Prediction of the Interface Temperature Rise in Tribochemical Polishing of CVD Diamond

Tribochemcial polishing is one of the most efficient methods for polishing CVD （Chemical Vapor Deposition） diamond film due to the use of catalytic metal. However the difficulty to control the interface temperature during polishing process often results in low material removal because of the unstable contact process. So this research investigates the contact process in the tribo- chemical polishing of CVD diamond film and proposes a dynamic contact model for predicting the actual contact area, the actual contact pressure, and the interface tem- perature in the polishing process. This model has been verified by characterizing surface metrology of the CVD diamond with Talysurf CLI2000 3D Surface Topography and measuring the polishing temperature. The theoretical and experimental results shows that the height distribution of asperities on diamond film surface in the polishing process is well evaluated by combining the height distribution of original and polished asperities. The modeled surface asperity height distribution of diamond film agrees with the actual surface metrology in polishing process. The actual contact pressure is very large due to the small actual contact area. The predicted interface temperature can reach the catalytic reaction temperature between diamond and polishing plate when the lowest rotation speed and load are 10 000 r/min and 50 N, respectively, and diamond material is significantly removed. The model may provide effective process theory for tribochemcial polishing.

Theoretical and experimental investigation of chemical mechanical polishing of W–Ni–Fe alloy

Fine finishing of tungsten alloy is required to improve the surface quality of molds and precision instruments. Nevertheless, it is difficult to obtain high-quality surfaces as a result of grain boundary steps attributed to differences in properties of two-phase microstructures. This paper presents a theoretical and experimental investigation on chemical mechanical polishing of W–Ni–Fe alloy. The mechanism of the boundary step generation is illustrated and a model of grain boundary step formation is proposed. The mechanism reveals the effects of mechanical and chemical actions in both surface roughness and material removal. The model was verified by the experiments and the results show that appropriately balancing the mechanical and chemical effects restrains the generation of boundary steps and leads to a fine surface quality with a high removal rate by citric acid-based slurry.

A review of low-temperature plasma-assisted machining:from mechanism to application

Materials with high hardness,strength or plasticity have been widely used in the fields of aviation,aerospace,and military,among others.However,the poor machinability of these materials leads to large cutting forces,high cutting temperatures,serious tool wear,and chip adhesion,which affect machining quality.Low-temperature plasma contains a variety of active particles and can effectively adjust material properties,including hardness,strength,ductility,and wettability,significantly improving material machinability.In this paper,we first discuss the mechanisms and applications of low-temperature plasma-assisted machining.After introducing the characteristics,classifications,and action mechanisms of the low-temperature plasma,we describe the effects of the low-temperature plasma on different machining processes of various difficult-to-cut materials.The low-temperature plasma can be classified as hot plasma and cold plasma according to the different equilibrium states.Hot plasma improves material machinability via the thermal softening effect induced by the high temperature,whereas the main mechanisms of the cold plasma can be summarized as chemical reactions to reduce material hardness,the hydrophilization effect to improve surface wettability,and the Rehbinder effect to promote fracture.In addition,hybrid machining methods combining the merits of the low-temperature plasma and other energy fields like ultrasonic vibration,liquid nitrogen,and minimum quantity lubrication are also described and analyzed.Finally,the promising development trends of low-temperature plasma-assisted machining are presented,which include more precise control of the heat-affected zone in hot plasma-assisted machining,cold plasma-assisted polishing of metal materials,and further investigations on the reaction mechanisms between the cold plasma and other materials.

Advances in molecular dynamics simulation of ultra-precision machining of hard and brittle materials

Hard and brittle materials, such as silicon, SiC, and optical glasses, are widely used in aerospace, military, integrated circuit, and other fields because of their excellent physical and chemical properties. However, these materials display poor machinability because of their hard and brittle properties. Damages such as surface micro-crack and subsurface damage often occur during machining of hard and brittle materials. Ultra-precision machining is widely used in processing hard and brittle materials to obtain nanoscale machining quality. However, the theoretical mechanism underlying this method remains unclear. This paper provides a review of present research on the molecular dynamics simulation of ultra-precision machining of hard and brittle materials. The future trends in this field are also discussed.

Atomistic understanding of interfacial processing mechanism of silicon in water environment:A ReaxFF molecular dynamics simulation

The interfacial wear between silicon and amorphous silica in water environment is critical in numerous applications.However,the understanding regarding the micro dynamic process is still unclear due to the limitations of apparatus.Herein,reactive force field simulations are utilized to study the interfacial process between silicon and amorphous silica in water environment,exploring the removal and damage mechanism caused by pressure,velocity,and humidity.Moreover,the reasons for high removal rate under high pressure and high velocity are elucidated from an atomic perspective.Simulation results show that the substrate is highly passivated under high humidity,and the passivation layer could alleviate the contact between the abrasive and the substrate,thus reducing the damage and wear.In addition to more Si-O-Si bridge bonds formed between the abrasive and the substrate,new removal pathways such as multibridge bonds and chain removal appear under high pressure,which cause higher removal rate and severer damage.At a higher velocity,the abrasive can induce extended tribochemical reactions and form more interfacial Si-O-Si bridge bonds,hence increasing removal rate.These results reveal the internal cause of the discrepancy in damage and removal rate under different conditions from an atomic level.

Smoothed-Particle Hydrodynamics Investigation on Brittle-Ductile Transition of Quartz Glass in Single-Grain Grinding Process

The smoothed-particle hydrodynamics(SPH)method was introduced to simulate the quartz glass grinding process with a single grain under micrp-nano scale.To investigate the mechanism of brittle-ductile transition,such factors as the machin-ing depth,grinding force,maximum equivalent stress,and residual stress were analyzed.The simulation results indicate that quartz glass can be machined in a ductile mode under a certain condition.In this paper,the occurrence and propaga-tion of cracks in quartz glass at different grinding depths(0.1-1μm)are observed,and the critical depth of brittle-ductile transformation is 0.36 pum.At different grinding depths,the grinding force ratio is greater than 1.When the cutting depth is 0.4 um,the crack propagation depth is about 1.2μm,which provides a basis for the prediction of subsurface damage depth.In addition,the correctness of the simulation result was verified by carrying out scratch experiments of varying cutting depth on optical quartz glass.

Design of ultrasonic elliptical vibration cutting system for tungsten heavy alloy

Nanoscale surface roughness of tungsten heavy alloy components is required in the nuclear industry and precision instruments.In this study,a high-performance ultrasonic elliptical vibration cutting(UEVC)system is developed to solve the precision machining problem of tungsten heavy alloy.A new design method of stepped bending vibration horn based on Timoshenko’s theory is first proposed,and its design process is greatly simplified.The arrangement and working principle of piezoelectric transducers on the ultrasonic vibrator using the fifth resonant mode of bending are analyzed to realize the dual-bending vibration modes.A cutting tool is installed at the end of the ultrasonic vibration unit to output the ultrasonic elliptical vibration locus,which is verified by finite element method.The vibration unit can display different three-degree-of-freedom(3-DOF)UEVC characteristics by adjusting the corresponding position of the unit and workpiece.A dual-channel ultrasonic power supply is developed to excite the ultrasonic vibration unit,which makes the UEVC system present the resonant frequency of 41 kHz and the maximum amplitude of 14.2μm.Different microtopography and surface roughness are obtained by the cutting experiments of tungsten heavy alloy hemispherical workpiece with the UEVC system,which validates the proposed design’s technical capability and provides optimization basis for further improving the machining quality of the curved surface components of tungsten heavy alloy.