Effect of nano-Al2O3 surface treatment on the tribological performance of phenolic composite coating

To overcome the disadvantages generated by the loosened nano-particle agglomerates dispersed in polymer composites, a chemical grafting method was applied to modify nano-Al2O3 by covalently bonding toluene-2, 4-diisocyanate (TDI) onto the particles. The tribological behaviors of the phenolic composite coating filled with nano-Al2O3 or toluene-2, 4-diisocyanate treated Al2O3 (TDI-Al2O3) were studied using a ring-on-block wear tester under dry sliding, and the worn surfaces of the filled phenolic coating and transfer films formed on the surface of the counterpart ring sliding against the composite coating were investigated by SEM and optical microscope (OM), respectively. Owing to the effective improvement of the interfacial adhesion between the nano-Al2O3 and phenolic matrix, compared with the cases of untreated nano-Al2O3, the employment of TDI modified nano-Al2O3 (TDI-Al2O3) provided the phenolic coating with much higher tribological performance enhancement efficiency. Compared with other filled phenolic coating, 3 wt.% TDI-Al2O3 reinforced phenolic coating has the lowest frictional property and the highest wear resistance. Furthermore, we studied effect of sliding speed and applied load on the friction and wear behaviors of the phenolic coating filled with 3 wt.% nano-Al2O3 or TDI-Al2O3. The results showed that the friction coefficient of the composite coating reinforced with different fillers decreased with increasing sliding speed and applied load, and the anti-wear behavior of the coating filled with 3.0 wt.% TDI-Al2O3 was the best under 320 N and at a speed of 3.0 m/s. The investigations of the frictional surfaces showed that low content nano-Al2O3 or TDI-Al2O3 was able to enhance the adhesion of the transfer films of the phenolic coating to the surface of counterpart ring, so they significantly reduced the wear of the phenolic coating. FTIR and TGA analyses indicated that the surface of Al2O3 was coated with TDI through the covalent bond between the hydroxy groups of Al2O3 and –OCN groups of TDI.