Processing YAG/α‐Al 2 O 3 composites via reactive sintering Y 2 O 3 /Al 2 O 3 NP mixtures. A superior alternative to bottom up processing using atomically mixed YAlO x NPs

This effort contrasts “bottom‐up” processing of YAG /α‐Al 2 O 3 composites where both elements (as 40‐50 nm APS s nanopowders) are present at close to atomic mixing with reactive sintering where ball‐milled mixtures of the individual nanopowders (40‐50 nm APS s) give uniform elemental mixing at length scales closer to 100‐800 nm with correspondingly much longer diffusion distances. In contrast to expectations, densification with control of final grain sizes is best effected using reactive sintering. Thus, reactive sintering to densities ≥95% occurs at only 1500°C with final grain sizes of ≈1000 nm for all samples. In contrast “bottom up” processing to ≥95% densities is only achieved at 1600°C, and with final grain sizes of 1700 nm. The reason for this unexpected behavior is that YAG phase forms early in the bottom up approach greatly inhibiting diffusion promoted densification. In contrast, in reactive sintering, YAG is prevented from forming because of the longer diffusion distances such that densification occurs prior to full conversion of the Y 2 O 3 component to YAG . The found hardness values are statistically superior to literature values for composites near the known eutectic composition. In an accompanying paper, the addition of a third component reverses this behavior.