Ni-based Superalloy Substrate Research Articles

Repairing weld for directly solidified Ni-based superalloy substrates using directed energy deposition of Inconel 625

Although the maintenance, repair, and operation of gas turbine systems are important issues in enhancing the operation of power units, advanced welding repair processes have limitations in depositing directionally solidified Ni-based superalloy components. To overcome this problem, in this study, a directed energy deposition method was applied to repair directionally solidified components using an Inconel 625 alloy. The rapid cooling rate and small laser beam diameter during directed energy deposition minimized the heat-affected zone and duration in the mushy zone, resulting in crack inhibition at the interface. The first laser scan partially melted the substrate and formed a mixing zone with a chemical composition gradient. By combining the chemical compositional gradient and dislocation accumulation due to repetitive solidification, that is, with the melting cycles of directed energy deposition, the thermal stability decreased, resulting in recrystallization and grain growth in the mixing zone. Consequently, the crack-free continuous interface of the partially repaired components did not induce tensile fracture near the interface. Although the mechanical properties of the IN625 deposit layer were degraded by severe heat treatment of the substrate, the results demonstrated that energy deposition with post-heat treatment is an effective approach in repairing Ni-based superalloy components without cracks and inclusion initiation.

Thermal Barrier Coatings in burner rig experiment analyzed through LAser Shock for DAmage Monitoring (LASDAM) method

This study investigates failure mechanisms in a typical thermal barrier coating (TBC) system comprising an EB-PVD columnar top coat, an aluminide bond coat, and a Ni-based single crystal superalloy substrate, simulating gas turbine operating conditions using a burner rig. TBC degradation, initiated by interfacial defects from the LASAT method, was studied during thermal gradient cycling under fast and slow cooling. In-situ optical and infrared imaging, along with ex-situ SEM cross-sectional analysis, monitored failure mechanisms. The Laser Shock for Damage Monitoring (LASDAM) technique provided insights into gradient and cooling rate impacts on columnar TBC damage. Results showed significant effects of cooling rate on delamination and localized failure at blister sites, with LASDAM revealing significant overheating at damage sites. Analysis included full-field temperature and damage assessment, emphasizing blister-driven delamination under severe thermal gradients. Discussion focused on elastic stored energy effects, noting that fast cooling induced transient conditions where reversed temperature gradients increased damage, limiting TBC lifespan.

Analysis of the Geometry of Wear Tracks in Laser Deposited Stellite 6 Coatings

A model has been developed to predict the wear groove geometry resulting from wear testing of Stellite 6 coatings on ferrous and nickel-based alloys produced by laser cladding with powers of 1 kW and 1.8 kW. Although the predictions of the model are close to the observed values, they differ in ways that can be accounted for by the incorrect assumption that only abrasive wear occurs. Abrasive wear is dominant, but there is also evidence that adhesive wear and plastic deformation occur and change the geometry and macrostructures of the wear tracks, particularly for the coatings on a Ni-based superalloy substrate. Significant differences were observed in wear track geometries and macrostructures for coatings on the substrates investigated and these differences correlate well with measured differences in the wear loss of the Stellite coatings.

Effect of coating degradation on the hot corrosion behavior of yttria-stabilized zirconia (YSZ) and blast furnace slag (BFS) coatings

In order to provide thermal insulation thermal barrier coatings (TBCs) used in gas turbine engine components under conditions of impure fuels and high turbine inlet temperatures are expected to have hot corrosion resistance. Techniques and material combinations used in the production of TBCs are primarily effective in the progression of failure mechanisms as a result of hot corrosion. In this study, the hot corrosion damage mechanism, which is caused by the impurities of the fuels used in the industry and causes great damage to the TBC systems, was simulated and evaluated in the laboratory environment. For the creation of TBC systems, CoNiCrAlY metallic bond coating powders were deposited on Ni-based super alloy substrates by the detonation gun (D-gun) process. Commercial yttria-stabilized zirconia (YSZ) and blast furnace slag (BFS) powders were deposited on the bond coating by Atmospheric plasma spraying (APS) technique. Hot corrosion testing of TBCs was performed at 800 °C for 8, 24, 50, and 75 h in an environmental condition consisting of a 45 wt% Na2SO4 and 55 wt% V2O5 corrosive salt mixture. Blast furnace slag, which is completely cost-free and a waste product, provided excellent protection against hot corrosion damage without deteriorating its stability at high temperatures. In this way, it is open to be evaluated as a new type of coating material alternative to the traditional top coating material YSZ, which has a very high production and cost, and to be used in different areas.

