Hydrolysis-Induced Aqueous Gelcasting: The Latest Concept for Net-Shape Consolidation of Ceramics—A Review

Abstract

This is a review article covering literature reported on consolidation of ceramic components out of aqueous-based Al₂O₃, MgAl₂O₄ spinel, β-Si₄Al₂O₂N₆, β-SiAlON-SiO₂ composite, and zirconia toughened alumina (ZTA) slurries following hydrolysis induced aqueous gelcasting (henceforth referred to as GCHAS) route. GCHAS is a combination of aqueous gelcasting (GC) and hydrolysis assisted solidification (HAS) routes. The advantages of GCHAS over other net-shape forming techniques in the fabrication of certain important ceramic structures such as thin wall radomes have been reviewed in this article. Interestingly, among the various colloidal-based net shape forming techniques available, GCHAS has been found to be a most promising one for consolidating complex-shaped ceramic structures with significantly high green strength and high product yield. Sintered properties of ceramics consolidated by GCHAS are compared and discussed with those of similar ceramics consolidated by aqueous slip casting (SC), GC, HAS, and a conventional dry-powder pressing (DP) of freeze dried granules in this article.

Keywords:

• Casting
• Consolidation
• Density
• Microstructure
• Sintering
• Strength

ACKNOWLEDGMENTS

The author wishes to thank Dr. G. Sundararajan, Director, ARCI, Hyderabad, and Prof. J. M. F. Ferreira, Ceramic and Glass Engineering Department, CICECO, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal, for their kind encouragements while writing this review.

Notes

†In the sample codes, A-DP, A-GC, A-HAS, and A-GCHAS denote alumina ceramics consolidated by following a conventional dry-powder pressing, aqueous gelcasting, hydrolysis assisted solidification, and hydrolysis-induced aqueous gelcasting route, respectively.

‡CPM stands for conventional premix solution, obtained by the dissolution of 17 wt% methacrylamide (MAM) and methylenebisacrylamide (MBAM) in the ratio of 6:1 ratio in distilled water.

*The setting time for A-GC and A-GCHAS systems is the time corresponding to the crossover point (G′ = G″). The values given in brackets represent the shortest time at which samples could be unmolded.

**RD stands for relative density, assuming a theoretical density of 3980 kg m⁻³ for Al₂O₃.

‡SiAlON stands for β-Si₄Al₂O₂N₆ and G for GCHAS route, H for HAS route. Numbers 0–15 represent the amount of as-purchased AlN (A-AlN) powder added to the suspension to promote suspensions setting process at room temperature according to the reaction, AlN + 3H₂O → Al(OH)₃ + NH₃. T-AlN stands for AlN powder that was surface treated against hydrolysis.

†CPM and NCPM stand for conventional and non-conventional premix solutions, respectively.