Penn and ExxonMobil researchers uncover mechanisms behind performance of major antiwear additive in lubricants
One of the main modern antiwear lubricant additives is zinc dialkyldithiophosphate (ZDDP)—widely used in automotive lubricants—which forms crucial antiwear tribofilms at sliding interfaces. However, despite its importance in prolonging machinery life and reducing energy use, the mechanisms governing its tribofilm growth are not well-understood. This limits the development of replacements with better performance and catalytic converter compatibility.
Now, in a study published in the journal Science, researchers from the University of Pennsylvania and ExxonMobil, have uncovered the mechanisms governing the growth of ZDDP antiwear tribofilms at sliding interfaces. The study provides a way forward for scientifically testing new anti-wear additives. Being able to pinpoint the level of stress at which they begin to break down and form tribofilms allows researchers to compare various properties in a more rigorous fashion.
ZDDP-derived tribofilms consist of rough, patchy, pad-like features that are composed of pyro- or ortho-phosphate glasses in the bulk with an outer nanoscale-layer of zinc polyphosphates and a sulfur-rich layer near the metal surface. However, the tribochemical film growth pathways are not established, and the factors which determine the film morphology and thickness (which tends to be limited to 50-150 nm) are unknown. Furthermore, ZDDP’s effectiveness as an antiwear additive for advanced engine materials is not yet clear. For low-weight materials (e.g., Al- and Mg-based alloys), ZDDP forms robust tribofilms primarily on load-bearing inclusions, but not on surrounding softer matrices. While ZDDP tribofilms can be formed between other non-ferrous material pairs, e.g., low-friction diamond-like carbon (DLC) films, they are often less durable than those formed when steel or iron is present for reasons not yet understood.
It is desirable to reduce or replace ZDDP as it often increases frictional losses, and produces Zn-, P- and S- containing compounds in the exhaust, reducing the catalytic converter’s efficiency and lifetime. Despite decades of research, no suitable substitute for ZDDP has yet been found, motivating research to understand the beneficial mechanisms underlying the growth and antiwear properties of ZDDP-derived tribofilms.—Gosvami et al.
The surfaces of a piston and cylinder in a car engine may look perfectly smooth to the naked eye, but, zoomed into the nanometer scale, they might look more like mountain ranges. Absent a buffering layer of a protective film, those peaks, known as asperities, would rub against each other and quickly wear down due to very high local stresses through direct steel-on-steel contacts. The resulting debris can further increase the friction between the surfaces and can cause severe abrasion, causing the system to catastrophically fail prematurely.
To see how the dynamics of this kind of sliding contributed to tribofilm growth, the researchers used the tip of an atomic force microscope to stand in for an individual point of roughness on those surfaces. Mounted on a flexible arm, or cantilever, the microscope measures the up-and-down movement of the arm as the tip is dragged over a surface, generating a topographical picture with nearly atomic resolution.
Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, the researchers monitored the growth and properties of the tribofilms in situ, observing surface-based nucleation, growth, and thickness saturation of patchy tribofilms versus sliding time.
The study was led by Robert Carpick, chair of the Department of Mechanical Engineering and Applied Mechanics in Penn’s School of Engineering and Applied Science, and Nitya Gosvami, a research project manager in his lab. Jason Bares and Filippo Mangolini, contributed to the study while members of Carpick’s lab. They collaborated with two researchers at Corporate Strategic Research, ExxonMobil Research and Engineering Company: Dalia Yablon, currently with SurfaceChar LLC, and Andrew Konicek.
In their experiment, the researcher immersed the entire cantilever-tip apparatus in ZDDP-infused oil, simulating the environment surrounding a single asperity on a piston surface. They then slid the tip over an iron surface, which simulated the ferrous composition of engine parts and recorded what happened as the tribofilm formed.
They found that films only began to form when the tip was slid at a certain pressure. This “stress activated” process meant that, the harder the tip squeezed and sheared the ZDDP-containing oil between the tip and sample, the faster the films grew.
The researchers also found an explanation to why these films grow to a certain thickness and then stop growing.
It’s essentially a cushion effect. The film that grows is not as stiff as the steel. When you push on a stiff surface, you get a high stress due to the concentration of force. When you push on a less stiff surface, the force is spread out, so the stress is lower. The thicker the film, the more it acts as a cushion to reduce the stress that is needed to cause the chemical reactions needed to keep growing. It’s self-limiting, or in other words, it has a way of cutting off its own growth.—Robert Carpick
The self-limiting nature of the films is beneficial, as they would otherwise quickly use up the small amount of ZDDP in the oil.
… ZDDP antiwear tribofilm growth increases exponentially with applied pressure and temperature under single-asperity contact, in very good agreement with stress-assisted reaction rate theory; the kinetic parameters are consistent with a covalent bond reaction pathway. Repeated sliding at sufficiently high loads leads to abundant tribochemical reactions and the associated nucleation and growth of robust tribofilms with a pad-like structure similar to macroscopically-generated films. The tribofilm is not a product of the weakly adsorbed thermal film, but instead is generated from molecular species fed continuously into the contact zone. We confirm the sacrificial nature of the tribofilm beyond a threshold thickness, indicating that layers grown at lower applied pressures are weaker. The observations support that ZDDP’s antiwear behavior derives from mechanical protection provided by the tribofilm, as opposed to corrosion inhibition. We suggest that this in situ approach can be directly applied to understand further molecular-level tribochemical phenomena and functionality, such as the behavior of other important lubricant additives like friction modifiers, or for films formed in vapor-phase lubrication.—Gosvami et al.
Such a discovery would not have been possible without the team’s nanoscale approach. Without being able to control the stress and geometry of a single point of contact and observe the film growth at the same time, there would be no way to connect the pressure threshold with the point at which the film begins to form and when it stops growing.
This is a fascinating example of what we call tribochemistry. The combination of friction and mechanical pressure enhances the probability of chemical reactions by reducing the energy needed to break or form bonds. In this case, it helps break down the ZDDP molecules and also helps them react to form the tribofilm on the surface. And when the pressure drops, the film growth stops as needed.—Robert Carpick
The research was partially supported by and the National Science Foundation under grant CMMI-1200019 and by the Marie Curie International Outgoing Fellowship for Career Development within the 7th European Community Framework Programme under contract PIOF-GA-2012-328776. ExxonMobil’s Corporate Strategic Research laboratory provided materials and financial support.
Jason Bares is now at BorgWarner Powertrain Technical Center. Dalia Yablon is now at SurfaceChar LLC.
N. N. Gosvami, J. A. Bares, F. Mangolini, A. R. Konicek, D. G. Yablon, and R. W. Carpick (2015) “Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts” Science doi: 10.1126/science.1258788