Use of Metal in Refractory

It is generally acknowledged in the abrasives industry that abrasive grains, such as brown fused alumina and silicon carbide, can be used as refractory materials. Recently, metal compound refractory have become a hot topic of research. Metals, such as Si, Al and Fe have been applied to refractory for Si-Si3N4-Conrundum ceramic cup, Al-Carbon-Corundum slide and Fe-AlN-MgO- Si3N4 compound, etc., and some of them have been successfully used in industrial production. It is noted that Si is nonmetal, but is still classified into metal materials due to its ductility and plasticity as metal.

According to research, once metal is combined as one element into refractory, it changes into compounds reinforcing the matrix, and promots sintering and antioxidation with the plastic forming property.

Usage of metal in refractory:

Plastic forming: metal lattices can flow under strain and is characterized with plastic properties. When it is applied into refractory, it can exert the plastic forming by contrast to the hard inorganic material particles, resulting in a higher density of the green-pressing.

Promoting sintering: two factors are responsible for the sintering promotion of refractory by metal. One is that the density of green-pressing is increased by plastic forming of metal, distance between particles is shortened and energy required for mass transfer is reduced. The other is that the melting point of metal is lower than that of refractory, thus the liquid phase of metal is easy to be generated. Capillary force and viscous flow by the liquid phase accelerate the atomic mobility and systole of the green-pressing, thus promoting the process of sintering. 

Toughness improvement: metal improves the toughness of compounds through crack-bridging, crack deflection and crack barrier etc., among which crack-bridging is the most effective: when cracks spread to the interface between metal materials and matrix, the cracks prolong the stress of metal particles, resulting in a bridging stress on the surface of cracks. In this way, the cracks are limited to further spread and intensity factor of the cracks is decreased. Besides, plastic deformation occurs as cracks spread, consuming the energy of cracks and improving toughness.

Antioxidation: In spite of the excellent thermal shock resistance and slag resistance of carbon compound refractory, once oxidation occurs all the advantages of carbon refractory would lose. To solve this problem, adequate additives are added to improve antioxidation. Metals tend to react with CO and O2 due to their active properties and are thus widely used as antioxidant such as Al, Si and Mg.

Reaction in-situ to generate non-oxides: during the sintering process, metal reacts with the raw materials or gases around to generate nonmetal reinforcement phase, improving the matrix properties at room temperature and high temperature as well as thermal shock resistance etc.