Abnormal Grain Growth In Alumina Research Articles

Microstructure and mechanical properties of MgO-stabilized ZrO2–Al2O3 dental composites

The aim of the present study was to investigate the production of tetragonal zirconia (t-ZrO2) particles (experimental t-ZrO2) from monoclinic zirconia (m-ZrO2) and to evaluate the effect of the t-ZrO2 content on the fracture toughness of alumina–zirconia composites by conducting ASTM E399 standard test. In the laboratory study, t-ZrO2 powder was produced by heat treating m-ZrO2 containing 10wt.% MgO. Alumina and alumina–zirconia composite powders containing various types and amounts of m-ZrO2 and t-ZrO2 were prepared (0–20wt.%), shaped by slip casting to achieve a uniform distribution and homogeneous microstructure in accordance with the dimensions of ASTM E399 standards, dried, sintered at three different temperatures: 1400, 1500 and 1600°C for two hours, and characterized. The results of the XRD analysis showed that t-ZrO2 was produced at 1400°C. In t-ZrO2-doped alumina composites, t-ZrO2 partially transformed to m-ZrO2 after sintering, whereas commercial t-ZrO2 (Tosoh TZ-3Y) remained intact. SEM studies on samples sintered at 1600°C revealed that the addition of zirconia inhibited abnormal grain growth of alumina, leading to a homogeneous and equiaxed grain structure, especially at high concentrations of zirconia. ZrO2-doping enhanced the fracture toughness of the composites, which increased with an increase in the t-ZrO2 content. The maximum fracture toughness was 11.5MPam1/2 and was observed when the t-ZrO2 content was equal to 20wt.%. Alternatively, the maximum fracture toughness for pure alumina was 5.9MPam1/2.

Effectively Inhibiting Abnormal Grain Growth of Alumina in ZTA with Low‐Content Fine‐Sized ZrO2 Inclusions Introduced by Infiltration and In Situ Precipitation

Previous literatures have all emphasized that the successful control of microstructure homogeneity of ZTA (ZrO2 toughened Al2O3) composites can only be achieved provided that the ZrO2 particles are homogeneously distributed and the volume content exceeds 5 vol%. In this paper, we report our strategy to incorporate fine‐sized ZrO2 into Al2O3 by infiltrating presintered alumina compact using zirconium ion‐containing solution and then in situ precipitation with ammonia solution. Homogeneous microstructure without AGG can be achieved in the prepared composite at a lower ZrO2 content (2.3 vol%). Such a result might be attributed to the excellent homogeneous distribution of fine‐sized incorporated ZrO2 particles in Al2O3 matrix.

Effect of ZrO 2 on phase transformation of Al 2O 3

This study investigates the effect of ZrO 2 on phase transformation of alumina. Alumina and alumina–zirconia composite powders were produced by the precipitation method from aluminum sulfate and zirconium sulfate precursors. Precipitates obtained at 15 °C were dried at 80 °C for 72 h, and then calcinated at four different temperatures; 1000 °C, 1100 °C, 1200 °C and 1300 °C for 1 h. Powders calcinated at 1300 °C were pressed uniaxially and sintered at 1600 °C for 1 h. XRD and DSC analyses showed that the presence of zirconia retarded the transformation to α-alumina. SEM studies on the powders calcinated at 1300 °C revealed that both alumina and alumina–zirconia particles were 100–300 nm in size and of near spherical shape. Zirconia additions inhibited abnormal grain growth of alumina and provided homogeneous, equaxied grain structure.

Effect of Chemical Composition on the Grain Orientation of Alumina in Oxide Laminate Composites

The effect of chemical compositions, such as CAS 2 (anorthite), Fe 2 O 3 , and TiO 2 , on the microstructure texturing of alumina grain have been investigated. Regardless of chemical compositions in alumina/alumina laminates, alumina grains were textured parallel with casting direction, such as (006) and (1010) planes. Compared to CAS 2 (anorthite), the addition of Fe 2 O 3 , and TiO 2 were influenced on the abnormal grain growth of alumina in untextured layer rather than textured layer. In the case of alumina/mullite laminates with TiO 2 addition, alumina grains were textured perpendicular to the casting direction, such as (110) and (300) planes.

