Effect of dewaxing condition on characteristics of grain boundary and translucency for the sintered alumina using supercritical fluids extraction in ceramic injection molding

Abstract

Powder injection molding (PIM) is an engineering process that merges the advantages of plastic injection molding and powder metallurgy technology. Used in complicated 3D product designs, PIM allows high formability and powder metallurgy uses the sintering technique that has been studied and practiced for several decades. In addition, PIM technology enables low-cost mass production of hard-to-form materials without the need for secondary processing.

In PIM, the powders are given flexibility so that they can be injected like plastics or macromolecules. The powders must be debind, however, prior to sintering in order to remove coupling agents that were used to improve powder fusion. Debinding is the most time consuming process during PIM, and is most likely to result in defects in the mold, as 15 to 50 volume percent of the coupling agents must be removed. The removal method is directly related to the composition of the agents and must, therefore, be considered during the design stage. Furthermore, increasing numbers of PIM-related patents and ongoing studies are aimed at removing the agents faster than conventional methods without making changes to the molds or causing defects.

Light transmission through translucent alumina is inhibited by the light-scattering interface, pores, defects, and impurities in the materials. Although the different refraction indexes of both sides cause light to scatter, light transmission is inhibited mainly by the pores and defects on the interface. In order to control these factors, the impurity level must be minimized and sintering densification technology should be implemented in order to remove pores and defects. Moreover, in order to obtain transparent ceramics, high-density sintering must be performed and there should be almost no pores on the grain boundary (sufficiently within the range of visible spectrum). There should also be almost no impurities or hyaline materials on the grain boundary, given that they have optical characteristics similar to those of the crystalline grains. Furthermore, their crystal grains should have uniform size and composition as well as slow growth rate. The manufacture of high-transparency parts using translucent alumina relies, therefore, on controlling the crystal structure of the pellets in order to ensure full densification of the microstructure during sintering. In this study, samples for the light transmission test were produced by injection molding, using the optimized feedstock produced using high-purity alumina powder. In the present study, carbon dioxide (CO2)in its supercritical state was used for debinding. In this state, the temperature and pressure are both higher than their respective critical points. A supercritical fluid is unique since its properties, for example, the density, change drastically in response to small changes in the pressure near the critical point. Moreover, like an ideal gas, this density can be changed continually from a lean to a high-density fluid-like state, which results in the high solubility of the supercritical fluid. During the study, the authors were able to selectively melt paraffin wax, which is a small molecule conjugate, by modifying the temperature and pressure of the supercritical CO2 and the dewaxing ratio changed with changing temperature and pressure. In fact, for a constant pressure of 25 MPa, the dewaxing ratio was smaller at high (65°C) than at low (35°C) temperature. This smaller ratio was obtained since the high-density, non-fully fluidic paraffin wax, can only be easily melted, even by high-density solvents, if dewaxing is performed at temperatures higher than the fusing point. The authors also observed the microstructure after supercritical and heat dewaxing in order to assess the effects of the removal rate of the original coupling agents on sintering densification and the grains. Pores in and between the agglomerates, and the corresponding difference in size, were identified during the observation of debind materials in the supercritical condition. High-speed release of the low molecular binder during their removal from the mold increased the pressure at the wax exit points. This increased pressure resulted in concentration, by capillary action, of the binder and hence, the creation of coarse pores inside the mold. As a result, the transmission rate was lowered significantly owing to the defects and pores on the grains of the sintered material. The control of defects and pores around the grains, rather than their average sizes, must, therefore, be prioritized as a determining factor in the light transmission rate of polycrystalline translucent alumina pellets.