Processing Mesocarbon Microbeads to High-Performance Materials fo Friction Applications

Honeywell, a leading manufacturer of friction materials for aerospace applications, manufactures brakes by densifying a carbon fiber preform using chemical vapor deposition. This has two major drawbacks, namely, the high cost of the fiber preform, and the time required to densify it via CVD. To address these issues, a novel material, mesocarbon microbeads, are investigated. This relatively inexpensive and rapidly processed material has shown good friction characteristics, but unacceptable fracture toughness. Therefore, major goals are to understand the behavior of MCMB during processing, and to investigate the most effective approach to producing high-toughness materials. Dilatometry, thermogravimetric analysis, x-ray diffraction, scanning electron microscopy, pycnometry, indentation, and compact tension were used to study sintering, graphitization, and in-situ reinforcement of MCMB-based materials. It is shown that low temperature sintering consists of two processes: neck formation via a non-densifying liquid-phase sintering mechanism < 800 K, and significant sample shrinkage due to changes of true density in the region 800 – 1200 K. This results in material densification without decreasing porosity. During high temperature treatment, shrinkage is again accompanied by increases in theoretical density, maintaining porosity. However, observation of pore microstructures indicate that high temperature sintering mechanisms are active, which could result in overall porosity elimination given sufficiently long sintering times. Activation energy of 100 kcal/mol was determined for the graphitization of MCMB. This is less than found for many other types of carbon, and indicates the effect of preferred orientation on graphitization. Also, it is demonstrated that exothermic reactions during in-situ reinforcement can lead to undesired swelling events. Through careful processing, these are reduced or eliminated, and additional sintering is achieved. Additionally, polymeric decomposition yields nanoscale reinforcements and enhanced sintering properties. Results show that this yields materials which are more tolerant of porosity content. Thus it is shown that the combination of excellent compressibility, low temperature sinterability, and rapid graphitization makes MCMB an attractive precursor for manufacturing carbon-based materials. Based on this, it is recommended that beta-resin content be optimized, methods of increasing initial sample density be investigated, and techniques of reinforcement without increasing porosity be explored.