NiCrAlY Coatings Research Articles

Improvement of oxidation-resistance of NiCrAlY coatings by application of CrN or CrON interlayer

NiCrAlY coatings with and without CrN or CrON interlayer as diffusion barrier were deposited on superalloy DSM11 by arc ion plating (AIP). The oxidation performance of the coating systems was evaluated by isothermal oxidation tests at 1100 °C for 100 h. The element interdiffusion and oxidation behavior of the coating systems were described. It was found that the NiCrAlY coatings provided protective effect for the DSM11 substrate. However, serious interdiffusion between the coatings and substrate resulted in rapid degradation of the coatings. The addition of CrN or CrON interlayer between the coatings and substrate markedly decreased the interdiffusion. CrON interlayer performed better than CrN interlayer, which was attributed to the excellent diffusion barrier ability of Al2O3 layer formed in the interlayer at high temperature. Also, the NiCrAlY/CrON coating system exhibited more effective protection for DSM11 than the NiCrAlY/CrN coating system.

High-temperature Oxidation Behavior and Surface Roughness Evolution of VPS NiCrAlY Coating

In this work, as a background to the development of high temperature environmental protection coating technology, the structural transformation of vacuum-plasma sprayed NiCrAlY coating has been studied, including the oxidation behavior, elastic modulus, microhardness, and surface roughness. Coating was oxidized isothermally at a 1,273 K for different time periods up to 1,000 h in order to form the oxide scale layer. The results showed that the oxide scale and weight gain increased proportional to the exposure time. The surface roughness and hardness decreased after isothermal exposure, but they are not proportional to the exposure time. The oxide scale growth was concentrated in the valleys of the surface at the early 1 h exposure (decreasing surface roughness), whereas the oxide scales over a longer perspective (time period), grew on the convex surface (increasing surface roughness).

High temperature behaviour of NiCrAlY coatings made by laser cladding

The oxidation behaviour of NiCrAlY coatings made by laser cladding on Hastelloy X is presented in this study. Laser cladding is an alternative method to thermal spraying for the production of bond coats. Comparable dense layers with approximately zero porosity should improve the oxidation behaviour. The oxidation behaviour of the coated specimens was assessed by air furnace oxidation tests at 1100 °C for up to 450 h. The coatings were analysed by means of light and electron microscopy techniques, microprobe analysis and X-ray diffraction analysis. The analysis was performed before and after the oxidation tests. The as-clad coating had a columnar dendritic structure and it did not show the presence of the relevant defects. After the oxidation tests an oxide scale was present which consisted of two distinct layers. The layers consisted of an outer layer of mixed spinel-type oxides and an inner continuous layer, in which alumina was present. The obtained results suggested that up to 450 h the system was able to form a continuous alumina layer that could protect the substrate from oxygen diffusion.

Characterization of NiCrAlY coatings for a high strength, high conductivity GRCop-84 copper alloy

NiCrAlY top coats are currently being considered as environmental protective coatings for copper alloy liners in rocket engine combustion chambers of reusable launch vehicles. The microstructure and mechanical properties of this top coat sprayed on GRCop-84 have been characterized as a function of thermal cycling between 298 and 873 K and no obvious degradation was observed. Interfacial microsample tensile tests developed to measure coating adhesion to the substrate revealed that the interfaces were stronger than the substrate in both the as-received and thermal cycled conditions. Finite element modeling was used to analyze the stresses in the microsamples and verify the strength of the interfaces. The formation of depleted zones, devoid of Cr 2Nb particles, was associated with plasma arc cleaning in a minority of the samples prepared for this study, and the presence of these depleted zones has been found to significantly decrease the adhesion of the NiCrAlY coating and to change the failure mode in thermal cycled specimens.

Oxidation Behavior and Residual Stress in Thermally Grown Oxides of Detonation and High Velocity Oxygen Fuel Sprayed NiCrAlY Coatings

NiCrAlY coatings were prepared by the detonation spraying (DS) and high velocity oxygen fuel (HVOF) spraying processes, respectively. The DS NiCrAlY coating has higher oxidation rate than the HVOF NiCrAlY coating at 1100 oC due to its rougher surface and more mixed oxides. The residual compressive stress in thermally grown oxides (TGO) on the DS NiCrAlY coating is higher than that on the HVOF coating because the θ-Al2O3 to α-Al2O3 transformation causes the tensile stress in the TGO on the HVOF coating.

