This Small Business Innovation Research Phase I project addresses the need for lower cost ceramic materials, specifically for reaction-bonded silicon carbide (RBSC) products. RBSC is used in a multitude of applications ranging from kiln furniture to body armor inserts to ultra-high purity semiconductor components. Lowering costs would make ceramic materials available for more wide-spread use. Currently, these products are limited in applications due to the high costs associated with expensive raw materials and high-temperature processing requirements. This project addresses these issues though the use of low cost preform materials and an innovative thermal processing technique. In prior work, a new method for producing RBSC was developed, through liquid infiltration of molten silicon by direct microwave heating. This innovative process allows for complete infiltration of porous preforms using microwaves, without the need for a high vacuum environment. However, one of the persistent technical issues is the formation of undesirable silicon veins in the RBSC. This may be caused by in part by a significant exothermic reaction during the infiltration. The veins can detrimentally affect the physical properties of the final RBSC. The anticipated technical results of this work are to identify the origin of silicon vein formation, and to develop methods to mitigate this issue.
The broader impact/commercial potential of this project is to lower the cost of RBSC ceramics, making them more economically viable in current applications, and increasing their use in previously unfeasible applications where RBSC could provide superior performance characteristics. The successful development of low-cost, higher strength, and higher purity RSBC would provide significant benefits to ceramic component manufacturers and end users. Some of the current applications for RBSC include kiln furniture and various burner parts for combustion. Areas targeted for expanded use are: wear resistant components (e.g., slip ring seals), body armor for soldiers, sand blasting nozzles, and diffusion components for the semiconductor industry. The semiconductor industry is of particular interest. As devices continue to get smaller, the purity of diffusion components is becoming a critical issue. The use of this RBSC for high-purity wafer carriers would be advantageous, as preforms in the green state can be heated and purified. Finally, this work will enhance scientific and technological understanding of high temperature exothermic reactions, explore methods to control exothermic rates of reaction, and quantify the energy benefit of microwave processing versus conventional methods.
This Small Business Innovation Research Phase I project provided proof-of-concept demonstration of a lower cost means for producing reaction bonded silicon carbide (RBSC) components which are used in a multitude of applications including mechanical seals, kiln furniture, radiant tubes, body armor inserts for soldiers, and ultra-high purity semiconductor components. Like most advanced ceramic materials, these products are limited in application due to the high costs associated with expensive raw materials and processing needs. For ultrahigh purity applications, a further limitation is the difficulty in procuring acceptable quality silicon carbide (SiC) powders with low enough impurity levels at reasonable cost. RBSC is commercially produced by the molten Si metal infiltration of SiC preforms that contain a small amount of carbon (C), typically only a few percent as a bonding phase. The Si fills the pores and reacts with any C that is present to form more SiC. Ceralink has been investigating the microwave processing (rather than radiant heating) of RBSC via the molten Si metal infiltration of all-carbon preforms (rather than mostly SiC). Microwave heating offers the potential to significantly reduce processing costs by substantially decreasing the processing time. The use of all-C preforms also reduces product costs since C is cheaper than SiC, and C is much easier to machine without the requirement of expensive tooling. In addition, the all-C preforms can be of finer grain size than the SiC preforms, which can result in much higher strength RBSC bodies. Finally, the all-C preforms can be purified to higher levels than SiC preforms, and as such, higher purity RBSC components can be produced for semiconductor applications. In Phase I, Ceralink advanced the understanding of the reaction of carbon particulate preforms with Si metal powders that is the prerequisite for achieving crack-free structurally stable RBSC bodies. The scientific value arises from an improved understanding of the interaction of microwaves with different materials, and a study of high temperature exothermic reactions and how critical microwave setup is for heating conductive materials such as metals. This lower cost process was demonstrated and resulted in RBSC samples with strength values comparable or better than commercially available RBSC products. Processing by microwave heating significantly decreased the time required for Si metal infiltration from several hours to about 30 minutes. Thus, the feasibility of the microwave processed RBSC composites from all-carbon preforms was demonstrated with the rapid fabrication of crack-free RBSC samples with equal or better properties compared to commercial RBSC products. The broader impacts of this technology to society are the reductions in energy use and greenhouse emissions. Further, the ability to produce higher purity RBSC could have benefits to the Nation by advancing the US technology over other countries to produce semiconductor components which increasingly must be of higher purity as device generations continue to decrease in size. Impurities in diffusion components have been described as critical issues impeding the future development of e.g., computer chips. The technology is poised to enable lower cost options for mechanical seals and other wear components. Finally, Ceralink, a women-owned business, has continued to support education of new and aspiring engineers through co-operative education experience directly engaged in innovative materials research.
