Gelcasting SiAION Radomes

Hydrolysis‐Induced Aqueous Gelcasting of β‐SiAlON–SiO2 Ceramic Composites: The Effect of AlN Additive

“…If AlN is utilized as one of the starting precursor materials without passivating it against hydrolysis in the processing of SiAlON following an aqueous colloidal processing route, this AlN will be decomposed in the suspension. This decomposition causes coagulation of the suspension in the vessel used for powder deagglomeration purpose, setting of the suspension within the vessel, and the overall change in the targeted chemical composition 26,27 …”

“…These degassed suspensions were then cast into an indigenously designed and fabricated split‐type Teflon‐lined aluminum radome‐shape mold, and inserted aluminum solid mandrel into the suspension. After mandrel insertion, the suspension was allowed to completely gel under ambient conditions in the mold 18–27 . Always, some excess suspension was poured in the mold so as to overflow this excess suspension from top of the mold along with air/gas bubbles (if any leftover after vacuum pumping) while inserting the mandrel into the suspension in the mold.…”

“…To withstand these severe environments while transmitting electromagnetic RADAR signals without any disturbance, the materials used for radome construction should have a low dielectric constant, a low loss tangent, a high flexural strength, high thermal shock resistance, high rain erosion, and particle impact resistance together with a high elastic modulus to keep the thin walls of the radome from buckling 4,5 . Although several materials such as fused silica, 6 Pyroceram 9606, 4 Rayceram 8, 4 barium aluminum silicate, 7 aluminum phosphate (commercially known as Cerablak ™ and manufactured by Applied Thin Films Inc., Evanston, IL), 8,9 SiO 2 –AlN ceramics, 10 SiO 2 –BN ceramics, 11 reaction‐bonded silicon nitride, 12 liquid‐phase‐sintered silicon nitride, 13 silicon nitride nanocomposite, 1 β‐Si 4 Al 2 O 2 N 4 , 4,5 etc., have been investigated for these applications, only the β‐SiAlON–SiO 2 ‐based ceramic composites have received much importance as these latter composites possess the desired flexural strength and dielectric properties and can withstand high temperatures (>1300°C) 3 . Recently, β‐Si 4 Al 2 O 2 N 4 ‐based radomes have been successfully test fired for certain standard missile applications 4,5 .…”

“…Although several materials such as fused silica, 6 Pyroceram 9606, 4 Rayceram 8, 4 barium aluminum silicate, 7 aluminum phosphate (commercially known as Cerablak ™ and manufactured by Applied Thin Films Inc., Evanston, IL), 8,9 SiO 2 –AlN ceramics, 10 SiO 2 –BN ceramics, 11 reaction‐bonded silicon nitride, 12 liquid‐phase‐sintered silicon nitride, 13 silicon nitride nanocomposite, 1 β‐Si 4 Al 2 O 2 N 4 , 4,5 etc., have been investigated for these applications, only the β‐SiAlON–SiO 2 ‐based ceramic composites have received much importance as these latter composites possess the desired flexural strength and dielectric properties and can withstand high temperatures (>1300°C) 3 . Recently, β‐Si 4 Al 2 O 2 N 4 ‐based radomes have been successfully test fired for certain standard missile applications 4,5 . Despite their several useful properties, there is no straight forward fabrication route for dense components of β‐SiAlON–SiO 2 ceramic composites 2 .…”

“…Despite their several useful properties, there is no straight forward fabrication route for dense components of β‐SiAlON–SiO 2 ceramic composites 2 . This is due to the fact that β‐Si 4 Al 2 O 2 N 4 with >9 wt% SiO 2 cannot be prepared following a conventional reaction sintering of the precursor mixture consisting Si 3 N 4 , AlN, and Al 2 O 3 powders, because they require an additional SiO 2 as one of the precursor materials 4,5 . As SiO 2 concentration increases in the precursor mixture, the diffusion paths between reactive Si 4+ and Al 3+ species also increase, which restricts the formation of the desired β‐SiAlON phase during sintering.…”