A new approach to ceramic lubrication: tribopolymerization
The lubrication of ceramic materials is a difficult problem; conventional lubrication techniques are limited or often ineffective. Therefore, the concept of tribopolymerization -- originally proposed by Furey and later modified by Furey and Kajdas -- is used as a new approach to boundary lubrication of ceramics. In this approach, potential polymer-forming compounds are used in minor concentrations in a carrier fluid, which polymerize at the contact region under the sliding action to form a protective layer at the contact.
Selected monomers -- including one condensation type, C₃₆ dimer acid/ ethylene glycol monoester, and five addition type, i.e., lauryl methacrylate, diallyl phthalate, vinyloctadecyl ether, vinyl acetate and methyl-2-acrylamido-2- methoxy acetate, were used at 1% concentration in hexadecane in pin-on-disk tests with sliding alumina and zirconia ceramic systems.
Results showed that wear reductions of alumina by up to 80% were achieved at room temperature. At elevated temperatures (up to 150°C), the monomers were also effective; one of the monomers reduced wear by over 90% at higher temperatures. In the zirconia system, the monomers in general were also effective in reducing wear, but not to the same extent as with alumina. There was no significant effect of the monomers on friction.
Detailed surface analytical studies of the worn surfaces lubricated with the monomer solutions using Fourier transform infrared microspectroscopy, X-ray photoelectron spectroscopy and mass spectrometry showed the complex nature of tribochemistry involved in the antiwear action of these monomers. In addition to the polymerization, evidence of chemical reactions of the monomers with the ceramic substrate was found.
Using an advanced infrared microscope system, surface temperatures at the lubricated contacts of alumina-on-sapphireand zirconia-on-sapphiresystems were measured for selected monomers. In general the temperatures were very low. Theoretical estimations of surface temperatures using Vick’s model were also carried out for several systems (including the ones studied in the past) and the role of surface temperature in the anti-wear action of the monomers was examined. The relationship is complex; but the general trend suggests that temperatures are important for tribopolymerization of the monoester, whereas not so much so for the addition monomers.
A molecular modeling software -- CHEM-X -- was also used to obtain additional insight into the mechanisms of anti-wear action of the monomers. In this exploratory study, 3-dimensional shapes of the monomers, their polymerization mechanisms, and possible orientation of a selected monomer on polymerization mechanisms, and possible orientation of a selected monomer on a ceramic surface was examined.
Possible mechanisms of anti-wear action of these monomers are proposed. For the monoester, the mechanism involves (a) an initial adsorption of the carboxylic end of the molecules on the surface, (b) chemical reaction with the surface to form a soap, and (c) the formation and outward growth of oligomer/polymer chains somewhat similar in structure to a Langmuir-Blodgett multi-layer. The mechanism of anti-wear action of the addition monomers is believed to be connected to the negative-ion radical action mechanism (NIRAM) as proposed by Kajdas. According to this mechanism, exoelectron emitted during sliding initiate tribopolymerization of vinyl monomers, and monomers polymerizing only by anionic or free radical mechanisms are capable of tribopolymerization on the ceramic surfaces.
It is proposed that the formed polymeric products act as a binding medium for fine wear debris particles generated during sliding. As a result, a strongly bonded debris layer -- somewhat similar to a ceramic powder reinforced polymer composite -- forms on the surface. This layer provides protection to the sliding surfaces against wear.