NiAl Coatings Research Articles

Microstructure stability of γ′ + β Ni–Al coated single-crystal superalloy N5 annealed at 1100 °C

γ′ + β Binary-phase Ni–Al coatings were prepared on second-generation single-crystal superalloy Rene N5 (N5) substrates by electron beam physical vapor deposition. Inter-diffusion behavior between coatings and substrates at 1100 °C and its effects on microstructure stability of the substrates were investigated. A 3-μm-thick substrate diffusion zone (SDZ) layer forms beneath the coating/substrate interface after 5-h heat treatment. The SDZ layer is composed of γ′-Ni3Al phases and needle-like precipitates growing along (100) $$\left\langle {110} \right\rangle$$ or (100) $$\left\langle { 1 {\bar{\text{1}}\text{0}}} \right\rangle$$ direction. After 100-h annealing, the thickness of the SDZ layer increases to ~17 μm, much lower than that of the single β-NiAl-coated substrate, while keeping single-crystal microstructure.

PERFORMANCE OF NI3AL AND TIO2 COATINGS ON T22 BOILER TUBE STEEL IN SIMULATED BOILER ENVIRONMENT IN LABORATORY

The corrosion of boiler steel tube in coal fired boiler is main matter during design and operation of thermal power plant and it leads to both direct and indirect cost and energy lost, including losses due to plant shut down time and inefficiency of operation. One of the feasible solution to solve the problem is providing surface coating on boiler steel tube, which on one hand protect the tube from highly corrosive flue gaseous and on other hand increase their operating life. Ni3Al and TiO2 and coatings were sprayed on ASTM SA213-T22 steel by using the HVOF process. Hot corrosion studies were conducted on the uncoated as well as coated sprayed specimens in molten salt environment of Na2SO4-60%V2O5 at 900ºC for 50 cycles. In the experiment each cycle consist of 1 hr of heating in tube furnace followed by 20 minute of cooling in air. The thermo-gravimetric technique was used in order to establish the kinetics of corrosion. XRD, SEM/EDS testing were used to examine the corrosion specimens. Theresult shows that both the coated samples are beneficial to corrosion resistance over uncoated samples. TiO2 coating was found to be most protective followed by the Ni3Al coating.

The oxidation performance for Zr-doped nickel aluminide coating by composite electrodepositing and pack cementation

Ni-Zr composite coatings were prepared by composite electrodepositing with the Zr particle loadings from 0.1g/L Zr to 0.7g/L Zr. Then the Ni-Zr coatings were aluminized by the pack cementation and heat-treatment to obtain Zr-doped β-NiAl coatings. The effects of Zr on the coating microstructure and oxidation performance were investigated. The spallation resistance for Zr-doped NiAl coatings was better than the coating without Zr. The coating of 0.5g/L Zr loading exhibited the best spallation resistance duo to a lower growth rate of the oxide scale and the formation of a mechanically interlocking oxide pegs structure.

A robust method to characterize rumpling in high-strength bond coats

A new method has been developed to nondestructively characterize bond coats that rumple during thermal cycling. This method employs Fourier transforms of 2D optical profilometer data and is applied to coatings that have differing tendencies to rumple such as (Pt,Ni)Al coatings and high-strength γ′ or γ+γ′ bond coats. The method isolates the 2D periodic undulations characteristic of rumpling from other inhomogeneous time-dependent phenomena that occur at the surface, including HfO2 and NiAl2O4 spinel oxide growth, bond coat swelling, and noisy data. The method is benchmarked against the traditional rumpling indicators seen in the literature, surface roughness and tortuosity, using synthetic datasets to explore the strengths and limitations of the algorithm. The versatility of this method to quantify rumpling is demonstrated on experimental datasets from a β-phase bond coat with high levels of noisy or missing data. Application of the method provides a quantification of the rumpling amplitude and wavelength that are confirmed with metallographic cross sections of the samples and predicted by the Balint and Hutchinson rumpling model. The algorithm correctly measures (in contrast to the RMS surface roughness) an increasing surface undulation amplitude during initial furnace cycling, followed by a decrease in undulation amplitude near the end of the coating life as TGO spallation becomes prevalent.

