Structural and interfacial features on macro- & micro-scales and at the atomic level affect mechanical behavior of ceramics. Knowledge of mechanisms plus theoretical modeling and systematic experimental studies are combined in the development of design concepts for advance ceramics & composites.

Office of Basic Energy Sciences
Division of Materials Sciences Programs

Atomic-Scale Microstructural Design of Advanced Ceramics

P. F. Becher, Task Leader

Theory and experiment combine to define (1) the relationships between the properties of ceramics and critical length-scale characteristics and (2) how to tailor these during processing. For example, the effects of sintering additives on the microstructure and interfacial properties are assessed experimentally. The atomic scale segregation of elements and interfacial debonding characteristics are assessed via advanced electron microscopy methods. These are combined with first principles theoretical calculations of the energies and forces associated with chemical bonding to determine the effects of chemical additives in regions of interest (e.g., interfaces) as well as the mechanisms controlling microstructure evolution. The nature of interfacial regions can then be incorporated into classical analyses to understand the effects of nano-scale interfaces on larger scale characteristics of ceramic monoliths and coatings. The combined theoretical and experimental findings provide a quantitative picture of the mechanisms controlling the behavior of ceramic-based systems and an understanding as to how to enhance their properties including fracture and/or deformation resistance.

• Microstructural Evolution: Segregation Effects
• Impact of Intergranular Phase on Mechanical Behavior at Elevated Temperatures 
• Deformation of Nanocrystalline Ceramics
• Tough Microstructures: Unraveling the Basis for Anisotropic Grain Growth in Si₃N₄ Ceramics
• Understanding How the Chemistry of Intergranular Films in Ceramics can be Used to Increase Fracture Toughness
• Contact Induced Radial Cracking in Layered Systems
• Piezospectroscopic Measurements of Toughness in SiC Whisker/Al₂O₃ Composites
• Multiple Cracking of Film on Substrate
• Effective Viscosity of Suspension of Spheres
• Hertzian Indentation on Coating/Substrate Systems
• Residual Stresses in Multilayers and Graded layers 
• Observation of Superionic Conductivity in Highly Oriented Nanometer Scale ZrO2 - 10 mol. % Y₂O₃ Films

• Future Program Directions
• Publications (2003-2005)

Microstructural Evolution: Segregation Effects

Attempts to address the mechanisms by which rare earths alter the anisotropy of grain growth, which is key to the introduction, or lack, of microscopic elongated reinforcing grains, have suggested that the c-axis growth rate is diffusion-controlled while that along the a-axis is slower, as it is attachment limited. Attachment limited growth is associated with a dearth of surface steps and recent observations reveal that the prismatic surfaces of the grains are indeed atomically smooth. This surface morphology, while necessary, is not sufficient to explain why certain rare earths are more effective in promoting anisotropic grain growth. Attempts at explaining this based on size of the rare earth ions or their cationic field strength are erroneous and inconsistent with the experimental observations. The present theoretical studies developed the concept of the driving forces that are imposed on competing cations within a chemical gradient. The differential binding energy (DBE) defines the energy change of a cation (e.g., La, Gd, Lu) between environments defining the gradient (e.g., in N-rich compared with O-rich regions), referenced with respect to that of the majority cation (e.g., Si in silicon nitride ceramics, where Si3N4 crystallites are surrounded by the amorphous silica-rich phase). First-principles local density calculations then defined the specific adsorption properties of the rare earth adsorbed at the nitride interface. Atomic resolution STEM observations confirm the predicted segregation-adsorption behaviors for the rare earths. The theoretical models now reveal the actual mechanisms responsible for the observed rare earth effects on grain growth anisotropy.

Effects of Segregation on Microstructural Evolution

The DBE results reveal that the differences in affinities of the rare earths (RE's) for the nitrogen-terminated Si3N4 grain surfaces also controls the density of filled adsorption sites on the prismatic plane surfaces of the grains. Combined with calculations of the RE binding energies, the site filling on the prismatic plane by the rare earths is found to be distinctly different (e.g., La > Gd > Lu), which is confirmed by the atomic resolution scanning transmission electron microscopy (STEM) images.

It is this variability in surface decoration and adsorbate binding for this series of rare earths that impacts various phenomena. Those RE's with increasing tendency to fill the adsorption sites on the prismatic grain surfaces more effectively impede the attachment of Si, a necessary step for diametrical grain growth. This would lead to greater grain shape anisotropy, which is consistent with the observed effects of RE's in silicon nitride ceramics. The result is a new fundamental understanding for the influence of RE's, as well as other elements, on the microstructural evolution in these ceramics. Note that previous studies had simply alluded to cation size effects as the source of this behavior.

The importance of these findings is seen in the fact that the elongated grains can serve to reinforce the ceramic, which results in much tougher, stronger materials. However during fracture, such reinforcing grains must at least partially separate/debond from the surrounding matrix, else the failure producing crack will cut through them and eliminate any toughening effects. Our results show this debond process can also be altered by RE segregation and binding strength at the Si3N4 grain surfaces.

Collaborators: G. Painter, K. van Benthem, S. Pennycook (ORNL); N. Shibata, Y. Ikuhara (U of Tokyo); M. Hoffmann, R. Satet (U. of Karlsruhe); F. Averill (U of Tennessee)

Impact of Intergranular Phase on Mechanical Behavior at Elevated Temperatures 

Deformation of Nanocrystalline Ceramics

 

Tough Microstructures: Unraveling the Basis for Anisotropic Grain Growth in Si₃N₄ Ceramics

Based on our recent studies, the differences in grain growth behavior of the rare earths (and likely other dopants) corresponds to a combination of (1) their preference for residing at or near the grain surfaces, (2) their observed adsorption at available surface sites, and (3) the strength of the adsorption bonds as related to desorption of the rare earth as predicted by atomic cluster calculations. Such overall behaviour ­ segregation, adsorption and desorption ­ provides a fundamental understanding of how various dopants can alter the anisotropic grain growth in Si₃N₄ ceramics where formation of elongated grains is one facet for achieving high toughness.

A combination of Z-contrast STEM observations with first-principles calculations reveals the atomistic origins of dopant-controlled grain growth behaviour in Si₃N₄ ceramics. Both the density of occupied dopant attachment sites on the grain's prism surface and the strength of the resultant bond depends on the rare earth species and correlates directly with the impediment of diametric grain growth. Thus, the chemical preference of the dopant for adsorption on the prismatic grain surface determines its effect on the anisotropic grain growth in Si₃N₄ ceramics. These results now identify guidelines to control ceramic toughening at the atomic-scale by selective choice of dopant additions using the theoretical analysis as opposed to empirical methods. These findings and newly established grain growth models also should help understanding the dopant controlled microstructure evolution in a wide range of liquid phase sintered materials.

Understanding How the Chemistry of Intergranular Films in Ceramics can be Used to Increase Fracture Toughness

Contact Induced Radial Cracking in Layered Systems

 

Piezospectroscopic Measurements of Toughness in SiC Whisker/Al₂O₃ Composites

 

Multiple Cracking of Film on Substrate

Effective Viscosity of Suspension of Spheres

 

Hertzian Indentation on Coating/Substrate Systems

 

Residual Stresses in Multilayers and Graded layers

 

Observation of Superionic Conductivity in Highly Oriented Nanometer Scale ZrO₂ - 10 mol. % Y₂O₃ Films

Future Program Directions

Related Links:

• OTT/HVPSM
• OPT/Microturbine