Faculty Publications
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Item Hot corrosion behaviour of plasma and d-gun sprayed coatings on t91 steel used in boiler applications(Institute of Physics Publishing helen.craven@iop.org, 2020) Sundaresan, C.; Rajasekaran, B.; Govindarajan, G.; Rao, D.S.Material degradation by salt induced hot corrosion is a serious threat to Coal fired Boiler components. Corrosion resistant thermal spray coatings is a proven solution for this problem. Thiswork compares the hot corrosion behaviour of a NiCoCrAlY coating on T91 boiler steel deposited using two different thermal spray methods namely Atmospheric Plasma Spray (APS) and Detonation Gun (D-Gun) spray. The coated samples were exposed to 650C for 100 cycles (1 cycle being 1 hour holding plus 20 minutes cooling in air) in a thermal cycling furnace under a mixed deposit containing Na2SO4, K2SO4 and Fe2O3 to simulate the real time coal ash environment. The exposed coatings were examined using FESEM/EDS and XRD to analyze the extent and nature of corrosion attack. The results indicate that the D-Gun sprayed coatings provided superior corrosion resistance over their APS sprayed counterparts. NiO and Spinel oxide NiCr2O4 which offers corrosion resistance were formed in both the coatingswith the APS sprayed coatingbeing rich in the fast growing NiO which affected its corrosion performance. The enhanced corrosion resistance of the D-Gun sprayed coatings was attributed to its dense microstructure with minimal porosity and good intersplat bonding which minimized corrosion ingress. © Published under licence by IOP Publishing Ltd.Item Sliding wear behaviour of Ni-5 %Al coating deposited by detonation spray on IN718(Elsevier Ltd, 2022) Purushotham, N.; Rajasekaran, B.; Parthasarathi, N.L.; Praveen, K.; Govindarajan, G.Ni-5 %Al metallic coating was deposited on Nickel-based superalloy (IN718) specimens using the detonation spray coating (DSC) method. Detonation spraying yielded coating with extreme chemical bond strength, hardness, and less porosity. The microstructure, microhardness, and room temperature pin-on-disc sliding wear behavior of the Ni-5 %Al coating and the as-received IN718 superalloy were evaluated. Sliding wear tests were done at room temperature (25 °C), under different loading conditions (6 N and 10 N), using an alumina (Al2O3) ball-on-disk tribometer and friction coefficients were measured. The study of worn surfaces conducted by SEM indicated that both Ni-5 %Al coating and the substrate suffered significant abrasive wear with occasional adhesion and spalling of the coating. The 3D topography of the wear track was examined by a 3D non-contact profilometer, which enabled the quantification of the wear. The friction coefficient values of the tests and the wear in terms of mass loss were in good correlation. © 2022Item 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 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 Effect of thermal expansion on the high temperature wear resistance of Ni-20%Cr detonation spray coating on IN718 substrate(Elsevier B.V., 2023) Purushotham, N.; Parthasarathi, N.L.; Babu, P.S.; Govindarajan, G.; Rajasekaran, B.The temperature-dependent materials properties on the dry sliding wear resistance of the detonation sprayed Ni-20%Cr coating have been studied. In-situ high-temperature X-ray diffraction (HT-XRD) was used to investigate high-temperature properties such as stress relieving, recrystallization, and thermal expansion. The dry sliding wear test was performed by using a ball-on-disc tribometer by sliding velocities (0.1 m/s), varying loads (6 N and 10 N), and temperatures (25 °C and 850 °C) against alumina (Al2O3) ball. The phase evolution, thermal expansion, crystallite size, and lattice strain were determined by the Williamson-Hall method. Field emission scanning electron microscopy and a non-contact optical profilometer was used to characterize the wear scar and calculate the wear rate. The wear test results demonstrated that the as-deposited coatings coefficient of friction (CoF) and wear rate (ω) continuously decreased as the temperature increased. The primary wear mechanism changed from abrasive and surface fatigue to adhesive and oxidative wear. The impact of stress relieving, recrystallization, and forming a composite tribolayer (Cr2O3, NiO) at elevated temperatures reduced the friction and enhanced the wear resistance. The effect of stress relieving, recrystallization, thermal expansion, and oxidation on the wear resistance of the coating has been discussed with a suitable mechanism. © 2023 Elsevier B.V.Item 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 The Effect of Detonation Frequency on the Linear Reciprocating Wear Behavior of Detonation Sprayed Ni-20%Cr Coatings at Elevated Temperatures(Springer, 2025) Prasad, R.; Purushotham, N.; Preetham Kumar, G.V.; Babu, P.S.; Govindarajan, G.; Rajasekaran, B.The study explores the impact of detonation frequency (3 and 6 Hz) on the temperature-dependent linear reciprocating wear behavior of Ni-20%Cr coatings deposited by detonation spraying on a nickel-based superalloy (IN718). Dry sliding experiments were carried out at both ambient (25 °C) and high (420 °C) temperatures, using an alumina (Al2O3) ball as the counter material and different loads (5, 10, and 20 N). HV0.2 microhardness indentations were used to test material hardness variations attributed to heat exposure. X-ray diffraction (XRD), Raman spectroscopy, and field emission scanning electron microscopy with energy-dispersive spectroscopy (FESEM with EDS) were used to investigate the wear characteristics and mechanisms. Furthermore, surface roughness and profiles of worn surfaces (including track depth, breadth, and wear volume) enabled the calculation of wear rates using confocal optical 3D profilometry. The results showed the 6 Hz Ni-20%Cr coating showed better wear resistance than the 3 Hz coating. However, a higher wear rate and low friction coefficient at 420 °C were observed due to partial oxide particles, which were insufficient to restrict direct ball-to-metal contact. The research delves into wear maps, tribolayer formation, wear mechanisms, and sub-mechanisms. © ASM International 2024.