The Abrasion Process

Research into types of abrasives suitable for bonding was carried out because of field failure of some bonded components. Grit recommendations based on research are given in Table 3.2. 

Particular care should be taken to avoid grits that apparently clean the surface but whose inherent deficiencies are masked to some degree by the ability of the primer to bond residual particles to the surface. Because there is no guarantee that all of the oxide has been removed by such grits there can be no guarantee of the reliability of such parts in service. The use of such grits that plate or incompletely clean the surface of the substrate are not recommended in FMEA terms. The use of such would increase the occurrence of higher R values in the RPN system. 

Wire drawn steel shot Hardness not listed in literature No 

HV - Vickers hardness HB - Brinell hardness HRc - Rockwell hardness  

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Since this appears to be the basis of the abrasion process it is profitable to be familiar with its shape. Figure 26.45 shows this relation for an unfilled NR compound [40,41]. It can be divided into four regions. 

If these large energy concentrations meet a flaw in the rubber, the resulting tearing energy will increase the flaw at each pass until a rubber particle is detached. This is essentially the mechanical basis of the abrasion process. 

For abrasion this is, however, a much more dominating process than for cut growth. The main reason is that the energy consumption in the abrasion process raises the temperature in the interface between rubber and track and thereby modifies this process. The temperature in the contact patch is a function of the power consumption and depends, therefore, also on the sliding speed. The temperature not only influences the oxidation and cut growth process, but also causes thermal degradation. 

As is seen from this latter figure, an increase of the solids loading results in a decrease in the cyclone attrition rate. This may be due to a cushioning effect of the increased solids concentration which is well known in comminution processes. This cushioning effect may be interpreted as a decrease in the efficiency T] of the abrasion process. If we assume r] to be a function of jJ and in its simplest form to follow an exponential function,... 

Chemical Reactivity. Any chemical interaction between abrasive grains and the material being abraded affects the abrasion process,... 

The caryopses of grasses and cereals are anatomically complex. The starch endosperm of the caryopsis is surrounded by the aleurone layer and pericarp, both of which can be partly or completely removed, along with the embryo, by the abrasive process known as pearling, which removes the bran. The flour obtained by milling a pearled grain consists of protein, starches, and a low proportion of endospermic hemicellulose and non-endospermic hemicellulose from the bran. 

The figure of this theoretical wear curve is coincident with that one of an experimental curve of the abrasion process (Figure 5). 

Laboratory abrasion tests are notorious for not correlating with service. The abrasion process is complicated, and the rate of abrasion is very much dependent on the particular conditions. Laboratory tests cannot often properly represent the conditions in service, and hence correlation is difficult if not impossible to find. This docs not mean that correlations never exist, but these will be application and test specific. The usual approach is to seek a correlation for the particular circumstances and to test materials on a comparative basis, rather than to predict service wear rate in absolute terms. 

The kinetics of the wear in a spherical aluminosilicate catalyst is shown in Figure 7.57a. This figure shows the relative percentage of mass removed as a function of time, m(f) = [Mq - M(t)]/Afo, where Mq is the initial material mass and M(t) is the residual mass after milling time, t. The three curves labeled 1,2, and 3 represent the abrasion process as a reaction of the zeroth, first, and second order, respectively. Curve 2, representing the exponential dependence m(f) = 1 - exp(-t/x), yields the best fit of the experimental data. The time constant, t, may serve as a measure of the catalyst s resistance to abrasion. 

Free radical formation was identified v/ith e.s.r. during the fracture process by Zurkov , Peterlin and DeVries . Thus, polymer wear due to the fatigue as well as the abrasive processes can lead to molecular degradation or thermal degradation of polymers. The degradation should predominantly take place on the polymer surface as shown by Bely and Sviridyonok. ... 

This phenomenon, which has been described and discussed by Schal lamach, clearly plays a very important part in the abrasion process. The detailed way it is developed, and the precise way it is deformed during abrasion is in general complicated. It is these complications that have perhaps been the main difficulty in producing a precise theory. 

The exponent a varies from about 2 for natural rubber to 4 or more for non-crystallizing unfilled rubbers such as SBR . With the above model of the abrasion process the rate of abrasion A, in cm per revolution of the wheel, will therefore be given by... 

In the grinding of many metals, reaction of the freshly cut metal surface with air forms an oxide layer which can effectively prevent adhesion of the metal surface to the abrasive, to the bond, or to itself. This facilitates the abrasive process. Substances which can react more rapidly with fresh metal surfaces than does air, or which can melt and cover the surface with a protective layer, are called active grinding aids. The most common ways such substances are used is in the oils or water-based coolants for wet grinding and as impregnants or fillers in the bond of the abrasive article for dry grinding. 

Before the abrasion process starts, irrespective of the method used, the substrate should undergo at least one of the cleaning processes highlighted earlier. 

Water-borne grit-blasting generally gives more control of the depth and severity of the abrasion process and is particularly recommended for plastic and reinforced plastic materials. 

This approach uses a vapour or solvent degreasing operation in accordance with one or other of several European and American specifications [6-12] followed by an abrasion of the aluminium surface using wire wool or Scotchhrite. This is followed by a second degreasing step. As the aluminium is relatively ductile, the abrasion process tends to cut easily into the surface. Although this produces a macro-rough surface, loosely attached aluminium detritus from the abrasion process will provide points of weakness in the bonded joint. Further, as Fig. 2... 

XPS survey spectra of low carbon steel sheet following (a) alkali cleaning and (b) energy abrasion. Note that the abrasion process, which removes the near-surface material, results in a much cleaner surface... 

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