Effect of nano-Al₂O₃ addition on the microstructure and erosion wear of HVOF sprayed NiCrSiB coatings

Abstract

Development of nanostructured high velocity oxy-fuel (HVOF) coatings with low porosity, high strength and increased wear resistance is still in its infancy. Combining nanoparticles with conventional microscale powders are increasingly being investigated to use with feedstock materials for thermal spray processes. Accordingly, this work investigates the addition of nano-Al 2 O 3 particles on the microstructure and erosion wear of NiCrSiB HVOF coating in a stainless steel (AISI 304) substrate. Particle analysis of the NiCrSiB feedstock was conducted and the maximum allowable addition of Al 2 O 3 nanoparticles have been identified using the 'mass mixture ratio' model considering both the particle size and density. Consequently, two cases are considered and their performance analysed: a maximum allowable case of 1.4 wt%, followed by a 0.17 wt% addition of nano-Al 2 O 3 with NiCrSiB. Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and x-ray Diffraction (XRD) analysis were employed to inform the microstructure, material composition and phase spectrum of the resulting coatings. Subsequently, the nanostructured coating was exposed to both a pull-off adhesion strength test and hot air jet (450 C) hard particle erosion to characterise its performance. It was found that the microhardness of the HVOF NiCrSiB coating improved from 576 HV 0.3 to 748 HV 0.3 with the addition of 1.4 wt% nano-Al 2 O 3 . Furthermore, the nanostructured coating also exhibited high erosion resistance at a 90erodent impact angle. The increase in erosion wear resistance was due to the increase in the hardness as a result of the nano-Al 2 O 3 addition.