The poly (N-isopropylacrylamide) brush was covalently bonded on an initiator-coated silicon wafer via surface-initiated atom transfer radical polymerization. The polymer brush was (76.2±0.1) nm in thickness (by ellipsometer) with a grafting density of ca. 0.27 chains/nm 2 . The tribological properties of the poly (N-isopropylacrylamide) brush were investigated by means of ball-on-disk tests in a rotational mode under water lubrication for tribological application. The experimental results exhibited a low friction coefficient of ca. 0.03. The excellent lubrication property of the brush was due to its amide groups in the polymer chains. It was supposed that the good lubrication property of the brush was attributed to the cross-linked polymer network formed by the hydrogen bond association of N-H…O==C and the water molecular layer adsorbed by the terminal amide groups in the brush. The poly (N-isopropylacrylamide) solution also exhibits a lubrication property due to physical adsorption of the polymer chains.
It is necessary to modify the running-in process for the application of ceramics using water as a lubricant in real conditions because ceramics sliding in water are characterized by a running-in period with severe friction and wear.Fullerenol,a kind of highly water-soluble nanoparticle,was synthesized and then used to ameliorate the tribological properties of Si 3 N 4 sliding against Al 2 O 3 in pure water.With the addition of fullerenol,the running-in period was shortened from 30 min to 100 s at a speed of 250 mm/s.The speed threshold above which ultralow friction can be obtained in a short time was expanded from 450 mm/s to 80 mm/s.Meanwhile,the load-carrying ability of water film was increased.The role of fullerenol was discussed based on observation of the wear scar by an optical interferometer and XPS characterization of the tribo-film on the wear track.
LIU YuHongWANG XiaoKangLIU PengXiaoZHENG JunPengSHU ChunYingPAN GuoShunLUO JianBin
The tribological properties of graphene oxide(GO) nanosheets and modified diamond(MD) nanoparticles with excellent water-solubility were investigated.GO nanosheets were synthesized using carbon fibers with a regular and uniform size,the lateral size being around 30 nm and the thickness being 2 or 3 nm,while MD nanoparticles were about 30 nm in the three dimensions.The friction properties of ceramics were improved by GO nanosheets or MD nanoparticles used as additives in water-based lubrication,though the effects of two nanoparticles were quite different.For GO nanosheets,the friction coefficient at the beginning decreased sharply from 0.6 to 0.1,as compared with the dionized water lubrication.At the same time,the running-in period was shortened from 2000 s to 250 s.A steady state characterized by ultralow friction(friction coefficient=0.01) was obtained after the running-in period.In the case of MD nanoparticles,the friction coefficient stayed at 0.1 without further decrease during the whole experiment.Based on the observation of wear scar and characterization of remains on the wear track,the positive effect of GO nanosheets was attributed to their lamellar structure and geometric size.MD nanoparticles reduced friction by forming the regularly grained surface on the mating surfaces,and prevented further reduction in steady-state friction coefficient owing to their larger size and hardness.In conclusion,GO nanosheets exhibited favorable potential as an effective additive for water-based lubrication.
