Maintaining low friction coefficient and ultralow wear in metal-filled PTFE composites

Many reinforcing fillers (particles, fibers, and platelets) can reduce the high wear rate of polytetrafluoroethylene (PTFE) (K∼10−4 to 10−3 mm3/Nm) by 10–1000 times. However, certain α-alumina particles can produce up to 10,000 times lower wear rate when composited with PTFE (K∼1.3 × 10−7 - 6.7 × 10−8 mm3/Nm). This ultralow wear behavior was attributed to two mechanisms: tribofilm generation and tribochemistry. However, while reducing the wear rate of PTFE, α-Al2O3 can also increase its friction coefficient up to two times (μ∼ 0.1 vs 0.2–0.25). This study revealed that the friction coefficient of PTFE-Cr against Brass 260 counterbody is up to three times lower than the ultralow wear PTFE α-alumina against 304 Stainless Steel (μ = 0.08–0.12 vs μ = 0.2–0.25) with up to 200 times lower steady-state wear rate (K∼1.4-34 × 10−9 mm3/Nm) than PTFE α-alumina against 304 SS (K∼6.7-13 × 10−8), and ∼3 × 105 times lower than unfilled PTFE. PTFE-Ti (μ = 0.09, Kss = 2.9 × 10−8 mm3/Nm) and PTFE-Mn (μ = 0.12, Kss = 1.2 × 10−8 mm3/Nm) against Brass 260 also performed better than PTFE-α-Al2O3 against 304 SS both in reducing wear rate and friction coefficient. The tribofilms created by the PTFE-Cr, PTFE-Mn, and PTFE-Ti composites were evaluated using Scanning White Light Interferometry (SWLI) and Infrared (IR) Spectroscopy. These analyses showed that these composites created thin (500–1500 nm) films on the Brass 260 counterbody and their worn surfaces exhibited new chemical groups (carboxylate salts, hydroxyl groups, and acetate ions). These new chemical groups signal the formation of bonds between the PTFE matrix, filler particles, and the metal counterbody, which protects both the composite and counterbody from wear.