Surface Formation Mechanism Research Articles

Abrasive machining induced surface layer damage behavior and formation mechanism of monocrystalline β-Ga2O3: A comparative study of nanoindentation and nanogrinding

Beta-phase gallium oxide (β-Ga2O3) has garnered significant attention in recent years as an ultra-wide bandgap semiconductor material. However, its surface layer damage behavior and machining deformation mechanism remain poorly understood. In this work, a systematic comparative study was conducted through nanoindentation and nanogrinding for monocrystalline β-Ga2O3 (−201) crystal plane. Transmission electron microscopy is employed as the primary characterization method. The results show that the microstructural evolution patterns of monocrystalline β-Ga2O3 can be categorized into four stages, i.e. stacking faults, twins, slip bands, and cleavage cracks are observed sequentially. According to the tests results, subsurface damage diagram models based on nanoindentation and nanogrinding are established, both of which exhibit the above evolution patterns as the indentation depth and grit depth of cut increases. However, there are some differences. In the nanogrinding process, nanocrystals and amorphous layers were found near the ground surface, which are related to the high strain rate grinding conditions and the unique material properties of monocrystalline β-Ga2O3 with low stacking fault energy. The results in this work contribute to an enhanced understanding of the deformation characteristics of monocrystalline β-Ga2O3 at macro and micro scales, while offering valuable insights for achieving damage control in ultra-precision machining processes.

A grinding force model and surface formation mechanism of cup wheels considering crystallographic orientation

As typical two-phase materials, tungsten heavy alloys (WHAs) are widely used in industry owing to their excellent mechanical properties, which also challenge high-quality machining. Grinding can lead to a better machining quality. Prediction and evaluation of grinding forces are essential for grinding quality control and process optimization. This study proposes a flow stress model for the WHA considering the strain hardening, strain rate hardening, and thermal softening effects. The difference between the two phases of WHA was confirmed by calculating the Taylor factors, where the average Taylor factor of the W phase was 3.011 and that of the matrix phase was 3.414. Therefore, dislocations are more likely to be generated and aggregated in the matrix phase during grinding process. The mechanism of subsurface formation during the grinding of WHA was analyzed by transmission electron microscopy. The results show that the plastic deformation layer consists of the fine grain layer, the high density dislocation zone and the substrate. A grinding force model for cup wheels in the vertical-spindle face grinding of WHA considering the grain orientation was developed, and the error between the model and experimental value was within 10%. This model can provide an in-depth understanding of the effects caused by the difference between the two phases during the grinding process, thus provide a theoretical basis for the realization of efficient and low-damage grinding of WHA and other composite materials.

Theoretical insights into the dissociation and oxidation of ethylene during vinyl acetate synthesis

Gas phase synthesis of vinyl acetate from ethylene over Pd-Au catalyst is one of the mainstream processes. During the synthesis of vinyl acetate, the surface C generated by the dissociation of ethylene can be doped into the lattice of Au to form interstitial C. The synergistic effect of interstitial C and Au can promote the production of vinyl acetate. However, the formation mechanism of surface C is not clear. In addition, CO2 is formed during the dissociation and oxidation of ethylene. In order to reduce carbon emissions, it is also necessary to clarify the formation mechanism of CO2. In this study, the reaction mechanism for the dissociation and oxidation of ethylene on the PdAu(100) surface was investigated by density functional theory and kinetic Monte Carlo, and the formation pathways of surface C and CO2 were clarified.The results show that the generation pathway of surface C is CH2CH2→ CH2CH→ CHCH→ CHC→ CC→ CCO→ C+CO. The rate control step is CCO→C + CO with energy barrier of 1.39 eV and reaction heat of 0.52 eV. The generation pathway of CO2 is C → COH → CO → CO2. The rate control step is C+OH→COH with energy barrier of 0.71 eV and reaction heat of -1.12 eV. Clarification of the reaction mechanism for dissociation and oxidation of ethylene on the PdAu(100) surface and the formation pathways of surface C and CO2 can more specifically reduce the consumption of ethylene for by-products, better construct the synergism of surface C and Au, and reduce the emission of CO2 in the process of catalyst improvement and optimization.

