J. N. Balaraju

Nanocrystalline electroless nickel poly-alloy deposition: incorporation of W and Mo

Abstract Autocatalytic quaternary Ni–W–Mo–P films were prepared using alkaline citrate based baths and compared with binary Ni–P and ternary Ni–W–P, Ni–Mo–P coatings. Energy dispersive X-ray analysis showed that the binary Ni–P deposit contained 12·2 wt-%P. Codeposition of tungsten in Ni–P matrix resulted in ternary Ni–W–P with 4·1 wt-%P and 5·2 wt-%W. Incorporation of molybdenum led to a ternary Ni–Mo–P deposit containing 4·1 wt-%Mo and 11·2 wt-%P. Presence of both sodium tungstate and sodium molybdate in the basic bath resulted in a quaternary coating with 3·6 wt-%W, 6·7 wt-%Mo and 2·5 wt-%P. X-ray diffraction patterns of all the deposits revealed a single peak for Ni (1 1 1). The quaternary alloy exhibited a sharper peak showing the more crystalline nature of the deposit. Field emission scanning electron microscopy studies of the deposits showed the presence of smooth nodules for ternary deposits, but coarse and well defined nodules for quaternary deposits. Phase transformation behaviour of the ternary Ni–W–P deposit revealed a single exothermic peak at 440°C. However, ternary Ni–Mo–P deposit exhibited a split type high temperature peak at 397 and 461°C and the quaternary Ni–W–Mo–P deposit showed a single high temperature peak at 485°C. Microhardness measurements showed that the quaternary Ni–W–Mo–P deposit exhibited increased hardness of 920 HV(50 gf) when heat treated for 1 h at 400°C.

Electrochemical behaviour of high phosphorus electroless Ni–P–Si3N4 composite coatings

Abstract A high phosphorus electroless nickel bath was used to prepare plain Ni–P and composite coatings containing submicrometre size silicon nitride particles. Deposits were characterised for their composition, morphology and electrochemical behaviour. Codeposition of particles in a Ni–P matrix has not influenced the phosphorus content (10 wt-%). Surface morphology of plain Ni–P deposits was smooth; the composite deposits became slightly rough with small nodules due to particle incorporation. Cross-sectional examination of composite coating revealed that the particles were uniformly distributed throughout the thickness of the coating. Potentiodynamic polarisation and electrochemical impedance studies were carried out in 3·5 wt-% sodium chloride solution in non-deaerated condition. Potentiodynamic polarisation studies showed that the corrosion current density value obtained for composite coatings is lower than that for plain Ni–P coatings. Electrochemical impedance spectroscopy studies showed that the coating resistance of the composite coating is higher than that of plain Ni–P coating. This was further confirmed by SEM analysis of corroded samples.

Comparison in the Oxidation and Corrosion Behavior of Aluminum and Alumina-Reinforced Ni/Ni-Co Alloy Coatings

In this study, a comparison in the oxidation and corrosion behavior of Ni/Ni-Co aluminum and alumina-reinforced electrodeposited composites has been made. The developed coatings were characterized for the morphology, structure, microhardness, oxidation, and corrosion resistance. It was found that the incorporation of Al particles in NiCo matrix is higher (9xa0wtxa0pct) compared to Ni matrix (1xa0wtxa0pct). In the case of aluminum oxide particles, about 5 and 7xa0wtxa0pct had been obtained in Ni and NiCo matrices respectively. The difference in the surface morphology was observed with respect to metallic (Al) and inert ceramic (Al2O3) particle incorporation. X-ray diffraction studies showed the presence of predominant Ni (200) reflection in the coatings. Also, peaks corresponding to Al and Al2O3 particles were present. The Ni/NiCo-Al coatings exhibited higher microhardness values at 1273xa0K (1000xa0°C) compared to alumina-reinforced coatings, indicating better thermal stability of the former coatings. The NiAl coating showed one and two orders of magnitude improved oxidation resistance compared to NiCoAl and Ni/NiCo-Al2O3 coatings, respectively. It was observed that the Ni-Al composite coating exhibited poor corrosion resistance in 3.5xa0pct NaCl solution compared to the other coatings studied.

Electrochemical behavior of nanocrystalline Ni–P alloys containing tin and tungsten

Autocatalytic ternary Ni-Sn-P, Ni-W-P and quaternary Ni-W-Sn-P films were prepared using an alkaline bath. Plain Ni-P films were also prepared for comparison. Corrosion resistance of the films was evaluated in 3.5% sodium chloride solution in non-deaerated condition by potentiodynamic polarization and electrochemical impedance spectroscopy methods. Deposits were also immersed in 3.5% sodium chloride solution for 7 days. All the coatings attained stable equilibrium potential within 30 minutes in NaCl medium. Lower corrosion current density values were obtained for ternary Ni-Sn-P coatings compared to the plain Ni-P coatings. Ternary Ni-W-P and quaternary Ni-W-Sn-P alloys did not show improved corrosion resistance compared to the ternary Ni-Sn-P coatings. Similar behavior of these coatings was further confirmed by the electrochemical impedance studies. After the potentiodynamic polarization test deposits were examined by scanning electron microscope. It was found that more corrosion occurred for the quaternary deposit compared to other deposits. Energy dispersive analysis of X-ray results indicated that more amount of Fe present on NiWP and NiWSnP coated samples. Similar behavior was confirmed from the optical images of the surfaces obtained for the deposits after the immersion test. The article is published in the original.

