Faculty Publications

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Microstructural characterization of low temperature plasma-nitrided 316L stainless steel surface with prior severe shot peening
(Elsevier Ltd, 2016) Jayalakshmi, M.; Huilgol, P.; Badekai Ramachandra, B.R.; Bhat, K.U.
Surface nanocrystallization by severe deformation has proven beneficial as pre-treatment to plasma nitriding. It aids in achieving thicker nitride layers at lower temperatures thus making the process more economical. In austenitic stainless steels, severe deformation leads to formation of strain induced martensite on the surface while plasma nitriding alone forms expanded austenite. However, structural characteristics of surface layer of pre-deformed steel after plasma nitriding is still a matter of debate. In present study, 316L stainless steel was subjected to severe shot peening: followed by plasma nitriding at 400 °C for 4 h. Characteristics of sample surface before and after treatment were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that, this duplex treatment leads to formation of about 45 ?m thick nitride layer; without CrN precipitation. This is significantly high compared to reported data considering the temperature and duration of nitriding treatment employed. Selected area electron diffraction pattern from topmost surface confirmed the co-existence of austenite and martensite while subsurface layer was predominantly consisting of lath martensite. This indicates that major phase in the nitrided layer is martensitic in nature and nitrogen supersaturation leads to transformation of small fraction of martensite to expanded austenite. © 2016 Elsevier Ltd
Structural and morphological changes with substrate heating in zinc films synthesized by thermal vapor deposition technique
(Springer New York LLC barbara.b.bertram@gsk.com, 2017) Sneha, C.; Prabukumar, C.; Jayalakshmi, M.; Bhat, K.U.
Zinc oxide (ZnO) films are used in numerous applications such as solar cells, gas sensors, nanogenerators, etc., owing to their large band gap, piezoelectric activity and versatile nanostructures. Deposition of zinc films and their subsequent oxidation is considered as one of the successful methods to obtain nanostructured ZnO films. It has been reported that the structural features of the oxide film depends on the characteristics of parent zinc film; which in turn depends on the deposition parameters. In the present work, zinc films were synthesized by thermal vacuum deposition route. In order to understand the effect of substrate heating during deposition, zinc films were deposited on glass with different substrate temperatures, in the range of room temperature to 180 °C. The structural and morphological properties of as-synthesized films were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) techniques. The XRD data confirmed that the as-synthesized films have strong (002) preferential orientation. Notable changes were observed such as change in crystallite size, texture coefficient and strain in the films, upon changing the substrate temperature. The morphology of as-synthesized zinc films found to consist of hexagonal-plate like structures. It was observed that the dimensions of the hexagonal-plates were changed in accordance with the substrate temperature. DSC results indicated a depression in the melting point of zinc films compared to bulk zinc and it is attributed to the nanoscale features constituting the film. © 2017, Springer Science+Business Media New York.
Effect of substrate temperature on the characteristics of ZnO films produced by a combination of thermal vapor deposition and oxidation processes
(Springer New York LLC barbara.b.bertram@gsk.com, 2017) Sneha, C.; Prabukumar, C.; Jayalakshmi, M.; Bhat, S.; Bhat, K.
In the present work, ZnO semiconductor films were prepared by following two step processes, namely, thermal vapor deposition of the zinc on the glass substrate followed by oxidative annealing. Substrate temperature during deposition of the zinc was varied. Structural properties and morphology of the ZnO films were investigated by using X-ray diffraction and scanning electron microscopy techniques. Hydrophobic nature of the film was confirmed by using contact angle analyser. Hall measurements facilitated the estimation of the carrier concentration, their mobility and their effect on the conductivity. Photoluminescence spectroscopy was used to analyse the lattice defect concentration in the film. Further, the sensor response of the ZnO film to CO gas was analysed. Sensor fabricated with ZnO films which were prepared by oxidizing Zn films deposited at higher substrate temperature were found to possess better response and faster response-recovery time than the film prepared using lower substrate temperature for zinc deposition. © 2017, Springer Science+Business Media, LLC.

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