Effect of tool wear on machining quality in milling Cf/SiC composites with PCD tool

Carbon fiber reinforced silicon carbide ceramic matrix (Cf/SiC) composites have great application potential in aerospace. But their high hardness and anisotropy cause rapid tool wear during machining, which affects machining quality and efficiency. However, tool wear mechanisms in milling Cf/SiC composites are still unclear. There is a lack of research on the relationship between tool wear and cutting performance. In this paper, milling experiments on 2.5D Cf/SiC composites were conducted using polycrystalline diamond (PCD) cutters. The tool wear mechanisms and effect of tool wear on surface roughness, milling force, burr damage, surface microstructure and chips were explored. The results show that abrasive wear caused by chips continuously scraping tool material and diamond particle fragmentation and falling off caused by sprouting and expansion of cracks inside grain were primary causes of tool failure. The formation mechanism of burr damage was analyzed. It was proved that the peak radius of cutting edge Rpeak and flank wear VB could reflect cutting capabilities. It was found that increasing flank wear had regrinding effects on the cutting edge, which affects the peak radius. The degree of burr damage was related to these two parameters. Tool wear changed the surface formation mechanism. Before tool wear, carbon fibers were primarily removed through the micro-brittle fracture, with low surface roughness. The flank face and chips scratched the workpiece like abrasive grains after tool wear, leaving scratches on the surface and increasing surface roughness from 1.27 μm to 2.25 μm.

Experiment study of surface formation mechanism during cryogenic turning of PEEK

Thermal deformation and local burnout are common machining defects of PEEK. At present, PEEK is mostly machined by conventional flood cooling, and there are relatively few studies based on LN2 cooling. In this paper, we conducted cryogenic turning experiments on PEEK and investigated the effect of LN2 internal injection cooling on surface quality and found for the first time that the fracture form of PEEK changes from microporosity-dominated plastic fracture to brittle fracture under the strong cooling effect of LN2. This study analyzes the influence of cutting parameters on machining quality through orthogonal experiments, and selects optimal parameters accordingly. Comparative turning experiments are conducted under three different conditions - dry, conventional flood cooling, and LN2 internal injection cooling - to obtain machining information such as cutting temperature, surface roughness, and machining accuracy under each condition. Based on microscopic morphological analysis, we propose a surface formation mechanism for cryogenic turning of PEEK. The study demonstrated that the use of LN2 internal injection cooling substantially decreases the cutting heat, suppresses the thermal deformation of the material, and promotes brittle fracture in PEEK, thereby enhancing machining quality and accuracy. The surface roughness of the workpiece decreased by 53.5 % and 39.2 %, respectively, while the machining accuracy error decreased by 40.4 % and 37.8 % compared to dry cutting and conventional flood cooling methods. These findings provide valuable guidance for efficient machining of PEEK.

Effects of cutting conditions on the surface formation mechanism in cutting of in-situ (TiBw+TiCP)/Ti composite

Particle damage and modified layer have a great influence on the cutting surface quality of in-situ (TiBw +TiCp) /Ti composite. Therefore, the particle removal mechanism and the formation mechanism of the modified layer under different cutting conditions are discussed based on the experimental and simulation results. The cutting experiments on the in-situ (TiBw +TiCp) /Ti composite prepared by laser cladding were performed, the particle removal modes and the microstructure of the modified layer were observed and analyzed. A multi-scale interface model was established by combining first-principle calculations with a cohesive zone model, and the interface strength was determined. A cutting finite element model was developed to simulate the particle removal process and stress distribution. The results demonstrate that the particle damage is determined by the material characteristics and the plastic deformation in the subsurface. Nano-scale TiCp particles are pressed into the machined surface due to good fluidity within the matrix. Some micron-scale TiCp particles are crushed because of stress concentration. Under high cutting parameters, the pulling out of the micron-scale particles in the (TiBw +TiCp) /Ti composite is determined by the mechanical properties of the interface and matrix. The damage mode of the TiBw is a brittle shear fracture due to its whisker morphology. The modified layer produced under high cutting parameters is caused by high cutting heat, stress, and strain rate.

