Thin film coatings of WO<Subscript>3</Subscript> by cold gas dynamic spray: A technical note

Dense and adhesive WO₃ films were prepared on a silicon substrate by the cold gas dynamic spray process (or cold spray). In contrast to standard metallic coatings, there was no sizable crater formation and plastic deformation. However, the aggregation of raw powder particles of a relatively large size was found to be destroyed upon impact on the substrate, forming a highly irregular surface with very fine secondary particles and providing good interlocking powder and void reduction among the particles in the coating. High-resolution images of the substrate interface showed that particles at the interface were more densely packed and that good adhesion was obtained. There fore, the particle bombardment onto the first layer of the coating could provide enhanced adhesion to the substrate mechanically and/or chemically.     

The effect of plasma spray parameters on the cavitation erosion of Al2O3-TiO2 coatings

This paper reports a study of how the choice of plasma spray parameters, used during deposition of Al₂O₃-13%TiO₂ coatings on carbon steel, influences the cavitation erosion properties of such coatings. The parameters studied are the power feeding rate and hydrogen flow rate. The surface and cross section of coatings before and after cavitation were also observed by scanning electron microscopy (SEM). The phases present in the coatings were characterized by X-ray diffraction method (XRD). The microscopic observations were used to study the inter-lamellar connection, porosity, unmelted particles and so on inside the coating. We also measured the roughness, microhardness, adhesion strength and cavitation erosion of the coatings. The XRD results showed that the coating includes different allotropes of Al₂O₃ such as α and γ. The cavitation erosion studies of the coatings were conducted by ultrasonic cavitation testing on the basis of ASTM G32 standard. It was found that cavitation erosion is accelerated around the unmelted particles and porosities. The results reveal that the cavitation resistance of the coating is determined by its microstructure and that increasing discontinuities (inside the coating) reduce its cavitation resistance. We have found that the coating obtained at hydrogen gas flow rate of 16 L/min and powder feeding rate of 20 g/min has the best cavitation resistance.     

Investigation on the corrosion and oxidation resistance of Ni-Al2O3 nano-composite coatings prepared by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique from a Watt's type electrolyte containing nano-Al₂O₃ particles. The corrosion resistance and high temperature oxidation resistance of resulting composite coatings were investigated. It was found that the incorporation of nano-Al₂O₃ particles in Ni matrix refined the Ni crystal and changed the preferential orientation of composite coatings. Meanwhile, the corrosion and oxidation resistance were improved after the incorporation of nano-Al₂O₃ particles into Ni matrix. The nano-Al₂O₃ content in deposits plays an important role for improving the corrosion and oxidation protection. The corrosion and oxidation resistance of Ni-Al₂O₃ nano-composite coatings produced via SCD technique are superior to that of CEP technique. Compared to pure Ni and Ni-Al₂O₃ composite coatings fabricated using CEP technique, the Ni-7.58 wt.% Al₂O₃ composite coating obtained by SCD technique exhibits better corrosion resistance and enhanced high temperature oxidation resistance. Moreover, the mechanism of corrosion and high temperature oxidation resistance of Ni-Al₂O₃ nano-composite coatings are discussed.     

Annealing temperature effect on the corrosion parameters of autocatalytically produced Ni-P and Ni-P-Al2O3 coatings in artificial seawater

Ni-P and Ni-P-Al₂O₃ amorphous alloy coatings with 9.3 and 8.3 wt.% P respectively were obtained by autocatalytic deposition at 90 °C on carbon steel substrates. The effect of annealing temperature (100, 200, 300, 400 and 500 °C) upon the corrosion parameters of the coatings in artificial seawater with pH 5.0 and 8.1 at room temperature was evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. It was found that deposits annealed at 400 and 500 °C presented an increase of the charge transfer resistance and negligible changes on samples annealed at lower temperature. Polarisation tests showed a charge transfer controlled anodic kinetics on both Ni-P and Ni-P-Al₂O₃ deposits and diffusion controlled cathodic reaction in artificial seawater at pH 5.0 and 8.1. The coatings did not present passive behaviour in the electrolytes and impedance measurements showed a single time constant for all cases with the lowest double layer capacitance (C_dl) for samples annealed at 400 and 500 °C. The best corrosion parameters were observed on Ni-P and Ni-P-Al₂O₃ coatings annealed at temperatures higher than 400 °C, which is the temperature where crystallisation of this kind of coatings takes place.     

