Indian scientists have claimed to have found a new technique of synthesizing thin hard surface coatings by high-velocity air fuel spraying. According to them, this has the potential of an environment-friendly and safer alternative to hard chrome plating used in car parts, tools, and kitchen utensils.

Hard chrome plating is typically used for surface coating as chrome is hard and wear-resistant. However, it consists of chromates, fluorites, and hexavalent chromium, making it carcinogenic (causing cancer) in nature. This has initiated the research for a safer and environment-friendly alternative with an equivalent or superior wear resistance but 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, the Indian Science and Technology Ministry said regarding the matter, quoting the study published in the Journal of Thermal Spray Technology.

Thermal spray is a group of techniques to deposit a wide variety of coatings for various industrial uses, reduce wear and tear and improve corrosion resistance. 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, says the study.

The new technique, called high-velocity air fuel, involves low temperatures and high particle velocities, and can deposit coatings using finer-sized powders, the study says. Scientists from Hyderabad-based Advanced Research Centre For Powder Mettalurgy, an autonomous institution of the Indian government's Department of Science and Technology, carried out the synthesis of thin hard coatings of a composite alloy of tungsten, cobalt, and chromium by high-velocity air fuel spraying. 

According to the study, thin coatings were deposited with torches with different capacities and by employing different nozzle sizes. Coatings with 50 µm thickness and surface roughness close to 1.5 µm were achieved on stainless steel substrates. The torch type and nozzle design influenced the coating's properties considerably. 

The study adds that superior sliding wear performance was noticed with high-velocity air fuel sprayed thin WC-10Co-4Cr coatings against conventional hard chrome plating. Corrosion studies carried out on the coating and compared with hard chrome plating showed that the new technique can be a better alternative for heavy-load applications like hydraulic shafts, valves, piston rods, balls, and so on. 

A comparison of the hard chrome plating with air fuel spray coating showed that the surface roughness of the latter is higher than that of hard chrome plating. Furthermore, the coating can be deposited on as-machined conditions to achieve a 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 hard chrome plating, according to the study.