Improving tribological and corrosion resistance of Ti6Al4V alloy by hybrid microarc oxidation/enameling treatments

A promising duplex coating was prepared by microarc oxidation(MAO) and enameling processes onto polished Ti6A14V alloy. The TiO_2 ceramic coating deposited by MAO was characterized and then combined with an enameling treatment in order to improve the tribological and corrosion resistance of Ti6A14V alloy. The morphology, phase composition, and hardness of MAO and MAO/enameling-coated Ti6A14V alloy were evaluated by scanning electron microscopy(SEM), X-ray diffraction(XRD), and Vickers microhardness tester, respectively.The tribological performance was investigated using a ballon-disk tribometer. The corrosion resistance was studied using immersion tests and potentiodynamic polarization.Wear tests show that the enamel coating on the MAOcoated surface causes a reduction in the friction coefficient.Immersion tests demonstrate that the duplex coating is more effective in improving the corrosion resistance of Ti6A14V than the MAO coating especially at high temperature(80 ℃). Potentiodynamic polarization curves reveal that the corrosion potential of the duplex coating increases by about 250 mV and the corrosion current density is slightly lower than that of the MAO coating. The duplex coating is superior to the stand-alone MAO coating in improving the tribological and corrosion behavior of Ti6A14V.     

微弧氧化工艺及封孔处理对镁合金耐蚀性能的影响

为寻求在低成本下获得高性能镁合金防护膜的方法,采用直流低压微弧氧化工艺,在硅酸盐体系中添加抑弧剂抑弧以获得均匀的膜层,并对随后获得的微弧氧化陶瓷膜进行封闭处理.通过形貌分析及腐蚀析氢的方法研究了膜层的耐蚀性能及腐蚀特征,结果表明:在低电流密度下适当延长微弧氧化时间,可获得更为致密的陶瓷膜层,抗腐蚀性能更好.碳酸盐封孔处理可使膜层的致密层变厚,并进一步强化膜层的致密度;能保证基体完全不受腐蚀的时间显著延长,并在较长时间内使膜层的抗腐蚀性能明显提高.     

微弧氧化法制备BaTiO3薄膜的微观结构分析

利用X射线衍射仪、透射电镜和能谱仪(EDS)等对采用微弧氧化技术在钛衬底上制备的钛酸钡铁电薄膜的微观结构进行了分析。结果表明,Ba(OH)2浓度为0.2 mol/L、电流密度为5A/cm2、电源频率为250 Hz、微弧氧化时间为30 min制备的薄膜以六方相、四方相的BaTiO3和非晶相为主;经过1200℃退火8 h后,薄膜中的BaTiO3以四方相为主,非晶相消失,但部分六方相仍然保留。     

Influence of microarc oxidation and hard anodizing on plain fatigue and fretting fatigue behaviour of Al-Mg-Si alloy

The present study compares the performance of microarc oxidation (MAO) and hard anodizing (HA) treated Al-Mg-Si alloy (AA6063) test samples under cyclic loading in uniaxial tension with a stress ratio of 0.1 (plain fatigue) and fretting fatigue loading. Fatigue test specimens were treated using MAO and HA techniques. MAO coated specimens were ground to reduce the surface roughness comparable with that in HA coated specimens. In that process the porous outer layer was removed. Characterization of coated and uncoated specimens was done with reference to the coating morphology, microhardness, surface roughness and residual stress. The specimens were tested under plain fatigue and fretting fatigue loading at ambient temperature. While the ground MAO coating exhibited relatively less amount of porosity, HA coating had through thickness cracks. MAO coating had compressive residual stress and it was very hard compared with HA coating. Both types of coated samples exhibited slightly higher friction force than that experienced by the uncoated specimens. Fretted region of the HA coated samples was rougher than that of the MAO coated specimens. Plain fatigue lives of both coated samples were inferior to those of the uncoated specimens. The inferior plain fatigue lives of MAO coated specimens compared with those of the substrate may be attributed to the tensile residual stresses supposedly present in the substrate leading to an early crack initiation in the substrate adjacent to the coating. As friction force of MAO coated samples was higher than that experienced by uncoated specimens, the fretting fatigue lives of MAO coated samples were slightly inferior to those of uncoated samples. As the anodized layer had preexisting through thickness cracks and strong adhesion with the substrate, cracks propagated from HA coating through the interface into the substrate easily. This may be the reason for the HA coated samples exhibiting inferior plain fatigue and fretting fatigue lives compared with MAO coated and uncoated samples.     

镁合金双脉冲微弧氧化表面膜层的形成及结构分析

利用XRD、SEM、EDX等方法,研究了纯镁与AZ91D镁合金在六氟铝酸钠(Na3AlF6)、氢氧化钾(KOH)、六偏磷酸钠((NaPO3)6)和三乙醇胺四元组分电解液中双脉冲微弧氧化膜层的形成、膜层形貌和成分结构.研究发现,负脉冲的载入极大地影响了微放电的数量与外观;氧化物层表面可以观察到3类微孔,它们的尺寸范围分别是0.5～1μm、1～2μm和4～7μm,同时在较大微孔的周围发现有微裂纹存在;基体与氧化物交界处存在氟化物富集区,纯镁和AZ91D合金表面膜层的氟化物富集区的尺寸范围分别是4～6μm和3～5 μm.     

