Ni-based Superalloy Substrate Research Articles

Pulsed Laser Deposition of the Ni-Base Superalloy Films

Ni-base superalloy films were deposited on single-crystal (SC) Ni-base superalloy substrates from a target with the same alloy composition by pulsed laser deposition (PLD) technique. Microstructure and growth behavior of the films deposited were investigated by X-ray diffraction and scanning electron microscopy, and atomic force microscope. The homoepitaxial growth of the SC Ni-base superalloy film occurred at the 1123 K (850 °C) substrate temperature and 2 J/cm2 pulse energy. Films generally exhibited a strong polycrystalline characteristic as the substrate temperature and pulse energy increased. The SC film had a smooth surface. The measured root mean square roughness of the SC film surface was ~6 nm. Based on the Taguchi analysis, the substrate temperature and pulse energy were the most significant process parameters influencing the structural characteristics of the films. Also, the influence of the pulse repletion rate and deposition time was not found to be significant.

Evaluation of hot corrosion behavior of plasma sprayed thermal barrier coatings with graded intermediate layer and double ceramic top layer

Abstract In the present work, the hot corrosion behavior of two types of multilayer plasma sprayed TBC were investigated and compared with functionally graded and conventional TBCs. These kinds of multilayer coatings consisted of nano/μ alumina as a top coat on YSZ layer, a metallic bond coat and a functionally graded intermediate layer deposited between YSZ and bond coat layers. All the layers were sprayed on the Ni-base super alloy substrate. The hot corrosion resistance of the plasma sprayed coatings was examined at 1050 °C for 40 h, using a fused mixture of 45 wt% Na 2 SO 4 + 55wt%V 2 O 5 . Before and after hot corrosion, the microstructure and phase analysis of the coating were studied using scanning electron microscope and X-ray diffractometer. The results showed that, the Al 2 O 3 top layer acted as a barrier against the infiltration of the molten salt into the YSZ layer during exposure to the molten salt mixture at the high temperature and the multilayer coatings of zirconia/alumina with the nanostructured alumina as a top coat showed higher hot corrosion resistance. Also, the failure mechanisms of the functionally graded coating and duplex TBC were investigated. The spallation occurred between the graded layer and the bond coat/top coat in functionally graded TBC and duplex TBC, respectively.

Stability and thermoelectric properties of ITON:Pt thin film thermocouples

Refractory thin film thermocouples can provide more accurate surface temperature measurement and faster response with excellent chemical and electrical stability at high temperatures in harsh oxidizing environment. Indium tin oxide (ITO) thin films were fabricated by radio frequency magnetron sputtering in gas mixture of Ar and N2. The introduction of nitrogen results in significant variations of the electrical and thermoelectric property of ITO thin films. This nitrogen doped ITO thin film was used as negative thermal element to form ITON:Pt thin film thermocouple on Ni-based superalloy substrate. The stability and thermoelectric performance of this thin film thermocouple was investigated up to 1000 °C.

Characterization and understanding of residual stresses in a NiCoCrAlY bond coat for thermal barrier coating application

The residual stresses in a NiCoCrAlY bond coat deposited on a Ni-base superalloy substrate after oxidation at 1150°C were studied by X-ray diffraction using the sin2Ψ technique. The stresses were found to be tensile; they first increased and then decreased with oxidation time. High temperature stress measurement indicated that the stress developed and built up upon cooling, predominantly within the temperature range from 1150°C to 600°C. Microstructural examination suggested that, due to the limited penetration depth into the bond coat, the X-ray only probed the stress in a thin surface layer consisting of the single γ-phase formed through Al depletion during oxidation. Quantitative high temperature X-ray diffraction analysis revealed that, above 600°C, the volume fraction of the β-phase in the bond coat increased with decreasing temperature. The mechanisms of stress generation in the bond coat were examined and are discussed based on the experiments designed to isolate the contribution of possible stress generation factors. It was found that the measured bond coat stresses were mainly induced by the volume change of the bond coat associated with the precipitation of the β-phase upon cooling.

RETRACTED: Microstructural stability oxidation and hot corrosion resistance of nanostructured Al2O3/YSZ composite compared to conventional YSZ TBC coatings

Abstract The spallation of ceramic coating from the bond coat is important problem for TBC systems. And the spallation is caused the oxidation and hot corrosion at the interface of ceramic layer and bond coat. In the present work oxidation and hot corrosion behavior of plasma sprayed nanostructured Al 2 O 3 /YSZ composite TBC coatings on Ni-base (IN-738LC) super alloy substrate was studied and compared to conventional YSZ. The coatings have been deposited by plasma spray method. High temperature oxidation test at 1100 °C and hot corrosion test at 1050 °C using Na 2 SO 4 and V 2 O 5 molten salts were performed on the coatings. The experimental results showed nanostructured Al 2 O 3 /YSZ composite coating had higher oxidation and hot corrosion resistance than those of the conventional YSZ coating. The microstructural analysis indicated that the growth of TGO was much less for this nanostructured Al 2 O 3 /YSZ composite coating while it reduced infiltration of both oxygen and aggressive molten salt.

