Qingshun Bai

Adsorption of tripeptide RGD on rutile TiO2 nanotopography surface in aqueous solution

Molecular dynamics simulations were carried out to investigate the adsorption mechanisms of tripeptide Arg-Gly-Asp (RGD) on the nanotopography and perfect rutile TiO(2) (110) surfaces in aqueous solution. It is shown that the amino groups (NH(2) and NH3+) and carboxyl group (COO(-)) of RGD are the main groups bonding to hydrophilic TiO(2) surface by electrostatic and van der Waals interactions. It is also demonstrated that RGD adsorbs much more rapidly and stably on the nanotopography surface than the perfect surface. On the hydrophilic TiO(2) surface, the water molecules occupy the adsorption sites to form hydration layers, which have a significant influence on RGD adsorption. On the perfect surface, since the fivefold titanium atom is surrounded by surface bridging oxygen atoms above it and has a water molecule bonding to it, the amino group NH(2) is the adsorption group. However, because the pit surface exposes more adsorption sites and has higher surface energy, RGD can adsorb rapidly on the surfaces by amino groups NH(2) and NH3+, and the carboxyl group COO(-) may edge out the adsorbed water molecules and bond to the surface titanium atom. Moreover, the surface with higher surface energy has more adsorption energy of RGD.

Modeling and experimental analysis of microburr formation considering tool edge radius and tool-tip breakage in microend milling

The microburr formation in the microend milling of aluminum alloy Al2024-T6 using tungsten-carbide cutter is investigated in this article. The three-dimensional finite element model is developed to analyze microburr formation in microend-milling process. This model predicts the effects of various tool edge radiuses and tool-tip breakage on the burr formation. The microburr formation is dynamically simulated. The simulation results show that there are three basic types of burrs (entrance burr, top burr, and exit burr) along the feature edges. The burrs formed in microend milling are larger than those formed in conventional milling in certain range. The effect curve of tool edge radius on the top-burr height is obtained. Various tool edge radii are found to have significant influence on the top-burr formation. The salient size effect of microburr morphologies is observed in the experiment and simulation. Experimental verification of the simulation model is carried out in the process of microend milling of a...

Hydrostatic spindle dynamic design system and its verification

A design system for hydrostatic spindle is presented in light of the dynamic synthesis, which is based on the laws of the fluid mechanics, engineering thermodynamics and rotor dynamics. The finite element theory and hydrostatic principle are integrated into the design process, which provides not only the analyses and determination of the stiffness and temperature rise of the hydrostatic bearing but also the dynamic performance optimization. The proposed design system was implemented through a hydrostatic spindle on ultra-precision machining tools with the flycutting process.

Multiscale simulation of nanometric cutting of single crystal copper—effect of different cutting speeds

A multiscale simulation has been performed to determine the effect of the cutting speed on the deformation mechanism and cutting forces in nanometric cutting of single crystal copper. The multiscale simulation model, which links the finite element method and the molecular dynamics method, captures the atomistic mechanisms during nanometric cutting from the free surface without the computational cost of full atomistic simulations. Simulation results show the material deformation mechanism of single crystal copper greatly changes when the cutting speed exceeds the material static propagation speed of plastic wave. At such a high cutting speed, the average magnitudes of tangential and normal forces increase rapidly. In addition, the variation of strain energy of work material atoms in different cutting speeds is investigated.

Three-dimensional molecular dynamics simulation of nanostructure for reciprocating nanomachining process

Three-dimensional molecular dynamics simulations are conducted to investigate the effect of reciprocating nanomachining process on the subsurface damaged layers, surface integrity, cutting force, stress variation of subsurface, and changes of energy and defects in the workpiece. Results show that there is no obvious shear zone ahead the tool during nanomachining. Dislocation nucleations are near the free surface ahead the tool and the interface of the tool and the workpiece and propagate in the surface and downward in the workpiece. There are the generations of dislocation jog and dislocation loops ahead the tool during the reciprocating cutting. The values of the reciprocating cutting force for the (111) orientation and (100) orientation under offset distance of 0a0 are not zero but 11.756 and 13.0498 nN, respectively. When the offset distance of the tool is up to 10a0, the ratio of primary cutting force to reciprocating force is nearly 90%. The shape of the machined groove in the (111) orientation remai...

