By Laurel M. Sheppard
The 105th Annual Meeting and Exposition of the American Ceramic Society, held the last week of April this year, covered a wide range of topics, from biomaterials to photonics and more. There were several sessions on composites that discussed reinforcements, processing and properties, and applications, among others. Hot topics included ceramic armor and geopolymer composites.
An Alternative to Polymer Composites
As part of the symposium on ceramic matrix composites, a special session on geopolymer composites with invited speakers was held. First developed in France several decades ago, geopolymers are being investigated as an alternative to organic matrices (see www.geopolymer.org, www.geopolymere.fr and www.geopolymer.com) for composites. The matrix is based on polymineral resins, which are alumino-silicate binders. Although these binders are processed at temperatures below 150°C (typically 80°C to 120°C, with techniques commonly used for thermoset organic resins), polymerized polymineral resins can resist temperatures up to 1200°C (long-term exposure). Polymineral resins therefore allow the production of ceramic-like materials and high temperature composites by using simple, low temperature processes.
The matrix is based on a poly(sialate) (Si-O-Al-O) structure whose atomic ratio of Si: Al determines the properties and application fields. A low ratio (1, 2, 3) results in a rigid three-dimensional network. A ratio above 15 results in a more polymeric material. A ratio of 1:1 can be used for bricks, ceramics and fire protection applications. A 2:1 ratio is suitable for cements and concretes. Fiberglass composites and tooling for titanium processing are based on a 3:1 ratio. These composites can be used between 200° and 1000°C.
High performance fiber composites are based on a two-dimensional crosslinking network with a ratio between 20:1 and 35:1. The working temperature and curing process is dependent on the type of fiber: for E glass it is room temperature for both; for carbon it is <400°C and room temperature up to 180°C, respectively, for steel it is <750°C and 80 to 180°C, respectively; and for SiC it is 1000°C and 80-1800°C, respectively.
Composites are made at room temperature or thermoset in a simple autoclave. A concrete type material is produced, which after four hours has higher strength and durability than the best currently-used concrete. The advantages of geopolymer composites over organic composites and other materials are: they are easy to make, as they handle easily and do not require high heat; they have a higher heat tolerance than organic composites (carbon reinforced geopolymer composites showed that they will not burn at all, no matter how many times ignition might be attempted); and mechanical properties are similar to those of organic composites. In addition, geopolymers resist all organic solvents (and are only affected by strong hydrochloric acid).
Research at Rutgers University (New Brunswick, NJ) has looked at various fiber compositions including high modulus C, SiC, steel, micofibers, rovings, fabrics, hybrids (glass mat carbon fabric). By combining a matrix hybrid organic with a geopolymer, a high strength and high temperature resistance material is obtained that is nontoxic. The composite consists of a core made from a glass mat carbon fabric impregnated with vinyl ester, which is coated with a skin of reinforced geopolymer. These C/polysilate composites have a tensile strength comparable to higher cost composites. Challenges remain regarding achieving uniform nanometer-sized particle dispersion, extending pot life, reducing shrinkage and increasing strain capacity.
Fiber reinforced geopolymers with alkali-activated alumino-silicate matrices are under development at The Swiss Federal Institute of Technology (ceramics.ethz.ch). The matrix is made from a liquid potassium silicate solution that is mixed with fly ash and an accelerator (NaF). After mixing, the material is cast, cured, annealed and dried. The NaF dissolution is a function of the annealing temperature.
The accelerator helps to form an amorphous binder phase. A low curing temperature and long reaction time result in a dense, amorphous microstructure. Fine pores disappear during curing. Alumina short fibers or platelets are added to improve mechanical properties. The Weibull modulus increases from 7 to 25. The composites also have excellent thermal shock resistance.
Other research at the University of Illinois (Urbana-Champaign) has investigated the microstructure of geopolymer composites reinforced with fibers and with metal particles added to improve ductility. The microstructure appears porous but is still stable up to 1000°C. These composites can be fabricated at ambient temperature and pressures. Potential applications include waste encapsulating materials and hermetic seals.
Applications for Geopolymer Composites
Geopolymer composites have been investigated and developed for a variety of applications. A five-year program funded by the Federal Aviation Association at Rutgers University, in conjunction with France's Geopolymer Institute, looked at developing low-cost, environmentally-friendly, fire resistant matrix materials for use in aircraft composites and cabin interior applications. The goal of the program was to eliminate cabin fire as cause of death in aircraft accidents. Unlike conventional polymer composites, carbon-fiber reinforced geopolymer composites did not ignite, burn, or release any smoke even after extended heat flux. Therefore, they are suitable as aircraft cabin materials for cargo liners, ceiling, floor panels, partitions and sidewalls, stowage bins, and wire insulation.
