Preparation and Bond Properties of Thermal Barrier Coatings on Mg Alloy with Sprayed Al or Diffused Mg-Al Intermetallic Interlayer

Sprayed Al or diffused Mg-Al layer was designed as interlayer between the thermal barrier coatings (TBCs) and Mg alloy substrate. The effects of the interlayer on the bond properties of the coats were investigated. Al layers were prepared by arc spraying and atmospheric plasma spraying (APS), respectively. Mg-Al diffused layer was obtained after the heat treatment of the sprayed sample (Mg alloy with APS Al coat) at 400 °C. The results show that sprayed Al interlayer does not improve the bond stability of TBCs. The failure of the TBCs on Mg alloy with Al interlayer occurs mainly due to the low strength of Al layer. Mg-Al diffused layer improves corrosion resistance of substrate and the bond interface. The TBCs on Mg alloy with Mg-Al diffused interlayer shows better bond stability than the sample of which the TBCs is directly sprayed on Mg alloy substrate by APS.     

On the Bonding Mechanism in Cold Spray of Deformable hex-BN-Ni Clusters

Bond strength and the lubrication potential of coatings made of 7 µm Hexagonal Boron Nitride particles encapsulated with nickel (hBN-Ni), and deposited onto aluminum 6061 substrates via cold spray were examined; for all tests, N₂ was used as the carrier gas at a temperature of 480 °C and pressure of 2.4 MPa. Results showed significant improvement in both wear resistance and reduced surface friction. Coated samples also demonstrated unexpected high bond strength, which was much greater than pure nickel cold sprayed onto aluminum. However, while the results were truly promising, the primary reason for the observed high bond strength could not be explained using existing cold spray theories which were primarily developed for pure metal particles. Based on the present findings compared to cold-sprayed layers of composite nickel-nickel (nickel particles encapsulated with nickel), a mechanism for bonding of hBN-Ni particles to aluminum based on the level of plastic deformation and hardenability is proposed. Indeed, the high bond strength between the coating and substrate is related to the relatively high initial ductility of the nickel encapsulation, compliance of the hBN, as well as the ensuing significant plastic deformation of the composite particles during cold spray deposition.     

The application of thermal sprayed coatings for pig iron ingot molds

Molds made of gray cast iron for casting pig iron ingots are subjected to severe temperature fluctuations. The main life- limiting factor for mold damage is the formation of surface cracks arising from thermal fa-tigue. Various flame and plasma sprayed coatings were investigated to extend the life of these molds. Coating materials studied include plasma sprayed ceramic coatings with bond coats as well as flame sprayed oxidation- resistant alloy powders. The results of cyclic furnace tests from room temperature to 1100 °C in air, simulating the thermal cycle in casting, indicated that failure occurred along the interface between the bond coat and the gray iron substrate because of iron oxidation, and not at the interface between the ceramic top coat-ing and the bond coating for a superalloy substrate. The field test results indicated that plasma sprayed alumina coatings with 200 μm top coating thickness are the most promising materials for pig iron casting.     

镍铝材料状态对火焰喷涂层组织结构和性能的影响

针对煤矿机械液压支柱热喷涂修复中涂层结合强度低的实际,研究了在27SiMn钢基体上火焰喷涂新研制的镍铝合金丝制备镍铝涂层,并与火焰喷涂镍包铝粉末、镍铝复合丝制备的镍铝涂层在组织结构、表面形貌、结合强度等方面进行了比较分析.结果表明,镍铝合金丝制备的涂层与基体之间主要为冶金结合,结合强度比镍包铝粉末涂层提高46.8％,比镍铝复合丝涂层提高8％,涂层组织结构致密,气孔率低、质杂少.     