High heat flux exposures of coated GRCop-84 substrates

The performance of several overlay coatings on Cu–8 at.%Cr 4%Nb alloy (GRCop-84) thermally cycled in a high heat flux hydrogen–oxygen combustion flame are reported. Finite element analyses (FEA) were conducted to evaluate the expected performances of coatings with different thermal conductivities as well as to determine a suitable thickness for coating deposition. Top coats consisting of Cu–26%Cr, NiAl and NiCrAlY along with a Cu–8%Cr bond coat were vacuum plasma sprayed on GRCop-84 thin sheet specimens. It was observed that the Cu–26%Cr coating formed blisters whereas the NiAl and NiCrAlY coatings showed no significant differences between the exposed and unexposed specimens. Water-cooled rectangular specimens specially designed to simulate a rocket engine liner were coated with a NiCrAlY-top coat using either a Cu–8%Cr or a Cu–8%Cr–1%Al bond coat. Preliminary tests conducted on these specimens showed no visible evidence of coating spallation after 40 cycles. The measured temperatures and heat fluxes were used in FEA models to estimate the stresses, strains and temperatures across the coating interfaces in these specimens.

The Oxide Scale Formation and Evolution on Detonation Gun Sprayed NiCrAlY Coatings during Isothermal Oxidation

NiCrAlY coatings were prepared by the newly-developed detonation gun spray process. The oxide scale formation and evolution on these coatings during isothermal oxidation in air at 1100 degrees C were investigated. It was found that semi-molten particles, particle debris and pores, are present in the surface layer of the as-sprayed coating. During 100 h oxidation, the particle debris and some semi-molten particles gradually change into oxide mixture consisting of spinel, chromia and nickel oxides. However, after removal of the surface layer of the coating by a grinding treatment, a dense and single-layer alpha-Al2O3 scale forms on the surface of the coating during the oxidation. The mechanisms governing the oxide scale formation and evolution are discussed in terms of atomic diffusion and thermodynamic stability. In addition, thermogravimetric analysis showed that the oxidation rate of the ground NiCrAlY coating at 1100 degrees C is much lower than that of the as-sprayed one. The residual stress in thermally grown oxide scales was investigated using photo-stimulated luminescence spectroscopy.

Improvement of the oxidation resistance of NiCrAlY coatings by the addition of rhenium

NiCrAlY coatings are commonly used to improve the oxidation resistance of gas turbine blade. In this research, the cyclic oxidation behavior at 1000 °C of NiCrAlY coatings with and without Re addition was investigated. XRD was used to identify the oxide phases formed on the coatings. The microstructural changes in the coatings during oxidation were examined by SEM, EDX and EPMA. The results indicate that the addition of Re decreases the oxidation rate of NiCrAlY coatings. After long-term oxidation, some less protective oxides (such as NiCr 2O 4 and NiAl 2O 4) were observed on the surface of the NiCrAlY coatings, while the scale formed on the NiCrAlYRe coatings was still composed of protective α-Al 2O 3. Therefore, it is concluded that the addition of Re clearly improves the oxidation resistance of NiCrAlY coatings in cyclic oxidation conditions.

Characterization and Erosion Behavior of Plasma Sprayed NiCrAlY and Ni-20Cr Coatings on an Fe-based Superalloy

