Faculty Publications
Permanent URI for this communityhttps://idr.nitk.ac.in/handle/123456789/18736
Publications by NITK Faculty
Browse
10 results
Search Results
Item Tailoring surface characteristics of bioabsorbable Mg-Zn-Dy alloy using friction stir processing for improved wettability and degradation behavior(Elsevier Editora Ltda, 2021) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.; Srinivasan, A.; Kailas, S.V.Magnesium (Mg) and its alloys are currently under consideration for use as temporary implants. However, early degradation and maintaining mechanical integrity is a significant concern. Surface modification techniques are used to improve mechanical and corrosion properties of Mg based alloys. In the present study, friction stir processing (FSP) was used to tailor the surface characteristics of Mg-1Zn-2Dy (wt.%) alloy for temporary implant applications. The FSPed alloy was characterized using EBSD to understand the influence of FSP on crystallographic texture, grain size and grain boundaries and thereby their effect on corrosion, wettability and hardness. Results showed that the grain size of stir zone (SZ) was refined to less than 3 ?m, as a result of dynamic recrystallization (DRX) during FSP and the FSPed alloy exhibited better wettability than as-cast alloy. An increase in the hardness (11.7%) and elastic modulus (6.84%) of FSPed alloy were also observed. Electrochemical corrosion and weight loss methods were conducted in Dulbecco's Modified Eagle's Medium (DMEM) with, 10% Fetal Bovine Serum (FBS) physiological solution. The lower degradation rate (0.72 mm/yr) of FSPed alloy has been attributed to the fine grains and evenly distributed secondary phase particles. Further, the influence of grain boundary characteristics and crystallographic texture on the corrosion behavior have been investigated. © 2021 The Author(s).Item Comparative investigation of coating and friction stir processing on Mg-Zn-Dy alloy for improving antibacterial, bioactive and corrosion behaviour(Elsevier B.V., 2021) Rokkala, U.; Jana, A.; Bontha, S.; Ramesh, M.R.; Balla, V.K.Magnesium based alloys are well-known materials for temporary implant applications. However, failures due to early degradation and bacterial infection are limiting their applications. To overcome these problems, in the present work a Mg-Zn-Dy alloy based composite surface was prepared using coating and friction stir processing (FSP) techniques. Herein, hydroxyapatite (HA) and silver (Ag) particles were deposited on Mg-Zn-Dy alloy to obtain HA and Ag coated surface (C-HAg). Later, FSP was carried out on the C-HAg surface to develop a Mg-Zn-Dy alloy based composite surface (F-HAg). Field emission scanning electron microscope (FESEM) and energy dispersive X-ray analysis (EDS) confirm the mixing of HA and Ag particles with the Mg-Zn-Dy substrate. Antibacterial studies reveal that both C-HAg and F-HAg samples inhibit Escherichia coli and Staphylococcus aureus bacteria. In vitro cytotoxicity study indicates that the both samples are non-toxic in nature. Results of in vitro corrosion study reveal a significant reduction (72%) in corrosion rate of F-HAg sample when compared to C-HAg sample. The F-HAg samples showed simultaneous improvement in corrosion resistance and antibacterial properties with good biocompatibility. The results of this study indicate that the developed composite surface is a promising material for antibacterial and biodegradable implant applications. © 2021 Elsevier B.V.Item Influence of friction stir processing on microstructure, mechanical properties and corrosion behaviour of Mg-Zn-Dy alloy(Springer, 2023) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.In the present study, friction stir processing (FSP) was carried out on as-cast Mg-Zn-Dy alloy to tailor grain size and texture which alter the mechanical properties and corrosion behaviour. The grain size of the as-cast alloy was reduced from 60 ± 2 µm to 3 ± 0.1 µm after FSP due to dynamic recrystallization. The effect of grain size, crystallographic orientation and fine precipitates on mechanical properties were investigated using field emission scanning electron microscope (FESEM) and electron back scattered diffraction (EBSD). The ultimate tensile strength, yield strength, % elongation and hardness of FSPed alloy improved by 55%, 60%, 53% and 46% when compared to as-cast alloy. The FSPed Mg-Zn-Dy alloy exhibited a 79% decrease in corrosion rate when compared to as-cast alloy which can be attributed to grain refinement, uniform distribution of secondary precipitates and strong basal texture. The surface of FSPed sample after immersion corrosion exhibited calcium phosphate rich minerals which help in apatite formation on the sample surface. Cytotoxicity studies using MTT assay revealed more than 80% cell viability for both as-cast and FSPed alloy illustrating non-toxic nature of both the samples. The results of this study indicate that FSPed Mg-Zn-Dy alloy is a potential material for biodegradable implants due to its high strength, corrosion resistance and biocompatibility. Graphical Abstract: [Figure not available: see fulltext.]. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item Comparative Study of Plasma Spray and Friction Stir Processing on Wear Properties of Mg-Zn-Dy Alloy(Springer, 2024) Rokkala, U.; Gudala, S.; Ramesh, M.R.Mg alloys are becoming increasingly popular as lightweight materials in recent years. Wear resistance, on the other hand, is a severe issue with Mg alloys. Plasma spray and friction stir processing (FSP) are being investigated in this context for developing composite surfaces with improved surface characteristics. Scanning electron microscopy (SEM), energy dispersive spectroscopy, and x-ray diffraction are used to examine the microstructural changes and phase changes of all materials. After FSP, SEM analysis indicated that the coated particles were equally dispersed throughout the Mg matrix. The composite samples had the lowest wear rate as compared to other samples, according to the wear tests. In comparison to AC, the F-1 sample surface has much higher wear resistance. As a consequence, the findings of this investigation for the F-1 sample appear encouraging for biological wear resistant applications. © ASM International 2023.Item Effect of dilution on the microstructure and high-temperature wear resistance of self-lubricating nickel alloy claddings(SAGE Publications Inc., 2024) Gudala, S.; Rokkala, U.; Rao Medabalimi, S.; M.r, R.; Konovalov, K.S.In this study, the impact of substrate dilution on the microstructure and tribological properties of tungsten inert gas (TIG)-deposited self-lubricating claddings was investigated. The dilution of Ti content on cladding increased as the TIG current increased, and the microhardness of the cladding decreased. The content of intermetallic phases such as TiNi and TiC increased with the increase in TIG current. The tribological studies revealed that coating dilution at higher TIG currents has prominent effects on wear behaviour at elevated temperatures. In both clads, the percentage decrease in hardness from higher TIG current to lower TIG current was noted as 31%. The higher dilution of Ti content in the cladding was found to be beneficial in tribological studies performed, especially at higher temperatures (≥400). Also, solid lubricants such as MoS2 and BaF2 encapsulation in the nickel alloy were found to be beneficial at both low and high temperatures. © The Author(s) 2024.Item Multi-step fabrication of bioactive Mg–Zn–Dy–AlO3/HA composites: exploring the synergistic effects of plasma spray and friction stir processing(Springer, 2024) Rokkala, U.; Bontha, S.; Ramesh, M.R.; Balla, V.K.Magnesium (Mg) alloys are gaining more attention in recent times as biodegradable materials. However, two major problems with Mg alloy implants are bacterial infections and poor corrosion resistance. In this context, a composite surface (Mg–Zn–Dy–Al2O3/HA) is developed using surface modification techniques. First, Al2O3 + HA composite powder is coated on Mg–Zn–Dy alloy to attain coated surface (C-AHa). Next, the C-AHa surface is subjected to friction stir processing to develop composite surface (F-AHa). Microstructural characterization reveals that, the Al2O3 + HA particles were distributed evenly into the Mg–Zn–Dy substrate. Antimicrobial activities against Escherichia coli and Staphylococcus aureus reveal low adhesion of bacteria on the F-AHa sample surface due to low surface energy (37.83 ± 0.22 mN/m) and low surface roughness (0.36 ± 0.1 µm). Further, the cytotoxicity tests confirm that the F-AHa sample shows significant improvement in cell viability (98%) after 7 days and non-toxic against the mouse osteoblast cells. In Vitro corrosion study observations demonstrate that the corrosion rate for the F-AHa sample is decreased by 72% compared to the C-AHa sample. Thus, the results of this study for the fabricated composites are promising for antimicrobial, biocompatible and bioabsorbable temporary implants. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.Item Studies on high temperature erosion behavior of HVOF-sprayed (Cr?C?-NiCr)Si and WC-Co/NiCrAlY composite coatings(Elsevier Ltd, 2025) Medabalimi, S.; Hebbale, A.M.; Gudala, S.; Rokkala, U.; Ramesh, M.R.The present study investigates the high temperature erosion behavior of HVOF sprayed composite coatings on T11 steel substrates by studying (Cr?C?-NiCr)Si and WC-Co/NiCrAlY coatings. Phase composition, cross sectional microstructure, mechanical properties, and erosion resistance were analyzed by XRD, EDS, SEM and three-dimensional optical profilography. The results demonstrate that the WC-Co/NiCrAlY coating has higher erosion resistance and oxidation stability for all temperatures and impact angles tested. Its enhanced performance in high temperature and erosive conditions is attributable to the formation of stable protective oxides such as Al?O? and Cr?O? and intermetallic phases such as Ni?Al and Cr?C?. The NiCrAlY matrix prevents significant decarburization of WC particles, and hence phase stability and oxidation resistance. The (Cr?C?