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Abstract: A low-cost, low-pressure cold spray unit was used to fabricate tungsten carbide (WC)-based metal matrix composite (MMC) coatings. A sintered and crushed WC-based powder was mixed with nickel powder, forming various compositions of mechanical blends. Scanning electron microscopy (SEM), coupled with image analysis, Vickers micro-hardness, and X-ray diffraction (XRD) was used to characterize the coatings. Image analysis was used to determine the WC content in the coating and the mean free path between the reinforcing particles. The WC content in the coating increased as the WC content in the powder blend increased. Each coating fabricated by the various mechanical blend compositions had a non-homogeneous distribution of the hard WC reinforcing particles embedded in the nickel matrix. However, as the WC content in the powder increased, the coating porosity decreased and the WC deposition efficiency increased. An inversely proportional relationship between hardness and mean free path between the reinforcing WC particles was observed, where a decrease in mean free path led to an increase in hardness. The hardness of each MMC coating was predicted using the rule of mixtures (ROM) method and compared to the experimental hardness values. However, the ROM method did not completely describe the strengthening mechanism of the MMC coatings because densification, plastic deformation, and adiabatic shear instability played a role, especially for the coatings with a lower WC content. [Copyright &y& Elsevier]

Abstract: Cold gas dynamic spraying at low pressure (1MPa gage or 150psig) was used to fabricate Al–Al2O3 metal matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained Al powder admixed with −10μm Al2O3 in fractions up to 90wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and volume fraction of reinforcing particles in the coatings. The coatings were then friction-stir processed (FSP) at tool rotation speeds of 894 or 1723RPM using a flat cylindrical tool. The Al2O3 content and hardness of the final MMC coatings increased with increasing fractions of Al2O3 particles in the feedstock powder, resulting in a maximum Al2O3 content of 48wt.% and a hardness of 85HV of the as-sprayed coating when 90wt.% Al2O3 was used in the feed powder blend. After FSP, the hardness of the MMC increased to a maximum of 137HV. The as-sprayed coatings contained Al2O3 particles that were segregated between the Al particles, and FSP was effective in dispersing these Al2O3 particles and decreasing their mean free path. It was suggested that this re-distribution and Al2O3 particle size refinement during FSP improved the hardness of the MMC coatings. [Copyright &y& Elsevier]

Abstract: Tungsten carbide (WC)-based metal matrix composite (MMC) coatings fabricated using a low-cost, low-pressure cold spray unit were subjected to ASTM standard G65 dry abrasion wear testing. A linear relationship was observed between the wear rate and hardness of the various MMC coatings. Rule of mixtures (ROM) and a modified wear theory was used to explain the wear rate and material removal from the various WC-based MMC coatings. In general, the MMC coatings had wear rates closer to the values predicted by the modified wear theory. Although the MMC coatings had low wear rates close to values predicted by the equal wear theory, the cross-sectional images of the worn coatings suggested that material removal may have occurred by mechanisms governed by the equal pressure theory, where portions of, or even, the entire reinforcing particle were removed with the nickel matrix. The mean free path between the WC-based reinforcing particles was used to explain the improvements in wear rate as a function of increasing WC content in the coating. The coatings with the highest WC content and lower mean free path between the reinforcing particles had the lowest wear rates that were comparable to high-velocity oxy-fuel (HVOF)-sprayed and high-pressure cold-sprayed WC-based coatings. [Copyright &y& Elsevier]