The silicon nitride reaction-bonding process

Abstract: The so called Reaction-Bonded Silicon Nitride (RBSN) materials are usually produced through the chemical reaction of silicon powder compacts in a nitriding atmosphere. This nitriding reaction is strongly exothermic and much heat is generated, which may cause to overheating, even melting of the powder compacts so to hinder the further reaction due to the reduction of specific area of the powder compacts, especially when the nitridation involves a rather large mass of silicon. The commercial process of reaction-bonded silicon nitride is usually slow and energy consuming. In order to optimise the production process and to achieve better properties of the materials obtained, mechanism of the enhanced nitridation needs to be further understood. In this thesis, a brief overview is presented of the current state of knowledge concerning the so called Reaction-Bonded Silicon Nitride (RBSN), including relevant crystallography, thermodynamics and kinetics on gas-solid reactions. The work concerns the reaction of silicon powder compacts with a nitrogen atmosphere. The kinetics and mechanisms of the nitridation process of silicon powder compacts have been studied by means of experiment and modelling. The majority of experiments have been carried out on pilot scale batches of silicon powder compacts in nitrogen environment. The XRD, HTXRD (high temperature X-ray diffraction) and SEM have been used to study the kinetics and mechanisms of the nitridation process. The experimental work has confirmed the existence of two reaction rate maxima at two different temperatures and has revealed the effects of selected process variables on these reactions, and explanation for the second reaction regions has been given in terms of possible reaction mechanism. The effect of the auxiliary gases (hydrogen, argon and helium, which were added to the nitriding atmosphere in order to minimise the overheating of the powder compacts) on the nitridation process has been discussed, relating with the sizes of the powder compact and the amount of silicon mass in the nitridation. Based on the understanding of the nitridation process, an advanced computer program that can monitor the gas pressure and temperature and can control the rate of reaction by tailoring both the supply of nitrogen gas and heating rate according to the instantaneous nitridation rate has developed. The nitridation has been investigated by high temperature X-ray diffraction and in situ information of the reaction kinetics and mechanism has been obtained. The insights obtained have formed the foundation for the creation of two theoretical models, one being a quantitative description of the solid-gas reaction as a complex combination of reaction steps, concerning the multiple reaction steps and many factors such as the changes of the specific area of the particle, the corporation of reaction and sintering and the volumetric change due to the reaction, etc.; and the other predicting the development of internal stresses arising when a reaction product forms on particle surface in a solid/gas reaction.