The effects of Na2WO4 concentration on the properties of microarc oxidation coatings on aluminum alloy

Alumina coating, approximately 30 μm, was deposited on an LY12 Al alloy substrate using a microarc oxidation (MAO) process in a H₃BO₃–KOH electrolyte solution with the Na₂WO₄ addition varying from 0 to 6 g/l. The MAO process was studied by measuring the voltage as a function of time. The coating layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with respect to the phases and microstructures and by measuring microhardness and wear resistance. The results show that the concentration of Na₂WO₄ has direct effects on the behavior of MAO process and the quality of the MAO coatings as well. The final phases in the coating were found to be α-Al₂O₃ and both γ-Al₂O₃ and a small amount of W. Without an addition of Na₂WO₄, the MAO coating process could not successfully proceed. With increasing the Na₂WO₄ concentration in the electrolyte, the working voltage at the microarc discharge stage decreased, the thickness and the content of α-Al₂O₃ phase in the coating both reduced. The microhardness and the wear resistance were both enhanced as the content of α-Al₂O₃ phase increased.     

Sliding wear behaviour and microstructure of PEO coatings formed on aluminium alloy

In this work, plasma electrolytic oxidation coating was formed on aluminium alloy in a cheap and inexpensive electrolyte to improve its wear resistance. It was found the micro-hardness of coatings increased first and then decreased with increasing the oxidised time. It was showed that the specimen treated under the time of 35 minutes exhibited the highest micro-hardness and lowest wear loss. The surface and cross-sectional morphology indicated that the coatings have a dense structure with low porosity. The presence of wear scars on the worn surface morphology demonstrates that the three-body rolling was the main wear mechanism for coated specimen. X-ray diffraction results showed the coating was formed mainly from α-Al₂O₃ and γ-Al₂O₃.     

Influence of cathodic current on composition, structure and properties of Al2O3 coatings on aluminum alloy prepared by micro-arc oxidation process

Alumina coatings on aluminum alloy were prepared by micro-arc oxidation (MAO) process using DC and AC power supplies, respectively. In comparison with the coating deposited by DCMAO, the influence of the cathodic current on the composition, structure and properties of the ACMAO coating was investigated. It is found that the coating deposited by DCMAO is composed of α-Al₂O₃, whereas the coating deposited by ACMAO has a mixture composition of α-Al₂O₃ and γ-Al₂O₃. The results of properties show that compared with the coatings deposited by DCMAO, the ACMAO coatings possess higher density, hardness and corrosion resistance. It can be attributed to that the DCMAO coating is rougher and existing much more micro-cracks in its inner layer. As a result, the adhesion of the DCMAO coating to the substrate decreases.     

Untersuchungen zur Beständigkeit thermisch gespritzter Schichten im Vergleich zu Hartchromschichten bei durch Abrasion dominierter tribologischer Beanspruchung

Investigations on thermal spray coatings resistance against abrasion dominated tribological load in comparison to hard chromium coatings HVOF iron and nickel based hard alloy as well as WC/Co(Cr) and Cr₃C₂/Ni20Cr coatings are compared to APS Al₂O₃/TiO₂ and Cr₂O₃, powder flame sprayed and fused composite coatings consisting of NiCrBSi and WC/Co and electrolytically deposited hard chromium coatings concerning their wear behavior for tribological load by lose abrasive particles (ASTM G65 and ASTM G75). Thereby the influence of newly developed HVOF torch combustion chambers with reduced critical diameter and divergent expansion nozzles that both permit increased combustion gas and therefore also particle velocities on microstructure and wear resistance of the produced coatings is studied. While there is no improvement of wear resistance for hard alloy coatings compared to mild steel substrates for the specific tribological boundary conditions of these tests, especially the carbide reinforced coatings permit improvement by more than one order of magnitude in ASTM G65 tests and even more than two orders of magnitude in ASTM G75 tests. Also, for both types of tribological load HVOF coatings with WC as reinforcing phase are clearly superior to electrolytically deposited hard chromium coatings. Both use of the combustion chamber with reduced critical diameter and the expansion nozzles with divergent contour result in improved wear resistance of the thereby produced coatings. The specific wear mechanisms are deduced based on SEM examination of worn specimen surfaces.     

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Abstract

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al₂O₃ nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.     

