Vapor adsorption and reaction at interfaces during tribological sliding

Friction and wear are not intrinsic material properties but instead depend on many extrinsic parameters of the sliding system. One important consideration when dealing with sliding surfaces is the vapor environment in which the two surfaces operate. Molecules from the vapor phase will adsorb onto the material surface, and even though these adsorbate layers are often less than one nanometer thick, they can influence friction and wear from the nano-scale to the macro-scale.;This study investigates the effect of different adsorbed vapor molecules on the friction and wear during sliding between different solid surfaces. A macro-scale reciprocating ball-onflat tribometer was used to examine the effect of water vapor and various organic vapors on lubrication and wear primarily between steel or copper surfaces, but also with ceramic, glass, and carbon surfaces. With the exception of engineered materials like diamond-like carbon (DLC), all tested materials experience poor lubrication and surface wear when rubbed in a dry, inert environment. In the presence of water vapor, a copper surface rubbed by a stainless steel ball undergoes three distinct lubrication and wear conditions depending on the surrounding relative humidity (RH). Dry conditions and low humidity (<20% RH) produces plastic deformation and mild abrasive wear of the copper, intermediate humidity (20-80% RH) produces catastrophic adhesive wear of the copper, and high humidity (>80% RH) produces galvanic corrosion of the stainless steel ball. Other classes of materials generally do not show the same complicated RH dependence although RH does not act as a lubricant for most materials. In contrast to the complicated and deleterious effects of water vapor, organic vapors act as a lubricant across nearly all solid surfaces. Organic molecules including linear alcohols, branched alcohols, fluorinated alcohols, alkanes, and ketones provided lubrication. A monolayer of n-pentanol vapor yields a friction coefficient of ~0.15 and minimal wear regardless of the friction or wear that a surface experiences in dry or humid conditions. This lubrication occurs without any tribochemical reaction occurring. However, this lubrication is not observed with surfaces covered by residue from evaporated solvents. Low vapor pressure residue from trace contaminants in solvents gives a low friction coefficient and minimal wear for some surfaces.;This study also investigates the effect of molecular structure of the adsorbate on friction and wear. Friction and wear decrease as hydrocarbon chain length increases for linear chain alcohols and branched alcohols have a higher friction coefficient than linear alcohols for the same number of carbons. Other molecular chemistries, including alkane, ketone, allyl alcohol, and fluorinated alcohol, had higher friction coefficients than n-pentanol. Unique to allyl alcohol is the formation of a large quantity of triboproduct during sliding. The triboproduct forms during sliding at all vapor partial pressures tested (15-80% p/psat) and increases in formation for higher applied loads. Infrared spectroscopy of the triboproduct shows that the allyl C-H stretch that is present in the adsorbed vapor spectrum is absent from the triboproduct, indicating the formation of a poly-alcohol. This triboproduct can lubricate the surface in the absence of vapor flow for more than an order of magnitude longer than it took to form.;The effect of adsorbed vapor on solid lubricants is also studied. Boric acid is a crystalline lamellar solid with a hexagonal structure similar to graphite and can exhibit a friction coefficient as low as 0.05. However, a coating of boric acid is quickly worn away in dry inert conditions, exposing the underlying surface. Water vapor and acetone vapor environments yield the expected low friction coefficient, while alcohol vapor yields a friction coefficient near 0.2. Further inspection of the coating during alcohol vapor flow shows that the boric acid coating reacts to form water and trialkyl borate, which desorbs from the surface, thus giving only lubrication by the adsorbed alcohol. The mechanism of lubrication for boric acid in water or acetone vapor appears to be due to edge site passivation in a similar fashion to graphite lubrication.