Preparation And Tribological Properties Of SrCrO₄Solid Lubricant Materials

In this paper, SrCrO₄powders with different crystallographic morphologiesand sizes were prepared by both hydrothermal synthesis and direct chemicalprecipitation methods. The single-phase SrCrO₄ceramics were prepared bypressureless sintering, while NiCr matrix composites incorporated with differentconcentrations of SrCrO₄were fabricated by hot pressing. The microstructure andtribological properties over a broad temperature range were investigated by meansof X-ray diffraction, scanning electron microscopy, Laser Raman spectroscopy andhigh temperature friction and wear tester. Hereafter, the self-lubricatingmechanisms of SrCrO₄ceramics and NiCr-SrCrO₄composites were proposed.Nanorods or nanobars of SrCrO₄can be prepared without any additive byhydrothermal synthesis route. With the addition of EDTA, the SrCrO₄crystals arefeatured to be the flake-type particles. EDTA exhibits a strong inhibitory role oncrystal growth. The addition of EDTA inhibits the homogeneous nucleation ofSrCrO₄crystals, as a result, a heterogeneous nucleation occurs on the inner wall ofTeflon container. Finally, the SrCrO₄crystals grow up into nanosheets along thewall of Teflon. However, spherical SrCrO₄powders were synthesized by addingboth anhydrous ethanol and EDTA as surfactants with the direct chemicalprecipitation method. The mixed solution of ethanol and EDTA can provide aspecial environment in which SrCrO₄crystals can nucleate and grow up into nanoparticles. As the environment of crystal growth is similar to each precipitate,SrCrO₄crystals gradually grow up into spherical precipitates containing a lot ofnanocrystals. The dispersion of crystals is improved due to the chemisorptions ofEDTA on the surface of SrCrO₄crystals.SrCrO₄ceramics were prepared by pressureless sintering. With increasing thetesting temperature, the wear rates of SrCrO₄ceramics decrease distinctly. The wearrates are3.7×10^-3mm³/（Nm） at room temperature,5.3×10-⁴mm³/（Nm） at400℃, and1.6×10^-4mm³/（Nm） at600℃, respectively. The wear mechanisms of SrCrO₄at roomtemperature are brittle fracture and abrasive wear, while the wear mechanisms atboth400℃and600℃are dominated as delamination wear. At high temperature, alot of nano-fine grains were formed on the worn surface during wear. These finegrains have a good self-lubricating ability of effectively reducing friction and wearat high temperatures.NiCr-SrCrO₄composites with different compositions were prepared by hotpressing. When the content of SrCrO₄is20wt.%, the composite has the best tribological performance, whose friction coefficient is0.28and wear rate is8.04×10^-6mm³/（Nm）, respectively. The wear mechanism is delamination wear atroom temperature. However, a self-lubricating transfer film containing SrCrO₄isformed on the worn surface at600℃, with which the composite exhibits anexcellent tribological performance at high temperature. In addition, an oxide layeris formed during tribological stressing, and this oxide layer is mainly composed ofNiO, Cr₂O₃and SrCrO₄. Therefore, the self-lubrication of NiCr-SrCrO₄compositesat high temperature is achieved by both the SrCrO₄-containing lubricating layer andthe oxide layer.