Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttriastabilized zirconia

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries(GBs) and the solidvapor interfaces(free surfaces), each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies(differently)as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.     

Constitutive modeling of alumina sintering: grain-size effect on dominant densification mechanism

In the present work, alumina powders with the initial grain sizes of 0.9 and 7.0 μm, respectively, were sintered at different temperatures. Constitutive laws for densification were employed to model the sintering process of alumina ceramics. Based on the constitutive laws employed and the experimental results obtained, the dominant densification mechanism was identified and the effect of grain size on dominant densification mechanism was discussed. The activation energy for densification was also evaluated. In the investigated sintering temperature range, interface reaction was identified as the controlling process in sintering of alumina powders with the initial grain size of 0.9 μm, while grain-boundary diffusion was identified as the dominant process in sintering of alumina powders with the initial grain size of 7.0 μm. The activation energies for densification of the finer and coarser grain size alumina ceramics were determined as 342 and 384 kJ mol⁻¹, respectively, which provided a strong support on the densification mechanism investigation.     

Grain boundary complexion and transparent polycrystalline alumina from an atomistic simulation perspective

Transparent alumina is often processed with sintering additives such as, Y, La, and Mg, in order to limit its grain growth at high sintering temperatures. Usually, these additives segregate to the grain boundaries due to their larger cationic size than Al and low solubility in bulk α-alumina. The grain boundary excess of these additives plays a key role in determining stable grain boundary complexions and thereby, the grain growth characteristics of the polycrystalline alumina. In the current work, the grain boundary segregation of trivalent (Y, La) as well as bivalent (Mg) dopants on several alumina grain boundaries was simulated using the force field based energy minimization method. Calculated segregation energy plots and atomistic structure analysis, for the case of trivalent dopants, suggest that there is a critical concentration (3–4 atoms/nm²) for achieving the lowest mobility monolayer grain boundary complexion. The bivalent dopant Mg plays a role in grain boundary complexion through creating ordered arrays of oxygen vacancies at the grain boundary and helps create the space for the easier occupation of the grain boundary cationic sites by the trivalent dopants in case of codoping. It was also observed that the twin grain boundaries are more preferable in comparison to general high angle grain boundaries to obtain monolayer complexions, which are necessary for limiting grain growth. The use of atomistic simulations can thus guide the experimentalist towards optimum dopant concentrations to better control ceramic microstructures. In a more general sense the possibility of second phase formation or an incipient second phase for high grain boundary concentrations >8 cations/nm² is briefly discussed.     

Sintering and grain growth of ultrapure alumina

The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900°C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina ( 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.     

陶瓷材料烧结致密化过程的元胞自动机模拟

为研究陶瓷材料烧结致密化过程,以晶界能和晶界曲率生长驱动力理论为基础,建立了含有气孔的二相晶粒生长的元胞自动机模型,对陶瓷材料烧结致密化过程进行了模拟,并与制备的Al2O3/TiN陶瓷材料进行对比.结果表明,模型可有效地模拟陶瓷材料烧结时晶粒的生长及气孔的湮灭情况,能较好地再现烧结致密化过程,模拟结果与制备的陶瓷材料微观形貌组织十分接近.     

Effect of particle size distribution on sintering

A sintering model, taking into account the effect of particle size distribution and the effect of grain growth, was developed previously. Experimental data from the sintering of high-purity alumina were used to testify this model. The sintering was carried out at 1500 °C in air for various lengths of time. The results agreed with the prediction of the model. The powders with a narrower starting particle size distribution exhibited a lower sintering rate prior to the occurrence of grain growth, but a higher densification rate after the grain growth took place. The grain size/density trajectory was applied to reveal the effect of particle size distribution on sintering behaviour. It is suggested that powders with a narrower size distribution are preferable.     

Characterization of Al2O3 composites with fine Mo particulates,I. Microstructural development

Alumina based composites containing nano- or submicron-meter Mo grains in the amounts of 20 vol% or less were prepared through a dissolution of molybdenum oxide in ammonium solution, followed by spray-drying, hydrogen reduction and sintering with or without hot-pressing. The properties of alumina/molybdate solutions and the ζ-potential of alumina particles in the solution were measured. By using electron microscopic and quantified X-ray diffraction techniques, the microstructural features and the evolution of Mo paniculate in spray-dried powder and sintered bodies were analyzed. The time dependent exponent and activation energy of grain growth of Mo between 600 to 900 °C were determined. There is no glassy phase or reaction at the interfaces between Mo/Al₂O₃ of dense composites. Only one coherent interface was found, and the others are incoherent. The results reveal that submicrometric Mo grains may grow by surface diffusion in reduction stage (≤ 900 °C) and greatly retard the densification and reduce the grain size of alumina matrix in sintering stage.     

