Photo: Siddharth Kankaria / Research Matters
In a recently published study, scientists at the Indian Institute of Science, Bangalore, are addressing one of the biggest challenges faced by many appliances - wear and friction due to usage. Wear and friction affects the lifetime of industrial equipment, which directly correlates with the profitability of the business. The teams of researchers, led by Prof. M.S. Bobji at the Department of Mechanical Engineering are now experimenting with alumina based nanocomposite coating for wear resistance.
Materials used to resist wear and friction have contrasting requirements – they should be hard enough to resist abrasive wear and, at the same time, need to be able to easily absorb and dissipate energy resulting from friction. A conventional method used to resist wearing is to coat it with another material. Hard coatings, used often, are brittle and fracture easily. They are also prone to delamination - a process where the coating on the base material separate. Since the material is prone to fractures, there is also a need to prevent the propagation of cracks.
This study has focused on preventing the propagation of cracks. Nano-porous alumina, an oxide of aluminum, can be a good wear resistant material due to its high hardness. The researchers found that by dispersing metal into the nanopores, the propagations of cracks through the material can be arrested.
Anodizing is an electrolytic process used to increase the thickness of the natural oxide layer on the surface of metal parts, thus making the metal surface durable and corrosion resistant. A coating of porous alumina can be obtained by anodisation of pure aluminum using a two-step anodisation process. A dense oxide layer, whose thickness varies with the applied anodisation voltage, exists at the interface between porous alumina and aluminum. Thick oxide layer called as barrier layer, which is at the interface between pure aluminium and porous alumina coating. This barrier layer offers high electrical resistance and prevents electrochemical deposition of metal. To overcome this, the thickness of the barrier layer has been reduced. Cathodic polarization, a method that involves reversing the polarity and electrochemically etching the barrier layer, is one method to achieve this. However, this process is difficult to control and produces a mechanically weak interface.
The researchers used stepwise voltage reduction for thinning the barrier layer. In this method, the anodizing voltage is reduced in steps after desired length of pores has been formed. Since the pore diameter depends on the anodizing voltage, this process results in a dendrite like structure at bottom of the pore. Then, the pores are filled with copper metal by electrochemical deposition. Tribological behaviour of this nano-composite coating was evaluated using a ball on flat reciprocating tribometer, an apparatus used to study wear resistance, under the dry contact conditions. The nanocomposite coating generated was found to have higher wear resistance compared to the porous alumina coating.
“Wear resistant nanocomposite coating has wide applications especially in protecting the internal surfaces of aluminium internal combustion engines and in the space industry. Aluminium alloy is also one of the potential candidates in the aerospace industry. Currently we are planning to extend this process to aluminium alloys and make nano-composite material, which will have huge industrial applications”, says Prof. Bobji on the next plans for this research.