Nanomechanical characterization of nanocrystalline Ni directly plated on aluminium

Abstract

Aluminium is the second largest consumer industrial item after steel. The high strength to weight ratio of aluminium allows a substantial savings in most applications. There is always a high demand for plating on Al and its alloys i.e. in automotive and aerospace products, house-hold goods, artificial jewellery, to name some. The achievable properties that can be added to aluminium are mechanical, magnetic, electrical, thermal, corrosive and decorative. As an example it could be interesting to add wear resistance (abrasive or adhesive wear), hardness and corrosion protection to aluminium. However, as soon as aluminium is exposed to the atmosphere a thin oxide film forms almost instantaneously. In its natural form this oxide film is less than 0.10µm thick. Although this natural oxide film is very hard, tenacious, abrasion resistant and an insulator but as it is attached to a soft ductile metal, it is easily damaged. The presence of the same oxide film (which makes the surface of the aluminium act as a non-conductor) has made it impossible to deposit any decorative or wear resistant coatings by electrodeposition on aluminium. This paper will for the first time discuss a new, an innovative and world's first ever process for directly electrodepositing nanocrystalline Ni on aluminium surfaces without the need to pre-treat the aluminium substrates with acid/s and pre-plate with an intermediate metallic layer. Nanomechanical test instrument was used to evaluate the mechanical properties of the as plated Ni/Al2O3/Al layers. Several partial-unloading nanoindentation tests were performed on the samples using a Berkovich probe to measure the hardness and reduced modulus of the samples as a function of contact depth. These test results will be discussed further to elucidate the mechanism of adhesion of the electrodeposited nanocrystalline Ni with the intermediate non-conductive aluminium oxide (Al2O3) layer at a nano-scale.