Structural characteristics, oxidation performance and failure mechanism of thermal barrier coatings fabricated by atmospheric plasma spraying and detonation gun spraying

In recent years, thermal barrier coatings (TBCs) are widely applied to the surface of gas-turbine engines to prolong the service life of high temperature components. The increase in raw material requirements and costs has led to the search for an alternative, cheaper ceramic based material to yttria stabilized zirconia (YSZ), which is the traditional TBC top coating material. In this study, CoNiCrAlY metallic bond coating powders were deposited on Ni-based superalloy substrates by the detonation gun (D-gun) process. Commercial YSZ and blast furnace slag (industrial waste) powders were deposited on the bond coating by atmospheric plasma spraying (APS) technique. TBCs were subjected to isothermal oxidation tests for 8, 24, 50, and 75 h at 800 °C. The microstructural properties of the TBCs were evaluated comparatively using advanced characterization techniques before and after oxidation tests. It has been observed that the coatings produced using BFS material have a potential to be used in different types of industries as a low-cost and environmental solution against high temperature oxidation conditions.

High temperature lattice structure evolution of C-axis preferred orientation AlN thin films and its application in temperature measurement

In order to measure the surface temperature of aerospace hot-end components accurately, AlN thin film temperature measurement technology was explored. In this research, C-axis preferred orientation AlN thin film was deposited on Ni-based superalloy substrates by medium frequency (MF) reaction magnetron sputtering. Effect of temperature on the lattice structure of AlN thin film was studied. X-ray diffraction results show that the diffraction angles (2 θ ) of AlN thin films shift to the left side monotonously with the increase of annealing temperature and annealing time. Based on this phenomenon, the maximum temperature experienced by the film can be deduced according to 2 θ values. A temperature interpretation algorithm equation was established with MATLAB. The algorithm is a binary linear equation, whose dependent variable is 2 θ and the independent variables are annealing temperature and annealing time. Then, temperature interpretation software was developed with Visual Studio 2019. The temperature interpretation range is 700–1200 °C and the relative interpretation error is less than 4.26%.

Microstructure Investigation and Cyclic Oxidation Resistance of Ce-Si-Modified Aluminide Coating Deposited by Pack Cementation on Inconel 738LC

The synergistic effect of Si and Ce addition on the oxidation resistance of a pack cementation aluminide coating applied on a Ni-based IN738LC superalloy substrate was investigated in this study. The structural and thermal influences of both Si and Ce, focusing on morphology, oxidation behavior, and scale spallation tendency, are accordingly discussed based on the experimental results using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray analyses (EDX). For this purpose, the oxidation resistance of the modified coatings was evaluated by measuring the weight gain of the coated samples after 16 h for each cycle at 1100 °C for a total of 50 cycles of the oxidation process. The investigations indicated that Si addition to the modified aluminide coating improves the oxidation resistance through the formation of β-NiAl and δ-Ni2Al3 phases, and also δ-Ni2Si phases. Furthermore, the addition of 1% Ce to the modified aluminide coating enhances the formation of the fine-grained microstructure of the β-NiAl and δ-Ni2Al3 and reduces the outward/inward diffusion of elements (so-called blocking effect), which significantly modifies the cyclic oxidation resistance. The oxidation enhancement also may be attributed to synergistic effects of Ce and Si addition during the deposition process that reduce the inward oxygen diffusion and reduce the growth rate of α-Al2O3 during oxidation tests.

