High-Temperature Hardness and Its Relationship with Plastic Yield Properties of Short Carbon-Fiber Reinforced Polyphenylene Sulfide (PPS)-Matrix Composite with Graphite Flakes and PTFE Particles

A study was conducted on high-temperature hardness of multifunctional polyphenylene sulfide (PPS)-matrix composite containing carbon fibers (CFs), graphite flakes (Gr) and polytetrafluoroethylene (PTFE). The PPS-matrix offers excellent mechanical properties, thermal stability, and chemical resistance at high temperature. Carbon fibers are added as reinforcement for improving strength and stiffness. Graphite flakes and carbon fibers are good thermal conductors for thermal management. PTFE and graphite are solid-state lubricants for tribological improvements. Thus CF/Gr/PTFE/PPS material is true multifunctional due to attractive combined mechanical, thermal, and tribological properties of its constituents. The high temperature referred to temperature near, at and above composite glass transition. The Rockwell Hardness (M scale) was determined from room temperature to 155°C. Hardness of the composite was found to decrease monotonically with increasing temperature, particularly above its glass transition temperature. At high temperature, rigid carbon fibers and graphite carried significant indentation load and constrained segmental chain mobility, thus, preserving composite hardness. The injection molded PPS-matrix composite had flow-induced carbon fiber orientation. The composite with transverse fibers had the highest transverse hardness while the composite with randomly oriented fibers saw an increase in hardness. The composite hardness was directionally dependent and had a linear relationship with its yield strength at high temperature, but its proportionality changed above and below its glass transition. The relationship enables subsequent prediction of composite tribological behavior based on basic yield property and provides insight into roles of high temperature and plastic yielding on friction and wear.