Testing Thermoplastic Composites against Steel
How do plastics wear against metals? In tests mating thermoplastic composites and AISI 1141 cold-rolled steel counter-faces, the highest composite wear factors (greatest wear) resulted with the smoothest range (8-12 µin.) of metal surface finishes. This result, contrary to what might be expected intuitively, is attributed to the adhesion-and-deformation friction mechanism of metal/plastic combinations. The 12 to 16 µin. range yielded the lowest wear factors, and the 50 to 70 µin. surface-finish range generally produced intermediate wear-factors for most composites (un-reinforced formulations were exceptions).
PTFW-lubricated, glass-fiber-reinforced polycarbonate was the only material to experience an increase in surface roughness when tested against a steel counter-face. Surface roughness of the other plastic composites was reduced to as little as one-eighth in the same test. In all cases the metal surface became rougher.
Wear Behavior of thermoplastic composites, as a function of the surface finish of stainless-steel and brass counter-faces, was tested at only two metal roughness values instead of three. As was expected, wear factors of all composites increased when the surface roughness of the mating 304 stainless steel counter-face was increased from the range of 8 to 16 µin. to that of 50 to 70 µin. Friction coefficients, however, tended to decrease as the surface roughness of the mating metal counter-face was increased.
Wear factors of the plastic composites and friction coefficients of the wear pairs were generally greater with 440 stainless than with the 304 grade. The 440 material also had a higher wear factor than the 304 grade-probably because the 304 grade work-hardens with wear.
Plastic wear factors against 8 to 16 µin. bras surface finishes are essentially the same as those for the stainless steels. However, the brass specimens wear up to eighty times faster than the stainless steel when mated with glass-fiber or carbon-fiber-reinforced composites. PTFE-lubricated nylon 6/6 and PTFE-lubricated, aramid-fiber-reinforced nylon 6/6 composites produced extremely low wear on the brass samples. However, the final surface finish of some specimens was as much as four times rougher than that of stainless in analogous wear pairs.
Compared to the other metal finishes, those on aluminum alloy 2024 counter-faces proved to have a narrower tolerance range yield acceptable wear factors for the plastic composites. The lowest wear rated occurred with the 12 to 16 µin. finish on the aluminum.
PTFE-lubricated unreinforced nylon 6/6 and the PTFE-lubricated, aramid-fiber-reinforced nylon 6/6 composited provided thr lowest wear rates of any mating pair involving thr aluminum alloy. However, the wear factors o all plastic composites tested against aluminum were higher the corresponding wear factors generated against steel, stainless steel or brass. Excessive system wear with phosphor bronze counter-faces was noted with all thermoplastic composites except for those tested with the same two PTFE-lubricated formulations.
Wear Behavior of Fluoropolymer-Based Composites
PTFE composites containing inorganic fillers such as graphite powder, coke flour and milled-glass fibers yielded higher wear factors and mating-surface wear when run against aluminum than against steel. In contrast, PTFE composites containing bronze and synergistic MoS2, yield lower composite and mating-metal wear rates against aluminum than against steel. PTFE composites containing PPS, polyoxybenzoate, and aromatic reinforcing polymer (ARP) organic fillers c similar ear rates and mating-surface wear when run against steel and aluminum. With all fluoropolymer composites, friction coefficients of friction were greater against aluminum n against steel.
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