Cr-WS2 nanocomposite solid lubricant coatings were prepared on mechanically textured stainless steel substrates. Micron-sized dimples of approximately 10-12 mu m depth and 100 x 80 mu m(2) area with uniform dimple spacing of 300 mu m were created on the substrates using a knurling tool. Substrate surface texturing improved the tribological properties of Cr-WS2 nanocomposite coatings significantly. Cr-WS2 nanocomposite coatings prepared on textured steel substrates exhibited a friction coefficient of 0.1 after 28,800 cycles compared to coatings prepared on non-textured substrates which exhibited a friction coefficient of 0.45. The dimpled regions acted as solid lubricant reservoirs and also helped in trapping wear debris. The retention of solid lubricant material in the dimpled region was confirmed using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy studies.

Nanocomposite coatings of Cr-Si3N4 exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the Cr-Si3N4 nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. Cr-Si3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam(1/2), respectively. Nanoscratch measurements indicated that Cr-Si3N4 nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ball-on-disc reciprocating tests at a load of 2 N showed that Cr-Si3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the Cr-Si3N4 nanocomposite coatings exhibited only mild wear due to oxidation. (C) 2011 Elsevier Ltd. All rights reserved.

Au-WS2 nanocomposite coatings with approximately 60 at.% Au were prepared using magnetron sputtering from high purity Au and WS2 targets. Structural characterization of the coatings using X-ray diffraction indicated the presence of (111), (200) and (220) peaks of cubic Au and broad bands corresponding to (002) and (004) planes of hexagonal WS2. Surface morphology of Au-WS2 coatings was studied using field emission scanning electron microscopy (FESEM) and atomic force microscopy which showed formation of uniformly distributed interconnected ligament-like features. High resolution transmission electron microscopy studies showed the presence of a two phase nanocomposite structure wherein nanocrystalline Au was dispersed in a matrix of nanocrystalline and amorphous WS2. The electrical resistivity of Au-WS2 coating measured using four-point-probe method was approximately 0.2 mu Omega m compared to Au and WS2 coatings which exhibited electrical resistivity values of approximately 0.04 mu Omega m and 0.057 Omega m, respectively. Ball-on-disc reciprocating tests at a load of 7 N showed that Au-WS2 nanocomposite coating exhibited a friction coefficient of 0.22 after 57,600 cycles and outperformed Au and WS2 coatings. Analyses of the ball and wear track of Au-WS2 coating using FESEM at the end of the wear test indicated formation of smooth transfer films, less accumulation of wear debris and reduced oxidation.

Nanocomposite coatings of CrN–WS2 were prepared at different Cr contents (approximately 8–39at%) using an unbalanced magnetron sputtering system. Structural changes in CrN–WS2 coatings with variation in Cr content were studied using X-ray diffraction. CrN–WS2 coatings displayed a dense, compact microstructure with reduced columnar growth in the field emission scanning electron microscopy data. Nanoindentation and nanoscratch data showed that CrN–WS2 coatings exhibited improved mechanical and adhesive properties, respectively. Micro-tribometer tests at a load of 2 N indicated that CrN–WS2 coatings prepared at 31at% Cr exhibited a stable friction coefficient of 0.20–0.24 even after 8h.

WS2 and Cr–WS2 nanocomposite coatings were deposited at different Cr contents (approximately 15–50at.%) 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 contents (≤23at.%), 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 ≤23at.% 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.

Solid lubricant coatings of WS2 and Cr–WS2 (15–50at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr–WS2 coatings. The adhesive strengths of Cr–WS2 coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr–WS2 coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS2 coatings exhibited large amount of plastic deformation compared to Cr–WS2 coatings which showed a combination of elastic–plastic deformation. However, micro-tribometer measurements at a load of 2N showed that the tribological properties of Cr–WS2 coatings deteriorated with an increase in the Cr content. For example, Cr–WS2 coatings prepared at Cr content ≥33at.% failed after a sliding distance of 1m. On the other hand, WS2 and Cr–WS2 coatings prepared at low Cr contents (15–23at.% Cr) exhibited a stable friction coefficient (50–60% relative humidity) in the range of 0.10–0.13 for a sliding distance of 14m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14m indicated the presence of WS2 transfer films for WS2 and Cr–WS2 coatings prepared at low Cr contents. For Cr–WS2 coatings with Cr content ≥33at.%, the worn films consisted predominantly of WO3. After an extended sliding distance of 50m, Cr–WS2 coatings (15–23at.% Cr) outperformed WS2 coating which failed after 20m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200m. At a higher load of 7N, Cr–WS2 coating with 15at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72m. These results indicate that the amount of Cr in the WS2 matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr–WS2 solid lubricant coatings.

