K.S. Rajam

Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings

About 1.5-um-thick single-layer TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings were deposited on silicon (111) substrates using a13; reactive direct current magnetron sputtering process. Structural characterization of the coatings was done using x-ray diffraction (XRD) and micro-Raman spectroscopy. All13; the coatings exhibited NaCl B1 structure in the XRD data. Raman spectroscopy data of as-deposited coatings exhibited two broad bands centered at 230x2013;250 and 540x2013;630 cmx2212;1.13; These bands have been assigned to acoustical and optical phonon modes, respectively. Thermal stability of the coatings was studied by heating the coatings in air in a13; resistive furnace for 30 min in the temperature range 400x2013;900 xB0;C. Structural changes as a result of heating were characterized using Raman spectroscopy and XRD. Raman13; data showed that TiN, CrN, TiN/CrN, TiAlN, and TiAlN/CrN coatings started to oxidize at 500, 600, 750, 800, and 900 xB0;C, respectively. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, samples were also annealed in vacuum at 800 xB0;C under similar conditions. The Raman data of vacuum-annealed coatings showed no phase transformation, and intensity of the optical phonon mode increased and shifted to lower frequencies. The origin of these spectral changes is discussed in terms of defect structure of the coatings. Our results indicate that the thermal stability of nanolayered multilayer coatings is superior to the13; single-layer coatings.

TiAlN∕TiAlON∕Si3N4 tandem absorber for high temperature solar selective applications

A tandem absorber of TiAlN∕TiAlON∕Si3N4 is prepared using a magnetron sputtering process. The graded composition of the individual component layers of the tandem absorber produces a film with a refractive index increasing from the surface to the substrate, which exhibits a high absorptance (0.95) and a low emittance (0.07). The tandem absorber is stable in air up to 600°C for 2h, indicating its importance for high temperature solar selective applications. The thermal stability of the tandem absorber is attributed to high oxidation resistance and microstructural stability of the component materials at higher temperatures.

Characterization of Cu/Ni multilayer coatings by nanoindentation and atomic force microscopy

Cu/Ni multilayer coatings prepared by RF/DC magnetron sputtering process were characterized using x-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) techniques. Films deposited under certain deposition conditions showed the appearance of satellite reflections around the principal reflections in the XRD data, indicating the formation of superlattice structure. Nanohardness measurements were performed on the films prepared under different deposition conditions, e.g. modulation wavelength (x39B;), copper to nickel thickness ratio (tCu/tNi) and substrate temperature (Ts). Nanohardness data revealed that the hardness was enhanced by a factor of 2.5 times that of the rule-of-mixtures value. This enhancement in hardness occurred over a limited range of x39B;. The results indicated that the hardness also depends on the copper to nickel thickness ratio and for tCu/tNi0.30 the films exhibited a maximum hardness. The hardness increased almost linearly with substrate temperature up to 200 xB0;C and got saturated at 250 xB0;C. No considerable change in the hardness was observed after vacuum annealing of the coatings. Imaging of the residual imprints by AFM revealed pile-up formation after indentation. The extent of pile-up, a measure of plastic flow of the material, was dependent on the preparation conditions of the multilayer coatings.

Structure and optical properties of pulsed sputter deposited CrxOy/Cr/Cr2O3 solar selective coatings

Spectrally selective CrxOy∕Cr∕Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottomlayer)∕Cr∕Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy∕Cr∕Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical const...

Investigation of surface composition of electrodeposited black chrome coatings by X-ray photoelectron spectroscopy

: Solar selective black chromium coating was electrodeposited on pre-treated electroformed nickel substrates from a hexavalent chromium containing bath. The composition of the film was investigated before and after annealing at 400 degreesC for different durations. In the as-deposited condition. the surface of the film was found to have trivalent chromium hydroxide and chromium in the chromate form contrary to previous studies which report the presence of hydroxides and metallic chromium. However in the present study, no evidence for metallic chromium was found. The major component. chromium hydroxide, was converted to Cr2O3 on annealing at 400 degreesC with the loss of water vapor. The chromate form remains but with a lowered concentration.