Degradation of materials due to solid particle erosion is encountered in a variety of engineering industries, either at room temperature or elevated temperatures. Nickel-based coatings are commonly used in applications where wear resistance, combined with oxidation or hot corrosion resistance, is required. In the present work, NiCrAlY and Ni-20Cr metallic coatings were deposited on an iron-based superalloy by a shrouded plasma spray process. The coatings were characterized by scanning electron microscopy, optical microscopy, microhardness testing, and x-ray diffractometry. Erosion studies were conducted using an air-jet erosion test rig at a velocity of 40ms−1 and impingement angles of 30 and 90deg. Scanning electron microscopy was used to analyze the eroded surfaces. 3D surface roughness profiles of the eroded samples were taken using a Veeco Optical Profilometer. NiCrAlY coatings had slightly lower average porosity and lower microhardness as compared to Ni-20Cr coatings. The observed erosion rate of the NiCrAlY coatings, however was lower than that of the Ni-20Cr coatings at both 30 and 90deg impingement angles. Ni-20Cr coating had shown higher erosion rate at 90deg impingement angle than that at 30deg, whereas the effect of impingement angle on the erosion rate is negligible for plasma sprayed NiCrAlY coating. The higher bond strength of NiCrAlY coating might be one of the major contributing factors for lower erosion rate of NiCrAlY coating as compared to Ni-20Cr coating under the tested conditions. Erosion mechanisms of plasma sprayed coatings are discussed.

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.

Corrosion behavior of plasma-sprayed coatings on a Ni-base superalloy in Na2SO4-60 Pct V2O5 environment at 900 °C

The shrouded plasma spray process was used to deposit NiCrAlY, Ni-20Cr, Ni3Al, and Stellite-6 metallic coatings on a Ni-based superalloy (62Ni-23Cr-1.48Al-0.80Mn-0.37Si-0.10Cu-0.025C-bal Fe). NiCrAlY was used as a bond coat in all cases. Hot corrosion studies were conducted on uncoated as well as plasma-spray-coated superalloy specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) and electron-probe microanalysis techniques were used to analyze the corrosion products. The uncoated superalloy suffered accelerated corrosion in the form of intense spalling of the scale. The NiCrAlY coated specimen showed a minimum weight gain, whereas the Stellite-6 indicated a maximum weight gain among the coatings studied. All the coatings were found to be successful in developing resistance against hot corrosion, which may be attributed to the formation of oxides, and spinels of nickel, aluminum, chromium, or cobalt.

The Influence of NiCrAlY Coatings Prepared by LPPS and EB-PVD on the Damping Properties of 1Cr18Ni9Ti Alloy

The Influence of NiCrAlY Coatings Prepared by LPPS and EB-PVD on the Damping Properties of 1Cr18Ni9Ti Alloy

Influence of Water Vapor on the Cyclic-Oxidation Behavior of a Low-Pressure Plasma-Sprayed NiCrAlY Coating

The oxidation of a low-pressure plasma-sprayed (LPPS) NiCrAlY coating on a nickel-base superalloy was studied at 1050 °C in flows of O2, and mixture of O2 and 5% H2O under atmospheric pressure. Water vapor has an obvious effect on the cyclic oxidation of the NiCrAlY coating. There is more decrease in weight gain when exposure to O2 is replaced by exposure to O2 + 5% H2O. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in pure oxygen is composed mainly of Cr2O3, and a thin Al2O3-rich layer is formed at the interface between the Cr2O3-rich layer and the coating. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in a mixture of O2 + H2O is composed of NiCr2O4, NiO and Cr2O3. The effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to an increase in Ni and Cr cation transport, stress-corrosion cracking of Al2O3 and moisture-enhanced volatility of the Cr2O3 scale.

Dilatometer studies of NiCrAlY coatings

The purpose of this work was to determine if lower thermal expansion bondcoat MCrAlY alloys could be developed for thermal barrier coating (TBC) systems. NiCrAlY coatings applied by plasma spray deposition were studied in the dilatometer for first-cycle sintering and in the thermally stabilized state for thermal expansion. Several different compositions were created and tested. A new composition was developed that has both low thermal expansion and avoids the expansion upsweep at 950 °C, common in current NiCrAlY coatings. Phase analysis of selected coatings was done to explain the control of thermal expansion. Comparisons were made to previous thermal expansion values of other MCrAlY coatings.

Evaluation of Thickness for NiCrAlY Coatings Sprayed on the Superalloy by Using Ultrasonic Spectral Analysis

Evaluation of Thickness for NiCrAlY Coatings Sprayed on the Superalloy by Using Ultrasonic Spectral Analysis

Morphology of the superficial oxides in conventional and nanocrystalline NiCrAlY coatings exposed to elevated temperature

HVOF thermal spray process was used to produce conventional and nanostructured NiCrAlY coatings. The oxidation behaviour at 1000°C of both coatings has been studied. The morphology of the oxides suggests that the nanostructured coating exhibited improved oxidation behaviour compared to conventional counterparts.

