Hello Everybody,

I am interested in mechanical properties of a coating made of carbide (SiC or WC) in Ni-Cr matrix.

For instance, we have a cast iron sample coated with a carbide/Ni-Cr coating. Now, this sample undergoes a mechanical shock, e.g. hammering or dropping down onto a concrete floor from height of two feet. Is this kind of coating enough malleable to withstand this shock without cracking or crumbling?

Hi, there,

Depending on how streneous your test is, there might be a chance of your coating passing the test. But, without knowing the details of the coating, I am almost certain that generally regardless of what coatings you may choose (certainly the hardfacing type), as long as they are from thermal sprayed, they'll not survive this type of test. Here we're talking about the fracture/impact toughness, which calls or high strength and good ductility, which is very much microstructure-dependent (grain size within splats of the coating) and there is a limit that changing the composition can do.

Coatings meet such a criterion must also have sufficient thickness (generally greater than the limit of normal thermal spray coatings) in order to avoid spalling at interface during testing.

Hong Wei Wang
ERA Technology, UK

Hi Hong,
Thank you for your answer!

Let me be more specific. At present time we have two choses to protect thermocouples against molten aluminum: 1) cast iron pipe coated by enamel and 2) sintered SiC pipes. I need to improve a life span of painted cast iron pipes but sintered SiC is not an alternative due to its fragility combined with bad handling at work place (they are just mechanically broken every second day).

Now questions:
Is carbide/Cr-Ni coating over cast iron as brittle as massive sintered SiC?
Is carbide/Cr-Ni coating over cast iron stronger (thermal and mechanical shock-wise) then enamel coating?

Regards,
Max

Hi KRYM2006

to the Surface Engineering Forum.

I have my doubts as to the suitability of any carbide/nickel chromium coating in this environment. Thermocouple tubes are commonly thermal sprayed with grey alumina or magnesium zirconate ceramic coatings for this type of application. Not too sure how they would compare to enamel coated tubes, but certainly worth trying.

Hi Gordon,
Thank you for greeting me on the forum! Nice to be here!

I appreciate your suggestion about spraying with grey alumina or magnesium zirconate ceramic coatings. But my question remains intact: How crackable is it comparing to bulk ceramic pipes and enamel coating? Does exist thermally sprayed coating (non-metallic but probably with metal matrix) that is ductile to a greater or lesser extent?

Hi KRYM2006

I would expect the thermal spray coated metal tubes to be more durable with respect to mechanical damage than the solid ceramic tubes and I guess similar or slightly better than the paint on ceramic glaze or enamel. This is simply because of the support from a metal substrate, not because the coatings are stronger (which they are not).

Not many metals will stand up to these temperatures or molten aluminium environment so I think a ceramic/metal composite would not be the answer. Oxide ceramic and non oxide ceramic composites could be worth investigating. The only existing coatings I know are from a mix of oxide ceramics with boron nitride currently being considered for use as high temperature abradable coatings.

Hi Gordon,

Thank you! Your answer gets me closer to understanding of this topic. But could you briefly explain when thermal spray coatings are preferred to ceramic glazes or enamels (which are cheaper and easier to apply)?

Max

Hi Max

Can't answer your question for sure. Many years ago, I coated thermocouple tubes with simple flame sprayed grey alumina (very open structured and relatively friable compared to plasma coatings) for a customer to test. The customer appeared happy as they purchased a flame spray set-up to coat their own tubes. Unfortunately, I don't know the environment they were place in. Searching for information on this topic yielded nothing useful. which is a bit disconcerting. I can only suggest that you get a few tubes coated and try them out.

Hi Max

Stumbled across this this web page which might be of interest:

http://www.zypcoatings.com/ProductPages/...elting.htm