NiCrAlY Coatings Research Articles

High temperature sulphidation of HVOF sprayed CeO2 modified NiCrAlY coatings in H2/H2S atmospheres

High temperature sulphidation is one of the most deleterious forms of surface degradation. In the present work, the high temperature sulphidation behaviours of high velocity oxyfuel sprayed NiCrAlY and NiCrAlYCe coatings on high pressure steels in H2–H2S mixtures were investigated. Results showed that the NiCrAlYCe coating provided much better sulphidation resistance than the NiCrAlY coating in the given environment. After adding minor CeO2, the outer layer of scales became compact and showed good adhesion, which decreased the sulphur vapour pressure inside the scale and promoted the formation of more protective inner layer of sulphospinels (Cr, Ni)Al2S2.

Influence of HVOF process parameters on microstructure and bond strength of NiCrAlY coatings

In the present study, NiCrAlY coatings were applied on steel substrate using high velocity oxygen fuel spraying process. The spray parameters were varied in order to study their influence on the coating properties. Powder and coating microstructures were investigated by a combination of X-ray diffraction, optical and scanning electron microscopy as well as energy dispersive spectroscopy analysis. Bond strength and microhardness tests were also performed on the coatings. It was found that the propane flowrate, the oxygen flowrate, the spray distance and powder feedrate have a significant effect on the coating characteristics. Finally, with the aim of minimising oxygen content and maximising bond strength, the optimum parameters were selected as the preferred spray parameters.

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.

激光熔覆复合涂层组织性能研究

激光熔覆复合涂层组织性能研究

激光熔覆复合涂层组织性能研究

激光熔覆复合涂层组织性能研究

Influence of Laser Power on the Microstructure and Anti-Corrosion Performance of LHPS NiCrAlY Coatings

The NiCrAlY coatings were deposited on 38CrMoAl substrate by a laser hybrid plasma spraying (LHPS) technology in this study. The coatings’ microstructures and anti-corrosion performances with different laser power were characterized using an optical microscope, a scanning electron microscope, and the Neutral Salt Spray(NSS) test. The test results indicate that the laser power has great effect on the microstructure and performances of the LHPS coatings. And when the laser power is appropriate, it can make the coating has more compact and homogeneous microstructure, exhibit less porosities or micro-cracks, higher bonding strength and more excellent anti-corrosion performance.

Optimization of Plasma Spray Process Using Statistical Methods

The microstructure features of coatings produced by a plasma spray process are affected significantly by the process parameters such as powder size, spray gun nozzle size, total plasma gas flow, ratio of H2 + N2 over total gas flow, and so on. This article presents a study of the effects of these parameters on the microstructure (porosity, formation of crack, unmelted particle and oxide phase) of NiCrAlY coatings deposited by the Mettech Axial III™ System. A Taguchi array is used to design the spraying process parameters. The results of the microstructure evaluation are used to generate regression equations for the prediction of coating microstructure based on process parameters. The results predicted from the regression equations are in good agreement with the experimental results according to a confidence level of 0.95. Among the parameters examined, the powder size and the ratio of H2 + N2 over total gas flow rate are the most significant parameters affecting the occurrence of crack, porosity, unmelted particle and oxide. Within the range of the designed process parameters, lower powder size and higher ratio of H2 + N2 over total gas flow rate lead to less cracks, pores, unmelted particles but more oxides. Nozzle size has marginal influence on oxides which increase with nozzle size. Gas flow rate has no direct influence on any coating feature evaluated with the range of variation.

Structure of NiCrAlY Coatings Deposited on Oriented Single Crystal Superalloy Substrates by Laser Cladding

In the present work single and multiple layer NiCrAlY coatings were produced by laser cladding on (100) single-crystalline substrates of SRR99 Ni-based superalloy. Detailed structural characterisation and texture analysis by optical microscopy, scanning electron microscopy, X-ray diffraction and Rutherford backscattering showed that the NiCrAlY coatings consisted essentially of gamma phase with yttrium oxide (Y2O3) and a small proportion of yttrium–aluminium garnet (Al5Y3O12) precipitated in the interdendritic regions. The coatings presented a columnar dendritic structure grown by epitaxial solidification on the substrate and inherited the single-crystalline nature and the orientation of the substrate. The coating material also showed a mosaicity and a defect density similar to those of the substrate. It can be expected that the protective effect of these coatings against oxidation is greatly enhanced compared with polycrystalline coatings because high diffusivity paths, such as grain boundaries, are eliminated in single-crystalline coatings, thus reducing mass transport through the coating.

