Near‐Net Shape β‐Si₄Al₂O₂N₆ Parts by Hydrolysis Induced Aqueous Gelcasting Process

Abstract

In this paper, a new net‐shaping process, an hydrolysis‐induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of β‐Si4Al2O2N6 ceramics from aqueous slurries containing 48–50 vol%α‐Si3N4, α‐Al2O3, AlN, and Y2O3 powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect‐free thin wall β‐Si4Al2O2N6 crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered β‐Si4Al2O2N6 ceramics exhibited comparable properties with those reported for similar materials in the literature.

Gelcasting of Magnesium Aluminate Spinel Powder

“…The RB flask was fitted with an equalization funnel and valve to pass dry‐nitrogen gas, and was placed in a thermostatic oil bath. In a separate experiment, 2 g of Al (H 2 PO 4 ) 3 was dissolved in 5 mL of hot H 3 PO 4 27 . This solution was then mixed with 50 mL ethanol and added drop by drop to the above alcohol‐based M‐MAS suspension with the help of an equalization funnel.…”

“…Owing to these reasons, it is rather difficult to obtain stable and concentrated aqueous suspensions, which are required for near‐net‐shape forming of ceramics by colloidal processing techniques. Near‐net shaping is of great importance because expensive postsintering machining operations can be minimized or even eliminated, reducing the production costs considerably 24–31 . Besides these advantages, aqueous processing offers several environmental and economic benefits apart from the ability to manipulate interparticle forces in aqueous suspensions.…”

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

“…Interestingly, all consolidates exhibit linear shrinkage values upon drying <3%, which was considered the safety range for a successful fabrication of complex-shaped ceramic components with near-net shape by gelcasting. 15,16,24 This obviously requires the use of suspensions with a fairly high solids content.…”

“…At least, three density measurements were averaged for each sample (±0.01 error). 16,17 XRD patterns were recorded on a Rigaku (Tokyo, Japan) using diffracted beam mono-chromated Cu K␣ (0.15406 nm) radiation source. 18 Crystalline phases were identified by comparison with PDF-4 reference data from International Centre for Diffraction Data (ICDD).…”

“…The thermal/moisture gradients generated during drying lead to appearance of cracks in the consolidated parts. [13][14][15] This paper reports on a new near-net shaping technique, "hydrolysis-induced aqueous gelcasting" (GCHAS), 16 which is a combination of the HAS and gelcasting (GC) processes aiming at addressing the problems associated to the drying step of consolidates formed by gelcasting. The synergetic effect of these two processes confers exceptionally high strength to the as consolidated parts and facilitates uniform drying of consolidates, as water is partially consumed upon hydrolysis of AlN.…”