The work aims to study the effect of ultrasonic pulse energy on the film formation process,microstructure,morphology,and corrosion resistance of micro-arc oxidation(MAO)coatings on AZ31B magnesium alloy,to further analyze the film formation mechanism.By varying the ultrasonic pulse frequency(50,100,150 kHz)to adjust the pulse energy,micro-arc oxidation was performed on the surface of the AZ31B magnesium alloy substrate in a silicate system.The surface morphology,elemental composition and phase structure of the coating were analyzed by laser confocal microscopy,SEM/EDS and XRD.The corrosion resistance of the coating was characterized through electrochemical testing.The phase composition of the micro-arc oxidation coating prepared in the silicate system mainly consisted of MgO and Mg2SiO4.As the ultrasonic pulse frequency increased,the pulse energy decreased,leading to an extended arc initiation time during the MAO process,a reduction of the micropores diameter,and an improvement in the uniformity of the coating surface.Compared with the substrate,the MAO coating prepared under ultrasonic pulse conditions exhibited an increase in self-corrosion potential by 1.5 V and a reduction in self-corrosion current density by 3 to 4 orders of magnitude.In conclusion,ultrasonic pulses voltage significantly affects the microstructure of magnesium alloy MAO coatings and enhances the uniformity of the coatings.With the increase of pulse frequency,which corresponds to a decrease of pulse energy,the size of the arcs during the MAO process reduces,and the number of arcs increases.Consequently,the size and diameter of the micropores on the surface of the MAO coating are reduced.At the same time,it also reduces the thickness of the loose layer and increases the thickness of the dense layer,which is great beneficial to improve the corrosion resistance of the coatings.
This paper employs numerical simulation to investigate the influence of thermal barrier coatings' (TBCs) thickness and surface roughness on the cooling and flow resistance characteristics of turbine blades. The results indicate that the application of TBCs significantly enhances the surface cooling efficiency of the blades. Turbine blades coated with a 0.4 mm thickness of TBC compared to blades without thermal barrier coatings, the average cooling efficiency of the blade surface increases by 12.3 %, and the maximum temperature drop at the leading edge(LE) is 317.8 K. However, the small increment in TBCs thickness leads to an increase in aerodynamic losses in the vane passage. The static pressure coefficient continuously decreases in the suction side(SS) within the interval 0.3 < x/C < 1.0. The average flow coefficient of the film holes exhibits distinct variations in different regions of the blade. As surface roughness increases, the cooling efficiency on the SS and pressure side(PS) of the blade decreases, while the heat transfer enhancement at the LE and cooling efficiency improve. Compared to a smooth coated surface, when the surface roughness height increases to 20 mu m, the blade surface cooling efficiency decreases by 1.12 %, and the average temperature rise is 10.3 K. Simultaneously, the energy loss coefficient and total pressure loss coefficient in the vane passage rise with the increase in surface roughness, while the variation in the average flow coefficient of the film cooling holes remains relatively small.
Moderate doping of heterogeneous elements can effectively solve the problem of insufficient adhesion between diamond-like carbon (DLC) thin films and substrates,leading to film detachment.In recent years,tantalum (Ta) has been introduced as a new metal dopant in carbon-based thin films,which can significantly improve the mechanical and tribological properties of the films.As a biocompatible metal with high melting point,wear resistance,corrosion resistance,high ductility,and excellent biocompatibility,tantalum exhibits structure and mechanical properties similar to human bones,promoting the proliferation and osteogenesis of human osteoblast cells.However,there is still limited research on tantalum-doped DLC films,especially regarding their biocompatibility. This study aims to explore the structural transformation of tantalum-doped DLC films and enhance their mechanical properties,tribological performance,and biocompatibility.The non-equilibrium magnetron sputtering technique was employed to deposit the films,and the tantalum doping level was controlled by adjusting the power of the tantalum target.DLC films with different Ta doping levels were prepared at power levels ranging from 0 to 0.5 kW.The films were characterized in terms of microstructure,chemical composition,friction performance,mechanical properties,and biocompatibility.The relationship between tantalum doping level and film performance was investigated to identify the optimal tantalum doping ratio and obtain DLC films with excellent performance,laying a foundation for their widespread applications in surface modification of artificial joints and other fields. The results showed that the inclusion of tantalum increased the carbon deposition rate,leading to an increase in film thickness.The sp3-C content in the films initially increased and then decreased with the increase of the Ta doping level.TaC crystals and Ta—Ta nanoclusters were observed in the DLC films doped with Ta at 0.2 kW and above,which resulted in an initial increase and subsequent decrease in surface roughness.Compared with undoped DLC films,Ta-doped DLC films exhibited several improvements.The film-based bonding force increased from 10 N to 25 N,leading to enhanced adhesion between the film and the substrate.The fracture toughness also improved from 0.6 Mpa·m1/2 to a value of 1.6 Mpa·m1/2 or higher.This indicated that the Ta-doped DLC films were more resistant to crack propagation,making them more mechanically robust.In terms of friction properties,the dry friction coefficient decreased from 0.45 to a range of 0.1 to 0.15.This meant that the Ta-doped DLC films experience reduced friction when in contact with dry surfaces.Similarly,the wet friction coefficient decreased from 0.35 to around 0.1,indicating improved lubrication and reduced friction under wet conditions.Moreover,the wear rate associated with dry friction diminished significantly from 4500×10-6 mm3/(N·m) to 7×10-6 mm3/(N·m) or lower.The wet friction wear rate also decreased to 1×10-6 mm3/(N·m).This suggested that Ta-doped DLC films exhibited superior wear resistance,making them more durable in both dry and wet environments.However,there were slight compromises in other aspects.The elastic modulus of Ta-doped DLC films experienced a slight decrease,indicating a slight reduction in their stiffness.Additionally,the wettability of the films also underwent a slight decrease.In addition,by simulating body fluid immersion,Ta doped thin films exhibited good capability in inducing hydroxyapatite formation,with a calcium-to-phosphorus (Ca/P) ratio ranging from 1.4 to 1.65,close to the Ca/P ratio in the human body.No cytotoxicity was observed for either doped or undoped films. In summary,the doping of Ta significantly improves the tribological and mechanical properties of DLC films,as well as the capability to induce hydroxyapatite formation.Therefore,these films have the potential to be used as a bio-protective layer on the surface of implants.The Ta-DLC film exhibits the best overall performance at Ta-0.4 kW.
