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

摘要

The primary objective of this program was to study the processing of coated fiberreinforced 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 BN(+C) for the Nicalon fiber composites, and BN, monazite (LaPO4), and fugitive carbon for the Nextel 720 fiber composites. The layered 3M BN(+C) 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 C/Al2O3/C coatings did not result in strong or tough' BMAS matrix composites for a variety of reasons, but primarily because of coating debonding leading to fiber/matrix 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.