Mechanical Properties and Thermal Shock Resistance of HVOF Sprayed NiCrAlY Coatings Without and With Nano Ceria

NiCrAlY coatings without and with 0.2?wt.% nano ceria were prepared by high velocity oxygen fuel spraying. The microstructure, mechanical properties, and thermal shock resistance of as-sprayed coatings were investigated. The results showed that in the as-sprayed coatings, the number of un-melted particles was reduced drastically, the microstructure was refined and compact due to the refinement of sprayable powders. Both the hardness and adhesive strength of the NiCrAlY increased due to the refinement of microstructure and the decrease of the defects, such as pores and oxides, after adding nano ceria. The thermal cycle life of NiCrAlY coatings was improved by 15% after adding 0.2?wt.% nano ceria, which is attributed to the low content of spinel NiCr₂O₄ and high content of Cr₂O₃ in the thermal cycling, the refined and compact microstructure, and increased interfacial boundary.     

Variation of Thermal Barrier Coating Lifetime Characteristics with Thermal Durability Evaluation Methods

The thermal durability of thermal barrier coating systems (TBCs) obtained using feedstock powders with different purity and phase content was investigated by thermal shock testing with different cycle times, including the effects on the sintering and phase transformation behaviors. Four 8 wt.% yttria-stabilized zirconia powders, with regular purity (TC1), high purity (TC2 and TC3), and without monoclinic phase (TC4), were employed to prepare the topcoat of TBC by atmospheric plasma spray on a NiCoCrAlY bondcoat deposited by high velocity oxy-fuel. The microstructure and phase stability of the topcoats affected the TBCs’ lifetime in the short-term (1 h) and long-term (24 h) furnace cyclic test (FCT) at 1100 °C and jet engine thermal shock (JETS) test. In the short-term FCT and JETS tests, in which coatings are severely subjected to thermal stress, the TBCs’ lifetime is most affected by the microstructure of the topcoat. The coating layer with the lowest monoclinic phase in the as-sprayed state showed the lowest phase-transformation characteristics in the isothermal oxidation test (1400 °C). These properties resulted in the best lifetime in the long-term FCT. Therefore, the coating material and evaluating methods of TBCs’ life should be selected depending on the usage environment.     

Microstructure and Corrosion Behavior of Electrodeposited Ni-Co-ZrC Coatings

In this present work, ZrC particles incorporated Ni-Co composite coatings were electrodeposited. The objective of this article is to study the influence of Co content on the microstructure and properties of Ni-Co-ZrC coatings. Pure Ni and Ni-ZrC coatings have also been electrodeposited for comparison. Surface morphology, chemical composition, microstructure, and microhardness of Ni-Co-ZrC coatings were characterized by scanning electron microscopy, energy dispersive spectrometer, x-ray diffractometer, and Vicker microhardness tester. The potentiodynamic polarization technique was applied to measure the corrosion behavior of the coatings. By increasing Co concentration in electrolyte, Co content of the coatings was modified from 0 to 80 wt.% and ZrC particles content of the coatings was reduced. As the Co content increased, the dominant phase structure was changed from face centered cubic to hexagonal close packed crystal structure. Surface morphology of the Ni-Co-ZrC coatings was changed from nodular to sharp corner structure, and finally branched morphology with increasing Co content of the coating. Among the electrodeposited coatings, Ni-Co-ZrC coating with 42 wt.% Co content exhibited the highest microhardness. The corrosion potential of the coating was shifted to more positive with increasing the Co content from 0 to 64 wt.%. The lowest corrosion rate of 4.507 × 10^?7 g·h^?1·cm^?2 was found for Ni-Co-ZrC coating at the Co content of 75 wt.%.     

