Formation and characterization of borohydride reduced electroless nickel deposits

The present work aims to study the formation of electroless Ni-B deposits and evaluation of their characteristic properties. An alkaline bath having nickel chloride as the source of nickel and borohydride as the reducing agent was used to prepare the electroless Ni-B deposits. The influence of variation in bath constituents as well as operating conditions on the plating rate, and, the nickel and boron content, of the resultant Ni-B deposits were studied. Selected deposits were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), evolved gas analysis (EGA), vibrating sample magnetometer (VSM) and transmission electron microscope (TEM), respectively, for assessing the phase content, phase transformation behaviour, liberation of hydrogen during crystallization, saturation magnetic moment and micro-structural features. The corrosion resistance of Ni-B deposits, in 3.5% sodium chloride solution, both in as-plated and heat-treated (450 ◦C/1 h) conditions, was also evaluated by potentiostatic polarization and electrochemical impedance studies. XRD patterns reveal that Ni-B deposits of the present study are amorphous in as-plated condition and undergo phase transformation to crystalline nickel and nickel borides upon heat-treatment. DSC traces exhibit two exothermic peaks at 306 and 427 ◦C, corresponding to the phase transformation of amorphous Ni-B to crystalline nickel and Ni3B phases and the transformation of a higher phase compound to Ni3B and Ni2B, respectively. TEM microstructures and EGA strongly support the occurrence of phase transitions at 306 and 427 ◦C. Electroless Ni-B deposits demonstrate a moderate corrosion resistance in 3.5% sodium chloride solution. The extent of corrosion resistance offered by electroless Ni-B deposits is relatively less compared to electroless Ni-9 wt.% P deposit

Electro- and electroless plated coatings for corrosion protection

This chapter presents an overview of the fundamental aspects of electro- and electroless deposition process, the mechanism of deposition, alloy deposition, etc. and the utility of these coatings for corrosion protection. Both electro- and electroless deposition techniques are simple, cost-effective and offer unique advantages for preparing deposits with desirable qualities. In electrodeposition, the plating rate, stability of the bath and the number of turnovers are very high but the resultant coatings lack uniformity on complex shapes and blind holes and they need a post-finishing treatment to achieve the desired performance. In electroless deposition, the plating rate, bath stability and the number of turnovers are relatively less but the resultant coatings are more uniform and do not require post-finishing treatment. Electroplated coatings offer corrosion protection to the substrate metals in three possible ways: (i) cathodic protection; (ii) barrier action; and (iii) environmental modification or control. The corrosion performance of electroplated coatings is influenced by a variety of factors, which include structure, crystallographic texture, grain size, porosity, impurities and triple junctions, interactions involving metallic underplates and cleanliness or freedom from processing contaminants. Electroless nickel does not perform as a sacrificial coating in the same way that electrodeposited Zn or Cd performs on steel substrate to provide protection against corrosion. It behaves as a true barrier coating, protecting the substrate by sealing it off from the corrosive environments. Consequently, the thickness of the deposit and the absence of porosity are of great importance. The electroless nickel coating shows superior corrosion resistance compared to electroplated nickel coatings. The most important factors that determine the corrosion resistance of electroless plated coatings are: substrate composition, structure and surface finish; pretreatment of the substrate to achieve a clean, uniform surface; adequate deposit thickness to meet the severity and time of exposure to the corrosive environment; the properties of the deposit (composition, porosity, internal stress etc.) which depends on pH, formulation and prolonged use (turnover) of the plating solution; post plating treatments of the coating such as passivation and annealing; and the aggressiveness of the corrosive environment condition. Electro- and electroless deposited ternary/quaternary alloy coatings, composite coatings, duplex coatings, graded coatings and multilayer coatings are some of the promising developments to achieve improved corrosion resistance

Cathodic electrosynthesis of alumina thin films and powders

The present work explores the utility of cathodic electrosynthesis methodology in the preparation of alumina thin films and powders

Electroless Ni–P/Ni–B duplex coatings: preparation and evaluation of microhardness, wear and corrosion resistance

The present work deals with the formation of Ni–P/Ni–B duplex coatings by electroless plating process and evaluation of their hardness, wear resistance and corrosion resistance. The Ni–P/Ni–B duplex coatings were prepared using dual baths (acidic hypophosphite- and alkaline borohydride-reduced electroless nickel baths) with both Ni–P and Ni–B as inner layers and with varying single layer thickness. Scanning electron microscopy (SEM) was used to assess the duplex interface. The microhardness, wear resistance and corrosion resistance of electroless nickel duplex coatings were compared with electroless Ni–P and Ni–B coatings of similar thickness. The study reveals that the Ni–P and Ni–B coatings are amorphous in their as-plated condition and upon heat-treatment at 450 ◦C for 1 h, both Ni–P and Ni–B coatings crystallize and produce nickel, nickel phosphide and nickel borides in the respective coatings. All the three phases are formed when Ni–P/Ni–B and Ni–B/Ni–P duplex coatings are heat-treated at 450◦C for 1 h. The duplex coatings are uniform and the compatibility between the layers is good. The microhardness, wear resistance and corrosion resistance of the duplex coating is higher than Ni–P and Ni–B coatings of similar thickness. Among the two types of duplex coatings studied, hardness and wear resistance is higher for coatings having Ni–B coating as the outer layer whereas better corrosion resistance is offered by coatings having Ni–P coating as the outer layer