Properties of Sputtered NiCrAlY Used for Protective Gun Tube Coatings

NiCrAlY coatings were deposited on CrNi3MoVA steel substrates by means of magnetron sputtering. The coatings were characterized in terms of their microstructure, hardness, friction coefficient, high-temperature oxidation resistance. Micro-indentation and tribometer testers were employed to measure the mechanical properties of NiCrAlY coatings and CrNi3MoVA steel. The results showed that the hardness of the coatings ranged from 5.7 to 5.9 GPa, with a higher value than that of CrNi3MoVA steel(4.1-4.3 GPa). The coefficient of steady-state friction of the coatings against 45-carbon-steel balls ranged from 0.35 to 0.40, with a lower value than that of CrNi3MoVA steel(0.63-0.68). The isothermal oxidation behavior at 850°C of the coatings were studied in comparison with CrNi3MoVA steel substrates. The results indicated that NiCrAlY coatings substantially increase the high-temperature oxidation resistance of CrNi3MoVA steel and the oxidation process was retarded mainly by the presence of outer complex oxide scales and a continuous Al2O3 inner layer on the coating.

Effects of surface modification on isothermal oxidation behavior of HVOF-sprayed NiCrAlY coatings

NiCrAlY coatings were deposited on Ni-based superalloy by high-velocity oxygen-fuel spraying (HVOF). Surface modification by means of grit-blasted, shot-peened and ground methods was used in order to study the effect of surface conditions on the isothermal oxidation behavior of HVOF-sprayed NiCrAlY coatings at 1 050 °C. The results showed that surface modification had an obvious effect on the isothermal oxidation behavior of the coatings. There was a large decrease in growth rate compared with the as-sprayed coating. The scale formed on the grit-blasted and shot-peened coatings was a mixture of Al2O3 and NiCr2O4, while the oxide formed on the ground coating was composed mainly of Al2O3. After surface modification, the content of NiCr2O4 spinels decreased compared with the as-sprayed coating.

OS1806 NiCrAlYコーティングの酸化層内残留応力に対する溶射方法の影響(OS18-2 薄膜・コーティング材の力学特性と微構造2,OS-18 薄膜・コーティング材の力学特性と微構造)

OS1806 NiCrAlYコーティングの酸化層内残留応力に対する溶射方法の影響(OS18-2 薄膜・コーティング材の力学特性と微構造2,OS-18 薄膜・コーティング材の力学特性と微構造)

Oxidation and interfacial fracture behaviour of NiCrAlY/Al 2O 3 coatings on an orthorhombic-Ti 2AlNb alloy

Al 2O 3 diffusion barriers of various thicknesses have been fabricated by filtered arc ion plating between the NiCrAlY coating and the O-Ti 2AlNb alloy. Isothermal oxidation tests and three-point bend tests have been conducted to investigate the influence of the Al 2O 3 diffusion barriers on the oxidation and interfacial fracture behaviour of the coatings. The results indicate that the Al 2O 3 diffusion barrier defers interdiffusion and gives oxidation resistance of the NiCrAlY coatings. The thickness of the Al 2O 3 interlayer not only influences the oxidation behaviour but also affects the interfacial fracture properties. Additionally, thermal exposure affects the critical load in three-point bend tests.

EFFECT OF POWDER STRUCTURE ON MICROSTRUCTURE OF THE OXIDE SCALES FORMED ON COLD-SPRAYED NiCrAlY COATINGS

Commercially available, gas-atomized NiCrAlY powder was milled in Ar atmosphere to prepare nanostructured powder. The NiCrAlY coatings in the conventional microstructure and the nanostructure were subsequently deposited by cold spraying using the as-atomized powder and milled powder. The influence of the powder structure on the microstructure of cold-sprayed coating was investigated. It was found that dense NiCrAlY coatings can be cold-sprayed using two powders. Isothermal oxidation was performed at 1000°C in air for up to 500 hours. The results showed that the growth rate of the oxide on the nanostructured NiCrAlY coating was almost comparable to that of the conventional NiCrAlY coating. A uniform Al 2 O 3 layer was formed on the surface of the nanostructured coating surface. Moreover, a dual-subscale oxide was formed on conventional coating composing of an inner Al 2 O 3 oxide and an external mixed oxide.

A study on the oxidation behavior of HVOF sprayed NiCrAlY–0.4 wt.% CeO 2 coatings on superalloys at elevated temperature

The high temperature oxidation behavior of HVOF sprayed ceria added NiCrAlY coatings on superalloys has been studied. Oxidation kinetics of the bare and coated superalloys in air at 900 °C under cyclic conditions was investigated. X-ray diffraction, field emission scanning electron microscope with energy dispersive spectroscopy and X-ray mapping were used to analyse the scales formed on the surface of the oxidised samples. The NiCrAlY–0.4 wt.% CeO 2 coated specimen showed negligible microspalling of the scales. The incorporation of ceria in NiCrAlY powder has contributed to the development of adherent oxide scale, in the coating, at elevated temperature and provided the better oxidation resistance.