Laser Beam Machining of Titanium Alloy—A Review

This study investigates the laser beam machining mechanism, surface formation mechanisms, heat-affected zone, taper formation, and the dimensional deviation of the titanium alloy, based on the information available in literature. The heat induced by the laser beam melts and vaporises titanium alloy, which is removed by a high pressure-assisted gas. The machined titanium alloy surface is expected to have craters and resolidified materials which were contributed by the low thermal conductivity of the titanium alloy. Taper and circularity error can be minimised by optimising the laser parameter, but it cannot be avoided in the laser beam machining of titanium alloy. Laser beam machining induces a non-diffusion phase transformation, which slightly changes the surface mechanical properties of the titanium alloys. Laser beam machining is gaining popularity as a way to improve the surface finish quality and properties of titanium components manufactured by additive manufacturing processes. To enhance the machining efficacy of titanium alloys, several hybrid machining processes were proposed.

A compressive-shear biaxial stress field model for analyzing the anisotropic mechanical behavior of nanotwinned diamond

Synthetic nanotwinned diamond (Nt-D) with unprecedented hardness and toughness is an ideal material for manufacturing ultra-precision cutting tools. It is urgent to investigate the surface formation mechanism of Nt-D under complex stress fields. Here, we developed a compressive-shear biaxial stress field model to study the extreme anisotropic mechanical behavior of Nt-D. It was found that graphitization occurred in all shear directions of the Nt-D (0001) plane through the microstructure evolution of 2H-diamond, 4H-diamond, 6H-diamond, and 9R-diamond, in which the stress release mechanism was accompanied by partial slips in certain specific crystal directions such as 〈1010〉. In addition, the generalized stacking fault energy calculation indicates that the partial slip in 6H- and 9R-diamonds is mainly attributed to the low slip energy barrier between adjacent interlayer. These results expand our understanding of the surface formation mechanism of Nt-D and will contribute to the manufacturing of new-generation ultra-precision tools.

Visible dynamic changes in the mechanism of water evaporation surface formation during wood drying

Visible dynamic changes in the mechanism of water evaporation surface formation during wood drying

Multi-dimensional ultrasonic-assisted belt grinding on the surface integrity of Inconel 718

In order to meet the requirements of the isotropic high-performance machining of complex surfaces, this paper proposed three-dimensional ultrasonic assisted belt grinding (3D-UABG) method that integrates the advantages of surface strengthening and isotropic machining, analyzed the motion characteristics and surface formation laws. Then, the influence of ultrasonic vibrations on the surface integrity of Inconel 718 was studied by a self-designed 3D-UABG device. Finally, the surface formation mechanism was revealed combined with the surface integrity and abrasive grain motion characteristic. Results showed that compared with conventional abrasive belt grinding (CABG), the abrasive grain in normal-ultrasonic assisted belt grinding (N-UABG) reduced the height of texture peak formed an intermittent texture, which makes it have smallest surface roughness. At the same time, the high-frequency impact of abrasive grains not only increased the surface plastic deformation, but also realized the impact strengthening. Planar two-dimensional ultrasonic assisted belt grinding (2D-UABG) reduced the directionality difference of surface quality. However, the deflection characteristics of abrasive grains make it have a large surface roughness. The 3D-UABG surface has good comprehensive mechanical properties. The abrasive grains have three-dimensional dynamic characteristics in space, which effectively inhibit anisotropic stress state, promote uniform deformation, and helpful to obtain approximately isotropic surface texture and small surface roughness. On the one hand, the in-plane motion characteristics of planar two-dimensional ultrasonic vibration gave grinding surface more uniform surface integrity. On the other hand, the spatial motion trajectory of abrasive grains driven by three-dimensional ultrasonic vibration made it have a slow increased cutting depth, which avoided the tear damage caused by materials plastic accumulation and the large roughness caused by abrasive deflection under large cutting depth. In 3D-UABG, lower feed speed and amplitude and linear velocity matched to grinding parameters contribute to the improved surface integrity. This study has guiding significance for the development and engineering application of ultrasonic-assisted belt grinding technology.

