A new technique of synthesising thin hard surface coatings by high velocity air fuel spraying, has the potential of emerging as an environment-friendly and safer alternative to hard chrome plating used for car parts, tools and kitchen utensils.

Chrome plating is generally used as it is hard and wear resistant. However, the presence of chromates, fluorites and hexavalent chromium makes it carcinogenic. This has lead the search for a safer, environment-friendly alternative with an equivalent or superior wear resistance and crack-free coating. Deposition of thin coating with industrially acceptable surface roughness is economical as it requires less powder and elimination of several grinding processes.

While with conventional thermal spray techniques, thickness build up is high and several grinding and polishing operations are needed to acquire the required thickness and roughness. A new technique called high velocity air fuel (HVAF), involving low temperatures and high particle velocities can deposit coatings using finer sized powders (5-15 µm).

Scientists from ARCI, Hyderabad, have carried out the synthesis of these thin hard coatings made of a composite alloy of tungsten, cobalt and chromium using high velocity air fuel spraying method. With this, thin coatings were deposited with torches of different capacities and by employing different nozzle sizes, says a press release.

The coating can be deposited on as-machined condition to achieve smooth surface and around 50 µm coating thickness. This significantly reduces the post coating finishing operations which reduces the processing and raw material cost significantly with better wear resistance than HCP, the release says.