Effect of substrate type on the lattice structure of AlN thin films annealed at high temperature

To measure the surface temperature of aerospace hot-end components accurately, AlN thin film temperature measurement technology is being explored. The core of this temperature measurement technology is the evolution law of the lattice structure with temperature, but the influence of substrate on its lattice structure is still unclear. In this research, C-axis preferred orientation AlN thin films were deposited on Ni-based superalloy and quartz glass substrates by Mid-frequency reactive magnetron sputtering. The effect of substrate type on the lattice structure of annealed AlN thin films was investigated. It was found that with the increase of temperature, the lattice structure parameters (2θ) of AlN thin films deposited on Ni-based superalloy gradually shifted to the left, while those of AlN thin films deposited on quartz glass keep a constant until 1300 °C. The results reveal that quartz glass substrate has no influence on the lattice structure of AlN thin films as long as the temperature is lower than 1300 °C. Quartz glass substrates are suitable for follow-up research of AlN thin film temperature measurement technology.

Investigation of HVOF sprayed novel Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 HEA coating as bond coat material in TBC system

The present study aims to investigate non-stoichiometric Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 high entropy alloy (HEA) as a bond coat material for the TBC (Thermal Barrier Coating) system. The mechanical activated synthesized HEA was sprayed on a Ni-based superalloy substrate by High-velocity oxy-fuel (HVOF) spraying, and 8 mol% Yttria Stabilized Zirconia (YSZ) was deposited on HEA by Air Plasma Spray (APS). X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM) were used to investigate the phases and microstructure of the as-synthesized HEA powder, Ni superalloy/HEA-bond coat, and Ni superalloy/HEA-bond coat/YSZ topcoat. The mechanical properties of the coating like microhardness, Young’s modulus and residual stress between bond coat and YSZ top coat was evaluated using the Nano-Hardness Tester (NHT). The TBC system was investigated for cyclic oxidation at 1050 ℃ for 100 cycles, and its cross-sections were analyzed for TGO (Thermally grown oxides) layer composition, thickness and interdiffusion of elements. The properties of the TBC system containing HEA as a bond coat were compared with those of the conventional TBC system comprising of MCrAlY (AMDRY 365–4) as a bond coat. It was observed that HEA containing TBC displayed exceptional high temperature properties and were comparable to MCrAlY.

Composition-dependent interdiffusivity matrices of ordered bcc_B2 phase in ternary Ni–Al–Ru system at 1273∼1473 K

Abstract In this paper, 5 Ni–Al–Ru ternary and 2 NiAl/RuAl binary equilibrium alloys were prepared, and their lattice parameters were evaluated by using the X-ray diffraction technique. The relation between the lattice parameters and Ru content satisfies Vegard’s law, confirming that a continuous single bcc_B2 phase exists between NiAl and RuAl in the ternary Ni–Al–Ru system. After that, three diffusion couples located in the bcc_B2 phase region were prepared at 1273, 1373 and 1473 K, respectively. With the presently measured concentration profiles, the composition-dependent interdiffusivity matrices along the entire diffusion paths were efficiently determined by using the High-throughput Determination of Interdiffusion Coefficients (HitDIC) software. The obtained interdiffusivities show a good agreement with the limited experimental data in the literature. Furthermore, the present results show that the addition of Ru to bcc_B2 NiAl alloys can significantly inhibit the interdiffusion rate of Ni, indicating that the bcc_B2 Ru-containing NiAl bond coats can serve as a potential diffusion barrier for Ni-based superalloy substrate.