Mold manufacture with plasma spraying

A process has been developed to produce molds or tooling using a steel or chrome- plated steel model. The investigation examined the effect of coating and model materials, model temperature and spray angle on the coating separation from the model surface, coating delamination, and surface quality. A polished model disk was heated and then plasma sprayed with iron, nickel, Ni- Al, or Ni- Cr- B- Si. It was found that the minimum temperature to facilitate entire coating removal was lower for steel models and varied be tween 200 and 450 °C depending on the material. However, at higher temperatures the higher bond strength produced by oxidation on the steel resulted in significant coating pullout. A chrome- plated model, heated to 600 to 700 °C, is required to produce a defect- free coating. The effect of substrate angle on open porosity is most critical for the Ni- Cr- B- Si alloy and least important for Ni- Al coatings. The sur face roughness of the plasma- sprayed molds is comparable to the corresponding models, permitting good surface detail reproducibility. Several molds and tools were produced for use in the glass, rubber, and plastics industries.     

Duplex coating of electroless nickel and HVOF (high-velocity oxygen fuel) sprayed WC-Co

The porosity of thermal sprayed coatings is usually a problem when coatings are used in wet corrosion application. The porosity allows media to penetrate to the surface of the base material. Corrosion spreads rapidly and the coating loses contact with the substrate and delaminates. This problem can be initiated by impregnating different polymers into the pores. An alternative approach has been tested in the present work to prevent corrosion of cemented carbide coated carbon steel in wet corrosion environments. Carbon steel substrates were coated with a thin film of electroless nickel (electroless nickel plating) and then HVOF (High-Velocity Oxygen Fuel) sprayed with cemented carbide. Reference specimens without electroless nickel were sprayed at the same time. The microhardness of the specimens was measured and the coating structure examined using optical microscopy and X-ray diffractometry (XRD). The bond between the layers and the base material was examined by means of a bend test. A salt chamber test was also performed for the specimens. The structure of the electroless nickel layer was crystalline as a result of the HVOF spraying. There were no cracks in the nickel layer, if the layer was about 20 μm thick. According to the results of the bend test, the adhesion between coatings and substrate was good, and there was no difference between the duplex-coated specimen and the reference specimen. A sample with a thin nickel layer under an HVOF sprayed cemented carbide did not exhibit corrosion after 8 h in the salt chamber test.     

The effect of oxide coatings on fatigue properties of 7475-T6 aluminium alloy

The effect of hard anodic oxide and plasma electrolytic oxide coatings on the fatigue strength of 7475-T6 aluminium alloy has been investigated. The coated aluminium alloy was tested using constant load uniaxial tensile fatigue machine. Hard anodising led to an appreciable reduction in the fatigue strength of 7475-T6 alloy of about 75% for a 60 μm thick coating. Further, plasma electrolytic oxidation resulted in reduction of the fatigue strength of about 58% for a 65 μm thick oxide coating. The decrease in fatigue strength of the hard anodic oxide coatings was associated with the stress concentration at the microcracks in the coating. The better fatigue performance of the PEO coatings was attributed to the development of the compressive residual internal stress within the coatings. The reduction in the fatigue strength of the PEO coatings as compared to the uncoated material was associated with the development of the tensile residual internal stress within the substrate. This may cause an early crack initiation in the substrate adjacent to the coating.     

The evaluation of powder processing on microstructure and mechanical properties of hydroxyapatite (HA)/yttria stabilized zirconia (YSZ) composite coatings

In clinical applications, the mechanical failure of HA-coated titanium alloy implants suffered at the interface of the HA coating and titanium alloy substrate will be a potential weakness in prosthesis. Yttria stabilized zirconia (YSZ) reinforced HA coatings have been proven to enhance the mechanical properties of the HA coating significantly and reduce the formation of calcium oxide (CaO). In this paper, HA/YSZ (30 wt.% YSZ) composite coatings were sprayed by the plasma technique. The effects of the powder processing–mechanical ball milling method and spheroidization method on the microstructure and mechanical properties of the HA/YSZ composite coatings were evaluated. The experimental results showed that the spheroidized powders melted better than the ball milled powders during plasma spraying and formed higher mechanical property coatings (1.6326±0.08 MPa m^−0.5 of fracture toughness, 58.59±2.91 GPa of elastic modulus and 43.42±2.53 MPa of tensile bond strength). HA/YSZ solid solution formed during deposition on the substrate, which played a very important role in the mechanical properties of the HA/YSZ composite coatings. Tensile bond strength tests showed that the fracture mode was cohesive and that failure occurred at the interface of HA and unmelted YSZ particles. The molten state of YSZ had a great influence on the properties of the HA/YSZ composite coatings.     

