Effect of nanoparticulate copper, zirconia and graphite: contribution to the friction coefficient and transfer layer formation in dry sliding tests with interfacial media addition.

Abstract

Copper is present in brake pads in the form of fibres, tough its suppression from pads' formulations is already a trend compared to the suppression of asbestos in the 1980's.As a contribution to this challenging scenario, this work investigated the behaviour of mixture additions containing copper, zirconia, graphite and magnetite in dry sliding tribotests. Pin-on-disc tribotests with interfacial media addition were performed in order to contribute on the mission of understanding how Cu, graphite and ZrO2 particles act on the oxide transfer layer formation on dry sliding conditions. Thus describing some of the tribological properties that a possible replacement for copper in brake pads should mimic. Tests were performed at 23°C in air, 400°C in air and 400°C in N2.Nanoparticulate Cu, ZrO2 and micron-sized graphite were mixed in a Fe3O4 basedmatrix. The ZrO2 particles were incorporated to the mixtures by two mixing methods: manual mixing and high-energy ball milling. This study sought to explain the coefficient of friction (CoF) behaviour of these additions individually, as well as together, by correlating the CoF with comprehensive characterization of the oxide transfer layer. The transfer layer was characterized by means of scanning electron microscopy (SEM), Focused Ion Beam (FIB) and Transmission electron microscopy. Graphite containing mixtures displayed low average CoF values (0.23 to 0.31) due to selective transfer of graphite films to the first bodies' tribosurfaces. Copper formed agglomerates and patches within the oxide transfer layer that were responsible for raising the CoF values at 23°C and acted as soft copper films at 400°C. The nanoparticulate addition of ZrO2 in the manual mixtures prevented the formation of Cu patches and larger agglomerates, promoting the formation of a smooth and compact oxide tribolayer, though graphite's selective transfer was still observed.

Use of synthetic Fe3O4-rich tribofilms to investigate the effect of microconstituents, temperature and atmosphere on the friction coefficient during pin-on-disc tribotest

“…Previous investigations [13,14] -using PoD tribotests under air atmosphere at room temperature and Fe 3 O 4 -Cu artificial third body -showed that copper promoted rougher sliding and CoF increase. Still, at 400 °C, copper promoted smoother sliding and lower CoF values [13][14][15]. Investigations [14,15] using PoD tribotests under air atmosphere at room temperature and Fe 3 O 4 -graphite artificial third body indicated that graphite causes a smoother sliding and reduces the CoF.…”

“…Still, at 400 °C, copper promoted smoother sliding and lower CoF values [13][14][15]. Investigations [14,15] using PoD tribotests under air atmosphere at room temperature and Fe 3 O 4 -graphite artificial third body indicated that graphite causes a smoother sliding and reduces the CoF. Still, the graphite's effect on the CoF depends on the tribotest conditions, such as temperature, mixing mode, and atmosphere [15,[18][19][20][21].…”

“…Artificial third body and PoD have been used to understand the effect of the chemical composition of the synthetic tribofilm on the CoF of less complex tribosystems [13][14][15][16][17]. For instance, Österle et al [16] used PoD and artificial third body to test raw materials ( Fe 3 O 4 , MoS 2 , graphite, potassium titanate, CNSL polymer, silica and silicon carbide ) for automotive brake pad formulations.…”

“…Investigations [14,15] using PoD tribotests under air atmosphere at room temperature and Fe 3 O 4 -graphite artificial third body indicated that graphite causes a smoother sliding and reduces the CoF. Still, the graphite's effect on the CoF depends on the tribotest conditions, such as temperature, mixing mode, and atmosphere [15,[18][19][20][21]. For instance, the structural ordering and reactivity of the graphite change with the testing atmosphere and temperature.…”

“…Copper fibres are still used in friction materials in Latin America, even though brake pads containing more than 5%Cu by weight have been banned in California since January 2021, andthe California Motor Vehicle Brake Friction Material Law diminishes this maximum Cu content to 0.5% by weight by 2025 [2,3,[13][14][15]. Copper is known to play an important role in controlling the tribocontact temperature (due to its good thermal conductivity) and stabilising the friction coefficient of brake pads, improving their performance [2,3,27].…”