Abstract: AlN/PTFE nanocomposites were prepared using an ultrasonic dispersion method. The tribological properties of the composites were tested in both humid air and dry argon using a linear reciprocating tribometer. The results revealed that the addition of 5% AlN nanofillers reduced the wear of PTFE by 4 orders of magnitude in the humid air. Whereas in a dry argon condition, the AlN nanofillers only reduced the wear of PTFE by 2 orders of magnitude. Calculated differential wear rate of AlN/PTFE in the humid air decreased with the increase of sliding distance in the wear experiment. While the differential wear rate of AlN/PTFE in dry argon did not change with the increase of sliding distance. The friction coefficient of AlN/PTFE composites fluctuated in humid air tests, which was suspected to be related to the growth and removal of transfer film. The friction coefficient of the composites in dry argon was relatively stable, which suggested the transfer film was chemically and structurally unchanged during the sliding. The morphological and chemical properties of the transfer film formed on the counterfaces were analyzed using a 3D profilometer, scanning electron microscope, infrared spectrometer (IR), and X-ray photoelectron spectroscopy (XPS). It was found that after the sliding in humid air, AlN/PTFE formed a continuous and high-covering transfer film with an average thickness of ~ 1 μm. The IR spectra of the low-wear transfer film had the obvious carboxylate signals. XPS measurement results showed the presence of high intensity of C=O bond on the low-wear polymer worn surface and transfer film. These polar groups and oxygen content improved the cohesion of polymer surface and the adhesion of transfer film onto metallic counterface thus building a protective interfacial layer for the tribo-system and improving the wear resistance of the composite. In dry argon tests, the absence of water oxygen inhibited the composite from generating carboxylate-rich transfer film via tribochemistry, which affected the improvement of the wear resistance of the polymer. Based on previous studies and present experimental results, the following assumptions for the wear reduction mechanism of AlN filler in the PTFE composite and carboxylated transfer film formation in the humid air were proposed. (a) During the run-in period, the aggregates of AlN nanoparticles in the polymer matrix were broken and dispersed by mechanical stress and shear with the large filler-polymer matrix interaction area near the frictional interface. (b) The surfaces of AlN fillers tended to dehydrate under the activation of friction heat and mechanical stress, exposing many Lewis acid sites, thus promoting the defluorination of PTFE molecules and generation of unsaturated carbon atoms on the polymer backbone. The unsaturated carbon atoms in the polymer chains were likely oxidized by environmental oxygen, then hydrated by ambient water thus forming carboxylic acid groups. (c) Modified PTFE chains with carboxylic acid groups were likely to chelate with metal counterface. This chelation caused by tribochemistry increased the adhesion of transfer film to the metal counterface and made the structure of transfer film more stable. This mechanistic study was expected to provide insights into the design of a wide range of polymeric solid lubricant materials.