Impact of Pore Structure on Densification Efficiency of 2-D Carbon- Carbon Composites and Its Relationship to Mechanical Properties

摘要

The purpose of this study is to experimentally determine the efficiency and effectiveness of AFRL's In-Situ densification process for 2-D carbon-carbon composites. This method of densification intends to provide competitive thermal and mechanical properties based on minimal matrix porosity and above average skeletal density faster and cheaper than more standard liquid phase carbon-carbon composite densification. In order to validate this hypothesis, small sections of carbon-carbon composite panels were cut out at each stage of fabrication and tested for open porosity and skeletal density. These properties were determined using two proven methods of porosity testing: boiling water immersion and mercury porosimetry. At key stages of processing, mechanical properties were determined to assess the direct correlation between porosity characteristics and matrix strength. Results were gathered for several panels with the same processing conditions to determine possible variation during processing. Once the porosity characteristics and trends were understood for the entire In-Situ densification process, the data was correlated with the experimentally determined mechanical properties. The results show that the majority of matrix porosity originates during matrix carbonization, and three cycles of the In-Situ densification process can reduce porosity from 25% to 10% by volume with minimal processing time. Skeletal density values between 1.70 and 1.75 g/cc were also obtained. The results also show an increasing trend in the flexural strength, elastic modulus, and interlaminar tensile strength of the composite throughout densification. The fully densified composite demonstrated flexural strength above 23,000 psi and an elastic modulus above 9700 ksi. Also the interlaminar tensile strength of the composite increased from 230 psi after the first densification cycle to nearly 500 psi after the third cycle.