The Effect of Additives and Processing Conditions on Properties of Sintered Mesocarbon Microbeads

Mesocarbon microbeads are an excellent precursor for high performance carbon materials because of their ability to self-sinter. The sintered product has many attractive properties such as high density, good thermal conductivity and high bending strength. However, the fracture properties need improvement for use in brake applications. To enhance the fracture properties, firstly, the carbonization behavior was systematically investigated and the processing conditions were optimized. Secondly, reinforcements, such as carbides and borides were introduced and the effect of heat treatment temperature on composite properties was studied. Thermogravimetry-mass spectroscopy analysis proved that the chemical processes associated with hydrogen release are responsible for the major shrinkage event. Cracks formed during heat treatment when microbead pellets were compacted above 100Mpa. The oxidation pretreatment increased plasticity of the pellets and effectively eliminated crack formation. However, x-ray diffraction and microstructure analysis indicated that the liquid phase sintering (<800K) that is critical to the development of 'bridges' between microbeads, is prohibited, and the fracture properties decreased.

Eutectic liquid formed when a TiB2/C composite was heat treated at 2800K or above, as evidenced by observation of a layered structure in the polished cross-section. At low TiB2 loading, liquid phase sintering leads to a homogeneous distribution of TiB2 particles in the carbon matrix of much smaller size and increased fracture properties. At higher loading, a complete transformation of mesophase carbon to graphite was observed and fracture toughness decreased. Diffusion of boron to the carbon matrix was proved by laser ablation ICP-MS. However, a control experiment indicates that it is not the main cause for the observed change in materials properties. Resistance to oxidation increased by more than two orders of magnitude due to TiB2 additions. Fracture properties of the TiB2/C composites increased further when finer TiB2 powders were used, and eventually a composite with fracture toughness as high as 1.92MPa.m0.5 was obtained. Average pore size decreased by one order of magnitude for the best composite. Similar results were also obtained for TiC/C and ZrB2/C composites but not for Ni/C composites, where the catalytic graphitization by nickel only leads to the formation of graphitic carbon. Further improvement maybe achieved by techniques such as hot-pressing to decrease the porosity level.