Microstructure and Properties of FeCrB Alloy Coatings Prepared by Wire-Arc Spraying

To improve the heat transfer ability and wear resistance of drying cylinders in paper production machines, a series of Fe_87?x Cr₁₃B _x (x = 1 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, and 4 wt.%) cored wires have been produced and used to prepare coatings by wire-arc spraying, in comparison with conventional X30Cr13 solid wire. All coatings presented dense layered structure with porosity of around 4%. The boron content in the cored wires significantly affected the thermal conductivity of the coating, which is attributed to the combined effects of the crystal structure, grain size, and oxide content of the coating. In the investigated range, the coating with 2 wt.% boron content exhibited the highest thermal conductivity, reaching 8.83 W/m-K, greater than that of X30Cr13 coating (5.45 W/m-K). Furthermore, the microhardness and relative wear resistance of the FeCrB coatings obtained from cored wires with boron addition were greatly increased compared with commercial X30Cr13 coating. Therefore, wire-arc-sprayed FeCrB coating has promise as an effective and economic approach to improve the heat transfer behavior and wear resistance of drying cylinders in the paper industry.     

Optimization of Air Plasma Sprayed Thermal Barrier Coating Parameters in Diesel Engine Applications

In the present paper, an optimization of thermal barrier coating parameters is performed for diesel engine applications. The substrate is A356.0-T7, a cast aluminum alloy which has a vast application in diesel engines, and the alloy is coated by plasma sprayed ZrO₂-8 wt.% Y₂O₃. Parameters including the feed rate of coating powders, the nozzle distance to specimen surfaces, and the coating thickness are optimized by thermal shock fatigue tests and bending tests. Optimum values of the feed rate and the nozzle distance are 30 g/min and 80 mm, respectively, when the objective is considered as maximizing the bending strength. Thermal shock tests demonstrate that lower thickness of coating layers has a better lifetime. By increasing the coating thickness, the thermal fatigue lifetime decreases. The reason is due to higher order of stresses near the interface of the substrate and the bond coat layer, calculated by a finite element simulation. One suggestion to improve the lifetime is to use multiple layers of coatings.     

Cold-Sprayed Ni-Al2O3 Coatings for Applications in Power Generation Industry

Cermets coatings are extensively used in energy applications both because of their high wear resistance as required, for example, in components like gas turbine sealants, and because of their specific functionality as required in solar absorbers. So far, high-temperature thermal spraying and physical vapor deposition have traditionally been used to deposit this kind of coatings. In this study, Ni-Al₂O₃ coatings have been deposited using a Kinetic^®3000 cold-spray system starting from Ni and Al₂O₃ powders blend; five blends have been prepared setting the alumina content in the feedstock to 10, 25, 50, 75, and 90 wt.%. The embedded alumina ranges between a few percent weight up to 16 and 31 wt.%, while the microhardness shows a deep increase from 175 Vickers in the case of pure Ni coatings up to 338 Vickers. The spray and coating growth mechanism have been discussed, with special attention to the fragmentation of the ceramic particles during the impact. Finally, the coating behavior at high temperature was analyzed by oxidation tests performed in air at 520 °C emphasizing a good oxidation resistance that could represent a very promising basis for application in power generation systems.     

Mechanical and Tribological Behavior of Ni(Al)-Reinforced Nanocomposite Plasma Spray Coatings

The mechanical and tribological behavior and microstructural evolutions of the Ni(Al)-reinforced nanocomposite plasma spray coatings were studied. At first, the feedstock Ni(Al)-15 wt.% (Al₂O₃-13% TiO₂) nanocomposite powders were prepared using low-energy mechanical milling of the pure Ni and Al powders as well as Al₂O₃-13% TiO₂ nanoparticle mixtures. The characteristics of the powder particles and the prepared coatings depending on their microstructures were examined in detail. The results showed that the feedstock powders after milling contained only α-Ni solid solution with no trace of the intermetallic phase. However, under the air plasma spraying conditions, the NiAl intermetallic phase in the α-Ni solid solution matrix appeared. The lack of nickel aluminide formation during low-energy ball milling is beneficial hence, the exothermic reaction can occur between Ni and Al during plasma spraying, improving the adhesive strength of the nanocomposite coatings. The results also indicated that the microhardness of the α-Ni phase was 3.91 ± 0.23 GPa and the NiAl intermetallic phase had a mean microhardness of 5.69 ± 0.12 GPa. The high microhardness of the nanocomposite coatings must be due to the presence of the reinforcing nanoparticles. Due to the improvement in mechanical properties, the Ni(Al) nanocomposite coatings showed significant modifications in wear resistance with low frictional coefficient.     

