A diamond-like carbon coating from IIT-Madras promises to make the insides of gun barrels tough and friction-free. DRDO is developing special coatings to reduce friction heat in gun barrels

The science of coating surfaces to increase their ability to withstand wear and tear and, sometimes, impart lubrication has immensely helped industry. The newly launched Advanced Conformal-Coating Technology (AdCoaTech) lab at IIT-Madras is developing technologies to give Indian defence capabilities an ‘Atmanirbhar Coat’.

The AdCoaTech lab was set up jointly by DRDO-CVRDE lab and IIT in August. “This is the first-of-its-kind set-up in any academic institute in India for physical vapour deposition technique for conformal coatings on the inner surface of long cylinders and tubes,” says Dr N Arunachalam, Associate Professor, Manufacturing Engineering Section, IIT-M.

As India aims for self-reliance (Atmanirbharta) in defence production, it would manufacture more weapons such as guns and tanks. As bullets and shells shoot through barrels, the friction generates tremendous heat. If a machine gun fires 100 rounds rapidly, the barrel can glow red hot — that’s why they have ‘barrel shrouds’ for the gunner to grip it safely. There is the same problem in the barrels of tanks and artillery guns.

It would greatly help if the insides of the barrels were ultra-smooth to reduce friction (and hence heat).

This calls for a suitable coating material for the insides of the barrels that can give both hardness and lubrication properties. Such coats find use in several other areas too. For example, the insides of the cylinders that house the pistons attached to a tank’s wheels — the up-down motion creates tremendous heat and wear-and-tear.

There are many methods for coating surfaces. Coating materials are deposited onto surfaces physically, chemically, in the form of plasma, or by cold spray — all of which are extensively used in industry — but coating the insides of surfaces is a challenge. The research in this area is for developing ‘recipes’ and the equipment that will do the coating. AdCoaTech lab has developed such coating equipment in collaboration with Excel Instruments, Mumbai.

As for the recipe, Dr Arunachalam’s team uses what is called ‘diamond-like carbon’. Diamond is the hardest material in the world, but how do you get it to stick to surfaces. Besides, aren’t they expensive? Well, no. In science, diamond is not necessarily a shining stone; it just refers to a particular structure of arrangement of carbon atoms.

Carbon atoms make three types of bonds — Sp1, Sp2 and Sp3. “The bonding arrangements between carbon atoms produce different types of carbon allotropes such as graphite or diamond,” notes Prof Abdul Wasy Zia, of the City University of Hong Kong, in a 2020 scientific paper. In graphite, the carbon atoms form sp2 bonds with neighbouring carbon atoms to form a honeycomb-like structure. But if the bonds are of sp3 type (formed under extreme pressure and temperature), carbon exists as diamond. Diamond is very hard — around 100 giga pascals, as compared with graphite’s 3 GPa.

‘Diamond-like carbon’ coatings are a mixture of sp1, sp2, and sp3 carbon. The more the sp3, the better the tribological characteristics. ‘Diamond-like carbon’ coatings have been used in industry for long. However, IIT-M is perfecting the technology for defence applications — the collaboration with DRDO will develop complex coatings for the special needs of the defence forces. “The aim of this project is to develop diamond-based coating technologies, which are essential for DRDO’s immediate and future needs for defence components,” says Dr Arunachalam.

Prof MS Ramachandra Rao of the Department of Physics at IIT-M says the AdCoaTech lab has “produced innovative technology to develop diamond-based coatings on inner surfaces of industrial-scale cylinders, which conventional coating technology cannot achieve”. He said the coating dissipates heat and can withstand tremendous loads. “Pneumatic and hydraulic systems, aerospace parts, and defence vehicles are a few of the applications for these coatings,” Prof Rao said.