Specific wear rate

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. 

Fig. 25 Specific wear rate ( ) of EPDM gum and PTFE-filled EPDM vulcanizates as a function of PTFE loading...

Fig. 32 Friction coefficient (a) and specific wear rate (b) of EPDM gum and PTFE-EPDM composites as a function of irradiation dose...

Fig. 34 Specific wear rate (k) as a function of (a) tan delta at 25°C, (b) hardness, and (c) mean agglomerate size. Squares indicate PTFE-EPDM composites containing nonmodified PTFE and circles modified PTFE...

Table 11. Order of Magnitude of the Specific Wear Rates for Various High Temperature Materials Sliding Against Themselves at 500°Ca...

Material Yield stress (MPa) Tensile strength (MPa) Strain at break (%) Young s modulus (MPa) Specific wear rate lO"8 (mm3/Nm)... 

However, the friction of the Duroid 5813 (about 0.34) was practically unaffected by fibre orientation, unlike the other two composites which gave the lowest friction when sliding was normal to the fibre orientation. This may indicate that in the Duroid 5813 the friction was to a considerable extent determined by the molybdenum disulphide, which was randomly oriented, or by the PTFE, whereas in the epoxy composites the friction was strongly influenced by the fibres themselves. Table 12.5 shows the relationship between fibre orientation and specific wear rate for the Duroid 5813. It should be noted that although the effect of fibre orientation is significant, ail three wear rates are of the same order of magnitude. 

Table 12.5 Relation Between Glass Fibre Orientation and Specific Wear Rate for Duroid 5813 (Ref.370 ...

Axis % of fibres oriented Specific wear rate... 

Suzuki et al ° made hot-pressed composites of molybdenum disulphide 80%, molybdenum dioxide 10% and niobium 10%. They were pressed at 25 MPa and 1500°C in carbon dies, and the flexural strength was 59 - 63 MPa and the elastic modulus 27.9 MPa. To improve the strength they added 5% of 304 stainless steel, and this gave a flexural strength of 69 - 80 MPa and elastic modulus 43.8 MPa. The coefficient of friction of the latter compact was 0.07 to 0.18 and the specific wear rate 2.2 x 10" mm Nm at 450°C in vacuum. 

Figure 7.9. Specific wear rate vs. angle of fiber orientation. [Adapted, by permission, from Wada N, Uchiyama Y, Hosokawa M, Int. Polym. Sci. Technol., 21, No.3, 1994, T/53-63.]...

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... 

Figure 15.8. Friction coefficient and specific wear rate of phenolic brake pads containing varying concentrations of aramid fiber. [Adapted, by permission, from Bijwe J, Polym. Composites 18 No.3, 1997, 378-96.]...

Figure 19.13. Specific wear rate of dental composites vs. filler volume. [Adapted, by permission, from Friedrich K, J. Mat. Sci. Mat. In Med., 4, No.3, 1993,266-72.]...

Coefficient of friction is inversely proportional to pressure and proportional to velocity. Wear rate of fluoropolymers is proportional to load (/ ) and velocity (V). Combinations of pressure and velocity are defined where the material can be used, thus a FV limit is defined. Above this PV limit, the wear increases exponentially because of the heat that is generated as a result of motion. Generally, a polymer or its compounds can be characterized by PV limit, deformation under load, and wear factor. Wear factor or specific wear rate is defined as the volume of material worn away per unit of sliding distance and per unit of load. 

Specific Wear Rate - Also known as wear factor, specific wear rate is defined as the volume of material worn away per unit of sliding distance and per unit of load. 

Figure IT. Variations of the specific wear rates with disk temperature in the steady states of wear of various polymers.

The arrows show the scales of specific wear rate at left or right side. 

PEEK shows a low specific wear rate of lO" nm /Nm or less and relatively low friction in the temperature range from 20to 250 C. 

Table II presents the specific wear rates (in mm /Nm, volume loss per unit load per unit sliding distance) and average kinetic friction coefficients of the composites sliding versus steel and silicon nitride, upon initial and continued sliding. Plots of friction coefficient versus sliding duration in kilocycles (1 kc = 107 m) are shown in Figures 2-5, for all experiments. All composites formed transfer films upon sliding against both steel and silicon nitride. The films were similar in appearance under optical microscopy, and covered approximately 30% of the contact region.

The uncertainties in calculating specific wear rates from in-service data are discussed, as well as the data from laboratory tests which attempt to simulate particular modes of wear (eg abrasion, fretting). 

All the above makes the comparison of the relative in-service performance of different materials far from accurate. The calculation of specific wear rate for a given application may require estimates to be made of the average load if it is dynamic, but this would not take account of the possible effect of the few high load conditions. Moreover estimating the relative distance of sliding between the surfaces may be very imprecise. 

SPECIFIC WEAR RATE, mVNm LABORATORY TESTS... 

Figure 8. Relationship between service and specific wear rate and application.

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