Anisotropy of Single-Crystal Silicon in Nanometric Cutting

The anisotropy exhibited by single-crystal silicon in nanometric cutting is very significant. In order to profoundly understand the effect of crystal anisotropy on cutting behaviors, a large-scale molecular dynamics model was conducted to simulate the nanometric cutting of single-crystal silicon in the (100)[0–10], (100)[0-1-1], (110)[−110], (110)[00–1], (111)[−101], and (111)[−12-1] crystal directions in this study. The simulation results show the variations of different degrees in chip, subsurface damage, cutting force, and friction coefficient with changes in crystal plane and crystal direction. Shear deformation is the formation mechanism of subsurface damage, and the direction and complexity it forms are the primary causes that result in the anisotropy of subsurface damage. Structurally, chips could be classified into completely amorphous ones and incompletely amorphous ones containing a few crystallites. The formation mechanism of the former is high-pressure phase transformation, while the latter is obtained under the combined action of high-pressure phase transformation and cleavage. Based on an analysis of the material removal mode, it can be found that compared with the other crystal direction on the same crystal plane, the (100)[0–10], (110)[−110], and (111)[−101] directions are more suitable for ductile cutting.

A novel dynamic modeling method for aerostatic spindle based on pressure distribution

The pressure distribution in an aerostatic bearing has an important effect on the performance of the associated mechanical equipment. To more accurately predict performance, a new dynamic modeling method has been developed that takes into account the pressure distribution in the bearing by integrating the principle of flow equilibrium and finite element theory. The direct corresponding relationship between the fluid film characteristics and spindle dynamic performance is established using this method. The simulation and experimental results show that the new dynamic modeling method for the aerostatic bearing is more efficient and reliable than traditional modeling methods.

Molecular dynamics simulation study on surface structure and surface energy of anatase

Molecular dynamics simulations were performed to investigate the relaxed structures and surface energies of perfect and pit anatase TiO2 surfaces. It is shown that the slab containing more than two unit-cell layers away from the fixed layer expresses the surface characteristics of perfect anatase TiO2 (1 0 1) and (1 0 0) surfaces well, while the slab containing more than one unit-cell layer away from the fixed layer expresses the surface characteristics of the (0 0 1) surface well. Their surface energies follow the sequence (0 0 1) < (1 0 1) < (1 0 0). Simulation results also indicate that the pit edges expose many undercoordinated atoms, and the more highly undercoordinated atoms exhibit the larger displacement vectors. Moreover, the surface energy of the pit surface is higher than that of the perfect surface. The surface energies of pit anatase (1 0 1) surfaces are linearly related to the pit sizes along the [ ] and [0 1 0] directions, and the changes in their surface energies are less than 0.05 J m−2, while the surface energies increase sharply with the increase in pit depth within 1 nm. Therefore, for anatase (1 0 1) surface, in order to obtain a higher surface energy, one may increase the pit sizes, particularly along the [1 0 1] direction.

Design and analysis of a novel large-aperture grating device and its experimental validation:

Large-aperture diffraction gratings are the key elements for high-power petawatt-class laser facility. At present, it is difficult to directly manufacture high-accuracy large-aperture gratings by using ultraprecision manufacturing techniques. Mechanical tiling technique with segmented gratings can be employed to cope with the challenge in large-aperture grating manufacturing. In this article, a novel tiling and adjusting device for obtaining large-aperture gratings was proposed and built to overcome the challenges in developing a Big Science Facility. The idea of integrated design and precision analysis has been introduced into the design process of the device. The theoretical analysis on static and dynamic characteristics has been conducted on the key components of the device by using finite element method. Under a series of mechanical tests, the performance of the large-aperture grating tiling and adjusting device developed was evaluated against the industrial requirements. The testing results show that the device has the positioning precision and stability in light of the design specifications. The results also indicate an optimal linear accuracy of 20 nm and a rotational accuracy of 0.4 µrad being achieved at the device.

Fourier transform based dynamic error modeling method for ultra-precision machine tool

In some industrial fields, the workpiece surface need to meet not only the demand of surface roughness, but the strict requirement of multi-scale frequency domain errors. Ultra-precision machine tool is the most important carrier for the ultra-precision machining of the parts, whose errors is the key factor to influence the multi-scale frequency domain errors of the machined surface. The volumetric error modeling is the important bridge to link the relationship between the machine error and machined surface error. However, the available error modeling method from the previous research is hard to use to analyze the relationship between the dynamic errors of the machine motion components and multi-scale frequency domain errors of the machined surface, which plays the important reference role in the design and accuracy improvement of the ultra-precision machine tool. In this paper, a fourier transform based dynamic error modeling method is presented, which is also on the theoretical basis of rigid body kinematics and homogeneous transformation matrix. A case study is carried out, which shows the proposed method can successfully realize the identical and regular numerical description of the machine dynamic errors and the volumetric errors. The proposed method has strong potential for the prediction of the frequency domain errors on the machined surface, extracting of the information of multi-scale frequency domain errors, and analysis of the relationship between the machine motion components and frequency domain errors of the machined surface.