Zonghan Xie

On the critical parameters that regulate the deformation behaviour of tooth enamel

Tooth enamel is the hardest tissue in the human body with a complex hierarchical structure. Enamel hypomineralisation--a developmental defect--has been reported to cause a marked reduction in the mechanical properties of enamel and loss of dental function. We discover a distinctive difference in the inelastic deformation mechanism between sound and hypomineralised enamels that is apparently controlled by microstructural variation. For sound enamel, when subjected to mechanical forces the controlling deformation mechanism was distributed shearing within nanometre thick protein layer between its constituent mineral crystals; whereas for hypomineralised enamel microcracking and subsequent crack growth were more evident in its less densely packed microstructure. We develop a mechanical model that not only identifies the critical parameters, i.e., the thickness and shear properties of enamels, that regulate the mechanical behaviour of enamel, but also explains the degradation of hypomineralised enamel as manifested by its lower resistance to deformation and propensity for catastrophic failure. With support of experimental data, we conclude that for sound enamel an optimal microstructure has been developed that endows enamel with remarkable structural integrity for durable mechanical function.

Effect of microstructure upon elastic behaviour of human tooth enamel

Tooth enamel is the stiffest tissue in the human body with a well-organized microstructure. Developmental diseases, such as enamel hypomineralisation, have been reported to cause marked reduction in the elastic modulus of enamel and consequently impair dental function. We produce evidence, using site-specific transmission electron microscopy (TEM), of difference in microstructure between sound and hypomineralised enamel. Built upon that, we develop a mechanical model to explore the relationship of the elastic modulus of the mineral-protein composite structure of enamel with the thickness of protein layers and the direction of mechanical loading. We conclude that when subject to complex mechanical loading conditions, sound enamel exhibits consistently high stiffness, which is essential for dental function. A marked decrease in stiffness of hypomineralised enamel is caused primarily by an increase in the thickness of protein layers between apatite crystals and to a lesser extent by an increase in the effective crystal orientation angle.

Transmission electron microscope characterisation of molar-incisor-hypomineralisation

Molar-incisor-hypomineralisation (MIH), one of the major developmental defects in dental enamel, is presenting challenge to clinicians due, in part, to the limited understanding of microstructural changes in affected teeth. Difficulties in the preparation of site-specific transmission electron microscope (TEM) specimens are partly responsible for this deficit. In this study, a dual-beam field emission scanning electron microscope (FESEM)/focused ion beam (FIB) milling instrument was used to prepare electron transparent specimens of sound and hypomineralised enamel. Microstructural analysis revealed that the hypomineralised areas in enamel were associated with marked changes in microstructure; loosely packed apatite crystals within prisms and wider sheath regions were identified. Microstructural changes appear to occur during enamel maturation and may be responsible for the dramatic reduction in mechanical properties of the affected regions. An enhanced knowledge of the degradation of structural integrity in hypomineralised enamel could shed light on more appropriate management strategies for these developmental defects.

Structural Integrity of Enamel: Experimental and Modeling

Tooth enamel is the hardest tissue in the human body and is directly responsible for dental function. Due to its non-regenerative nature, enamel is unable to heal and repair itself biologically after damage. We hypothesized that with its unique microstructure, enamel possesses excellent resistance to contact-induced damage, regardless of loading direction. By combining instrumented indentation tests with microstructural analysis, we report that enamel can absorb indentation energy through shear deformation within its protein layers between apatite crystallites. Moreover, a near-isotropic inelastic response was observed when we analyzed indentation data in directions either perpendicular or parallel to the path of enamel prisms. An “effective” crystal orientation angle, 33°–34°, was derived for enamel microstructure, independent of the loading direction. These findings will help guide the design of the nanostructural architecture of dental restorative materials.

