Antiwear Tribofilm Research Articles

X-Ray Photoelectron Spectroscopy Analysis of Antiwear Tribofilms Produced on Boundary-Lubricated Steel Surfaces from Sulfur- and Phosphorus-Containing Additives and Metal Deactivator Additive

X-ray photoelectron spectroscopy (XPS) was performed on AISI 52100 steel surfaces subjected to sliding in the boundary lubrication regime at 32 and 100°C. The specimens were lubricated with base oil blended with individual additives containing sulfur (S), phosphorus (P), or metal deactivator, as well as base oil with all the previous additives in the same amounts as in the single blends. XPS spectra were analyzed to confirm the formation and determine the chemical composition of the antiwear tribofilms produced on the steel surfaces during sliding. The use of S- and P-containing additives on the tested disk surfaces revealed that tribochemical reactions resulted in the formation of antiwear tribofilms containing S- and P-rich components. Results for the multi-additive blend provided evidence for two components in the produced tribofilm, appearing to consist primarily of sulfide and phosphate. This investigation provides new insight into the competing roles of these compounds on the tribological properties of the antiwear tribofilms. The significance of the sulfide components is demonstrated by the more pronounced antiwear effect of the S-containing additive in the multi-additive formulation.

Effect of Sulfur- and Phosphorus-Containing Additives and Metal Deactivator on the Tribological Properties of Boundary-Lubricated Steel Surfaces

The tribological behaviors of various blends consisting of base oil and additives containing sulfur and phosphorus and a metal deactivator, in conjunction with those of a blend of the same base oil and all the studied additives in the same amounts as in the single blends were investigated at 32°C and 100°C. Balls and flat disks of AISI 52100 steel were tested in baths of the different blends using a ball-on-disk tribometer under a load of 147 N and sliding speed of 0.34 m/s that resulted in sliding in the boundary lubrication regime. Measurements of the coefficient of friction and electrical contact voltage were used to characterize the tribological performance of each blend and to confirm the formation and removal of antiwear tribofilms. Surface profilometry was performed to evaluate the wear resistance of the tribofilms, and scanning electron microscopy to characterize the dominant wear mechanisms. Endurance tests were conducted with the phosphorus-containing blend that presented in-situ evidence of tribofilm formation through the contact voltage response, in order to determine the tribofilm's durability at room temperature in the absence of oil. The results from various tests demonstrate the effects of temperature and additive(s) type on the tribological properties. While the additives reduced the coefficient of friction at 100°C moderately, the effect on the wear resistance at both 32°C and 100°C was more pronounced compared to the base oil. Adhesion, abrasion, surface plastic flow, and microcracking were found to be the prevailing steady-state wear mechanisms, depending on the additive type(s) and oil temperature. Scheduled for Presentation at the 58th Annual Meeting in New York City April 28–May 1, 2003

Antiwear Tribofilm Formation on Steel Surfaces Lubricated With Gear Oil Containing Borate, Phosphorus, and Sulfur Additives

The formation of antiwear tribofilms plays a critical role in the longevity of automotive gears. The focus of this experimental study was on the lubrication efficacy of gear oils with different contents of borate-, phosphorus-, and sulfur-containing additives leading to the formation of protective tribofilms. Experiments were performed with AISI 52100 steel balls sliding against AISI 52100 steel disks in baths of different oils at ambient (∼32 °C) and elevated (∼100 °C) temperatures under load and speed conditions favoring sliding in the boundary lubrication regime. Friction coefficient responses accompanied by electrical contact voltage measurements provided real-time information about the formation and durability of the antiwear tribofilms. The wear resistance of the tribochemical films was quantified by wear rate data obtained from surface profilometry measurements of wear tracks on the disk specimens and sliding tests performed at ambient temperatures after the formation of the tribofilms during elevated-temperature sliding. Results indicate a strong dependence of tribofilm formation on temperature and type of additives. The slightly lower friction and higher wear resistance obtained at elevated temperatures with blended oils is attributed to the increased chemical reactivity of additives containing borate, phosphorus, and sulfur, leading to the formation of durable tribofilms. Relatively higher wear resistance and faster tribofilm formation were obtained with the borate-enriched gear oil formulations. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Micellar calcium borate as an antiwear additive

The mechanisms of action of a new generation of antiwear additives is studied here by means of energy‐filtering transmission electron microscopy (EFTEM) carried out on the wear particles generated during friction tests between two ferrous surfaces (under boundary lubrication conditions). This paper deals with the structural and physico‐chemical changes that colloidal particles, calcium carbonate (CC) and calcium borate (CB) overbased salicylates detergents, have undergone during the build‐up of the interfacial antiwear tribofilm. EFTEM allowed us to investigate the nature of wear fragments originating from the film, stemming from CC and CB micelles, and to make a comparison regarding the tribofilm formation mechanisms. It appears that the CC wear debris are mainly crystalline and contain a high concentration of iron (as abrasive iron oxide Fe2O3), limiting their antiwear action. Consequently, CC micelles do not lead to an effective protective tribofilm. On the other hand, CB micelles do have an antiwear action, which we explained by the formation of a glassy iron borate tribofilm during the friction tests. Many of the CB wear fragments are composed of this amorphous material containing very small crystallites of residual calcite. Boron (contained in the CB micelles) is responsible for the presence of amorphous zones of the film and acts as a glass former, in a comparable way to phosphorus in zinc dithiophosphate.