Electroless Ni–B coatings: preparation and evaluation of hardness and wear resistance

The present work aims to study the hardness and wear resistance of electroless Ni–B coatings. An alkaline bath having nickel chloride as the source of nickel and borohydride as the reducing agent was used to prepare the electroless Ni–B coatings. The structure, microhardness and wear resistance of electroless Ni–B coatings, both in as-plated and heat-treated conditions, were evaluated using X-ray diffraction (XRD), Leitz microhardness tester and a pin-on-disc wear test apparatus. XRD patterns reveal that electroless Ni–B coatings are amorphous in as-plated condition and undergo phase transformation to crystalline nickel and nickel borides upon heat-treatment. The microhardness of the electroless Ni–B coatings increases with increase in heat-treatment temperature and exhibit two maxima in the hardness vs. heattreatment temperature curve. The specific wear rate increases with increase in applied load from 20 to 40 N and at all applied loads, the specific wear rate and coefficient of friction are less for heat-treated electroless Ni–B deposits compared to that obtained for as-plated ones. The wear process of electroless Ni–B coatings is governed by an adhesive wear mechanism

Corrosion resistance of electrodeposited Ni–B and Ni–B–Si3N4 composite coatings

Corrosion resistance of electrodeposited (ED) and electroless (EL) composite coatings have been a debatable issue in the published literature. The present paper aims to compare the corrosion resistance of ED Ni–B–Si3N4 composite coating with its plain counter part. The ED Ni–B coatings were prepared using Watt’s nickel bath modified with the addition of dimethylamine borane and the ED Ni–B–Si3N4 composite coatings were prepared using the same bath in which Si3N4 particles (mean diameter: 0.80m) were dispersed in it. The structural and morphological characteristics of ED Ni–B and Ni–B–Si3N4 composite coatings were determined using X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). The corrosion resistances of ED Ni–B and Ni–B–Si3N4 composite coatings, both in asplated and heat treated conditions, in 3.5% NaCl, were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies. The study reveals that the extent of shift in corrosion potential (Ecorr) towards the noble direction, decrease in corrosion current density (icorr), increase in charge transfer resistance (Rct) and decrease in double layer capacitance (Cdl) values with the incorporation of Si3N4 particles in the ED Ni–B matrix is not appreciable, both in as-plated and heat-treated conditions. The occurrence of the second phase angle maximum suggests penetration of the electrolyte via the pores/micro-pores in these coating to create another interface, namely, the electrolyte/substrate. Unlike the nanosized particles, the micron size Si3N4 particles (mean diameter: 0.80m) used in this study is not capable of completely filling all the pores in the coating and allowed diffusion of chloride ions along the interface. The marginal improvement in corrosion resistance observed for ED Ni–B–Si3N4 composite coatings compared to its plain counterpart could have resulted from the decrease in effective metallic area prone to corrosion

Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO

We discuss the properties of semiconducting bulk ZnO when substituted with the magnetic transition metal ions Mn and Co, with substituent fraction ranging from $x$ = 0.02 to $x$ = 0.15. The magnetic properties were measured as a function of magnetic field and temperature and we find no evidence for magnetic ordering in these systems down to $T$ = 2 K. The magnetization can be fit by the sum of a Curie-Weiss term with a Weiss temperature of $\Theta\gg$100 K and a Curie term. We attribute this behavior to contributions from both \textit{t}M ions with \textit{t}M nearest neighbors and from isolated spins. This particular functional form for the susceptibility is used to explain why no ordering is observed in \textit{t}M substituted ZnO samples despite the large values of the Weiss temperature. We also discuss in detail the methods we used to minimize any impurity contributions to the magnetic signal.Comment: 6 pages, 4 figures (revised

Control of superluminal transit through a heterogeneous medium

We consider pulse propagation through a two component composite medium (metal inclusions in a dielectric host) with or without cavity mirrors. We show that a very thin slab of such a medium, under conditions of localized plasmon resonance, can lead to significant superluminality with detectable levels of transmitted pulse. A cavity containing the heterogeneous medium is shown to lead to subluminal-to-superluminal transmission depending on the volume fraction of the metal inclusions. The predictions of phase time calculations are verified by explicit calculations of the transmitted pulse shapes. We also demonstrate the independence of the phase time on system width and the volume fraction under specific conditions.Comment: 21 Pages,5 Figures (Published in Journal of Modern Optics