Multifunction Sr doped microporous coating on pure magnesium of antibacterial,osteogenic and angiogenic activities

Strontium (Sr) ions were added to porous magnesium (Mg) oxide with silicon and fluorine by microarc oxidation (MAO) to improve its osteogenic and pro-angiogenic properties. First, pure Mg was oxidized by MAO, and Sr was added by electrolysis. The surface of the resulting Sr coating was characterized by SEM, EDS, and EDS mapping. The release of Sr ions was monitored by ICP-OES. The antibacterial property of the coating was assessed against Staphylococcus aureus. The effect of Sr coating on osteogenesis was tested in MC3T3-E1 cell line by performing cell adhesion and proliferation tests, alkaline phosphatase (ALP) activity detection, cell morphology characterization, alizarin red staining, and osteogenic-related gene expression analysis. Finally, HUVECs cells were used to test the effect of Sr coating on angiogenesis through cell migration and tube formation assays, VEGF quantification, chicken embryo chorioallantoic membranes (CAM) test, and angiogenic-related gene expression analysis. The results showed that Sr coating had a hierarchical microstructure with a microporous structure evenly covered with nano-grains and that the Sr elements from the coating were released slowly and continuously. Sr coating had effective antibacterial properties and promoted cell adhesion, proliferation, ALP release, calcium nodule formation, and upregulated osteogenic gene expression. Moreover, the coating could promote migration, tube formation, VEGF expression, and angiogenic gene upregulation in endothelial cells. Sr coating also enhanced angiogenesis of CAM. This study supports that Sr coating on Mg- MAO enhances osteogenesis and angiogenesis.     

Electrochemical properties of microarc oxidation films on a magnesium alloy modified by high-intensity pulsed ion beam

Microarc oxidation (MAO) films on AZ31 magnesium alloy were treated by high-intensity pulsed ion beam (HIPIB) irradiation with ion energy of 300 keV at ion current density of 200 A/cm². A remelted layer of a few micrometers was produced on irradiated MAO films. The corrosion resistance of MAO films was characterized by potentiodynamic electrochemical test in 3.5% NaCl solution. The anodic polarization behavior of MAO samples exhibited a transition from the active anodic dissolution for the original one to the passivation-pitting breakdown for the modified films. The passivation-pitting breakdown voltage of modified films increased with multi-shot irradiation, i.e. from a value of − 1420 mV(SCE) at 1 shot to − 800 mV(SCE) at 5 shots, and the corresponding passivation current density decreased by two orders of magnitude. The irradiated MAO films have a higher corrosion potential than the original one, reaching a maximal value of − 1350 mV(SCE) at 5 shots from the original − 1580 mV(SCE). The electrochemical impedance spectrum (EIS) of modified MAO films was measured with varying the immersion time in 3.5% NaCl solution. The Nyquist impedance plots from modified films may show only a capacitive loop at the immersion time of 5 h, and the inductive loop occur at longer immersion time, whereas original films presenting both capacitive and inductive loops at all the immersion time. Bode plots of MAO films before and after irradiation were obviously distinguished in shape at low frequency range, especially at short immersion time. The evolution of Nyquist and Bode plots with immersion time was discussed in association with the processes of electrolyte penetration into the MAO structure and resultant reaction at the film-substrate interface. It is concluded that the improvement in the continuity and compaction of MAO films accounts for the enhanced corrosion resistance of the films irradiated by HIPIB.     

钛合金表面微弧氧化／水热处理复合陶瓷层的性能研究

对微弧氧化后的Ti6Al4V合金表面进行了水热处理,对处理后涂层的成分、形貌及其在Hank's模拟体液中的腐蚀性能进行了研究.试验结果表明:水热处理后,微弧氧化层中以无定形态钙磷酸盐存在的Ca、P元素转化为针状羟基磷灰石,厚度约为5μm,并在微弧氧化膜微孔边缘和底部优先生长.与微弧氧化层相比,水热处理后涂层的粗糙度明显下降,同时显微硬度下降约1/3.电化学腐蚀试验表明:微弧氧化显著改善了钛合金在模拟体液中的耐腐蚀性能;水热处理钛合金自腐蚀电位低于纯钛合金,但当处于人体电位0.4～0.6V(vs.SCE)范围时,微弧氧化/水热处理钛合金的腐蚀电流比纯钛合金低1个数量级.     

The outward-inward growth behavior of microarc oxidation coatings in phosphate and silicate solution

Microarc oxidation (MAO) coatings have a unique outward-inward growth behavior, which is crucial to the dimensional matching of precision-matching parts after surface treatment. Here, a notably different outward-inward growth behavior of MAO coatings was observed in phosphate solution and silicate solution, respectively. The results indicate that the outward-growth dimension of MAO coating in Na₂SiO₃ solution is notably larger than that in (NaPO₃)₆ solution, especially at the initial stage of MAO treatment, but as an increase of oxidation time, the inward-growth behavior enhances in both solutions. It is considered that the different adsorptive capacity of solute anions leads to the various outward-inward growth behaviors of MAO coatings in phosphate solution and silicate solution.     

Effect of Negatively Charged Ions on the Formation of Microarc Oxidation Coating on 2024 Aluminium Alloy

The present study deals with the effect of negatively charged ions on the ceramic coating formation on 2024 aluminium alloy during microarc oxidation (MAO) process. On the basis of the experimental results, two steps (the formation of an incipient film without arc presence and the growth of a ceramic coating with arc discharge) of MAO process have been observed. For comparison, four different negatively charged ions studied. It is proved that negatively charged ions strongly participated in the formation of an incipient film with high impedance value at the first step. The growth of ceramic coating depends on the combination between Al of the substrate and O from the electrolyte, and the negatively charged ions are little consumed. As an anodic oxide coating is prepared on the sample surface instead of the incipient film, the first step occurs easily and the growth of ceramic coating is accelerated. Furthermore, the mechanism of negatively charged ions in the formation of the MAO coating has been proposed.