NiCrAlY Coatings Research Articles

Thermophysical Properties of Cold- and Vacuum Plasma-Sprayed Cu-Cr-X Alloys, NiAl and NiCrAlY Coatings II: Specific Heat Capacity

Part I of the paper discussed the temperature dependencies of the electrical resistivities, thermal conductivities, thermal diffusivities and total hemispherical emissivities of several vacuum plasma-sprayed (VPS) and cold-sprayed (CS) copper alloy monolithic coatings, VPS NiAl, VPS NiCrAlY, extruded GRCop-84 and as-cast Cu-17(wt.%)Cr-5%Al. Part II discusses the temperature dependencies of the constant-pressure specific heat capacities, C P, of these coatings. The data were empirically regression-fitted with the equation: $${\varvec{C}}_{\mathbf{P}} = {\mathbf{ AT}}^{\mathbf 4} + {\mathbf{BT}}^{\mathbf 3} + {\mathbf{CT}}^{\mathbf 2} + {\mathbf{DT}} + \varvec{E}$$ where T is the absolute temperature and A, B, C, D and E are regression constants. The temperature dependencies of the molar enthalpy, molar entropy and Gibbs molar free energy determined from experimental values of molar specific heat capacity are reported. Calculated values of C P using the Neumann–Kopp (NK) rule were in poor agreement with experimental data. Instead, a modification of the NK rule was found to predict values closer to the experimental data with an absolute deviation less than 6.5%. The specific molar heat capacities for all the alloys did not agree with the Dulong–Petit law, and C P > 3R, where R is the universal gas constant, were measured for all the alloys except NiAl for which C P < 3R at all temperatures.

Properties of NiCrAlY coatings fabricated on superalloy GH4169 by electrospark deposition

NiCrAlY coatings were deposited on superalloy GH4169 by electrospark deposition technique. Influences of the depositing parameters (100 V/40 μF, 100 V/80 μF, 100 V/120 μF, 150 V/40 μF, 150 V/80 μF, 150 V/120 μF) on the microstructure and the properties of the coatings were experimentally investigated. Results show that the depositing parameters affect defect density and consequent properties of coatings. The deposition voltage has greater influences on the deposition velocity and on the properties of the coatings than the deposition capacity. The weight gain per area of the coatings deposited under 150 V increases faster than that under 100 V. The coating deposited under 100 V/40 μF has the minimum defect density and the best properties.

Alumina composite coatings with enhanced high-temperature electrical insulating properties on Ni-based superalloy substrates

Electrical insulation of nickel-based superalloy substrate, especially at high temperature range, is one of the major challenges for the reliability and stability of the integrated thin-film sensors. Here, we report a solution-processed approach to fabricating high-temperature, electrically insulating coatings on Ni-based superalloy substrates. NiCrAlY coatings were fabricated by DC magnetron sputtering and heat-treated, and then Al2O3 films were deposited by sol-gel method. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the composition, phase, microstructure and morphology of these composite coatings. The electrical resistance of the composite coating was measured as a function of temperature up to 800°C. Electrical resistance greater than 1MΩ were consistently achieved from 600°C to 800°C. Moreover, this insulating coating survived thermal shock and thermal fatigue tests without cracking or delaminating. A type S thin-film thermocouple was prepared on the composite coating to verify its high-temperature electrical insulation performance.

Heat protective properties of NiCrAlY/Al2O3 gradient ceramic coating fabricated by plasma spraying and slurry spraying

