Zhongwei Zhang

To enhance the interfacial properties of carbon fiber (CF) reinforced epoxy composites without damaging the CF strength. The polydopamine (PDA) was used as the precursor layer, and polyethyleneimine (PEI) and nano-silica (nano-SiO₂) were deposited on the surface of CF by layer-by-layer electrostatic self-assembly. The layered strengthening materials (CF-Ln) were prepared and the microstructure of the CF was systematically characterized. The results showed that when the number of self-assembled layers is 5 (CF-L5), nano-SiO₂ covers the surface of CF uniformly, and the tensile strength of CF-L5 monofilament reaches the maximum value, which is 12.1% higher than that of untreated CF (U-CF). Meanwhile, the interface properties between CF-L5 and epoxy matrix have been significantly improved. The interfacial shear strength (IFSS) between CF-L5 and epoxy matrix is 99.4% higher than that of U-CF. The improvement of the interfacial properties between CF-L5 and epoxy matrix is due to the introduction of alarge number of active groups (-NH₂, -OH) in the PEI/SiO₂ multilayer film deposited on the surface of CF, which improves the wettability between the CF and epoxy matrix. Also, the nano-SiO₂ increases the roughness of the CF surface and improves the mechanical engagement between the CF and epoxy resin. Graphical abstract

The TiB₂ reinforced Inconel 718 alloy composite coatings were prepared on carbon steel by laser cladding. The energy input (the ratio of laser power and scanning speed, E) was introduced to investigate the effects of process parameters on geometrical characteristics, microstructure and tribological properties of composite coatings. The results show that the dilution rate decreases and the contact angle increases, respectively, as E decreases from 200.00 J/mm to 80.00 J/mm, which causes the decrease of the boding strength from 1.91 GPa to 0.39 GPa. The composite coatings contain the phases of γ-(Ni, Fe), Laves, TiB₂, TiB, TiC, Ni₃B, CrB₄ and Cr₂₃C₆. As E decreases from 200.00 J/mm to 133.33 J/mm, the primary dendrite arm spacing (PDAS) and the Laves content of the composite coating decrease for the increase of cooling rate, leading to the increase of microhardness, the improvement of tribological properties and the change of wear mechanism from the severe multi-plastic deformation wear, adhesive wear and oxidative wear to the slight abrasive wear. However, as E decreases from 133.33 J/mm to 80.00 J/mm, for the weak Marangoni convection, TiB₂ particles are prone to aggregate in the top of the composite coating which causes the increase of the PDAS and the Laves content, leading to the decrease of the microhardness and the degradation of tribological properties, and the wear mechanism change to the multi-plastic deformation wear and abrasive wear gradually.

In order to improve the mechanical and tribological properties of epoxy composites, the hyperbranched polyesters with terminal carboxyl (HBP) were used for covalent functionalization of multi-walled carbon nanotubes (MWCNTs). The dispersibility and wettability of functionalized MWCNTs (HBP-NH₂-MWCNTs) in epoxy resin (EP) were analyzed. The HBP-NH₂-MWCNTs reinforced EP composites (HBP-NH₂-MWCNTs/EP) were prepared. The microhardness and fracture toughness of HBP-NH₂-MWCNTs/EP were tested, and the tribological behavior and mechanism under dry friction conditions were investigated. The results show that the hyperbranched polyesters coated on the surface of MWCNTs forms a physical barrier layer, blocking the van der Waals force between MWCNTs, effectively reducing the agglomeration phenomenon and significantly improving its dispersibility and wettability. Meanwhile, The flexible hyperbranched polyester on the surface of MWCNTs alleviates the stress concentration of the interface and improves the load transfer efficiency, which greatly improves the microhardness and fracture toughness. Under dry friction conditions, the friction coefficient and wear loss of HBP-NH₂-MWCNTs/EP composites showed a trend of decreasing first and then increasing as the content of HBP-NH₂-MWCNTs increased from 0.3 phr to 1.2 phr. When the content of HBP-NH₂-MWCNT is 0.6 phr, the composites has the lowest friction coefficient and the wear loss, and the wear mechanism is mainly characterized by slight fatigue wear.

In order to improve the interfacial properties of graphene oxide (GO) and epoxy resin (EP), hyperbranched polyesters with terminal carboxyl (HBP) non-covalently functionalized graphene oxide (HBP-GO) was achieved by strong π-π coupling between hyperbranched polyesters and GO nanosheets. The effects of non-covalent functionalization of GO on the dispersibility, wettability and interfacial properties were analyzed. The mechanical properties and enhancement mechanism of HBP-GO/EP composites were investigated. The results show that the hyperbranched polyesters is embedded in the GO layer due to its highly branched structure, which forms the steric hindrance effect between the GO nanosheets, effectively prevents the agglomeration of GO nanosheets, and significantly improved the dispersibility of GO. Simultaneously, the contact angle of HBP-GO with EP is reduced, the surface energy, interfacial energy and adhesion work are increased, then the wetting property of HBP-GO is significantly improved. The main toughening mechanism of HBP-GO is microcrack deflection induced by HBP-GO and plastic deformation of the EP matrix. In the microcrack propagation zones, HBP-GO may produce the pinning effect near the microcrack tips and change their stress state, resulting in microcrack deflection and bifurcation. So, the microcrack propagation path is more tortuous, which will consume much more fracture energy. Therefore, the mechanical properties of the HBP-GO/EP composites are greatly improved.