High temperature electrical insulation and adhesion of nanocrystalline YSZ/Al2O3 composite film for thin-film thermocouples on Ni-based superalloy substrates

Electrical insulation and adhesion are two vital aspects for the integrated thin film sensors with Ni-based superalloy substrates. In this paper, nanocrystalline yttria-stabilized zirconia/alumina (YSZ/Al2O3) composite film was fabricated on Ni-based superalloy substrate by reactive magnetron sputtering. The X-ray diffraction and scanning electron microscope results show that tetragonal zirconia (t-ZrO2) nanocrystalline is homogeneously dispersed in the amorphous Al2O3 matrix. The crystal size of the t-ZrO2 nanocrystalline is well maintained with the temperature up to 1000 °C. The resistivity of the film is about 108 Ω·cm at 1000 °C, which is three orders of magnitude higher than that of the pure Al2O3 film. Furthermore, this YSZ/Al2O3 film demonstrates superior toughness and adhesion with the Ni-based superalloy substrate even after undergoing high-temperature cycles. Finally, a S-type thin-film thermocouple was fabricated on the YSZ/Al2O3 film shows good stability, repeatability, and linearity in multiple thermal cycling tests up to 900 °C exceeded 48 h.

Hot corrosion studies of nanostructured gadolinium zirconate thermal barrier coatings

Improvement of hot corrosion resistance is one of the important parameters governing the lifetime and efficiency of the thermal barrier coatings (TBCs). In this study, the Gadolinium Zirconate (GZ) was synthesized by ball milling method and deposited by Electron Beam-Physical Vapour Deposition (EB-PVD) on Ni-based superalloy substrate with NiCrAlY as an intermediate bond coat. The effect of nanostructured GZ TBCs on hot corrosion resistance were studied under three different salt mixture environments viz; SM1, SM2 and SM3 in isothermal condition at 900 °C for 12 h. The results indicated that EB-PVD coated nanostructured GZ TBCs have improved the hot corrosion resistance and performed well under SM1 and SM3 conditions with minimal weight gain and without any spallation, whereas, the TBC suffered severe spallation under of SM2 salt condition with higher weight gain among the other two conditions. The formation of microcracks along the columnar gaps of the topcoat were found in the SM2 condition, have allowed the molten salts infiltration up to the coating interface. The formation of dense corrosive products GdVO4 and m-ZrO2 phases were identified after hot corrosion in SM1 and SM3 condition, which were absent in SM2 condition.

Oxidation Resistance of Double-Ceramic-Layered Thermal Barrier Coating System with an Intermediate Al2O3-YAG Layer

In the present study, APS Al2O3-YAG layers containing 10, 20, and 40 wt.% YAG were deposited as the intermediate layer between the MCrAlY bond coat and the YSZ top coat of double-ceramic-layered TBC (DCL-TBC) on the Ni-based superalloy substrate. Then, the effect of its YAG content on the oxidation behavior of the coated sample was investigated. Microstructural studies of the coatings showed that adding Al2O3-YAG intermediate layer to DCL-TBC system reduced the growth rate and thickness of the TGO layer due to the influence of reactive element, reduction in oxygen inward diffusion, and inhibition in the formation of detrimental mixed oxides as the TGO layer. The cyclic oxidation test at 1050 °C indicated that the oxidation resistance of the coating with the Al2O3-20 wt.% YAG layer was better than that of the other samples.

A combinatorial approach to study the phase constitution of the Ni-Al-Pt system

Platinum has a beneficial effect on improving alumina scale adhesion and decreasing the alpha alumina growth of aluminide bond coats in thermal barrier coatings. However, complex phases form among Ni, Al and Pt during service due to diffusion between bond coat and underlying Ni-base superalloy substrate. A combinatorial/high-throughput approach to study alloy phase equilibria with precious compositions is high-efficient and cost-saving. The microstructural evolution of a Ni-Al-Pt diffusion triple fabricated by annealing at 1100°C for 96 h followed by water-cooling was characterized by transmission electron microscopy and electron probe microanalysis. The high temperature Pt3Al(h) phase is stabilized due to dissolved Ni, indicating that Ni is a stabilizer for Pt3Al(h). A ternary phase, Ni2PtAl, predicted to exist, has been observed experimentally with L12 structure adjacent to (Ni,Pt)3Al, while the L10-structured tetragonal NiPt2Al phase is also confirmed in the Ni-Al-Pt system. Upon cooling, the Ni-rich β-(Ni,Pt)Al undergoes a martensitic transformation from β (B2) to β′-(Ni,Pt)Al (L10). The orthorhombic D2h19-structured (Ni,Pt)5Al3 phase with lattice parameters a = 0.747 nm, b = 0.682 nm, and c = 0.376 nm was found to form from β′-(Ni,Pt)Al martensite. The orientation relationship of D2h19 and β′ phases is (100)[010]D2h19 // (100)[001] β′. (221) twins are observed in lamellar (Ni,Pt)5Al3. Phase constitution is determined in the Ni-Al-Pt system at 1100°C. The mechanism of formation of (Ni,Pt)5Al3 from β′-(Ni,Pt)Al was analyzed.

Cold spray for production of in-situ nanocrystalline MCrAlY coatings – Part II: Isothermal oxidation performance

Cold spray (CS) MCrAlY coatings have been widely explored as potential bond coats for thermal barrier coatings used on the hot-section components of modern gas turbines. In this study, NiCoCrAlTaY coatings were applied on CMSX-4 single-crystal Ni-base superalloy substrates using the CS technique. Nitrogen was used as process gas with low spray parameters to allow the production of in-situ nanocrystalline NiCoCrAlTaY coatings while using conventional powders of large grain size, as demonstrated in Part I of the study using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). In this part of the study (Part II), the coated samples were examined under isothermal oxidation conditions at 1100 °C for 1 h to 500 h. The as-deposited NiCoCrAlTaY coatings, characterized by dense microstructure, low surface roughness, nanoscale grains, and a large number of crystallographic defects such as grain boundaries and dislocations, promoted the formation of a protective α-Al2O3 scale during short-term oxidation. Fast diffusion via these crystallographic defects facilitated Al diffusion in the coatings. This provides sufficient Al replenishment to the coating subsurface region, sustaining the growth of the α-Al2O3 scale during prolonged oxidation. The relatively low scale growth kinetics found in the study demonstrated enhanced oxidation performance of the CS nanocrystalline NiCoCrAlTaY coatings.

Cold spray for production of in-situ nanocrystalline MCrAlY coatings – Part I: Process analysis and microstructure characterization

Nanocrystalline MCrAlY coatings have been shown excellent oxidation performance as bond coats in thermal barrier coating (TBC) systems. In the first part of the present study, a cold spray (CS) process is proposed, aiming at producing in-situ nanocrystalline NiCoCrAlTaY coatings on CMSX-4 single-crystal Ni-base superalloy substrates while using conventional powders of large grain sizes. Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD) were used to characterize the resulting coating-substrate system. Results show that the feedstock powder particles experienced substantial plastic deformation, which promoted dynamic recrystallization and grain refinement resulting in extended nanostructures in the CS NiCoCrAlTaY coatings. Furthermore, interfacial features such as materials mixing and intimate metal-metal contacts without pores or oxide fragments suggest the presence of mechanical and potentially metallurgical bonding between the CS NiCoCrAlTaY coatings and the CMSX-4 substrate. Isothermal oxidation testing results are presented in Part II of the study, which reveals the oxidation performance of the NiCoCrAlTaY coatings produced using the proposed CS process.

Oxidation Behavior of the Monolayered La2Zr2O7, Composite La2Zr2O7 + 8YSZ, and Double-Ceramic Layered La2Zr2O7/La2Zr2O7 + 8YSZ/8YSZ Thermal Barrier Coatings.

