Wun Chet Davy Cheong

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.

A molecular dynamics study of scale effects on the friction of single-asperity contacts

The apparent scale effect on friction of single-asperity sliding was investigated with the aid of a molecular dynamics analysis. The specimen material used was mono-crystalline copper, while the asperity materials were diamond and copper, respectively. It was found that a friction transition does exist but depends on interface conditions between the asperity and specimen.

Effect of repeated nano-indentations on the deformation in monocrystalline silicon

The study of a single nano-indentation using molecular dynamics simulation has provided us with much important knowledge on the deformation mechanisms in monocrystalline silicon indentation [1–3]. Previous simulations have shown the phase transformation of silicon from diamond cubic structure to the metallic phase and finally to the amorphous phase during a nanoindentation [2]. A failure criterion to predict this transformation has been laid down [1]. Studies have also achieved an understanding of the overall response of silicon in a complete loading–unloading cycle of nanoindentation. These include the load–displacement relationship, the load–contact area relationship, the number of silicon atoms that have transformed from its original diamond cubic structure to other forms, the volume of the transformed zone, the ratio of the average density of transformed silicon to the density of crystal diamond cubic silicon and the average temperature rise in the specimen during indentation [1]. However, the results obtained from a single indentation are not sufficient to provide a complete understanding of the mechanism of deformation in many processes. For example, in grinding and polishing silicon wafers, the material is actually subjected to repeated tool-workpiece interactions. When the first interaction has created a damaged zone, the material may deform differently under subsequent operations. The purpose of this short paper is to understand how monocrystalline silicon will behave under repeated indentations. Using the same conditions as the previous simulation [1–3], the molecular dynamics method is used to simulate the behavior of silicon monocrystal under three indentations repeated at the same location with the same speed and maximum indentation depth. Further details about carrying out adequate molecular dynamics simulation can be found elsewhere [1–5]. Fig. 1 shows the location of the atoms of a silicon specimen at different stages during the second and third indentations. The big spheres represent the silicon atoms while the smaller ones represent the carbon atoms of the hemispherical diamond indentor. The residual amorphous zone after the first indentation seems to remain amorphous throughout the second and third indentations. There is no significant change in size of the amorphous zone. The residual indentation depths also appear to be consistent after the first, second and third indentations. Fig. 2 shows the volume of the transformed zone during indentation. The loading and unloading curves for the first, second and third indentations are almost identical to that of the number of transformed silicon atoms during indentation. There is only a small increase in the volume of amorphous silicon owing to the second and third indentations. This supports our earlier claim that the amorphous zone does not increase very much in size even after the second and third indentations. However, by considering the number of the nearest neighbor atoms, it is observed that there is an increase of atoms with six nearest neighbors during the loading phase of each indentation. This suggests the recovery of the metallic phase from the amorphous phase, mentioned in greater detail in a previous paper [2]. Fig. 3 shows the load–displacement curves for all three indentations. The indentation depths are taken from the same reference, which is the initial untouched surface of the monocrystal silicon. This explains why both the load–displacement graphs for the second and third indentations start at the indentation depth of about 0.6 nm that is the residual depth after the first indentation. The behavior of the second and third indentations are similar to the first in the sense that they too consist of an initial attractive phase between the silicon and carbon atoms when the indentor is brought near the work-piece. This is followed by a phase of increasing load with increasing indentation depth. On unloading, the graphs given by the second and third indentations are also very similar to that of the first indentation. Despite the similarities mentioned above, there are however, significant differences between the load–displacement curve of the first indentation and those of the second and third. Firstly, there is a marked increase in the resistance to the indentation during the loading phase of the second and third indentation because the gradient of the load–displacement curve increases. This is due to the residual stresses acting within the residual amorphous zone after the first indentation to resist the second and subsequent indentations, as shown in Fig. 4. Secondly, though all three indentations start with an initial attractive phase as mentioned before, it must be noted that the attractive forces between the work-piece and indentor during the second and third indentations are considerably greater than that of the first. This shows that the amorphous silicon atoms experience greater attraction to the diamond indentor compared to diamondcubic silicon atoms. It should also be noted that the contact area between the work piece and the indentor

