激光熔覆Al+SiC涂层对镁合金表面耐磨性能的改性

通过脉冲激光器(Nd-YAG)在AZ91D镁合金基底上熔覆Al+SiC粉体。采用扫描电子显微镜、能量色散谱(EDS)和X-射线衍射测定分析熔覆层的显微组织、化学成分和物相组成。研究表明:Al+SiC涂层主要由SiC,β-Mg_(17)Al_(12)及Mg和Al相组成,激光熔覆层与镁合金基底表现出良好的冶金结合。所有样品都具有树枝状结构,且随着SiC质量分数的增大,树枝状和胞状结构的间隔变得更大。熔覆涂层的表面硬度高于基底,并且随着熔覆层中的SiC质量分数的增加而增大,SiC质量分数为40%的熔覆层具有最大的显微硬度,达到180 HV,然而质量分数为10%的熔覆层硬度为136 HV。销盘滑动磨损试验表明,复合涂层中的SiC颗粒和原位合成的Mg_(17)Al_(12)相显著提高了AZ91D镁合金的耐磨损性,其中,SiC质量分数从10%增加到30%过程中磨损体积损失逐渐减少,SiC质量分数在20%~30%时熔覆层具有最好的耐磨性。     

Analysis and optimization of process parameters in Al–SiCp laser cladding

The laser cladding process parameters have great effect on the clad geometry and on dilution in the single and multi-pass aluminum matrix composite reinforced with SiC particles (Al/SiCp) coatings on ZE41 magnesium alloys deposited using a high-power diode laser (HPLD). The influence of the laser power (500–700 W), scan speed (3–17 mm/s) and laser beam focal position (focus, positive and negative defocus) on the shape factor, cladding-bead geometry, cladding-bead microstructure (including the presence of pores and cracks), and hardness has been evaluated. The correlation of these process parameters and their influence on the properties and ultimately, on the feasibility of the cladding process, is demonstrated. The importance of focal position is demonstrated. The different energy distribution of the laser beam cross section in focus plane or in positive and negative defocus plane affect on the cladding-bead properties.     

Microstructure and corrosion behavior of coated AZ91 alloy by microarc oxidation for biomedical application

Magnesium and its alloy currently are considered as the potential biodegradable implant materials, while the accelerated corrosion rate in intro environment leads to implant failure by losing the mechanical integrity before complete restoration. Dense oxide coatings formed in alkaline silicate electrolyte with and without titania sol addition were fabricated on magnesium alloy using microarc oxidation process. The microstructure, composition and degradation behavior in simulated body fluid (SBF) of the coated specimens were evaluated. It reveals that a small amount of TiO₂ is introduced into the as-deposited coating mainly composed of MgO and Mg₂SiO₄ by the addition of titania sol into based alkaline silicate electrolytic bath. With increasing concentration of titania sol from 0 to 10 vol.%, the coating thickness decreases from 22 to 18 μm. Electrochemical tests show that the E_corr of Mg substrate positively shifted about 300500 mV and i_corr lowers more than 100 times after microarc oxidation. However, the TiO₂ modified coatings formed in electrolyte containing 5 and 10 vol.% titania sol indicate an increasing worse corrosion resistance compared with that of the unmodified coating, which is possibly attributed to the increasing amorphous components caused by TiO₂ involvement. The long term immersing test in SBF is consistent with the electrochemical test, with the coated Mg alloy obviously slowing down the biodegradation rate, meanwhile accompanied by the increasing damage trends in the coatings modified by 5 and 10 vol.% titania sol.     

Ti6Al4V合金表面激光沉积复合涂层的组织和性能

为了增强Ti6Al4V钛合金的耐磨性,采用激光沉积制造方法在其表面上制备了以原位生成的TiC颗粒和直接添加的WC颗粒为增强相的耐磨涂层,观察了各涂层的微观组织,并测量了涂层的显微硬度和涂层在室温大气条件下的摩擦磨损性能。结果表明各涂层和基体呈现冶金结合,原位自生的TiC和部分熔化的WC颗粒均能够均匀弥散分布于基体上,由于增强相颗粒的弥散强化及激光沉积组织的细晶强化作用,基材的硬度和耐磨性均得到了提高。原位自生的TiC涂层比WC涂层硬度梯度分布平缓,但耐磨性稍差。     

The microstructure, mechanical and friction properties of protective diamond like carbon films on magnesium alloy