Cladding of Cr- Modified NiAl Coating on 310 Stainless Steel Weld Cladding and Evaluation of its Wear Behavior.

Cladding of Cr- Modified NiAl Coating on 310 Stainless Steel Weld Cladding and Evaluation of its Wear Behavior.

Empirical alloys-by-design theory calculations to the microstructure evolution mechanical properties of Mo-doped laser cladding NiAl composite coatings on medium carbon steel substrates

An experimental investigation is performed into the microstructure and mechanical properties of Mo-doped (0, 3, 6, 9,12 and 15 wt% Mo) NiAl coatings deposited on medium carbon steel substrates with a laser cladding process. It is shown that as the Mo content increases, the density of the Mo-rich phase network structure increases and the grain size of the laser-clad coating reduces. Moreover, the hardness and wear test results indicate that a higher Mo addition not only increases the strength of the NiAl coating, but also improves its toughness and wear resistance. With a combination of alloy design and the role of laser cladding process, this study provides a potential guideline for practical application of intermetallic compound in the future. In general, with a combination of alloy design and the role of laser cladding process in this study provide a useful source of reference for extending the applications of NiAl composite coatings in the aerospace field and other advanced industries.

Microstructure Effect of Intermediate Coat Layer on Corrosion Behavior of HVAF-Sprayed Bi-Layer Coatings

The inherent pores and carbides of Cr3C2-NiCr coatings significantly reduce the corrosion resistance, the former by providing preferential paths for ion diffusion and the latter by forming cathodic sites in galvanic couples (between NiCr and Cr3C2). Adding a dense intermediate layer (intermediate coat layer) between the Cr3C2-NiCr coating (top coat) and substrate increases the corrosion protection of the coating if the layer acts as cathode in connection to the top coat. In the present work, NiCr, NiAl, and NiCoCrAlY layers were deposited by high-velocity air-fuel process as intermediate coat layers for the Cr3C2-NiCr top coat. Effects of coating microstructure on corrosion behavior of single- and bi-layer coatings were studied by open-circuit potential and polarization tests in 3.5 wt.% NaCl at room temperature. A zero resistance ammeter technique was used to study the galvanic corrosion of the coupled top and intermediate coat layers. Methods such as SEM and XRD were employed to characterize the as-sprayed and corroded coatings and to investigate the corrosion mechanisms. The results showed that the NiCoCrAlY coating not only presented a more positive corrosion potential (Ecorr) than the Cr3C2-NiCr coating, but also provided a better passive layer than the single-phase NiCr and NiAl coatings.

A Comparative Study on Ni-Based Coatings Prepared by HVAF, HVOF, and APS Methods for Corrosion Protection Applications

Selection of the thermal spray process is the most important step toward a proper coating solution for a given application as important coating characteristics such as adhesion and microstructure are highly dependent on it. In the present work, a process-microstructure-properties-performance correlation study was performed in order to figure out the main characteristics and corrosion performance of the coatings produced by different thermal spray techniques such as high-velocity air fuel (HVAF), high-velocity oxy fuel (HVOF), and atmospheric plasma spraying (APS). Previously optimized HVOF and APS process parameters were used to deposit Ni, NiCr, and NiAl coatings and compare with HVAF-sprayed coatings with randomly selected process parameters. As the HVAF process presented the best coating characteristics and corrosion behavior, few process parameters such as feed rate and standoff distance (SoD) were investigated to systematically optimize the HVAF coatings in terms of low porosity and high corrosion resistance. The Ni and NiAl coatings with lower porosity and better corrosion behavior were obtained at an average SoD of 300 mm and feed rate of 150 g/min. The NiCr coating sprayed at a SoD of 250 mm and feed rate of 75 g/min showed the highest corrosion resistance among all investigated samples.