The Cr-coated Zr alloy materials appear to be one promising short-term accident tolerant fuel cladding concept, due to their outstanding high-temperature oxidation resistance and improved resistance to fretting wear. In addition, there are few barriers or challenges to be applied to nuclear reactors for coated Zr alloy claddings. In this work, uniform and dense Cr coatings with thickness of 12-15 mu m were deposited by magnetron-sputtering process on the outer surface of Zr-1Nb alloy tubes. Two-sided isothermal oxidation tests were performed in flowing steam by synchronous thermogravimetric analyzer, at temperatures ranging from 1000 degrees C to 1200 degrees C and for oxidation time ranging from 300 s to 5000 s, with the object of systematically studying the high-temperature steam oxidation behavior of Cr-coated Zr alloy cladding during reactor accidents. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction were utilized to characterize the microstructure, oxide layer thickness, element distributions and phase of oxide scale to study the oxidation kinetics and mechanisms of Cr coating. Based on these analyses, dense chromia scale is developed on the outer surface of cladding during steam oxidation, preventing the oxygen atoms from diffusing into the substrate, to improve the high-temperature resistance of the composited claddings. In addition, the oxidation kinetics of the Cr coating is nearly parabolic and the rate constants is at least two orders of magnitude lower than the Zr alloy, enhancing high-temperature steam oxidation resistance of Zr alloy claddings.
The surface of GCr15SiMn bearing steel was strengthened by supersonic fine particle bombardment (SFPB), and the effects of different impact times on the microstructure, mechanical properties and friction and wear properties of the material were studied. The results show that the surface roughness of GCr15SiMn steel samples increases and the grain is refined after SFPB. As the impact time prolongs, the grain size gradually decreases. When the bombardment time is 180 s, the surface grain size of the sample reaches 12.7 nm. The microhardness of the sample gradually increases and tends to stabilize with the increase of the impact time. The residual compressive stress layer is produced on the surface of the sample after SFPB and the residual compressive stress shows a trend of first increasing and then decreasing with rising the impact time. The friction coefficient and wear rate of GCr15SiMn bearing steel decreases after SFPB. The wear mechanism of the sample changes from mixture of adhesive wear and oxidation wear, accompanied by slight abrasive wear (before SFPB) to the abrasive wear, supplemented by adhesive wear (after SFPB). The wear resistance is improved by the of the grain refinement and the formation of a high residual compressive stress layer.
TC4 titanium alloy has low density,corrosion resistance,high strength ratio,good fracture toughness and other characteristics,so it is widely used in the aerospace field.When the aircraft passes through the low-temperature cloud in the air,the intake part of the engine will freeze due to the existence of supercold water droplets,which will affect the flight safety.Therefore,it is particularly important to prepare superhydrophobic surface on TC4 titanium alloy.In order to improve the hydrophobicity,corrosion resistance and mechanical properties of the superhydrophobic surface of TC4 titanium alloy,a combined method of chemical etching and anodic oxidation was proposed to prepare the superhydrophobic surface.Firstly,TC4 titanium alloy was treated with chemical etching to prepare micron-scale structure,and then nano-scale structure was prepared by anodic oxidation method.Finally,superhydrophobic surface with micro-nano structure was prepared on the surface of the sample.The superhydrophobic surfaces prepared by H2O2 etching,strong acid etching,anodic oxidation,H2O2 etching-anodic oxidation and strong acid etching-anodic oxidation were compared by Tafel test,linear wear test,impact resistance test and anti-icing performance test respectively.It can be seen from the experimental results that the contact angle of the superhydrophobic surface prepared by the mixed solution of hydrogen peroxide and sodium bicarbonate is 156.4° and the rolling angle is 2.7°.The contact angle of the superhydrophobic surface prepared by the mixed solution of sulfuric acid and hydrochloric acid is 153.1° and the rolling angle is 7.6°.The contact angle of the superhydrophobic surface prepared by anodic oxidation is 156.3° and the rolling angle is 4.2°.The surface contact angle is 157.6° after etching with hydrogen peroxide and sodium bicarbonate mixed solution and anodizing treatment,and 155.9° after etching with sulfuric acid and hydrochloric acid mixed solution and anodizing treatment,and the rolling angle of both is less than 2°.Scanning electron microscopy(SEM)is used to observe the microscopic morphology of the superhydrophobic surface prepared by the five methods.It can be seen that the superhydrophobic surface prepared by the combined method presents a micro-nano hierarchical structure,which effectively reduces the contact area between the water droplets and the surface and significantly reduces the rolling angle.Therefore,the hydrophobic property of the surface prepared by the method method is superior to that prepared by a single method.The OCP of superhydrophobic samples is higher than that of TC4 titanium alloy.Especially the OCP of superhydrophobic samples after strong acid etching and anodic oxidation treatment is moving to 0.08 V,and the corrosion resistance is significantly improved.According to the polarization curve,the Jcorr of the superhydrophobic surface prepared by the combined method is reduced by one order of magnitude,and the Rp is increased by one order of magnitude,which also indicates that the corrosion resistance has been greatly improved.The superhydrophobic surface prepared by the combined method can still maintain the contact angle above 150° and the rolling angle about 10° after several linear wear and 200 g sand fall impact,and still maintain the superhydrophobic property.Therefore,the superhydrophobic surface with micro-nano hierarchical structure prepared by combined chemical etching-anodizing method has better hydrophobicity,corrosion resistance,wear resistance and impact resistance than the superhydrophobic surface with a single structure.