电弧喷涂锡基巴氏合金涂层的组织与性能

针对电弧喷涂巴氏合金涂层组织与铸造组织的显著差异,借助于扫描电子显微镜、能谱分析和X射线衍射分析涂层的微观组织结构;测试了巴氏合金涂层在钢铁基体上的结合强度,研究了涂层与碳钢及铸铁组成摩擦副时的磨损表现.结果表明,电弧喷涂巴氏合金涂层的组织细小、均匀,SnSb和Cu6Sn5化合物的形态趋于不规则的块状或近于球形;施加过渡底层可以使涂层更可靠地与钢铁基体结合;在润滑条件下,涂层表现出比铸造合金更好的耐摩性能.     

The deformation and cooling of ceramic particles sprayed with a thermal radio-frequency plasma under atmospheric conditions

Common thermal-spray techniques use the strong acceleration of powder particles to produce dense ceramic coatings with high bond strength. The residence time of the powder particles within the plasma jet is correspondingly low, and only relatively small particles can be molten. In this work, on the contrary, an inductively coupled radio-frequency (RF) inductively coupled plasma (ICP) torch was used to spray large oxide-ceramic powder particles under atmospheric conditions. The slow plasma flow of a RF plasma leads to large residence times of the powder particles, so that the powder size of the feedstock can be 100 μm and more. It was observed that these particles will not be strongly accelerated in the plasma and that their velocity at the moment of impact is in the range of 10 to 20 m/s. Ceramic coatings were ICP sprayed with a low porosity and a high bond strength, similar to direct current (DC) or high-velocity-oxygen-fuel (HVOF) sprayed coatings. The morphology of ICP-sprayed particles on smooth steel surfaces, as a function of the surface temperature, is described and compared with DC plasma-sprayed splats. Furthermore, the degree of deformation was measured and determined by different models, and the pronounced contact zones formed between the pancake and the substrate were investigated. The ICP-sprayed ceramic coatings show some special properties, such as the absence of metastable crystalline phases, which are common in other spray technologies.     

高温树脂基复合材料防护用轻质陶瓷涂层的制备

研究了采用Ni-3％Al粉末和纯铝、纯锌作为打底材料在碳纤维增强聚酰亚胺复合材料(PMC)基体上制备Al2O3轻质陶瓷防护涂层的可行性,测试了涂层的剪切结合强度和耐热循环性能。结果表明,等离子喷涂Ni-3％Al粉末会对PMC基体造成破坏,不适合于作为PMC基体上的打底材料。电弧喷铝也会对基体造成一定程度的破坏,结合强度和耐热循环性能较低。采用低电压、小电流电弧喷锌,可以获得和基体结合良好打底涂层,涂层剪切结合强度达10．45MPa。在其上制备的Al2O3陶瓷防护涂层耐热循环性能良好。     

Influence of Plasma Remelting on the Microstructure and Cavitation Resistance of Arc-Sprayed Fe-Mn-Cr-Si Alloy

Surface remelting is an important technique for modifying the microstructure of thermally sprayed coatings as it reduces the porosity and promotes a metallurgical bond between substrate and coating. Many studies have been carried out in the field of materials selection and surface engineering in an attempt to reduce cavitation damage. In this work, an Fe-Mn-Cr-Si alloy was deposited by arc spraying and then remelted by a plasma-transferred arc process. The base metal was a soft martensitic stainless steel. The influence of remelting current on coating and base metal microstructure and cavitation resistance was studied. The use of a lower mean current and a pulsed arc reduced the thickness of the heat-affected zone. In specimens remelted with constant arc current, dendrites were aligned parallel to the path followed by the plasma torch; while in those remelted with a pulsed plasma arc, the alignment of the microstructure was disrupted. The use of a higher peak current in pulsed-current plasma transferred arc remelting reduced mass loss due to cavitation. Fe-Mn-Cr-Si coatings exhibited cavitation-induced hardening, with martensite formation during cavitation tests. This transformation helps to increase the cavitation resistance of the remelted coating compared with the soft martensitic stainless steel base metal.     