Effect of the Liquid‐Forming Additive Content on the Kinetics of Abnormal Grain Growth in Alumina

Alumina specimens with various amounts of CaO and SiO2 (1:2 ratio) were prepared, and their abnormal grain growth (AGG) kinetics were investigated. A plot of the area fraction covered by abnormal grains versus log (sintering time) had a sigmoidal shape with an apparent incubation period before the onset of AGG. The overall kinetics of AGG was similar to that of a phase transformation controlled by nucleation and growth. The incubation time and the end point of AGG were strongly dependent on the amount of liquid‐forming additives. Correspondingly, the final microstructure was affected by the liquid content: a large grain size and a high aspect ratio at low liquid content and a small grain size and a low aspect ratio at high liquid content.

Abnormal Grain Growth in Alumina: Synergistic Effects of Yttria and Silica

Abnormal grain growth without strong anisotropy or faceting of the grains has been observed in high‐purity yttria‐doped alumina specimens, often starting at the surface and spreading right through the bulk at higher sintering temperatures. This appears to occur because of an interaction between Si contamination from sintering and the yttria doping; no such effect is seen for undoped samples. Similar microstructures were observed after deliberate Y/Si codoping. Analytical STEM showed that some grain boundaries bordering on large grains contained more Si than Y. HRTEM and diffuse dark‐field imaging revealed thin (0.5–0.9 nm) disordered layers at some boundaries bordering large grains. It appears that Si impurities are accumulating at some boundaries and together with the Y inducing a grain boundary structural transformation that accounts for the dramatically increased mobility of these boundaries.

Abnormal Grain Growth of Alumina: CaO Effect

The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination‐free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.

Abnormal Grain Growth in Alumina with Anorthite Liquid and the Effect of MgO Addition

Abnormal grain growth (AGG) in alumina with anorthite liquid has been observed with varying anorthite and MgO contents, at 1620°C. When only anorthite is added to form a liquid matrix, the grain–liquid interfaces have either flat or hill‐and‐valley shapes indicating atomically flat (singular) structures. The large grains grow at accelerated rates to produce AGG structures with large grains elongated along their basal planes. This is consistent with the slow growth at low driving forces and accelerated growth above a critical driving force predicted by the two‐dimensional nucleation theory of surface steps. With increasing temperature, the AGG rate increases. The number density of the abnormally large grains increases with increasing anorthite content. The addition of MgO causes some grain–liquid interfaces to become curved and hence atomically rough. The grains also become nearly equiaxed. With increasing MgO content the number density of the abnormally large grains increases until the grain growth resembles normal growth. This result is qualitatively consistent with the decreasing surface step free energy associated with partial interface roughening transition.

Effect of anorthite liquid on the abnormal grain growth of alumina

A small amount of anorthite (2CaO·Al2O3SiO2) powder was placed on the top-center of the pure Al2O3 powder compact and then sintered at 1600°C. During sintering some grains grew extensively along the radial direction from the liquid source. By considering an atomically smooth solid–liquid interface structure of alumina, grain growth controlled by 2-dimensional (2-D) nucleation was suggested. The elongation has been explained in terms of nucleation advantage of grain-boundary re-entrant edges (GBRE) formed dominantly at non-basal planes.

Effect of Liquid Content on the Abnormal Grain Growth of Alumina

Alumina specimens with small amounts of CaO and TiO2 were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO2 content was small (≤0.04 wt%). When the CaO + TiO2 content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.

