Comparison of 10 μm and 20 nm Al-Al2O3 Metal Matrix Composite Coatings Fabricated by Low-Pressure Cold Gas Dynamic Spraying

Cold-gas dynamic spraying ("cold spraying") was used to deposit aluminum-alumina (Al-Al2O3) metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powders consisted of -45 mu m commercially pure Al that was admixed with either 10 mu m or agglomerated 20 nm Al2O3 in weight fractions of 25, 50, 75, 90, and 95 wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and the volume fractions of reinforcing particles in the coatings, which was then converted to weight fractions. As the weight fraction of the Al2O3 in the coatings increased, the hardness values of the MMC coatings increased. A maximum hardness of 96 +/- A 10 HV0.2 was observed for the MMC coating that contained the agglomerated 20 nm Al2O3 particles, while a maximum hardness of 85 +/- A 24 HV0.2 was observed for the coatings with the 10 mu m Al2O3 particles. The slight increase in hardness of the coating containing the agglomerated 20 nm Al2O3 particles occurred in a coating of Al2O3 content that was lower than that in the coating that contained the 10 mu m reinforcing Al2O3 particles. The increased hardness of the MMC coatings that contained the agglomerated 20 nm Al2O3 particles and at lower reinforcing particle content was attributed to the increased spreading of the nanoagglomerated particles in the coating, which increased load-sharing and reinforcement capability of the particles. These results suggest that the use of nanoagglomerated, reinforcing hard-phase particles in cold-sprayed MMC coatings may be a more efficient alternative to the use of conventional micronsized reinforcing particles.