HVOF 용사공정에 의한 단결정 초내열합금 코팅계면 이차 반응층 생성기구 고찰

The formation of the secondary reaction zone (SRZ) at the interface between the CoNiCrAlY coating and experimental Ni-based single crystal superalloy substrate was investigated. Transmission elec- tron microscope (TEM) observation revealed that the SRZ was composed of recrystallized γ΄ grains with a high density of fine topologically close-packed (TCP) precipitates. The TCP precipitate was identified as the faulted and twinned rhombohedral μ phase. TCP precipitation appears to be analogous to the discontinuous precipitation. The γ΄ grains nucleated epitaxially with grains in the coated layer. Preferential formation of the SRZ was observed along the direction. This anisotropy of SRZ formation may be essentially related to the preferential dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways. Based on the suggested SRZ formation mechanism, the intermediate annealing is proposed to suppress SRZ formation in terms of mitigation of strain energy and interdiffusion. †(Received September 26, 2013)

On the mechanism of secondary reaction zone formation in a coated nickel-based single-crystal superalloy containing ruthenium

The secondary reaction zone (SRZ) formation at the interface between CoNiCrAlY coating and experimental Ni-based single-crystal superalloy substrate with 1 wt.% Ru has been investigated. The γ′ grains nucleated epitaxially with grains in coated layer. Preferential formation of the SRZ was observed along the 〈1 1 0〉 directions. The factors affecting the SRZ kinetics are discussed in terms of the preferential dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways.

AlPO4–C composite coating for high emissivity and oxidation protection applications

AlPO4–C coating deposited by sol–gel technique on Ni base superalloy substrate Nimonic-75 has been examined for high emissivity applications. The coating showed a spectral (2–25 μm wavelength) emissivity in the range of 0·8–0·91, and the emissivity increased with coating thickness. The coating exhibited a reasonably good adhesion with the substrate, as determined from nanoscratch test. The above coating also had a good oxidation resistance in air at 800 and 900°C under cyclic heating and cooling conditions.

Porous Ceramic Coating for Transpiration Cooling of Gas Turbine Blade

A transpiration cooling system for gas turbine applications has significant benefit for reducing the amount of cooling air and increasing cooling efficiency. In this paper, the porous ceramic coating, which can infiltrate cooling gas, is developed with plasma spraying process, and the properties of the porous coating material such as permeability of cooling gas, thermal conductivity, and adhesion strength are examined. The mixture of 8 wt.% yttria-stabilized zirconia and polyester powders was employed as the coating material, in order to deposit the porous ceramic coating onto Ni-based super alloy substrate. It was shown that the porous ceramic coating has superior permeability for cooling gas. The adhesion strength of the porous coating was low only 20% compared with the thermal barrier coating utilized in current gas turbine blades. Simulation test of hot gas flow around the gas turbine blade verified remarkable reduction of the coating surface temperature by the transpiration cooling mechanism. It was concluded that the transpiration cooling system for the gas turbine could be achieved using the porous ceramic coating developed in this study.

Optimizing structural and compositional properties of electrodeposited ceria coatings for enhanced oxidation resistance of a nickel-based superalloy

Cathodic electrodeposition was used to generate ceria coatings onto a Ni-based superalloy substrate. The electrochemical parameters were optimized so as to obtain relatively thick but adherent films with tailored composition and microstructural features (multi-cracked network) designed for high temperature applications. Whereas the applied current density was shown to mainly influence the appearance and size of the cracks as well as the amount of oxygen vacancies, the linear trend between the deposited mass and the deposition time allowed a good control of the deposited thickness. A crystallization/diffusion thermal treatment was then applied to promote the dehydration of the deposit, thus resulting in a network of refined cracks, and in the complete crystallization of nanometric CeO2−x exhibiting either a needle-like or a quasi “foam-like” microstructure depending on the applied temperature. This also provided the establishment of an inwardly grown α-Al2O3 scale at the substrate/coating interface expected to further increase the high temperature oxidation resistance of the coated superalloy.

Interfacial reaction between SiO2–Al2O3–ZnO–CaO based glass coatings and K38G superalloy substrates

In order to extend the use of a SiO2–Al2O3–ZnO–CaO based glass coating for higher temperature applications on Ni-base superalloy substrates, its interfacial reaction with the K38G superalloy substrates at 1000°C was investigated. Results indicated that the interfacial reaction resulted in bubbling of the glass coatings due to Zn gas volatilization and formation of a loose TiO2 interlayer which finally accounted for the peeling of the coatings. With a pre-crystallization process of the glasses, its interfacial reaction was controlled and the bubbling of the coatings was avoided. After the heat-treatment at 1000°C for 20h, the glass coatings were still very dense and bonded well to the K38G superalloy substrates. Instead of a loose TiO2 interlayer, an Al-riched layer formed at the interfaces.