Effect of plasma spraying regimes on structure and properties of Ni3Al coatings

The structure and tribological properties of coatings made of PN85YU15 powder were studied. The coatings were deposited on the mild steel blanks by the technology of air-plasma spraying using a unit of annular input and gas-dynamic powder focusing. Efficiency of heating and acceleration of powder particles was studied preliminarily. Measurement results on temperature and velocity distributions of particles at a certain spraying distance by the method of spectral pyrometry and time-of-flight method are presented. The effect of plasmatorch arc current and amount of propane-butane in the plasma flow on the structure and properties of coatings is analyzed in this paper. It is determined that the phase composition of coatings and initial powder is the same: the main phase is Ni3Al compound; moreover, the structure contains Ni5Al3 phase. It is shown that an increase in the amount of propane-butane increases coatings porosity. The densest coatings (5.77%) were obtained at the plasmatorch arc current of 200 A with the reduced amount of propane-butane. The coatings obtained at the minimal arc current of 100 A with an increased amount of propane-butane are characterized by maximal porosity (20.38%). The results of tribological testing of the coatings under the conditions of sliding friction with a lubricant by the disc-plane scheme are presented. From the standpoint of obtaining the densest coatings with high performance, the optimal regimes of plasma spraying of PN85YU15 powder are the current from 140 A to 200 and using the air and propane-butane mixture only as the shielding gas (anode curtain).

Microstructural study of as sprayed and heat treated Ni3Al coatings deposited by air plasma spraying technique

Air plasma spraying system was utilized to deposit Ni3Al coatings on AISI 321 steel samples. After plasma spraying the coatings were heat treated at different temperatures i.e. 500 °C to 800 °C for 10 to 100 hours. The characterization tools such as, X-Ray diffraction analysis, optical and scanning electron microscopy were used. By comparing the XRD scan data of as sprayed and heat treated coating, it was observed that the formation of NiO increases drastically with time and temperature. Due to the formation of NiO, hardness was also enhanced. The oxidation behavior was observed by using optical microscope and when it was studied that the oxidation was increasing with time and temperature. Further, the SEM tool was utilized to study the detail microstructural behavior such as shrinkage cavity and oxide particles. The other phases like alumina and spinel phases were determined by using Energy dispersive spectrometer method.

Synthesis of advanced aluminide intermetallic coatings by low-energy Al-ion radiation.

Metals that work at high temperatures (for instance, superalloys in gas-turbines) depend on thermally grown oxide (TGO, commonly alumina) to withstand corrosion attack. Nickel Aluminide (NiAl) as one superior alumina TGO former plays an important role in protective coatings for turbine blades in gas-turbine engines used for aircraft propulsion and power generation. Lowering TGO growth rate is essentially favored for offering sustainable protection, especially in thermal barrier coatings (TBC). However, it can only be achieved currently by a strategy of adding the third element (Pt or reactive elements) into NiAl during traditional diffusion- or deposition-based synthesis of the coating. Here we present a highly flexible Al-ion radiation-based synthesis of advanced NiAl coatings, achieving low TGO growth rate without relying on the third element addition. Our results expand the strategy for lowering TGO growth rate and demonstrate potentials for ion radiation in advancing materials synthesis.

Characterization and Wear Behavior of Heat-treated Ni3Al Coatings Deposited by Air Plasma Spraying

Air plasma spraying was utilized to deposit Ni3Al coatings on AISI-321 steel substrate. The deposited coatings were isothermally heat-treated at various temperatures from 500 to 800 °C for 10, 30, 60, and 100 h. The x-ray diffraction analysis revealed NiO formation in Ni3Al at 500 °C after 100 h, and the percentage of NiO increased with increasing exposure time and temperature. The hardness of the coating increased with the formation of NiO. The DSC test showed the formation of minor phases, Al3Ni and Al3Ni2, within the coating along with the major phase Ni3Al. TGA revealed a slowing down of the oxidation rate upon surface oxide formation. The pin-on-disk wear test on the as-sprayed and heat-treated coatings showed that wear rate and coefficient of friction decreased with an increase in the NiO phase content.

Selection of Protective Coatings Obtained by Plasma Spraying Method for Foundry Industry

The paper presents the results of studies of protective coatings for graphite moulds, which were used in copper casting process. The following coatings were involved: Mo, NiAl, NiCrAl, Cr3C2-NiCr, WC-CrC-Ni. All tested coatings were applied on the graphite substrate by plasma spraying method. The microstructure was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The microhardness of coatings was measured by Vickers method. The wettability of coatings by liquid copper was evaluated. It was found that NiAl and NiCrAl coatings had the highest limiting wetting angle and consequently the lowest wettability among all the examined coatings. It was also found that chemical composition strongly influenced the coating properties.

