Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering
Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering
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DOI：
10.1016/j.jeurceramsoc.2020.08.018
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发表时间：
2021
影响因子：
5.7
通讯作者：
Haiyue Xu;J. Zou;Weimin Wang;Hao Wang;Wei Ji;Z. Fu
中科院分区：
文献类型：
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作者：
Haiyue Xu;J. Zou;Weimin Wang;Hao Wang;Wei Ji;Z. Fu
采用高压烧结法可以在相对较低的温度下有效地获得具有延迟晶粒生长的全致密陶瓷。但在致密化机理、晶粒生长过程、晶界表征和残余应力等方面缺乏深入的研究。采用碳纤维增强碳（Cf/C）复合材料制造的坚固可靠的火花等离子烧结模具，在相对较低的温度和高达200 MPa的高压下烧结了平均粒径为220 nm和3 μm的两种工业纯α- al2o3粉末。烧结致密化温度和晶粒生长起始阈值温度（Tsg）由施加的压力和相对于晶粒尺寸的表面能决定，因为它们都随施加的压力而增大，随施加的压力而减小。220nm al2o3粉末和3 μm al2o3粉末分别在1050°C和1400°C的施加压力下发生致密化和有限的晶粒粗化。高压低温烧结陶瓷的晶界能、残余应力和位错密度均高于无附加压力烧结陶瓷。证明了相邻颗粒接触区域发生的塑性变形是高压烧结的主要机制，并建立了基于塑性力学和等球紧密堆积的数学模型。基于数学模型，通过塑性变形机制预测al2o3致密体的相对密度可达~ 80%，与实验结果吻合较好。从烧结参数，即保温温度、保温时间和施加压力等方面研究了致密化动力学。应力指数和微观组织演变表明，扩散、晶界滑动和位错运动是烧结最后阶段的辅助机制。在1125℃和100 MPa下烧结220 nm氧化铝时，变形倾向于增加缺陷和空位的产生，这两者都加速了晶格扩散，从而促进了晶粒的生长。
Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (Cf/C) composite for spark plasma sintering, two kinds of commercially pure α-Al2O3powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (Tsg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al2O3powder and 1400 °C for the 3 μm Al2O3powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al2O3compact can reach ∼80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.