Atomistic structure of monocrystalline silicon in surface nano-modification

This paper presents both experimental and theoretical studies on the atomic structure changes of monocrystalline silicon brought about by surface nano-modification. The experiment revealed amorphous transformations with boundaries featuring faceting along {111} planes near the sample surface, which were altered to a random nature at the bottom of the transformation zone. The deformation outside the zone was minor near the surface, but advanced to heavy bending, extensive dislocations and plane shifting in the depth of the samples. Theoretical analysis closely reproduced this deformation, highlighting some scaling effects.

Monocrystalline silicon subjected to multi-asperity sliding: nano-wear mechanisms, subsurface damage and effect of asperity interaction

Nano-sliding between two surfaces often involves the interaction of many asperities. With the aid of molecular dynamics analysis, this study uses a three-asperity model to investigate the effects of the relative orientation and position of the asperities on the nano-wear mechanism of silicon. It was found that when the first asperity has created a damaged layer, the material would deform differently under subsequent sliding. A thin amorphous layer always remains and there is also an absence of dislocations when the depth of asperity penetration is small. On the other hand, when the asperities are not traversing a damaged zone, the forces experienced by the asperities are independent of their relative positions. The results suggest that the microstructural changes are localised and the initial sliding affects the subsequent deformation over a damaged region.

Study of Residual Stress Relaxation Using X-Ray Diffraction

An x-ray diffraction system was used to monitor and investigate the relaxation of residual stresses in Ti-6Al-4V material under low cycle fatigue (LCF) loading conditions. The surfaces of the specimens were treated by either shot peening or low-plasticity burnishing. After either surface treatment, the Ti-6Al-4V specimens were subjected to low cycle fatigue loading conditions. After varying LCF cycles, residual stresses at the surface of the specimens were measured using an x-ray diffraction system. The LCF tests were carried out in four-point bending at a cyclic frequency of 0.5 Hz. Residual stress relaxation is studied considering the LCF cycles and applied stress. Introduction Compressive residual stresses induced by shot peening and low-plasticity burnishing can be highly beneficial to the fatigue resistance of gas turbine engine components where the operating loads are dominated by tensile stress. However, relaxation of compressive residual stresses due to fatigue loading can reduce the benefit [1]. It has been found that residual stresses induced by either shot peening or low-plasticity burnishing can be relaxed due to fatigue load and thermal exposure during engine operations [2]. In this paper, only relaxation during low cycle fatigue loading condition is studied using x-ray diffraction method. To compare the effect of shot peening with low-plasticity burnishing, two groups of Ti-6Al-4V specimens were manufactured. Subsequently, one group was surface treated by shot peening, the other by low-plasticity burnishing. After either surface treatment, the specimens were subjected to low cycle fatigue loading conditions. After varying LCF cycles, residual stresses at the surface of the specimens were measured using an x-ray diffraction system. The LCF tests were carried out in four-point bending at a cyclic frequency of 0.5 Hz. The influence of the LCF loading parameters on the relaxation of residual stresses is investigated. Material and Specimen The material used in this study has been chosen to be a Forged ST(1700F/1h)+VA(1300F/2h) Ti-6Al-4V titanium alloy plate because of its wide use in engine components for disks and blades. The general mechanical properties of the material are listed in Table 1. Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 871-876 doi:10.4028/www.scientific.net/KEM.274-276.871

Molecular Dynamics Simulation of Nano-Indentation of Carbon Coated Monocrystalline Silicon

This paper presents the molecular dynamics (MD) simulation of nano-indentation of diamond-like carbon (DLC) coating on silicon substrates. It is found that the mechanisms of nanoindentation of coated systems on the nanometre scale defers considerably from the same process on the micrometre scale. The coating thickness affects the mechanisms of plastic deformation both in the coating and the substrate.