This study successfully synthesizes CoAl-hexametaphosphate layered double hydroxide (HMP-LDH) via anion exchange. The application of HMP-LDH leads to substantial wear and friction reduction of 88% and 25%, respectively, outperforming the PAO-4 base oil. In-depth characterizations of tribofilms formed on wear scars and tracks were conducted using various techniques, including Raman, X-ray photoelectron spectroscopy, scanning transmission electron microscopy - energy dispersive X-ray spectroscopy, and cryo-electron energy loss spectroscopy. It was found that the predominant composition of the tribofilm consists of mixed oxide and phosphate species. Notably, metallic cobalt is observed near the tribofilm-steel interface. Nanoindentation and nanoscratch were performed on the HMP tribofilm. The relationship between the nanostructure and nanomechanical properties and their influence on friction and wear have been revealed.
Layered double hydroxides (LDHs) have demonstrated excellent tribological performance in various studies. However, there remains a fundamental knowledge gap concerning the chemical composition of tribofilms across the cross-section. In this study, we seek to elucidate this question by tribologically testing various binary LDHs. It was found that five out of eight LDHs, denoted as CoAl-, CoFe-, MgAl-, NiFe-, and ZnAl-LDHs could induce hierarchical protective tribofilms due to the formation of rigid oxide layers and in situ carbon-based films. The five tribofilms helped to reduce wear loss and friction by approximately 90% and 20%, respectively. Remarkably, the Co-based tribofilms showed the best antiwear performance because of the thick carbon tribofilm formation and the strong bonding at the metallic/tribo-oxide interfaces. The formation mechanisms of the carbon tribofilm and its characteristic nature were evaluated and revealed by scanning transmission electron microscopy and electron
Carbon-containing tribofilms have attracted significant interest in the lubrication research despite a scarcity of information on their high-temperature performance under severe boundary conditions. In this study, high-temperature lubrication of the carbon tribofilm produced from cyclopropane carboxylic acid (CPCa) and NiAl-layered double hydroxide (LDH) nanoparticles was evaluated. NiAl-LDH nanoparticles significantly enhanced the friction stability and antiwear performance of CPCa by over 90% at 50°C and 100°C, comparable to the benchmark zinc dialkyldithiophosphates (ZDDPs). The highly graphitic amorphous carbon tribofilms and the fine-grain intermediate tribolayer constructed by the thermal decomposition products of NiAl-LDH contributed to such excellent lubrication performance. This study paves a pathway in developing functional anti-wear additives for the durable and high-performance carbon-containing tribofilms at high temperatures.
The Mg–Al layered double hydroxide (LDH) conversion coatings were first synthesized in situ to modify the AZ91D alloy through urea hydrolysis to adjust the pH values (9.4, 10.4, 11.2 and 11.4). The pH 11.2 Mg–Al LDH possessed the best compactness and good crystallinity compared to other in situ LDH coatings and obtained the lowest corrosion current density (icorr) of (2.884 × 10−6 ± 0.345 × 10−6) A·cm−2, which was attributed to the anion-exchange reaction of LDH and the physical barrier against corrosion owing to the twisted penetration pathway of the interlaced LDH sheets. Core–shell structured Zn–Al LDH@ZIF-8 powder modified with stearic acid (SA) was further wrapped with polyvinylidene fluoride (PVDF) to prepare a hydrophobic double-layered coating on the underlying pH 11.2 Mg–Al LDH (SLLZ). The water contact angle (CA) of the SLLZ coating reached 105.6°, and its icorr decreased to (3.524 × 10−7 ± 0.214 × 10−7) A·cm−2 compared with a single pH 11.2 fil