Microstructure effects on surface integrity in slot micro-milling multiphase titanium alloy Ti6Al4V

Material microstructure in micro-cutting, such as the phase boundary for multiphase material, plays a dominant role in governing the surface integrity that strongly influences the fatigue life of micro-components with complex structures. In the present study, surface formation, subsurface deformation, and subsurface microstructure evolution for the machined surface were investigated by slot micro-milling multiphase titanium alloy Ti6Al4V in terms of chip morphology, burrs formation, surface roughness, surface defects, and subsurface deformation. In particular, the effect of β-Ti was considered in the study. The results show that the machined surface shows the obvious feed marks and adhesive materials accompanied by squeezed β-Ti, split β-Ti and plastic side flow β-Ti. The subsurface of the machined surface can be divided into the amorphous layer (around 24 nm), nanocrystalline layer (around 124 nm), elongated grain layer (around 197 nm), and bulk material. The deformation layer thickness of the β-Ti zone is larger than the α-Ti zone. In the deformation layer of the machined subsurface, the synergetic deformation between α-Ti and β-Ti can lead to the formation of a large number of nanocrystalline grains and some enlarged grains in the phase boundary of the α-Ti side, and the β-Ti side is full of elongated grains and sporadic dynamic recrystallization nanocrystalline. Our study enhances the understanding of the surface formation and subsurface formation mechanism in micro-milling Ti6Al4V, which can provide a theoretical basis and practical reference for achieving high surface integrity for polycrystalline or multiphase material by micro-milling.

Origin of tearing topography surface in hydrogen-charged pearlitic steel

In the present study, the formation mechanism of tearing topography surface (TTS), which is observed after slow strain-rate tensile tests of H-charged pearlitic specimens, was investigated in detail. The TTS always appeared at the subsurface of failed H-charged specimens, where H atoms were concentrated after room-temperature electrochemical H-charging. The TTS consisted of stepped flat surfaces and dimpled surfaces. The flat surface was caused by the coalescence of sharp micro-shear cracks, not micro-voids, of H-enriched cementite (θ) platelets in a pearlite colony, namely brittle transcolonial fracture. The regions surrounding the flat surface were H-depleted due to the migration of H atoms into neighboring micro-shear cracks during tensile deformation. As a result, the H-depleted regions were fractured by the typical coalescence of micro-voids, namely relatively ductile shear cracking, resulting in dimpled surfaces after tensile fracture. These results mean that the TTS region, strictly speaking, the flat surface region, which is generated by transcolonial fracture inside the TTS region, is an initial site of H-induced cracking of pearlitic steel. The ab-initio calculations of the energy value of H desorption from a C vacancy in θ supported the recent report that H atoms are concentrated inside the θ platelets after H charging.

Formation mechanism of DKDP surface in single point diamond fly-cutting process and the resulting degradation of laser-induced damage performance.

Laser damage performance of DKDP (KD2xH2(1-x)PO4) crystal is largely determined by the surface microstructures generated in the manufacturing process, more specifically, single point diamond fly-cutting process. However, because of the lack of knowledge about the formation mechanism and damage performance of the microstructures, laser induced damage of DKDP crystal remains a key issue limiting the output energy of the high power laser systems. In this paper, the influence of fly-cutting parameters on the generation of DKDP surface and the underlying material deformation mechanism have been investigated. Except for cracks, two kinds of new microstructures, namely micro grains and ripples, have been found on the processed DKDP surfaces. GIXRD, nano-indentation and nano-scratch test results prove that the micro grains are generated by the slip motion of the crystal, while the simulation results show that the cracks are induced by the tensile stress formed behind the cutting edge. Moreover, the formation of micro grains can facilitate the plastic chip flow through the mechanism of grain boundary sliding, which will further lead to a periodic fluctuation of the chip separation point and the formation of micro ripples. Finally, laser damage test results demonstrate that cracks will degrade the damage performance of DKDP surface significantly, while the formation of micro grains and micro ripples has little impact. The results of this study can deepen the understanding of the formation mechanism of the DKDP surface during the cutting process and provide guidance to improve the laser-induced damage performance of the crystal.