Thermal stability of TiAlN∕TiAlON∕Si3N4 tandem absorbers prepared by reactive direct current magnetron sputtering

Spectrally selective TiAlN∕TiAlON∕Si3N4 tandem absorber was deposited on copper, stainless steel, and Nimonic substrates using a reactive direct current magnetron sputtering system. The absorptance and the emittance of the tandem absorbers were characterized using solar spectrum reflectometer and emissometer. The surface morphology of the tandem absorbers was studied using atomic force microscopy. The compositions and the thicknesses of the individual component layers have been optimized in such a way to achieve high absorptance (0.958) and low emittance (0.07 at 82°C). In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The structural changes as a result of heating of the tandem absorbers were studied using micro-Raman spectroscopy. The tandem absorbers deposited on copper substrates exhibited high solar selectivity in the order of 9–10 even after heat treatment in air up to 600°C for 2h. These...

Deposition of TiN/CrN hard superlattices by reactive d.c. magnetron sputtering

Multilayer superlattice coatings of TiN/CrN were deposited on silicon substrates using a reactive d.c. magnetron sputtering process. Superlattice period, also known as modulation wavelength (A), was controlled by controlling the dwell time of the substrate underneath Ti and Cr targets. X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the films. The XRD data showed 1st and 2nd order satellite reflections along the principal reflection for films having 132 Å > Å > 84 Å, thus confirming the formation of superlattice. The multilayer coatings exhibited hardness(H) as high as 3200 kg/mm2, which is 2 times the rule-of-mixtures value (i.e.)HTiN= 2200 kg/mm2 andHCrN= 1000 kg/mm2). Detailed investigations on the effects of various process parameters indicated that hardness of the superlattice coatings was affected not only by modulation wavelength but also by nitrogen partial pressure and ion bombardment during deposition.

Effect of substrate roughness on the apparent surface free energy of sputter deposited superhydrophobic polytetrafluoroethylene coatings: A comparison of experimental data with different theoretical models

We have studied the effect of substrate roughness on the wettability and the apparent surface free energy (SFE) of sputter deposited polytetrafluoroethylene (PTFE) coatings deposited on untreated glass (average roughness, Ra=2.0 nm), plasma etched glass (Ra=7.4 nm), and sandblasted glass (Ra=4500 nm) substrates. The wettability of the PTFE coatings deposited on substrates with varying roughnesses was evaluated by measuring the apparent contact angle (CA) using a series of probe liquids from nonpolar aprotic to polar protic. The wettability measurements indicate that an apparent water CA of 152° with a sliding angle of 8° was achieved for PTFE coatings deposited on a substrate with Ra=4500 nm. The superhydrophobicity observed in these coatings is attributed to the presence of dual scale roughness, densely packed microstructure and the presence of CF3 groups. Unlike the bulk PTFE which is mainly dispersive, the sputter deposited PTFE coatings are expected to have some degree of polar component due to the pl...

Effect of substrate roughness on the apparent surface free energy of sputter deposited superhydrophobic polytetrafluoroethylene thin films

Surface morphology plays an important role in determining the superhydrophobic behavior of solids. To provide insights on the influence of roughness over a wide range, we have carried out a quantitative study of effect of substrate roughness on contact angle and surface free energy (SFE) of sputter deposited superhydrophobic polytetrafluoroethylene thin films. An optimum value for substrate roughness has been derived and a practical methodology (sand blasting) for its fabrication has been proposed. The SFE values were calculated using Zisman method and validated by Fowkes–Girifalco–Good theory.

A Raman-scattering study on the interface structure of nanolayered TiAlN∕TiN and TiN∕NbN multilayer thin films grown by reactive dc magnetron sputtering

Nanolayered multilayer coatings of TiAlN∕TiN and TiN∕NbN were deposited on Si (100) substrates at various modulation wavelengths (i.e., bilayer thickness, Λ) using a reactive dc magnetron sputtering system. These coatings were characterized using micro-Raman spectroscopy to study the effect of interfaces on the optical-phonon modes. For TiAlN∕TiN multilayers, the optical-phonon band shifts to higher frequencies with a decrease in the modulation wavelength. Furthermore, the optical-phonon band shifts to higher frequencies with an increase in the substrate temperature for TiAlN∕TiN multilayers deposited at Λ=80A. No such shift was observed for single-layer TiN and TiN∕NbN multilayer coatings. This observed shift has been attributed to interdiffusion between the layers during deposition, which is more for TiAlN∕TiN multilayers as compared to TiN∕NbN multilayers. The x-ray-diffraction data showed well-defined satellite reflections for TiN∕NbN multilayers at low modulation wavelengths and very weak satellite r...