Hot corrosion behavior of Cr-modified NiAl coatings on 310 stainless steel produced by a gas tungsten arc cladding process

The microstructure and hot corrosion behavior of Ni/Al–Cr composite claddings produced by gas tungsten arc welding (GTAW) on a 310 stainless steel substrate were studied. The phase analyses and the microstructure of the cladding layers were evaluated using optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy. The influence of Cr addition (0, 5, 10 at% Cr) on the microstructure and hot corrosion behavior of the NiAl coatings was assessed. The cyclic hot corrosion behavior of the base metal and different claddings was investigated at 900°C and in static air, with a 2–3 mg/cm2 Na2SO4–10%NaCl (wt%). It was found that a dendritic microstructure was formed on the clad surfaces. The results of the XRD analyses indicated that a NiAl phase was synthesized in situ during GTAW cladding and the presence of Cr reduced the intensity of diffraction peaks of NiAl. Hot corrosion experiments also revealed that the addition of Cr had a crucial influence on the hot corrosion behavior of NiAl coatings. It was found that the larger the amount of Cr, the superior the resistance of the coatings to hot corrosion. This improvement was attributed to the formation of Al2O3 as a protective oxide layer, as evidenced by XRD patterns. However, the iron containing phases produced as a result of interactions with the substrate were found to be a detrimental factor influencing the corrosion properties of different cladded layers.

Experimental and Finite Element Simulation of Wear in Nanostructured NiAl Coating

In this paper, the wear of nanostructured NiAl coating was studied both experimentally and numerically. First, the nanocrystalline NiAl intermetallic powder was synthesized by mechanical alloying (MA) of aluminum and Ni powders. The coatings were deposited onto the low carbon steel substrate using high velocity oxy-fuel (HVOF) technique. Nanoindentation test was conducted to find out the mechanical properties of the coating. The dry wear tests were then performed using a pin-on-block test rig under different operating conditions. Finally, finite element (FE) method was employed to model the wear characteristics of the prepared nanostructured material. A three-dimensional (3D) FE model was created and used to simulate the pin-on-block experiments. The results show that the volume losses predicted by the numerical analysis are in good agreement with the experimental data.

Synthesis and heat treatment of sprayed high-temperature NiAl–Ni3Al coatings by in-flight combustion synthesis (CAFSY)

Combustion-Assisted Flame SpraYing (CAFSY) of intermetallic coatings is a new, cost-efficient and on-site-applicable thermal spraying process for applying Ni–Al intermetallic overlays or bond-coats on metallic substrates for protection at high temperature. This is the synthesis of desired intermetallic phases inflight and in-situ on the substrates during oxy-acetylene thermal spraying, using only low cost base-metal powders. By adjusting the spraying conditions (initial composition, spraying distance, substrate temperature, and flame temperature), excellent Ni–Al-based coatings have been produced on various substrates, including mild steel, stainless steel, and aluminum alloys. In some cases, however, the intermetallic phases formed on the substrates during CAFSY have been found to be metastable or the nickel and aluminum powders have not reacted sufficiently. In such cases, post-spraying heat treatments of the coatings allow the solid-state combustion reactions to proceed to completion in the coating. In many cases, it was found that increasing the temperature and the duration of the heat treatment increased the amount of intermetallic compounds (NiAl, Ni3Al, NiAl3, and Ni2Al3) in the coating up to as much as 90 vol %. In particular, all remaining aluminum reacts completely by forming Ni–Al intermetallics in the coatings. In all cases, porosity of the coatings remains below 3% while adhesion strength increases and reaches 57 MPa. The CAFSY method is a special manifestation of combustion synthesis, along the lines of the well-known SHS method. Without the need for expensive pre-alloyed intermetallic powders, optimization of spraying conditions allows very fast, in-flight reactions between component base metal powders to produce the required coating alloys when they reach the surface of the substrate. The actual mixture of the intermetallic phases of the coatings and their properties can be optimized for any industrial use by controlling spraying conditions and composition of powder mixtures.