Microstructural and tribological characterization of air plasma sprayed nanostructured alumina–titania coatings deposited with nitrogen and argon as primary plasma gases

Air plasma sprayed nanostructured Al₂O₃–13wt%TiO₂ coatings were deposited as a function of critical plasma spray parameter (CPSP), defined as the ratio of arc power to primary gas flow rate, using nitrogen and argon as the primary plasma gases. Microstructural features including percentage of α-Al₂O₃ phase, percentage of partially melted/unmelted regions, microhardness, and wear characteristics were evaluated for the deposited coatings. Effect of CPSP on microstructural and wear characteristics of coatings deposited with nitrogen was found to be relatively small. In contrast, significant effect of CPSP on coating characteristics was found for coatings deposited with argon. In wear tests, while strong effect of normal load on weight loss was observed for coatings deposited with nitrogen, weight loss for coatings deposited with argon was nearly independent of applied normal load, at least for coatings deposited at the highest CPSP.     

Linear reciprocating wear behaviour of plasma-sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings at different loading and sliding conditions

Linear reciprocating wear test is carried out on atmospheric plasma-sprayed Al₂O₃–Cr₂O₃ coatings applied on steel substrates. Linear bi-directional sliding wear test of the coatings is performed at different loading and sliding conditions such as load (10, 20 and 30 N), reciprocating amplitude (1.5, 3 and 6 mm) and frequency (5, 10, 15 and 20 Hz) using ball on flat linear reciprocating tribometer. The patterns of tribological behaviour of the coatings, as manifested at the tribo-contact surface, are judged. Results have shown that the wear rate increases with increasing applied load and frequency and that decreases with increasing reciprocating amplitude. Plastic deformation, detachments of unmelted core, reattachments, delamination and adhesive wear dominate the main failure mechanism of coating.     

Preparation and tribological properties of polyurethane/α-aluminum oxide hybrid films

This study focused on the preparation and tribological properties of polyurethane/α-aluminum oxide (PU/α-Al₂O₃) hybrid films. PU/α-Al₂O₃ hybrid films containing various nanoscaled α-Al₂O₃ contents were prepared by an effectively mechanical stirring method. The tribological properties of PU/α-Al₂O₃ hybrid films were investigated by a TABER type abrasion tester after 2000 cycles. The results of abrasion tests showed the abrasion resistance of the PU/α-Al₂O₃ hybrid film was increased as the α-Al₂O₃ content was increased. The abrasion resistance of the PU/α-Al₂O₃ hybrid film was significantly improved up to 27.4% by adding 2 wt.% nanoscaled α-Al₂O₃ particles. The surface morphologies of PU/α-Al₂O₃ hybrid films, before and after abrasion tests, were examined by scanning electron microscopy (SEM). For the loading of 2 wt.% α-Al₂O₃ particles, the SEM image of the worn surface of the PU/α-Al₂O₃ hybrid film showed much smoother than those of pure PU film and other PU/α-Al₂O₃ hybrid films.     

Evaluation on tribological design coatings of Al2O3, Ni–P–PTFE and MoS2 on aluminium alloy 7075 under oil lubrication

The current work evaluated the friction and wear properties of tribological design surface coatings on aluminium alloy 7075 under various speed and nominal contact pressure. Hard-anodized Aluminium Oxide (Al₂O₃), burnished Refractory Metal Sulfide (MoS₂) and composite electroless nickel coatings with polytetrafluoroethylene (Ni–P–PTFE) particles were subjected to pin-on-disc sliding test against grey cast iron (GCI) under Mach 5 SL SAE 10 W-30 lubrication. The results indicated that Ni–P–PTFE composite coating possessed excellent friction–reduction capability but limited wear resistance due to low mechanical strength. Al₂O₃ coated sample showed outstanding wear resistance with high friction characteristic leading to high surface contact temperature. Furthermore, MoS₂ coating improved the wear resistance of the aluminium alloy.     

Hard coatings on aluminium alloy surfaces produced using microplasma oxidation

Abstract

The present paper reports on the production of oxide coatings on an aluminium alloy by microplasma oxidation and on their properties. The surface characteristics of the coatings were determined by surface and structural analytical techniques, i.e. SEM and X-ray diffraction. Thermo-analysis of the coatings was evaluated by means of differential scanning calorimetry and thermogravimetric analyses. It was ascertained that the coatings with a mixture of crystallised γ-Al₂O₃ and α-Al₂O₃ have significant microhardness, good electric resistance and good thermostability.     

电解质浓度对TiB2基刀具表面MAO涂层组织和摩擦性能的影响

研究了以高浓度Na₅P₃O₁₀作为电解液对钛合金实施微弧氧化,并对其表面结构和各成分组成情况进行了分析,之后测试了氧化层的表面硬度及其耐磨特性。研究结果表明:提高Na₅P₃O₁₀浓度后,发生了电导率明显增大现象,反应时间显著缩短。加入50 g/L的Na₅P₃O₁₀涂层的表面形成了尺寸差异较大的微孔,这些微孔的尺寸都在1μm以内,表面较平滑。所有微弧氧化涂层内都存在B、Ti、O。各膜层中主要存在B₂TiO₅,同时形成了B₂O₃与金红石TiO₂衍射峰。逐渐提高Na₅P₃O₁₀含量后,加入50 g/L的Na₅P₃O₁₀涂层试样获得了更高硬度,但硬度变化速率发生了降低。加入50 g/L的Na₅P₃O₁₀涂层具备很高极限载荷,迅速达到了稳定的摩擦状态,因此摩擦系数快速减小到稳定的程度。基体的表面受到了较大破坏,产生了许多磨屑颗粒与犁沟;涂层表现为粘着磨损的特征,可以显著降低粘着磨损的程度。     