A reactive direct current magnetron sputtering system was used to prepare NbAlN coatings at different nitrogen flow rates and substrate bias voltages. Various properties of NbAlN coatings were studied using x-ray diffraction, scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, nanoindentation, the four-probe method, a solar spectrum reflectometer and emissometer, spectroscopic ellipsometry, micro-Raman spectroscopy, and potentiodynamic polarization techniques. Single-phase NbAlN with B1 NaCl structure was obtained for the coatings prepared at a nitrogen flow rate in the range of 1.5–3 sccm, a substrate bias voltage of −50 to −210 V, and a substrate temperature of 300 °C. Nanoindentation data showed that the optimized NbAlN coating exhibited a maximum hardness of 2856 kg/mm 2 . An approximately 100-nm-thick NbAlN–NbAlON tandem on copper substrate exhibited a high absorptance (0.93) and a low emittance (0.06), suitable for solar-selective applications. The spectroscopic ellipsometry and resistivity data established the metallic nature of NbAlN and the semitransparent behavior of NbAlON coatings. The corrosion resistance of NbAlN coatings was superior to that of the mild steel substrate. The addition of aluminum in NbN coatings increased the onset of oxidation in air from 350 to 700 °C. Vacuum-annealed NbAlN coatings were structurally stable up to 700 °C and retained their high hardness up to a temperature of 650 °C.

Thin films of TiO2 were grown on n-type Si substrate by thermal oxidation of Ti films deposited by dc sputtering. The phase purity of TiO2 was confirmed by Raman spectroscopy, and secondary ion mass spectroscopy was used to analyze the interfacial and chemical composition of the TiO2 thin films. Metal–oxide–semiconductor capacitors with Al as the top electrode were fabricated to study the electrical properties of the TiO2 films. The current conduction mechanisms in thermally grown TiO2 films were observed to follow the space charge-limited current mechanism followed by a Schottky emission process both at and above room temperature. Three orders of magnitude of reduction in current density were observed for thermally grown samples while measured the I–V characteristics at 77 K and Fowler–Nordheim (F–N) tunneling was found to be a dominant conduction mechanism at higher biasing voltages.

TiAlN and CrAlN coatings were prepared using a reactive direct current magnetron sputtering system from TiAl and CrAl targets. Structural characterization of the coatings using x-ray diffraction (XRD) revealed the B1 NaCl structure of TiAlN and CrAlN coatings with a prominent reflection along the (111) plane. The XPS data confirmed the bonding structures of TiAlN and CrAlN single layer coatings. Subsequently, nanolayered multilayer coatings of TiAlN∕CrAlN were deposited on silicon and mild steel (MS) substrates at different modulation wavelengths (Λ) with a total thickness of approximately 1.0μm. The modulation wavelengths were calculated from the x-ray reflectivity data using modified Bragg’s law. TiAlN∕CrAlN multilayer coatings were textured along (111) for Λ<200Å and the XRD patterns showed the formation of superlattice structure for coatings deposited at Λ=102Å. The x-ray reflectivity data showed reflections of fifth and seventh orders for multilayer coatings deposited at Λ=102 and 138Å, respectively, indicating the formation of sharp interfaces between TiAlN and CrAlN layers. The cross-sectional scanning electron microscopy image of TiAlN∕CrAlN multilayer coatings indicated a noncolumnar and dense microstructure. A maximum hardness of 39GPa was observed for TiAlN∕CrAlN multilayer coatings deposited at Λ=93Å, which was higher than the rule-of-mixture value (30GPa) for TiAlN and CrAlN. Study of thermal stability of the coatings in air using micro-Raman spectroscopy indicated that the TiAlN∕CrAlN multilayer coatings were stable up to 900°C in air. TiAlN∕CrAlN multilayer coatings also exhibited improved corrosion resistance when compared to the MS substrate.

