TRIBOLOGICAL PROPERTIES OF RESINS AT CRYOGENIC TEMPERATURES

摘要

Highly efficient rocket engines use powerful fuels and oxidisers like liquid hydrogen (LH_2 at 20 K) and liquid oxygen (LO_X at 90 K) under high pressure (around 200 bar). The flow of such fluids is controlled by valves including seals operating under severe conditions of temperature and pressure. The seals must operate reliably during a large number of actuations, with contact pressure higher than 20 MPa and reciprocating sliding speed up to 0.3 m/sec. Under these conditions, the tribological behaviour of the seals becomes a critical issue for the design of valves. Due to a lack of available documented design data, it was decided to launch a campaign of tribological tests to acquire information on materials to be used under the above-mentioned conditions. To help designers, data must be gathered in a model (equations) easy to use and sufficiently accurate. Fnction coefficients and wear rates were obtained by means of a pin-on-disk tribometer on pairs made of fluoropolymers or polyimide rubbing against an aluminium alloy at room temperature and at 77 K. To optimise results, data points were distributed in a contact pressure/sliding speed range according to a test plan and a corresponding regression analysis. Experimental aspects are presented and discussed. The good agreement with mathematical approximation by second degree polynomials is demonstrated through response surfaces. As a consequence the engineer/designer can use the appropriate formulas to calculate values of the friction coefficient and wear rate for the applicable speed and pressure conditions of various polymer/metal pairs.