To improve the grinding wheel wear and enhance the surface quality of the workpiece when grinding engineering ceramic materials with a traditional diamond grinding wheel.This paper proposes a design and preparation method for a nano self-lubricating diamond grinding wheel imitating a bird feather structure.Firstly,the bronze bond nano self-lubricating diamond grinding wheel was prepared by taking artificial diamond as abrasive,bronze bond as binder,and adding a certain mass fraction of nano-molybdenum disulfide and nano-titanium dioxide as filler materials.Secondly,the secondary feather trunk structure of swallows with drag-reducing properties was borrowed to be applied to the surface of grinding wheels.The optimal geometric model and dimensional parameters of the imitation bird feather structure applied to the grinding wheel surface were designed.Through orthogonal tests,the optimal laser processing parameters for machining the designed dimensions of the bird-feather imitation structure on the surface of a bronze bond diamond grinding wheel are derived.A new self-lubricating diamond grinding wheel with a bird-feather-like structure was obtained by ablating the designed bird-feather-like drag-reducing geometrical structure on the surface of a nano self-lubricating diamond grinding wheel using a pulsed laser.Four kinds of grinding wheels with different working conditions,namely,traditional bronze diamond wheel(TGW),nano self-lubricating diamond wheel(NGW),bird-feather structured diamond wheel(FGW),and bird-feather structured nano self-lubricating diamond wheel(FNGW),are prepared to compare the differences in their grinding performances.To investigate the grinding performance of FNGW,SiC ceramic grinding experiments were conducted.By analyzing the surface morphology and mechanical properties of FNGW,it was found that the addition of nanoparticles does not degrade the mechanical properties of the wheels and that the bird feather structure on the surface of the wheels has a high-quality laser-formed ablation and does not negatively affect the unabraded areas of the wheels.The grinding performance of the FNGW was evaluated in terms of grinding force,surface quality,and wheel wear.The results show that the addition of nanoparticles does not degrade the mechanical properties of the grinding wheel,and the bird feather structure on the surface of the wheel has a high laser-formed ablation quality and has no effect on the unabated area of the wheel.Compared with TGW,FGW showed significant improvement in grinding performance,but the improvement in surface roughness and wheel wear was not obvious.NGW also showed some improvement in grinding performance,but the overall improvement was not obvious.In the case of FNGW,the combination of the bird-like feather structure and the nanoparticles resulted in a significant improvement in the grinding performance.Compared with TGW,FNGW reduces the grinding force by up to 65.1%,the workpiece roughness value by up to 21.5%,and the grinding wheel wear is significantly reduced,which effectively prolongs the service life of the grinding wheel.The proposed FNGW utilizes the controlled release film-forming effect of self-supplied nanocomposite solid particles in the grinding arc to enhance the lubrication and material removal effect during the grinding process.In addition,it achieves efficient cooling and smooth chip removal by means of a bird feather dampening structure,which improves the anti-wear performance of the grinding wheel surface structure and enhances the surface quality of ceramic material parts.
The work aims to review the relevant research reports on magnesium silicate hydroxide in the field of lubrication inChina and abroad in detail.Then,two commonly used preparation methods of magnesium silicate hydroxide nanoparticles are listed,explaining the rationality,advantages and disadvantages of each method.The chemical methods for the synthesis of magnesium silicate hydroxide nanoparticles are highlighted.The effects of different reaction conditions(pH value,molar ratio:Si/Mg,temperature and holding time)on the morphology control were explored.It was found that with the increase of pH,temperature and holding time,the morphology of magnesium silicate hydroxide changed from flake/flower to tubular/fibrous.Secondly,the methods used to improve the dispersion stability of magnesium silicate hydroxide in lubricating oil were summarized.Specifically,it included:surfactant based on oleic acid.Through the carboxyl functional group at its end and the hydroxyl functional group on the surface of magnesium silicate hydroxide,with the physical action of van der Waals force and hydrogen bonding,it was coated on the surface of nanoparticles.The hydroxyl group that can be further reacted and the surfactant with hydroxyl or carboxyl group are selected,and the long organic chain is introduced on the surface through condensation reaction or esterification reaction and hybridization with other substances,etc.Then,the changing rules of the tribological properties of magnesium silicate hydroxide nanoparticles under different friction parameters,special matching pairs and hybridization with rare earth elements are discussed.The formation mechanism and action mechanism of the tribofilm of magnesium silicate hydroxide on the surface of the friction pair are analyzed.Based on the reported literature,the potential lubricating mechanism(surface repair effect,tribochemical reaction film,microstructural transformation,synergistic effect and internal oxidation mechanism)of magnesium silicate hydroxide nanoparticles for anti-wear and anti-friction properties is summarized.Finally,the problems and challenges in the application and popularization of magnesium hydroxy silicate nanoparticles in the field of lubrication in the future are put forward.The corresponding solutions are given for the existing problems.
