| Summary: | We present the Hall–Petch correlation between microhardness values and grain sizes for single, binary (50-50 vol.%), and ternary (33-33-33 vol.%) phase composites of yttria-stabilized zirconia (8Y–ZrO2), spinel (MgAl2O4), and alumina (Al2O3) samples produced by spark plasma sintering. Both the intrinsic grain microhardness values and the Hall–Petch constants were found to be significantly higher for alumina among the single-phase samples, and for the alumina-containing composites for the multiphase specimens. The microhardness values of all binary and ternary composites were observed to follow a rule of mixtures for both sub-micron (250–400 nm) and micrometer (1–1.5 μm) grain sizes. The fracture toughness values for all the specimens were also determined using the Vickers indentation method. Among all studied ceramic composites, single phase Al2O3 and a binary composite of 8Y–ZrO2/Al2O3 attained the highest hardness and toughness values of 20 GPa and 5.8 MPa•m1/2, respectively. The number of slip systems could be the main reason for the improved mechanical performance of the alumina and alumina-containing composites. Particularly, alumina consists of only 17 slip systems, while both zirconia and spinel are more ductile and contain 12 principal and 24 secondary, and 24 slip systems, respectively. These results are of significant interest, since they connect the grain sizes of the materials produced to their corresponding mechanical response.
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