Kaiyang Zeng

Extracting the mechanical properties of a viscoelastic polymeric film on a hard elastic substrate

A semi-analytical approach was developed to study the creep response of a viscoelastic polymeric thin film on a hard elastic substrate under flat-ended punch indentation. This approach allows one to separate mechanical properties of the film from its substrate. To verify the approach, flat-ended punch creep indentation tests were performed to obtain the long-time viscoelastic behaviors of a polymethyl-methacrylide film overlying on an aluminum substrate. Three viscoelastic models describing the constitutive equations of the polymer film are compared in this paper. “Film-only” viscoelastic parameters were extracted by fitting with the experimental creep curves of flat-ended punch indentation. It is shown that the present approach provides an effective way to characterize the mechanical properties of a polymeric film on an elastic substrate.

Determining interfacial properties of submicron low-k films on Si substrate by using wedge indentation technique

This article presents studies on using a wedge indentation technique to determine interfacial adhesion properties of low-k dielectric films, namely, methyl-silsesquioxane (MSQ) and black diamond (BD™)films, both on a Si substrate. Interfacial crack initiation and propagation processes in the MSQ/Si system are studied by using focused-ion-beam sectioning of the indentation impressions created by wedge tips with 90° and 120° of inclusion angles, respectively. Furthermore, the indentation induced stress is found to be proportional to the ratio of the indentation volume and the interface delamination crack volume for both plane strain and nonplane strain cases. With this analysis, the interface toughness of the MSQ/Si and BD/Si system, in terms of the strain energy release rate, is determined. The interface toughness for the MSQ/Si system is found to be a value of 1.89±0.28J∕m2 for the 90° wedge tip indentation and 1.92±0.08J∕m2 for the 120° wedge tip indentation. In addition, using the 120° wedge tip, the in...

Understanding nature's residual strain engineering at the human dentine-enamel junction interface.

UNLABELLEDnHuman dental tissue is a hydrated biological mineral composite. In terms of volume and mass, a human tooth mainly consists of dentine and enamel. Human dental tissues have a hierarchical structure and versatile mechanical properties. The dentine enamel junction (DEJ) is an important biological interface that provides a durable bond between enamel and dentine that is a life-long success story: while intact and free from disease, this interface does not fail despite the harsh thermo-mechanical loading in the oral cavity. The underlying reasons for such remarkable strength and durability are still not fully clear from the structural and mechanical perspectives. One possibility is that, in an example of residual stress engineering, evolution has led to the formation of a layer of inelastic strain adjacent to the DEJ during odontogenesis (tooth formation). However, due to significant experimental and interpretational challenges, no meaningful quantification of residual stress in the vicinity of the DEJ at the appropriate spatial resolution has been reported to date. In this study, we applied a recently developed flexible and versatile method for measuring the residual elastic strain at (sub)micron-scale utilising focused ion beam (FIB) milling with digital image correlation (DIC). We report the results that span the transition from human dentine to enamel, and incorporate the material lying at and in the vicinity of the DEJ. The capability of observing the association between internal architecture and the residual elastic strain state at the micrometre scale is useful for understanding the remarkable performance of the DEJ and may help the creation of improved biomimetic materials for clinical and engineering applications.nnnSTATEMENT OF SIGNIFICANCEnWe studied the micron-scale residual stresses that exist within human teeth, between enamel (outer tooth shell, hardest substance in the human body) and dentine (soft bone-like vascularised tooth core). The dentine-enamel junction (DEJ) is an extremely interesting example of natures design in terms of hierarchical structuring and residual stress management. Key developments reported are systematic focused ion beam (FIB) milling and digital image correlation (DIC) micrometre scale residual strain evaluation, and the determination of principal strain direction near DEJ, correlated with internal architecture responsible for remarkable strength. This work helps understanding DEJ performance and improving biomimetic materials design for clinical and engineering applications.

Domain characterization of Pb(Zn1/3Nb2/3)O3−(6%–7%)PbTiO3 single crystals using scanning electron acoustic microscopy

Domain structures of [001]T and [011]T-cutu2002Pb(Zn1/3Nb2/3)O3−(6%−7%)PbTiO3 (PZN-PT) single crystals are studied using scanning electron acoustic microscope (SEAM) technique. The observation of the orientation of domain walls agree reasonably well with the trigonometric projection of rhombohedral and orthorhombic dipoles on the (001) and (011) surfaces, respectively. After mechanical loading with microindentation, domain switching is also observed to form a hyperbolic butterfly shape and extend preferentially along four diagonal directions, i.e., ⟨110⟩ on (001) surface and ⟨111¯⟩ on (011) surface. The critical shear stress to cause domain switching for PZN-PT crystal is estimated to be approximately 49 MPa for both {110} and {111¯} planes based on theoretical analysis. Generally, the SEAM technique has been successfully demonstrated to be a valid technique for observation of domain structures in single crystal PZN-PTs.

