Tribological Properties Of Modified Light Metals And Alloy Based On Self-Assembled Technology

With the extensive application in MEMS,the lubrication of light metals and alloy is studied in the paper.The self-assembled monolayers were prepared on light metals and alloy. The characterization of modified surfaces were explored by contact angle measuring meter and atomic force microscopy,the tribological properties of samples in nanometer and millinewton scale were measured by friction force microscopy and CETR UMT-2 tribometer. The effects of ultraviolet radiation,sliding velocity and load on tribological properties of samples were analyzed.The condensation reaction between hydrolyzed self-assembled molecule and hydroxyl on metal surface is to form Si-O-M bond.In the same time,the reaction among hydrolyzed self-assembled molecules is to form Si-O-Si bond.Different substrates are hydroxylated by different methods such as metal film with UV radiation,magnesium and aluminum with alkali solution combining UV radiation,titanium and titanium alloy with Piranha solution combining UV radiation.The optimal solution concentration is 0.015 ml/L,adsorption time is 12h.Light metals and alloy surfaces modified by SAMs are uniform and exhibit good hydrophobic and tribological properties.In nanometer scale,friction force of modified aluminum film increases with load and sliding velocity increasing.Friction force of modified titanium film increases with sliding velocity increasing,the friction force of modified titanium film increases with load increasing when substrate roughness is small,but decreases when substrate roughness is big.In millinewton scale,friction force of modified samples increases with load and sliding velocity increasing.The durability of samples decreases with load and sliding velocity increasing.5 min UV radiation can improve the modified surface state,which makes the contact angle become big and the surface energy become small.The friction coefficient of sample becomes smaller,but a little decrease in the durability.Lubrication effect of samples with 15 min UV radiation is weakened because of the destroy of compact net structure of SAMs.The modified surface with a high friction coefficient becomes hydrophilic.The main reason is that: there is a basic photoreaction of oxygen under UV radiation,in which atomic oxygen may highly be generated.Then the oxygen atom reacts with trace amounts of water to form hydroxyl.Next step is hydrogen abstraction from alkyl chains by attacks of hydroxyl radicals, thereby forming alkyl radicals.The alkyl radicals can be degraded byβ-scission to form shorter chain radicals.When being prepared on the titanium film,the value of friction force is shown as follow: OTS＜FOTS＜APS＜MPS.The friction force of OTS SAMs is the smallest because of the lowest chemical activity of its terminal groups.The durability of SAMs is shown as follow: FOTS＞OTS＞APS＞MPS.The durability of FOTS SAMs is the best because of biggest chemical adsorption energy between hydrolyzed FOTS molecule and substrate.When the OTS SAMs are prepared on four kinds of substrates such as magnesium,aluminum,titanium and titanium alloy,the tribological properties of magnesium modified by OTS SAMs is the best because of its highest chemical activity,which can make magnesium easily react with OTS molecules to form compact lubrication film.Calculated results of molecular dynamics simulation on chemical adsorption energy between self-assembled molecules and substrate are shown as follow: MPS＜APS＜OTS＜FOTS.The chemical adsorption energy between FOTS molecule and substrate is the biggest.Calculated results of molecular dynamics simulation on physical adsorption energy between self-assembled molecules and Si₃N₄ are shown as follow: OTS＜FOTS＜APS＜MPS.The physical adsorption energy between Si₃N₄ and MPS molecule is the biggest.The results of molecular dynamics simulation can clearly explain the friction property and durability of modified samples.