viernes, 1 de mayo de 2009

Sintering of new SiC-Phosphate composite materials

The chemistry of different kind of materials bonded by phosphate binders was reviewed and classified by Kingery (1) and Cassidy (2). The good adhesive properties, the high refractoriness and thermal resistance, the superior mechanical resistance and the ability of phosphate binders to hardening by polycondensation, gives an opportunity for their industrial application as special kinds of glues, refractories, cements, ceramics and metal coatings production (3). Usually, these binding materials are phosphate solutions, namely hydrophosphates, yielded by dissolving oxides or hydroxides in water and neutralization with phosphoric acid. Such binders are saturated molecular solutions. Through the hardening process the binder turns to a disperse system due to the hard phase separation. The physical and the mechanical properties of aluminum phosphate binders are improved by B2O3 modification, leading to formation of condensed aluminum phosphates, at lower temperatures than in the case of pure aluminum phosphate binders, or to BPO4 formation (4,5). For example, the strength of porcelain is known to considerably increase by addition of BPO4 (6). The insertion of inert additives allows varying the mechanical, chemical, thermal and electrical properties in a wide range. Various kinds of materials based on aluminum phosphate binders with SiC additions were developed for refractory production (7,8). In spite of this, the use of phosphate binders in abrasive grinding tools hasn’t been researched yet. Hence, we are focusing our work on the replacing of conventional binders by boron phosphate matrices in such articles, which most important characteristics are: i) Self-sharpening – the ability of the grinding tool to loose by bonding strength lack, the abrasive grains that reached a critical radius of roundness of their cutting edge, and to replace them by new sharp grains at the surface; ii) Structure – the abrasive material, binder and pores amount and volume; iii) Hardness – the capacity of the binder to hold the abrasive grains (9).
The use of colloidal solutions of B2O3 into H3PO4 are a very promising alternative route for binder preparation for SiC abrasive tools, as relatively low processing temperatures are used and final thermal and mechanical properties are kept. The presence of BPO4 in the phosphate matrix compositions lowers the temperature needed for amorphous phase formation. The amount of the amorphous phase increases the final mechanical strength of the SiC-phosphate composite materials. Also, by increasing the sintering temperature and the heating step time, the mechanical strength rises as the time and energy for the composite consolidation and amorphous phase formation is guaranteed.

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