Magnetic moment studies on nickel coated titanium powders

Abstract

Titanium aluminide intermetallics are gaining greater importance due to their low density, high melting temperature, good high temperature properties and oxidation resistance. Their brittle behaviour at low temperatures has resulted in several investigations aimed at improving their ductility. This can be improved either by suitably alloying or by microstructure control [1–5] and in some cases by both. In general, microstructural refinements developed through processing lead to improvements in both strength and ductility [6]. Thus, processing techniques that increase the range of attainable microstructures are fundamental to the development of new engineered materials. These alloys cannot be produced by the traditional metallurgical processes, because of the restricted range of the atomic percentage of the alloying elements. Conventional powder metallurgical (P=M) techniques can be used for preparing these alloys, with an additional step involving coating on the elemental powders. There are several coating methods available like vapour phase deposition techniques, spray deposition, electroplating, and electroless coating. Each of these has its advantages and disadvantages. Vapour phase deposition methods can be used for coating towbased fibres such as carbon fibres [7]. Electroplating methods can also be used for carbon fibres, for which the substrate should be a conducting material. Coating a powder with a particular element is easier by the electroless coating method because it gives a uniform coating on the substrate. The electroless plating process is an intermediate step in the fabrication of ceramic–metal and metal– metal composites, because it gives a uniform adherent metal coating on substrates. Electroless nickel can be applied to metallic surfaces as well as non-metallic materials to provide a coating of uniform thickness with excellent chemical and physical properties [8]. The production of the titanium aluminide intermetallic alloy from mixtures of the elemental powders may not produce a chemically homogeneous product under normal P=M processes. With pre-alloyed powders, the problem of heterogeneity can be largely eliminated, but it is not cost-effective. Electroless coatings on metal powders enable the coated powder to exhibit a high level of homogeneity during P=M preparation. The aim of this work was to investigate the feasibility of obtaining a satisfactory electroless nickel coating on Ti elemental powders and to analyse this coating with the help of magnetic properties. Measurement of magnetic moment is a novel idea to confirm the presence of nickel on the titanium elemental powders. The presence of nickel coating on titanium powders was further confirmed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies. Elemental powders of commercially pure titanium (,75 im) were used for the coating of nickel. The composition of the chemical deposition bath used for the coating of nickel on titanium powder is given in Table I. The proportion of elemental powders added to the coating bath was 70 g to 1 l. A pH of 8–9 was maintained by adding ammonia solution to the bath. With slow stirring, using a magnetic stirrer, all of the particles were allowed to come into contact with the solution. A temperature of 88 8C was maintained throughout the experiment. The coating time was varied (20, 30 and 40 min.) to increase the amount of nickel coated on the elemental powders. The coated powders were rinsed in distilled water, filtered out and dried under infrared (IR) light. Chemical analysis for the presence of nickel was done using dimethyl glyoxime dissolved in methanol. A purple precipitate confirmed the presence of nickel. The presence of coating was analysed with the help of SEM. The presence of nickel as a coating on the elemental powders was confirmed by EDS analysis. Figs 1 and 2 show the SEM micrographs of the uncoated and coated titanium powders. Fig. 3 shows the EDS analysis of nickel-coated titanium powders. Figs 2 and 3 establish the presence of nickel as a coating on the substrate. The magnetic moment test was conducted on the coated particles to confirm the increase in the coating thickness with time. The coating material and the substrate are ferromagnetic and paramagnetic, respectively. From the theory of magnetism, the magnetic moment increases directly with the increase in susceptibility of the material. This concept is used here to determine the increase in coating thickness. Fig. 4 shows magnetic moment plotted against magnetic field for different coating durations. The magnetic moment slowly increases