Copper addition provides good thermal conductivity to automotive brake pads, but its contribution as a friction material during service conditions is still under investigation. In this sense, the role of copper in the formation of a compact nanocrystalline transfer layer during pin-on-disc tribotest was investigated for different temperatures and interfacial media mixtures. The analysis of the evolution of the friction coefficient (CoF) was associated with the microstructural characterization of the tribofilms and tribosurfaces to better understand the role of Cu, in association with magnetite and/or graphite, on the CoF.
A high-vacuum tribometer was used to assess the effect of the environment (air or N2) and the testing temperature (25 °C and 400 ºC) on the CoF and the tribofilm microstructure. Pin-on-disc tribotests using steel counter bodies and various interfacial media additions (IMA) were performed for 60 min using normal load of 20 N and speed of 0.05 m/s. The IMA consisted of 0.1 gram of material (pure hematite; pure magnetite and binary and ternary mixtures of magnetite and copper nanoparticles, and graphite). These blends were either mixed manually (MM) or by high energy ball milling (BM). The tribosurfaces and tribolayers microstructures were characterized using SEM associated with FIB, EDS and XEDS mapping techniques.
The transformation of magnetite into hematite was observed for all the mixtures tested in air at 400 °C. Tribotests using pure iron oxides (magnetite and hematite) and others IMA resulted on CoF of approximately 0.4 and 0.5, respectively. Tribotests at 400 °C (air and N2) using magnetite-graphite and magnetite-graphite-copper MM mixtures presented higher CoF values (0.30 to 0.35) than magnetite-graphite and magnetite-graphite-copper MM mixtures tested at room temperature (~0.2). As a rule, BM mixtures presented higher CoF values than the respective MM mixtures: the CoF values measured for the BM mixtures were around 0.35 (400 °C, N2 atmosphere), 0.45 (400 °C, air) and 0.40 (25 °C, air). The microstructural characterization of the tribosurfaces of the BM mixtures revealed the presence of exogenous Zr-rich particles. Limitations of this study: The BM mixture was expected to result in lower values of CoF in comparison to the MM mixtures. The BM mixture was thought to provide a more homogenous mixing of particles in the nanoscale. However, the BM mixture caused ZrO2 contamination, which acted as a friction material, preventing the selective transfer from graphite and copper to the tribosurfaces. What does the paper offer that is new in the field in comparison to other works of the author? The authors had already shown for binary magnetite-copper mixtures that non-homogenous distributions of nanocrystalline Cu formed patches at the tribosurfaces, increasing the CoF and producing peaks in the CoF curve. Moreover, at higher temperatures (400 °C), Cu acted as a solid lubricant for binary magnetite-copper mixtures. Ternary mixtures, however, had not yet been tested at 400 °C. The extensive investigation of the behaviour of higher order mixtures may help to unveil the role of these constituents on the formation of tribofilm, generating useful data for the development of new hybrid materials for friction.
For MM mixtures, graphite played a dominating role on the CoF, even at 400 °C, acting as a solid lubricant. Unlike the MM mixtures, the BM mixtures featured higher CoF values (0.45) for tests at 400 °C in air when compared to the results for pure magnetite at the same conditions. The transformation of magnetite into hematite in air at 400 °C explains the higher CoF values measured for BM and MM mixtures. The contamination of the BM ternary mixture with ZrO2 was responsible for the higher CoF values as the presence of this exogenous abrasive material prevented the selective transfer of graphite into the tribosurfaces. At 400 °C, Cu acted as solid lubricant only for BM mixtures tested in N2.
Bilgic Istoc Sami, Winner Hermann Institute of Automotive Engineering, Technische Universität Darmstadt, Germany