Al2O3 non-gradient ceramic coating (marked as 1# coating) and NiCrAlY/Al2O3 gradient ceramic coating (marked as 2# coating) were prepared with the plasma spraying and slurry spraying methods, respectively. The coating specimens were characterized by scanning electron microscopy and X-ray diffractometry. The high temperature oxidation kinetics curves of the coatings were examined; the bonding strength between the coatings and the metal substrates was studied, and analyses were conducted on the thermal shock resistance and the failure mechanism of the coatings. The results show that spraying of Al2O3 ceramic slurries can effectively fill the cracks generated during the process of plasma spraying of NiCrAlY coatings, and metallurgical bonds form between the coatings and the metal substrates. α-Al2O3 and rutile TiO2 are the main crystalline phase structures of the temperature resistant ceramic coatings. The oxidation rate constant of 2# coating was 1.323×10−4mg2cm−4h−1, which shows the best resistance to high temperature oxidation. The effect of high-temperature melting of the Al metal in the bonding layer was found to be beneficial to the bonding strength. Under the same temperature conditions, 2# coating had a higher bonding strength than 1# coating, and when the oxidation time was 96h, its bonding strength reached up to 26.9MPa. The thermal expansion coefficient of the NiCrAlY bonding coating fell between those of the metal substrate and ceramic top coating, thus reducing the thermal expansion coefficient difference between the two and decreasing the rate of propagation of the cracks to improve the thermal shock resistance of the coating. An integrated study indicated that 2# coating had a better heat protective performance for the metal substrates.

Oxidation behavior of NiCrAlY coatings prepared by arc ion plating using various substrate biases: Effects of chemical composition and thickness of the coatings

NiCrAlY coatings with various Cr and Al contents were prepared on Ni-based superalloy K438G from a single NiCrAlY target by adjusting the substrate bias during high vacuum arc ion plating. The oxidation behavior of the coatings follows the oxide map laws proposed by Wallwork and Hed rather than the guide line based on Cr/Al=4. As the coatings degrade due to Al depletion, formation of Cr2O3 scales beneath the external α-Al2O3 scales was observed. The mechanism for the oxidation of Ni-Cr-Al alloys is discussed.

Vibration Damping Properties of NiCrAlY Coating Deposited by Arc Ion Plating

Abstract NiCrAlY coating was deposited on 3Cr13 stainless steel substrates by an arc ion plating technique. This paper is to explore the feasibility of NiCrAlY coating for the damping application and to characterize the method that may be used in defining the vibration features of the coatings. The phases of the NiCrAlY coating were determined by X-ray diffraction (XRD) technique. The surface morphology and chemical composition of the NiCrAlY coating were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively. The damping properties were characterized by dynamic mechanical analyzer (DMA) and sine sweeps. Results show that the damping properties of the substrate are considerably improved by the NiCrAlY coating.

Isothermal oxidation of HVAF-sprayed Ni-based chromia, alumina and mixed-oxide scale forming coatings in ambient air

The power generation industry has been progressively shifting towards higher operating steam temperatures and pressures to increase efficiency and reduce CO2 emissions. However, higher operating temperatures lead to more aggressive oxidation of the boiler components. A promising route to improve the durability of degradation-prone components is through deployment of high-performance coatings. In the present work, four Ni-based coatings - Ni21Cr, Ni5Al, Ni21Cr9Mo, and Ni21Cr7Al1Y - thermally sprayed by the high-velocity air fuel (HVAF) technique on boiler steel (16Mo3) substrates were investigated. The isothermal oxidation behavior of the coatings was studied in ambient air environment at 600°C for different time intervals i.e. 1, 5, 10, 24, 48, 96, and 168h. The oxidation behavior of the as-sprayed and polished coatings was compared. The protective α-Al2O3 was not detected on the exposed alumina-forming NiAl coating. On the other hand, Cr2O3 along with a small amount of NiO were the main oxidation products on the surface of the NiCr and NiCrMo coatings, and were found to be relatively less protective. The mixed-oxide scale forming NiCrAlY coatings showed the best oxidation resistance due to the formation of a thin and slow-growing Al2O3 scale along with Ni(Al,Cr)2O4 and Cr2O3. The polished coatings were found to significantly reduce the oxidation rate in each case as the protective scale-forming elements were more uniformly supplied to the surface oxide scale by removing the surface asperities.