In France, a fire-resistant geopolymer-encased electronic flight recorder has been patented by S.F.I.M. and jets are also being equipped with a highly advanced fireproof air filter from Sofiltra-Camfil. For Northtrop Aviation, a geopolymer composite tooling prototype (self-heated carbon/SiC/geopolymer composite) was used in the fabrication of a carbon composite designed for a new US Airforce bomber. Formula 1 racing cars have also been built with carbon/geopolymer composites as thermal shields in the exhaust system, replacing titanium.
Other applications are used in the processing of several materials. During float glass manufacture, composite rings are installed on lehr rollers to withstand operating temperatures of 750°C. Hot glass handling applications during the manufacture of glass bottles include take-out tongs, transfer wheels and pushers on the stacker bar.
Joint research between Catawba Resources Inc. (Stow, OH) and the University of Illinois at Urbana-Champaign is also developing geopolymer composites for molds used in metal processing. The production of ferro-silicon and ferro-manganese products for the specialty steel market includes the casting of final molten metal product at a temperature of 1425°C into large nickel molds. Geopolymer composite molds are much cheaper than nickel molds and meet most of the following requirements for mold materials: high thermal stability (1528°C), a strength of 100 MPa, a low coefficient of thermal expansion, high heat conductivity, high oxidation temperature, self releasing molds, high cycle life (change molds once a month), and stability in an alkaline environment. In addition, no preheating of molds is required.
Out of seven mold compositions tested, a potassium silicate system containing chromium, tantalum and C fiber worked the best (60, 15, 15, 5%, respectively). The cermet metals are used as additives. In preliminary testing, all molds could be used twice; those containing cermet metals could be cast four times. Despite some microcracking the molds remained intact and the C fibers remained stable in the geopolymer matrix. The molds provided rapid heat dispersion and metal plugs were readily ejected three minutes after pouring. However, the molds need to survive 100 cycles to be commercially viable.
Improving Ceramic Armor Performance
Conventional ceramic armor is based on alumina or silicon carbide materials. In an effort to improve ballistic performance, researchers are investigating reinforcing these materials with whiskers or particles. Another approach involves combining multilayers of several different materials to form a laminate composite.
Joint research between Bechtel BWXT (Idaho, ID) and Superior Graphite Co. (Chicago, Illinois) is adding SiC whiskers to their low-cost pressureless sintered SiC material in an effort to improve fracture toughness. The starting powder is a submicron alpha SiC that is mixed with an organic binder and a boron carbide sintering aid. After spray drying, the powder is pressed into thick tiles (102 mm2, 19 mm or 27.9 mm thick) and sintered at <2200°C.
Whiskers, 1.5 micron by 18 micron long, are heat treated to eliminate stress and improve toughness by modifying their shapes at 700°C/2 h and 1700°C/2 h. This heat treatment produces rounded edges. Prototype composite tiles show porosity and grain growth; fabrication and sintering methods are being optimized to improve density and other properties. Ceramic armor costs are expected to be reduced to $140/kg.
Tailoring microstructures to produce in situ composites is another strategy to improving properties. At Ceramic Protection Corp. (Calgary, Canada), a manufacturer of ceramic armor, composites based on coarse hard particles bonded by a hard microcrystalline/glassy matrix are under development. The hard particles are SiC and the matrix is based on a SiC-SiO2-Al2O3-Si3N4-sialon combination. A multi-hit ballistic performance is achieved because the particles change the direction of crack propagation, bridging of microcracks occurs, and a high friction is produced between damaged projectile and hard grains.
Other work funded by the U. S. Army Research Laboratory (Baltimore, MD) at Ceradyne Inc. (Costa Mesa, Cal.) is developing multilayered functionally gradient boron carbide/silicon carbide laminates. Green tapes of 500-micron thickness are made by a rolling process and then hot pressed at 2150°C. Three layer and nine layer systems are being tested. The three-layer system shows similar ballistic performance to conventional materials and it is expected the nine-layer system will have better performance.
Israel's Ben-Gurion University is also investigating adding Fe to reaction bonded Si/SiC/B4C composites in an effort to complete the reaction between C and free Si because unreacted Si reduces hardness. The Fe addition also reduces the reaction bonding temperature by decreasing the melting temperature of Si from 1340 to 1430°C. However, Fe also reduces the bending strength.
No matter what the type of composite is or its application, economics is still an issue for accelerating commercialization of ceramic matrix composites. Both raw materials and processes must be improved so that an affordable product can be made. In only a few cases will the performance benefits outweigh the costs.
About the Author
Laurel M. Sheppard is President of Lash Publications International (www.lashpublications.com) and Contributing Editor of Ceramic Industry (www.ceramicindustry.com). She has a B.S. in ceramic engineering and has written numerous articles on ceramic technology and manufacturing, as well as a market report on ceramic matrix composites for Business Communications Co. (www.bccresearch.com)