几种金属基板上冷喷涂铜涂层的试验与模拟

采用自主研制的冷喷涂设备在三种典型基板上进行喷涂试验,相同的工艺参数下,在铜和铝基板上得到良好的铜涂层,而在钢基板上则没有沉积.实验结果表明:涂层与基板界面、涂层内部颗粒界面结合良好,铜涂层组织致密,显微硬度高达150HV0.1;从涂层表面形貌扫描电镜(SEM)照片中可以观察到射流状的金属,说明颗粒发生了巨大变形,经计算知颗粒在碰撞中压缩率达69%;粉末和涂层的X射线衍射(XRD)结果表明铜粉末在冷喷涂过程中没有发生氧化.同时,数值模拟了铜颗粒与三种基板的碰撞过程,讨论了形成有效结合的判断准则,根据该准则,计算出铜颗粒在铜、铝、钢基板上的临界沉积速度分别为600m/s,500m/s,800m/s,从而解释了铜颗粒在三种基板上不同的沉积行为.     

Cold spray of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with soft metal incorporation: A technical contribution

Al + SiC, Al + Al₂O₃ composites as well as pure Al, SiC, and Al₂O₃ coatings were prepared on Si substrates by the cold gas dynamic spray process (CGDS or cold spray). The powder composition of metal (Al) and ceramic (SiC, Al₂O₃) was varied into 1:1 and 10:1 wt.%, respectively. The propellant gas was air heated up to 330 °C and the gas pressure was fixed at 0.7 MPa. SiC and Al₂O₃ have been successfully sprayed producing coatings with more than 50 μm in thickness with the incorporation of Al as a binder. Also, hard ceramic particles showed peening effects on the coating surfaces. In the case of pure Al metal coating, there was no crater formation on hard Si substrates. However, when Al mixed with SiC and Al₂O₃, craters were observed and their quantities and sizes depended on the composition, aggregation and size of raw materials.     

Microstructure and properties of WC reinforced Ni-based composite coatings with Y2O3 addition on titanium alloy by laser cladding

ABSTRACT

In this study, WC reinforced Ni-based composite coatings with Y₂O₃ addition were deposited on Ti-6Al-4V titanium substrate by laser cladding. The phases, microstructure, microhardness and wear resistance of the composite coatings were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, microhardness tester and wear tester. The results showed that good metallurgical bonding was achieved between the composite coatings and substrate. The phases mainly were γ-Ni, TiC, TiB₂, Ni₃B, M₂₃C₆ and WC. Most of the WC was dissolved in small pieces of WC during the laser cladding process. The microhardness of the composite coatings was about 3 times that of the titanium substrate and the wear resistance of the composite coatings had a significant increase.     

Electrochemical corrosion behavior of plasma sprayed Al2O3‐13%TiO2 coatings in aqueous hydrochloric acid solution

One kind of conventional and two kinds of nanostructured Al₂O₃‐13%TiO₂ coatings were prepared by plasma spray process. The phase composition and microstructure of coatings were examined by means of scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to investigate the corrosion behavior of coatings in aqueous hydrochloric acid solution. The results showed that nanostructured coatings had superior corrosion resistance compared to conventional Metco 130 coating. The corrosion resistance of coatings was mainly related to their microstructure and defects density. The EIS measurement for long time immersion in hydrochloric acid solution revealed that the corrosion resistance of coatings decreased with the increasing of immersion time. During the immersion period, electrochemical corrosion mainly occurred on the carbon steel substrate under NiCrAl coatings. In addition, the Al₂O₃‐13%TiO₂ coatings were also failed during corrosion in aqueous hydrochloric acid solution.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

Plasma spray coating of spray-dried Cr2O3/wt.% TiO2 powder

An agglomerated Cr₂O₃/wt.%TiO₂ powder has been fabricated by the spray drying process under different parameters. The spray-dried powder has well-agglomerated particles of spherical shape. In the conditions of the high slurry feed rate and low binder concentration in the slurry, the powder has large cavities inside some particles and ruggedness over their surface. The optimum plasma spray feed rate has been found by examining the spraying behavior of the powder and melted state of particles. The plasma spray coating has been performed under different process variables such as spraying distance and plasma power. These parameters strongly affect the characteristics of the coated layer: microstructure, hardness, and bond strength.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Effect of applied current on the electrodeposited Ni-Al2O3 composite coatings

