Wear rates

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

Fig. 10. Wear rates for iron, as measured in a pin-on-disk machine (-) unimplanted and implanted with ( ) nitrogen at rates in atoms /cm of A,...

High speed machines such as impact mills begin to suffer high wear rates when processing materials above Mohs s hardness 3, unless very special wear-resisting measures can be taken. 

Roller Mills Pendulum, Table, and Bowl Type. This is a group of machines commonly applied for grinding of mineral powders down to approximately 97% below 75 p.m, or even finer in some instances. The mills operate at medium speed, up to approximately 30 m/s, and can handle materials with up to Mohs hardness 5 before wear rates become prohibitive. Many different designs are available the two most commonly encountered variants are pendulum mills and the table roUer mill. 

As an example in estimating wear rate in a nylon bushing consider a 10-rnm diameter shaft miming 900 rpm (0.47 m/s) under 0.5 x 10 N/m (70 psi) load. The P/ of 0.235 X 10 N/m -m/s(6510 psi-fpm) and if = 0.24 x 10 m /N for filled nylon in Table 9 gives a wear rate of 0.20 mm/1000 h. Since Pp test results vary widely, these wear estimates are only guides. For maximum utiHty, the test materials, finishes, temperature, load, speed, and lubrication should dupHcate as nearly as possible those in the planned appHcation. 

Table 11 gives order of magnitude wear rates for high temperature materials sliding against themselves in pia-on-disk tests (30). 

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

Requirements. Automotive brakes must satisfy a certain set of consumer expectations, which iacludes safety, comfort, durabiUty, and reasonable cost. In technical terms, these expectations are translated iato a set of specific requirements such as high and stable friction, no or minimal vibration and noise, and low wear rates for the friction material and rotor mating surfaces, all of which have to be achieved simultaneously at a reasonable cost. Particularly, the performance has to be stable under varying appHcation conditions over extremes ia temperature, humidity, speed, and deceleration rate for occasional or many consecutive stops. The requirements for use ia machines are less stringent. 

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). 

Wear is an economic consideration. Wear resistance generally, but not always, is inversely related to friction level and other desirable performance charactenstics within any class of friction matenal. The objective is to provide the highest level of wear resistance in the normal use temperature range, a controlled moderate increase at elevated temperatures, and a return to the original lower wear rate when temperatures again return to normal. Contrary to common behef, maximum wear life does not require maximum physical and mechanical properties. 

Cermet or carbon friction matenals operate at substantially higher temperatures than the normal automotive or tmck materials. Still the wear rates of these materials increase with the brake temperature. One unique feature of these materials is the formation of a glazed friction film at high temperatures that reduces the wear rate and stabilizes the friction level. Without this glazed layer the wear rate is usually very high. 

Generally the harder the ceramic, the better its wear resistance however, other properties such as fracture toughness may play the dominant role. If a ceramic is mated with a metal hardness is the determining factor, but when a ceramic is mated with another ceramic fracture toughness appears to determine the wear rate (54). 

G Layering rate cmVs inVs V Volumetric wear rate cmVs inVs... 

The energy requirement for reducing MSW to 90 percent passing 10 cm is typically 6 kWh/ton, or 50 kWh/ton for passing 1 cm. Wear is also a major cost, and wear rates are shown in Fig. 20-60. The maximum capacity of commercially available hammer mills is about 100 ton/h. 

Each mechanism of breakage implies a different functional dependence of breakage rate on material properties. For the case of abrasive wear of ceramics due to surface scratching by loaded indentors, Evans Wilshaw [Acta Metallurgica, 24, 939 (1976)] determined a volumetric wear rate V of... 

FIG. 20-75 Fluid-bed erosion or wear rate as a function of granule material properties. Kq is fracture toughness and H is hardness as measured hy three-point bend tests. [Ennis [Pg.1888]

Sealless pumps rely on the process fluid to lubricate the bearings. If the wear rate of the bearings in the fluid being handled is not known, the bearings can wear unexpec tedly, causing rupture of the can. 

Coefficients of friction and relative wear rates for plastics... 

The wear rate of plastics is governed by several mechanisms. The primary one is adhesive wear which is characterised by fine particles of polymer being removed from the surface. This is a small-scale effect and is a common occurrence in bearings which are performing satisfactorily. However, the other mechanism is more serious and occurs when the plastic becomes overheated to the extent where large troughs of melted plastic are removed. Table 1.7... 

The diesel engine, being a reciprocating machine, is mechanically complex, and in arduous environments its wear rate can be high. Major overhauls on high-speed engines are usually stipulated at 15,000 running hours, which extends to 20,000 and 30,000 on medium- and low-speed machines, respectively. 

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