Papers by Author: Liang Chi Zhang

Abstract: This paper investigates the mechanical properties of potassium dihydrogen phosphate (KDP) crystals with the aid of nanoindentation using a conical diamond indenter. It was found that when unloading is after the first pop-in, the common method of obtaining elastic modulus from the unloading curve of nanoindentation is no longer applicable, because the unloading is inelastic. The study revealed that the pop-in could be due to dislocation nucleation and propagation, and that the first pop-in occurs under a stress below that of the major dislocation burst. Hence, the macroscopic yielding point, which is usually regarded as the onset of plasticity of a material, is nanoscopically not a critical point of the first dislocation in KDP. The study found that the elastic modulus of KDP indenting on its (001) plane is 52.8±3.8GPa. The hardness of the material is 1.89±0.05GPa.

Abstract: Glass transition temperature T_g is the most important parameter affecting the mechanical properties of amorphous and semi-crystalline polymers. However, the atomistic origin of glass transition is not yet well understood. Using Polyethylene (PE) as an example, this paper investigates the glass transition temperature T_g of PE with the aid of molecular dynamics (MD) simulation. The effects of PE chain branches, crystallinity and carbon-nanotube (CNT) additives on the glass transition temperature are analyzed. The MD simulations render a good agreement with the relevant experimental data of semi-crystalline PE and show the significant effects of crystallinity and addition of CNTs on T_g.

Abstract: Although localized atomic rearrangements have been considered to be the underlying mechanism of plastic deformation in metallic glass, the nucleation and evolution of such plasticity events are still elusive. With the aid of molecular dynamics analysis, this study revealed that a series of localized atomic rearrangement events would occur in metallic glass as demonstrated by the formation of high-kinetic-energy clusters. It was found that atomic clusters of average sizes of 1 to 2 nm nucleate during elastic deformation, and become prevailing after yielding. The cores of these clusters contain several high-velocity atoms, which drive the local structural change and accommodate plastic strain. The nucleation and evolution of the local plasticity events are shown clearly by the strong dynamic signature, attributed to the spontaneous structural reshuffling after crossing an energy barrier.

Abstract: This study aims to investigate the microscopic origin of viscosity by simplifying an amorphous system to a mixture of many independent atomic subsystems. The response of the macroscopic system is then taken as an ensemble average of the relaxations of such subsystems. The result shows that with the reduction of temperature, the overall viscosity changes from the harmonic mean of the subsystems, which is dominated by the fast relaxations, to the arithmetic mean governed by the slowest relaxation. The successful application of our model to the amorphous Selenium indicates the model captures the fundamental mechanism of the viscosity variation.

Abstract: This paper investigates the applicability of the dynamic friction polishing (DFP) technique to process the chemical vapour deposition (CVD) diamond surfaces. Two types of CVD diamond specimens were studied. A stepwise polishing process was introduced to minimise the cracking in CVD thin films. The investigation focused on the polished surface quality in relation to the polishing conditions and material remove rates. It was found that by selecting proper polishing parameters, surfaces of quality finish with a roughness of less than 70 nm Ra could be obtained in 15 minutes when the specimens were CVD diamond wafers with an initial roughness of 17 μm. The polishing time could be reduced to only 2.5 minutes in the case of diamond thin film specimens of initial roughness of 1.6 μm.

Abstract: This paper reports the specimen preparation using an advanced dual beam focused ion beam (FIB) technique for bulk polycrystalline diamond (PCD) composites after dynamic friction polishing (DFP). The technique adapted allows for precisely processing diamond materials at the specific polishing track sites of PCD surface, from which large cross-sectional specimens for SEM/EDS/Raman microanalysis could be successfully created. In addition, an in-situ lift-out method was developed to prepare the site-specific HRTEM specimens which were thin enough for imaging the atomic lattice of diamond and for conducting EELS analysis.

Abstract: To meet different electrical or optical functionalities, thin films are often of multiple layers processed at high temperatures. Substantial residual stresses can therefore develop in such thin film systems due to the disparate thermal properties of the individual material layers. High stresses can lead to mechanical failure of the systems and thus understanding the residual stresses in thin film systems is important. This paper presents a systematic way to characterize the residual stresses in epitaxial, polycrystalline and amorphous layers by using X-ray diffraction (XRD) techniques. The single-point XRD pattern renders the stresses of crystalline layers and the scanning XRD gives the curvature of the whole film. Based on the newly-developed analytical model, the residual stresses of each layer can all be determined.

Abstract: Fibre-reinforced polymer (FRP) composites have been widely used in industry. However, the machining of FRP products is difficult, because of very different properties of the fibres and matrix. This paper discusses the development and implementation of a microstructure-based three-dimensional finite element model for the elliptic vibration-assisted (EVA) cutting of unidirectional FRP composites. The results showed that the EVA cutting has a good potential to the machining of FRP composites, featured a much reduced cutting force, better surface integrity and controllable chip size.

Abstract: This paper investigates the high speed nano-grooving on mono-crystalline copper by nano-end-milling and nano-peripheral-milling. The molecular dynamics method was used for simulation. The milling forces and grooving quality were analysed in detail. The comparison showed that peripheral-milling requires a higher force, but the milled groove has a better quality. Nevertheless, both the milled groove subsurfaces are free of major dislocations.

Abstract: Mono-crystalline silicon experiences various phase transformations under different loading conditions. This paper reveals, with the aid of molecular dynamics simulations, that scratching the silicon {001} surface along the [110] direction under a load of 0.8 µN or more would produce stable 5 coordinated body centered tetragonal (bct-5) silicon in the subsurface. By examining the effect of this bct-5 silicon on indentation, it was found that the resistant to deformation of bct-5 silicon is higher than a-Si but lower than diamond Si.