Gayle S. Painter

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Debonding behavior between β-Si3N4 whiskers and oxynitride glasses with or without an epitaxial β-SiAlON interfacial layer

Abstract In order to gain insight on the influence of intergranular glass on the fracture toughness of silicon nitride, the debonding behavior of the interface between the prismatic faces of β-Si3N4 whiskers and oxynitride glasses was investigated in model systems based on various Si–(Al)–Y(Ln)–O–N (Ln: rare-earth) oxynitride glasses. It was found that while the interfacial debonding strength increased when an epitaxial β′-SiAlON layer grew on the β-Si3N4 whiskers, the critical angle for debonding was lowered with increasing Al and O concentrations in the SiAlON layer. Only in the absence of a SiAlON epitaxial layer, were debonding conditions altered by residual stresses imposed on the interface due to thermal–mechanical mismatch. A possible explanation for the effect of SiAlON formation and its composition on the debonding behavior is suggested by first-principles atomic cluster calculations. It is concluded that by tailoring the densification additives and hence the chemistry of the intergranular glass, it is possible to improve the fracture resistance of silicon nitride.

The importance of amorphous intergranular films in self-reinforced Si3N4 ceramics

Abstract High-fracture-strength and high-toughness β-Si 3 N 4 ceramics can be obtained by tailoring the size and number of the elongated bridging grains. However, these bridging mechanisms rely on debonding of the reinforcing grains from the matrix to increase toughness. Interfacial debonding is shown to be influenced by sintering aids incorporated in the amorphous intergranular films. In one case, the interface strength between the intergranular glass and the reinforcing grains increases with the aluminum and oxygen content of an interfacial epitaxial β-SiAlON layer. In another, the incorporation of fluorine in the intergranular film allows the crack to circumvent the grains. Atomic cluster calculations reveal that these two debonding processes are related to (1) strong Si–O and Al–O bonding across the glass/crystalline interface with an epitaxial SiAlON layer and (2) a weakening of the amorphous network of the intergranular film when difluorine substitutes for bridging oxygen.

Atomic ordering at an amorphous/crystal interface

In this study, the authors report atomic-resolution images that illustrate the transition from a crystalline Si3N4 grain across the interface into an amorphous Lu–Si–Mg–N–O glassy phase. The interface is not atomically abrupt, but is comprised of sub-nanometer-scale ordered regions that resemble a LuN-like structure. These ordered clusters bind to the prismatic surface of the Si3N4 grains at specific low energy positions for Lu adsorption as predicted by first-principles calculations. The ordered regions are filamentary in nature, extending for at least two atomic layers into the amorphous pockets at multigrain junctions before disappearing.

Experimental probe of adsorbate binding energies at internal crystalline/amorphous interfaces in Gd-doped Si3N4

Electron beam irradiation during scanning transmission electron microscopy has been used to probe the relative abundance and stabilities of gadolinium adsorption sites in polycrystalline silicon nitride ceramics. Site-specific binding strengths in the interface plane between β-Si3N4 grains and the adjacent amorphous triple pockets were found to be consistent with theoretical predictions. Decreasing stability was found for Gd within partially ordered planes further from the interface. Atomic level characterization such as that reported here provides detailed insights that will allow one to tailor new functional ceramic microstructures with improved macroscopic mechanical properties.

First-principles atomic cluster study of boron interactions in Ni3Al

First-principles atomic cluster calculations have been carried out in the local density approximation to understand the segregation behavior and strengthening effects of boron in Ni3Al. The binding energy of boron is calculated in lattice fragment clusters representing the perfect crystal, as well as various defect sites. The agreement between trends in energetics determined for small clusters and periodic supercells indicates the dominant role of boron’s interaction with nearest-neighbors of the host. The stereochemical factor underlying boron’s preferential bonding to nickel atoms in four-fold planar coordination (i.e., sp3 hybridization) suggests a mechanism for the boron-effect in Ni3Al: increased cohesion provides a driving force for B segregation to open sites, such as at Ni-enriched grain boundary sites, and the strengthening is a result of strong localized Ni–B covalent bond formation.