Structure of NiCrAlY coatings deposited on single-crystal alloy turbine blade material by laser cladding

In the present work single and multiple layer NiCrAlY coatings were produced by laser cladding on (100) single-crystalline substrates of SRR99 Ni-based superalloy. Detailed structural characterisation and texture analysis by optical microscopy, scanning electron microscopy, X-ray diffraction and Rutherford backscattering showed that the NiCrAlY coatings consisted essentially of γ phase with yttrium oxide (Y 2O 3) and a small proportion of yttrium–aluminum garnet (Al 5Y 3O 12) precipitated in the interdendritic regions. The coatings presented a columnar dendritic structure grown by epitaxial solidification on the substrate and inherited the single-crystalline nature and the orientation of the substrate. The coating material also showed a mosaicity and a defect density similar to those of the substrate. It can be expected that the protective effect of these coatings against oxidation is greatly enhanced compared with polycrystalline coatings because high diffusivity paths, such as grain boundaries, are eliminated in single-crystalline coatings, thus reducing mass transport through the coating.

Hot corrosion behaviour of plasma sprayed YSZ/Al 2O 3 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 Na 2SO 4 and V 2O 5 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 YVO 4 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.% Al 2O 3 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.

Stainless Steel Bipolar Plates.-Commercial and Technical Feasibility Study for Mass Production in Mexico

Stainless steel Type 444 is an excellent material for use in the manufacture of bipolar plates. It is comparable to austenitic stainless steel Type 316 in its corrosion resistance but has the advantage of being half less expensive. The coatings that had the best performance were chromium carbide and gold. Corrosion resistance of ferritic stainless steels 439 and 441, with a Ni-Cr and NiCrAlY coatings, were evaluated under simulated cathodic PEMFC environment. The corrosion current density of the coatings is higher than the US Department of Energy target value and they are not a feasible option to be used in bipolar plates for fuel cell applications. A possible explanation of the inadequate corrosion resistance is the segregation of the elements. The XRD and SEM measurements indicate that the corrosion mechanisms are different for NiCr and NiCrAlY. We have concluded that it is technically possible to produce bipolar plates in Mexico.

Young's moduli of cold and vacuum plasma sprayed metallic coatings

Monolithic metallic copper alloy and NiCrAlY coatings were fabricated by either the cold spray (CS) or the vacuum plasma spray (VPS) deposition processes. Dynamic elastic modulus property measurements were conducted on these monolithic coating specimens between 300 and 1273 K using the impulse excitation technique. The Young's moduli decreased almost linearly with increasing temperature at all temperatures except in the case of the CS Cu–23%Cr–5%Al and VPS NiCrAlY, where deviations from linearity were observed above a critical temperature. It was observed that the Young's moduli for VPS Cu–8%Cr were larger than the literature data compiled for Cu. The addition of 1%Al to Cu–8%Cr significantly increased its Young's modulus by 12–17% presumably due to a solid solution effect. Comparisons of the Young's moduli data between two different measurements on the same CS Cu–23%Cr–5%Al specimen revealed that the values measured in the first run were about 10% higher than those in the second run. It is suggested that this observation is due to annealing of the initial cold work microstructure resulting form the cold spray deposition process.

Evaluation of Coated Stainless Steels for Bipolar Plates in Fuel Cells Type PEM

According to the electrochemical study of Ni20Cr, NiCrAlY and Cr3C2(NiCr) alloys in coating form made with the Tafel extrapolation and Resistance techniques to polarization, the icorr values obtained for Ni20Cr and NiCrAlY coatings, the material dissolution, and the little resistance to corrosion discard the possibility to consider them as candidates to be used as bipolar plates for fuel cells type PEM; however with the Cr3C2(NiCr) coating, icorr values below 1.6X10-5A/cm2 were obtained. Materials must fulfill this criterion to be considered as candidates in applications such as a bipolar plate.

Characterizations of AMT-200 HVOF NiCrAlY coatings

The development of high velocity oxygen-fuel (HVOF) process shows a clear trend toward the design of a new gun in which high pressure and large power are expected in order to obtain a stable flame and a high powder feed rate for industrial applications. In this work, a new HVOF gun, AMT-200, was used to prepare NiCrAlY coatings. The influences of thermal treatment and spraying parameters, i.e. oxygen/fuel gas stoichiometric ratio and spraying distance on coating structures and properties were studied. Artificial neural networks (ANN) was implemented in this study to predict the coating structural attribute (porosity, hardness). The results show that the spraying parameters play significant roles on the structures and properties of NiCrAlY coatings. Employing optimized parameters, a dense NiCrAlY coating, with a high coating/substrate bonding strength was obtained by this system. Moreover, thermal treatment reduced significantly the coating porosity and increased greatly coating/substrate bonding strength.