhesion process, it is not unanticipated that the average size of the transferred and detached particles as well as their number will be decreased by lubrication. It is to this latter type of mechanistic process that we must look for an explanation of why such parameters as contact pressure, rubbing speed and material properties affect the balance between the inhibition or promotion of wear by additive action and the transition from smooth lubricated wear to catastrophically damaging wear behavior such as scuffing. 

The second set of experiments was also characterized by a period of no wear with low friction followed by wear track Initiation and an Increase In the friction. In these experiments the tests were continued and measurements of the wear track were made every 2000 cycles up to 16 kc of disk rotation. After the first 2000 to 4000 cycles the wear rate was constant and the wear rates were calculated using a linear regression on the wear vs. number of cycles data. 

A linear wear rate ]B crack-growth constant F frictional force per width K coefficient... 

Under constant pressure, against stainless steel, interrupted sliding for different periods had no effect on the tissue s wear rates. This was not the case when the tissue was worn against slightly cross-linked cartilage. (In these... 

Figure llo Steady state wear rates of cartilage when worn against cartilage that was reacted with formaldehyde to different extents. The tissues were worn under a constant pressure of 2.07 MN/m in these ex-perimentSo By comparison against Surface A of the stainless steel plate the tissue wore at a constant rate of 1 ugm carto/mln. 

The reasoning is consistent with the greater differences in wear rates observed between the 0.69 - 6.9 MN/m cycle and those that would have been attained under an equivalent constant pressure as opposed to the 0.69 - 4.14 MN/m cycle and its... 

The lower steady state wear rates, as compared with those under equivalent constant pressures were obtained for all frequencies tried (Fig. 8) and for the two loading patterns at the same frequency (Fig. 7 ). However, if the pressure differences in these experiments were much less or the frequencies considerably lower, the differences in wear rates would probably not be so evident. With lower frequencies the importance of stress dissipation on the tissue s overall wear... 

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