The work aims to achieve efficient and nondestructive removal of organic coatings on the surface of automotive carbon fiber reinforced resin matrix composites(CFRP)structural parts.In this work,recyclable melamine plastics were selected to prepare abrasives.A new machining method with air jet carrying and high-speed jetting was proposed.The polyurethane-coated CFRP specimens were eroded by single-factor control of the erosion angle with 500 μm abrasive at different erosion angles under 0.3 MPa jet pressure.The erosion morphology was observed with an SEM and a super depth-of-field 3D microscopy.The coating removal mechanism was illustrated by developing a micro cutting and repetitive deformation model based on the law of energy conservation.The particle velocity and contact stress,and quantitatively calculating the mass of coating loss were analyzed.The effects of abrasive shape,rotation and rebound on erosion mechanism were investigated.The results showed that the material removal rate of the coating was the largest when the erosion angle was 30°,and the removal rate was 5.8×104 g/s,which showed ductile erosion behavior,and the erosion mechanism was micro-plowing and micro-cutting at this time.The material removal rate decreased with the increase of the erosion angle,and when the erosion angle was 90°,the removal rate was 1.2×104 g/s.The erosion mechanism of the coating was repeated plastic deformation removal.Sharp angular particles impacted the coating with concentrated stress and removed the material in the form of cutting at the inclined angle.Whereas abrasives evolved from sharp angles to sub-spherical particles due to wear(which can be cycled 15 times)and impacted the coating with distributed stress.In addition,the removal of coatings by abrasives reverse rotation was greater than that of forward rotation attributed to the reverse rotation increasing the effective cutting speed.Large particle size rebound produced incomplete cutting paths and small particle size rebound produced tearing of the coating.In order to achieve non-destructive removal of substrate materials,it is recommended to use high erosion angle so as to retain primer and achieve non-destructive removal of substrate materials.
The work aims to introduce composite anti-/deicing methods that combine superhydrophobic surfaces with active anti-icing methods of photothermal treatment,heating,magnetic energy,and acoustic energy,along with their mechanisms and applications.The advantages and shortcomings of the composite anti-/deicing technologies were summarized.Among them,the photothermal composite technology showed low energy consumption and diverse choice on materials.The electric heating composite technology possessed a simple process and was beneficial to preparation in a large area.The magnetic energy composite technology satisfied the needs of anti-/deicing in complex scenes.The acoustic wave composite technology had a simple structure.In addition,the simultaneous combination of multiple energy fields could further enhance the efficiency of anti/deicing performance,so it was one of the trends for developing active-passive composite technologies.Finally,the shortcomings of the current active-passive composite technologies were elaborated based on superhydrophobic surfaces,and the application of those composite anti-/deicing technologies was prospected.
Atmospheric plasma spraying(APS)is an advanced surface modification technology,which can improve surface properties of titanium alloy without changing the substrate material,such as wear resistance,corrosion resistance,oxidation resistance and other properties.Many researchers have studied the abrasion resistance and corrosion resistance of Al2O3-13％TiO2 coating prepared by APS on titanium alloy at room temperature.However,temperature has an important effect on the performance of oxide ceramic coatings and related researches are rarely reported.The work aims to study the effect of temperature on the friction and wear properties of Al2O3-40％TiO2(AT40)ceramic coating and explore the friction and wear mechanism of the coating at high temperature. Commercially available HasC-276(NiMo16Cr15Fe6W4,wt.％)powders and Al2O3-40％TiO2(wt.％)with a nominal particle size distribution of-45-+15 μm and-35-+5 μm were prepared as spray powder,respectively.Plain TC4(Ti-6Al-4V,wt.％)titanium alloy plates(30 mm×15 mm×8 mm)were used as substrate materials.Prior to APS spraying,the substrates were sand blasted with corundum grit(50～70 mesh)in order to improve the bonding strength between coating and substrate.The coatings were deposited by atmospheric spraying equipment(PRAXAIR 3710M,America)manipulated with a robot(ABB,Sweden). The plasma deposition process was carried out with optimal process parameters.The HasC-276 layer acted as bonder coating in order to reduce the difference in mechanical properties between TC4 substrate and ceramic coating,which reduced the crack sensitivity and improved the adhesion.The spray thickness of HasC-276 layer and AT40 coating was about 100 µm and 250-300 µm,respectively. AT40 coating samples were cut with wire cutting and the cross section and surface were polished to a smooth surface with Ra of(0.15±0.02)μm.The friction and wear properties of AT40 ceramic coating were tested on a multi-function friction wear tester(MFT-5000,China)at 200℃,350℃and 500℃and as well as the in-situ online automatic 3D morphology characterization.Scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)were used to qualitatively analyze the micro morphology and phase of AT40 ceramic coating.The section micro-hardness distribution of AT40 ceramic coating at room temperature and at high temperature was studied with the Vickers micro-hardness tester. The results show that the AT40 ceramic coating presents a typical thermal spraying layered structure,with uniform distribution of all phases and dense coating structure.The average micro-hardness is 81％higher than that of the TC4 titanium alloy substrate.The high temperature hardness of AT40 ceramic coating at 200,350 and 500℃is 513,463 and 448HV0.3 respectively.At 200℃and 350℃,the average friction coefficient of AT40 ceramic coating is 0.18±0.02 and 0.38±0.03 respectively,and the wear rate is(7.8±0.01)×10-5 mm3/(N·m)and(37.2±0.01)×10-5 mm3/(N·m)respectively and the coating shows excellent high temperature friction and wear resistance.At 500℃,the average friction coefficient and wear rate of the coating are 0.77±0.02 and(134.4±0.01)×10-5 mm3/(N·m)respectively,the wear scar depth and wear volume increase significantly,and the wear resistance decreases. A few small holes and micro-cracks are observed on the surface morphology of AT40 as-sprayed coating,which acts as initial-cracking.During the high temperature wear process,the surface of AT40 coating under the action of friction and compressive stress will generate high local stress,which will cause these initial micro-crack to grow and expand along the oxide structure boundary and the holes of the coating and generate longitudinal through cracks,forming micro brittle fracture.The wear mechanism of AT40 ceramic coating is mainly micro brittle fracture at 200℃and 350℃. Moreover,with the temperature increasing to 500℃,the thermal stress inside the coating is the major factor that promotes the crack propagation.It causes the coating to delaminate and peel off,forming peeling pits and wear debris.These peeled particles remain on the surface of the sample and will be crushed to fine debris.These pulled out debris will act as abrasive particles leading to the three-body abrasive wear.Many wear grooves,micro-cracking,wear debris and pores are obviously observed on the worn surface of the AT40 coating at 500℃indicating that delamination and peeling caused by crack propagation and slight abrasive wear are the main wear mechanism.
