coating thickness

[Brent]

I have a question on coating thickness that I require some help with. Right now when we have a job that requires a Wc based coating build up that would be in excess of .014"(.35mm) per side we first build up with twin arc wire, re-machine, grit blast then hvof spray. One of the reasons we do this is to save on the cost of powder.
It has been asked of me if we could first hvof spray a base coating which would be a austentic stainless steel powder ( 10Ni 16Cr ) to a desired diameter. Then change the powder and continue to build up the part to the finish dia.
Is changing to a different powder midspray advisable?
Wondering what my concerns should be if this is possible. Most of the substrate would be stainless.

[KevGarty]

Hi Brent,

Is there any reason you machine the ARC Spray then grit blast it ?
It is possible (I have seen it done many times) to HVOF straight onto ARC Spray.

In relation to changing powders its common practice to bond coat then top coat using different powders as long as you use a powder that is equivalent to a bond coat or that has excellent bond strength

Only thing I will say is stainless likes expand a great deal you would have to keep an eye on the temperatures in relation to cooling and heating up

Instead of changing powders etc perhaps go with the HVOF straight onto the ARC ?

Like I said I have seen this done many times maybe some of the more experienced guys can confirm it too ???

[kschewe]

You do not require lots of material, only .014. For cost you would spray .007 bond coat wire arc then spray the top coat on right afterward with out any intermediate maching. This is a common practice in the industry. Best quality coating since you dont contaminate the bond coat interface with imbedded grit. When parts need lots of material and need to be put in round etc then that is an acceptable practice by many aerospace oems, bond then machining etc.
For me I would spray the minimum amont of top coat with out any bond to allow for a nice finish, probably .005 more. Hvof can spray much thicker coatings for Wc.

[Brent]

That is very interesting I am still very new to thermal spray and follow the procedures set out by my employer. My thinking is that we do the second operation to give a more even Wc coating thickness which is easier to mic as it is going over a grit blasted surface only. I recently sprayed a large sleeve with a really coarse feed on it and that feed profile was maintained right to the end of my spray run. It was more difficult to mic as I had to figure what my size would be once the crests of high spots were gone. Would this not be the case with a rough twin arc wire finish also??

[sreenuvundela]

Brent

I totally agree with Mr.Kevgarty comments.

There is no need of machining and grit blsating on twin wire arc spraying, you can directly coat HVOF coating. But, my point is why you are depositing very high bond strength HVOF coating on low bond strength twin wire coating, because finally the bond strength of entire coating will be decided by low bond strength twin wire arc process only.

My suggestion is carry bond coat also with HVOF only.

Regarding roughness of the coating, in twin wire arc spray process coating roughness can be controlled by changing the process parameters.
we have to consider roughness of the thermal spray coating and necessary grinding/machining allowances to be added.

Regards,

SREENIVAS

[shantanu]

Brent

I totally agree with Mr.Kevgarty comments.

There is no need of machining and grit blsating on twin wire arc spraying, you can directly coat HVOF coating. But, my point is why you are depositing very high bond strength HVOF coating on low bond strength twin wire coating, because finally the bond strength of entire coating will be decided by low bond strength twin wire arc process only.

My suggestion is carry bond coat also with HVOF only.

Regarding roughness of the coating, in twin wire arc spray process coating roughness can be controlled by changing the process parameters.
we have to consider roughness of the thermal spray coating and necessary grinding/machining allowances to be added.


As you want to put in a bond coat only to save money for powder, you could also try spraying a Ni Al bond coat using simple powder combustion gun to say 0.05-0.1 mm thickness per side and then spray your WC. However, 0.35 mm per side is very STD thickness of WC...also I would think a rough ARC sprayed bond coat may affect the final Surface Ra of your coating if left as sprayed after WC...these are requirements you would need to factor in...

Regards

Shantanu

Regards,

SREENIVAS

[Brent]

I agree with the thinking that your coating is only as strong as the lowest bond strength sprayed. Using the hvof process for both coatings would be beneficial so what is mostly used under a Wc based coating on stainless substrate? Is the powder being recommended to me similar?

Typical Composition:
Carbon: 0.03
Silicon: 1.0
Molybdenum: 2.5 – 3.0
Nickel: 10.0 – 14.0
Chromium: 16.0 – 18.0
Iron: Balance

I realize that .35 is a standard Wc build up I am looking for options for parts requiring more than a standard build up.

[sreenuvundela]

Hi Brent,

Bond coating materials are 95Ni-5Al, 80Ni-20Cr and MCrAly (M= Ni/Co).

These bond coats will help you as below

1) take care of thermal expansion differences between base material and top coating

2) It provides very good bonding surface for top coat (in some cases, where shot blasting impossible to carry in such areas these bond coats provides required surface roughness to carry top coat.

3) These bond coats provide very good corrosion resistant layer below the top coat.

I hope there will not be much price variation for your SS Powder and these bond coats.


Regards,

SREENIVAS