Direct microscopic evidence is presented in support of an explanation of R-curve behavior in monophase ceramics by grain-localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such “activity sites” along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface-adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal-zone microcracking and cracktip/internal-stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.

Crack-Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near-theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis–processing–property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green-body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an expose of the state of the art in transparent polycrystalline spinel fabrication.

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

A review of the densification mechanisms and the microstructural development for transparent spinel made by free sintering and by hot pressing is given. The paper is divided into two main parts. The first part considers spinel without any sintering additives because there still is some controversy concerning the role of cation stoichiometry on sintering and grain growth. The second part discusses the role of the classic sintering aid, LiF, in processing transparent spinel. LiF is shown to have multiple behaviors: (1) it initially wets spinel and forms a liquid phase at relatively low temperatures, which affects early-stage densification and also grain growth; (2) upon cooling from intermediate temperatures, or even from higher temperatures if microstructure evolution (e.g., formation of closed porosity) prevents volatization, the LiF-containing liquid dewets and resides in isolated pockets; (3) LiF alters the cation stoichiometry, thereby enhancing diffusion via an increase in the concentration of oxygen vacancies; this affects both the densification rate and grain growth; and (4) it reacts with impurities in the system, thereby acting as a cleanser. For the production of transparent spinel, it is critical that LiF or associated reaction products not be retained as a secondary phase.

A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)

The effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied. The separate effects of the additive on densification and grain growth were assessed. Magnesia was found to increase the densification rate during sintering by a factor of 3 through a raising of the diffusion rate. The grain-size dependence of the densification rate indicated control primarily by grain-boundary diffusion. Magnesia also increased the grain growth rate during sintering by a factor of 2.5. The dependence of the grain growth rate on density and grain size suggested a mechanism of surface-diffusion-controlled pore drag. It was argued, therefore, that MgO enhanced grain growth by raising the surface diffusion coefficient. The effect of MgO on the densification rate/grain growth rate ratio was, therefore, found to be minimal and, consequently, MgO did not have a significant effect on the grain size/density trajectory during sintering. The role of MgO in the sintering of alumina was attributed mainly to its ability to lower the grain-boundary mobility.

Effect of MgO Solute on Microstructure Development in Al2O3

The grain-growth behavior of Al{sub 2}O{sub 3} compacts with small contents ({le}10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO{sub 2}, or CaO + TiO{sub 2}. The platelike grains, defined as the abnormal grains larger than 100 {mu}m with an aspect ratio {ge}5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na{sub 2}O + SiO{sub 2}, CaO + SiO{sub 2}, SrO + SiO{sub 2}, or BaO + SiO{sub 2}. However, an addition of MgO to the Al{sub 2},O{sub 3} doped with CaO + SiO{sub 2} resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO{sub 2} doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains is explained by an increased interfacial reaction rate due to the codopants.

Origin and growth kinetics of platelike abnormal grains in liquid-phase-sintered alumina

Additive and impurity distributions at grain boundaries in sintered Al2O3 were determined using a scanning Auger microprobe, transmission electron microscopy, metallography, and electron probe analysis. The results indicated that Ca was present uniformly at the boundaries as a segregant, whereas the Mg or Ni levels observed resulted from nonequilibrium spinel particles. The Ca concentrations were consistent with equilibrium segregation theory and followed an equilibrium segregation isotherm.

Additive and Impurity Distributions at Grain Boundaries in Sintered Alumina

Auger electron spectroscopy was used to characterize grain-boundary chemistry in MgO, Al2O3, and MgAl2O4 (spinel) hot-pressed with LiF or NaF. The presence of additives (F, Na) at the grain boundaries was confirmed; observation of grain-boundary impurities in MgO extends previous studies.

D. F. Stein

Papers

Additive and Impurity Distributions at Grain Boundaries in Sintered Alumina

Analysis of Grain‐Boundary Impurities and Fluoride Additives in Hot‐Pressed Oxides by Auger Electron Spectroscopy

Additive and impurity distributions at grain boundaries in sintered alumina

Analysis of grain‐boundary impurities and fluoride additives in hot‐pressed oxides by auger electron spectroscopy