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ifriction.com comments: Specific gravity 1.7 gms/cc, Moh hardness 2. One of the cheapest fillers available. Low specific gravity - a little goes a long way. Used in particle form. Very popular in the 1930's in rolled or extruded linings for light duty applications. Still found in low cost material for bonding segments, but its use is decreasing because of it acting as a lubricant at medium high temperatures. It just burns out - the gases and juices formed act to reduce friction . Beware some coals contain large quantities of ash. Usual to chose coal with less than 6%. Check for abrasives. See scratch test recommended for graphites. Coal is not good for high temperature applications. Foundry coke is harder than coal, usually imparts lower friction. Not in popular use today. A little more heat stable. |