National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Studies on electroless Ni–W–P and Ni–W–Cu–P alloy coatings using chloride-based bath

Abstract: In the present investigation ternary electroless Ni-W-P alloy coatings were prepared using alkaline citrate-based bath. Nickel chloride and sodium tungstate were used as nickel and tungsten sources, respectively, and sodium hypophosphite was used as a reducing agent. Deposits were characterized for their structure, morphology, chemical composition and microhardness. A single peak was obtained in XRD for ternary Ni-W-P alloy. Addition of 3 mM of copper chloride in the bath has resulted in a very smooth quaternary Ni-W-Cu-P alloy. Also, codeposition of copper in ternary alloy has increased the peak sharpness and the calculated grain sizes for ternary and quaternary deposits are 2.5 and 8.8 nm, respectively. XPS studies show that in ternary Ni-W-P deposits from the present bath nearly 60 at.% of tungsten is present in the elemental form, Wlt;supgt;0lt;/supgt;, as compared to that from nickel chloride and sulphate additives bath in which Wlt;supgt;0lt;/supgt; state is only 27 at.%. Addition of copper chloride to the present bath has increased this elemental form by nearly 10 at.% in the quaternary deposit. SEM micrographs show that ternary alloy films are nodular whereas quaternary alloy films are smooth and nodule-free. Microhardness measurements show a marginal difference in the hardness between ternary and quaternary deposits. [All rights reserved Elsevier]

Structure, morphology and chemical composition of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

Abstract: WS2 and Cr–WS2 nanocomposite coatings were deposited at different Cr contents (approximately 15–50 at. %) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr–WS2 coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr–WS2 coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO3 on the surface of Cr–WS2 coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS2 and Cr–WS2 coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS2 strongly influences the structure and morphology of Cr–WS2 coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr–WS2 coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS2 coating showed a porous and columnar microstructure. For the Cr–WS2 coatings, a mixture of columnar and featureless microstructure was observed at low Cr ontents (≤23 at.%),whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr–WS2 coatings prepared at Cr content ≤23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS2 phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr–WS2 coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.

Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing

Abstract: Dense ZrB2–SiC (25–30 vol%) composites have been produced by reactive hot pressing using stoichiometric Zr, B4C, C and Si powder mixtures with and without Ni addition at 40 MPa, 1600 °C for 60 min. Nickel, a common additive to promote densification, is shown not to be essential; the presence of an ultra-fine microstructure containing a transient plastic ZrC phase is suggested to play a key role at low temperatures, while a transient liquid phase may be responsible at temperatures above 1350 °C. Hot Pressing of non-stoichiometric mixture of Zr, B4C and Si at 40 MPa, 1600 °C for 30 min resulted in ZrB2–ZrCx–SiC (15 vol%) composites of ∼98% RD.