We have prepared nanolayered thin films of ZrO2/Al2O3 and ZrO2/Y2O3 using pulsed sputtering to study the feasibility of stabilization of various polymorphs of ZrO2. These films have been deposited at various substrate temperatures (350–700 °C), individual layer thicknesses (1.9–25.2 nm), and modulation wavelengths (Λ, 9.6–32.7 nm). The cross section of ZrO2 and Y2O3 films consisted of microcolumns, whereas the ZrO2/Al2O3 and ZrO2/Y2O3 multilayer systems exhibited a less columnar microstructure. X-ray diffraction (XRD) data showed monoclinic, cubic, and amorphous structures for ZrO2, Y2O3, and Al2O3 thin films, respectively. The tetragonal phase of ZrO2 (t-ZrO2) was stabilized for ZrO2/Al2O3 nanolayered thin films prepared at Λ≤13.8 nm. Our studies demonstrated that a critical ZrO2 thickness (≤10.5 nm at a substrate temperature of 700 °C) is required in order to stabilize the t-ZrO2 phase, which was in agreement with thermodynamically derived critical radius for the stabilization of the tetragonal phase. In the case of ZrO2/Y2O3 nanolayered thin films, the cubic phase of ZrO2 was stabilized for films prepared at 24.4 nm≥Λ≥9.9 nm. The nanolayered thin films prepared at Λ=9.9 nm exhibited satellite reflections of third order, suggesting the formation of superlattice structure. The formation of various polymorphs of ZrO2 was also confirmed by micro-Raman spectroscopy. XRD studies of nanolayered thin films heat treated in air demonstrated the stability of the tetragonal and the cubic phases of ZrO2 in the ZrO2/Al2O3 and ZrO2/Y2O3 multilayers, respectively, up to 1100 °C. ZrO2/Al2O3 and ZrO2/Y2O3 nanolayered thin films exhibited maximum nanoindentation hardness values of 30.5 and 28 GPa, respectively, which were very high as compared to the rule-of-mixture values. Detailed studies on elastic/plastic behavior of the multilayers indicated highest resistance to plastic deformation for the ZrO2/Al2O3 multilayers.

Nanocomposite coatings of CrN/Si3N4 and CrAlN/Si3N4 with varying silicon contents were synthesized using a reactive direct current (DC) unbalanced magnetron sputtering system. The Cr and CrAl targets were sputtered using a DC power supply and the Si target was sputtered using an asymmetric bipolar-pulsed DC power supply, in Ar+N-2, plasma. The coatings were approximately 1.5 mu m thick and were characterized using X-ray diffraction (XRD), nanoindentation, X-ray photoelectron spectroscopy and atomic force microscopy. Both the CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings exhibited cubic B1 NaCl structure in the XRD data, at low silicon contents (< 9 at.%). A maximum hardness and elastic modulus of 29 and 305 GPa, respectively were obtained from the nanoindentation data for CrN/Si3N4 nanocomposite coatings, at a silicon content of 7.5 at.%. (cf,. 24 and 285 GPa, respectively for CrN). The hardness and elastic modulus decreased significantly with further increase in silicon content. CrAlN/Si3N4 nanocomposite coatings exhibited a hardness and elastic modulus of 32 and 305 GPa, respectively at a silicon content of 7.5 at.% (cf., 31 and 298 GPa, respectively for CrAlN). The thermal stability of the coatings was studied by heating the coatings in air for 30 min in the temperature range of 400-900 degrees C. The microstructural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data of the heat-treated coatings in air indicated that CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings, with a silicon content of approximately 7.5 at.% were thermally stable up to 700 and 900 degrees C, respectively. (c) 2007 Elsevier B.V. All rights reserved.

