Grain-boundary diffusion coefficient in α-Al₂O₃ from spark plasma sintering tests: Evidence of collective motion of charge disconnections

Abstract

The sintering of fine-grained α-alumina by spark plasma sintering (SPS) was performed to study grain growth under SPS conditions. Grain growth is found to be extensive at relative densities above 95%. A grain growth versus dwell time analysis during SPS allows for the determination of the grain-boundary diffusion coefficient. This study shows that the remarkable enhancement of grain-boundary diffusion derived from a previous analysis could be a consequence of the presence of the recently discovered “disconnections” at the grain boundaries of α-alumina. Their presence, together with their electric charge and the external electric field at the boundaries, are the key ingredients for a violation of the typical grain growth kinetic law. When they are introduced appropriately, an updated value of the grain-boundary diffusion coefficient is achieved. A comparison with other values reported previously in the literature through other techniques and a critical analysis are also carried out.

Introduction

Alumina-based materials have widely attracted the interest of the international science community due to their physical-chemical properties, such as mechanical, electrical and chemical stability and abrasion resistance. Alumina ceramics represent the highest investment in Japanese industry compared to the other ceramic systems [1]. Although alumina is a promising candidate for high temperature structural applications, its brittleness is a limiting factor for a broader range of applications. One classical approach to improve the brittleness and other properties of alumina is the achievement of a fine microstructure [2], [3]. In fact, the combined use of a starting powder of fine particles and a controlled sintering process, which could minimize grain growth, may lead to the manufacturing of a dense sintered body with a fine-grained microstructure. One of the most studied processes to control grain size is the use of additives that limit grain growth [4]. However, even a small concentration of additives in a highly stoichiometric compound can generally modify the material properties, such as oxidation resistance, thermal transport or high temperature creep behaviour.

The sintering of dense materials by spark plasma sintering (SPS) has been extensively studied since the early 1990s [5]. The raw material powder is placed in a graphite die and pressed with two graphite punches to a certain pressure in a chamber under vacuum. An electric current is then applied through the mould producing Joule heating. Joule heating enables high heating and cooling rates, which minimize grain growth during sintering, as well as lowering the sintering temperature and dwell time compared to other techniques, such as pressureless sintering or hot pressing [6], [7]. Although an explanation for the fundamental mechanism is still under debate, SPS has been intensively investigated by several authors to determine the effect of SPS conditions, such as sintering temperature, sintering dwell time, heating rate and mechanical pressure on the relative density and grain size of alumina [8], [9], [10], [11], [12], [13], [14]. The smallest grain size reported in a dense alumina specimen is 0.2 µm, with a relative density of 98%, a sintering temperature of 1550 °C, a dwell time of 2 min and a heating rate of 200 °C/min. The applied mechanical pressure was 100 MPa and a starting alumina powder size of 0.2 µm was used [8]. In the case of relative densities higher than 99.7%, the smallest grain size achieved is 0.3 µm, resulting from a sintering temperature of 1200 °C, a sintering dwell time of 3 min, a heating rate of 150 °C/min and a mechanical pressure of 100 MPa [9]. Aman et al. [11] carried out an experimental screening design method with the aim of obtaining an insight into the relationship between density or grain size and SPS parameters, such as green-shaping processing, heating rate, sintering dwell time and sintering temperature. From the results in [8], [9], [10], [11], [12], [13], [14], it is clear that the SPS parameter with the major influence on the final grain size is the sintering temperature.

One pending question that has not received attention so far is the kinetics of grain growth during the dwell time after reaching the sintering temperature. As far as we are concerned, grain size versus dwell time determinations in the SPS sintering of α-Al₂O₃ have been reported by Shen et al. [13] only. They only consider two temperatures, namely, 1200 and 1300 °C. However, these authors do not use their results to measure a diffusion coefficient.

This study is devoted to a rigorous study on the grain growth kinetics during SPS sintering of α-Al₂O₃. To this purpose, two requisites must be satisfied. First, the range of densities in which grain growth kinetics are dominant over pure densification must be carefully measured. Second, a strict control of the local temperature at the specimen during grain growth must be assured. The first two sections are focused on these preliminary conditions. The third section reports the grain growth analysis and the value of the diffusion coefficient measured accordingly.