Contact Mechanics · Tribofilms
Tribofilms and third bodies: what forms between sliding surfaces
Trapped debris and new films soon sit between sliding surfaces and set the friction and wear.
1 Trapped debris 2 ZDDP film 3 PTFE film Questions
What is a third body in a sliding contact?
A third body is the debris layer between the two sliding parts; watch it fill and drain.
Try it: set the share that comes back in to 80%: debris inside settles at 133 units, not 53, but wear out stays 4 units/s.
How does a ZDDP antiwear tribofilm grow on steel?
ZDDP (zinc dialkyldithiophosphate), an oil additive, grows phosphate pads on rubbed steel; heat and pressure speed it up.
Try it: drop the oil from 120 to 80 °C: the start rate falls from 66 to 6.8 nm per hour.
Why does PTFE friction drop once a transfer film forms?
PTFE (Teflon) smears a thin film of itself onto the steel, then slides PTFE on PTFE.
Try it: choose "Too smooth": the film keeps peeling, so cover settles at 33% instead of 91%.
What to take away
Wear is an outflow
Debris that stays in the contact is not yet wear. Wear is what leaves.
ZDDP needs heat and stress
In this model, oil 40 °C cooler grows the film about ten times slower.
A film has to stick
A PTFE film lowers friction only if the steel holds it. Too smooth or too rough, and it is lost.
More detail: the equations, the numbers and their limits
Third-body flows (Godet, Berthier)
Godet (1984) pointed out that the two parts are separated by a third body most of the time. Berthier and co-workers called its movements the tribological circuit: a source flow Qs (debris detached from the first bodies), an internal flow Qi (debris moving inside the contact, which takes up the speed difference), an ejection flow Qe at the edges, part of which comes back in (recirculation Qr), and the rest, which leaves for good (wear flow Qw).
The mass balance is dM/dt = Qs − Qw (Fillot, Iordanoff and Berthier 2007). Here the ejection is taken as Qe = ke M and Qw = (1 − r) Qe, so M settles at Qs / (ke (1 − r)) and then Qw equals Qs. This first-order form is a teaching choice: real ejection depends on the contact shape, load and particle size.
The friction trace is schematic. For dry steel, friction often rises during running-in as loose oxide debris builds up (Hutchings and Shipway). At higher temperature, compacted oxide can sinter into a smooth "glaze" layer that lowers friction and wear (Stott 1998).
ZDDP growth law (Gosvami et al. 2015)
In an AFM with a silicon tip sliding on iron-coated silicon in ZDDP-containing oil, the growth rate fitted Γ = Γ0 exp(−(ΔU − σΔV) / kT), with σ the mean Hertz contact pressure (several GPa in that work). The page uses ΔV = 3.8 ų (0.0038 nm³, the value Zhang and Spikes 2016 quote from that paper), Γ0 = 10⁻² m/s (the value Akchurin and Bosman 2017 take from the same paper) and ΔU = 0.8 eV. The ΔU value is approximate and not checked against the original paper or a source that quotes it, so treat the absolute rates as illustrative. The trends with temperature and pressure are what the widget is for. Settings outside the tested range are extrapolations.
Thickness is h = hcap (1 − exp(−Γ t / hcap)). The cap is an added assumption: in rubbing tests, ZDDP films level off after about an hour at roughly 60 to 150 nm, depending on the ZDDP (Spikes 2004; Dawczyk et al. 2019). Single-asperity rates at GPa pressures do not map directly onto a full-size contact.
Zhang and Spikes (2016) found the growth follows shear stress rather than pressure in their tests, with 53 kJ/mol and 0.18 nm³. Both views agree that stress lowers the barrier.
PTFE transfer film
The model: each pass lays down film on bare steel (rate a) and removes some (rate b), so cover settles at a/(a+b). Bahadur (2000) reviews why a moderate roughness anchors the film best. Plain PTFE on steel wears at about 10⁻⁴ to 10⁻³ mm³/N m; good fillers bring that to about 10⁻⁶ to 10⁻⁷ (Blanchet and Kennedy 1992; Burris and Sawyer 2006).
