Steady-state wear

The tests in which the epoxy pins were rubbed on steel disks showed that the pins were initially worn by the abrasive action of the asperities on the steel surface. This initial wear correlated with the inverse of the values. During the initial wear, the steel surface was smoothed by the transferred epoxy material. The steady state wear which followed the initial wear was lower in magnitude than the first stage of wear. The highest wear rate was obtained with 15 wt,-% of the dimethyl siloxane modifier. 

The initial and steady state wear rates of the siloxane-modified epoxy pins on the steel disks correlated with the inverse of the KIC values which agrees with previous abrasive wear tests 47>. The steady state wear rates on the smooth glass disks were comparable to those on the steel disks. Thus in both cases the wear mechanism is abrasive wear by the wear particles trapped in the interface between the pin end and the disk. 

Figure 5.6. Steady state wear rates of materials tested against X5CrNil8-10, [Cop = 80% UHMWPE and 20% HDPE, Cop 2%p = Cop + 2% MWCNT (pre-treated), Cop 2%u = Cop + 2% MWCNT (un-treated), Xue et al., (16)].

In functioning machinery the contacting parts repeatedly rub one another many times. The interaction of two surfaces on reiterated contact will in part depend on the condition in which the previous iteration left them. Under ordinary circumstances, with the machinery operating satisfactorily, each iteration is much like the one before and an analysis of steady-state wear or friction can be made on the basis of one cycle of surface interaction. Generally in such cases, but not necessarily always, asperity deformation is elastic rather than plastic. Whether an adhesive junction forms depends on the condition of the asperity surface. If the materials f>e.n. 4e are easily adhesive but the surfaces are covered by a film which inhibits adhesion, then to initiate adhesion obviously the film must first be removed, broken up or penetrated. The subsequent course of adhesive contact will then be governed by such factors as the size of the contact, the shape of the asperity, the impressed load, the strength of the material, etc., in accordance with the fundamental modes of behavior. 

The most comprehensive fatigue wear model (2) proposed In the literature was used to predict wear rates to within 30 percent or less of the experimentally measured values (J). The exponent t was determined from notched cylindrical specimens In reverse bending. The number of contacts and the areas were determined from surface profiles of the polymer and the counterface after steady-state wear was attained. The wear data was obtained from a polymer pin sliding on a rotating cylinder. 

When polymers slide on machined metal surfaces, it is quite possible that steady-state wear Involves a combination of abrasive, fatigue, and adhesive wear mechanisms. To study fatigue wear, it would be desirable to minimize the contributions of the abrasive and adhesive wear modes. In this paper, the following polymers polycarbonate, polyvinyl chloride, ultra-high molecular weight polyethylene, siloxane modified epoxies, and polylmldes are tested in experiments in which the fatigue wear mode is predominant. 

The Initiation of the wear track occurred simultaneously with the abrupt rise in the friction. The wear rate decreased from its initial value to a steady state value in less than 4 kc after initiation (see Figure 4). The steady state wear rates, expressed in square micrometers of track cross section area per kilocycle of sliding, are given in Table III. 

The wear results were characterized by an Initial period of no wear followed by catastrophic initiation of the wear track. This Initiation was distinguished by a sharp Increase In friction coefficient and a wear rate which was higher than the subsequent steady state wear. Fatigue wear Is proposed as the predominant wear mechanism because of the multiple stress cycles required to Initiate wear and because of the positive correlation between the wear rates and the elastic moduli of the polylmldes. 

Inserting Equations (23), (28), (5), and (21) into Equation (24) the modification of the steady-state wear equation will be obtained... 

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. 

Figure 13o Steady state wear of the tissue under inter-mittant sliding. Curve A oscillation stopped for 55 seconds, resumed for 1 miuo Curve B oscillation stopped for 15 seconds, resumed for 1 minb Pressure 2,07 MN/m. ...

The tissue s steady state wear rates, under cyclic pressures... 

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... 

HOPE sliding against the abraded steel surface, a steady-state wear rate of 0.01 mg/hr and a coefficient of fraction of about 0.28... 

In the sliding of 30% CuS, 70% HOPE against the rolled brass, the steady-state wear rate was 0.78 mg/hr (Figure 13) compared to... 

Thirty percent CuS as a filler in high density polyethylene produces a dramatic reduction in the steady state wear rate to... 

Also, the addition of a 4.5% volume fraction of surface-modified AI2O3 nanoparticles to an unsaturated polyester led to a significant increase (by almost 100%) in its fracture toughness [270]. The wear resistance of polytetrafluor-oethylene (PTFE) can be similarly increased by a factor of 600 upon addition of 20 wt% alumina nanoparticles [271]. The steady-state wear rates of polyphenylene... 

Table 7.2 A summary of the average steady-state wear rates of CFR PEEK against various counterface materials...

Counterface material CFR PEEK composition Average steady-state wear rate (10 mm N m" Reference )... 

In addition, as a typical failure mechanism, fiber/matrix debonding occurs due to the shear and tension type loading. If fiber/matrix debonding has taken place, the local separation initiates additional fiber cracking, wear debris formation, and a more intensive wear process. In the steady state wear process, a so-called compacted wear debris layer (CWDL) covers the surface it is composed of pulverized wear debris and matrix material. During the wear process, this layer is continuously formed and removed by the surfaces sliding over each other. 

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