Lingzhong Du

Microstructure, mechanical and tribological characteristics of plasma, detonation gun and HVOF sprayed NiCr/Cr3C2–BaF2.CaF2 coatings

Abstract The microstructure, mechanical properties and tribological behaviour of NiCr/Cr3C2–BaF2.CaF2 composite coatings prepared with atmospheric plasma spraying, detonation gun spraying (DS) and high velocity oxyfuel spraying (HVOF) technologies were systematically investigated. X-ray diffraction and transmission electron microscopy analyses showed that the main phases of the three kinds of NiCr/Cr3C2–BaF2.CaF2 coatings were mainly consistent with the designed feedstock powders. The three kinds of NiCr/Cr3C2–BaF2.CaF2 coatings have similar and high microhardness. The HVOF and DS NiCr/Cr3C2–BaF2.CaF2 coatings have lower porosity and higher adhesive strength in comparison to the plasma sprayed NiCr/Cr3C2–BaF2.CaF2 coating. The friction coefficients of the HVOF and DS coatings were lower than that of the plasma sprayed coating, and the HVOF and DS coatings showed better wear resistance. The lower friction coefficients and wear rates of the three coatings and coupled Si3N4 balls at elevated temperatures were attributed to the formation of a compact BaF2.CaF2 eutectic lubricating layer on the worn surface.

Wear Behavior of a NiCr/AgVO3 Self-Lubricating Composite

NiCr/AgVO3 self-lubricating composite was prepared by powder cold-pressed method with the NiCr alloy as the matrix and 10 wt.% additive of AgVO3 as solid lubricant. The AgVO3 additive powder was synthesized by the precipitation method which exhibits a melting point of 460 °C. Microstructure, phase composition and thermal properties of the AgVO3 powder, as well as the composite of NiCr/AgVO3 were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The friction and wear behavior of the specimens from room temperature (R.T.) to 800 °C was evaluated using a ball-on-disk tribometer and 3D white light interference (WLI). The results showed that the friction coefficient of this material under atmosphere decreases with temperature increasing from R.T. to 800 °C. However, the wear rate firstly increases from R.T. to 200 °C, almost remains stable from 200 °C to 600 °C, and then decreases with further increasing the temperature up to 800 °C. It is also found that the prepared composite materials show a better frictional behavior than NiCr alloy over the whole range of temperatures, which is mainly attributed to solid lubrication of AgVO3 exhibiting a lamella-slip structure at temperatures below 460 °C and forms liquid-film at elevated temperatures above the melting point.