Questions people ask
What is a tribofilm?
A tribofilm is a thin layer that forms on a surface only because of rubbing. It can come from an oil additive (like ZDDP), from oxidised wear debris, or from material moved from the other surface. It often controls friction and wear more than the base metal does.
What is the difference between a third body and a tribofilm?
"Third body" is Godet's name for anything between the two parts, loose or attached. A tribofilm is the part that sticks to a surface as a layer. Loose debris can be packed into a tribofilm, and a tribofilm can break up into loose debris.
How thick is a ZDDP tribofilm?
On steel in rubbing tests, typically about 50 to 150 nm, made of pads a few micrometres across with deep valleys between them. The thickness levels off after roughly an hour, when growth and removal balance.
Why does ZDDP need high temperature to work?
The film grows by a chemical reaction with an energy barrier. Heat and contact stress both help molecules get over it. At room temperature and moderate stress, growth is very slow.
Why does PTFE have low friction on steel?
PTFE chains are long and smooth and slide past each other easily. Once a thin, well-attached PTFE film covers the steel, the pin slides on PTFE, and the chains line up in the sliding direction. Friction is lowest when this film is thin and stays put.
What is an oxide glaze?
A smooth, hard layer of packed and partly sintered oxide debris. It forms on alloys sliding at raised temperature. It can lower wear a great deal, as Stott (1998) reviews for iron and nickel alloys.
Related: Friction is not a constant · Flash temperature · Wear mechanisms · Stribeck curve · Running-in
References
Show the 12 references
- M. Godet, The third-body approach: a mechanical view of wear, Wear 100, 437 to 452 (1984). doi:10.1016/0043-1648(84)90025-5
- Y. Berthier, M. Godet and M. Brendle, Velocity accommodation in friction, Tribology Transactions 32, 490 to 496 (1989). doi:10.1080/10402008908981917
- N. Fillot, I. Iordanoff and Y. Berthier, Wear modeling and the third body concept, Wear 262, 949 to 957 (2007). doi:10.1016/j.wear.2006.10.011
- N. N. Gosvami, J. A. Bares, F. Mangolini, A. R. Konicek, D. G. Yablon and R. W. Carpick, Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts, Science 348, 102 to 106 (2015). doi:10.1126/science.1258788
- H. Spikes, The history and mechanisms of ZDDP, Tribology Letters 17, 469 to 489 (2004). doi:10.1023/B:TRIL.0000044495.26882.b5
- J. Zhang and H. Spikes, On the mechanism of ZDDP antiwear film formation, Tribology Letters 63, 24 (2016). doi:10.1007/s11249-016-0706-7
- A. Akchurin and R. Bosman, A deterministic stress-activated model for tribo-film growth and wear simulation, Tribology Letters 65, 59 (2017). doi:10.1007/s11249-017-0842-8
- J. Dawczyk, N. Morgan, J. Russo and H. Spikes, Film thickness and friction of ZDDP tribofilms, Tribology Letters 67, 34 (2019). doi:10.1007/s11249-019-1148-9
- S. Bahadur, The development of transfer layers and their role in polymer tribology, Wear 245, 92 to 99 (2000). doi:10.1016/S0043-1648(00)00469-5
- T. A. Blanchet and F. E. Kennedy, Sliding wear mechanism of polytetrafluoroethylene (PTFE) and PTFE composites, Wear 153, 229 to 243 (1992). doi:10.1016/0043-1648(92)90271-9
- D. L. Burris and W. G. Sawyer, Improved wear resistance in alumina-PTFE nanocomposites with irregular shaped nanoparticles, Wear 260, 915 to 918 (2006). doi:10.1016/j.wear.2005.06.009
- F. H. Stott, The role of oxidation in the wear of alloys, Tribology International 31, 61 to 71 (1998). doi:10.1016/S0301-679X(98)00008-5
BibTeX
@misc{tripathy2026tribofilms,
author = {Tripathy, Manisha},
title = {Tribofilms and Third Bodies Lab},
year = {2026},
howpublished = {\url{https://untetheredatom.com/tribology/tribofilms-and-third-bodies}},
note = {Interactive web tool}
}