Good biosafety,biocompatibility and valuable self-degradation properties endow medical magnesium and magnesium alloys with great potential to replace inert implant materials in the field of traditional clinical applications.The excessive degradation rate of magnesium alloy,however,leads to its premature loss of structural integrity and mechanical support,being unable to complete the effective service time necessary for tissue healing of the implant site.At the same time,it is also its excessive degradation rate that leads to the intensification of hydrogen evolution reaction of magnesium alloy.Because it cannot be absorbed by the human body in a short time,the excessive H2 will easily gather around the implant or form a subcutaneous airbag,which will not only cause the inflammation of the implant site,but also hinder the adhesion and growth of cells in the implant,limiting its clinical application prospects.Surface modification technology can effectively delay the degradation rate of medical magnesium and magnesium alloys,and reduce the rate of hydrogen evolution. Firstly,starting from the structure and performance characteristics of organic materials(phytic acid(PA),chitosan(CS),stearic acid(SA),dopamine(DA),polylactic acid glycolic acid copolymer(PLGA),polylactic acid(PLA),and polycaprolactone(PCL)),the mechanism of improving the corrosion resistance of magnesium and magnesium alloys by a single organic coating was analyzed,and the performance weaknesses of a single coating were also pointed out:①Micro arc oxidation(MAO)is an anodic oxidation process that generates a highly adhesive ceramic oxide coating on the surface of an alloy immersed in an electrolyte through high voltage(up to 300 V)spark discharge.The continuous high voltage discharge and the bubbles generated by the reaction bring about the inevitable occurrence of a large number of volcanic micropores and cracks in the coating.The diversity of discharge modes also gives rise to the unpredictable morphology of micropores and cracks.Therefore,the preparation of a single MAO coating on different alloy surfaces does not only require proper adjustment of MAO electrical parameters(current density,voltage,duty cycle,frequency,oxidation time)and the coupling effect of its electrolyte system to decrease(small)the pores and cracks on the MAO coating surface,but also increases the sealing process at the later stage.② A single organic coating has a low bonding strength with magnesium alloy,being easy to flake off.These performance weaknesses limit the protection effect of a single coating on magnesium alloy degradation. Secondly,from the perspectives of bonding strength,corrosion resistance,and versatility(biosafety,biocompatibility,induced regeneration,antibacterial and antibacterial properties,drug loading and sustained-release properties,and so on),the structural characteristics and advantages of each MAO/organic composite coating were elaborated in detail.It has revealed that MAO/organic composite coating has an enormous application potentiality in the field of surface modification of medical magnesium and magnesium alloys,thanks to its good corrosion inhibition and degradation performance.On this basis,it is clearly pointed out that,in order to achieve the biological activity and versatility of medical magnesium alloy implant materials,the best way is to adopt the MAO/PCL(MAO/CS)composite coating as the base coating and make the cross combination of PCL(CS)coating and other coatings.Finally,the evolution direction of magnesium alloy MAO/organic composite coating is scientifically predicted.
There is a process signature problem related to clean machining surface integrity in metal cutting,which is extremely important to the service life and service performance of high efficiency and clean manufactured parts.Exploring the relationship between energy consumption changes and machined surface integrity during clean cutting of titanium alloy,will help optimize the cutting parameters to control the machined surface quality,and eventually improve the service life and performance efficient and clean manufacturing parts of titanium alloy. In this paper,a process signature method based on energy consumption was proposed to describe the interaction between multi-step clean machining process and machined surface integrity.The calculation model of specific cutting energy was established related to the process field parameters and cutting parameters.Combined with the two-step milling experiments of titanium alloy,the influence of the variation of rough machining parameters on the cutting force and specific cutting energy of rough machining and finish machining was analyzed,and the specific cutting energy of the machined surface of two-step machining was further studied,the variation laws of residual stress and crystallite size of rough machined and finish machined surfaces under different rough machining parameters were studied.The results showed that the changes in cutting force and cutting parameters could both affect the magnitude of specific cutting energy.In the multi-step machining process,the cutting speed in rough machining had the most significant impact on the cutting force,residual stress,and surface microcrystalline size of finish machining.The selection of cutting parameters for rough and finish machining in multi-step cutting processes could cause changes in surface integrity by changing the specific cutting energy.The radial cutting depth had the greatest influence on the specific cutting energy,followed by the feed rate and cutting speed.With the increase of feed rate and radial cutting depth,the specific cutting energy decreased.With the increase of cutting speed,the specific cutting energy first increased and then decreased.The microcrystalline size on the surface of precision machining was larger under higher cutting speed range,indicating that the larger rough machining cutting speed weakened the phenomenon of microcrystalline refinement on the surface of precision machining.As the radial cutting depth of rough machining and the feed rate per tooth increased,the surface microcrystalline refinement of precision machining showed a trend of weakening and then strengthening.When the specific cutting energy was large,the residual stress was large and the crystallite size was small. The establishment of a prediction relationship model for specific cutting energy consumption and machined surface quality is beneficial for process planning before actual machining process,and is of great significance for the reasonable selection of cutting parameters and the improvement of energy efficiency.On the premise of ensuring the machining quality,from the perspective of energy saving and consumption reduction,appropriate cutting speed,large cutting depth and feed rate should be selected,so as to reduce the specific cutting energy,reduce energy consumption and improve the machined surface integrity,which will help advancing the realization of clean and energy-saving manufacturing.
