Zirconia Bicrystals Research Articles

Role of grain boundaries in determining strength and plastic deformation of yttria-stabilized tetragonal zirconia bicrystals

Mechanical properties of yttria-stabilized tetragonal zirconia (YSTZ) bicrystals under compressive loading are investigated by atomistic simulations. Previous studies on deformation of single-crystal YSTZ showed that some specific orientations promote dislocation emission, tetragonal to monoclinic phase transformation, or both. In this work, nanograins with different orientations are selectively combined to generate bicrystals with various grain boundaries (GBs). Simulation results show that regardless of orientation of nanograins, the strength of YSTZ bicrystals is higher when the GB plane is parallel to the loading direction, and in the case of [011]/ $$ \left[ {01\bar{1}} \right] $$ -oriented YSTZ bicrystal, the strength even exceeds that of the single crystal. Independent plastic deformation of individual grains and their interactions at the GB plane are believed to be responsible for the observed increase in strength. GB plane inhibits the volume expansion of transformed monoclinic phase and therefore serves as a source of strengthening. In contrast, YSTZ bicrystal displays softer behavior when GB plane is perpendicular to the loading direction. GB plane acts as the source of softening by initiating local amorphous phase formation.

Dislocation structures and strain fields in [111] low-angle tilt grain boundaries in zirconia bicrystals

Structures of <111> low-angle tilt grain boundaries in yttria-stabilized cubic zirconia bicrystals were characterized by conventional transmission electron microscopy, high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. It is found that the 0.4 and 4.0 degrees tilt grain boundaries are composed of periodic arrays of edge dislocations with Burgers vectors . The experimentally estimated strain field of each dislocation in the 0.4 degrees tilt boundary was in good agreement with the theoretically predicted strain field from the Peierls-Nabarro model. On the other hand, the estimated strain field of each dislocation in the 4.0 degrees tilt boundary was clearly different from that in the 0.4 degrees tilt boundary, which suggests that the strain fields of neighbouring dislocations interact when the separation distance between dislocations is shorter than a critical value.

Oxygen diffusion blocking of single grain boundary in yttria-doped zirconia bicrystals

Yttrium-doped zirconia bicrystals with [001] symmetric tilt Σ 5 grain boundaries were fabricated by a diffusion bonding technique, and oxygen diffusion behavior across the grain boundary was measured by secondary ion mass spectrometry (SIMS), tracing 18O isotope. It was found that the 18O fraction across the boundary exhibited explicit decrease around the boundary plane, indicating that the oxygen-diffusion is blocked by the presence of the Σ 5 grain boundary. This is the first experimental detection of oxygen diffusion blocking at a single grain boundary in zirconia ceramics. From high-resolution transmission electron microscopy observations and energy dispersive X-ray spectroscopy analysis, neither amorphous layers nor Si impurity segregation were found at the grain boundary. The grain boundary blocking effect of the Σ 5 boundary must be an intrinsic feature arising from its core structure and yttrium solute segregation of the grain boundary.

Direct Evidence of Dopant‐Enhanced Grain‐Boundary Sliding in Yttria‐Stabilized Zirconia Bicrystals

Compression tests were conducted at 1400°C in air for undoped and Si‐doped yttria‐stabilized zirconia (YSZ) bicrystals with the same orientation relationship (Σ=5, [001]/{210} grain boundary). It was found that the macroscopic grain‐boundary slidings are observed during the deformation in both undoped and doped bicrystals, and the sliding displacements increase almost linearly with increasing total displacements. It is distinctly demonstrated that the Si‐doped bicrystal exhibits a sliding displacement that is a few times larger than the undoped bicrystal. In addition, the total strain of the Si‐doped bicrystal at a failure is much larger than that of the undoped bicrystal. It can be stated that Si doping in YSZ not only enhances the grain‐boundary sliding but also suppresses its failure.

Systematic study of grain boundary atomistic structures and related properties in cubic zirconia bicrystals

Abstract Systematic grain boundary study of cubic zirconia has been conducted by using bicrystals. It is clearly demonstrated that grain boundary atomistic structures dramatically change according to the misorientations and plane orientations of the boundaries, resulting in a dramatic change of excess energies and solute segregation behaviors. Combining with theoretical calculations, it is found that grain boundaries possess unique coordination-deficient cation sites at the cores, and their densities have a clear correlation with these properties in high-angle grain boundaries. This result indicates that grain boundary properties in ceramics are possibly determined by the accumulation of coordination-deficient sites. Thus, systematic grain boundary study using bicrystal offers fundamental understandings of the relationship between atomistic structures and properties in ceramic grain boundaries.