Al–O–N based duplex coating system for improved oxidation resistance of superalloys and NiCrAlY coatings

Performance of an Al–O–N based duplex coating system (DCS) was examined in order to improve the high-temperature oxidation resistance of superalloys and the low-pressure plasma sprayed NiCrAlY coatings. The DCS (∼5 μm thick) was deposited using pulsed DC reactive magnetron sputtering as an external coating on Rene'N5 superalloy with and without NiCrAlY coating. DCS coatings were characterized using photo-stimulated luminescence spectroscopy, optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy before and after high-temperature oxidation. As-deposited DCS contained varying amounts of α-, γ-Al 2O 3 and amorphous phases depending on the chemistry and surface characteristics of the substrate. Heat treatment at 1121 °C (2050 °F) for 2 h in air facilitated complete phase transformation to equilibrium α-Al 2O 3 in the DCS. Improvement in oxidation resistance was observed for DCS specimens during 250-h oxidation at 1121 °C (2050 °F). In particular, DCS was found to be very effective as an external coating on the rough (as-sprayed) NiCrAlY surface. The superior oxidation resistance of the specimens with DCS may be attributed to the formation of slow-growing and adherent alumina scale and the prevention of Ni/Cr-rich oxide formation.

Al–O–N based duplex coating system for improved oxidation resistance of superalloys and NiCrAlY coatings

Performance of an Al–O–N based duplex coating system (DCS) was examined in order to improve the high-temperature oxidation resistance of superalloys and the low-pressure plasma sprayed NiCrAlY coatings. The DCS (∼5 μm thick) was deposited using pulsed DC reactive magnetron sputtering as an external coating on Rene'N5 superalloy with and without NiCrAlY coating. DCS coatings were characterized using photo-stimulated luminescence spectroscopy, optical microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy before and after high-temperature oxidation. As-deposited DCS contained varying amounts of α-, γ-Al2O3 and amorphous phases depending on the chemistry and surface characteristics of the substrate. Heat treatment at 1121 °C (2050 °F) for 2 h in air facilitated complete phase transformation to equilibrium α-Al2O3 in the DCS. Improvement in oxidation resistance was observed for DCS specimens during 250-h oxidation at 1121 °C (2050 °F). In particular, DCS was found to be very effective as an external coating on the rough (as-sprayed) NiCrAlY surface. The superior oxidation resistance of the specimens with DCS may be attributed to the formation of slow-growing and adherent alumina scale and the prevention of Ni/Cr-rich oxide formation.

A comparative study of DS NiCrAlY coating and LPPS NiCrAlY coating

Two thermal spraying techniques, namely, detonation gun spraying (DS) and low pressure plasma spraying (LPPS), were used to prepare different NiCrAlY coatings on a single crystal (SC) Ni-based superalloy. Microstructure, surface hardness, adhesion strength and high-temperature oxidation behavior of DS NiCrAlY coating and LPPS NiCrAlY coating were compared. The results showed that the two coatings in as-sprayed state had the same splat layer structure. Somewhat differently, oxidation reaction took place in DS process and Al2O3 sandwiched between the splats in DS NiCrAlY coating. It is found that the surface hardness of DS coatings are higher than those of LPPS coating. But the oxidation resistance of DS coating is relatively lower.

Improvement of Oxidation-Resistance of TiAl Alloy by Electric Discharge Nicraly Coating

The effect of NiCrAlY and NiCrAiY-50%NiAl coating by electric discharge technique on oxidation of TiAl alloy at 800°C in air was studied. The mass gain of the non-coated specimen increased linearly with exposure time, while that of the coated specimen increased at first then saturated to a lower value. The coating was further carried out on notch root surface of notched compact tension specimen, which was low cycle-fatigued at 800°C in air. The coating had no deteriorative effect on low cycle fatigue crack initiation.