Key Engineering Materials Vol. 875

Abstract: Yttria stabilized zirconia (YSZ) based composite topcoats were prepared with three advanced ceramic materials as second phase component; CaZrO₃, (La_0.75Nd_0.25)₂Zr₂O₇ and Nd₂Ce₂O₇. The solid particles erosion (SPE) testing of the air plasma sprayed composite topcoats was carried out at room temperature and 900 °C to study the effects of second phase on erosion behavior. The erodent was angular fused alumina of 40-45 mm size. The erosion mechanism was followed in all topcoats with cracking and fracturing of the plasma sprayed splats due to erodent particles impact and impingement, whereas micro-ploughing was observed after 900 °C SPE testing as an additional feature. The hardness and intrinsic properties of second phase component in the composite topcoats played a crucial role in improving the erosion rate (ER) at ambient as well as 900 °C. The Nd₂Ce₂O₇/YSZ topcoat show lower ER due to combatively higher hardness of Nd₂Ce₂O₇ and good interfacial bonding with YSZ. The overall lowering of ER at 900 °C as compared to that of at ambient conditions was ascribed to the anelastic mechanical response of the ceramic topcoats due to thermal cycling involved in the SPE testing at high temperature.

Abstract: The external surface of the pipeline steels can be protected from corrosion by cathodic protection and a suitable coating system. But to protect the internal surface of steel pipelines from corrosion is always a challenge. In the current study, a protective aluminum coating was applied on the internal surface of steel pipeline grades API X56 and API X70 by hot dip aluminization process to minimize the cost of inhibitors. The steel substrates were dipped in the hot aluminum bath, whose temperature was maintained at 720 °C, for three different dipping times (5, 7.5 and 10 minutes) and then permitted to cool to room temperature. The coated specimens were characterized in terms of their structure and composition by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM micrographs analysis reveal that the hot dip aluminized specimens gave an unwavering and well adherent interface which is essential for corrosion resistance of the substrates. The corrosion behavior was evaluated by weight loss method in case of sweet corrosion (in 1.0 % NaCl + CO₂ environment) and, by electrochemical potentiodynamic polarization tests for marine corrosion (in 3.5% NaCl environment). The sweet corrosion rates showed severe material loss (14.17 mpy for API X56 and 6.6 mpy in case of API X70) of uncoated samples as compared to the aluminized samples which showed no corrosion at all even after one week due to the passive layer of Al₂O₃. Whereas the marine corrosion of aluminized specimens was also reduced to 0.11 mpy (coated specimens) from 0.57 mpy (uncoated specimens).

Abstract: Traditionally superhydrophobic surfaces are prepared by applying liquid repellant organic coatings or nano-based coatings. These superhydrophobic coatings are prone to wear and can be easily damaged by abrasion and cleaning. Recently researchers are switching interest to more efficient and promising technology of pulse laser texturing for engineering sub-micron topographies to have superhydrophobic surfaces. In this research, the micro-second Laser Pulses are used to feature sub-micron textures on titanium nitride coated aluminum and polished aluminum surfaces in order to achieve the water contact angle greater than 150^°. Titanium nitride coated aluminum surface with scan line separation of 50 µm shows superior hydrophobicity having a water contact angle of 156º. These superhydrophobic aluminum surfaces have applications for anti-water clogging and anti-corrosion use.

Abstract: Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

Abstract: AA2219 is a superior performance Al-base alloy which promises toughness, strength and creep resistance which allows the deposition of titanium nitride (TiN) coating at 200 °C. The present research addresses the issue of poor wear properties of the alloy, using state of the art technologies, to deposit hard and adherent thin TiN layer followed by laser surface texturing. The thickness of sub-micron size coating is determined by appropriate modification of the existing mathematical model and composite microhardness values. Laser energy density of 20.8 J/cm² and 4 repeated pulses is optimized to produce regular size and shape of micro-holes on TiN-coated samples. Epoxy-based MoS₂ lubricant is deposited on laser textured samples to produce ultra-low friction surfaces. The micro-holes act as a micro-reservoir of MoS₂ solid lubricant. Field emission scanning electron microscope and optical profilometer were used to estimate the topology, shape, size, and depth of micro-holes. The cross-sectional view shows the successful impregnation of epoxy-based MoS₂ due to the chemisorption of functional groups with an Al oxide surface.

