Reactive sintering of electroless nickel-plated aluminum powders

Abstract

Fine (about 2 μm) particle size aluminum powders were produced in a plasma reactor, in a flow of argon gas, and the particle size distribution analyzed. A uniformly thick nickel film was deposited on the aluminum powder particles by electroless nickel plating, thus providing more efficient surface contact than can be obtained using conventional methods such as mechanical mixing of nickel and aluminum powder. Nickel-plated aluminum compacts were subsequently sintered at various temperatures in argon and in vacuum, and Ni₃Al intermetallic alloys produced. Differential thermal analyses, scanning electron microscopy, and X-ray diffraction analyses of the sintered samples were carried out. Satisfactory densities of 99.8% were achieved by sintering in vacuum at temperatures as low as 680°C.

Introduction

Intermetallic compounds are potentially useful structural materials that form long-range ordered crystal structures below their melting points. Aluminum-based intermetallic compounds have excellent potential for use at elevated temperatures because of their low density and a high strength that increases with increasing temperature. Because of their high corrosion and creep resistance, they are used in corrosive environments such as oil and gas wells, chemical process industry, marine environments, as automotive pistons, turbochargers, valves, aircraft fasteners, dental drills, forging dies, bearings, tubing, casting moulds for aluminum and glass, and heating elements for appliances [1]. Many of these compounds have provided sufficient justification for replacing conventional nickel-based superalloys [1], [2], [3].

Despite their attractive properties, intermetallic alloys have limited applications due to their brittleness. Liu et al. [4], [5], [6], [7] have worked on improving their ductility at room temperature, culminating in an elongation of over 50% in boron-doped, nickel-rich, 24% at. Al Ni₃Al. It was this work that led to the conclusion that the boron preferentially segregates to the grain boundaries [8], [9].

Reactive sintering occurs by the formation of a transient liquid phase during the exothermic reaction between metal powders, with reactions between the constituent powders characterized by large heat releases. Reactive sintering has been applied to the formation of intermetallic alloys such as NiAl, TiAl, TiC, TiN and various silicon-nitride materials, in the search for a method of production of high-density intermetallic alloys such as Ni₃Al at temperatures as low as 500°C [10]. The reactive sintering process used in the production of nickel–aluminum powders results in Ni₃Al samples with the general appearance of a face-centered cubic phase [11], [12], [13]. It has also been reported [14], [15], [16] that the structure of Ni₃Al remains ordered at temperatures up to very close to its melting point.

In the present work, the focus was on making fine aluminum powder using a plasma reactor, coating the powder with nickel via an electroless nickel plating process, then using reactive sintering to produce high-density (99.8%) Ni₃Al alloy samples. Powders made up of coarse particles result in a poor distribution of liquids in the microstructure during sintering, leading to final microstructures containing large pores. The use of finer particle sizes means that there is more surface contact between the particles and conditions are thus optimized for the production of a high-density, homogeneous microstructure. The use of electroless nickel plating process results in a fine and homogeneous distribution of nickel over the aluminum powder particles, leading to more interdiffusion during heating. Finally, reactive sintering allows the production of high-density (99.8%) Ni₃Al alloy at temperatures (e.g., 680°C) much lower than those needed when conventional methods, such as hot isostatic pressing, are employed.