Studies on introducing phyllosilicate minerals into polymers to improve their tribological properties can provide some ideas for the development of intelligent self-healing composite materials. Polytetrafluoroethylene (PTFE) is widely used as one of engineering materials and solid lubricating materials due to its good self-lubricating ability, thermal stability, corrosion resistance and machinability. However, the molecular bond structure of PTFE exhibits unique slip characteristics during crystallization, making it prone to creep and resulting in a poor wear resistance. It is thus necessary for the modification of PTFE to improve the mechanical properties and tribological performance for engineering applications. The modification methods mainly include surface modification, blending modification, and filling modification. Recent studies indicate that the introduction of minerals such as serpentine, attapulgite, and sepiolite into PTFE can obtain PTFE-based composites with uniform mineral dispersion and excellent thermal stability, mechanical properties, and tribological properties due to the property of phyllosilicate minerals to intercalate monomer or polymer molecules. In this paper, antigorite and wollastonite minerals reinforced PTFE-based composites were prepared. The tribological properties of the composites were investigated, and the mechanism of the tribological properties improvement of PTFE caused by antigorite and wollastonite was discussed based on the worn surface analysis.

Antigorite (Atg) was extracted from natural deposited serpentine mineral in Anshan, China. Wollastonite (Wl) was provided by Dalian Global Mineral Co., Ltd., China. Polytetrafluoroethylene (PTFE) was purchased from Shandong Dongyue Co., China. The phase structure, functional groups and thermodynamic behaviors of the mixed raw powders were characterized by a model D8 Advance X-ray diffractometer (XRD), a model Nicolet 6700Fourier infrared spectrometer (FT-IR) and a model NETZSCH STA 449C simultaneous thermal analyzer (TG-DSC), respectively. The Shore hardness of the sintered composite samples was measured by a Shore hardness tester. The tribological properties of the composites were tested by a model Optimal SRV-IV ball-disc friction and wear tester. The 3D morphology, surface roughness and wear volume of the wear scars on the composites were determined by a model Olympus LEXT OLS4000 laser confocal microscope. The morphology and chemical composition of the worn composite and steel surfaces were examined by a model FEI Nova Nano SEM 450 field emission scanning electron microscope (FESEM) and an attached X-ray energy spectrometer (EDS). The chemical state of the major elements on the worn steel surface was analyzed by a model ESCALAB 250Xi X-ray photoelectron spectrometer (XPS).

Compared to pure PTFE, the peak intensity of composite materials with different components obtained by adding minerals is reduced. The possible reason is that the addition of minerals hinders the nucleation and crystallization of PTFE, leading to a decrease in the crystallinity. In addition, no chemical reaction occurs during the process of introducing serpentine and wollastonite into PTFE by a pressureless sintering technology.

The powders of antigorite and wollastonite are evenly dispersed in PTFE, with slight aggregation of antigorite. The mineral particles are tightly bound to the PTFE matrix, and there are no obvious defects such as pores and cracks in the composite. The average friction coefficient and wear volume of PTFE-based composites with simultaneous addition of 10% (in mass fraction) antigorite and 20% wollastonite are reduced by 44.2% and 71.4%, respectively, compared to pure PTFE. The composites exhibit excellent tribological properties due to the reinforcement of PTFE by minerals and the formation of a tribo-layer on the dual steel surface, which has high hardness and self-lubricating properties. Compared with the addition of a single antigorite, the addition of wollastonite minerals lowers the phase transition temperature of antigorite and promotes the formation of the tribo-layer, which has a good synergistic effect on the improvement of the tribological properties of the composites.

The mineral reinforced PTFE composite material was obtained via introducing antigorite and wollastonite minerals into PTFE and pressureless sintering, which had a dense structure, a uniform distribution of reinforcements, and an increased Shore hardness (from 11.5% to 23.7%), compared to pure PTFE.

Single addition of antigorite could improve the wear resistance of PTFE and increased the frictional stability, while the simultaneous introduction of antigorite and wollastonite could improve both the anti-wear and friction reducing properties of PTFE. The average friction coefficient and wear volume of (10Atg+20Wl)/PTFE were reduced by 44.2% and 71.4%, respectively, compared to pure PTFE.

The improvement of the tribological properties of mineral containing PTFE was attributed to the strengthening effect of minerals on the polymer and the formation of a tribo-layer with high hardness and self-lubricating properties on the dual surface. Compared with the addition of single antigorite, the simultaneous addition of wollastonite reduced the phase transformation temperature of antigorite, and promoted the formation of tribo-layer on the worn surface, thus having a good synergistic effect.