Bond Characterization of Plasma Sprayed Zirconium on Uranium Alloy by Microcantilever Testing

The future production of low enriched uranium nuclear fuel for test reactors requires a well-adhered diffusion barrier coating of zirconium (Zr) on the uranium/molybdenum (U-Mo) alloy fuel. In this study, the interfacial bond between plasma sprayed Zr coatings and U-Mo fuel was characterized by microcantilever beam testing. Test results revealed the effect of specific flaws such as cracks and pores on the bonding of interfaces with a sampling area of approximately 20 μm². TEM examination showed the Zr/U-Mo interface to contain rows of very fine grains (5-30 nm) with the Zr in contact with UO₂. Bond characteristics of plasma sprayed samples were measured that are similar to those of roll bonded samples showing the potential for plasma sprayed Zr coatings to have high bond strength.     

Vacuum plasma spraying of high-performance electrodes for alkaline water electrolysis

Electrode coatings for advanced alkaline water electrolysis were produced by applying the vacuum plasma spraying (VPS) process. The characteristics of the used VPS equipment that were essential for the development of effective electrocatalytic electrode layers are presented. Molybdenum-containing Raney nickel coatings were applied for cathodic hydrogen evolution, and Raney nickel/Co₃O₄ matrix composite layers were developed for the anodic oxygen evolution reaction. For the preparation of Raney nickel coatings, a precursor alloy such as Ni-Al was sprayed that had to be leached subsequently in caustic solution to remove the aluminum content, forming a porous, high-surface-area nickel layer. The spray powders and the resulting VPS layers were studied by metallography, x-ray diffraction (XRD), and scanning electron microscopy/energy dispersive analysis by x-ray (SEM/EDX). For spraying of thermally sensitive oxide electrocatalysts (e.g., Co₃O₄), special process conditions involving plasma-chemical effects (reactive plasma spraying) had to be developed. The electrocatalytic activity of the electrode coatings was investigated by performing polarization curves free of ohmic losses (IR-free) and long-term tests under conditions of continuous and intermittent operation, which showed excellent electrochemical properties.     

铝合金表面电弧喷涂Ag基涂层及其低温钎焊行为

在波导器件中,铝合金壳体较差的润湿性制约了其与微带电路板之间大面积、可靠低温钎焊连接. 通过电弧喷涂技术在5A06铝合金表面制备了厚度约为80 μm的Ag-15%Ni(质量分数)单一涂层和Ni-5%Al/Ag-15%Ni(质量分数)复合涂层,以提升Sn-Pb钎料在其表面的润湿性. 对比研究了两种涂层的显微结构、涂层界面结合性能、低温钎焊行为及钎焊接头剪切失效机制. 结果表明,涂层与铝合金基板间形成了良好的界面结合,并且两种涂层均具有较好的低温焊接性. 其中,Ag-15%Ni单一涂层与铝合金基板的结合强度为40 MPa,喷涂后铝合金基板与T2紫铜形成的钎焊接头抗剪强度为26 MPa. 相较而言,Ni-5%Al /Ag-15%Ni复合涂层展现出更佳的涂层结合强度(42 MPa)和钎焊接头抗剪强度(31 MPa).     

Improving stability of thermal barrier coatings on magnesium alloy with electroless plated Ni–P interlayer

Thermal barrier coatings (TBCs) of zirconia stabilized by 8 wt.% yttria (8YSZ) on MB26 rare earth–magnesium alloy with MCrAlY as bond coat were prepared by air plasma spraying (APS). In order to improve the thermal shock resistance of the coatings, an interlayer of Ni–P alloy between the substrate and bond coat was prepared by electroless plating. The preparation, microstructure, bond strength and thermal shock resistance of the coatings were investigated. The results indicate that Ni–P interlayer not only has favorable effects on the protection of Mg alloy substrate from thermal oxidation during thermal spraying, but also significantly improves the bond strength of TBCs. The thermal shock life of TBCs was enhanced from 5 cycles to longer than 130 cycles with the application of Ni–P interlayer. The failure of TBCs in thermal shock test was mainly induced by the corrosion of Mg alloy substrate.     

