Liangchi Zhang

Molecular dynamics simulation of phase transformations in silicon monocrystals due to nano-indentation

This paper discusses the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential. By monitoring the positions of atoms within the model, the microstructural changes as silicon transforms from its diamond cubic structure to other phases were identified. The simulation showed that diamond cubic silicon transforms into a body-centred tetragonal form (β-silicon) upon loading of the indentor. The change of structure is accomplished by the flattening of the tetrahedron structure in diamond cubic silicon. Upon unloading, the body-centred tetragonal form transforms into an amorphous phase accompanied by the loss of long-range order of the silicon atoms. By performing a second indentation on the amorphous zone, it was found that the body-centred-tetragonal-to-amorphous phase transformation could be a reversible process.

An experimental investigation into the orthogonal cutting of unidirectional fibre reinforced plastics

Abstract This paper aims to understand the machinability of epoxy composites reinforced by unidirectional carbon fibres when subjected to orthogonal cutting. It was found that the subsurface damage and its mechanisms of a machined component are greatly influenced by fibre orientation. The material’s bouncing back is a characteristic phenomenon associated with the cutting of a fibre-reinforced composite. Three distinct deformation zones appear, i.e., chipping, pressing and bouncing when the fibre orientation is

Towards a deeper understanding of wear and friction on the atomic scale—a molecular dynamics analysis

This study aims to gain an in-depth understanding of the mechanisms of wear and friction on the atomic scale through the investigation of a typical diamond-copper sliding system with the aid of molecular dynamics analysis. The study revealed that there generally exist four distinct regimes of deformation, i.e. the no-wear regime, adhering regime, ploughing regime and cutting regime. The transition between these regimes is governed by key sliding parameters such as indentation depth, sliding speed and surface lubrication conditions. The no-wear regime exists for a wide range of indentation depths and thus a no-wear design of practical sliding systems is possible even under chemically clean conditions. In the cutting regime, the frictional behaviour of the system follows a proportional law. In all the other regimes, however, the variation of the frictional force is complex and cannot be described by a simple formula. The study also pointed out that on the atomic scale the slip lines generated by dislocation motion are very different from those predicted by the slip-line theory of plasticity. A new theory needs to be developed to bridge the gap between atomic and micro analyses.

Atomic scale deformation in silicon monocrystals induced by two-body and three-body contact sliding

This paper discusses the deformation of silicon monocrystals subjected to two-body and three-body contact sliding with the aid of the molecular dynamics analysis. It was found that amorphous phase transformation is the main deformation in silicon and the onset of such inelastic deformation can be well predicted by a stress criterion. In a two-body contact sliding, the deformation of silicon falls into no-wear, adhering, ploughing and cutting regimes, while in a three-body contact sliding it follows the regimes of no-wear, condensing, adhering and ploughing. Under certain conditions in three-body sliding, wear without any subsurface damage can also occur when the bonding strength among surface silicon atoms is weakened and material removal takes place through the mechanism of adhesion. Based on the detailed deformation analysis, a new friction law and a new concept for wearability evaluation were proposed.

A critical assessment of the elastic properties and effective wall thickness of single-walled carbon nanotubes

This paper discusses the fundamental issues of the elastic properties and effective wall thickness of single-walled carbon nanotubes (SWCNTs). It provides an in-depth analysis based on the rationale of the nanoscale-to-macroscale deformation relationship of SWCNTs and carries out a critical assessment of the diverse theoretical predictions in the literature. It was found that the in-plane stiffness of SWCNTs is a mechanics quantity that has been consistently reflected by the majority of the existing models. However, a further systematic study is necessary to clarify the dilemma of the wall thickness of SWCNTs.

Towards a deeper understanding of plastic deformation in mono-crystalline silicon

Abstract This paper investigates the plastic deformation in mono-crystalline silicon under complex loading conditions. With the aid of various characterization techniques, it was found that the mechanism of plasticity in silicon is complex and depends on loading conditions, involving dislocations, phase transformations and chemical reactions. In general, plastic deformation in silicon is the coupled result of mechanical deformation controlled by the stress field applied, chemical reaction determined by the external loading environment, and mechanical–chemical interaction governed by both the loading type and environment. Temperature rise accelerates the penetration of oxygen into silicon and reduces the critical stress of plastic yielding. When the chemical effect is avoided, the initiation of plasticity is enabled by octahedral shear stress but the further development of plastic deformation is influenced by hydrostatic stress. Plasticity of silicon in the form of phase transformations, e.g., from the diamond to amorphous or from the amorphous to bcc structures, is determined by loading history.

Wear of ceramic particle-reinforced metal-matrix composites

Pin-on-disc dry sliding tests were carried out to study the wear mechanisms in a range of metal-matrix composites. 6061-aluminium alloys reinforced with 10 and 20 vol% SiC and Al2O3 particles were used as pin materials, and a mild steel disc was used as a counterface. A transition from mild wear to severe wear was found for the present composites; the wear rate increased by a factor of 102. The effects of the ceramic particles on the transition load and wear with varying normal pressure were thoroughly investigated. Three wear mechanisms were identified: abrasion in the running-in period, oxidation during steady wear at low load levels, and adhesion at high loads. A higher particle volume fraction raised the transition load but increased the wear rate in the abrasion and adhesion regimes. Increase of particle size was more effective than increase of volume fraction to prolong the transition from mild wear to adhesive wear. The reasons for different wear mechanisms were determined by analyses of the worn surfaces and wear debris.

Cold rolling of foil

A theory of cold rolling of thin gauge strip is presented which, within the idealizations of homogeneous deformation and a constant coefficient of Coulomb friction, rigorously models the elastic deformation of the rolls and the frictional traction at the interface. In contrast with classical theories (3) it is shown that, for gauges less than a critical value, plastic reduction takes place in two zones, at entry and exit, which are separated by a neutral zone in which the rolls are compressed fiat and there is no slip between the rolls and the strip. Roll load and torque are governed by five independent non-dimensional parameters which express the influence of gauge, reduction, friction and front and back tensions. Values of load and torque have been computed (for zero front and back tensions) for a wide range of thickness, reduction and friction and have been found to collapse approximately on to a single master curve.

Structure changes in mono-crystalline silicon subjected to indentation — yexperimental findings

This paper investigates the subsurface deformation in silicon induced by indentation. With the precise cross-sectioning technique and high-resolution electron microscopy, a number of new phenomena were discovered. These include the phase transformation to amorphous, bcc and tetragonal silicon, the emergence of planar defects and the onset of microcracking. The study drew a relatively complete picture of silicon deformation under a range of indentation conditions.

Particle effects on friction and wear of aluminium matrix composites

Particle effects on friction and wear of 6061 aluminium (6061 Al) reinforced with silicon carbide (SiC) and alumina (Al2O3) particles were investigated by means of Vickers microhardness measurements and scratch tests. Unreinforced 6061 Al matrix alloy was also studied for comparison. To explore the effect of heat treatment, materials subjected to three different heat treatment conditions, i.e. under-aged, over-aged and T6, were used. Multiplescratch tests using a diamond and a steel indentor were also carried out to simulate real abrasive wear processes. Vickers microhardness measurements indicated that T6 heattreated composites had the highest hardness. Single-scratch tests showed that the variation of friction coefficient was similar to that of Vickers hardness and the peak-aged composites exhibited the best wear resistance. The wear rate of fine particle-reinforced composites was mainly affected by hardness. However, the wear rate of large particle-reinforced composites was influenced by both the hardness and fracture of the particles.