Wear silicon carbides

The engineering properties of electroless nickel have been summarhed (28). The Ni—P aHoy has good corrosion resistance, lubricity, and especiaHy high hardness. This aHoy can be heat-treated to a hardness equivalent to electrolytic hard chromium [7440-47-3] (Table 2), and the lubricity is also comparable. The wear characteristics ate extremely good, especiaHy with composites of electroless nickel and silicon carbide or fluorochloropolymers. Thus the main appHcations for electroless nickel are in replacement of hard chromium (29,30). 

In the Premier Mill the rotor is shaped hke the frustrum of a cone, similar to that in Fig. 20-53. Surfaces are smooth, and adjustment of the clearance can be made from 25 [Lm (0.001 in) upward. A small impeller helps to feed material into the rotor gap. The mill is jacketed for temperature control. Direct-connected hquid-type mills are available with 15- to 38-cm (6- to 15-in) rotors. These mills operate at 3600 r/min at capacities up to 2 mVh (500 gal/h). They are powered with up to 28 kW (40 hp). Working parts are made of Invar alloy, which does not expand enough to change the grinding gap if heating occurs. The rotor is faced with Stellite or silicon carbide tor wear resistance. For pilot-plant operations, the Premier Mill is available with 7.5- and 10-cm (3- and 4-in) rotors. These mills are belt-driven and operate at 7200 to 17,000 r/min with capacities of 0,02 to 2 mVh (5 to 50 gal/h). 

A wide range of applications for hard, wear-resistant coatings of electroless nickel containing silicon carbide particles have been discussed by Weissenberger . The solution is basically for nickel-phosphorus coatings, but contains an addition of 5-15 g/1 silicon carbide. Hiibner and Ostermann have published a comparison between electroless nickel-silicon carbide, electrodeposited nickel-silicon carbide, and hard chromium engineering coatings. 

Silicon Carbide. SiC has low thermal expansion, high hardness, and good resistance to oxidation. It is used extensively in the coating of graphite and carbon to impart wear and oxidation resistance. 

Wear shell specifications, for wet drum ore concentrator, 15 449 Wear surfaces, silicon carbide in, 22 538 Wear testing, 9 713 Weatherability... 

Silicon carbide is marketed as a surface enhancer (see Table 3.19). At levels of less than 10% it significantly improves the coefficient of friction and the wear resistance. 

The ultrafine silicon carbide produced in an electric arc is used as an insulation in cryogenic applications (152) (see Cryogenics). It generally increases the wear resistance of the paint film when added to paint formulations. 

Meial Mairix Composites. Silicon carbide particles are contributing to easy-to-cast metal-matrix composites (MMCs). When compared with their non-reinforced counterparts, the SiCp/Al components are more wear resistant, stiffer, and stronger, accompanied by improved thermal stability. Additional advantages include lower density and lower cost. Nearly all prior aluminum MMCs required labor-intensive methods, such as powder metallurgy, diffusion bonding, squeeze casting, or thermal spraying. 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see LUBRICATION AND LUBRICANTS). Silicon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

SiC, like boron carbide, silicon carbide is nearly completely covalent and is e.g. used as grinding powder, in the manufacture of ball bearings, balls, outlet casings and jet nozzles, as well as in packing rings of pumps used to transport materials which are likely to cause wear. 

Silicon nitride is prized for its hardness (9 out of 10 on the Mohr scale), its wear resistance, and its mechanical strength at elevated temperatures. It melts and dissociates into the elements at 1,900 °C, and has a maximum use temperature near 1,800 °C in the absence of oxygen and near 1,500 °C under oxidizing conditions.41 It also has a relatively low density (3.185 g/cm3). Unlike silicon carbide, silicon nitride is an electrical insulator. The bulk material has a relatively good stability to aggressive chemicals. This combination of properties underlies its uses in internal combustion engines and jet engines. 

C. R. Blanchard and R. A. Page, Effect of Silicon Carbide Whisker and Titanium Carbide Particulate Additions on the Friction and Wear Behavior of Silicon Nitride, J. Am. Ceram. Soc., 73[11], 3442-3452 (1990). 

In the case of the alloyed carbon, microcrystalline silicon carbide particles are randomly interspersed in the carbon matrix. The presence of this SiC phase greatly contributes to the hardness and wear resistance of these alloys compared to pure pyrolytic carbon. 

H. Tomizawa and T.E. Fischer, Friction and wear of silicon nitride and silicon carbide in water Hydrodynamic lubrication of low-sliding speed obtained by tribochemical wear, ASLE Trans., 30, 41-46, 1986. 

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