The two experimental techniques, spacer layer interferometry imaging (SLIM) and atomic force microscopy (AFM), have been used to measure the thickness and roughness of zinc dialkyl dithiophosphate (ZDDP) reaction films formed in a rolling-sliding minitraction machine (MTM) tribometer. The AFM method has been complemented by a novel ZDDP film removal method based on ethylenediaminetetraacetic acid (EDTA) solution. It has been found that the two approaches measure very similar ZDDP film thickness values, lending credence to both methods. However the AFM approach measures much rougher ZDDP reaction films than MTM-SLIM and it is believed that SLIM underestimates the film roughness. Based on this, the use of MTM-SLIM is recommended for monitoring the evolution of antiwear film thickness during rubbing, while AFM should be employed for studying the morphology of antiwear films.There has been a great deal of research to characterize the reaction films formed by the antiwear additive zinc dialkyl dithiophosphate (ZDDP). Particular attention has focused on the thickness of ZDDP films since it is known that thick ZDDP surface films enhance mixed lubrication friction [1] and influence wear [2].In practice, ZDDP film thickness is difficult to quantify precisely since ZDDP forms patchy, uneven reaction films. It is thus important to represent film thickness by more than a single value: the maximum and mean film thickness, the kinetics of the film thickness, the root mean square roughness and average slope are all likely to be important.Measurements of the thickness of antiwear films formed by ZDDP solutions on rubbing surfaces published over more than the last 50 years agree surprisingly well, despite the fact that the ZDDP films studied were formed in different rubbing experiments and that various film thickness characterization methods were employed.Soon after ZDDPs were introduced as lubricant additives it was recognized that they form relatively thick films on rubbed surfaces compared to most other lubricant additives. In the 1950s, 32 P radiotracing was used to show that ZDDP solutions rapidly formed films on rubbed cast iron surfaces that stabilized at approximately 120 nm thickness [3]. Using X-ray fluorescence (XRF) Rounds [4] found that films formed by ZDDP on rubbed surfaces were Ôseveral hundred monolayers thickÕ.

Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces to produce thick, viscous boundary films. These films enhance lubricant film formation in slow-speed and high temperature conditions and thus produce a significant reduction in friction. This article describes a systematic study of this phenomenon, which makes use of the versatile nature of polymethacrylate (PMA) chemistry. Dispersant polymethacrylates with a range of different functionalities, molecular weights, and architectures have been synthesized using controlled radical polymerization techniques. The influence of each of these features on boundary film formation and friction has been explored using optical interferometry and friction versus speed measurement. From the results, guidelines have been developed for designing PMAs having optimal boundary lubricating and, thus, friction-reducing properties.

The friction properties of a range of boundary lubricating additives in DLC-DLC rolling-sliding contact have been investigated. Two types of commercial DLC have been studied, one hydrogenated diamond-like and the other Cr-doped, nonhydrogenated and graphitic.The graphitic-type DLC coating initially gave very low boundary friction with additive-free base oil but after a few minutes of rubbing in thin film conditions friction rose to a higher value, similar to that produced immediately by the diamondlike carbon. Some organic friction modifiers were able to preserve this low boundary friction behaviour of the graphitic-type DLC for a full two hour rubbing test. One organic friction modifier, glyceryl monooleate produced a considerable reduction in friction with both types of DLC coating in intermediate speed, though notin very slow speed conditions. The additive molybdenum dithiocarbamate formed a low friction film on both DLC surfaces, with lower friction on the graphitic than on diamond-like carbon one. AFM measurements showed that this was due to the formation of regions of low lateral force on asperities within the rubbed track. ZDDP provided some reduction in friction on both DLC surfaces after rubbing but did not form thick ZDDP tribofilms like those generated on steel surfaces.

The film-forming and friction properties of overbased calcium sulphonate (OBCaSu) detergents in rolling-sliding, thin film, lubricated contact have been investigated. All of the commercial detergents studied form thick, solid-like, calcium carbonate films on the rubbed surfaces, of thickness 100-150 nm. The films have a padlike structure, interspersed by deep valleys in which practically no film is present. These films have the effect of increasing friction in intermediate speed conditions, an effect which is believed to occur because the pad structure of the film inhibits fluid entrainment and thus postpones the formation of an EHD film to higher entrainment speeds. Large differences were noted between the boundary friction coefficients of the various detergents tested. Two of them gave very low boundary friction coefficients, in the range 0.06-0.08, while the other two gave considerably higher friction. These differences are believed to originate from differences in the structures of the alkyl chains in the sulphonate detergent molecules. The films formed by OBCaSu detergents can be removed very effectively by treatment with EDTA solution and this shows that the films are effective in preventing wear.

The friction properties of a range of boundary lubricating additives in DLC-DLC sliding-rolling, lubricated contact have been investigated. Two types of DLC have been studied, one hydrogenated diamond-like and the other a non-hydrogenated graphitic type. The graphitic-type DLC coating gave initially very low boundary friction with additive-free base oil but after a few minutes of rubbing in thin film conditions this rose to the higher value produced immediately by the diamond-like DLC. With the graphitic-type DLC, ZDDP formed a thin patchy film while MoDTC produced a marked reduction of friction. Lateral force microscopy showed that the latter resulted from the formation of low friction material on the asperity peaks. Some organic friction modifiers preserved the low friction of g-DLC during prolonged rubbing. MoDTC and ZDDP were also effective in reducing friction on diamond-like DLC.