Faculty Publications
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Publications by NITK Faculty
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Item Influence of carbon nanotube reinforcement on the heat transfer coefficient, microstructure, and mechanical properties of a die cast Al-7Si-0.35Mg alloy(Elsevier Ltd, 2021) Usef, A.P.; Bhajantri, V.; Kannoth, V.; Jambagi, S.C.Al-7Si-0.35Mg or A356 alloy is most widely used in aircraft and automobile industries owing to its high strength to weight ratio. This alloy has been reinforced with a 1 wt% carbon nanotube (CNT) to improve its properties in this investigation. First, A356/1 wt% CNT powders were ball milled in the presence of ethanol and subsequently consolidated using gravity die casting. Ball milling was effective in achieving homogeneous dispersion of CNT. The microstructural study revealed the segregation of the Al4C3 phase at the grain boundary. This mechanism is known as grain boundary precipitation. Also, the grain size has decreased by ~44%. Next, the casting-die interfacial heat transfer coefficient (IHTC) has been evaluated using Beck's inverse heat transfer algorithm. With the reinforcement, the IHTC has increased by ~2.5%, which indicates the rise in heat transfer rate during solidification. Then, the experimental and theoretical tensile properties of A356 were correlated using simulation software. The experimental results showed the synergistic effect of grain size, Al4C3, and IHTC improving yield strength by ~19.8%, ultimate tensile strength by ~14.13%, elongation by 7%, and hardness ~22%. Therefore, a meagre 1 wt% CNT has improved the heat transfer rate of the melt as indicated by IHTC values. This effect was further corroborated by evaluating the thermal conductivity of the sample. The thermal conductivity has improved by 10% that resulted in finer grain size of the sample. Therefore, such reinforced alloys are expected to display higher strength demanded in various industrial applications. © 2021Item An Investigation into the Relative Efficacy of High-Velocity Air-Fuel-Sprayed Hydroxyapatite Implants Based on the Crystallinity Index, Residual Stress, Wear, and In-Flight Powder Particle Behavior(American Chemical Society, 2023) Jagadeeshanayaka, N.; Kele, S.N.; Jambagi, S.C.Due to its resemblance to the bone, hydroxyapatite (HA) has been widely used for bioactive surface modification of orthopedic implants. However, it undergoes significant thermal decomposition and phase transformations at a high operating temperature, leading to premature implant failure. This investigation uses high-velocity air-fuel (HVAF) spray, an emerging low-temperature thermal spray technique, to deposit HA over the Ti-6Al-4V substrate. Coating characteristics, such as the crystallinity index and phase analysis, were measured using X-ray diffraction, Raman analysis, and Fourier transform infrared spectroscopy, residual stress using the sin2ψ method, and tribological performance by a fretting wear test. The coating retained an over 90% crystallinity index, a crystallite size of 41.04 nm, a compressive residual stress of −229 ± 34.5 MPa, and a wear rate of 1.532 × 10-3 mm3 N-1 m-1. Computational in-flight particle traits of HA particles (5 to 60 μm) were analyzed using computational fluid dynamics; it showed that 90% of particles were deposited at a 700 to 1000 m/s velocity and a 900 to 1450 K temperature with a 2.1 ms mean residence time. In-flight particle oxidation was minimized, and particle impact deformation was maximized, which caused severe plastic deformation, forming crystalline, compressive residual stressed coatings. The thermal decomposition model of low-temperature HVAF-sprayed HA particles helped to understand the implants’ crystallinity index, residual stress, and tribological characteristics. Hence, this experimental and computational analysis shows that the HVAF process can be a promising candidate for biomedical applications for having strong and durable implants. © 2023 American Chemical Society.Item Thermal spray processes influencing surface chemistry and in-vitro hemocompatibility of hydroxyapatite-based orthopedic implants(Elsevier Ltd, 2024) Shankar, D.; Jayaganesh, K.; Gowda, N.; Lakshmi, K.S.; Jayanthi, K.J.; Jambagi, S.C.Orthopedic implants made from titanium are a popular choice in the medical field because of their remarkable strength-to-weight ratio. Nevertheless, they may not interact well with human blood, resulting in thrombosis and hemolysis. In fact, non-hemocompatibility is believed to be responsible for about 31 % of medical device failures in the US alone, requiring painful and expensive revision surgery. To address this issue, bioactive hydroxyapatite coatings are applied to Ti-6Al-4V implants using thermal spray techniques. However, the temperature used during thermal processing impacts the coating's surface properties, affecting the mechanical and biological properties. Furthermore, the effectiveness of HA coatings on titanium for orthopedic applications has not been validated by biocompatibility tests, particularly hemocompatibility. In this study, we aimed to investigate the relative efficacy of three thermal spray processes of different temperature ranges: Atmospheric plasma spray (APS) (high temperature), Flame spray (FS) (moderate temperature), and