粉末中C抑制FeAl熔滴氧化机制及其对等离子喷涂层组织与性能的影响*

大气等离子喷涂（APS）金属时,熔滴不可避免地发生氧化是难以获得粒子间结合充分的致密涂层的主要原因。以FeAl金属间化合物为例,提出一种在粉末中添加亚微米金刚石颗粒引入碳源,以期利用碳在高温下优先氧化的特性抑制等离子喷涂飞行粒子中Fe、Al元素的氧化,获得无氧化物的高温熔滴从而制备低氧含量（质量分数）、粒子间充分结合的FeAl金属间化合物涂层的新方法。采用APS制备Fe Al涂层,研究金刚石的添加对涂层氧含量、碳含量、涂层内粒子间结合质量与硬度的影响规律,探讨FeAl熔滴飞行中的氧化行为。采用商用热喷涂粒子诊断系统测量APS喷涂中的粒子温度,通过SEM与XRD表征了涂层的组织结构,并表征涂层的结合强度与硬度。结果表明,在等离子射流的加热和Fe、Al元素放热反应的联合作用下,飞行中FeAl熔滴的表面温度可达2 000℃以上,满足C原位脱氧的热力学条件。与不含碳的传统Fe Al涂层中的氧含量随喷涂距离的增加而显著增加的规律完全不同,用Fe/Al/2.5C粉末喷涂时涂层中的氧含量随距离的增加而减小,表明飞行中熔滴的氧化得到抑制,实现了C原位脱氧抑制金属元素氧化的自清洁氧化物的效应。FeAl/...     

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.     

Correlation of microstructure and high temperature oxidation resistance of plasma sprayed NiCrAl, NiCrAlY, and TiAlO composite coatings on Ti−6Al−4V

In the present study Ni−18Cr−6Al, Ni−22Cr−10Al−1Y and TiAlO composite powders were coated on Ti−6Al−4V substrates by atmospheric plasma spraying, and the coated specimens were evaluated by isothermal and cyclic oxidation resistance tests at 800°C. The oxidation kinetics of the plasma sprayed NiCrAl, NiCrAlY, and TiAlO composite coated specimens obey a parabolic rate law. The oxidation resistance of the plasma sprayed NiCrAl and NiCrAlY coatings is superior to that of plasma sprayed TiAlO composite coating. The best oxidation resistance was observed in the plasma sprayed NiCrAlY coatings. This is mainly attributed to the formation of Y−Al−O complex oxides and Ni₃Al with higher thermal stability on the coatings.     

Microstructure and Mechanical Properties of FeBSiNb Metallic Glass Coatings by Twin Wire Arc Spraying

In this study, FeBSiNb coatings were prepared by twin wire arc spraying process. The microstructure and mechanical properties of as-sprayed coatings were characterized. The results show that the coating is adhering well and very compact with porosity of 1.2% (the value range is 0.9-1.7%). The microstructure of the coating consists of full glassy structure. The crystallization temperature, microhardness, elastic modulus, and average adhesive strength of the coating are 819 K, 16.42 GPa (the value range is 14.38-18.46 GPa), 219 GPa (the value range is 201-237 GPa), and 57.4 MPa (the value range is 55-64 MPa), respectively. The relatively wear resistance of the coating is about three times than that of 3Cr13 martensite stainless steel coating. The reasons for excellent wear resistance of FeBSiNb metallic glass coating are attributed to a uniform amorphous structure, the high ratio of hardness to elastic modulus (H/E) and the ratio of the elastic deformation energy to the total deformation energy (η value). The main failure mechanism of the coating is brittle failure and fracture.     

