One purpose of the present study is to evaluate the tribological behvior of a fast-carbonized (1000°C/min) C/C composite. One other purpose of the study is to enhance the tribological performance of the composite by applying a post-treatment comprising re-impregnation of a carbonaceous additive-doped liquid precursor. The results indicate that average coefficient of friction (COF) values of non-post-treated composites prepared with three different carbonization rates (1100 and 1000°C/min) are similar (0.40-0.45). The average wear rate of samples carbonized at 1000°C/min is about twice as large as samples carbonized at 1 and 100°C/min. Great majority of the samples demonstrate an increase in density and a decrease in porosity in porosity after the post-treatment. Pitch-group samples generally have larger changes in density and porosity than furan-group samples. After the post-treatment, all samples demonstrate decreases in both COF and specific wear rate coefficient. Pitch-group samples generally exhibit lower wear rate than furan-group samples. Samples post-treated with pitch/carbon blackand pitch/mesophase pitch demonstrate the lowest wear rates among all samples tested. These results indicate that an appropriate post-treatment, especially a pitch treatment, may dramatically improve the tribological performance of fast-carbonized C/C composite.
Carbonization is one of the most time and energy-consuming steps in the entire fabrication process of carbon/carbon composites, especially for those densified by liquid phases. Not only properties, the economical efficacy of a C/C composite is also critically dependent on a careful control of its carbonization process. Logically the simplest way to reduce the manufacturing cost of C/C composites is to increase the carbonization rate, which is usually very low. However, carbonization is primarily a process of pyrolysis of hydrocarbons of a carbon precursor. The pyrolysis of hydrocarbons generally involves such processes as cleavage of C-H and C-C bonds to form reactive free radicals, molecular rearrangement, thermal polymerization, aromatic condensation and elimination of side chains. To minmize such adverse effects as shrinkage, cracking and thermal stresses that may build up during carbonization, low carbonization rates are usually required.
An earlier study of the present authors investigated the effect of an extremely high carbonization rate (1000°C/min) on properties of a polyacrylonitrile (PAN)/phenolic-based C/C composite. One major finding was that, after second graphitization, the bending properties of the composite carbonized at 1000°C/min were comparable to that carbonized at 1°C/min. Furthermore, the composite carbonized at 1000°C/min had higher fracture energy than those carbonized at lower rates. Apparently, this fast carbonization process has demonstrated a great potential from a practical point of view. Since C/C composites have been widely used as a friction material, one purpose of the present study was to evaluate the tribological behavior of the same fast-carbonized C/C composite.
The tribological behavior of C/C composites may be affected by many material, fabrication and testing factors, such as precursor fiber and matrix materials, woven structure, process parameters, loading condition and environment humidity. Several methods have been used or proposed to improve the tribological performance of C/C composites. For example, Matsumoto and Forsythe discovered that a small amount of crystalline silicon carbide uniformly distributed throughout the composite could reduce wear with either no change or a slight increase in coefficient of friction. Murphy introduced a C/C brake disc with a protective outer layer of ceramic to adjust coefficient of friction and/or wear resistance. Johnson used a laminate of carbon fiber precursor material incorporated with silicon powder. Carbon was then deposited on the silicon particles by chemical vapor infiltration and reacted with silicon to form silion carbide to improve wear resistance of the composite. Matsui and Ysutake impregnated a porous C/C composite with liquid hydrocarbon and aqueous solution of various inorganic compounds and found that the COF of the composite incorporated with oxide was relatively low and stable in the temperature range from room temperature to 1000°C.
As shown in the present results, the tribological performance of the present fast-carbonized composite is found inferior to that prepared with lower carbonization rates. Therefore one other purpose of the present study is to improve the tribological performance of the fast-carbonized composite by applying, after second graphitization, and additional cycle of impregnation, curing, carbonization and graphitization. In this post-treatment, two different liquid precursors and three different additives were used.