Protective hard coatings deposited on magnesium alloys are believed to be effective for overcoming their poor wear properties. In this work, diamond-like carbon (DLC) films as hard protective films were deposited on AZ91 magnesium alloy by arc ion plating under negative pulse bias voltages ranging from 0 to −200 V. The microstructure, composition and mechanical properties of the DLC films were analyzed by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation. The tribological behavior of uncoated and coated AZ91 magnesium alloy was investigated using a ball-on-disk tribotester. The results show that the negative pulse bias voltage used for film deposition has a significant effect on the sp³ carbon content and mechanical properties of the deposited DLC films. A maximum sp³ content of 33.3% was obtained at −100 V, resulting in a high hardness of 28.6 GPa and elastic modulus of 300.0 GPa. The DLC films showed very good adhesion to the AZ91 magnesium alloy with no observable cracks and delamination even during friction testing. Compared with the uncoated AZ91 magnesium alloy, the magnesium alloy coated with DLC films exhibits a low friction coefficient and a narrow, shallow wear track. The wear resistance and surface hardness of AZ91 magnesium alloy can be significantly improved by coating a layer of DLC protective film due to its high hardness and low friction coefficient.     

Effect of α-Al2O3 on the properties of cold sprayed Al/α-Al2O3 composite coatings on AZ91D magnesium alloy

Composite coatings using pure Al powder blended with α-Al₂O₃ as feedstock were deposited on AZ91D magnesium alloy substrates by cold spray (CS). The content of α-Al₂O₃ in the feedstock was 25 wt.% and 50 wt.%, respectively. The effects of α-Al₂O₃ on the porosity, microhardness, adhesion and tensile strength of the coatings were studied. Electrochemical tests were carried out in neutral 3.5 wt.% NaCl solution to evaluate the effect of α-Al₂O₃ on the corrosion behavior of the coatings. The results showed that the composite coatings possessed lower porosity, higher adhesion strength and tensile strength than cold sprayed pure Al coating. The corrosion current densities of the composite coatings were similar to that of the pure Al coating and much higher than that of bare AZ91D magnesium alloy.     

Mechanical properties and wear and corrosion resistance of electrodeposited Ni-Co/SiC nanocomposite coating

Ni-Co/SiC nanocomposite coatings with various contents of SiC nano-particulates were prepared by electrodeposition in a Ni-Co plating bath containing SiC nano-particulates to be co-deposited. The influences of the nanoparticulates concentration, current density, stirring rate and temperature of the plating bath on the composition of the coatings were investigated. The shape and size of the SiC nano-particulates were observed and determined using a transmission electron microscope. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The wear behavior of the Ni-Co/SiC nanocomposite coatings was evaluated on a ball-on-disk UMT-2MT test rig. The worn surface morphologies of the Ni-Co/SiC nanocomposite coatings were observed using a scanning electron microscope. The corrosion behavior of the nanocomposite coatings was evaluated by charting the Tafel curves of the solution of 0.5 mol L⁻¹ NaCl at room temperature. It was found that the cathodic polarization potential of the composite electrolyte increased with increasing SiC concentration in the plating bath. The microhardness and wear and corrosion resistance of the nanocomposite coatings also increased with increasing content of the nano-SiC in the plating bath, and the morphologies of the nanocomposite coatings varied with varying SiC concentration in the plating bath as well. Moreover, the co-deposited SiC nano-particulates were uniformly distributed in the Ni-Co matrix and contributed to greatly increase the microhardness and wear resistance of the Ni-Co alloy coating.     

Electrochemical deposition and tribological behaviour of Ni and Ni-Co metal matrix composites with SiC nano-particles

Metal matrix composites reinforced with nano-sized particles have attracted scientific and technological interest due to the enhanced properties exhibited by these coatings. Ni-SiC composites have gained widespread application for the protection of friction parts in the automobile industry. The influence of variables like SiC content, current density and stirring speed on microhardness of nano-composite coatings has been studied. The improved microhardness was associated with the reduction in crystallite size determined by X-ray diffraction studies. The influence of incorporation of nano-SiC in hardened Ni-Co alloy matrix was also studied. It was observed that for 28 wt.% Co content in the matrix the microhardness was higher compared to 70 wt.% for a given nano-SiC content. This was associated to the crystal phase of Ni-28Co-SiC being fcc compared to hcp phase exhibited by Ni-70Co-SiC. The wear resistance of pure Ni, Co and nano-composite coatings was studied using pin-on-disc wear tester under dry sliding condition. The volumetric wear loss indicated that, the wear resistance of Ni-SiC nano-composite is better than that of pure nickel deposit. The wear resistance of Ni-Co composites was observed to be superior to Ni composite. The wear behaviour of Ni and Ni-28Co composite was in accordance with the Archard's law. However, the superior wear characteristic exhibited by Ni-70Co-SiC composite followed the reverse Archard's behaviour.     

