Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined single-asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.

https://science.sciencemag.org/content/sci/348/6230/102.full.pdf

Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts

During the last two decades, the industry (including scientists) has focused on diamond-like carbon (DLC) coating because of its wide range of application in various fields. This material has numerous applications in mechanical, electrical, tribological, biomedical, and optical fields. Severe friction and wear in some machine parts consumes high amount of energy, which makes the process energy inefficient. Thus, DLC coating can be an effective means to lower the friction and wear rate. Some important process variables that affect the tribological characteristics of DLC coating are adhesion promoter intermediate layer, substrate surface roughness, hydrogen incorporation or hydrogen non involvement, and coating deposition parameters (e.g., bias voltage, etching, current, precursor gas, time, and substrate temperature). Working condition of DLC-coated parts also affects the tribological characteristics, such as temperature, sliding speed and load, relative humidity, counter surface, and lubrication media (DL...

An updated overview of diamond-like carbon coating in tribology

In the last ten years significant progress has been made on development and understanding of diamond-like carbon coatings (DLC), which enabled them to become one of the most promising types of protective coatings for various mechanical applications. Their main advantages are low friction, good anti-wear properties, and adhesive protection. However, due to their low surface energy their reactivity with conventional oils and additives is limited, which makes achieving effective boundary lubrication a complex task. For a qualitative step-change that would improve the performance and allow effective optimizing and tailoring of these boundary-lubricated tribological systems, it is necessary to understand the mechanisms of why, how, under which conditions, and with which materials and lubricants, the actual boundary lubrication is possible—if at all. The current results on boundary lubrication of DLC coatings are not many, and are due to the different types of coatings, lubricants and additives used in these studies, often difficult to compare and sometimes even contradictive. However, a recent great demand in different industries to apply the DLC coatings to lubricated systems requires a better understanding of these phenomena and overall performance. Therefore, if we wish to see a more effective continuation of the research and a better understanding of the scattered results, an overview of today’s state-of-the-art of lubricated DLC contacts is needed. In this paper we analyse the behaviors and suggested mechanisms from already-published studies and we summarize the present understanding of the boundary lubrication of DLC coatings. We focus on the DLC-lubricant interaction, thus we analyse only self-mated DLC/DLC contacts in order to avoid the inevitable effects from interactions with other counter materials such as steel.

Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications

Tribological properties of tribofilms formed from ZDDP in DLC/DLC and DLC/steel contacts

Electrochemical and structural properties of electroless Ni-P-SiC nanocomposite coatings

Tribofilm formation from ZnDTP on diamond-like carbon

Highly homogeneous Ni–P/SiO2 nanocomposite coatings were prepared by electroless deposition from composite baths containing well dispersed SiO2 nanoparticles. The particle concentration in the coatings increases gradually with the SiO2 concentration in the bath and reaches an observed maximum of 25 vol.% at a bath concentration of 10 g/l. Tribological measurements of the heat treated coatings indicate that while the hardness of the coating is decreased compared to that of the Ni–P coating without particles, an improvement in Pin-on-Disk wear resistance up to a factor of 2 can be achieved. The aspect ratio of the wear track of samples heat treated above 270 °C is found to increase with the concentration of nanoparticles in the coatings. This observation indicates that the nanoparticles also modify the wear behavior of the coatings.

Sigfried Roos

Papers

Tribofilm formation from ZnDTP on diamond-like carbon

Homogeneous electroless Ni–P/SiO2 nanocomposite coatings with improved wear resistance and modified wear behavior

Reactions of zinc-free anti-wear additives in DLC/DLC and steel/steel contacts

Materials failure mechanisms of hybrid ball bearings with silicon nitride balls