Dynamic elastic modulus and vibrational damping in nicalon SiCxOy fiber/borosilicate glass composites:: Effects of thermal cycling

Measurements of dynamic elastic modulus and vibrational damping were made at room temperature for DURAN (a borosilicate glass) and SiCxOy Nicalon(TM)/DURAN (a glass matrix composite), Both sets of materials had been thermal cycled to 500 and 700 degrees C which are below and above the glass transition temperature (T-g = 530 degrees C), respectively. The piezoelectric ultrasonic composite oscillator technique (PUCOT) was used to determine the values of the Young's modulus and damping. Archimedes' method was used to find the density of the specimens, and the impulse excitation technique was used to find the flexural modulus, Microstructural examinations were made on selected specimens. The experimental results showed that thermal cycling of the composites below T-g had no distinguishable effect on the density, dynamic Young's modulus or flexural modulus values; however, an increase in damping of 56% was observed. For thermal cycling above T-g, the density decreased by about 0.5%, the Young's modulus decreased by 8%, the flexural modulus decreased by 15% and the damping increased by 608%. The simultaneous decrease of elastic modulus and density, and increase of damping in the composites with increasing thermal cycling temperature were analyzed in terms of microstructural degradation due to thermal effects on the matrix, fibers and interfaces.