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The Effect of Processing on the Interface, Microstructure, and Properties of Coated Fiber Reinforced Glass-Ceramic Matrix Composites

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Final rept. 1 May 95-31 Mar 98

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The primary objective of this program was to study the processing of coated fiber reinforced glass-ceramic matrix composites that will result in reproducible, reliable fiber coatings and the composites that incorporate them, for use to 1200C in advanced gas turbine engine applications. The primary glass-ceramic system was a barium magnesium aluminosilicate BMAS, while the fibers were Ceramic Grade Nicalon Si-C-O and Nextel 720 aluminosilicate fibers, as well as new advanced high temperature SiC based fibers. The primary fiber coating system was layered BNC for the Nicalon fiber composites, and BN, monazite LaPO4, and fugitive carbon for the Nextel 720 fiber composites. The layered 3M BNC coatings on Nicalon fibers produced very strong and tough BMAS glass-ceramic matrix composites, due to the limited matrix element diffusion through the coating and matrix crack deviation within the layered coating structure. These composites exhibited excellent tensile fatigue and stress-rupture properties, although oxidation of the layered BN coatings at intermediate temperatures 700-900C under conditions of high fatigue stress 138 MPa may potentially limit the stress at which these composites can be utilized for structural applications. Alternate BN and Si-doped BN coatings from other vendors did not perform as well, with excessive BN crystallization occurring during composite fabrication. Nextel 720 aluminosilicate fibers with either BN, fugitive carbon, or CAl2O3C coatings did not result in strong or tough BMAS matrix composites for a variety of reasons, but primarily because of coating debonding leading to fibermatrix interactions. Monazite LaPO4 coated Nextel 720 fibers reacted excessively with the BMAS matrix during composite fabrication. The crystalline SiC fibers from the U. of Fla., especially with the in-situ BN coatings, appear to offer future promise for high temperature CMC reinforcement.

Subject Categories:

  • Laminates and Composite Materials
  • Manufacturing and Industrial Engineering and Control of Production Systems
  • Jet and Gas Turbine Engines

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