High-temperature oxidation behavior of laser-aided additively manufactured NiCrAlY coating

In this study, NiCrAlY coatings were additively manufactured on the surface of IN738 substrate through a laser-aided additive manufacturing approach. The high-temperature oxidation behavior of freestanding coatings at 1200°C was also investigated. The coatings were thick, dense, and well-bonded with dendritic γ/β microstructure. The results proved that the coatings were able to withstand the operating temperature of 1200°C for up to ten cycles. After ten cycles of oxidation, the oxide scales formed on the coatings were mainly composed of α-Al2O3 and Al5Y3O12 phases; however, NiO, Cr2O3 and Ni(Cr,Al)2O4 phases were not detected.

High Temperature Oxidation Behaviour of HVOF Thermally Sprayed NiCrAlY Coating on T-91 Boiler Tube Steel

In this work, NiCrAlY coating was deposited on T-91 boiler tube steel by high velocity oxy fuel (HVOF)thermal spraying system. The comparative cyclic high temperature oxidation behavior of NiCrAlY coating with uncoated substrate has been investigated experimentally. The cyclic oxidation studies were carried out at 900 °C for 50 cycles. Each cycle consists of 1 hour heating in silicon carbide tube furnace followed by 20 minutes cooling in air. The kinetics of oxidation of coated and uncoated boiler steel has been established with weight change measurements after each cycle. The corrosion products were characterized by X-ray diffractometry, Scanning Electron Microscopy (SEM) and Electron Dispersive Spectroscopy (EDS). The NiCrAlY coating showed better resistance to high temperature oxidation and corrosion due to the formation of Al2O3, Cr2O3, NiO and CrO protective oxides which lead to a reduced rate of corrosion. The uncoated steel suffered high rate of corrosion in form of spallation and peeling of unprotected Fe2O3 oxide scale.

Analysis of the Metallurgical Bonding between Inconel and NiCrAlY Coatings by HVOF and with CO&lt;sub&gt;2&lt;/sub&gt; Laser Beam

Coatings are applied on turbine blades’ surfaces to provide protection not only against high temperature but also against aggressive environment. Ceramic coatings are employed to avoid metallic substrate overheating, while at the same time increasing turbine work temperature and performance. A bond coat (BC) base of particulate material based on Ni-Al powders is necessary to assure oxidation protection, a good adhesion and gradual decrease in thermal expansion coefficient between the blades’ metallic substrate and the ceramic top coating. One of the most important parameters of such coatings is the adhesion strength. In this work, a NiCrAlY bond coat was deposited on Inconel 625 substrate employing High-Velocity Oxygen-Fuel (HVOF) thermal spraying technology and CO2 laser beam irradiation to enhance coating–substrate adherence and metallurgical bonding. Microstructural features were examined by optical and scanning electron microscopy (SEM), X-ray diffraction and microhardness analysis. The results indicate that the laser treatment provided an efficient metallurgical bond between the (BC) and Inconel 625 substrate.

Microstructure and Oxidation Resistance of Laser Remelted Plasma Sprayed Nicraly Coating

Abstract The article presents results of research relating to the impact of laser treatment done to the surface of plasma sprayed coatings NiCrAlY. Analysis consisted microstructure and oxidation resistance of coatings subjected to two different laser melting surfaces. The test were performed at a temperature 1000°C the samples were removed from the furnace after 25, 300, 500, 750 and 1000 hours. The investigations range included analysis of top surface of coatings by XRD characterization oxides formed types and microscopic investigations of coatings morphology