High-Velocity Oxy-Fuel spray (HVOF) (low temperature), and study their impact on coating's surface properties, affecting blood components and implant's strength. The crystallinity of the HA coating increased by 32 % with a decrease in the operating temperature (APS < FS < HVOF). HVOF coating exhibited a ~ 34 % and ~ 120 % improvement in adhesion strength and ~ 31 % and 59 % increment in hardness compared to APS and FS coating, respectively, attributed to its low porosity, low coating thickness (~55 μm), and high degree of crystallinity. The HVOF coating showcased a significant increase in non-hemolytic behavior, with hemolysis rates ~8 and ~ 11 times lower than APS and FS coatings, respectively, owing to its smooth texture and high degree of crystallinity (p < 0.05). Furthermore, the HVOF coating exhibited minimal blood clotting based on the whole blood clotting assay, again confirmed by PT and aPTT assays showing delayed clotting time, indicating its non-thrombogenic behavior. The number of platelets adhered to the three coatings showed no significant difference compared to Ti-6Al-4V. APS and FS coatings showed low platelet activation, unlike HVOF coating and titanium, which revealed round platelets, similar to the negative control. Neither titanium nor HA coatings exhibited antibacterial properties, which may be due to their high affinity for organic substances, which promotes bacterial adhesion and replication. Among the three thermal processes, HVOF coating displayed good apatite growth, non-hemolytic, and non-thrombogenicity with no platelet activation owing to its low processing temperature, high degree of crystallinity (89.7 %), hydrophilicity, smooth (~4 μm) and dense (~97 %) microstructural properties. The results demonstrated that the HVOF-HA coating presented in this work meets the hemocompatible requirements and shows promise for prospective application as an orthopedic implant. Furthermore, this study has the potential to significantly reduce the use of animals in in-vivo research and improve their welfare while also cutting costs. © 2024 Elsevier B.V.Item Comparative analysis of flipped and overlapped microwave sintered plus friction stir processed in-situ Al-Cu composites(Springer Nature, 2024) Vinayak, V.R.; Bajakke, P.A.; Jambagi, S.C.; Bhajantri, V.; Deshpande, A.S.The in-situ Al-Cu composites were fabricated with Cu content more than the solubility limit (5.65%) and the critical composition of an Al-Cu alloy (4.6%). A powder metallurgy route with state-of-the-art microwave sintering and friction stir process with overlapped and flipped conditions was attempted. The overlapped friction stir resulted in a minimum area of nugget region, unidirectional material mixing, high heat input in successive passes, and brittleness in the material. This accounted for the material to exhibit high strength with low ductility. Whereas flipped friction stir leads to a maximum area of stir zone, bidirectional material mixing, and similar thermal cycles in individual passes and avoids excessive heating. This facilitated the material to possess maximum strength by retaining ductility. The newly developed in-situ Al-Cu composite material (with Cu wt% in ranges of 8–12) possesses equivalent strength, ductility, electrical conductivity, and rate of corrosion compared to copper. Notably, these attributes, combined with its cost-effectiveness, position this material as a promising alternative to copper conductors in electrical applications. From the present investigation, it is strongly recommended to choose a flipped friction stir for better properties. © Qatar University and Springer Nature Switzerland AG 2024.Item Numerical and Experimental Investigation of Thermal Barrier Effects of CNT-Reinforced Fly Ash/Alumina Coatings in Diesel Engine Pistons(American Chemical Society, 2025) Chavana, N.; Sarkar, B.; Jambagi, S.C.Fly ash (FA), an industrial byproduct from coal combustion, presents significant disposal challenges, especially in developing nations. Given its mineralogical properties, FA shows potential in thermal spray coatings. This study evaluates FA-based coatings for pistons to improve thermal management in internal combustion engines through numerical simulations, analyzing their effects on the temperature distribution, thermal stress, and combustion efficiency. FA coatings were also applied to marine-grade steel with additives (50 wt % Al2O3 and 0-2 wt % CNT) to assess high-temperature performance. Microstructural analysis revealed that 2 wt % CNT-reinforced (2CAF) coatings showed agglomeration, reducing microhardness by ?9.27% compared to 1 wt % CNT-reinforced (1CAF) coatings. The XRD analysis of 1CAF indicated ?56.51% transformation of corundum to ?-alumina, lowering thermal conductivity by ?15.40% compared to alumina/FA (AF) coatings, while 2CAF coatings showed increased conductivity due to CNT inhomogeneity. For piston applications, simulations showed an ?24.59% rise in maximum surface temperature, from 241.39 to 300.76 °C, and an ?62.06% reduction in heat flux, indicating enhanced durability and reduced cold-start emissions. Thermal cycling demonstrated that 1CAF coatings outlasted AF and 2CAF, suggesting FA-based TBCs as sustainable and economical options for enhanced engine performance and waste valorization. © 2025 American Chemical Society.