• S Prakash

      Articles written in Bulletin of Materials Science

    • Characteristics of porous zirconia coated with hydroxyapatite as human bones

      V V Narulkar S Prakash K Chandra

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      Since hydroxyapatite has excellent biocompatibility and bone bonding ability, porous hydroxyapatite ceramics have been intensively studied. However, porous hydroxyapatite bodies are mechanically weak and brittle, which makes shaping and implantation difficult. One way to solve this problem is to introduce a strong porous network onto which hydroxyapatite coating is applied. In this study, porous zirconia and alumina-added zirconia ceramics were prepared by ceramic slurry infiltration of expanded polystyrene bead compacts, followed by firing at 1500°C. Then slurry of hydroxyapatite–borosilicate glass mixed powder was used to coat the porous ceramics, followed by firing at 1200°C. The porous structures without the coating had high porosities of 51–69%, high pore interconnectivity, and sufficiently large pore window sizes (300–500 𝜇m). The porous ceramics had compressive strengths of 5.3∼36.8 MPa, favourably comparable to the mechanical properties of cancellous bones. In addition, porous hydroxyapatite surface was formed on the top of the composite coating, whereas a borosilicate glass layer was found on the interface. Thus, porous zirconia-based ceramics were modified with a bioactive composite coating for biomedical applications.

    • Ceramic coated Y1 magnesium alloy surfaces by microarc oxidation process for marine applications

      V V Narulkar S Prakash K Chandra

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      The magnesium alloys occupy an important place in marine applications, but their poor corrosion resistance, wear resistance, hardness and so on, have limited their application. To meet these defects, some techniques are developed. Microarc oxidation is a one such recently developed surface treatment technology under anodic oxidation in which ceramic coating is directly formed on the surface of magnesium alloy, by which its surface property is greatly improved. In this paper, a dense ceramic oxide coating, ∼ 20 𝜇m thick, was prepared on an Y1 magnesium alloy through microarc oxidation in a Na3SiO3–Na2WO4–KOH–Na2EDTA electrolytic solution. The property of corrosion resistance of ceramic coating was studied by CS300P electrochemistry–corrosion workstation, and the main impact factor of the corrosion resistance was also analysed. Microstructure and phase composition were analysed by SEM and XRD. The microhardness of the coating was also measured. The basic mechanism of microarc coating formation is explained in brief.

      The results show that the corrosion resistance property of microarc oxidation coating on the Y1 magnesium surface is superior to the original samples in the 3.5 wt% NaCl solutions. The microarc oxidation coating is relatively dense and uniform, mainly composed of MgO, MgAl2O4 and MgSiO3. The microhardness of the Y1 magnesium alloy surface attained 410 HV, which was much larger than that of the original Y1 magnesium alloy without microarc oxidation.

    • High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

      Subhash Kamal R Jayaganthan S Prakash

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      Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na2SO4–60% V2O5) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr2O3, NiO and spinel NiCr2O4 has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.

    • Erosive wear of a surface coated hydroturbine steel

      Akhilesh K Chauhan D B Goel S Prakash

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      In the present investigation, stellite-6, Cr3C2–NiCr and WC–Co–Cr coatings were deposited by DGun on a hot rolled 21Cr–4Ni–N steel meant for fabrication of hydro turbine underwater parts. The coatings have been characterized for microstructure, porosity, microhardness and crystalline nature. The erosion experiments were carried out using an air jet erosion test rig at a velocity of 120 ms-1 and impingement angles of 30° and 90°. Silicon carbide particles of size ranging between 500 and 700 𝜇m were used as erodent. Scanning electron microscopy (SEM) technique was used to analyse the nature and mechanism of erosion. Erosion behaviour is observed to be influenced largely by the nature and extent of porosity in the surface coatings.

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