Interface bonding of NiCrAlY coating on laser modified H13 tool steel surface

Bonding strength of thermal spray coatings depends on the interfacial adhesion between bond coat and substrate material. In this paper, NiCrAlY (Ni-164/211 Ni22 %Cr10 %Al1.0 %Y) coatings were developed on laser modified H13 tool steel surface using atmospheric plasma spray (APS). Different laser peak power, Pp, and duty cycle, DC, were investigated in order to improve the mechanical properties of H13 tool steel surface. The APS spraying parameters setting for coatings were set constant. The coating microstructure near the interface was analyzed using IM7000 inverted optical microscope. Interface bonding of NiCrAlY was investigated by interfacial indentation test (IIT) method using MMT-X7 Matsuzawa Hardness Tester Machine with Vickers indenter. Diffusion of atoms along NiCrAlY coating, laser modified and substrate layers was investigated by energy-dispersive X-ray spectroscopy (EDXS) using Hitachi Tabletop Microscope TM3030 Plus. Based on IIT method results, average interfacial toughness, Kavg, for reference sample was 2.15 MPa m1/2 compared to sample L1 range of Kavg from 6.02 to 6.96 MPa m1/2 and sample L2 range of Kavg from 2.47 to 3.46 MPa m1/2. Hence, according to Kavg, sample L1 has the highest interface bonding and is being laser modified at lower laser peak power, Pp, and higher duty cycle, DC, prior to coating. The EDXS analysis indicated the presence of Fe in the NiCrAlY coating layer and increased Ni and Cr composition in the laser modified layer. Atomic diffusion occurred in both coating and laser modified layers involved in Fe, Ni and Cr elements. These findings introduce enhancement of coating system by substrate surface modification to allow atomic diffusion.

Characteristics and properties of cryomilling-induced columnar growth in NiCrAlY coatings on Ni-based superalloy by laser induction hybrid cladding

Abstract

Effects of Cr–Si Diffusion Barrier Layer on the Oxidation Resistance of NiCrAlY Coating System with Aluminized Top Layer

A multilayer coating, which consisted of a Cr–Si co-deposited layer as the diffusion barrier, a plasma sprayed NiCrAlY middle layer, and an aluminized top layer, was developed. During the aluminizing treatment, Cr in the NiCrAlY layer was released as the γ/γ′ structure of this layer transformed to the β phase. The released Cr was inhibited by the inner Cr–Si layer to diffuse into the substrate, and a Cr layer eventually formed over the Cr–Si layer. The Cr layer impeded the inward diffusion of Al due to the low solubility of Al in the Cr layer so that more Al atoms remain in the coating and contributed to the oxidation resistance of the coating. The multilayer coating exhibited better oxidation and spallation resistance than coatings without a Cr–Si layer, at least at 1050 °C for up to 1000 h.

Influence of milling time on the microstructure evolution and oxidation behavior of NiCrAlY coatings by laser induction hybrid cladding

The influence of milling time on the microstructure evolution and oxidation behavior of NiCrAlY coatings generated by laser induction hybrid cladding was investigated. Mechanical milling induced γ’-Ni3Al formation in the NiCrAlY powder. With longer milling times, the NiCrAlY coatings experienced an equiaxed-to-columnar transition, and their compositions became homogenized. Furthermore, the relative intensities of the diffraction peaks of α-Al2O3, Cr2O3 and NiCr2O4 decreased and the oxidation resistance of the NiCrAlY coatings improved. Therefore, milling time is another critical parameter for the control of the columnar growth in NiCrAlY coatings, whose oxidation kinetics depend on both the grain boundaries and lattice diffusion.

High vacuum arc ion plating NiCrAlY coatings: Bias effect and approach to preparation of functional gradient coatings

The substrate bias effects on high vacuum arc ion plating (HV-AIP) of NiCrAlY coatings on superalloy were investigated at the vacuum level of 10−3–10−4Pa, which are 2–3 orders in magnitude better than that of conventional low vacuum arc ion plating (LV-AIP). The negative bias applied to the substrate was in the range of −50V to −450V. Microstructures, deposition rates, and chemical compositions of the coatings are significantly influenced by the negative bias. Except for commonly observed bias induced densification in microstructure and decrease in deposition rate, strong preferential sputtering effect at high bias was observed. The sputtering yield for Cr was estimated to be ranged from ~0.1 to ~0.2 at bias from −50V to −450V, while those for Ni and Al were from ~0.1 to 1.5. As a result, the major phase constitution of the coatings changed from γ-Ni/γ′-Ni3Al phases at bias of −50V to α-Cr phase at bias of −450V. Such stronger preferential sputtering effect in HV-AIP fosters a strategy for preparing compositionally gradient NiCrAlY coatings just by control of the substrate bias without the need of further treatments or multi-targets. Based on this concept, preparation of several types of gradient NiCrAlY coatings with great flexibilities by one target and one process was demonstrated.

