K. Kempa, S. Boncel, A. Marek, A. Kulczycki, J. Merkisz, T. Runka, M. Nowicki, J. Kałużny
Poznan University of Technology,
Poland
Keywords: carbon nanotubes, friction
Summary:
Carbon Nanotubes (CNTs) ‘re-discovered’ by Sumio Iijima in 1991 have been intensively studied recently; their often-reported unique combination of excellent physicochemical properties is extremely beneficial for tribological performance. Despite the promising preliminary laboratory test results presented in the literature the early attempts of CNTs commercialization were disappointing. In this paper the reasons behind this were identified and the possible solutions for overcoming the problems were discussed. The research results were based primarily on multiple engine test results, with the engine used as an example of a comprehensive tribological system with a multitude of friction components, made out of various metal and non-metallic materials, working at different speed, entrainment velocities and pressures, and at different temperatures. Additionally, a set of relevant tribometer tests was discussed in this paper in an effort to achieve a detailed understanding of the physical phenomena related to the CNT effects in lubricants. In our opinion there are two main aspects that must be addressed before making large-scale industrial application of CNTs in lubricants possible. These aspects are: the stability of CNT dispersion, and understanding of the mechanisms of CNT tribological effects. Thus, the impact of the CNT dispersion in oil, comparing in details multiple chemical and mechanical methods for CNT dispersion in oil is discussed first. The presented engine tests lead to the statement, that only well dispersed CNTs contribute to any significant friction reduction. The same level of total engine friction reduction reaching a notable value of about 5% was measured both for CNT concentration in oil equal to 0.5% and in the case for concentration as low as only 0.03%. In the first case only standard ultrasound homogenization was used, but a major improvement in the CNT dispersion was achieved for the low CNT concentration using a simple and effective chemical method. Finally, a smart solution was presented for constant CNT release into the lubricating oil, leading to a perfect dispersion and an unprecedented friction reduction of up to 16%. Depending on the actual local friction conditions CNTs may reveal various functional mechanisms occurring simultaneously during the friction process. Basing on the follow-up tribological tests and friction track characterization a new role of CNT action in the friction process related strictly to the energy transfer was proposed. Furthermore a unique, strong interaction of CNTs and the surface of polyacetal during the friction process was described. Finally, a risk analysis for the proposed application solutions was attempted.