Tribological Properties of Carbon Nanotubes-Carbon Fiber Reinforced WS₂/PAI Composite Coatings

In this study, carbon nanotubes-carbon fiber (CNTs-CF) multiscale reinforcements were prepared by grafting carbon nanotubes (CNTs) onto the surface of carbon fibers (CF) through a rare-earth grafting method. The CeCl₃ modified CF (RCF) was obtained by the same preparation method without adding CNTs. TheWS₂/PAI composite coating was prepared with polyamideimide (PAI) as binder and WS₂ as solid lubricant filler. The CNTs-CF was added to the WS₂/PAI composite coating at mass fractions of 0.0%, 0.5%, 1.0%, 1.5% and 2.0% respectively. The WS₂/PAI composite coating with CNT-CF was coated on the surface of the aluminum alloy sample and formed the CNT-CF/WS₂/PAI composite coating. In addition, in order to study the reinforcement mechanism of CNTs-CF, the WS₂/PAI composite coatings adding CF and RCF were prepared, which was denoted as CF/ WS₂/PAI and RCF/ WS₂/PAI. In order to confirm whether the preparation of CNTs-CF was successful, the morphology and structure of CNTs-CF were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectra, and scanning electron microscope (SEM). The microhardness of the coating was measured using a micro-Vickers hardness tester. The tribological properties of CNTs-CF on the tribological properties of WS₂/PAI composite coating was tested under different experimental conditions by using a friction and wear tester and an ultra-depth 3D microscope. The experimental results showed that since rare earth elements had a large coordination number and extremely high chemical activity, the oxygen and nitrogen on the CF surface would react with cerium, causing more oxygen-containing functional groups to exist on the CF surface. There was an obvious characteristic peak of Ce-O at 530.46 eV, indicating that cerium formed a chemical bond with oxygen to adsorb CNTs on the CF surface. Raman spectroscopy showed that the degree of graphitization of CNTs-CF was higher than that of carbon fiber. This was because the degree of graphitization of CNTs was high. The results of the microhardness test showed that the CNTs-CF/WS₂/PAI composite coating had the highest hardness. This was because Ce³⁺ can coordinate with the oxygen-containing functional groups and nitrogen-containing functional groups in PAI to limit the relative slip between polymer chains, thereby improving the mechanical strength and hardness of PAI. The results showed that the appropriate amount of CNTs-CF could improve the bond strength, wear resistance and lubrication effect of WS₂/PAI composite coating. The Ce³⁺ did not affect the lubrication performance of the coating, but improved the enhancement effect of carbon materials on the mechanical properties of the coating, thereby improving the wear resistance of the coating. The friction coefficient and wear rate of WS₂/PAI composite coating were lower under different loads when the mass fraction of CNTs-CF was 1.5%. The average friction coefficient of the CNTs-CF/WS₂/PAI composite coating under 10 N load was only 0.165, and the wear rate was only 1.38×10^-2 mm³. Cerium improved the interfacial bonding strength between the carbon material and the coating. CNTs-CF can form a stable skeleton structure in the WS₂/PAI composite coating, and can form a stable transfer film together with WS₂ during the friction process. In addition, CNTs dispersed the load borne by CF to avoid stress concentration causing CF to break. The excellent tribological properties of CNTs-CF/WS₂/PAI composite coatings were mainly attributed to the lubrication effect of WS₂, the coordination reaction between rare-earth Ce and oxygen-containing functional groups, and the enhancement of CNTs-CF.