Wear behavior of polymeric compositions in dry reciprocating sliding

The development of Stirling engines as high efficiency motors for automobiles is critically dependent on the ability of sliding seals to operate for long periods (up to 3500 h) without excessive leakage or wear. Piston rings must run completely dry while sealing against hydrogen leakage under a pressure drop of 5–20 MPa. Main seals, cooled by oil, must seal against similar pressures without admitting any oil which could contaminate the high temperature sections of the engine.The results of an initial search for materials with lower wear rates than the polytetrafluoroethylene (PTFE) polymer compositions currently used in test engines are described in this paper. On the basis of a literature review, the results of engine development tests and experience with solid lubricants, several commercially available formulations were selected for a screening test program. In addition, some custom formulations were prepared by commercial vendors.A reciprocating screening tester, operating at 1200 cycles min^?1 with a stroke of 40 mm, was used to determine material wear rates at a PV factor below the first PV limit of the reference PTFE material. Materials with wear rates in the range of 0.13 × 10^?6–0.16 × 10^?6 mm³ N^?1 m^?1 (the reference material wear rate was 0.40 × 10^?6 mm³ N^?1 were found.Friction coefficients were also measured since friction power loss in the seals represents a significant portion of the total mechanical engine losses. Compositions which significantly reduced the friction coefficient while retaining a low wear rate were found.