Preparation And Properties Of B₄C/PTFE Composites

Though polytetrafluoroethylene (PTFE) is a well-known polymer which has been applied in friction field due to its various tribological advantages, its application in anti-wear area has been somewhat limited by its poor bending strength and poor wear resistance. Plastic deformation and serious wear happen when PTFE is used under high load or at high sliding speed. Boron carbide, one of famous friction materials, shows its appealing prospect owing to excellent properties including ultra-high hardness, corrosion and wear resistance. Nevertheless, the brittleness of B4C limits its application. Recently, the PTFE composites reinforced by using various fillers, mainly including carbon fiber, nano-alumina, silicon carbide, have been extensively studied. However, the reports on the B4C strengthened PTFE composites are still very rare.In this study, the B4C/PTFE composites were fabricated via vacuum sintering method. The mechanical and tribological properties of the composites were studied. The effect of load, sliding speed, mass fraction and size of B4C particles on friction coefficient, wear rate and wear mechanism were investigated. Moreover, the effect of the B4C particles treated by silane coupling agent on the tribological properties of composites were also analyzed. The obtained results are as follows:(1) the bending strength and fracture toughness of the B4C/PTFE composites increase with the increase of content of PTFE. The bending strength of B4C/PTEF composites with w(PTFE) of50%is60MPa, which is5times of pure PTFE, and3times of Ba2Ti9O20/PTFE composites. It is also found that when the content of PTFE keeps constant, the mechanical properties of the composites decrease with the increase of content of coarse B4C. This is because the coarse particles become source of stress concentration, leading to the decrease of the mechanical properties.(2) Wear resistance of the composites filled with fine B4C becomes better than that of pure PTFE, and the friction coefficient of the composites is higher than that of pure PTFE too. It is demonstrated that the friction coefficient of composites approachs that of PTFE (0.04) in the conditions of high load and high sliding speed. The composites filled with fine B4C, whose wear mechanism is adhesion wear, can protect parts well, so it can be used as anti-wear material.(3) It is known from the study that the composites filled with coarse B4C have much better wear resistance than pure PTFE. However, the friction coefficient increases sharply. Furthermore, the coarse B4C on the surface of composites plough the friction pair severely, and its wear mechanism is abrasive wear caused by the B4C abrasive from cracked coarse B4C particles. Therefore, this kind of composites has a high potential application in grinding field.(4) The surface modification of B4C particle using silane coupling again can improve the interfacial strength between the polymer and particle thus resulting in the improvement in the tribological properties.