The unique surface properties of superhydrophobic surfaces make them capable of sealing the air film underwater.With excellent underwater drag reduction effects,it has become an international research hotspot.The maintenance of air films on superhydrophobic surfaces is the key to its application in underwater drag reduction.Some studies show that the air film at large scales has a good drag reduction effect.However,previous studies have mostly focused on the maintenance of air films at millimeter and smaller scales,and the maintenance of air films at centimeter scales has not yet been reported.In this paper,in order to overcome the problems of poor air film maintenance on existing fully superhydrophobic surfaces and insufficient drag reduction effect of millimeter-scale air films,the idea of centimeter-scale air film maintenance on alternant hydrophilic and superhydrophobic surfaces underwater was proposed to obtain high drag reduction performance underwater.Firstly,the k-ω turbulence model was used to simulate the water scouring of the centimeter-scale alternant hydrophilic and superhydrophobic surface and fully superhydrophobic surfaces at a flow rate of 0.5 m/s,in which the contact angle of the superhydrophobic surfaces in the groove on the alternant hydrophilic and superhydrophobic surface was set to 165°,and the hydrophilic gaps were set to 45°.The contact angles of the fully superhydrophobic surfaces were all set to 165°.Then,based on the simulation results,water scouring experiments were carried out on the two surfaces at a centimeter scale under different water flow rates and air supply conditions,and the changes in the air film shapes of the two surfaces under different experimental conditions were investigated.The simulation results showed that a hydrophilic gap made the gas in the groove subject to a pinning and binding effect,which allowed gas film maintenance.The experimental results showed that the air film on the fully superhydrophobic surface at the centimeter scale was maintained better at a flow rate of 0.55 m/s.However,when the flow rate reached 0.94 m/s,with the calculated Reynolds number of Re =14030,the flow state in the experimental section was turbulent,the area covered by the air film in the groove changed from time to time,and the area that could be covered was generally no more than 50%of the area of the groove,which made it difficult to maintain the complete air film.While in the flow rate range of 0～1.2 m/s,the alternant hydrophilic and superhydrophobic surface and hydrophobic spacer surfaces could maintain a stable air film surface in the superhydrophobic region,which had a good effect on maintaining the air film.Based on the experimental results and mechanism analysis,it is concluded that,compared with the fully superhydrophobic surface,the alternant hydrophilic and superhydrophobic surface can provide the maximum binding force to the air film due to the difference in advancing and receding contact angle of the alternant hydrophilic and superhydrophobic surface,which has good air film maintenance performance.The greater thickness of the air film and the greater surface velocity slip result in better underwater drag reduction.
Metal pipeline is an important carrier for continuous and rapid transportation of marine resources such as gas,oil and ore.In the complex and harsh marine environment,the inner and outer walls of the metal pipeline have to withstand the coupling operating conditions of chemical corrosion and physical friction brought by the seawater environment and substances transported,leading to the damage and failure of the pipeline.This seriously shortens the service life of the metal pipeline in the marine environment.Surface coating for the metal pipeline is a direct and effective treatment to modify and strengthen the workpiece,and therefore support it work safely and stably with extended service life.Thus,the development of high-performance protective coatings for metal pipeline adapted to the harsh marine service environment has both significant economic and social values. In this paper,the harsh corrosion environment and various corrosion behaviors of marine metal pipelines are firstly introduced,and then the common protective coatings for marine metal pipelines are classified and summarized.Besides,this review also concludes the advantages and disadvantages of the different protective coating systems by comparing their microstructure,corrosion resistance,wear resistance and the corresponding failure mechanisms,which provides an important reference for the innovation,design and optimization of the new coating materials and structures.In detail,the existing metal coating for marine pipelines is mainly alloys such as nickel alloys,zinc alloys and aluminum alloys.The main advantages of the metal coatings are high strength,high hardness,good toughness,strong binding force of the coating and substrate,and good compatibility with the physical and chemical properties of the substrate materials.Ceramic coatings can be mainly subdivided into nitride ceramics,oxide ceramics and composite ceramics,which has been widely employed for surface protection because of its dense structure,high hardness and good electrochemical impedance.In addition,polymer coating is also a sort of widely used marine anticorrosive coating because of the outstanding chemical corrosion resistance,high density,excellent flexibility and easy maintaining.Typical and representative polymer coatings are ethylene oxide,vinyl resin,polyurethane and so on.In general,polymer coating is mainly used in the external side of marine metal pipelines.In general,it is necessary to comprehensively consider the corrosion environment,pipeline matrix,economic cost and other factors,and then select the proper coating materials system. On the basis,this paper also looks forward to the future research directions of the marine metal pipeline protective coating materials.In the future,researchers may pay more attention to the development of new coating systems with multiple functions,e.g.,high-entropy alloys,high-entropy ceramics,high-performance carbon materials,and amorphous materials.Also,developing new coating structures,such as functional graded coatings,inorganic/organic composite coatings,alloy/ceramic composite coatings,and metal-organic/graphene-modified composite coatings,could be another effective option to offer excellent protective performance for the metal pipeline.
