Interface energy and its influence on interface fracture between metal and ceramic thin films in nanoscale

A theoretical model about the size-dependent interface energy between two thin films with different materials is developed by considering the chemical bonding contribution based on the thermodynamic expressions and the structure strain contribution based on the mechanical characteristics. The interface energy decreases with reducing thickness of thin films, and is determined by such available thermodynamic and mechanical parameters as the melting entropy, the melting enthalpy, the shear modulus of two materials, etc. The predicted interface energies of some metal/MgO and metal/Al2O3 interfaces based on the model are consistent with the results based on the molecular mechanics calculation. Furthermore, the interface fracture properties of Ag/MgO and Ni/Al2O3 based on the atomistic simulation are further compared with each other. The fracture strength and the toughness of the interface with the smaller structure interface energy are both found to be lower. The intrinsic relations among the interface energy, the interface strength, and the fracture toughness are discussed by introducing the related interface potential and the interface stress. The microscopic interface fracture toughness is found to equal the structure interface energy in nanoscale, and the microscopic fracture strength is proportional to the fracture toughness. (C) 2010 American Institute of Physics. [doi:10.1063/1.3501090]

First principles based predictions of the toughness of a metal/oxide interface

We describe a first-principles-based strategy to predict the macroscopic toughness of a gamma-Ni(Al)/alpha-Al2O3 interface. Density functional theory calculations are used to ascertain energy changes upon displacing the two materials adjacent to the interface, with relaxation conducted over all atoms located within adjoining rows. Traction/displacernent curves are obtained from derivatives of the energy. Calculations are performed in mode I (opening), mode II (shear) and at a phase angle of 45 degrees. The shear calculations are conducted for displacements along < 110 > and < 112 > of the Ni lattice. A generalized interface potential function is used to characterize the results. Initial fitting to both the shear and normal stress results is required to calibrate the unknowns. Thereafter, consistency is established by using the potential to predict other traction quantities. The potential is incorporated as a traction/displacement function within a cohesive zone model and used to predict the steady-state toughness of the interface. For this purpose, the plasticity of the Ni alloy must be known, including the plasticity length scale. Measurements obtained for a gamma-Ni superalloy are used and the toughness predicted over the full range of mode mixity. Additional results for a range of alloys are used to demonstrate the influences of yield strength and length scale.

Nano-size Effect of Interface Energy and Its Effect on Interface Fracture

An analytical model about size-dependent interface energy of metal/ceramic interfaces in nanoscale is developed by introducing both the chemical energy and the structure stain energy contributions. The dependence of interface energy on the interface thickness is determined by the melting enthalpy, the molar volume, and the shear modulus of two materials composing the interfaces, etc. The analytic prediction of the interface energy and the atomic scale simulation of the interface fracture strength are compared with each other for Ag/MgO and Ni/Al2O3 interfaces, the fracture strength of the interface with the lower chemical interface energy is found to be larger. The potential of Ag/MgO interface related to the interface energy is calculated, and the interface stress and the interface fracture strength are estimated further. The effect of the interface energy on the interface strength and the behind mechanism are discussed.

First principles based predictions of the toughness of a metal/oxide interface

We describe a first-principles-based strategy to predict the macroscopic toughness of a gamma-Ni(Al)/alpha-Al2O3 interface. Density functional theory calculations are used to ascertain energy changes upon displacing the two materials adjacent to the interface, with relaxation conducted over all atoms located within adjoining rows. Traction/displacernent curves are obtained from derivatives of the energy. Calculations are performed in mode I (opening), mode II (shear) and at a phase angle of 45 degrees. The shear calculations are conducted for displacements along < 110 > and < 112 > of the Ni lattice. A generalized interface potential function is used to characterize the results. Initial fitting to both the shear and normal stress results is required to calibrate the unknowns. Thereafter, consistency is established by using the potential to predict other traction quantities. The potential is incorporated as a traction/displacement function within a cohesive zone model and used to predict the steady-state toughness of the interface. For this purpose, the plasticity of the Ni alloy must be known, including the plasticity length scale. Measurements obtained for a gamma-Ni superalloy are used and the toughness predicted over the full range of mode mixity. Additional results for a range of alloys are used to demonstrate the influences of yield strength and length scale.

Coupling of thermocapillary convection and evaporation effect in a liquid layer when the evaporating interface is open to air

The coupling mechanism of thermocapillary convection and evaporation effect in evaporating liquids was studied experimentally. The experiments were carried out to study a thin evaporating liquid layer in a rectangular test cell when the upper surface was open to air. By altering the imposed horizontal temperature differences and heights of liquid layers, the average evaporating rate and interfacial temperature profiles were measured. The flow fields were also visualized by PIV method. For comparison, the experiments were repeated by use of another two non-evaporating liquids to study the influence of evaporation effect. The results reveal evidently the role that evaporation effect plays in the coupling with thermocapillary convection.

Interface delamination of the thermal barrier coating subjected to local heating

To investigate the possible failure modes of the thermal barrier coating (TBC) used to protect the scramjet combustion chamber, the local heating via laser beam irradiation was utilized to simulate the service condition of high thermal flux and high temperature gradient. Firstly, the experimental method and process were described and the typical fracture morphology of the TBC under test were provided. Then, the theoretical and finite element modeling were carried out to study the temperature, deformation and stresses of the specimen when the top ceramic coat was subjected to local heating, and to demonstrate the mechanism on the failure of the TBC. It is revealed that the interface delamination shall appear and ultimately lead to the failure of the TBC under such thermal loading of local quick heating. According to the outcome of this study, the driving force of the interface delamination is influenced greatly by the key structural parameters and performance matching. Moreover, by utilizing the rules of the effects of these parameters on the fracture driving force, there is some possibility for the designer to optimize the performances of the TBC.

Enhanced nucleation of Ag nanoparticles by vacancy-type defects in Ar-ions implanted spinel

Vacancy-type defects are introduced into magnesium aluminate spine] (MgAl2O4 (1 1 0)) by Ar-ions implantation, and then Ag-ions are implanted into the depth rich in vacancy-type defects. The ultraviolet-visible spectrometry (UV-VIS) and positron annihilation spectroscopy (PAS) are used to study the influence of vacancy-type defects on nucleation of Ag nanoparticles. After introduction of vacancy-type defects the pronounced increase of surface plasmon resonance (SPR) absorbance intensity indicates that defects enhance the nucleation of Ag nanoparticles. The PAS results reveal that vacancy-type defects provide pre-nucleating centers for Ag nanoparticles nucleation and growth. (C) 2010 Elsevier B.V. All rights reserved.