Formation and Study of Porous Alumina and Catalytic Coatings by the Use of Cold Gas Dynamic Spraying Method

Cold gas dynamic spraying was used to obtain coatings based on γ-Al₂O₃ and Pd-CeO₂/γ-Al₂O₃ catalyst on a stainless steel foil. The coatings were studied by thermal cycling, BET, SEM, TEM, and XPS methods. The textural and adhesion characteristics of the coatings were found to depend on the composition of initial alumina powders with metallic aluminum (weight ratios of the components were varied from 2:8 to 8:2). It was shown that a mixture containing 25% γ-Al₂O₃ and 75% Al provides the optimal properties of the coatings. The effect produced by a method of the active component introduction into the coating on the catalytic properties of samples was examined. A considerable advantage of Pd introduction by impregnation in the preliminarily sprayed γ-Al₂O₃ layer over the direct spraying of the finished catalytic composition on the foil was demonstrated.     

Tribological properties of surface modified nano-alumina/epoxy composites

Nano-sized Al₂O₃ particles grafted with polystyrene or polyarcrylamide were employed as fillers for fabricating epoxy based composites. Curing habit, mechanical properties and tribological performance revealed by sliding wear tests of the composites were investigated. The experimental results indicated that the nanoparticles accelerate curing of epoxy, increase composites' impact strength and decrease wear rate and frictional coefficient of the composites. The surface modification by means of grafting polymerization can further enhance the properties improvement of epoxy due to the increased filler/matrix interfacial interaction. Compared to frictional coefficient, wear rate of epoxy can be decreased more remarkably by the addition of nano-alumina when rubbing against steel. The wear mode changes from severe peeling off of unfilled epoxy to mild micro-ploughing in the case of nano-alumina filled composites.     

Preparation and investigation of the structure and properties of Al2O3 plasma-detonation coatings

Using a specially designed and constructed plasmatron, combining the plasma and detonation technologies, aluminum oxide coatings with high adhesion and strength (19.8 GPa) were obtained for the first time. The samples were studied by the transmission electron microscopy and X-ray diffraction methods and characterized for the first time by the Rutherford backscattering and proton elastic resonance measurements. It was found that the aluminum oxide coatings are composed of 0.20–0.25 μm crystallites, belonging to the α-, β-, γ-, and δ-Al₂O₃ crystal modifications, and have a transition region containing AlFe nanocrystals with a size of 20–40 nm and a considerable dislocation density. The aluminum oxide coatings exhibit significant carbonization, with a maximum carbon concentration on the surface reaching 20 at. %. The relative phase composition calculated using the X-ray diffraction data is as follows: γ-Al₂O₃, 60%; α-Al₂O₃, 30%; β-and δ-Al₂O₃, balance to 100%.     

Lubrication mechanisms of C‐MoS2‐Fe2O3 (Fe3O4) nano‐composite lubricants at the rubbing interfaces of non‐copper coated solid wires against the contact tube

Graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) nano‐composite lubricating coatings were prepared on the surfaces of non‐copper coated solid wires by a mechanical coating technique. The tribological behaviours of graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) coatings at the rubbing interfaces of welding wires against the contact tube were investigated. The results demonstrate that the lubricating properties of graphite‐Fe₃O₄ coatings outperform the lubricating properties of graphite‐Fe₂O₃ coatings. The anti‐wear performance of the contact tube is strengthened with increasing nano‐MoS₂ contents. Layers of protective tribofilms are formed at the rubbing interfaces of welding wires against a contact tube by tribochemical reaction among lubricants. The tribofilms are composed of FeO, MoO₃ and FeMoO₄ with excellent lubricating properties. They can avoid direct contact of welding wires against the contact tube, thus decreasing contact tube wear. With the transition of the contact tube wear from mild to severe, the dominant wear mechanisms of contact tube change from fatigue peeling and oxidative wear to abrasive wear and arc ablation.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Influence of SiC, Si3N4 and Al2O3 particles on the structure and properties of electrodeposited Ni

