Tribological Investigation Of PAAm Hydrogel/UHMWPE Water Lubricated Stern Bearing Composite Materials

Because of the low viscosity of water and the low bearing capacity of waterlubricated film,the lubrication failure under the heavy load and low speed conditions limit the development and application of water-lubricated bearings.Soft matters in living body have solved this problem very well by hydration lubrication,and provide excellent lubrication for the moving tissues and organs.In order to using the hydration lubrication of soft matters in the engineering fields,the polyacrylamide(PAAm)hydrogel microparticles were synthesized,and the PAAm/ultra highmolecular weight polyethylene(UHMWPE)composites were prepared in this work.The physical,chemical properties and tribological performances of composites had evaluated,and the lubricating mechanisms of composites were verified by comprehensive analyzing of the experimental results.The main conclusions were shown as follow:(1)The feasibility of composite design was evaluated based on the theoretical analyses.The analyses of swelling kinetics and thermodynamics show that the diameter size and cross-linking degree of PAAm hydrogel microparticles were two influence factors on the internal diffusion of water molecules.The diffusion rate of water molecules was increased with the decrease of the diameter size or cross-linking degree.The thermodynamic analyses show that the decrease of cross-linking degree of PAAm hydrogel microparticles was conducive to the increase of swelling degree.Based on the analyses of swelling and hydration mechanisms of hydrogel,the hydration lubrication effectively combined the high bearing capacity with low shear fluidity.The improving designs of double network structure and graphene oxide(GO)for PAAm hydrogel microparticles were also discussed theoretically.The analysis results provided a good theoretical support for the design of PAAm/UHMWPE composites.(2)PAAm hydrogel microparticles/high density polyethylene(HDPE)composites were prepared to verify the accuracy of theoretical analysis and evaluate the feasibility of test.The poor wear resistance of the HDPE is helpful to observe the formation process of lubricating film clearly and reveal the lubrication mechanism of the composite.At the same time,HDPE has the same chemical structure as UHMWPE,which can provide guidance for the preparation of UHMWPE composites.The test results show that the mechanical properties of composite have no obvious change when the PAAm hydrogel particle content is less than 1 wt%,and have obviously declined when the PAAm hydrogel particle content is more than 1 wt%.The swelling behavior of PAAm hydrogel particles in water confirms that the PAAm hydrogel particles in composites have good hydration capacity.The experimental results show that the friction and wear behaviors of HDPE can be obviously reduced by adding appropriate amount of PAAm hydrogel particles.The formation of hydration lubrication layer composed of PAAm fragments is determined by the characteristic analysis of wear surface.The relationships between particle content and properties of composite have been identified,and the lubrication mechanism of composite has also been revealed.The results show that the heterogeneous structure of composite can effectively maintain the relative independence of the two components.(3)In order to further improve the quality of hydrogel particles and prepare composite materials with engineering application value,the PAAm hydrogel microspheres with different particle sizes and crosslinking degrees were prepared by emulsion polymerization,and PAAm microsphere/UHMWPE composites were prepared by melt blending.The SEM results show that PAAm hydrogel microspheres have a regular spherical structure and can maintain their initial spherical shape after processing.The mechanical strength of the composite has been significantly affected by the particle size and filling content of PAAm microsphere,and the tribological properties of composite have been significantly affected by the particle size,crosslinking degree and filling content of PAAm microsphere.The PAAm microsphere/UHMWPE composite has better mechanical and tribological properties than PAAm particle/HDPE composite,which make the UHMWPE composite has a good practical application value.However,the testing results also show that the swelling behavior of PAAm microsphere will reduces the lubricating capacity of composite to some extent.(4)In order to improve the stability of PAAm microsphere in water,PAAm/reaformaldehyde resin(UF)double network(DN)microspheres were prepared by free radical polymerization and condensation polymerization.The soaking tests show that the introduction of UF hard network effectively improves the stability of PAAm microspheres.Compared with PAAm microsphere/UHMWPE composite,the soaking strength,tribological properties and stability of DN microsphere/UHMWPE composite have been effectively improved.The bearing and tribological properties of the composite have been obviously improved,especially under the heavy load condition(30 N).This also confirms that the DN structure of microsphere can effectively improve the bearing capacity and tribological properties of DN composite under the conditions of heavy load or low speed.(5)The PAAm-GO flakes were prepared to further enhance the bearing capacity of the composite.The FTIR analysis results show that PAAm hydrogel layer can be grafted on the surface of GO flake by chemical bonds,which is benefit for improving the interface quality between UHMWPE and GO flakes.This can further improve the bearing capacity of composite.Meanwhile,the PAAm-GO flake has combined with the hydration lubrication of PAAm hydrogel and the 2D shape effect of GO flake.Therefore,the lubrication layer composed of PAAm-GO fragments can also significantly reduce the friction and wear behaviors.The testing results can also prove that the PAAm-GO/UHMWPE composite can effectively avoid the local damage of UHMWPE under heavy load of 50 N,and have good bearing capacity and tribological properties.