Effect of Ti Addition on Microstructural Evolution of Ni3Si

Laser cladding technique was used to form Ni3Si intermetallic coating by Ti addition on Ni-based superalloy substrate. The coating was investigated by optical microscopy and X-ray diffraction (XRD). The clad coating was dense, compact and smooth. An excellent bonding between the coating and the substrate was ensured by a strong metallurgical interface. The laser clad Ni3Si-Ti intermetallic coating has a rapidly solidified microstructure consisting of Ni (Si), Ni3Si (L12) and Ni31Si12 . The grain refinement effect has been achieved due to the occurrence of Ti in cladding layers. By increasing the proportion of Ti, the hardness decreased as the Ti content increasing, the peaks intensity of Ni31Si12 decreased.

Effects of negative substrate bias voltage on the structure and properties of aluminum oxide films prepared by DC reactive magnetron sputtering

Aluminum oxide thin films (AlxOy) were deposited onto Ni-based superalloy substrates by DC reactive magnetron sputtering using Al target in an Ar and O2 gas environment. Subsequently post-deposition annealing was carried out in air ambient at 500°C for 1h with heating rate of 25°C/min and then some annealed samples were heated at 800°C for 1h to simulate the application environment. Effects of the negative substrate bias voltage (Vb) on the structure and properties of the as-deposited, the annealed and the heated films, such as the deposition rate, elemental composition, surface morphology, microstructure, optical transmittance and microhardness, have been studied. It is found that with the increase of negative Vb (0V, −20V, −30V and −40V), the deposition rate decreases sharply and so do the O/Al ratios of the as-deposited films. While for the annealed films, the O/Al ratios show an increasing trend. All the as-deposited and annealed films are amorphous. However, the heated films are all crystallized and the crystallization gets weaker with the negatively increasing Vb. Grain refinement for the as-deposited films and loose crack surface for the annealed ones are found in the process of negatively increasing Vb to −40V. And all the annealed films exhibit higher transmittance but lower microhardness than the as-deposited ones.

Oxidation Behavior of NiAl-TiB2 Coatings Prepared by Electrothermal Explosion Ultrahigh Speed Spraying

Abstract The electro-thermal explosion ultrahigh speed spraying technology was employed to prepare the NiAl coatings with 0, 10, and 20 wt.% TiB2 on the Ni-based super-alloy substrate. The Microstructure, phase structure, and hardness of coatings were analyzed by scanning electron microscopy with energy dispersive X-ray spectrometry, X-ray diffraction, and Vickers micro-hardness tester, respectively. The results showed that the coatings had dense microstructure with submicron grains. The addition of TiB2 as a particulate reinforcement increased the hardness of NiAl. The isothermal oxidation test of coatings at 1000 °C for 140 h in air was investigated. The results showed that the oxidation resistance of NiAl coatings was higher than that of NiAl-TiB2 coatings at high temperature. The oxides and morphologies of the above coatings were investigated. The Al2O3 was formed on NiAl coating surface, while Al2O3 and TiO2 were formed on NiAl-TiB2 coating surface after oxidation.

Thermal Diffusivity Measurement for Thermal Spray Coating Attached to Substrate Using Laser Flash Method

Ceramic-based thermal barrier coatings are used as heat and wear shields of gas turbine blades. There is a strong need to evaluate the thermal conductivity of coating for thermal design and use. The thermal conductivity of a bulk material is obtained as the product of thermal diffusivity, specific heat capacity, and density above room temperature in many cases. Thermal diffusivity and thermal conductivity are unique for a given material because they are sensitive to the structure of the material. Therefore, it is important to measure them in each sample. However it is difficult to measure the thermal diffusivity and thermal conductivity of coatings because coatings are attached to substrates. In order to evaluate the thermal diffusivity of a coating attached to the substrate, we have examined the laser flash method with the multilayer model on the basis of the response function method. We carried out laser flash measurements in layered samples composed of a CoNiCrAlY bond coating and a 8YSZ top coating by thermal spraying on a Ni-based superalloy substrate. It was found that the procedure using laser flash method with the multilayer model is useful for the thermal diffusivity evaluation of a coating attached to a substrate.