Nano-sized Al₂O₃ ceramic particles (50 nm) were co-deposited with nickel using electrodeposition technique to develop composite coatings. The coatings were produced in an aqueous nickel bath at different current densities and the research investigated the effect of applied current on microstructure and thickness of the coatings. The variation in some mechanical properties such as hardness, wear resistance, and the adhesive strength of the composite coatings is influenced by the applied current and this was also studied. The morphology of the coatings was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The hardness, wear resistance, and bond strength of the coatings were evaluated by Vickers micro-hardness test, pin-on-disc test, and tensile test, respectively. Results showed that the Al₂O₃ particles were uniformly distributed in the coatings, and the coatings deposited at a current density of 0.01 A/cm² was most favorable in achieving a maximum current efficiency which causes the co-deposition of a maximum amount of Al₂O₃ particles (4.3 wt.%) in the coatings. The increase in Al₂O₃ particles in the coatings increased the mechanical properties of the Ni-Al₂O₃ composite coatings by grain refining and dispersion strengthening mechanisms.     

氧化铝含量对冷喷涂Al-Al2O3复合涂层表观形貌和显微结构的影响

目的 通过改变原料粉末中氧化铝的添加量,研究氧化铝含量的变化对冷喷涂Al-Al2O3复合涂层形貌和显微结构的影响,为冷喷涂金属/陶瓷复合涂层的制备及结构性能的优化提供参考.方法 将商用纯Al粉和Al2O3粉按照不同的配比进行混合,利用冷喷涂技术在Q235钢表面制备不同Al2O3含量的Al-Al2O3复合涂层.通过X射线...     

Investigation and characterization of Cr3C2-based wear-resistant coatings applied by the cold spray process

The purpose of this study was to explore the potential of the cold spray (CS) process in applying Cr₃C₂-25wt.%NiCr and Cr₃C₂-25wt%Ni coatings on 4140 alloy for wear-resistant applications. This article discusses the improvements in Cr₃C₂-based coating properties and microstructure through changes in nozzle design, powder characteristics stand off distance, powder feed rate, and traverse speed that resulted in an improved average Vickers hardness number comparable to some thermal spray processes. Cold spray process optimization of the Cr₃C₂-based coatings resulted in increased hardness and improved wear characteristics with lower friction coefficients. The improvement in hardness is directly associated with higher particle velocities and increased densities of the Cr₃C₂-based coatings deposited on 4140 alloy at ambient temperature. Selective coatings were evaluated using x-ray diffraction for phase analysis, optical microscopy (OM). and scanning electron microscopy (SEM) for microstructural evaluation, and ball-on-disk tribology experiments for friction coefficient and wear determination. The presented results strongly suggest that cold, spray is a versatile coating technique capable of tailoring the hardness of Cr₃C₂-based wear-resistant coatings on temperature sensitive substrates.     

Influence of nitrogen in the shielding gas on corrosion resistance of duplex stainless steel welds

The influence of nitrogen in shielding gas on the corrosion resistance of welds of a duplex stainless steel (grade U-50), obtained by gas tungsten arc (GTA) with filler wire, autogenous GTA (bead-on-plate), electron beam welding (EBW), and microplasma techniques, has been evaluated in chloride solutions at 30 °C. Pitting attack has been observed in GTA, electron beam welding, and microplasma welds when welding has been carried out using pure argon as the shielding gas. Gas tungsten arc welding with 5 to 10% nitrogen and 90 to 95% argon, as the shielding gas, has been found to result in an improved pitting corrosion resistance of the weldments of this steel. However, the resistance to pitting of autogenous welds (bead-on-plate) obtained in pure argon as the shielding gas has been observed to remain unaffected. Microscopic examination, electron probe microanalysis (EPMA), and x-ray diffraction studies have revealed that the presence of nitrogen in the shielding gas in the GTA welds not only modifies the microstructure and the austenite to ferrite ratio but also results in a nearly uniform distribution of the various alloying elements, for example, chromium, nickel, and molybdenum among the constitutent phases, which are responsible for improved resistance to pitting corrosion.     