Macro- to Atomic-Scale Tailoring of Si3N4 Ceramics to Enhance Properties

Silicon nitride ceramics are finding uses in numerous engineering applications because of their tendency to form whisker-like microstructures that can overcome the inherent brittle nature of ceramics. Studies now establish the underlying microscopic and atomic-scale principles for engineering a tough, strong ceramic. The theoretical predictions are confirmed by macroscopic observations and atomic level characterization of preferential segregation at the interfaces between the grains and the continuous nanometer thick amorphous intergranular film (IGF). Two interrelated factors must be controlled for this to occur including the generation of the elongated reinforcing grains during sintering and debonding of the interfaces between the reinforcing grains and the matrix. The reinforcing grains can be controlled by (1) seeding with beta particles and (2) the chemistry of the additives, which also can influence the interfacial debonding conditions. In addition to modifying the morphology of the reinforcing grains, it now appears that the combination of preferential segregation and strong bonding of the additives (e.g., the rare earths, RE) to the prism planes can also result in sufficiently weakens the bond of the interface with the IGF to promote debonding. Thus atomic-scale engineering may allow us to gain further enhancements in fracture properties. This new knowledge will enable true atomic-level engineering to be joined with microscale tailoring to develop the advanced ceramics that will be required for more efficient engines, new electronic device architectures and composites.

Embedded-cluster self-consistent partial-wave method: Extending the spatial scale of electronic structure calculations

An efficient approach to extending the spatial scale of electronic structure calculations is described in this work. The method is formulated as a combination of the interacting fragments concept of Harris [J. Harris, Phys. Rev. B 31, 1770 (1985)] and the D&C method of Yang [W. Yang, Phys. Rev. Lett. 66, 1438 (1991)], which recognizes the intrinsic locality of electron bonding and is devised to optimize the total electron charge density within an approximate representation of partitioned components. Beginning with a brief review of D&C concepts, we report results from this new method using the D&C as an embedding method for coupling an atomic cluster to its extended environment. The convergence properties as implemented within the self-consistent partial wave linear variational method (SCPW) are illustrated through various applications. In particular, results from a study of the adsorption of La atoms at the prism plane of -Si3N4 demonstrate the practicality of the SCPW using D&C as an embedding technique. PACS numbers: 71.15.Mb, 31.15.Ew, 31.50.Bc

First-principles study of rare earth adsorption at {beta}-Si{sub 3}N{sub 4} interfaces

Structural characterization of rare earth adsorption at surfaces or interfaces of {beta}-Si{sub 3}N{sub 4} grains within silicon nitride ceramics has recently been reported by three different groups using Z-contrast scanning transmission electron microscopy (STEM) imaging. Here we report the electronic structure basis for these observations and discuss the origin of similarities and differences among the lanthanides characterized in that work. Along with the features that are well described by a first-principles cluster and surface slab models, we identify those differences in the experiment and theory that warrant further investigation. Stereochemical bonding factors are found to determine adsorption site preferences as opposed to ionic size effects. The set of possible bond sites is a characteristic of the {beta}-Si{sub 3}N{sub 4} interface; however the strength of the rare earth-interface bonding is determined by the electronic structure of the nitride surface and the specific adsorbate. This is the principal factor controlling the effects of dopants on the {alpha}{yields}{beta} phase transformation and on the {beta}-Si{sub 3}N{sub 4} grain growth at high temperature as well as the subsequent microstructure of the ceramic.

Tailoring the Composition of Self-Reinforced Silicon Nitride Ceramics to Enhance Mechanical Behavior

Studies have shown that seeding allows for greater control of the microstructure of self-reinforced beta-silicon nitride based ceramics, which when combined with tailoring of the sintering additives can results in significant improvements in fracture toughness and strength. Similar behavior was noted in alpha-SiAlONs. In beta-Si3N4 ceramics in which alumina serves as one of the additives, the larger elongated reinforcing grains result from the epitaxial deposition of Si6-zAlzOzN8-z layers on the Si3N4 core. Structural models show that the bond strength of the interface between the SiAlON and the amorphous intergranular film increases as the Al and O contents of the SiAlON increase. This is consistent with experiments that revealed interfacial debonding was promoted when the z-value of the SiAlON epitaxial layer was reduced. Compositional tailoring that leads to enhanced interfacial debonding and an increase in the fracture toughness.