Thermal Diffusivity Measurement for Thermal Spray Coating Attached to Substrate Using Laser Flash Method

Ceramic-based thermal barrier coatings are used as heat and wear shields of gas turbine blades. There is a strong need to evaluate the thermal conductivity of coating for thermal design and use. The thermal conductivity of a bulk material is obtained as the product of thermal diffusivity, specific heat capacity, and density above room temperature in many cases. Thermal diffusivity and thermal conductivity are unique for a given material because they are sensitive to the structure of the material. Therefore, it is important to measure them in each sample. However it is difficult to measure the thermal diffusivity and thermal conductivity of coatings because coatings are attached to substrates. In order to evaluate the thermal diffusivity of a coating attached to the substrate, we have examined the laser flash method with the multilayer model on the basis of the response function method. We carried out laser flash measurements in layered samples composed of a CoNiCrAlY bond coating and a 8YSZ top coating by thermal spraying on a Ni-based superalloy substrate. It was found that the procedure using laser flash method with the multilayer model is useful for the thermal diffusivity evaluation of a coating attached to a substrate.

Application of Thermodynamic and Kinetic Modeling to Diffusion Simulations in Nickel-Base Superalloy Systems

This paper presents a brief review, followed by some new results from recent diffusion simulations in Ni-base superalloy systems, performed by means of a thermodynamic and kinetic modeling approach as taken in the commercial finite-difference code DICTRA. The DICTRA code solves the multi-component diffusion equations, combining assessed thermodynamic and kinetic data in order to determine the full composition dependent interdiffusion matrix. The link between fundamental physics based models and critically assessed data allows simulations to be performed with realistic conditions on alloys of practical importance. Emphasis in this paper is on modeling and simulation of interdiffusion occurring between NiAl coatings and Ni-base superalloy substrates. For this purpose we have used the so-called homogenization approach to diffusion in multi-phase systems, recently implemented into the DICTRA software. The simulation results have been validated against experimental data and the agreement is very satisfactory given the complexity of the problem.

Cyclic oxidation behavior of HVOF bond coatings deposited on La- and Y-doped superalloys

One suggested strategy for improving the performance of thermal barrier coating (TBC) systems used to protect hot section components in gas turbines is the addition of low levels of dopants to the Ni-base superalloy substrate. To quantify the benefit of these dopants, the oxidation behavior of three commercial superalloys with different Y and La contents was evaluated with and without a NiCoCrAlYHfSi bond coating deposited by high velocity oxygen fuel (HVOF) spraying. Cyclic oxidation experiments were conducted in dry O2 at 1050°, 1100° and 1150°C. At the highest temperature, the bare superalloy without La showed more attack due to its lower Al content but no difference in oxidation rate or scale adhesion was noted at lower temperatures. With a bond coating, the alumina scale was non-uniform in thickness and spalled at each temperature. Among the three coated superalloys, no clear difference in oxide growth rate or scale adhesion was observed. Evaluations with a YSZ top coat and a bond coating without Hf are needed to better determine the effect of superalloy dopants on high temperature oxidation performance.

K-Type Thin Film Thermocouples Deposited on Ni-Based Superalloy Substrates

NiCr-NiSi K-type thin film thermocouples with multi-layer structure were fabricated on Ni-based superalloy substrates (95 mm×35 mm×2 mm). The multi-layer structure contains NiCrAlY buffer layer (2 μm)/ thermally grown Al2O3 bond coating (200 nm)/ Al2O3 insulating layer (8 μm)/ NiCr-NiSi thin film thermocouples (1 μm)/ Al2O3 protecting layer (500 nm). The samples were statically calibrated in a tube furnace in the temperature range from 170 °C to 610 °C. The results show that the resistance of Al2O3 insulating layer is about 14.6 kΩ at 800 °C and exceeds 100 MΩ at room temperature. The Seebeck coefficient a of the samples is about 34 μV/°C, and the sensitivity coefficient K is greater than 0.8 in the temperature range from 170 °C to 610 °C. The maximal sensitivity coefficient is about 0.97 at 265 °C.

Oxidation resistance of YSZ-alumina composites compared to normal YSZ TBC coatings at 1100 °C

In the present work oxidation behavior of plasma sprayed YSZ-alumina composite TBC coatings on Ni-base (IN-738LC) super alloy substrate was studied and compared to normal YSZ. Cyclic oxidation process in 4 h intervals was performed in an air electrical furnace at 1100 °C and the specimens were cooled in the furnace during each cycle. Preliminary checking was done with naked eye and further investigation was achieved using scanning electron microscopy. If there were any cracks or spallation in the coating's edge, the tests were stopped, the time was recorded and coating microstructure was studied. YSZ-alumina composites were made by applying alumina layer at the top of YSZ or mixed with YSZ as a TBC layer on the bond coat. Composite coatings of YSZ-alumina having alumina as a top coat and the mixed YSZ-alumina layer, showed better resistance than normal YSZ in oxidation test. It was observed that alumina overlay on YSZ has promoted the oxidation resistance of the coatings for longer times by preventing infiltration of oxygen through YSZ layer.