The structure and properties of electrodeposited nickel composites reinforced with inert particles like SiC, Si₃N₄ and Al₂O₃ were compared. A comparison was made with respect to structure, morphology, microhardness and tribological behaviour. The coatings were characterized with optical microscopy, Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) technique. The cross-sectional microscopy studies revealed that the particles were uniformly distributed in all the composites. However, a difference in the surface morphology was revealed from SEM studies. The microhardness studies revealed that Si₃N₄ reinforced composite showed higher hardness compared to SiC and Al₂O₃ composite. This was attributed to the reduced crystallite size of Ni — 12 nm compared to 16 nm (SiC) and 23 nm (Al₂O₃) in the composite coating. The tribological performance of these coatings studied using a Pin-on-disk wear tester, revealed that Si₃N₄ reinforced composite exhibited better wear resistance compared to SiC and Al₂O₃ composites. However, no significant variation in the coefficient of friction was observed for all the three composites.     

An Approach to Synthesize Thick α- and γ-Al2O3 Coatings by High-Speed Physical Vapor Deposition

Crystalline α- and γ-Al₂O₃ exhibit in many applications high wear resistance, chemical resistance, and hot hardness, making them interesting materials for production engineering. To synthesize α-Al₂O₃ with high coating thickness of s ≥ 10 μm, chemical vapor deposition at temperatures T > 1000 °C is well established. However, there are almost no studies dealing with the synthesis of thick α-Al₂O₃ by physical vapor deposition (PVD) at high temperatures T > 700 °C. High-temperature deposition of thick coatings can be realized by means of the dense hollow cathode plasma, combined with the transport function of the plasma gas in high-speed (HS) PVD. Herein, crystalline α- and γ-Al₂O₃ films are deposited on cemented carbides at substrate temperatures T _s ≈ 570 °C and T _s ≈ 780 °C by HS-PVD. These coatings exhibit a thickness up to s = 20 μm. Moreover, phase analysis presents α-phases in coatings synthesized at substrate temperature of T _s ≈ 780 °C with significant higher hardness than films by T _s ≈ 570 °C. These release the potential of HS-PVD to synthesize α-Al₂O₃ coatings with high thickness. Thereby, a higher thickness of these coatings is beneficial for the wear protection of turning and die casting tools.     

Wear behaviors of Fe-based amorphous composite coatings reinforced by Al2O3 particles in air and in NaCl solution

In this paper, the influence of the addition of Al₂O₃ particles on the microstructure and wear properties of Fe-based amorphous coatings prepared by high velocity oxygen fuel (HVOF) has been studied. The wear behaviors of the composite coatings were evaluated against Si₃N₄ in a pin-on-disk mode in air and in 3.5 wt.% NaCl solution. It was found that the Al₂O₃ particles were homogenously distributed in the amorphous matrix and the composite coatings exhibited improved wear resistance and reduced coefficient of friction (COF) in both air and wet conditions as compared to the monolithic amorphous coating. The composite coating reinforced with 20 wt.% Al₂O₃ particles exhibit the best wear performance, which, for example, has extremely low COF (< 0.2) and high wear resistance (2–3 times higher than monolithic amorphous coating). Detailed analysis on the worn surface indicated that the wear mechanism for the amorphous and composite coatings is similar and is dominated by oxidative delamination in air and by corrosion wear in 3.5% NaCl solution. The enhanced wear resistance is mainly attributed to the addition of Al₂O₃ particles which exhibit high hardness, good corrosion resistance and excellent chemical and thermal stability.     

电解液中Na_2WO_4对Ti_2AlNb微弧氧化膜结构及摩擦磨损性能的影响

选取硅酸盐/磷酸盐体系在Ti_2AlNb表面制备微弧氧化陶瓷膜,利用SEM,XRD,XPS等研究了电解液中Na_2WO_4对氧化膜生长过程、微观结构及成分的影响,同时评价了Ti_2AlNb微弧氧化膜的摩擦磨损行为。结果表明:硅酸盐/磷酸盐电解液中,氧化膜生长速率仅为0.08μm/min,膜层较疏松,表面存在大孔相连的"网状"结构,主要相组成为金红石TiO_2、锐钛矿TiO_2、Al_2O_3及Nb_2O_5。电解液中加入Na_2WO_4,缩短了Ti_2AlNb合金的起弧时间、提高了氧化膜的生长速率、改善了膜层均匀性,同时在膜层中引入了少量WO_3。此外,在Na_2WO_4参与下制备的微弧氧化膜的耐磨性更好。与Si_3N_4对磨时,Ti_2AlNb合金发生严重的磨粒磨损,摩擦因数高达0.5~0.7;含4g/L Na_2WO_4电解液中制备的Ti_2AlNb微弧氧化膜的摩擦因数、比磨损率分别为0.24及6.2×10~(-4) mm~3/(N·m),表面仅出现"鱼鳞状"疲劳磨损特征。