Thermal-assisted cold sintering study of Al2O3 ceramics: Enabled with a soluble γ-Al2O3 intermediate phase

Thermal-assisted cold sintering process (TA-CSP) has been applied to fabricate high dense α-Al 2 O 3 ceramics with submicron grain sizes. The α-Al 2 O 3 (80 wt%) and γ-Al 2 O 3 (20 wt%) powders are firstly mixed and then cold sintered at 300 o C to produce a green bulk with a relatively high density of ~86.9%, and then later a second heat treatment (800–1350 o C) is applied to finally fabricate (~98% dense) α-Al 2 O 3 ceramics with grain sizes of 720 nm. A microstructural analysis with XRD and TEM suggests that the TA-CSP samples not only complete the final densification but also drive a phase transition of γ-Al 2 O 3 to α-Al 2 O 3 . To put into perspective the Hardness and Young's modulus of TA-CSP samples reach ~14 GPa and ~335 GPa, respectively, which is comparable to conventional sintered samples processed at higher temperatures of 1500–1700 °C. Therefore, it is feasible to utilize TA-CSP to prepare α-Al 2 O 3 ceramics with small grain sizes at low sintering temperatures. • Thermal-assisted cold sintering process (TA-CSP) is proposed to fabricate high dense α-Al 2 O 3 ceramics. • A phase transition from γ-Al 2 O 3 to α-Al 2 O 3 promotes the densification process. • TA-CSP enables highly dense (~98%) α-Al 2 O 3 ceramics with submicron grain sizes. • Hardness of α-Al 2 O 3 ceramics reaches ~14 GPa via TA-CSP at 1350 o C.