Study on the structure and properties of fine-grained alumina fast sintered with high heating rate

Fine-grained alumina was obtained in 2 min by a new densification method based on Self-propagating High-temperature Synthesis plus Quick Pressing (SHS/QP). The sample was densified to more than 99% of theoretical density under a large mechanical pressure (100 MP) and a fast heating rate (1600 °C/min). Compared with the alumina sample obtained by spark plasma sintering (SPS) at lower heating rates (100 or 500 °C/min), almost no grain growth was found in the sample obtained in this work. The microstructure and mechanical properties were studied. Hardness value of 18.6 ± 0.4 GPa and fracture resistance value of 3.4 ± 0.3 MPa m^1/2 were measured for fine-grained alumina of this work. The densification mechanism was discussed.     

Three dimensional simulation of microstructure evolution for ceramic tool materials

The observation and scientific quantitative characterization of three dimensional microstructure evolution during sintering process of ceramic tool materials is important to investigate the influence of nano-particles on mechanical properties. The relationship between microstructure and mechanical properties of ceramic tool materials can be established to direct the development of nano-composite ceramic tool materials by the research of the grain growth, grain boundary migration, distribution of nano-particles and microstructure densification at the different sintering temperature and pressure. In this paper, a 3D Monte Carlo model of three-phase nano-composite ceramic tool material is built and applied to simulate the microstructure evolution during sintering process. In this model, the grain boundary energy of each phase and interfacial energy between two phases are taken into consideration as the driving forces for grain growth. The sintering temperature and pressure are successfully coupled into the Monte Carlo simulation model. The microstructure evolution of defect free three-phase nano-composite ceramic tool materials is successfully simulated at different sintering temperature and pressure. The simulation results show that the higher the sintering temperature is, the faster the grain growth. However, the sintering pressure has little effect on the grain growth.     

Experimental and Numerical Studies of Densification and Grain Growth of 8YSZ during Flash Sintering

As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra-high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering ($Q_{\mathrm{d}}$ =424 kJ mol⁻¹) compared to the conventional sintering ($Q_{\mathrm{d}}$ = 691 kJ mol⁻¹). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

Processing of alumina-based composites via conventional sintering and their characterization

The present work relates to the processing of dense alumina-based composites, their microstructural characterization and study of mechanical properties. Alumina ceramic material and alumina-based composites with m-Zirconia and Ceria addition are sintered at 1600°C, 1650°C and 1700°C temperatures via conventional sintering. Solid-state diffusion during sintering led to volume diffusion in alumina, and volume and grain boundary diffusion in alumina composite. In the present sintering conditions alumina is found to be the least dense as improper solid-state diffusion resulted in porosity, whereas alumina–zirconia composite achieved the highest density of 97%. Scanning electron microscope (SEM) micrograph shows homogeneous distribution of fine zirconia particles inside the alumina matrix, filling the voids of the alumina skeletal structure. Zirconia connects to alumina particles, restricting its abnormal grain growth. It results in strong bonding and grain refinement. Alumina–zirconia composite exhibits the highest hardness and fracture toughness of 14.37?GPa and 4.6?MPa?·?m^1/2 at 1700°C. Alumina suppresses the transformation of m-t zirconia, resulting in high toughness of alumina composites. Alumina–zirconia–ceria composite revealed the presence of porosity, which led to less densification and low mechanical properties.     

Computer-aided control of the evolution of microstructure during sintering

Control of the microstructure is crucial for realizing optimum properties in advanced sintered materials. We describe a mathematical model for solid state sintering of ceramic systems, which is based on a population balance paradigm for pore shrinkage and grain growth. It incorporates the interplay between densification and grain growth and tracks the trajectories of pore and grain size spectra in a reasonably realistic manner. The model is employed in a simulation mode to identify the effect of initial grain (particle) size distribution and to optimize the time–temperature sintering cycle based on our own work on sintering of composite alumina–zirconia. The utility of this approach for computer-aided control of the evolution of microstructure is discussed.     

Pressureless sintering and related reaction phenomena of Al2O3-doped B4C

The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B₄C were studied. Pressureless sintering was conducted at various temperatures for 15 min in a flowing Ar atmosphere. The addition of alumina improved the densification of boron carbide. Maximum density of 96% theoretical was obtained with the 3 wt % alumina-doped B₄C sintered at 2150°C. Abnormal (or exaggerated) grain growth was observed in the specimen containing more than 4 wt % alumina. In the B₄C-Al₂O₃ reaction couples, good wetting of the liquid phase on the boron carbide grains was observed. X-ray diffraction and Auger electron spectra showed that the AlB₁₂C₂ phase was formed by the reaction between boron carbide and alumina. It is suggested that these phenomena promote the densification of boron carbide.     