Microstructure and Oxidation Behavior of Al and Al/NiCrAlY Coatings on Pure Titanium Alloy

To improve the oxidation resistance of Ti alloys, a NiCrAlY coating was deposited as diffusion barrier between aluminum overlay coating and pure Ti substrate by air plasma spraying method. The microstructure and oxidation behavior of Al coatings with and without NiCrAlY diffusion barrier were investigated in isothermal oxidation tests at 800 °C for 100 h. The results indicate that the weight gain of the Al/NiCrAlY coating was 4.16 × 10^?5 mg² cm^?4 s^?1, whereas that of the single Al coating was 9.52 × 10^?5 mg² cm^?4 s^?1 after 100 h oxidation. As compared with single Al coating, the Al/NiCrAlY coating revealed lower oxidation rate and excellent oxidation resistance by forming thin Al₂O₃ + NiO scales at overlaying coating/diffusion barrier and diffusion barrier/substrate interfaces. Meanwhile, the inward diffusion of Al and the outward diffusion of Ti were inhibited effectively by the NiCrAlY diffusion barrier.     

Influence of Powder Porous Structure on the Deposition Behavior of Cold-Sprayed WC-12Co Coatings

The limited deformation of hard cermet particles and impacted coating makes it difficult for conventional thermal spray powders to continuously build up on impact in cold spraying. In this study, three nanostructured WC-12Co powders with different porous structure and apparent hardness were employed to deposit WC-Co coatings on stainless steel substrate by cold spraying. The deposition characteristics of three powders of porosity from 44 to 5% were investigated. It was found that WC-Co coating is easily built-up using porous powders with WC particles bonded loosely and a low hardness. The microhardness of WC-12Co coatings varied from 400 to 1790 Hv with powders and spray conditions, which depends on the densification effects by impacting particles. With porous WC-Co powders, the fracture of particles on impact may occur and low deposition efficiency during cold spraying. The successful building up of coating at high deposition efficiency depends on the design of powder porous structure.     

Microstructure,Mechanical Properties,and Two-Body Abrasive Wear Behavior of Cold-Sprayed 20 vol.% Cubic BN-NiCrAl Nanocomposite Coating

20 vol.% cubic boron nitride (cBN) dispersoid reinforced NiCrAl matrix nanocomposite coating was prepared by cold spray using mechanically alloyed nanostructured composite powders. The as-sprayed nanocomposite coating was annealed at a temperature of 750 °C to enhance the inter-particle bonding. Microstructure of spray powders and coatings was characterized. Vickers microhardness of the coatings was measured. Two-body abrasive wear behavior of the coatings was examined on a pin-on-disk test. It was found that, in mechanically alloyed composite powders, nano-sized and submicro-sized cBN particles are uniformly distributed in nanocrystalline NiCrAl matrix. Dense coating was deposited by cold spray at a gas temperature of 650 °C with the same phases and grain size as those of the starting powder. Vickers hardness test yielded a hardness of 1063 HV for the as-sprayed 20 vol.% cBN-NiCrAl coating. After annealed at 750 °C for 5 h, unbonded inter-particle boundaries were partially healed and evident grain growth of nanocrystalline NiCrAl was avoided. Wear resistance of the as-sprayed 20 vol.% cBN-NiCrAl nanocomposite coating was comparable to the HVOF-sprayed WC-12Co coating. Annealing of the nanocomposite coating resulted in the improvement of wear resistance by a factor of ~33% owing to the enhanced inter-particle bonding. Main material removal mechanisms during the abrasive wear are also discussed.     

Development and Investigation on New Composite and Ceramic Coatings as Possible Abradable Seals

To improve gas turbine performance, it is possible to decrease back flow gases in the high-temperature combustion region of the turbo machine by reducing the shroud/rotor gap. Thick and porous thermal barrier coating (TBC) systems and composite CoNiCrAlY/Al₂O₃ coatings made by air plasma spray and composite NiCrAlY/graphite coatings made by laser cladding were studied as possible high-temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al₂O₃ and NiCrAlY/graphite coatings after 1000 h at 1100 °C do not show noticeable microstructural modification. The oxidation resistance of the new composite coatings satisfied original equipment manufacturer (OEM) specifications. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specifications for abradable coatings, the hardness evaluation suggests that these kinds of coatings must be used with abrasive-tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.     