Laser surface patterning to enhance adhesion of plasma sprayed coatings

In thermal spraying, adhesive bond strength is a feature of surface properties. An adapted surface is studied with prior-surface treatments to enhance interface energy. This study deals with Ni–Al coatings on 2017 aluminum alloy substrate produced by atmospheric plasma spraying. The adherence was evaluated with several controlled surface topographies obtained by grit-blasting and laser surface texturing technique. Adherence has been tested with two different techniques: pull-off test and LASer Adhesion Test. They induce different stresses at the interface. The results showed that the adhesive strength is mostly controlled by a contact adhesion area. A large contact area increases the energy release rate at the interface during coating failures. The bond strength tendency for the two adherence tests is similar: apparent adherence is tripled thanks to laser surface patterning. Fracture propagation is stopped nearby laser-induced holes due to the complex shape and has to deviate inside the coating to maintain crack propagation (inter-splat cracks). The energy at the interfaces being stored locally due to pattern: pattern morphology, pattern localization and powder feed rate are important factors that control the adhesion strength of the thermally sprayed coatings.

Effects of crystalline growth on corrosion behaviour of nanocrystalline NiAl coating

In the current work, the effect of crystalline growth on the corrosion behaviour of nanocrystalline NiAl coating was investigated. In this regard, NiAl coatings with different crystalline sizes in the range of 20–110 nm were produced by mechanical alloying, high-velocity oxy-fuel processing (HVOF) and isothermal annealing at 600°C for 0–30 h. The produced and annealed samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The corrosion behaviours of coatings were examined in NaCl 3/5% electrolyte by potentiostat analysis. The nanocrystalline NiAl coating with the average crystalline size of about 20 nm and the porosity content of about 2% was successfully produced by mechanical alloying and HVOF processing. By annealing the coating, the NiAl crystalline sizes increased sharply, approaching a constant value of about 110 nm. It was found that the corrosion resistance of nanocrystalline coating increased with the increase in the crystalline size.

A study on nanostructured in-situ oxide dispersed NiAl coating and its high temperature oxidation behavior

Improving the efficiency and lifetime of high temperature coatings in a cost-effective way has always been receiving more attention. In this study, it was focused to investigate the high temperature oxidation behavior of thermally sprayed Ni-5wt.% Al coating as a foundation of protective Ni-based alloys under the influence of in-situ formation and dispersion of oxide nanoparticles and grain size refinement. Therefore, high-energy ball-milling process and High Velocity Oxy-fuel (HVOF) system were employed to produce nanocrystalline powders and coatings. Isothermal oxidation evaluation of free-standing coatings was carried out at 950°C. Also the microhardness of the nanostructured coatings was examined and compared with conventional Ni–5Al coatings. The oxidation rate constant (kP) of the modified coating was considerably lower than that of the conventional coating. This phenomenon happened because the continuous protective alumina layer was formed as a thermally grown oxide (TGO) on the nanostructured dilute NiAl from the beginning of the oxidation. The presence of the in-situ alumina nanodispersoids, formed during the milling process, as nucleation sites and numerous grain boundaries as easy diffusion paths facilitates the α-Al2O3 protective layer. The microhardness test results also confirmed the higher hardness of nanostructured coating.

Characterization and Evaluation of Cyclic Hot Corrosion Resistance of Detonation-Gun Sprayed Ni-5Al Coatings on Inconel-718

The high temperature hot corrosion behavior of bare and detonation-gun-sprayed Ni-5Al coatings on Ni-based superalloy Inconel-718 is comparatively discussed in the present study. Hot corrosion studies were carried out at 900 °C for 100 cycles in Na2SO4-60% V2O5 molten salt environment under cyclic heating and cooling conditions. The thermo-gravimetric technique was used to establish the kinetics of hot corrosion. X-ray diffraction, SEM/EDAX, and X-ray mapping techniques were used to analyze the hot corrosion products of bare and coated superalloys. The results indicate that Ni-5Al-coated superalloy showed very good hot corrosion resistance. The overall weight gain and parabolic rate constant of Ni-5Al-coated superalloy were less in comparison with the bare superalloy. The D-gun-sprayed Ni-5Al coating was found to be uniform, adherent, and dense in hot corrosion environment. The formation of nickel- and aluminum-rich oxide scale might have contributed for the better hot corrosion resistance of the coated superalloy.