KEY CONCEPTS
•
This research reveals the formation of multilayer chemistries resulting from tribochemical reactions between the steel interfaces, featuring evidence of oxygen diffusion, voids and metallic clusters.
•
The polyurea grease tribofilm’s dense amorphous nature, the size of the voids formed and the top iron sulfide layer provides exceptional friction and wear properties compared with the tribofilm generated using greases with lithium complex thickeners.
•
A fundamental understanding of the tribolayer crystallinity and composition plays a pivotal role in not only understanding the wear mechanisms but also in developing new-generation lubricant chemistries for superior performance.
Meet the Presenters
This article is based on an Industry Insights presentation titled
Atomistic Understanding of Grease Tribolayers on Bearing Surfaces by Dr. Kuldeep Mistry and Dr. Vikram Bedekar on April 19. Industry Insights is a monthly virtual event series available for free to STLE members. These events provide a forum for members to collaborate with STLE industry peers and experts on a specific topic. The presenters give a brief presentation, then facilitate conversation by asking discussion questions and providing actionable takeaways. For more information, click
here.
Dr. Kuldeep K. Mistry is formerly a program manager for The Timken Co.’s industrial bearings product portfolio in North Canton, Ohio. Dr. Mistry has successfully led multiple lubrication-related projects in The Timken Co. and developed lubrication solutions for several applications (e.g., automotive, industrial, wind, rail, heavy truck) within the company during his 15 years of experience.
Dr. Mistry received his doctorate from the University of Leeds, UK, and his graduate studies were sponsored by ExxonMobil. As a principal investigator and postdoctoral scientist at the U.S. Department of Energy (DOE), he operated the Argonne National Laboratory in Lemont, Ill. Dr. Mistry received the U.S. DOE R&D 100 Award in 2015: “Versatile Hard Carbon Microspheres Made from Plastic Waste.” Dr. Mistry is actively engaged in the tribology and lubrication research community through peer-reviewed journal publications, presentations and technical leadership. Mistry has been on the STLE Board of Directors since 2019.
Dr. Mistry joined Chevron’s Lubricants Product and Technology Division in July 2022 as a formulator. You can reach him at
kuldeep.mistry@chevron.com.
Dr. Kuldeep Mistry
Dr. Vikram Bedekar is a manager for The Timken Co.’s R&D division in North Canton, Ohio, and has been a technical lead in advanced material characterization, induction hardening for ultra large wind bearings, heat treatment, coatings, hard machining and surface engineering for more than 17 years.
Dr. Bedekar received his doctorate from The Ohio State University while working full time at Timken. In 2013, the College International pour la Recherche en Productique (The International Academy for Production Engineering, CIRP) awarded Dr. Bedekar the prestigious F.W. Taylor Medal for his work in hard turning. He has written more than 25 peer-reviewed articles in prominent journals such as Acta Materialia, CIRP Annals, Materials Science and Engineering and the International Centre for Theoretical Physics (ICTP). You can reach him at
vikram.bedekar@timken.com.
Dr. Vikram Bedekar
Grease-lubricated bearings are common in industrial applications ranging from construction, mining, cement, steel and aluminum mills, food machinery, paper and pulp and wind turbines. The grease and the thickener chemistries have a large impact on bearing lubrication. The research presented in this article investigates the morphological and compositional characteristics of grease-generated tribolayers at the atomistic level.
Lithium-based and polyurea-based grease thickeners are studied. The tribolayers formed on 52100 bearing steel are investigated using atom probe tomography (APT) and transmission electron microscopy (TEM) to answer questions regarding how grease stays in place on components during severe sliding conditions.
Studying tribolayers
The thickeners added to the base oil to form a grease comprise a three-dimensional network of fibers or surface-active particles suspended in the grease. With 80% of bearings lubricated with grease, lubrication failure is the main reason for damage to bearings during operations. Speed of operation and temperature are the dominating mechanisms for grease deterioration. NLGI grades correspond to a specific worked penetration value range. The harder the grease, the higher the NLGI grade number.
In industry, hundreds of bearings are in operation at any given time, meaning that careful bearing, seal and lubricant selections are critical to help ensure equipment operates reliably. High loads, low rotational speeds, elevated temperatures and an environment heavily contaminated with water, steam and scale are reasons why the formation of a tribochemical or boundary film is key. These films contribute to performance and protect the metal component surfaces, keep water away or handle water.
Tribology tests conducted for this research include bench tests and component tests. Bench test results identify wear, contact potential and friction coefficients. Using a Plint tribometer for bench testing, the lithium complex grease shows the highest wear and lowest contact potential. The polyurea grease shows the lowest wear and friction coefficient and the highest contact potential.
APT is a technique that allows the individual atoms in a sample to be studied. This research is performed using equipment at the Oak Ridge National Laboratory in Oak Ridge, Tenn. The atom probe sample preparation involves coating the sample with platinum or nickel. A conical sample with a 150 nanometer (nm) base diameter and 150 nm length is lifted out from the sample and attached to a silicon post. A pulsing laser is focused on the sample’s tip to evaporate atoms from the sample surface that fall onto the detector. The measurement of the atom’s time of flight and momentum identify the type of atom, allowing the material in the tribolayer to be identified. The small relative sample size can limit this technique.
Lithium complex grease tribofilm
TEM shows layer thickness of 50 nm for lithium complex tribofilm. Iron oxide and iron sulfide are identified in the initiation layer. Iron sulfide has lubricating properties that lower friction. The tribolayer thickness is approximately 40-50 nm with an interface between the steel and the layer. No calcium is identified in the tribolayer—however, traces of carbon and oxygen are seen. APT analysis showed 5-10 nm clusters of lithium and bismuth that are difficult to detect using traditional characterization techniques.
Polyurea grease
Polyurea grease is much thicker than the lithium complex grease. The layer thickness is 100-200 nm and shows three distinct layers. Near the steel the base tribolayer shows calcium oxide enrichment with the concentration of iron in the tribolayer decreasing close to the surface. Relatively large (10-20 nm) voids are present in the middle tribolayer that are formed during the sliding motion. The surface tribolayer shows iron sulfide enrichment. The overall tribolayer structure is amorphous as indicated by diffused microdiffraction patterns.
The polyurea grease is wear resistant, corrosion resistant and self-lubricating.
Conclusions
The results of this research show the formation of multilayer chemistries resulting from tribochemical reactions between steel interfaces. These chemistries feature oxygen diffusion, voids via mechanical deformation and metallic clusters. The results indicate that tribolayer thickness, crystallinity, amorphicity and atomic substructures play influential roles in the performance of the tested greases.
The lithium complex grease analysis show the presence of high iron sulfide compounds that result in low friction and high wear.
The polyurea grease tribofilm is very uniform with a dense structure. The layer is amorphous and contributes to low friction structure. The 100-200 nm tribolayer shows multilayer structure with sulfide enrichment near surface.
A fundamental understanding of the tribolayer crystallinity and composition plays a pivotal role in not only understanding the wear mechanisms but also in developing new-generation lubricant chemistries for superior performance.
Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.