Exploration of Flash Sintering Mechanisms and Increase Mechanical Strength of YSZ for Dental Application

Abstract

Flash sintering is a rapid ceramics sintering method with an external electric field assistance. Compared to conventional sintering by which YSZ should be heated to 1200~1400OC and hold for 2~4 hours to be sintered, flash sintering can lower the sintering temperature and shorten the sintering time markedly. In addition, the grain size of YSZ observed after flash sintering is often smaller than those sintered by conventional sintering approach. However, it is still not well understood how ceramics could be sintered in a few seconds and accompanied by a sudden increase of electric conductivity of the ceramic material. One proposed explanation of the electric field effect during flash sintering is the Joule heat generation within the sample, leading to significantly higher local temperature that in turn enhances mass transfer. However, the in-situ observed temperature of flash-sintered samples in literatures suggests a diffusion rate based on Arrhenius equation insufficient to meet the required mass transfer rate for rapid densification. Flash sintering process is commonly divided into three stages. Stage I is considered an incubation time while a constant voltage is applied. Stage II is the period during which the flash event occurred, accompanied by a sharp increase of current. Stage III is the constant-current period after flash sintering when the pre-set limiting power is reached. In order to understand how electric field affects the sintering exactly, we performed flash sintering on 3YSZ compact and investigated the sample at the end of each aforementioned stages: case A, before the flash event; case B, at flash event and cut off the electric field instantly; case C, constant current mode for 300 s, as illustrated in figure (d). All of the samples are flash sintered at 850OC with an applied field of 150 V/cm and a current limit of 0.5 A. SEM images are shown in figure (a), (b), and (c), which correspond to case A, B, and C, respectively. As shown in figure (a) and (b), very little grain growth is observed under constant voltage, and then grain growth accelerated after the flash event. If we extend the holding time of constant current period, as demonstrated in figure (c), the grain boundaries merging is more obvious and larger grain sizes are observed. However, too large a grain size of YSZ could lead to phase transformation from tetragonal to monoclinic structure and in turn reduce its mechanical strength. In this work, we focus on enlarging current limit and shorten holding time to sinter YSZ of smaller grain sizes and elevate uniformity of grain distribution by improving the electric contact. The goal is to raising the hardness of sintered YSZ to 1200 HV in Vickers hardness and density to 6 g/cm3 while controlling the proportion of phase transformation to be less than 5%. We aim to flash sinter YSZ of smaller grain size and higher mechanical strength for the dental application while reducing energy consumption during its manufacture. Figure 1