Nature of contact deformation of TiN films on steel

Nanoindentation experiments were carried out on a columnar ∼1.5-m-thick TiN film on steel using a conical indenter with a 5-m tip radius. Microstructural examination of the contact zone indicates that after initial elastic deformation, the deformation mechanism of the TiN is dominated by shear fracture at inter-columnar grain boundaries of the TiN film. A simple model is proposed whereby the applied load is partitioned between a deforming TiN annulus and a central expanding cavit yi n the steel substrate. It is possible to obtain a good fit to the experimental load–displacement curves with only one adjustable parameter, namely the inter-columnar shear fracture stress of the TiN film. The implication of results in the context of the performance of TiN films in service is also discussed.

Contact damage evolution in diamondlike carbon coatings on ductile substrates

Rajnish K. Singha, M.T. Tilbrook, Z.H. Xie, A. Bendavid, P.J. Martin, P. Munroe and M. Hoffman

Role of microstructure in the grinding and polishing of α-sialon ceramics

The grinding and polishing behaviour of three tailored α-sialon microstructures made of the same chemical composition: fine-grained, bimodal and large elongated-grained, were determined. The fine-grained microstructure was characterised by extensive lateral cracks in the grinding process and widespread grain pullout during the mechanical polishing; in contrast, the large elongated-grained microstructure exhibited high grinding damage resistance, and a defect-free, mirror-like surface after mechanical polishing. The bimodal microstructure showed an intermediate behaviour during both processes. The material removal mechanisms during grinding and polishing were analysed and modelled as a function of grain size and aspect ratio.

Characterization of surface contact-induced fracture in ceramics using a focused ion beam miller

Focused ion beam (FIB) milling and imaging are powerful techniques for evaluation of surface contact-induced crack structures and the effect of microstructure on crack growth in ceramics. Two distinct α-sialon microstructures made from the same composition were tested under indentation, scratching and grinding conditions. Following each test, the FIB was used to analyze fracture events in both the surface and subsurface, and reveal the factors that control material removal during surface contact.

Electrochemical behavior of (Ti1-xNbx) 5Si3 nanocrystalline films in simulated physiological media

In this paper, (Ti1-xNbx)5Si3 nanocrystalline films were synthesized and their potential as highly corrosion-resistant coatings for the biomedical alloy Ti-6Al-4V was explored. To assess the electrochemical behavior of the as-deposited films, the samples were immersed in Ringers solution open to air at 37°C. The processes that govern the electrochemical reactions at the film surface were analyzed using a combination of complementary electrochemical measurement techniques such as potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. The results show that the (Ti1-xNbx)5Si3 nanocrystalline films offer Ti-6Al-4V a strong shield from corrosive attack and the addition of Nb in the films greatly enhances their resistance to corrosion, and in so doing, minimizes metal ion release. Collectively, our data suggest that (Ti1-xNbx)5Si3 nanocrystalline films as supreme coatings with anti-corrosive properties have potential to improve the performance and extend the service life of orthopedic and cochlear implants.

A critical role for Al in regulating the corrosion resistance of nanocrystalline Mo(Si1−xAlx)2 films

Novel nanocrystalline Mo(Si1−xAlx)2 films, with differing Al contents were synthesized by double cathode glow discharge. The films exhibited a compact columnar microstructure having a pronounced (111) preferred orientation. The corrosion behaviour of these films were characterized by using various electrochemical techniques including open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. The corrosion resistance of the films increased with increasing Al content in the as-synthesized films. The composition and chemical state of the passive layers formed on the films were investigated by X-ray photoelectron spectroscopy (XPS). It was demonstrated that the passive layer formed on the binary MoSi2 film was highly enriched in SiO2 with minor amounts of MoO42−, MoO2 and SiOx. With the increase of Al content in the films, Al2O3 was generated and incorporated into the passive layers, enhancing the corrosion resistance of the films by inhibiting the dissolution of Mo. Built upon the experimental results, the first-principles density-functional theory was applied to calculate the inter-atomic bonding strength in Mo(Si1−xAlx)2 and elucidate the role of Al in controlling the corrosion resistance of the films. The new findings lay a solid basis for the development and application of MoSi2 based corrosion-resistant films.