Superhard nanocomposite coatings of TiAlN/Si3N4 with varying silicon contents were synthesized using reactive direct current (DC) unbalanced magnetron sputtering. The Si and TiAl targets were sputtered using an asymmetric bipolar-pulsed DC power supply and a DC power supply, respectively, in Ar+N2 plasma. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The elemental composition of the TiAlN/Si3N4 nanocomposite coatings was determined using energy-dispersive X-ray analysis and the bonding structure was characterized by X-ray photoelectron spectroscopy. The surface morphology of the coatings was studied using atomic force microscopy. The XRD data showed that the nanocomposite coatings exhibited (111) and (200) reflections of cubic TiAlN phase. The broadening of the diffraction peaks with an increase in the silicon content in the nanocomposite coatings, suggested a decrease in the average crystallite size. The TiAlN/Si3N4 nanocomposite coatings exhibited a maximum hardness of 43GPa and an elastic modulus of 350GPa at a silicon concentration of approximately 11at%. The hardness and the elastic modulus of the nanocomposite coatings decreased significantly at higher silicon contents. Micro-Raman spectroscopy was used to characterize the structural changes as a result of heating of the nanocomposite coatings in air (400–850°C) and in vacuum (900°C). The Raman data of the nanocomposite coatings annealed in air and vacuum showed better thermal stability as compared to that of the TiAlN coatings. Similarly, the nanocomposite coatings deposited on mild steel substrates exhibited improved corrosion resistance.

Approximately 1.5 mu m thick CrN and CrAIN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC) magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD data showed that the CrN and CrAIN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN and CrAIN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM and AFM showed a more compact and dense microstructure for CrAIN coatings. The thermal stability of the coatings was studied by heating the coatings in air from 400 to 900 degrees C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data revealed that CrN coatings got oxidized at 600 degrees C, whereas in the case of CrAIN coatings, no detectable oxides were formed even at 800 degrees C. After annealing up to 700 degrees C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAIN coatings, which exhibited hardness as high as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAIN coatings exhibited superior corrosion resistance as compared to CrN coatings. (c) 2006 Elsevier B.V. All rights reserved.

Single-phase CrN and CrAlN coatings were deposited on silicon and mild steel substrates using a reactive DC magnetron sputtering system. The structural characterization of the coatings was done using X-ray diffraction (XRD). The XRD data showed that both the CrN and CrAlN coatings exhibited B1 NaCl structure with a prominent reflection along (200) plane. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy and the surface morphology of the coatings was studied using atomic force microscopy. Subsequently, nanolayered CrN/CrAlN multilayer coatings with a total thickness of approximately 1μm were deposited on silicon substrates at different modulation wavelengths (Λ). The XRD data showed that all the multilayer coatings were textured along {200}. The CrN/CrAlN multilayer coatings exhibited a maximum nanoindentation hardness of 3125kg/mm2 at a modulation wavelength of 72Å, whereas single layer CrN and CrAlN deposited under similar conditions exhibited hardness values of 2375 and 2800kg/mm2, respectively. Structural changes as a result of heating of the multilayer coatings in air (400–800°C) were characterized using XRD and micro-Raman spectroscopy. The XRD data showed that the multilayer coatings were stable up to a temperature of 650°C and peaks pertaining to Cr2O3 started appearing at 700°C. These results were confirmed by micro-Raman spectroscopy. Nanoindentation measurements performed on the heat-treated coatings revealed that the multilayer coatings retained hardness as high as 2250kg/mm2 after annealing up to a temperature of 600°C.

A simple approach has been developed for the solid phase extraction of chromium(VI) based on the adsorption of its ion-association complex with cetyltrimethylammoniumbromide (CTABr) on an alumina column. The effect of various parameters such as acidity, stability of the column, sample volume, effect of diverse ions, etc. have been studied in detail. The adsorbed complex could be eluted using sodium hydroxide and the concentration of chromium has been established using visible spectrophotometry after complexation with diphenyl carbazide. The calibration graph was linear in the range 0-0.5 microg mL(-1) chromium(VI) with a detection limit of 5 microg L(-1). A highest preconcentration factor of 25 could be obtained for 250 mL sample volume. The data from the column studies were also studied using the Thomas model of adsorption. The experimental results obtained were correlated with the proposed model of adsorption. The Thomas rate constant k was found to be 0.0025 L/min mg and the maximum adsorption capacity q(o) was found to be 0.36 mgCr/g alumina at an initial chromium(VI) concentration of 1 mg L(-1). The validity of the method has been checked by applying it to study the recovery of chromium in spiked water samples and electroplating wastewater.