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Graded composite coatings show potential for better performance

Connect, no. 106, May/June 2000, p.8

A Group Sponsored Project (GSP) is currently underway at TWI to develop graded composite coatings using the high velocity (HVOF) spraying process and demonstrate their performance as protective coating systems for components in severe wear/corrosion environments.

So far, the first phase of the project, a feasibility study, has successfully achieved the construction of a graded coating as shown in the Figure. The coating combines a hard wear resistant chromium carbide cermet material on the outer surface with a corrosion resistant nickel alloy on the substrate surface. The result is a coating structure that combines properties not achievable with either coating materials on their own. The coating was produced using conventional HVOF equipment with a dual powder feed system.

Graded coating microstructure
Graded coating microstructure

For this particular graded composite coating, wear properties were tested using a dry sand rubber wheel abrasion test and the corrosion performance determined by polarisation scans in seawater. The wear performance closely matches that of the cermet material, whilst its corrosion performance is considerably enhanced compared to that for the cermet coating alone.

The project is now entering its second phase where protective coating systems based on cermet/metallic and ceramic/metallic grade composites are to be developed for corrosive environments of particular interest to the sponsors. In addition to the seawater environment mentioned above, this may include gaseous corrosion with erosion damage.

Following the successful results of Phase I, additional sponsors are being sought to expand the range of protective coating systems and environments that can be included in the project.

If you wish to join this project or would like further information, please contact the surfacing team at TWI. E-mail:

For more information please email: