Faculty Publications
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Item Comparative hot corrosion performance of APS and Detonation sprayed CoCrAlY, NiCoCrAlY and NiCr coatings on T91 boiler steel(Elsevier Ltd, 2021) Sundaresan, C.; Rajasekaran, B.; Varalakshmi, S.; Santhy, K.; Rao, D.S.; Govindarajan, G.Hot corrosion performance of Atmospheric Plasma Spray (APS) and Detonation spray (DSC) CoCrAlY, NiCoCrAlY, and NiCr coatings on T91 steel were investigated at 650 °C for 100 cycles under Na2SO4-K2SO4-Fe2O3 mixed salt deposit in ambient air. The hot corrosion resistance of DSC coatings was found to be superior to their APS counterparts. Chromia and spinel oxides provided excellent corrosion resistance while no ?-Al2O3 was observed. DSC NiCr and APS NiCoCrAlY offered the most and the least corrosion resistance, respectively. DFT calculation was performed to validate the thermodynamic stability of each oxide and identify the precise oxide formation. © 2021 Elsevier LtdItem Understanding the initial stage oxidation and microstructural evolution of detonation sprayed NiCoCrAlY bond coat using in-situ high-temperature X-ray diffraction(Elsevier Ltd, 2022) Kala, V.; Santhy, K.; Govindarajan, G.; Rajasekaran, B.This study focuses on in-situ high-temperature X-ray diffraction (HT-XRD) investigation to understand the oxidation, microstructural evolution, and recrystallization of the NiCoCrAlY bond coat. HT-XRD analysis was carried out at a constant temperature of 1423 K for 3 h in a 10−4 Pa vacuum. The formation of α-Al2O3, Cr2O3, Co3O4, and NiCr2O4 oxides and microstructural evolution, such as homogenization of phases (γ and β) during in-situ HT-XRD, has been analyzed with FE-SEM (EDS), Density Functional Theory calculations and Rietveld refinement's support. β-(Ni,Co)Al phase depletion (by 77%) was found at 1423 K during the HT-XRD; however, no TGO formation was noticed. © 2022 Elsevier LtdItem Thermal expansion of Crofer 22 APU steel used for SOFC interconnect using in-situ high temperature X-ray diffraction(Elsevier Ltd, 2023) Manjunath, N.; Santhy, K.; Rajasekaran, B.Crofer 22 APU is ferritic stainless steel extensively used as metallic interconnect material in Solid Oxide Fuel Cell (SOFC) applications. The interconnects are exposed to both oxidizing and reducing atmospheres at high temperatures. As SOFCs are operated above 700 °C, understanding the thermal expansion behavior of the interconnect material with other components (anode, cathode, electrolyte) of the fuel cells is essential. Metallic interconnects should have a matchable thermal expansion to other ceramic materials such as anode, cathode, and solid electrolyte used in SOFCs. The present study evaluates the thermal expansion of Crofer 22 APU steel from 25 to 950 °C in a controlled atmosphere (10-4 mbar pressure) using in-situ high-temperature X-ray diffraction (XRD). The XRD patterns were analyzed using the ‘High Score Plus Software’ attached to the system, and the phases were identified using the standard Crystallographic Open Database (COD). The coefficient of thermal expansion (CTE) was determined based on the change in lattice parameter/peak shift to a lower 2θ value as a function of temperature. The normal XRD data showed no oxide formation on the Crofer steel after heating until 950 °C in in-situ high-temperature conditions. The peak shift to the lower 2θ degree observed in the XRD data was due to the relaxation of residual stress upon heating. The isothermal section and phase fraction of Crofer 22 APU alloys are analyzed with the help of thermo-calc with the iron database of TCFE7. The Fe-rich bcc phase was found to be stable up to high temperatures. The major phases are the Fe-rich bcc, Cr-rich BCC, and sigma phase in the solid state. The minor phases are FCC, M3P, TiC, Laves, and Ti4C2S2. The calculated lattice parameter of the Fe-rich BCC phase matches with the experimentally calculated data using XRD. The thermal expansion of Crofer 22 APU was found to be 11.9181 × 10-6 /°C at 950 °C. The in-situ high-temperature XRD technique has been an effective methodology for determining the thermal expansion behavior of the as-received Crofer steel. © 2023Item Thermal expansion and microstructure evolution of atmospheric plasma sprayed NiCrAlY bond coat using in-situ high temperature X-ray diffraction(Elsevier B.V., 2023) Abhijith Vijay, V.; Santhy, K.; Govindarajan, G.; Rajasekaran, B.The paper focuses on in-situ high-temperature X-ray diffraction (HT-XRD) study on atmospheric plasma sprayed NiCrAlY coating. The sample was in-situ heated from 25 °C to 1150 °C in a controlled atmosphere (3 × 10−4 bar), and the corresponding X-ray diffraction patterns for different temperatures were recorded. The effect of temperature on crystallite size, lattice strain, and coefficient of linear thermal expansion was studied. Major phases identified are γ-Ni, γ’-Ni3Al, β-NiAl, and α-Cr. The formation of stable α-Al2O3 and spinel was found above 1000 °C. The transformation of β to γ’ and γ phase was observed as a function of temperature. The equilibrium phases and the thermal expansion of disordered Face Centered Cubic (FCC) and Body Centered Cubic (BCC) phases were predicted and supported by Thermo-Calc prediction for the stable temperature range. Results showed that the non-equilibrium microstructure produced by thermal spray process did not alter the thermal expansion behaviour. In-situ treatment resulted in microstructure and elemental homogenization. The thermal expansion and mechanism of phase evolution were discussed. © 2022 Elsevier B.V.Item The effect of strain induced phase transformation on the thermal expansion compatibility of plasma sprayed spinel coating on SOFC metallic interconnect – A study using in-situ high temperature X-ray diffraction(Elsevier Ltd, 2023) N, M.; Santhy, K.; Rajasekaran, R.A new and novel approach has been adopted in this study to evaluate thermal mismatch induced by thermal expansion in substrate-coating contact pairs using in-situ high-temperature X-ray diffraction (HT-XRD). Atmospheric plasma sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) coating on Crofer 22 APU steel interconnect was investigated. In-situ HT-XRD was performed individually for substrate and coating from 25 °C to 900 °C. Diffraction data were recorded for different temperatures to obtain lattice parameters and strain as a function of temperature. The coefficient of thermal expansion (CTE) of MCF coating was slightly higher than steel substrate and showed no significant thermal expansion mismatch till 700 °C. The increasing lattice strain measured by Scherrer and Williamson-Hall methods indicates strain-induced phase transformation of MCF coating with temperature, supporting the phase transformation-induced self-healing phenomenon of MCF coating. The merit of in-situ HT-XRD as a tool for optimizing operating temperature and measuring thermal mismatch of solid oxide fuel cell (SOFC) stacks has been discussed. © 2023 Hydrogen Energy Publications LLCItem In Situ High-Temperature X-ray Diffraction Study on Atmospheric Plasma and Detonation Sprayed Ni-5 wt.%Al Coatings(Springer, 2023) Purushotham, N.; Santhy, K.; Suresh Babu, P.; Govindarajan, G.; Rajasekaran, R.In situ high-temperature x-ray diffraction (HT-XRD) was used in the present study to assess the coefficient of thermal expansion and recrystallization of Ni-5 wt.%Al coatings. Atmospheric plasma spray (APS) and detonation spray (DSC) techniques were used to deposit Ni-5 wt.%Al coatings on IN718 substrates. The coatings were examined using HT-XRD at ambient conditions (25 °C) up to high temperatures (1150 °C) under a vacuum pressure of around 10−4 mbar. Coefficients of thermal expansion (CTE), crystallite size (D) and lattice strain (ε) were determined by the Scherer and Williamson-Hall (W-H) method with a uniform strain model (UDM) using x-ray peak profile analysis (XPPA). The microstructure of the Ni-5 wt.%Al coatings was analyzed by field emission scanning electron microscopy (FESEM). No phase changes were observed in either coating, as the Ni-5 wt.%Al coatings consisted mainly of γ-Ni crystals with a face-centered cube (FCC) phase in both coating techniques. Lattice parameters as a function of temperature were used to calculate linear thermal expansion coefficients. The linear thermal expansion of Ni-5 wt.%Al coatings deposited by both thermal spray methods was discussed on the basis of process-induced microstructures. © 2023, ASM International.Item Understanding early-stage oxidation mechanism of Crofer 22 APU solid oxide fuel cell steel interconnect using glow discharge optical emission spectroscopy and grazing incidence X-ray diffraction(Elsevier B.V., 2023) Naik, M.; Santhy, K.; Rajasekaran, R.Crofer 22 APU is used as metallic interconnects in stacking solid oxide fuel cells (SOFCs) operated at elevated temperatures (above 700 °C) owing to their excellent oxidation resistance. Understanding the protective layer formation in the initial oxidation stage would be useful for optimizing and designing protective coatings for extended life. Initial stage oxidation of Crofer 22 APU steel using surface analytical tools such as glow discharge optical emission spectroscopy (GD-OES), grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM) are studied in the paper. An oxidation test on as-received Crofer 22 APU steel was carried out in a controlled atmosphere (0.01 Pa) in an in-situ high-temperature X-ray diffraction (XRD) stage at 950 °C. Normal XRD showed no indication of oxidation, while GIXRD revealed the formation of two-layer oxides: Top layer spinel MnCr2O4 and fine-grained inner layer Cr2O3, which was confirmed and quantified by GD-OES depth profiling. The Cr2O3 formed initially led to the formation of MnCr2O4 spinel during the initial stage. The rapid diffusion of Mn through the fine-grained Cr2O3 layer results in an increased growth rate of MnCr2O4 spinel on the top of the fine-grained Cr2O3 layer. © 2023 Elsevier B.V.Item Effect of microwave treatment on structural characteristics and energy bandgap of electrochemically synthesized hydrated tungsten oxide quantum dots(Elsevier Ltd, 2024) Salot, M.; Santhy, K.; Pramanick, A.K.; Rajasekaran, B.; Awasthi, G.; Singh, S.G.; Chaudhury, S.K.Quantum Dots (QDs) of hydrated tungsten oxide were synthesized via electrolysis using sintered tungsten carbide-6 wt% cobalt (WC–6Co) scrap as anode, Ti plate as cathode, and sulfuric acid as electrolyte at room temperature. The as-synthesized powder was characterized using X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Electron paramagnetic resonance spectroscopy (EPR), and Ultraviolet–visible spectroscopy (UV–Vis). The XRD analysis confirmed the formation of orthorhombic hydrated tungsten oxide (WO3.H2O) QDs via electrochemical oxidation of WC. As-synthesized WO3.H2O QDs were thermally-treated using microwave radiation and conventional furnace at 150 °C for 8 min and 45 min, respectively. Thermal treatment of as-synthesized QDs produced partially dehydrated powder consisting of both orthorhombic WO3.H2O and cubic WO3.H0.5 crystal structures. The TEM analysis showed that the average particle size of QDs was 7.60 nm. Further, an increase in lattice strain was observed on microwave treatment owing to the non-equilibrium phase transformation (i.e., rapid heating) from orthorhombic to cubic crystal structure resulting in the generation of oxygen vacancies. The increase in oxygen vacancy concentrations in QDs on microwave heating was confirmed by XPS, FTIR, EPR, and Raman spectroscopy. The energy bandgaps of as-synthesized and thermally-treated QDs were in the range of 2.4307–2.4979 eV. The relatively low energy bandgap of QDs is attributed to the change in crystal structure and increase in the oxygen vacancy concentration. An improved CO gas sensing characteristics of microwave-treated QDs was noted. © 2024 Elsevier Ltd and Techna Group S.r.l.Item A study on the kinetics and structure of tungsten oxide nanopowder synthesized by an electrochemical oxidation process(John Wiley and Sons Inc, 2025) Salot, M.; Santhy, K.; Mandal, D.; Pramanick, A.K.; Rajasekaran, B.; Avasthi, G.; Chaudhury, S.K.Tungsten oxide possesses unique properties owing to its multiple oxidation states. They are produced by several techniques with each having their advantages and limitations. In this study, the hydrated tungsten oxide nanopowders with varied morphology were synthesized by electrochemical oxidation of WC-6Co scrap at room temperature. This process is efficient and requires low capital investment. The effect of processing parameters, namely voltage, molarity, temperature, and electrolyte stirring on yield, structure, morphology, and energy bandgap is studied. The X-ray diffraction (XRD) analysis showed that at low voltage and low molarity monoclinic WO3.2H2O nanoparticles are synthesized. In contrast, at high molarity and high voltage, orthorhombic WO3.H2O nanoparticles are synthesized. Further, the size of crystal decreases with the increase in voltage during electrochemical oxidation of WC-6Co pellet. The in-situ XRD analysis showed progressive transformation of as-synthesized nanopowder from orthorhombic to cubic crystal structure. Thermal treatments using microwave radiation and muffle furnace resulted in partial phase transformation of hydrated tungsten oxide to cubic WO3.H0.5 phase. The scanning electron microscopy and transmission electron microscopy analyses confirmed the formation of nanoplates, nanorods, and quantum dots depending on the processing parameters. The ultraviolet-visible spectroscopy showed a relatively lower energy bandgap of as-synthesized tungsten oxide nanopowder. © 2025 The American Ceramic Society.Item On the merit of solute segregation and low angle grain boundary for thermal stability and thermal expansion of cold-sprayed CuCrZr(Elsevier Ltd, 2025) Abhijith Vijay; Sreerag, M.P.; Varalakshmi, S.; Santhy, K.; Singh, R.; Kondás, J.; Makineni, S.K.; Rajasekaran, B.The precipitation hardenable CuCrZr alloy is a potential alternative to copper for inner liners in rocket thrust engines. Cold spray manufacturing has been seen as a promising processing route to manufacture bulk additive structure of CuCrZr. This work reveals that the cold-sprayed as-deposited Cu-Cr-Zr alloy, in its inherent non-equilibrium state. It is highly stable up to 950 °C and exhibits lower thermal expansion than the equilibrium Cu-Cr-Zr alloy, deduced using HT-XRD and Thermo-Calc. Atomic-scale compositional and diffraction analysis using Atom Probe Tomography (APT) and Electron Backscatter Diffraction (EBSD) support the Zener pinning effect of Cr segregation near the grain boundaries, along with a large fraction of low-angle grain boundaries (LAGBs), that contribute to the high thermal stability and controlled thermal expansion of the deposit. Cold spray deposition naturally yields microstructural features that are conducive to high thermal stability and controlled thermal expansion, features which are comparable to the self-organized microstructures observed in segregation engineering (SE). © 2025 Elsevier B.V.