Microstructure,creep, and tensile behaviour of a Ti–15Al–33Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti–15Al–33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including:?β?(disordered body-centred cubic), α₂ (ordered hexagonal close-packed based on Ti₃Al) and O (ordered orthorhombic based on Ti₂AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α₂-phase precipitated within the?β?grains. The fine-grained as-processed microstructure, which exhibited 90?vol.% β-phase, exhibited excellent strength (UTS?=?916?MPa) and ductility (?_f>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ? _f values with UTS values ranging between 836–920?MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63?vol.% O-phase. Specimens subjected to 650°C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650–710°C and stress range 49–275?MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti–12Al–38Nb O?+?β alloy.     

Effect of La surface treatments on corrosion resistance of A3xx.x/SiCp composites in salt fog

The influence of the SiC_p proportion and the matrix concentration of four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) modified by lanthanum-based conversion or electrolysis coating was evaluated in neutral salt fog according to ASTM B 117. Lanthanum-based conversion coatings were obtained by immersion in 50 °C solution of La(III) salt and lanthanum electrolysis treatments were performed in ethylene glycol mono-butyl ether solution. These treatments preferentially covered cathodic areas such as intermetallic compounds, Si eutectic and SiC_p. The kinetic of the corrosion process was studied on the basis of gravimetric tests. Both coating microstructure and nature of corrosion products were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDS) and low angle X-ray diffraction (XRD) before and after accelerated testing to determine the influence of microstructural changes on corrosion behaviour during exposure to the corrosive environment. The corrosion process was more influenced by the concentration of alloy elements in the matrix than by the proportion of SiC_p reinforcement. Both conversion and electrolysis surface treatments improved the behaviour to salt fog corrosion in comparison with original composites without treatment. Additionally, electrolysis provided a higher degree of protection than the conversion treatment because the coating was more extensive.     

Laser (a pulsed Nd:YAG) cladding of AZ91D magnesium alloy with Al and Al2O3 powders

The laser surface cladding of an AZ91D magnesium alloy with Al and Al₂O₃ powders was investigated using a pulsed Nd:YAG laser. The optimum ratio of Al to Al₂O₃ and the suitable range of laser processing parameters were identified. The resulting microstructure in the modified surface layer was examined and the wear resistance property was evaluated. The results show that the wear resistance of the laser treated samples was much superior to that of the untreated samples.     

The effect of cerium oxide argon-annealed coatings on the high temperature oxidation of a FeCrAl alloy

Ceria coatings were applied in order to improve the adherence of alumina scales developed on a model Fe–20Cr–5Al alloy during oxidation at high temperature. These coatings were performed by argon annealing of a ceria sol–gel coating at temperatures ranging between 600 and 1000 °C. The influence of these coatings on the alloy oxidation behaviour was studied at 1100 °C. In situ X-ray diffraction (XRD) was performed to characterize the coating crystallographic nature after annealing and during the oxidation process. The alumina scale morphologies were studied by means of scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The present work shows that the alumina scale morphology observed on cerium sol–gel coated alloy was very convoluted. On the cerium sol–gel coated alloy, argon annealing results in an increase of the oxidation rate in air, at 1100 °C. The 600 °C argon annealing temperature results in a good alumina scale adherence under thermal cycling conditions at 1100 °C.     

Improvement in the properties of plasma-sprayed metallic, alloy and ceramic coatings using dry-ice blasting

Dry-ice blasting, as an environmental-friendly method, was introduced into atmospheric plasma spraying for improving properties of metallic, alloy and ceramic coatings. The deposited coatings were then compared with coatings plasma-sprayed using conventional air cooling in terms of microstructure, temperature, oxidation, porosity, residual stress and adhesion. It was found that a denser steel or CoNiCrAlY alloy coating with a lower content of oxide can be achieved with the application of dry-ice blasting during the plasma spraying. In addition, the adhesive strength of Al₂O₃ coating deposited with dry-ice blasting exceeded 60 MPa, which was nearly increased by 30% compared with that of the coating deposited with conventional air cooling. The improvement in properties of plasma-sprayed metallic, alloy and ceramic coatings caused by dry-ice blasting was attributed to the decrease of annulus-ringed disk like splats, the better cooling efficiency of dry-ice pellets and even the mechanical effect of dry-ice impact.     

Microstructure and infrared emissivity property of coating containing TiO2 formed on titanium alloy by microarc oxidation

Ceramic coatings containing TiO₂ were formed on Ti6Al2Zr1Mo1V alloy surface by microarc oxidation (MAO) method. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The coating mainly consists of rutile TiO₂ and a small amount of anatase TiO₂. The infrared emissivity values of coated and uncoated titanium samples when exposed to 700 °C were tested. It was found that the coating exhibits a higher infrared emissivity value (about 0.9) in the wavelength range of 8–14 μm than that of the uncoated titanium alloy, although which shows a slight increase from 0.1 to 0.3 with increasing exposure time at 700 °C. The relatively high infrared emissivity value of the MAO coating is possibly attributed to the photon emission from the as formed TiO₂ phase.     

