Pressureless sintering of plasma-reacted nanosemicrystalline silicon nitride ceramics with doped sintering additives

For the past few years, synthesis and consolidation of ceramics using nanoparticles have been a focus of much research. The obvious advantage is their low process temperatures (<200°C). In addition, the process results in super-plastic deformation, and drastically enhances the mechanical properties of the ceramics. Recently, a plasma-assisted chemical reaction was adopted to produce nanosize Si₃N₄ powders that have the characteristics of sintering additive elements on individual particles. Homogeneous size distribution of the additives can eliminate inhomogeneous shrinkage of the sintered body during the mixing process.; In this work, the behavior of plasma-reacted nano-size Si₃N ₄ powders intrinsically doped with sintering additives was explored. These particles were doped with Y₂O₃ and Al₂O ₃, and processed under a low process temperature in the absence of pressurizing equipment. Several experiments were performed to address the effects of the powders on the phase transformation rate from $a$to $b$ phase, and on the mechanical properties due to the microstructure evolution under different sintering conditions.; The existence of sintering additive elements in each individual powder was verified by electron microscope and energy dispersive spectroscopy. The doped powders were 60% crystalline and had a specific surface area of ∼24 m2/g. The powders showed excellent sinterability at low temperature, compared to the other powders used in commercial processes. In spite of the low process temperature, the $a\to b$ phase transformation occurred rapidly. The homogeneous distribution of Si₃N₄ powders was speculated as the reason for fast phase transformation. In addition, evolution of the microstructures in different sintering conditions produced materials with vastly different mechanical properties. Thus, one may obtain ceramics with desired mechanical properties by carefully controlling the sintering additives.; Finally, a mathematical model, based on the previous work done by Krämer et al., was developed to describe the kinetics of the grain growth and the phase transformation. The model was employed to illustrate the effects of ill-defined constants, such as, diffusion coefficients and reaction constants, on the grain growth rate.