Microstructure and 3D microtomographic characterization of porosity of MAO surface layers formed on aluminium and 2214-T6 alloy

A fine characterization of the microstructure of the MAO coatings formed on aluminium (purity 99,999%) and its alloy 2214-T6 was observed using field emission gun scanning electron microscopy (FEG-SEM) with energy dispersive spectrometry (EDS) and X-ray microtomography (XMT). In addition, the influence of current frequency (pure aluminium) and the precipitates of substrate were investigated (case of 2214-T6 alloy).The MAO surface layers formed on aluminium and its alloy 2214-T6 can be divided into two parts: the outer layer with high porosity (so-call porous layer) and the inner layer with low porosity (so-call dense layer). However, the porosity of the inner layer increases toward the MAO layer/substrate interface. It is found that, both the thickness and the porosity increase when the current frequency decreases. The porosity of the MAO coating can be attributed to discharges formation in the vicinity of the MAO coating/substrate interface.In the particular case of 2214-T6 aluminium alloy (3.9% Cu), with coarse non-valve metal rich precipitates aligned perpendicular to the surface of the work electrode; the transformation of precipitates under discharge effect can provoke the formation of big channels, which go through the MAO surface layer. This last one induces local formation of a layer rich electrolyte species toward the substrate.     

Study of microstructure and thermal shock behavior of two types of thermal barrier coatings

Gas turbines provide one of the most severe environments challenging material systems nowadays. Only an appropriate coating system can supply protection particularly for turbine blades. This study was made by comparison of properties of two different types of thermal barrier coatings (TBCs) in order to improve the surface characteristics of high temperature components. These TBCs consisted of a duplex TBC and a five layered functionally graded TBC. In duplex TBCs, 0.35 mm thick yittria partially stabilized zirconia top coat (YSZ) was deposited by air plasma spraying and ～0.15 mm thick NiCrAlY bond coat was deposited by high velocity oxyfuel spraying. ～0.5 mm thick functionally graded TBC was sprayed by varying the feeding ratio of YSZ/NiCrAlY powders. Both coatings were deposited on IN 738LC alloy as a substrate. Microstructural characterization was performed by SEM and optical microscopy whereas phase analysis and chemical composition changes of the coatings and oxides formed during the tests were studied by XRD and EDX. The performance of the coatings fabricated with the optimum processing conditions was evaluated as a function of intense thermal cycling test at 1100 °C. During thermal shock test, FGM coating failed after 150 and duplex coating failed after 85 cycles. The adhesion strength of the coatings to the substrate was also measured. Finally, it is found that FGM coating has a larger lifetime than the duplex TBC, especially with regard to the adhesion strength of the coatings.     

电热爆炸定向喷涂Stellite 6合金涂层

采用电热爆炸定向喷涂工艺在45号钢基体上制备stellite6合金涂层。借助光学显微镜，扫描电镜和图像分析软件等对涂层厚度均匀性、孔隙率，显微组织、晶粒度以及涂层基体界面结合情况进行了分析。借助显微硬度仪对涂层的硬度进行了测试。对不同尺寸喷涂材料所得涂层进行了比较。结果表明，两种不同尺寸喷涂材料制备的涂层组织晶粒均大大细化，涂层孔隙率都比较低；小截面积的喷涂材料制备涂层的厚度均匀性好于大截面的喷涂层；涂层与基体界面结合良好，在界面附近发生了扩散现象；涂层硬度均远远高于原始stellite6喷涂材料硬度，最高分别达到997HV和738HV，为原始硬度的2～3倍。且小尺寸喷涂材料涂层硬度高于大尺寸喷涂材料涂层硬度。