Superabrasive grinding wheels are important grinding tools for difficult-to-cut materials that are increasingly used in aerospace,automotive industry,engineering machinery,and other fields.The design of their matrix and surface structure is directly related to the overall performance of the grinding wheel,which in turn affects the grinding accuracy,workpiece quality,and processing efficiency.Currently,the primary bottlenecks restricting the development of superabrasive grinding wheels are grinding chatter caused by excessive grinding force,thermal damage to the workpiece caused by high grinding temperature,and decrease of accuracy caused by clogging of the grinding wheel,and the structuring design and preparation of superabrasive grinding wheel is an effective way to break through the bottlenecks.In view of this,the work aims to explore the effective measures of superabrasive structured grinding wheels in reducing grinding force and temperature,inhibiting thermal damage to the workpiece surface,and improving the workpiece surface integrity from reducing friction to reducing grinding force,guiding chips to promote removal,and storing grinding fluid to exchange heat.The basic principles and latest progress in the design and preparation of the superabrasive structured grinding wheels are comprehensively discussed and summarized based on the effect of factors such as the geometry,three-dimensional size,and arrangement of the grinding wheel structure on the grinding performance.It focused on revealing the intrinsic relationship between the surface/matrix structure characterization parameters of the superabrasive grinding wheel-grinding wheel grinding performance-workpiece surface quality and profoundly analyzed the superiority of structured grinding wheels in grinding. Regarding the grooved structured grinding wheel,it is pointed out that to improve the surface quality of the workpiece,the design of grooved structured grinding wheels should be based on the optimization of groove parameters and strive to reveal the intrinsic relationship between structural characterization parameters-grinding wheel grinding performance-workpiece surface quality,thus providing a reliable theoretical reference for the optimal design of grooved structured grinding wheels.Regarding structured grinding wheels with holes,it is pointed out that the precise mapping relationship between the three-dimensional size and arrangement of blind holes on the surface of the grinding wheel and the liquid storage and heat exchange,friction reduction and wear resistance has not yet been established.Regarding the convex hull structured grinding wheel,it is pointed out that it can reduce the interference between the grains and effectively improve the utilization rate of grains by designing the arrangement style,optimizing the arrangement parameters,controlling the grinding wheel speed/feed speed,etc.,thereby improving the surface quality of the workpiece after grinding.Regarding the matrix structured grinding wheel,it is pointed out that it dramatically improves the flow characteristics and the heat exchange performance of the grinding fluid in the grinding arc area by regularly grooving or drilling holes inside the matrix and rationally designing the size,quantity,arrangement and other characterization parameters of the groove/hole structure.At the end of this work,the future development trend of structured grinding wheels is predicted,which aims to provide theoretical guidance and practical experience for developing structured design and preparation technology of superabrasive grinding wheels.
Titanium alloy has the advantages of high strength,good ductility,strong corrosion resistance and elastic modulus close to human bone,which is suitable for making human lower limb joints.According to the requirements of pharmaceutical industry standards,the surface roughness Ra of joint implants generally does not exceed 0.1 μm,and the surface should be free of defects such as oxide skin,cracks,pits,edges,burrs and so on.Therefore,the high efficiency and high quality preparation of nano-scale ultra-smooth surface is the premise of achieving the wide application of titanium alloy in medicine.In order to further improve the polishing efficiency and polishing quality of titanium alloy,this article combined the high efficiency of electrolytic polishing and the high quality characteristics of magnetorheological polishing,and proposed a magnetorheological electrolytic composite polishing method for titanium alloy structural parts.The effects of electrolyte mass fraction,machining voltage,machining gap,tool speed and other polishing parameters on the surface quality of titanium alloy were discussed.The effects of different polishing parameters on the surface morphology of the titanium alloy were analyzed.The formation mechanism of the titanium alloy surface under electrolysis and magnetorheological polishing was elucidated.The feasibility of magnetorheological electrolytic composite polishing of titanium alloy artificial joint prostheses was verified.The experimental results show that with the increase of NaNO3 mass fraction in the electrolyte,the surface roughness of titanium alloy first decreases and then increases.Under the conditions of 1.0wt.%-2.5wt.%mass fraction of NaNO3,the titanium alloy surface quality of magnetorheological electrolysis composite polishing is better than that of magnetorheological polishing.The effects of different machining voltages on the polishing quality of titanium alloy are analyzed,and the results show that when the machining voltage is 0.1 V,the surface roughness of the titanium alloy after polishing reaches 18 nm,and the surface quality is better than that of magnetorheological polishing.Then,as the machining voltage increases,the surface roughness of the polished surface shows an increasing trend.The effect of different machining gaps on the polishing quality of titanium alloy was compared.The results show that as the machining gap increases,the surface roughness of magnetorheological electrochemical composite polishing shows a trend of first decreasing and then increasing.The effect of different machining gaps on the polishing quality of titanium alloy was compared.The results show that as the tool speed increases,the surface roughness of the titanium alloy after polishing first decreases and then increases.When the tool speed is 300 r/min,the surface roughness of the surface polishing is reduced to the minimum.A comparative experiment was conducted on the magnetorheological and magnetorheological electrochemical composite polishing of titanium alloy artificial joint prostheses.The results showed that,compared with magnetorheological polishing,the efficiency of magnetorheological electrochemical composite polishing was improved by 62.5%.At the same polishing time,the polishing quality of the magnetorheological electrolytic composite polishing process is better.The magnetorheological electrolysis composite polishing method can be used for the high efficiency and high quality polishing of titanium alloy joint prosthesiss.
Cassie-Wenzel wetting state transition is a typical phenomenon in the failure of superhydrophobic surfaces and has attracted much more attention recently.The force response curve method,which is carried out by squeezing a liquid droplet on the tested superhydrophobic surfaces,is a typical method to characterize the Cassie-Wenzel wetting state transition.In this force response curve method,extracting the wetting state transition information from the force response curve is critical,which requires choosing suitable value of experimental parameters.In order to find out the suitable experimental parameter range for obtaining stable transition information of wetting state,the work aims to investigate the effect of experimental system error,experimental process parameters,surface wettability and wettability transition conditions on the wettability transition information in the force response curve.According to the theory of the force response curve method,a series of force response curves with the Cassie-Wenzel wetting state transition information were calculated for the squeezing droplet processes with different experimental parameters.The calculated results were verified by comparison with the force response curve from the former experiments.When the volume of droplet was 0.1 mL in the squeezing droplet experiment,a distance error of 1 μm and a force error of 0.8 mN could ensure the obvious wetting state transition information in the force response curve.More errors in both the distance and the force would make a considerable fluctuation on the force response curve,which might cover up the wetting state transition information.In the general situation of testing systems with a less precision,a droplet with volume above 0.050 mL could be used in the experiments to obtain the wetting state transition information.A volume above 0.010 mL could be used in experiments on testing system with a higher precision.The optimum value range of loading step was 10～25 μm,which could guarantee a reasonable wetting state transition information over the fluctuation on the force response curve.The above calculated results were verified by comparison between the former experiments from two different groups.Moreover,it was observed that both the wettability of the loading surface and the wettability of the surface to be measured in Cassie state had few effect on the wetting state transition information on the force curve.However,increasing the contact angle of the surface to be measured in Wenzel state might decrease the width of the bulge that represented the wetting state transition information.The wetting state transition information on the force response curve can be enhanced by using a smaller distance error and a force error,a large droplet size and a variable loading step length.Another wetting state transition information,a much more deflection on the force response curve induced by the droplet filling into the microstructure of the tested superhydrophobic surface during the wetting state transition process,can be used in the squeezing droplet experiment.By optimizing the experimental parameters based on the guidance in this work,the mechanism of the Cassie-Wenzel wetting state transition may be further reliably explored.