过渡材料对等离子喷涂Al2O3梯度陶瓷涂层性能影响

针对Al2O3陶瓷涂层结合强度低、孔隙率高的实际,选择NiAl金属间化合物和金属铜粉作为过渡材料,利用等离子喷涂制备Al2O3梯度陶瓷涂层,并对梯度涂层进行组织形貌观察,测试结合强度和孔隙率.结果表明,梯度涂层的组织表现出宏观的不均匀性和微观连续性的分布特征,NiAl和Cu是金属基体与Al2O3涂层之间过渡层的理想材料,可以有效地提高涂层的结合强度,而Cu-Al2O3梯度涂层又比NiAl-Al2O3梯度涂层结合强度高;梯度涂层的孔隙率远低于双涂层的孔隙率,在Cu-Al2O3梯度涂层中随Al2O3含量的增加,涂层的孔隙率降低,而且孔隙率低于NiAl-Al2O3梯度涂层.     

Effects of critical plasma spray parameter and spray distance on wear resistance of Al2O3-8 wt.%TiO2 coatings plasma-sprayed with nanopowders

Effects of plasma spraying conditions on wear resistance of nanostructured Al₂O₃-8 wt.%TiO₂ coatings plasma-sprayed with nanopowders were investigated in this study. Five kinds of nanostructured coatings were plasma-sprayed on a low-carbon steel substrate by varying critical plasma spray parameter (CPSP) and spray distance. The coatings consisted of fully melted region of γ-Al₂O₃ and partially melted region, and the fraction of the partially melted regions and pores decreased with increasing CPSP or decreasing spray distance. The hardness and wear test results revealed that the hardness of the coatings increased with increasing CPSP or decreasing spray distance, and that the hardness increase generally led to the increase in wear resistance, although the hardness and wear resistance were not correlated in the coating fabricated with the low CPSP. The main wear mechanism was a delamination one in the coatings, but an abrasive wear mode also appeared in the coating fabricated with the low CPSP. According to these wear mechanisms, the improvement of wear resistance in the coating fabricated with the low CPSP could be explained because the improved resistance to fracture due to the presence of partially melted regions might compensate a deleterious effect of the hardness decrease.     

Toughening and strengthening mechanism of plasma sprayed nanostructured Al2O3–13 wt.%TiO2 coatings

The starting materials of Al₂O₃, TiO₂, ZrO₂ and CeO₂ nanoparticles were agglomerated into sprayable feedstock powders and plasma sprayed to form nanostructured coatings. There were net structures and fused structures in plasma sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings. The net structures were derived from partially melted feedstock powders and the fused structures were derived from fully melted feedstock powders. The nanostructured Al₂O₃–13 wt.%TiO₂ coatings possessed higher hardness, bonding strength and crack growth resistance than conventional Metco 130 coatings which were mainly composed of lamellar fused structures. The higher toughness and strength of nanostructured Al₂O₃–13 wt.%TiO₂ coatings were mainly related to the obtained net structures.     

Effect of Y2O3 additions on the plasticity of sintered Cr2O3

We have carried our constant strain-rate compression tests on polycrystalline Cr₂O₃ and Cr₂O₃ doped with 0.09 wt. % Y₂O₃ to establish whether there exists an effect of Y₂O₃ on the plasticity of Cr₂O₃. This study is motivated by previous work on the oxidation of alloys containing reactive-element additions. In that work, it has been observed that the addition of oxygen-active elements, such as Y to alloys that form Cr₂O₃ or Al₂O₃ oxide layers upon exposure at high temperature, strongly enhances the adhesion of the oxide layer to the base alloy as compared with alloys without reactive-element additions. We have found that at 1200°C (1) chromia exhibits limited plasticity at high temperatures, and (2) the presence of Y in the oxides does not enhance plasticity compared with addition-free oxides.     

Enhanced corrosion resistance of porous NiTi with plasma sprayed alumina coating

In this study, corrosion behaviour of porous NiTi modified by plasma sprayed alumina coating has been investigated. Scanning electron microscopy and X-ray diffraction techniques were applied for the morphology and microstructure characterisation, while linear sweep voltammetry and electrochemical impedance spectroscopy were used for investigation of corrosion behaviour of coated and uncoated NiTi specimens. Induced couple plasma was conducted to measure ion release of the specimens in simulated body fluid at 37°C. The plasma sprayed Al₂O₃ coating on the porous NiTi improved the surface characteristics for biomedical applications. The alumina coating significantly hampered Ni ion release from the surface. In spite of slight decrease in corrosion resistance of the coated specimens, the corrosion mechanism changed from pitting to general corrosion. The breakdown phenomenon was not detected in the coated specimens, as well. Overall, it can be concluded the longevity of the coated specimen in the simulated biological system was enhanced, comparing to bare NiTi specimens.