Transfer Film Tenacity: A Case Study Using Ultra-Low-Wear Alumina–PTFE

The transfer film, a protective barrier that forms when a solid lubricant slides against a hard and high-surface-energy counterface, plays an important role in friction and wear reduction. The transfer films of many solid lubricants are removed and replenished during sliding. However, one particularly low wear rate solid lubricant has been shown to produce persistent transfer films that thicken and homogenize over the course of a test. Based on a mass balance of the system, transfer film growth can only occur if its wear rate is less than that of the parent polymer. However, recent measurements of the wear rates of these transfer films show that they are likely orders of magnitude higher than those of the parent polymer. The goal of this paper is to elucidate the origins of this apparent contradiction. In this study, a high-density polyethylene (HDPE) probe was used in an effort to approximate the contact conditions under which the film originally formed. Transfer films formed during run-in of the parent solid lubricant were removed immediately by the HDPE pin (k ~ 10⁰ mm³/Nm). However, transfer films formed after the solid lubricant had transitioned to ultra-low wear rates themselves exhibited ultra-low wear rates in the range from 10⁻⁸ to 10⁻¹⁰ mm³/Nm; to our knowledge, this is the first direct observation of ultra-low wear rate transfer films, a condition that was previously assumed necessary for ultra-low wear rates of the solid lubricant system. Follow-up measurements showed that the wear rate of the transfer film was extremely sensitive to the surface energy of the probe; the wear rate increased by orders of magnitude when the surface energy of the probe exceeded a critical value near ~35 mJ/mm². These results provide fresh insights into the wear behaviors of transfer films and the processes governing ultra-low wear of solid lubricant materials.