The degradation process of thermal barrier coatings (TBCs) such as monolayered La2Zr2O7, composite 50% La2Zr2O7 + 50% 8YSZ, and double-ceramic layer (DCL) La2Zr2O7/50% La2Zr2O7 + 50% 8YSZ/8YSZ was investigated. Coatings were deposited using the atmospheric plasma spraying (APS) process (ceramic layer and bond-coat) on the Ni-based superalloy substrate with Ni-22Cr-10Al-1Y bond-coat. The thickness of the ceramic top-coats in all cases were 300 µm. In the case of La2Zr2O7/8YSZ, the internal sublayer was built from 8YSZ powder whereas the outer from La2Zr2O7. Between both sublayers’ “composite” a 50% La2Zr2O7 + 50% 8YSZ zone was present. The “composite” 50% La2Zr2O7 + 50% 8YSZ TBC system was sprayed from two different feedstock powders with equal weight ratios. In the first part of the investigation, the microstructural characterization of the TBCs was presented. The main goals were related to the characterization of the degradation processes in different TBC systems with special emphasis on the phenomenon in the thermally grown oxide (TGO) zone related to oxidation, and the phenomenon related to phase stability in ceramic top-coats as related to temperature influence. The oxidation test was carried out in air at 1100 °C for 500 h. In the second step of the investigation, the numerical simulation of the monolayered TBC 8YSZ and La2Zr2O7 systems was analyzed from the stress distribution point of view. Additionally, the two-layered TBC coating of the DCL type was also analyzed.

Effect of powder composition upon plasma spray-physical vapor deposition of 8YSZ columnar coating

Agglomerated 8YSZ nano-powders for thermal barrier coating (TBC) were prepared by spray drying method. The morphology and diameter of 8YSZ powders were adjusted through controlling the solid content of the suspensions. Using prepared agglomerated 8YSZ nano-powders, columnar-structured coatings were obtained on Ni-based superalloy substrates by plasma spray-physical vapor deposition (PS-PVD). The experimental results proved that the spray dried powders characteristics were related to the solid content of suspensions. During the PS-PVD process, the deposition efficiency of the powders was improved with the increase of powder diameter due to the existence of thermophoresis. Moreover, it can be concluded by mathematical derivations that the top area of the plasma flame was sufficient for powder evaporation.

Microstructural aspects of the protective ceramic coatings applied on the surfaces of refractory alloys produced by additive manufacturing

The possibility of depositing multi layers made of metals/alloys and high temperature ceramics by electron beam physical evaporation process under high vacuum (EB-PVD) on the surface of a parallelepiped sample made by selective laser melting (SLM) from a Ni base refractory super alloy was experimentally tested. The SEM-EDAX micro structural analysis revealed the morphology and thickness of the coatings consisting of a NiCrAlY base alloy as bonding layer and three successive YSZ, LZO and GZO ceramic layers on the Ni-based super alloy substrate obtained by additive manufacturing. The adhesion of the layer deposited was evaluated by the scratch test method. The analysis highlighted the importance of pre-preparing the surface of the additive manufactured substrates, in order to control the adhesion and uniformity of the deposited layers.

Thermal Durability of YSZ/Nanostructured Gd2Zr2O7 TBC Undergoing Thermal Cycling

Inferior fracture toughness (KIC), low thermal expansion coefficient (CTE), thermochemical incompatibility with bond coat and growth of thermally grown oxide (TGO) are the main destruction factors in conventional Gd2Zr2O7 (GZO) thermal barrier coating (TBC) undergoing thermal cycling. In this paper, to overcome these drawbacks, nanostructured GZO and an intermediate conventional YSZ layer between top and bond coats were used in the TBC system. Single-layer nanostructured GZO and dual-layer YSZ/nanostructured GZO coatings were deposited with atmospheric plasma spray technique on Ni-based superalloy (IN738LC) substrates as a topcoat with a CoNiCrAlY bond coat. Oxidation behaviour of samples was studied and compared in the cyclic mode at 1100 °C for 4 h in each cycle. Also, CTE and KIC of the nanostructured GZO and intermediate YSZ layers were investigated. The results indicated that the oxidation life of dual-layer YSZ/nanostructured GZO coating was 3.2 times more than that of single-layer nanostructured GZO. The microstructural analysis indicated that the growth rate of TGO was considerably slower for dual-layer coating due to reduced oxygen infiltration and cracks propagation. Also, nanostructured topcoat improved the thermochemical compatibility and mechanical properties in TBCs.