Phase composition and tribological properties of Ti-Al coatings produced on pure Ti by laser cladding

Ti-Al coatings with ∼14.7, 18.1, 25.2 and 29.7 at.% Al contents were fabricated on pure Ti substrate by laser cladding. The laser cladding Ti-Al coatings were analyzed with X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). It was found that with the increase of Al content, the diffraction peaks shifted gradually to higher 2θ values. The laser cladding Ti-Al coatings with 14.7 and 18.1 at.% Al were composed of α-Ti and α₂-Ti₃Al phases, while those with 25.2 and 29.7 at.% Al were composed of α₂-Ti₃Al phase. With the increase of Al content, the cross-sectional hardness increased, while the fracture toughness decreased. For the laser cladding Ti-Al coatings, when the Al content was ≤18.1 at.%, the wear mechanism was adhesive wear and abrasive wear; while when the Al content ≥25.2 at.%, the wear mechanism was adhesive wear, abrasive wear and microfracture. With the increase of Al content, the wear rate of laser cladding Ti-Al coatings decreased under 1 N normal load, while the wear rate firstly decreased and then increased under a normal load of 3 N. Due to its optimized combination of high hardness and high fracture toughness, the laser cladding Ti-Al coating with 18.1 at.% Al showed the best anti-wear properties at higher normal load.     

Corrosion and wear properties of PEO coatings formed on AM60B alloy in NaAlO2 electrolytes

Coatings with a thickness of 22-32 μm were formed on an AM60B magnesium alloy by plasma electrolytic oxidation (PEO) in electrolytes containing 12.0-24.0 g/l NaAlO₂ and other additives. SEM analyses of the coated samples showed that the coatings were compact with relatively low porosity. X-ray diffraction revealed that the coatings consisted of mainly MgAl₂O₄ and MgO phases. The relative amount of MgAl₂O₄ in the coating increased with increasing NaAlO₂ concentration. The relatively compact and thick coatings provide good corrosion protection for magnesium, as indicated by the results of potentiodynamic polarization tests. In addition, the PEO treatment also significantly improved the wear resistance of the alloy. Pin-on-disk wear tests showed that the PEO treatment reduced the wear volume loss by a factor of 10.     

Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes

Microarc oxidation coatings on AM60B magnesium alloy were prepared in silicate and phosphate electrolytes. Structure, composition, mechanical property, tribological, and corrosion resistant characteristics of the coatings was studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and microhardness analyses, and by ball-on-disc friction and potentiodynamic corrosion testing. It is found that the coating produced from the silicate electrolyte is compact and uniform and is mainly composed of MgO and forsterite Mg₂SiO₄ phases, while the one formed in phosphate electrolyte is relatively porous and is mainly composed of MgO phase. The thick coating produced from a silicate electrolyte possesses a high hardness and provides a low wear rate (3.55 × 10⁻⁵ mm³/Nm) but a high friction coefficient against Si₃N₄ ball. A relatively low hardness and friction coefficient while a high wear rate (8.65 × 10⁻⁵ mm³/Nm) is recorded during the testing of the thick coating produced from a phosphate electrolyte. Both of these types of coatings provide effective protection for the corrosion resistance compared with the uncoated magnesium alloy. The coating prepared from the silicate electrolyte demonstrates better corrosion behavior due to the compacter microstructure.     

Laser surface alloying of an electroless Ni–P coating with Al-356 substrate

Laser surface alloying of an electroless plating Ni–P coatings on an Al-356 aluminium alloy was carried out using a 1-kW pulsed Nd:YAG laser. The microstructure, chemical composition and phase identification of the alloyed layer were determined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD), respectively. It was shown that laser surface treatment produced a relatively smooth, crack-free and hard surface layer. The hardness of the surface significantly increased due to the formation of the uniformly distributed fine Ni–Al intermetallic phases. The corrosion behaviour of the surface alloyed specimens in 3.5% NaCl solution at 23 °C was also determined by electrochemical techniques. The laser-alloyed surface showed an improved corrosion and pitting potential compared to the substrate as well as the plated Ni–P coating.     

Production of hard metal-like wear protection coatings by CO2 laser cladding

Protective coatings with hard metal-like wear properties could be obtained by laser beam surfacing of powder mixtures consisting of coarse-grained tungsten carbide and a nickel or cobalt hard alloy. The microstructure of the clad composite layers was investigated by metallography and SEM. The four coating systems produced were found to differ strongly in the degree of partial carbide dissolution and the formation of new hard phases. This also influences the hardness and wear behaviour.