Effects of SiO2, CaO2, and MgO Additions on the Grain Growth of Alumina

When pure alumina is sintered at 1620°C, normal grain growth occurs with equiaxial grains and curved grain boundaries. When 100 ppm of SiO2 together with 50 ppm of CaO is added, abnormal grain growth (AGG) occurs with large grains elongated with straight grain‐boundary segments in the direction of the basal planes. Some of the fine matrix grains also have straight grain boundaries, and ∼10% of the grain boundaries of the matrix grains are faceted when observed by transmission electron microscopy (TEM). Some of these grain boundaries are expected to be singular with low‐energy structures corresponding to the cusps in the polar plot of the grain‐boundary energy against the inclination angle. No frozen liquid is found at the grain triple junctions and grain boundaries by TEM. When 600 ppm of MgO is added together with 100 ppm of SiO2 and 50 ppm of CaO normal growth occurs. The grain boundaries are curved when observed via optical microscopy and TEM and show that all the grain boundaries are defaceted, indicating that they become atomically rough. When sintered at 1900°C after adding 150, 300, or 500 ppm of SiO2, AGG occurs with straight and faceted grain boundaries, similar to the specimens sintered at 1620°C after CaO and SiO2 are added. When MgO is added together with SiO2 and sintered at 1900°C, normal grain growth occurs with rough grain boundaries. High‐resolution TEM observation shows no frozen liquid layer at a grain boundary. The results indicate that the occurrence of AGG in alumina with SiO2 or together with CaO is correlated with the formation of faceted and straight (on large and atomic scales) grain boundaries. It is proposed that these grain boundaries have singular ordered structures with low boundary energies and their growth by lateral step movement can cause the AGG. The addition of MgO causes grain‐boundary roughening and, thus, normal grain growth. The grain boundaries in pure alumina also appear to be rough, and, hence, normal grain growth occurs.

Abnormal Grain Growth of Alumina

Abnormal grain growth (AGG) is not one of the intrinsic properties of alumina but rather is an extrinsic property that is controlled by certain impurities that are introduced during powder synthesis, processing, or sintering. When small amounts of glass‐forming impurities are introduced, some portion beyond their solubility limits will accumulate at grain boundaries at the final stage of densification, form thin intergranular glass films of thermodynamically stable thickness, and induce the sudden appearance of abnormal grains by increasing the rate of grain‐boundary migration abruptly. The proposition has been tested experimentally with small, but varying, amounts of silica in ultrapure alumina (99.999%) that has been sintered in a contamination‐free condition. Average grain sizes for the appearance of AGG are inversely related to the doping concentration of silica. The thickness of intergranular silicate glass films at the onset of AGG in alumina is constant and estimated to be }3.7 nm.

Abnormal Grain Growth in Alumina

Abnormal Grain Growth in Alumina

ChemInform Abstract: Critical Concentration of MgO for the Prevention of Abnormal Grain Growth in Alumina.

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Critical Concentration of MgO for the Prevention of Abnormal Grain Growth in Alumina

The effects of MgO on sintering and grain growth of alumina in the absence of any other impurities as well as in the presence of various amounts of CaO were investigated using ultrapure (>99.999%) alumina and sintering at 1900°C for 1 h in a clean contamination‐free condition. Critical concentrations of MgO required for the prevention of abnormal grain growth were linearly dependent on the CaO concentration. For a given concentration of CaO, at least the same amount of MgO has to be added to prevent abnormal grain growth. MgO addition alone to the ultrapure alumina enhanced both grain growth and densification kinetics during pressureless sintering. The beneficial effect of MgO doping could not be explained based on the solute drag (or pinning) model. It was more likely to be understood in terms of either a glass modification model or a solid–liquid interface modification model.

Determination of Critical Concentrations of Silica and/or Calcia for Abnormal Grain Growth in Alumina

Minimum amounts of SiO2 and CaO required for inducing abnormal grain growth in alumina were determined using ultrapure alumina (>99.999%) and sintering at 1900°C for 1 h in a contamination‐free condition. The critical concentrations of silicon in cationic mole fractions in alumina were 300 ppm without calcium, 200 ppm with 10 ppm calcium, and 150 ppm with 20 ppm calcium. The critical concentration of calcium alone was 30 ppm. These concentrations seemed to match approximately the solubility limits reported in the literature, which suggested that the abnormal grain growth in commercially pure alumina was indeed related with the formation of a small amount of liquid phase during sintering.

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.