Due to its excellent creep and fatigue resistance at high temperatures,the microstructure control during the forming and mechanical processing of single crystal superalloy widely used in hot end components of aircraft engines has always been a hot topic of concern.At present,the theory of dislocation slip and recrystallization at high strain rates for this special material without original grain boundaries is currently unclear.Thus,the mechanism of microstructure evolution on the surface of single crystal superalloy parts was obtained through grinding experiments in this work.The sample for transmission electron microscopy(TEM)of ground surface was prepared by focused ion beam(FIB)directed cutting technique.The microstructure,orientation,grain size,and dislocation density distribution on the ground surface were characterized using field emission scanning electron microscopy(FE-SEM),X-ray diffraction technology(XRD),TEM,and transmission Kikuchi diffraction(TKD)technology,respectively.The results indicate that a gradient structure with white layer,plastic deformation layer,and bulk material are formed below the ground surface,and dynamic recrystallization phenomenon is generated on the subsurface.The SEM results show that there is no obvious deformation feature in the white layer,while a large number of slip traces are formed in the plastic deformation layer.The original structure of γ/γ′ phase is destroyed,exhibiting typical shear deformation characteristics.XRD results show that the original single orientation of the single crystal superalloy is transformed into multiple diffraction peaks(111),(200),(220),(311)and(222).TEM detection indicate that dynamic recrystallization occurred in the top surface layer of single crystal parts during grinding,resulting in grain refinement at the nanoscale.The surface of single crystal parts has transformed from a special organizational structure with only a single grain to a polycrystalline structure characterized by the coexistence of nano equiaxed grains and subgrains.A large number of high-density dislocation structures are formed on the subsurface.TKD results show that under the high-speed rotation and shear action of the grinding wheel,plastic deformation are generated on the ground surface along the shear direction,and the original Cube texture transformed into R-Cube texture and F texture.Grains with{111}<112>,{001}<110>,and{001}<100>orientations were formed along the grinding depth direction,respectively.The evolution of microstructure orientation caused by shear strain during grinding indicates that the recrystallized grains and deformed microstructure are generated by lattice rotation,driving the{111}plane parallel to the shear plane,while the<110>direction is consistent with the shear direction.Moreover,the high-density crystal defects formed during plastic deformation can adapt to strain,leading the crystal direction to develop towards the direction with the highest shear stress.This lattice rotation method can provide a macroscopic direction that is easy to shear for grinding,resulting in a smaller grinding force during the grinding process and facilitating material removal.The microstructure changes on the ground surface of single crystal superalloy are the evolution mechanism of dynamic recrystallization dominated by lattice rotation and dislocation slip motion.The research on the microstructure during the grinding process of single crystal parts has improved the recrystallization theory of single crystal materials,providing experimental and theoretical basis for controlling orientation changes in actual machining of single crystal blades.
The directional flow characteristics of droplets on the inclined cone surface were studied to reveal the directional transport mechanism of droplets on the inclined cone surface.This article focuses on the motion behavior of droplets on the inclined cone surfaces.By using numerical simulation technology to extract droplet dynamics parameter data,the influence of different inclined cone structural parameters on droplet self-transport behavior were explored.The research found that the uneven distribution of fluid velocity within the liquid lead to the generation of velocity vortices within the droplet.Under the action of Taylor capillary rise,a main velocity vortex with a higher vorticity value was generated in the inclined cone gap,and a velocity secondary vortex with a lower vorticity value was generated inside the droplet.During the directional transport of droplets,the rotation direction of the main and secondary vortexes remained consistent with the transport direction of the droplet.And the self-transport process of the droplet was also accompanied by the mutual conversion of the surface energy and kinetic energy.During the expansion and contraction stages,the droplet underwent significant deformation,and the solid-liquid contact area first increased and then decreased.The surface energy of the droplet also increased and then decreased,but the movement speed of the droplet first decreased and then increased.During the stable transmission stage,the surface energy and velocity of the droplets remained basically unchanged.The Taylor capillary rising and the imbalanced capillary force in the inclined cone gap drove the liquid to continuously fill the inclined cone gap.The unbalanced pinning resistance of the droplet on the inclined conical surface made it easier for the left side of the droplet to separate,ensuring that the entire droplet was transported to the right side without damage.The liquid droplet was subjected to the combined action of unbalanced capillary force and Taylor capillary rise on the surface of the inclined cone,resulting in the formation of velocity vortexes inside the liquid,prompting the fluid to fill the wedge-shaped space between the inclined cones.The nailing force on the left and right sides of the droplet was different,and the left side of the droplet was more prone to detachment,which also ensured that the droplet moved to the right as a hemispherical whole.And the self-transport process of the droplet was accompanied by the mutual conversion of surface energy and kinetic energy.When the shape variation of the droplet was large,the solid-liquid contact area first increased and then decreased,and the surface energy of the droplet also increased and then decreased.However,the velocity of the droplet movement first decreased and then increased.When the deformation of the droplet reached a stable state,the surface energy and velocity of the droplet remained basically unchanged.This study provides theoretical support for revealing the flow characteristics and self-transport mechanism of liquid droplets on the inclined cone surface,and is used to guide the development of mechanical functional surfaces using the inclined cone structure as a biomimetic prototype.
