Conference Papers
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Item Oxidation and corrosion resistance of TiAl3 coatings(Indian Institute of Metals, 2008) Udayashankar, N.K.; Rajasekaran, S.; Nayak, J.Titanium aluminides based on TiAl, TiAl3 and Ti3Al are potential materials for high temperature aerospace applications. Their low density, high temperature creep resistance, high temperature strength and high oxidation resistance make them excellent coating materials. However mese coatings are likely to be subjected to high temperature and corrosive environments during service. Hence it is aimed to study the oxidation and corrosion resistance of TiAl3 coatings on various types of substrates. In me present work, TiAl3 is coated on high speed steel, stainless steel 304, stainless steel 316, copper and aluminum substrates by physical vapor deposition technique. X-ray diffraction analysis confirms the presence of TiAl3 phase. The hardness studies reveal that better hardness can be achieved with thick coatings. The oxidation behavior of the coatings is studied by carrying out step stress experiments at elevated temperatures. Coated samples are heated up from 400°C in the steps of 100°C for 1h in each step to 3000°C. The mass gain caused by oxidation was determined. The oxidation curve drawn as a function of mass gain versus temperature reveals that TiAl3 film started to oxidize above 800°C, where as oxidation of me uncoated substrates began at a much lower temperature of 550°C. The excellent oxidation resistance of the coatings can be attributed to the formation of an amorphous Al2O3 film. Scanning electron microscope (SEM) and EDAX analysis confirm the presence of an amorphous Al2O3 film. The corrosion behavior of TiAl3 coatings are investigated by the polarization resistance experiments in NaCl aqueous solution at ambient temperature. According to the Tafel plot analysis, the coatings show lower corrosion rate than the untreated substrates. The major corrosion in the coatings arose from electrolyte penetration into the pores of the coatings. In fact, a dense coating showed a high corrosion resistance in an aqueous medium.Item The effect of aging and the protective coating on the oxidation behavior of 6061Al/SiC composite at high temperatures(Trans Tech Publications Ltd ttp@transtec.ch, 2011) Rajasekaran, S.; Udayashankar, N.K.; Nayak, J.This paper analyses the effect of ageing on the oxidation behavior of 6061Al/SiC composite material at temperatures ranging from 500 to 800 K. Also aluminum coating is employed as protective coating in order to improve the oxidation resistance of the composite. SEM, EDAX, XRD and stepped oxidation measurement techniques are used to study the oxidation behavior and to characterize the composite specimens. Oxidation of the composite material without protective coatings is seen to be very rapid during the initial stages of exposure to the high temperatures but subsequently slowed down due to the formation of a protective surface layer of oxide. Among the artificially aged composites, peak aged specimens are more prone to oxidation. The oxidation was especially severe above 600 K. The interface between the matrix and reinforcement particles and the grain boundary regions of the matrix enhance this oxidation process since they provided sites for oxidation initiation. Aluminum coating on the composite obtained by DC magnetron sputtering technique, reduce the oxidation rate effectively since the interface regions between the matrix and reinforcement, grain boundary regions of the matrix are unexposed to the atmosphere. Aluminum coating provides better oxidation resistance for the artificially aged composites. © (2011) Trans Tech Publications.Item Effect of aging and protective coating on the corrosion behavior of 6061 Al / SiCp composite in chloride solutions(Trans Tech Publications Ltd ttp@transtec.ch, 2011) Rajasekaran, S.; Udayashankar, N.K.; Nayak, J.Metal matrix composites considered especially for marine application are typically aluminum-based. Utilization of the composites in marine environments requires adequate corrosion resistance. In this work, the effect of aging and aluminum as resistive coating on the corrosion behavior of 6061 Al / SiC p composite in 1N and 3.5N NaCl solution in the temperature range of 30°C-50°C is studied by means of electrochemical techniques, scanning electron microscope (SEM) and weight loss measurements. Aging results in precipitation of intermetallics in the matrix, this in turn leads to galvanic and pitting corrosion. Pitting initiation and propagation as the main corrosion mechanism was driven by the aging kinetics which is ruled by the reduction in the retained vacancy concentration and at the same time by dislocation density due to the presence of SiCp. Sputtered aluminum coating on the composite effectively resists the pitting initiation and propagation and hence corrosion process. Also the intermetallic precipitates are unexposed to the Chloride solution by the presence of aluminum coating. Hence aluminum coating acts as corrosion resistive coating for the 6061 Al / SiCp aged composites. © (2011) Trans Tech Publications.Item Regression model of oxidation behavior of 6061 Al/SiC composite with and without protective coatings(2011) Priyamvada, B.; Rajasekaran, S.; Udayashankar, N.K.; Nayak, J.This paper analyses the variation of weight gain, Δm, of 6061 Al/SiC composite due to oxidation with time, t, using regression model. Using curve fitting technique, the mathematical equations for the oxidation behavior of the composite are formulated. The generated data according to the mathematical equations are analyzed and compared with the experimental data. More specifically, regression analysis helps in understanding how the typical value of the mass gain (dependent variable) changes when the time of oxidation (independent variable) is varied, while the other independent variable (Temperature) held fixed. Since the oxidation resistance of the 6061Al/SiC composite decreases due to the presence of alloying elements precipitates in the matrix, the effect of aging treatment and protective coatings like Aluminium and AlCrN on the oxidation behavior of the composite is studied. It is observed that the coatings increase the oxidation resistance of the composite. The regression analysis carried out shows a threefold linear variation of weight gain (Dependent variable) with respect to time and temperature of oxidation (Independent variables). © 2011 IEEE.Item The effect of protective coatings on the oxidation behavior of 6061Al/SiC composite at high temperatures(2012) Udayashankar, N.K.; Rajasekaran, S.; Nayak, J.This paper analyses the effect of protective coatings on the oxidation behavior of 6061Al/SiC composite material at temperatures ranging from 500 to 800 K. Aluminum and AlCrN coatings are employed as protective coatings in order to improve the oxidation resistance of the composite. SEM, EDAX, XRD and oxidation measurement techniques are used to study the oxidation behavior and to characterize the composite specimens. Oxidation of the composite material without protective coatings is seen to be very rapid during the initial stages of exposure to the high temperatures but subsequently slowed down due to the formation of a protective surface layer of oxide. The oxidation was especially severe above 600 K. The interface between the matrix and reinforcement particles and the grain boundary regions of intermetallic precipitates in the matrix enhance this oxidation process since they provided sites for oxidation initiation. Aluminum coating on the composite obtained by DC magnetron sputtering technique and AlCrN coating obtained by low voltage electron beam evaporation technique reduce the oxidation rate effectively since the interface regions between the matrix and reinforcement, grain boundary regions of the matrix are unexposed to the atmosphere. Aluminum coating provides better oxidation resistance for 6061 Al/SiC composites.Item Synthesis and studies on microhardness of alkali zinc borate glasses(American Institute of Physics Inc. subs@aip.org, 2014) Subhashini, u.; Bhattacharya, S.; Shashikala, H.D.; Udayashankar, N.K.The mixed alkali effect on zinc borate glasses have been reported. The glass systems of nominal composition 10Zn+xLi2O+yNa2O+80B2O3 (x = y = 0, 5, 10, 15 mol%) were prepared using standard melt quenching method. The structural, physical and mechanical properties of the samples have been studied using X-ray diffraction(XRD), density measurement and Vickers hardness measurement, respectively. A consistent increase in the density was observed, which explains the role of the modifiers (Li2O and Na2O) in the network modification of borate structure. The molar volume is decreasing linearly with the alkali concentration, which is attributed to the conversion of tetrahedral boron (BO4/2)- into (BO3/2)-. The microhardness studies reveals the anisotropy nature of the material. It further confirms that the samples belong to hard glass category. © 2014 AIP Publishing LLC.Item Studies on mechanical properties of porous anodic alumina membranes using microindentation test(American Institute of Physics Inc. subs@aip.org, 2015) Reddy, P.R.; Udayashankar, N.K.; Ajith, K.M.Porous anodic alumina (PAA) membranes were prepared using two-step self-organized anodization process carried out in 0.3 M oxalic acid at the temperature of about 5 o C. PAA membranes fabricated underconstant potential of 40 V gave optimum ordering of hexagonal pores. Microhardness of the prepared PAA membranes were studied using Vickers hardness test rig. It was observed that microhardness of prepared PAA membraneswere 2.20 GPa and 0.21 GPa for 100 g and1000 g loads respectively. © 2015 AIP Publishing LLC.Item Effect of pore widening time on the structural aspects of self-organized nanopore arrays formed by anodization of aluminum in chromic acid(Elsevier Ltd, 2016) Suchitra, S.M.; Ramana Reddy, P.; Udayashankar, N.K.Anodic aluminium oxide (AAO) membranes were prepared by two-step anodization of aluminum formed in 0.3M chromic acid at the ambient temperature of ~ 30° C with anodization potential of 50 V. The radial average fast Fourier transform based study of nanopores has been carried out. The duration of the pore widening was varied from 0min to 60min in steps of 15 minutes. The effect of the pore widening time on the structural parameters of porous anodic alumina such as pore diameter (Dp), interpore distance (Dc), porosity (P) and pore density (n) were studied. It was observed that pore diameter and porosity increased with increasing pore widening time. Highly ordered and better nanopore array of AAO membrane was obtained for the sample anodized in chromic acid with 60 min pore widening time. © 2016 Elsevier Ltd.Item Structural and wetting properties of porous anodic alumina templates prepared by different electrolytes(American Institute of Physics Inc. subs@aip.org, 2016) Suchitra, S.M.; Ramana Reddy, P.R.; Udayashankar, N.K.Porous anodic alumina (PAA) has been extensively studied in recent years due to their unique properties and applications for manufacturing nanostructured materials. In this article, we report our studies on structural and wetting properties of PAA membranes prepared using different electrolytes such as sulphuric, oxalic and phosphoric acids. The morphological parameters such as pore diameter and porosity were measured using SEM and analysed using image-J software. The structural investigation of PAA membranes was carried out through X-ray diffraction analysis and it was confirmed that PAA membranes were amorphous in nature. The wetting behaviour of PAA membranes were measured using contact angle measurement technique. The results show that PAA membranes were hydrophilic in nature with contact angles 26.03°, 35.21° and 42.0° for sulphuric, oxalic and phosphoric acids respectively. © 2016 Author(s).Item Effective role of P2O5 on in-vitro bioactivity of soda lime phosphosilicate glasses(Elsevier Ltd, 2017) Kiran, P.; Udayashankar, N.K.; Shashikala, H.D.In-vitro bio active properties have been studied on Sol-gel derived 58SiO2-33CaO-(5-X) Na2O-(4+X) P2O5 (where X=0, 5) bio glass samples soaked for seven days in a simulated body fluid (SBF) solution. X-ray diffraction studies indicated the presence of Hydroxyl Apatite (HA) crystalline phase in the synthesized samples. The spherical shaped hydroxyl carbonated apatite (HCA) nuclei were observed under scanning electron microscope. The Fourier transform infrared (FTIR) and Raman spectroscopic analysis confirmed the presence of carbonates in the glass matrix. In particular, larger concentration of P2O5 in the bioglass resulted in superior bioactivity compared to low concentration of P2O5. © 2017 Elsevier Ltd.