Surface morphology in high-speed grinding of TMCs fabricated by selective laser melting

TiC-reinforced titanium matrix composites (TMCs) are typically difficult-to-machine materials. It is prospective to combine the advantages of selective laser melting (SLM) and high-speed grinding to manufacture TMCs parts. This study investigates the surface morphology in high-speed grinding of TMCs fabricated by SLM. Grinding experiments with 28.75–232.66 m/s are conducted. Two surface features are found in the ground surface of TMCs, i.e. smearing at low grinding speeds and scratch at high grinding speeds. Surface morphologies are evaluated based on fractal dimension FD and surface roughness Ra. The results show that the correlations between FD and Ra indicate the ground surface formation mechanisms of TMCs. It is found that submicron TiC generated in SLM cooperating with high grinding speed achieves a good surface quality in terms of reduced surface roughness and subsurface damages for TMCs, which could be beneficial to the fabrication of other metal matrix composites.

Molecular dynamics simulation of the nano-cutting mechanism of a high-phosphorus NiP coating

Electroless nickel-phosphorus workpieces undergo a variety of surface/subsurface deformations and internal organizational changes throughout the nano-cutting process that have a major impact on the machining quality of microstructured functional surfaces. In this paper, molecular dynamics simulations were performed for nano-cutting of electroless nickel-phosphorus (Ni–P) workpieces obtained at different cooling rates (1e11 K/s, 1e12 K/s, 1e13 K/s). To research the mechanisms of material removal and surface formation during nano-cutting, the impacts of various cutting depths on atomic ordering, cutting force, cutting temperature, shear strain, material pile-up, and surface shape were examined. The simulations showed that material removal of NiP coatings was mainly based on nanoscale extrusion rather than macroscale shear. The radial distribution function of NiP coatings was dominated by the Ni–P pair, the cutting force of the nano-cutting process was more stable, the thrust force was smaller, and the machined surface roughness Ra was smaller when the cooling rate was higher. However, the number of atoms on the upper surface of the workpiece increased more, and the actual machined surface contour line deviated from the ideal surface contour when the cooling rate was faster. To increase the forming accuracy of the machined surface on NiP without destroying the machined surface roughness, the cooling rate must be slowed appropriately for the NiP alloy to form a tiny amount of crystalline organization or to minimize the nano-cutting depth.

Experimental and FEM study of surface formation and deformation mechanism of SiCp/Al composites in laser-ultrasonic vibration assisted turning

Experimental and FEM study of surface formation and deformation mechanism of SiCp/Al composites in laser-ultrasonic vibration assisted turning

Conical Grinding Wheel Ultrasonic-Assisted Grinding Micro-Texture Surface Formation Mechanism

The rotating ultrasonic-assisted grinding (RUAG) experiment of the conical grinding wheel generated the intermittent pit-shaped micro-texture on the surface of the workpiece, reducing thermal damage and improving the lubrication characteristics compared with conventional grinding (CG). To further optimize the surface properties, this paper studied the formation mechanism of micro-texture. This study used as basis the theory that micro-debris volume equals the macroscopic material removal one to establish the mathematical equation of grinding depth. Thereafter, formulas of micro-texture feature parameters, including pit length, pit depth, and texture spacing were deduced. The solved microscopic grinding depth was alternatingly positive and negative, indicating that the alternating separation between the grinding grain and workpiece caused intermittent pits in the grinding. Through response surface analysis (RSA), this paper analyzed the relationships among macroscopic grinding depth, micro-texture feature parameters, and machining parameters (i.e., amplitude, feed rate, and rotational speed). Single-factor experiments of machining parameters, with finite element simulation and experiment methods, were performed to verify the theoretical micro-texture features. The simulated program formed three-dimensional surfaces with micro-textures. Their measurement results were consistent with the theoretical ones. Experimental results proved that the range of pit length covers the theoretical ones, further verifying the accuracy of the grinding depth model. For this grinding wheel, the 8–10 μm amplitude was optimal for better roughness, lubrication, and thermal damage. Roughness was improved when increasing the rotational speed or reducing the feed rate based on the experiment. If the rotational speed and feed rate exceed the limiting values, then continuous grinding will break down the abrasive grains and even damage the cubic boron nitride (CBN) coating. Experimental results likewise showed that the pit shape was closely related to the surface properties, which deserves further investigation.