Structure, energy and solute segregation behaviour of [110] symmetric tilt grain boundaries in yttria-stabilized cubic zirconia

Yttria-stabilized cubic zirconia bicrystals with [110] symmetric tilt grain boundaries are systematically fabricated by the diffusion bonding method. It is revealed that the grain-boundary atomistic structures, excess energies and solute segregation behaviours are strongly dependent on the macroscopic geometries of the boundaries. High-resolution transmission electron microscopy combined with lattice statics calculations suggests that the grain-boundary structures are characterized by the accumulation of coordination-deficient cation sites at their cores, whose densities have a clear correlation with excess energies and amounts of solute segregation. The orientation dependence of grain-boundary properties in cubic zirconia can thus be linked and understood via local grain-boundary atomistic structures with the characteristic miscoordinated cation sites.

High-temperature grain boundary sliding behavior and grain boundary energy in cubic zirconia bicrystals

Misorientation dependence of grain boundary energy and grain boundary sliding at high temperature were examined in cubic zirconia bicrystals with [1 1 0] symmetric tilt boundaries, which were fabricated by diffusion bonding method from two cubic zirconia single crystals. High-resolution transmission electron microscopy observation revealed that the grain boundary in cubic zirconia bicrystals was clean and atomically sharp without any void or grain boundary amorphous layer. Grain boundary energy of the tilt boundaries was estimated from the dihedral angles on thermal grooved surface measured with atomic force microscope techniques. The misorientation dependence of the grain boundary energy in cubic zirconia bicrystals shows similar tendency to that of fcc metal such as aluminum and copper. Grain boundary sliding associated with intragranular dislocation slip in cubic zirconia bicrystals was observed for all specimens. The amount of the grain boundary sliding showed a good correlation with the misorientation factor of each boundary. Grain boundary migration also took place accompanying with the grain boundary sliding. The observed grain boundary sliding and migration can be explained based on a dislocation mechanism for sliding which is based on the movement of lattice dislocations along the grain boundary by a combination of climb and glide.

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.

Structure of [110] tilt grain boundaries in zirconia bicrystals.

Cubic stabilized zirconia bicrystals with [110] symmetric tilt grain boundaries were fabricated by diffusion bonding of two single crystals with the composition of ZrO2-9.6mol%Y2O3. The structures of symmetric tilt small angle grain boundary and two types of symmetric tilt sigma3 grain boundaries with different grain boundary planes were observed by transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HREM) observations clarified that the [110] small angle tilt grain boundary consists of periodic array of b = a/2[110] type edge dislocations. This result is consistent with Frank's dislocation model for small angle grain boundary. HREM observation also revealed that the 70.5 degrees sigma3 grain boundary shows atomically coherent grain boundary structure with the boundary plane of [111], while the 109.5 degrees sigma3 grain boundary accompanies grain boundary facets taking [111]/[115] asymmetric grain boundary plane. Because of the very low surface energy of [111] plane and/or high lattice matching of [111] and [115] type planes, the grain boundary faceting may be preferred in spite of increasing grain boundary area to about 6%. TEM-energy-dispersive X-ray spectroscopy (EDS) analyses were performed on both sigma3 grain boundaries, and the segregation of yttrium ions to the boundaries was detected in both cases. The amount of segregation is about the same in both sigma3 boundaries. It can be concluded that the segregation of yttrium ions to sigma3 grain boundary exists in cubic zirconia.

Shunted bicrystal Josephson junctions arrays for voltage standards

Series arrays of 365 shunted YBa2Cu3O7−x Josephson junctions on yttria-stabilized zirconia bicrystals were investigated under microwave irradiation in the K-band. Sub-arrays of up to 39 junctions were synchronized to the external microwave power at a temperature of 15 K. Although there was a considerable spread in the critical currents of the junctions and in the microwave bias current distribution, a first Shapiro step with a height of 30% of the total critical current of the sub-array was obtained by using an external shunt resistor, thus reducing the spread in normal resistances to 5%. Such high-Tc cryoelectronic circuits are applicable for voltage standards and precision digital-analog converters.