The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites

The sintering and grain growth behaviour of alumina + 2, 3.5 and 5 wt.% carbon nanotubes (CNTs) and alumina + 2 wt.% carbon black nanocomposites prepared by Spark Plasma Sintering (SPS) were studied. The addition of CNTs to ceramics produces a large reduction in the sintering temperature required for their complete densification and a significant grain size refinement by a previously unreported mechanism. The CNTs form a strong entangled network around the grains, which constrains the normal and abnormal grain growth. An alumina/alumina + 2 wt.% CNT/alumina laminate structure was prepared to demonstrate directly the large grain-growth retardation effect of CNTs. These effects open up the possibility of using CNTs as a sintering aid to control the sintering behaviour and microstructures of ceramics in bulk, laminate and functionally gradient (FGM) form.     

升温速率对氧化铝粉末烧结行为的影响

采用不同的升温速率,在膨胀计中对脱脂后的氧化铝粉末射成形坯件进行一系列的烧结试验.结果表明,烧结致密化过程主要发生在升温阶段,快速升温有利于致密化的进行和抑止晶粒长大,但由于烧结时间较短和烧结炉最高温度的限制,产品的最终致密化程度不高.在低温时快速升温,高温时缓慢加热,可以获得较好的致密化效果和微观结构.试验和分析结果将为建立非等温烧结模型和烧结工艺参数的优化方法提供依据.     

Abnormal grain growth enhanced densification of liquid phase-sintered WC–Co in support of the pore filling theory

This study examines the effect of grain growth on densification during liquid phase sintering of compacts with faceted grains. Two kinds of WC powders with different sizes were used to produce WC–Co alloys. Large pores of ~5 μm size were generated in 95WC–5Co (wt%) using spherical Co particles of the same size. The overall sintering behavior was observed by measuring grain growth and densification as a function of sintering time at a sintering temperature of 1350 °C. When the WC powder was fine (0.4 μm), large pores disappeared upon filling of pores by liquid with the formation of abnormal grains. On the contrary, when the WC powder was large (4.2 μm), grain growth is not observed, and large pores remained intact even after a long period of sintering (24 h). These observations confirm that densification during final stage liquid phase sintering occurs via filling of pores by liquid as a result of grain growth. This finding is consistent with the model of densification predicted by the pore filling theory.     

Grain growth during spark plasma and flash sintering of ceramic nanoparticles: a review

Spark plasma and flash sintering process characteristics together with their corresponding sintering and densification mechanisms and field effects were briefly reviewed. The enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms. When the densification is aided by plastic deformation, the kinetics of grain growth depends on the particles’ rotation/sliding rate and is controlled by lattice and pipe diffusion. When the densification is aided by spark, plasma, and the particles’ surface softening, grain growth kinetics is controlled by viscous diffusion and interface reactions. Grain growth in both cases is hierarchical by grain rotation, grain cluster formation and sliding, as long as the plastic deformation proceeds or as long as plasma exists. Densification by diffusion in a solid state via defects leads to normal grain growth, which takes over at the final stage of sintering. Various field effects, as well as the effect of external pressure on the grain growth behaviour were also addressed.     

Sintering study of nanocrystalline tungsten carbide powders

WC powder with an average grain size of 6 nm was obtained after high energy ball milling under protective gas atmosphere. The kinetics of densification was studied during sintering the powder in a dilatometer up to 1450 °C. The microstructure was investigated by TEM and high resolution SEM after various stages of sintering. The green density of the specimens was 45%. Three stages of sintering were defined: (a) rearrangement of particles at low temperature (850 °C) without grain or particle growth, (b) neckformation between powder particles at 1000–1250 °C and initial grain growth at 1200 °C, (c) pore elimination accompanied by massive grain growth at 1300–1450 °C.     

SPS制备亚微米晶氧化铝陶瓷

以商业α-Al₂O₃粉体为原料, MgO为烧结助剂, 采用放电等离子烧结技术(SPS)制备亚微米晶氧化铝陶瓷. 系统研究了烧结温度、烧结助剂含量对亚微米晶氧化铝陶瓷的致密化过程及显微结构的影响. 分析结果表明, 1250℃以及0.05wt%分别是最佳的烧结温度和烧结助剂含量; 在此条件下获得的亚微米晶氧化铝陶瓷, 其相对密度达到99.8%TD(theoretical density),平均晶粒尺寸约0.68μm,显微硬度(HV5)达到20.75GPa,在3～5μm中红外范围内直线透过率超过83%. 当MgO掺杂量超过0.1wt%时, 第二相MgAl₂O₄形成, 引起光散射, 降低红外透过率.