http://www.greencarcongress.com/2015/03/20150313-penn.html
Go to the link for the complete article.
"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."
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.
Go to the link for the complete article.
"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."
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.
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