Importance of NiCr in Chromium Carbide Coatings

[DJr9515]

Hello, I'm a newbie and I was wondering what is the significance of NiCr in Cr3C2 coatings. I came across one explanation but it was very short and I wanted to find out if there was anything more to it then that. Here is what I found:

"In thermal spray applications chromium carbide is combined with a metal matrix such as nickel chrome. There is typically a 3:1 ratio by weight of carbide to metal matrix. The metal matrix is present to bond the carbide to the substrate that has been coated and to provide a high degree of corrosion resistance. The combination of corrosion and wear resistance means that the thermally sprayed CrC-NiCr coatings are suitable as a barrier for high temperature wear. It is for this reason that they are finding increasing application in the aerospace market. Typical uses here are as coatings for rod mandrels, hot forming dies, hydraulic valves, machine parts, wear protection of aluminium parts and general applications with good corrosion and abrasion resistance at temperatures up to 700-800?C."

I found it here:
https://www.azom.com/details.asp?ArticleID=524


On a side note, what are the process parameters commonly used to make these Cr3C2 coatings using thermal spray (APS, HVOF, etc)? If there are any websites or specific publications which outline established methods of producing these coatings, please feel free to add!

Thank you

[Fergus]

Hello DJr9515

As far as I'm aware, most carbides used in TS coatings come in the form of cermets or carbide/metal matrix composites like WC/Co. I think the primary reason that carbides are not sprayed alone is that they oxidise or decompose or volatilise readily at their rather high melting points. Coatings produced from pure carbides I guess would have very poor properties. Additions of a metal matrix allows carbides to be sprayed at a lower temperature or heat input. The metal matrix gives much needed ductility/toughness to the coating, while benefiting from the very hard wear resistant carbide particles.

There is typically a 3:1 ratio by weight of carbide to metal matrix.

Typically I would say they range from 7-50% NiCr matrix

The metal matrix is present to bond the carbide to the substrate

Its needed to bond everything together and provide some ductility and toughness

and to provide a high degree of corrosion resistance.

Chromium carbide by itself is corrosion and temperature resistant and is not really made any more so by the addition of NiCr apart from dramatically increasing coating density so protection the substrate better.

NiCr general is used as a matrix or binder for chromium carbide because they are metallurgically compatible, have similar high temperature and corrosion resistance. The composite nature of hard particles in softer tougher matrix gives them very useful coating properties.

HVOF is now the preferred method of application, even though many old aerospace specs still call up plasma spray.

Hoping that's of some help I'll come back if I find any decent links.

Fergus

[Gordon]

Hi DJr9515

to the Surface Engineering Forum.

General agreement with Fergus.

Metco use to sell Metco 70C-NS a coarse grade of pure chromium carbide powder (not sure if they still do, Sulzer Metco that is). Below I quote part of the technical bulletin summary which might be of interest:

Metco 70C-Ns is a chromium carbide without admixture. Sprayed coatings of neat chromium carbide are theoretically capable of withstanding temperatures above 1000 F. However, the coatings are porous, contains a considerable proportion of oxides and exhibit relatively poor inter-particle cohesion. These factors lower temperatures and wear-resistance.

The full benefits of flame-sprayed (I think they mean plasma sprayed here) chromium carbide are obtainable when it is mixed with a matrix material. Neat chromium carbide is recommended only when its use is required by a process specification.

Not like their usual sales patter No Applications are indicated only aerospace specification conformance. So I'm not sure what its good for.