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 alpha-Al2O3 ceramics with submicron grain sizes. The alpha-Al2O3 (80 wt%) and gamma-Al2O3 (20 wt%) powders are firstly mixed and then cold sintered at 300 degrees C to produce a green bulk with a relatively high density of -86.9 %, and then later a second heat treatment (800-1350 degrees C) is applied to finally fabricate (-98 % dense) alpha-Al2O3 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 gamma-Al2O3 to alpha-Al2O3. 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 degrees C. Therefore, it is feasible to utilize TA-CSP to prepare alpha-Al2O3 ceramics with small grain sizes at low sintering temperatures.