Demonstration of Minimally Machined Honeycomb Silicon Carbide Mirrors
- Sunday, 01 July 2012
This manufacturing process eliminates machining and steps for mirrors and optomechanical structures.
Honeycomb silicon carbide composite mirrors are made from a carbon fiber preform that is molded into a honeycomb shape using a rigid mold. The carbon fiber honeycomb is densified by using polymer infiltration pyrolysis, or through a reaction with liquid silicon. A chemical vapor deposit, or chemical vapor composite (CVC), process is used to deposit a polishable silicon or silicon carbide cladding on the honeycomb structure. Alternatively, the cladding may be replaced by a freestanding, replicated CVC SiC facesheet that is bonded to the honeycomb. The resulting carbon fiber-reinforced silicon carbide honeycomb structure is a ceramic matrix composite material with high stiffness and mechanical strength, high thermal conductivity, and low CTE (coefficient of thermal expansion). This innovation enables rapid, inexpensive manufacturing.The web thickness of the new material is less than 1 mm, and core geometries (pocket depth, pocket size) are easily tailored. These parameters are based on precursor carboncarbon honeycomb material made and patented by Ultracor. It is estimated at the time of this reporting that the HoneySiC™ will have a net production cost on the order of $38,000/m2. This includes an Ultracor raw material cost of about $97,000/m2, and a Trex silicon carbide deposition cost of $27,000/m2. Even at double this price, HoneySiC would beat NASA’s goal of $100,000/m2. Cost savings are estimated to be 40 to 100 times that of current mirror technologies.
The organic, rich prepreg material has a density of 56 kg/m3. A charred carbon-carbon panel (volatile organics burnt off) has a density of 270 kg/m33, which is about half that of beryllium and about one-third the density of bulk silicon carbide. It is also estimated that larger mirrors could be produced in a matter of weeks.
Each cell is completely uniform, maintaining the shape of the inserted mandrel. Furthermore, the layup creates pressure that insures node bond strength. Each node is a composite laminate using only the inherent resin system to form the bond. This contrasts starkly with the other known method of producing composite honeycomb, in which individual corrugations are formed, cured, and then bonded together in a secondary process.
By varying the size of the mandrels within the layup, varying degrees of density can be achieved. Typical sizes are 3/8 and 3/16 in. (≈10 and 5 mm). Cell sizes up to 1 in. (≈25 mm) have been manufactured. Similarly, the shape of the core can be altered for a flexible honeycomb structure.