Study on the Matching Characteristics between Common Tribological Materials and Lubricant Anti-wear Additives

The matching tests between the conventional S,P,S-P-con-taining anti-wear (AW) additives, a newly synthesized organo-boron anti-wear agent and the commonly used tribological materials, such as steel, copper and cast iron, on Falex tester have been carried out in this paper. The P-V (load-velocity) diagrams for four kinds of AW agents matching with six pairs of friction couples were determined. The results of P-V diagram suggested that ZDDP and the organo-boron AW agent shows the weak selectivity on tribological materials. The P-V diagrams also demonstrate that the organo-boron AW agent exhibits fine AW properties when it is matched with copper-steel friction couple, the P-containing AW agent is suitable for the lubrication of copper-steel and cast iron-steel friction couples, whereas the S-containing AW agent and ZDDP are more suited to the lubrication of steel-steel friction couple. It has been found that the P-V diagrams with and without AW agents exhibit distinct discrepancies. No overlap between the first and the second transitional curves has been observed except for the P-V diagram of the S-containing AW agent vs copper-steel friction couple. It suggests that the boundary lubrication (BL) can still exist at high operating velocities due to the formation of BL films on the surfaces of friction couples. Typical matching mechanisms between the S-containing AW agent, the organo-boron AW agent and friction couples were examined and ana- lyzed by means of surface analysis techniques such as Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). It has been suggested that the S-containing AW agent reacts chemically with the steel surface to produce iron sulfate and iron sulfide films, thus protecting the steel surface from being worn; on the other hand, the Cu2S film with bad toughness, formed by the reaction between S-containing AW agent and copper surface can be easily erased from the copper substrate, resulting in bad AW properties. Additionally, the organo-boron AW agent absorbs on the friction surface in the form of tri bo-polymers, and the B atoms, isolated from tribo-excitation , can permeate through the friction surface. The boron permeation through the steel and copper surfaces can increase their surface hardnesses and results in the increasing of AW properties of the steel and the copper surfaces.