Abstract
High-temperature solid lubricants play a significant role in the hot metal forming process. However, preparing high-temperature solid lubricant is formidably challenging due to the stern working conditions. Here we successfully develop a new type of eco-friendly high-temperature graphite-based solid lubricant by using amorphous silica dioxide, aluminum dihydrogen phosphate, and solid lubricant graphite. The solid lubricating coating exhibits excellent tribological properties with a very low friction coefficient and good wear protection for workpiece at high temperature under the air atmosphere. An array of analytical techniques reveals the existence of solid lubricant graphite in the lubricating coating after the high-temperature friction test. A synergistic effect between the protective surface film and the solid lubricant graphite is proposed to account for such superior lubricating performance. This work highlights the synergistic effect between the protection layer and the lubricant graphite and further provides the insight in designing the high-temperature solid lubricant.
Highlights
Metal forming is vital to manufacturing metals into various shapes for different applications
During the running-in period, the coefficient of friction (COF) fluctuated heavily due to the intensive deformation at the contact interface, while the COF remained relatively stable in the steady period since a dynamic balance was reached with a stable coating formed at the interfacial area
The present work demonstrates an eco-friendly graphite-based solid lubricating coating which was successfully prepared using a readily achievable method by an appropriate mixture of the amorphous SiO2, solid lubricant graphite, and Al(H2PO4)3 binder. This solid lubricating coating displayed a stable adhesion to the substrate and a superior lubricant performance with ultralow COF of about 0.05 at high temperature under the air atmosphere regardless of the operational sliding speed
Summary
Metal forming is vital to manufacturing metals into various shapes for different applications. The removal of CO and CO2 from the surface could further promote the oxidation reaction and the burnout of graphite [19] Given those detrimental factors, graphite has been mainly employed as a lubricant for medium-temperature metal forming processes like casting [20], forging [21, 22], and extrusion [23], etc. Despite its effective protection at such high temperatures, the performance within the temperature range between 800 and 1,200 °C is unsatisfactory due to the defects like cracks and holes generated during the preparation process or cooling stage Those formed defects could not be healed at this temperature range because of the high viscosity of the SiO2-based layer. The evaluation via various analytical techniques like micro-Raman, X-ray diffraction (XRD), thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) and scanning electron microscopy (SEM) illustrates the protective effect of the composite coating for the interior graphite against its oxidation, enabling the superior lubricating performance
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