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Metallic foams are among the most promising class of materials due to their unique mechanical properties combining low mass with high stiffness, excellent energy absorption, and vibroacoustic damping. Consequently, noise control using methodically engineered metallic foams has received increased attention from both industrial and scientific community. Accordingly, this paper aims to present the mechanism of sound absorption along with the experimental and theoretical procedure that can be used to classify metallic foams. Additionally, the influence of design parameters on the resulting sound absorption coefficient of closed and opencell metallic foams are explored. While aluminium foams used to dominate the literature when it comes to acoustics, recent studies have reported Nickel-Inconel superalloy and Copper foams as having superior sound absorption coefficients.

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 90°erodent 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. IntroductionSolid particle erosion wear plays an important role in the material degradation process of engineering components [1-3], including turbines, thermal power plants, pipelines, hydropower machinery and combustion systems [4,5]. According to Martinella [6], one of the major industries affected by solid particle erosion are coal based electric power plants. This is caused as a result of the fly ash interaction with the wall surfaces facilitated by the high-temperature flue gases. The prolonged effect of this causes the boiler components to fail and currently account for ∼25% of the down time [7]. The subsequent cost effect of erosion wear damage in such cases is often as high as 54% of the overall maintenance cost [8,9]. Consequently, coatings that can improve the surface resistance to solid particle erosion wear along with high-temperature oxidation protection systems are increasingly being sought.Among the numerous solution available to prevent or control the fly ash erosion, surface coatings [10] are the most widely adopted technique [11][12][13]. When it comes to coating methodology, several techniques are available including vapour deposition [14], electrochemical, sol-gel [14] and thermal spraying [14]. Among these different coating techniques, thermal spray techniques [15,16] have received increased attention largely due to their capacity to accommodate a range of ...