Lubrication of oxygenated diamond-like carbon surfaces

Abstract

Methods are provided for lubricating oxygenated diamond-like carbon surfaces to reduce friction while reducing or minimizing wear on the surface. A diamond-like carbon surface layer having a surface ratio of oxygen to carbon of 1:15 or more can be lubricated using a lubricant oil that includes a molybdenum-based friction modifier additive, a tungsten-based friction modifier additive, or a combination thereof. The Mo-based friction modifier (and/or other friction modifier based on a Group VI metal) can be selected based on the Gibbs free energy of adsorption (ΔG) for the friction modifier on an oxygenated diamond-like carbon surface. Use of a Group VI metal-based friction modifier having a ΔG of adsorption with a sufficiently large magnitude can reduce friction at the surface of the oxygenated diamond-like carbon while causing a reduced or minimized amount of wear during lubrication.

1 . A method for lubricating a machine surface comprising:
 supplying a lubricant comprising a Group VI metal-containing friction modifier to a surface layer comprising diamond-like carbon with a surface oxygen to carbon ratio of 1:15 or more, the lubricant comprising 0.05 wt % to 0.2 wt % of the Group VI metal-containing friction modifier, the Group VI metal-containing friction modifier comprising a ΔG of adsorption on the diamond like carbon of −25 kJ/mol or less.   
     
     
         2 . The method of  claim 1 , wherein the surface oxygen to carbon ratio is 1:10 or more. 
     
     
         3 . The method of any of  claim 1 , wherein the surface oxygen to carbon ratio comprises a top 0.7 nm at the surface of the diamond-like carbon. 
     
     
         4 . The method of  claim 1 , wherein the diamond-like carbon comprises borated diamond-like carbon. 
     
     
         5 . The method of  claim 4 , wherein the borated diamond-like carbon comprises a ratio of boron to carbon of 0.5:100 or more. 
     
     
         6 . The method of  claim 1 , wherein the surface oxygen to carbon ratio is 1:10 or more. 
     
     
         7 . The method of  claim 1 , wherein the diamond-like carbon comprises 15 mole % or more of hydrogen. 
     
     
         8 . The method of  claim 1 , wherein the lubricant comprises 50 wppm or more of Group VI metal. 
     
     
         9 . The method of  claim 1 , wherein the Group VI metal-containing friction modifier further comprises a polar site diameter of 7.0 Angstroms or more. 
     
     
         10 . The method of  claim 1 , wherein the Group VI metal-containing friction modifier comprises an Mo-containing friction modifier. 
     
     
         11 . The method of  claim 10 , wherein the lubricant comprises 50 wppm or more of Mo. 
     
     
         12 . The method of  claim 1 , wherein the friction modifier comprises MoDTP, [Mo 3 S 4 ](dtc) 2 , a trinuclear molybdenum compound, or a combination thereof. 
     
     
         13 . The method of  claim 1 , wherein the surface layer comprising diamond-like carbon comprises a layer depth of 2.0 μm or more of diamond-like carbon. 
     
     
         14 . The method of  claim 1 , wherein the lubricant further comprises a friction coefficient of 0.06 or less. 
     
     
         15 . The method of  claim 1 , wherein the lubricant is supplied to a surface layer positioned in opposition to a steel surface layer. 
     
     
         16 . The method of any of  claim 1 , wherein a ratio of sp 2  to sp 3  hybridized carbons in the diamond-like carbon is 0.5 to 2.5. 
     
     
         17 . A method for lubricating a machine surface comprising,
 supplying a lubricant comprising a Group VI metal-containing friction modifier, to a surface layer comprising diamond-like carbon with a surface oxygen to carbon ratio of 1:15 or more, the lubricant comprising 0.05 wt % to 0.2 wt % of the Group VI metal-containing friction modifier, the Group VI metal-containing friction modifier comprising a polar site diameter of 5.0 Angstroms or more and a ΔG of adsorption of −25 kJ/mol or less.   
     
     
         18 . The method of  claim 17 , wherein the Group VI metal-containing friction modifier further comprises a polar site diameter of 7.0 Angstroms or more. 
     
     
         19 . The method of  claim 17 , wherein the diamond-like carbon comprises borated diamond-like carbon, or wherein the diamond-like carbon comprises a surface oxygen to carbon ratio of 1:10 or more, or a combination thereof. 
     
     
         20 . The method of  claim 17 , wherein the Group VI metal-containing friction modifier comprises an Mo-containing friction modifier, the lubricant comprising 50 wppm or more of Mo.