Polymer-Derived Ceramic Coil Springs

Polymer-derived ceramic coil springs were manufactured by thermoplastic coiling of extruded filaments. Feedstocks for extrusion were processed from polymethylsilsesquioxane loaded with 34 vol% FeSiCr (reactive filler) and 16 vol% Si3N4 (inert filler). The pyrolytic conversion into a composite residue, the surface reactions, and the mechanical properties were analyzed. Annealing the extruded spring preforms in a reactive nitrogen atmosphere at 1300 degrees C resulted in the formation of a surface reaction zone with a thickness <10 mu m where nitridation reaction products are likely to heal cracks and pores. The as-pyrolyzed ceramic springs (without surface machining) were loaded in compression. Ceramic coil springs with a filament diameter of 2.5 mm were characterized by a spring constant of 14.3 N mm(-1) and attained a fracture stress of 187 +/- 40 MPa.