Abstract: The article aims to comprehend the microstructural changes, in Plasma Transfer Arc (PTA) deposited M2 high speed steel (HSS) hardfacings upon incorporation of 10 wt% Mo alloying during deposition followed by laser surface melting. PTA deposited hardfacings were produced over 4140 steel. Then Mo alloyed and unalloyed PTA deposits were subjected to laser surface melting (LSM) process. A comprehensive microstructural characterization for all the resultant structures was carried out. Optical metallography using appropriate etching reagents and SEM microscopy in conjunction with XRD techniques were employed to ascertain the matrix structure and carbides morphology. The PTA microstructure was close to equilibrium structure of M2 HSS containing mixture of ferrite/austenite/martensite along with MC, M₂C and M₆C type carbides. While the LSM of M2 HSS caused higher fraction of martensite and finer grains in the structure resulting in increment in hardness. 10-wt% Mo addition changes the carbides from MC and rod like M₂C to fibrous M₂C and fishbone like M₆C carbides. The LSM of Mo alloyed M2 HSS PTA deposits led to an overall decrease in the fraction of M₆C carbides and fibrous M₂C carbides accompanied by a decrease in hardness.

Abstract: The aim of this study is to investigate the photoluminescence (PL) properties of europium (Eu) doped alumina as potential platform for simultaneous bio Imaging and drug delivery. Synthesis of Eu doped alumina is done by a facile two step method. In the first stage, hydrothermal synthesis is used to prepare the Eu doped ammonium aluminum carbonate hydroxide which is then calcined to get a crystalline Eu doped alumina. Structural characterization of the prepared sample is done through XRD and SEM. Photoluminescence spectroscopy is performed in order the study the PL response. The SEM images of the Eu doped sample revealed whisker shaped morphology, the porosity in the inter and intra whisker region is beneficial for the high drug loading capacity. The length of the bundle after annealing was about 5 µm with the bundle diameter of 0.45 µm. XRD patterns of the prepared sample has sharp peaks, showing a high degree of crystallinity corresponding to the α-alumina phase. Finally PL response was checked at an excitation wavelength of 393 nm. A dominant peak was observed at a wavelength of 613 nm corresponding to the 5D0 to 7F2 transition.The3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide MTT assay confirms the cell viability of more than 100% at even a concentration of 500 nano molar alumina in phosphate-buffered saline (PBS). These results show that the Eu doped alumina having optimum PL response, high biocompatibility and drug loading capacity which makes it a promising candidate for theranostic applications.

Abstract: The drug loading capability and inherent cytotoxicity of mesoporous silica particles are two prime considerations for targeted drug delivery applications. In current study, uncoated mesoporous silica (UMS) carrier particles were synthesized by sol-gel emulsion approach. The morphology and structure of UMS was thoroughly characterized using atomic force microscope (AFM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmett–Teller (BET). The scanning electron microscopy (SEM) and dynamic light scattering (DLS) measurements reveal that mono dispersed silica particles have an average size of 250 nm with narrow size distribution. The pore size was measured as 47nm. Concentration dependent biocompatibility of UMS was evaluated using MTT assay with Hep-2c cancer cell line and cell viability of ~65% at concentrations of 7.5 nM was observed. Finally, the drug loading capability of UMS carrier was studied using ibuprofen as a model drug.

Abstract: Restorative material is a class of dental materials used for direct filling and fabrication of indirect restoration. NiCr alloy is a restorative material frequently used for dental prostheses due to its properties and economic reasons. In present work beryllium free NiCrMo alloy was developed and studied for dental restoration application. The alloy have unique characteristics of resistance to oxidation and biocompatibility; the requisites for dental prostheses. NiCrMo alloy is found to possess mechanical strength and fabrication properties suitable for dental repairs. In this study the developed alloy was tested for its mechanical properties, biocompatibility and corrosion resistance. An in-vitro biocompatibility study was carried out. No signs of toxicity and no signs of cell growth inhibition, in presence of NiCrMo alloy specimen, were observed. Mechanical properties and corrosion resistance are found in the range that is suitable for dental prostheses and easy fabrication.