Role of Magnesia Stabilized Zirconia Dispersion on the Hot Corrosion Behaviour of NiCrAlY Plasma-Sprayed Coatings in Na2SO4 + 50 wt% V2O5 and Na2SO4 + 10 wt% NaCl Melts at 800 °C

In order to understand the possible role of magnesia stabilised zirconia (MSZ) on hot corrosion in graded thermal barrier coatings, plain and 25 and 50 wt% MSZ dispersed NiCrAlY coatings, developed using air plasma spray technique was examined by exposing them to Na2SO4 + 50 wt% V2O5 and Na2SO4 + 10 wt% NaCl environments at 800 °C. The study shows that MSZ dispersion significantly increases the hot corrosion tendency of NiCrAlY coatings in both the environments and that the MSZ phase suffers destabilization into monoclinic ZrO2 in both the environments suggesting that it cannot be used for producing graded bond coats. Mg ions leach out from MSZ phase forming Mg3V2O8 and MgCl2 on exposure to vanadates and chlorides respectively. The study further demonstrates that chloride is more detrimental to the coatings than vanadate.

Influences of MCrAlY Coatings and TBCs on Oxidation Behavior of a Ni-Based Single Crystal Superalloy

DD98M alloy is a second generation nickel-based single crystal superalloy without Re addition, which is developed by Institute of Metal Research, Chinese Academy of Sciences. It is a combination of attractively strong points including high strength and low cost. In this work, Ni-based single crystal superalloy DD98M was adopted as the substrate material. Two types MCrAlY (M denote metal) coatings were deposited on DD98M specimen by arc ion plating and YSZ topcoat (TC) were deposited on the substrate by electron beam physical vapor deposition (EB-PVD) on the NiCrAlY bond coat (BC). Experimental results showed that the application of the NiCrAlY and NiCoCrAlYHfSi coatings improved the oxidation resistance of DD98M obviously at 1000 °C. The adhesion of oxide scale of NiCoCrAlYHfSi coating was much better than that of NiCrAlY coating. TBCs application greatly enhanced the operating temperature and significantly improved the durability of the substrate. The thermal growth oxidation (TGO) between the bond coat and topcoat play an important role in adhesion of the whole coating system.

High-Temperature Oxidation and Hot Corrosion Studies on NiCrAlY Coatings Deposited by Flame-Spray Technique

The NiCrAlY coatings deposited by flame-spray technique on the superalloy substrate were oxidized in the presence of air and Na2SO4 + V2O5 salt at 900 °C for 100 cycles. The kinetics of oxidation showed that the coatings deposited by flame-spray technique possess better oxidation resistance compared with coatings deposited by high-velocity oxy fuel (HVOF)-sprayed technique. The oxidized coatings were further characterized by XRD, FESEM/EDS, and x-ray mapping techniques. The mechanisms of the oxidation and hot corrosion were substantiated by analyzing the results obtained from the various characterization techniques.

High vacuum arc ion plating NiCrAlY coatings: Microstructure and oxidation behavior

High vacuum (HV) arc ion plating (AIP) was developed for depositing NiCrAlY coatings at vacuum levels of 10−3–10−4Pa, which are 2–3 orders in magnitude better than that of conventional low vacuum (LV) AIP. Microstructure and oxidation behavior of the NiCrAlY coatings prepared by HV- and LV-AIP were comparatively investigated. As compared with the LV coatings, the HV coatings showed dramatically lower porosity and better oxidation resistance by forming purer alumina scales during oxidation. The significant improvements are considered to be attributed to that HV facilitated forming denser NiCrAlY coatings and eliminating gas inclusions in the coating.