-NiCr)Si coating has higher microhardness due to silicide phases, but is more vulnerable to direct impacts and inferior oxidation resistance. The phase transformations for both coatings are favorable at elevated temperatures which enhances erosion resistance. The WC-Co/NiCrAlY coating is smooth and shallower in erosion craters and is perfectly suited for harsh environments demanding high toughness, impact resistance and oxidation stability. For applications in which high hardness is needed in less severe conditions, the (Cr?C?-NiCr)Si coating is more suitable. © 2024Item Microstructure and elevated temperature wear behavior of HVOF-sprayed SS304L stainless-steel coating(Springer Nature, 2025) Medabalimi, S.; Gudala, S.; Rokkala, U.; Hebbale, A.M.; Ramesh, M.R.The paper aims to investigate the performance of the SS304L stainless steel coating on wear properties by varying load, temperature and velocity. Stainless-steel coatings were fabricated by high-velocity oxy-fuel spraying (HVOF) on superfer800. Surface morphology, elemental distribution and phase analysis were expressed by SEM, EDS, and XRD, respectively. The porosity, average surface roughness, and average microhardness of HVOF stainless steel coating are 2%, 7 µm, and 1167 ± 54 HV0.3, respectively. The wear rate of stainless-steel coating is 0.5 × 10?3 mm3/m at 600 °C with 20 N loads, which is about 16 times lower than the substrate. Adhesion and abrasion are the main wear mechanisms of HVOF stainless steel coatings during high-temperature tests. Comparing to superfer800 substrate, stainless steel coatings showed superior wear resistance at all the loads, temperature and velocities. © The Author(s) 2025.Item Microstructural Evolution of Mg-Zn-Gd Alloy Using Equal Channel Angular Pressing to Enhance Mechanical and Corrosion Properties(Springer, 2025) Rokkala, U.; Patil, A.; Bontha, S.; Ramesh, M.R.; Balla, V.K.; Srinivasan, A.Equal channel angular pressing (ECAP) was used on the Mg-Zn-Gd alloy in this study to improve its corrosion and mechanical properties. Microstructural and phase analysis reveal that, after ECAP, a substantial grain refinement occurred, and secondary phases were observed. The grain size of the as-cast (AC) sample is reduced from 20 ± 1 to 0.88 ± 0.6 µm, attributed to dynamic recrystallization. The mechanical properties of the ECAP sample were significantly improved when compared to the AC sample. An improvement in the microhardness (43%), ultimate tensile strength (73%), yield strength (76%), and ductility (50%) were observed for the ECAP sample. A decrease in the corrosion rate was observed for ECAP sample (9 ± 1 mm/year) compared to the AC (16 ± 2 mm/year) sample. The grain refinement and crystallographic orientation of the ECAP samples contributed to the enhancement of corrosion resistance. © ASM International 2025.Item Wear and frictional behaviour of partially oxidized and plasma sprayed NiCr and NiCrBSiFe coatings(Nature Research, 2025) Medabalimi, S.; Rokkala, U.; Gudala, S.; Behera, N.; Ramesh, M.R.; Dejene, M.The wear performance of partially oxidized NiCr and NiCrBSiFe coatings were investigated by varing load and speed. The partial oxidized powders were processed from the alloy powder using a flame spray process that involved spraying into distilled water. The partially oxidized powder was then plasma-sprayed onto MDN321 steel. The coatings were characterized for adhesive strength, microhardness, and density. The wear behavior was evaluated at disc speeds of 1, 2, and 3 m/s, with loads ranging from 10 to 50 N, over a 3000 m sliding distance. A significant difference in wear rates between the coating and substrate was observed. Operating at a sliding velocity of 1 m/s under a 10 N load, the substrate’s wear rate was found to be 3.56 times higher than that of the NiCrBSiFe coating, whereas for NiCr coating, it was 2.78 times higher. Wear rate coefficient performance shift takes place between the coatings at 12 N-m/s, product of applied load (C) and sliding velocity (V). In NiCrBSiFe coating, the wear mechanism observed at lower speeds and loads is micro-brittle and mechanism shifts to abrasive wear at higher speeds and loads. In the NiCr partially oxidized coating, the wear mechanism observed involves spallation of the coating at higher loads and adhesive wear at lower loads. Thermo gravimetric analysis of the coatings revealed a weight loss percentage of 1.42 for NiCrBSiFe and 14.09 for NiCr coatings. These findings highlight the NiCrBSiFe partially oxidized coating as being tenfold more stable at high temperatures compared to the NiCr partially oxidized coating. © The Author(s) 2025.