Thermal Cycling Behavior of Quasi-Columnar YSZ Coatings Deposited by PS-PVD

Columnar-structured thermal barrier coatings, owing to their high strain tolerance, are expected for their potential possibilities to substantially extend turbine lives and improve engine efficiencies. In this paper, plasma spray-physical vapor deposition (PS-PVD) process was used to deposit yttria partially stabilized zirconia (YSZ) coatings with quasi-columnar structures. Thermal cyclic tests on burner rigs and thermal shock tests by heating and water-quenching method were involved to evaluate the thermal cycling and thermal shock behaviors of such kind of structured thermal barrier coatings (TBCs). Evolution of the microstructures, phase composition, residual stresses and failure behaviors of quasi-columnar YSZ coatings before and after the thermal tests was investigated. The quasi-columnar coating obtained had an average life of around 623 cycles when the spallation area reached about 10% of the total coating surface during burner rig tests with the coating surface temperature of ~1250 °C. Failure of the coating is mainly due to the break and pull-out of center columnar segments.     

Influence of the Spray Angle on the Properties of HVOF Sprayed WC–Co Coatings Using (−10 + 2 μm) Fine Powders

The application of fine powders in thermal spray technology represents an innovative approach to apply dense and smooth near-net shape coatings on tools with complex geometry. However, this aim can only be achieved as long as the influence of the handling parameters of the spray process, such as the spray angle, is sufficiently understood. In this study, the influence of the spray angle on the deposition rate as well as on the coating properties (microhardness, roughness, and porosity) of HVOF-sprayed, fine-structured coatings are investigated. A fine, agglomerated, and sintered WC-12Co powder (agglomerate size: 2-10 μm, WC-particle Fisher sub-sieve size = 400 nm) was used as feedstock material. It has been shown that HVOF spraying of fine powders is less susceptible to an alteration of the spray angle than most other thermal spray processes such as plasma- or arc-spraying. The reduction of the spray angle results in a decrease in the deposition rate, while no significant degradation of the coating properties is found up to 30°. However, at spray angles below 30° the coating strength is negatively affected by the formation of pores and cracks.     

Characterization of Yttria-Stabilized Zirconia Coatings Deposited by Low-Pressure Plasma Spraying

The research presented here aimed to apply plasma spraying at a low pressure of 100 Pa for fabricating the columnar structure or dense coatings. These coatings with different structures were elaborated from the vapor condensation and molten droplets, respectively, using the agglomerated YSZ powders and a relatively low power commercial F4-VB torch. It was shown that the crystallite size of coating deposited from the vapor condensation at a spraying distance of 200 mm was reduced to 17.1 nm from 43.7 nm of the feedstock. Observations indicated that a thin columnar structured coating was produced out of the line of sight of projection. In the line of sight of projection, the hybrid structured coating was obtained. The relatively dense coating was fabricated using a specifically designed extended nozzle. Investigations by means of optical emission spectroscopy were performed to analyze the nature of the plasma jet with YSZ powders. The Vickers microhardness was also conducted. It was found that the relatively dense coating showed a higher value in comparison to the hybrid structure coating, up to 1273 ± 56 Hv_100g.     

Influence of Pulse Electrodeposition and Heat Treatment on Microstructure,Tribological, and Corrosion Behavior of Nano-Grain Size Co-W Coatings

In the present study, Co-W nano-structured alloy coatings are produced on low-carbon steel substrate by means of pulse electrodeposition from a citrate-based bath under different average current densities and duty cycles. The results indicate that the coating deposited under 60% of duty cycle and 1 A/dm² of average current density exhibit optimum pulse plating conditions with 44.38 wt.% W, 37 nm grain size, and 758 HV microhardness. The effect of heat treatment temperature on microstructure, composition, corrosion behavior, and morphology of amorphous deposited Co-W alloy with 44 wt.% W was investigated. The microhardness of the coating increased to 1052 HV after heat treatment at 600 °C, which is due to the formation of Co₃W and CoWO₄ phases in the deposit. Furthermore, the coatings heat-treated at 600 °C had lower friction coefficients and better wear resistance under various loads than before heating.     

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.