The development of glass-ceramic matrix composites and the crystallization of two glass-ceramic matrices

In order to develop ceramic matrix composites for high temperature applications, two glass-ceramic matrices, one an yttrium aluminosilicate (YAS), the other a magnesium aluminosilicate (cordierite), were chosen as possible matrix compositions. Two separate sets of experiments were performed. Specimens were hot pressed and heat treated then analyzed for the crystallization products and C.T.E. Specimens were also prepared containing commercially available ceramic fibers and coated ceramic fibers to investigate fiber/matrix interactions and the effect of fiber additions on the crystallization of the glass matrices. DTA was employed to approximate the crystallization temperature of the glasses and to determine the effect of dopant and fiber additions on the glass crystallization. Glass specimens hot pressed above 1000° C for 15 minutes would crystallize while specimens hot pressed for shorter times, i.e., 2 minutes, would remain amorphous. The cordierite matrix crystallized to α-cordierite. The YAS glass would crystallize to mullite and α-Y₂Si₂O₇ when hot pressed at, or below 1260 °C and to mullite and ß-Y₂Si₂O₇ when hot pressed at 1300 °C. Thermal expansion measurements on YAS glass specimens showed the α - to ß-Y₂Si₂O₇ phase transformation to be displacive and occur at 1280 °C. Fiber additions enhanced crystallization, but without protective coatings, they chemically bonded to the matrix eliminating fiber pullout and causing brittle failure of the composite during testing. Fiber coatings would effectively reduce this interaction and allow fiber pullout to occur. --Abstract, page iii