Predictive modelling for enhanced scratching of brittle ceramics with magneto-plasticity

The micro-cutting of brittle ceramics starts with elastic-plastic deformation followed by severe brittle fracture, which significantly restricts the machinability and application of these materials. Magnetic field-enhanced plasticity (i.e., magneto-plasticity) has shown a positive potential to augment the ductile-brittle transition and machinability but the material removal mechanism of brittle ceramics with magneto-plasticity and its dependence on strain rate remains unclear. In this study, a deformation energy-based model was developed to bring light to the deformation mechanisms affecting the ductile-brittle transition under magneto-plasticity and the corresponding dependency on strain rate. MD simulations assisted the analytical modeling by deriving the orientation-dependent dislocation movement and stress distribution that contribute to single-crystal anisotropy in the surface formation and ductile-brittle transitions during scratching. The proposed model accurately predicted the improvement in ductile-brittle transition under various magnetic field intensities and orientations as confirmed in the experimental magnetic field-enhanced micro-scratching tests on single-crystal calcium fluoride (CaF2). Scratched surface morphologies, acoustic emission (AE) signals, and critical loading conditions demonstrated the improvements in ductile-brittle transition with magneto-plasticity. Additionally, the progression of lateral slip trace and tangential surface pile-up under various magnetic field conditions confirm the direct influence of magneto-plasticity on dislocation mobility and the change in surface formation mechanism of brittle ceramics. This study not only deepens the understanding of the manifestation of magneto-plasticity during the deformation of brittle ceramics, but also opens new avenues for the integration of magnetic field-assisted technology in manufacturing processes.

Grinding Force and Surface Formation Mechanisms of 17CrNi2MoVNb Alloy When Grinding with CBN and Alumina Wheels

The 17CrNi2MoVNb alloy is widely used for manufacturing heavy-duty gears in vehicles' transmission systems, where grinding is a significant process affecting gears' working performance and service life. This work comprehensively analyzed the grinding force, surface morphology, and surface roughness when grinding 17CrNi2MoVNb alloy using alumina and CBN grinding wheels. Results showed that the maximum normal grinding force from the CBN wheel was only ~67% of the one from the alumina wheel. Due to the small size and limited cutting depth of CBN grains, the grinding force increased by about 20% when the grinding depth increased from 0.02 to 0.03 mm for CBN grinding wheels. Surface defects, including cavities and material tearing, were mainly found on the ground surface when using an alumina grinding wheel. The surface roughness Ra recorded from the CBN grinding wheel mainly ranged from 0.263 to 0.410 μm, accounting for less than 40% of the one from the alumina grinding wheel. The information from this work could provide benchmark data and references for optimizing grinding tools and parameters when manufacturing gears in the vehicle industry.

Formation mechanism of surface modified iron oxide nanoparticles using controlled hydrolysis reaction in supercritical CO2

In the synthesis of surface modified nanoparticles (NPs), supercritical CO2 is one of appealing reaction fields to realize simple and green chemistry approach without using an organic solvent. In this work, surface modified iron oxide NPs was synthesized in supercritical CO2 with various amounts of water between 5.0 and 90.0 mmol to elucidate the formation mechanism of surface modified NPs with the hydrolysis reaction. The synthesis was performed at 30.0 ± 0.9 MPa of CO2, 18 h and 100 °C, where iron(Ⅲ) acetylacetonate, water and oleic acid were used as starting materials. As a result, the smaller amount of water was revealed to be key factor to synthesize more densely modified iron oxide NPs without the aggregation and iron oxyhydroxide formation while the larger amount of water caused the sever aggregation of less modified NPs and the formation of iron oxyhydroxide. Additionally, the detailed analysis of iron oleate complex and the phase state prediction suggested that the smaller amount of water allowed more dominated formation of iron oleate and subsequent its hydrolysis reaction in uniform supercritical CO2 phase, resulting in the formation of densely modified NPs without the aggregation.