Ti Reactive Sintering of Electrically Conductive Al2O3–TiN Composite: Influence of Ti Particle Size and Morphology on Electrical and Mechanical Properties

In the current study, Al2O3–TiN composites were successfully fabricated with various particle sizes (10, 20, 30, and 50 μm) and concentrations (5, 10, 15, and 20 vol %) via a novel ball milling + Ti reactive sintering process. By applying the reactive sintering, Ti powders will transform into TiN particles, which act as mechanical reinforcements and electrical conductors in the Al2O3 matrix. The ball milling process alters the Ti powder morphology from a low-aspect-ratio sphere into a high-aspect-ratio disc, which reduces the electrical percolation threshold value from 29% to 15% in the current setup. However, such a threshold value is insensitive to the particle size. Meanwhile, the Ti particle size has a significant influence on the material’s mechanical properties. A small particle size results in less porosity and hence higher flexural strength of the composite.

Instability pathways of hydrogel microlenses under concentrated loadings

We investigate the mechanical instabilities of hydrogel microlenses triggered by concentrated loadings. Depending on hydrogel microlenses, geometries, and loading conditions, a series of nontrivial instability pathways is observed, including transitions of localized polyhedral faceted shapes with multifold vertices created by bifurcation, snapping through curvature inversion and large load drop, and wrinkling driven by combined in-plane stretching and compression. These pathways reveal the richness of the mechanical behavior of hydrogel microlenses and provide potential strategies to control the design and shape of complex hydrogel microlenses.

Nanoscale domains and preferred cracking planes in Pb(Zn1/3Nb2/3)O3–(6–7)% PbTiO3 single crystals studied by piezoresponse force microscopy and fractography

This paper presents recent studies on surface and cross-sectional domain structures of Pb(Zn1/3Nb2/3)O3–(6–7)%u2002PbTiO3 (PZN–PT) single crystals using piezoresponse force microscopy and three-point bending technique. The surface domain structures for the rhombohedral-based single crystals in (001) orientation are found to be influenced by polishing process, whereas the surface domains on the (011)-oriented single crystals are aligned along [011¯] direction, unaffected by the polishing process. On the other hand, the domain structures on the cross-sectional fracture surface show preferential alignment which agrees reasonably with the rhombohedral dipoles on the {100} and {110} planes. The differences between the surface and cross-sectional domain structures could be attributed to stress compensation between the surface strain effect and the minimization of elastic energy. In addition, both surface and cross-sectional surface demonstrate nanoscale domains, about 100–200 nm in size. Further fractography observ...

Structure-Function Correlative Microscopy of Peritubular and Intertubular Dentine

Peritubular dentine (PTD) and intertubular dentine (ITD) were investigated by 3D correlative Focused Ion Beam (FIB)-Scanning Electron Microscopy (SEM)-Energy Dispersive Spectroscopy (EDS) tomography, tapping mode Atomic Force Microscopy (AFM) and scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) mapping. The brighter appearance of PTD in 3D SEM-Backscattered-Electron (BSE) imaging mode and the corresponding higher grey value indicate a greater mineral concentration in PTD (~160) compared to ITD (~152). However, the 3D FIB-SEM-EDS reconstruction and high resolution, quantitative 2D map of the Ca/P ratio (~1.8) fail to distinguish between PTD and ITD. This has been further confirmed using nanoscale 2D AFM map, which clearly visualised biopolymers and hydroxyapatite (HAp) crystallites with larger mean crystallite size in ITD (32 ± 8 nm) than that in PTD (22 ± 3 nm). Correlative microscopy reveals that the principal difference between PTD and ITD arises primarily from the nanoscale packing density of the crystallites bonded together by thin biopolymer, with moderate contribution from the chemical composition difference. The structural difference results in the mechanical properties variation that is described by the parabolic stiffness-volume fraction correlation function introduced here. The obtained results benefit a microstructure-based mechano-chemical model to simulate the chemical etching process that can occur in human dental caries and some of its treatments.