Surface and mechanical properties of transparent polycrystalline YAG fabricated by SPS

• Ultrasonication as effective, un-polluting dispersion route for YAG powders.
• Annealing in the 700–900 °C range to increase the transparency.
• Oxygen vacancies more crucial on the transmittance than C contamination.
• SPS reliable method for transparent and ultra-fine polycrystalline YAG.
• Very high hardness (16.5–17 GPa) for the fully dense, fine materials.

YAG powder was synthesised by reverse-strike co-precipitation, calcined at 1000 °C and dispersed by either ball-milling with α-alumina (BMA) or zirconia (BMz) spheres or by ultrasonication (US). All the dispersed powders were consolidated by SPS to nearly theoretical density, but only the US powder gave rise to a transparent material (transmittance of about 60% at 600 nm, 1 mm thickness), characterised by an ultra-fine microstructure (average size of 330 nm). In the BM materials, Raman spectroscopy allowed to evidence some phonon vibrational shifts due to secondary phases deriving from pollution by the milling media, not detectable by XRD because present in small amounts. The transmittance of the as-sintered US sample was further increased by annealing in air at 900 °C; this was assigned to the restoration of some oxygen vacancies created in the reducing environment of the SPS chamber, as evidenced by XPS (X-ray photoelectron spectroscopy). Finally, US samples sintered in the 1250–1400 °C were submitted to a basic mechanical characterisation, showing a very good hardness, in spite of a moderate fracture toughness, especially for the fully dense and fine-grained materials sintered at 1300–1350 °C.

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