Preparation And Properties Of Polyimide/MoS₂ Lubricant Coatings

The solid lubricant coatings based on plyimide have been applied on the piston skirt. These coatings can provide better performance and durability, as well as control friction and wear. As a kind of solid lubricant, MoS2 effectively provides lower friction coefficient. TiO2, as nanoparticle, possesses good physical and chemical properties, thermal stability and corrosion resistance.In this paper, we mixed PAA with MoS2/TiO2 by using high speed ball mill about 15 min. Then we added defoamer (0.4 wt%) and flatting agent (0.4 wt%) into the mixture, at the same time, producing a kind of adhesive. We printed PI/MoS2/TiO2 coatings to the surface of Al-alloy matrix by screen printing technique. To form PI lubricating coating, the samples were heated at 210℃ for 30 min. During this period, PAA turned into PI via imidation reaction reaction.We analyzed the crystal structure of MoS2 in PI lubricating coatings, as well as the surface profile and bonding strength with metal by using different methods, such as XRD, Raman, SEM, EDS, and so on. In addition, ultra-functional machine of testing (UMT-3) was used to get the friction coefficient-time curves, according to which we analyzed the friction and wear mechanism of PI/MoS2 lubricant coatings.Among the four samples of PI/MoS2 lubricant coatings with different MoS2 content, the lubricant coatings presented the highest bonding strength when the content of MoS2 was 10 wt%. X-ray diffraction analysis results showed that diffraction peaks of (004) and (103) crystal face in MoS2 changed, length increasing along c-axis because of PI molecules seeping into.The friction coefficient of PI/MoS2 coating initially decreased and then increased at room temperature, fixed load (1 N) and rubbing speed (10 mm/s), and appeared the minimum mean value about 0.21 when the content of MoS2 was 16.7 wt%.TiO2 can improve abrasion resistance of PI/MoS2 lubricant coatings. The results indicated that wear-out time of PI/MoS2/TiO2 coatings was the longest, about 45 min, when the content of TiO2 was 1.5 wt%. PI/MoS2 coatings were worn out after 20 min. At the other hand, the minimum friction coefficient (mean value) was about 0.22 the content of TiO2 was 1.0 wt%.During the friction process, PI and MoS2/TiO2 shared part of friction force to improve the performances of these coatings. Using the low shear strength of PI and self-lubricating property of fillers, friction coefficient was decreased. During the process of friction, there were cracks on the surface produced by cyclic shear stress. At last, these coatings come off because of fatigue wear, failing to protect the metallic matrix.