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.
