Tribological Properties And Friction-reducing And Anti-wear Mechanisms Of Layered Double Hydroxides And The Calcined Products

Layered double hydroxides(LDHs)with typical layer structure,which have various kinds of cations in brucite-like layers and exchangeable anions in the interlayer,have been widely applied in the areas of catalysis,ion-exchange and adsorption.The layers of LDHs slip relatively under shearing force,which have special advantages as friction-reducing and anti-wear materials in the area of lubrication.The tribological properties of Mg/Al-,Zn/Al-and Zn/Mg/Al-LDHs as lubricant additives have been studied systematically under various friction contact modes and tests conditions in the doctoral dissertation,including four-ball friction and wear tests,pin-on-block friction tests(reciprocating type)and pin-on-disc friction tests(rotating type).The research about friction properties of the calcined products of LDHs under different temperatures has been conducted innovatively in the dissertation.The comparison of friction properties of LDHs with traditional solid additives graphite and MoS2 has been discussed.Through XRD,SEM,EDS,XPS and AES test methods,the composition,structure and micro-morphology of LDHs and the calcined products have been characterized.The compositions and element distribution rules of worn surfaces and the tribological mechanisms have been studied deeply.The research results are as follows:The calcined product of Mg/Al-LDHs under 1000 oC,the Zn/Al-LDHs and the calcined product under 850 oC had excellent anti-wear properties which decreased the wear scar diameter by above 18%,respectively.The lubricating oil with 0.5 wt% Zn/Mg/Al-LDHs powders showed best friction-reducing and anti-wear properties.The Zn/Mg/Al-LDHs had better friction properties under higher normal load(200 N~400 N)and suitable speed.Compared with traditional solid additives graphite and MoS2,the LDHs had better anti-wear properties under specific test conditions,which is attributed to higher hardness.The friction properties of LDHs were influenced by normal load in the pin-on-block tests.The worn surfaces lubricated by oil with LDHs consisted of ZnO(or MgO),ZnS,various iron oxides,organic compounds with iron,organics with C-O and C=O bonds and graphite without the element of Al.The surfaces had more O and less C.The Zn/Al-and Zn/Mg/Al-LDHs had a synergistic effect with ZDDP to increase the content of Zn in the surfaces.The calcined products of LDHs promoted more Zn to deposite on the worn surfaces to improve the anti-wear properties.More Zn and O deposited in the smooth areas of worn surfaces lubricated by oil with the calcined products of LDHs and the areas had less Fe and C.The grooves and furrows had more element of C.The LDHs and the calcined products had similar tribological mechanisms which is the self-repairing protective layers theory.The tribofilms consisted of carbon-enriched layer,oxygen-enriched layer and transition layer,which had thickness of about 100~200 nm.Zn and Mg almost distributed uniformly in the oxygen-enriched layer.The LDHs deposited on the rubbed surfaces and the laminates slipped relatively under shearing force and active oxygen was released.Nanoparticles filled in the microcrack of surfaces.The protective layers were formed on the worn surfaces through complex tribochemistry reactions,which had friction-reducing and anti-wear effects.The excellent anti-wear properties of the calcined products were attributed to the supporting and mechanical polishing effects of hard spinel particles which are the in-situ phase transition products of LDHs.