Silicon nitride ceramics are widely used in structural applications because they have low density yet are very tough. Their damage tolerance is based upon an interlocking microstructure of elongated grains, which is developed by heating the appropriate powder compacts to temperatures above 1,700°C for extended periods, at which point densification, phase transformation and grain growth take place simultaneously. Consequently, it is extremely difficult to clarify the mechanism behind grain growth.
What is known is that the process takes place slowly over a period of several hours, even at high temperatures. Now, Zhijan Shen and colleagues at the Department of Inorganic Chemistry at Stockholm University, using a spark plasma sintering (SPS) technique, have developed a method that allows greater control over the final structure of the silicon nitride ceramic.
Liquid Phase Sintering Mechanisms
Previously, during the liquid phase of sintering, the formation of the final phases from the mixture of oxides and nitrides that compromise the starting material was driven by chemical forces. Small crystals that are initially present in the starting material dissolve in the liquid and reform into larger more thermodynamically stable larger crystals. The two phases are generally close to thermodynamic equilibrium, giving the process little momentum, which is why the process takes hours.
Spark Plasma Sintering Mechanisms
In Shen’s process the sintering temperature is reached in less than five minutes, causing the liquid phase to be grossly out of equilibrium with grain forming matrix, creating a strong chemical driving force for the two to equilibrate. The resulting transport of materials gives long needle-shaped grains in substantially shorter time.
Although the work was carried out in a research-scale SPS furnace, the dynamic ripening principle seems applicable to all nitrogen ceramics - further use of SPS will allow more detailed study of the grain growth mechanism. As a result, the principle behind the development of tough needle-shaped microstructures will be better understood, giving rise to new high-toughness ceramics.