Mechanical Properties of Titanium–Titanium Boride Composites Through Nanoindentation and Ultrasonic Techniques — An Evaluation Perspective

In this work, the mechanical properties such as elastic moduli, shear moduli, and hardness of titanium–titanium boride composites with 20% and 40% of titanium boride reinforcements were estimated by ultrasonic and nanoindentation techniques. The estimated values obtained from the both measuring techniques are compared. The composites were processed by three powder metallurgical techniques such as spark plasma sintering, hot isostatic pressing, and vacuum sintering. The composites processed through spark plasma sintering and hot isostatic pressing showed better mechanical properties compared to the vacuum sintered composites. The effects of titanium boride reinforcements and their morphological influences on the mechanical properties are also described.

Surface and Electrochemical Characteristics of Novel Chromate-Free Mn-V Oxyanion Sealed Tartaric–Sulfuric Acid Anodized Coating

In the present investigation, a novel chromate-free Mn and V oxyanions sealed tartaric–sulfuric acid anodic oxide coating (TSA) was developed on airframe grade aluminum alloy (AA2024). The results obtained from field emission scanning electron microscopy and energy-dispersive x-ray analysis showed effective surface modification of TSA after Mn-V oxyanions sealing (TSAMnVO). Potentiodynamic polarization results showed no significant difference in corrosion current density value (0.01xa0µA/cm2) after 1xa0h and 336xa0h of immersion in 3.5% NaCl solution. EIS result exhibited barrier layer resistance (Rb) value of TSAMnVOxa0>xa0106xa0kΩxa0cm2 even after 336xa0h of immersion in NaCl solution. The 3D profile and Raman analysis showed no significant difference in the topography and composition, respectively, on the surface of TSAMnVO even after 1000xa0h of salt spray test. TSAMnVO also showed excellent adhesion with aircraft grade epoxy primer as per the ASTM 3359. Tensile and constant amplitude fatigue test results of TSAMnVO exhibited ultimate tensile strength and number of cycles to failure comparable with chromic acid anodization (CAA) process. Hence, the developed novel TSAMnVO coating system eliminates the usage of chromate ions completely in anodization process since it provides comparable corrosion resistance and mechanical properties with CAA.

FESEM and XPS studies of ZrO2 modified electrodeposited NiCoCrAlY nanocomposite coating subjected to hot corrosion environment

The aim of this study is to investigate the influence of Co and reactive element oxide, i.e. ZrO2, addition on the hot corrosion behavior of NiCrAlY electrodeposited nanocomposite coatings. The hot corrosion behavior of the coatings was investigated in a medium comprising 75% Na2SO4 and 25% NaCl and expressed as weight change. The addition of ZrO2 resulted in a reduction of Co content from 40 wt% to 20 wt%, as shown by EDX. The hot corrosion studies revealed that the addition of cobalt and yttrium to the NiCrAl coating enhanced its corrosion resistance. However, the effect of Y was more pronounced in the case of NiCrAl compared to NiCoCrAl coatings. The introduction of small amounts of ZrO2 further resulted in a reduction in weight gain. The formation of oxide of Ni on the surface of NiCrAlY and that of Co on the surface of NiCoCrAlY coatings was confirmed by FESEM and XPS analysis. XPS analysis revealed the presence of oxide of Al as well as small amounts of oxide of Cr on the surface and near surface of NiCrAlY and NiCrAlY(Z) coatings, while only oxide of Cr was observed in NiCoCrAlY and NiCoCrAlY(Z) coatings and this was responsible for their improved hot corrosion resistance.

Effect of Anodized Oxide Layer Aging on Wettability of Alkyl Silane Coating Developed on Aerospace Aluminum Alloy

Incipient nanoporous texture was developed by phosphoric acid anodization (PAAO) process on AA2024 specimen. The developed oxide layer was modified with low surface energy material fluoroalkyl-silane (FAS-13) by simple immersion technique to improve the hydrophobic property of the surface. Atomic force microscopy results showed that there was a significant reduction (30xa0pct) in surface roughness due to the aging of PAAO. The water contact angle measurement revealed that there was a gradual increase in WCA from 130 to 160 deg due to the aging of PAAO. A systematic composition analysis was carried out to understand the interfacial chemical reaction and also to differentiate the coating formation mechanism between the natural and artificial aging processes. The potentiodynamic polarization results revealed that the superhydrophobic (SH) surface exhibited about 15 and 4 times improved corrosion resistance compared to bare specimen and PAAO, respectively. Electrochemical impedance spectroscopy results also showed the improved corrosion inhibition behavior of vacuum heat-treated and FAS-modified SH surface among the developed coatings.