Buffer or build up layers under HVOF Carbide Coatings

[Stephen Booth]

Basic Question
What process, steps, materials, and limitations for thickness are best for building up under HVOF Carbide coatings?

Background:
I have tried to anticipate several questions in this message, sorry it is so long. In our experience, as well as common experience in the field, HVOF Gas Fuel coatings of Carbides are limited to thickness of 300 microns (radial thickness after grinding of the coating assumption being this is a cylindrical component).

Since we are doing repair, often components are required for repair with thicknesses exceeding 300 microns, often up to 1500 micron or more. Obviously no TS process adds mechanical strength to the component so in most cases components with wear exceeding 1500 micron radial we would scrap or possibly PTAW weld .

In a situation where the build up is more than the 300 micron of HVOF Carbide, which process do our members recommend from experience as effective, efficient, strong, and practical, and technically correct.

In our own shop we have the HVOF Top Gun and the DJ2700 equipment and have used the following variety of processes, procedeures:

1. NiAl HVOF with top laver HVOF Carbide
   
2. Arc Spray NiAl bond, build up arc AISI 410, machining, blasting on AISI 410, HVOF top layer
   
3. NiAl Arc Spray thick, machining, blasting, HVOF top layer
   
4. NiAl Arc buildup thick, as sprayed, with top layer HVOF Carbide
   

We have also used PTAW, GTAW, etc, etc. looking forward to practical aspects of this. I prefer NiAl Arc build up as sprayed, or NiAl HVOF build up as sprayed as the base layer for the HVOF Carbide

[Gordon]

Hi Stephen

Great post I agree with most of your comments.

I know some may say their HVOF systems can apply very much thicker carbide coatings with no problems and for some applications with large wear allowances this can be beneficial. But in many cases wear allowances are small, so to my mind it makes sense to make up the majority thickness with an intermediate layer using a relatively stress free material which is cheaper and easier to apply.

I am pleased you made the important point that thermal spray coatings add little to the mechanical strength of the component. While thermal spray coatings are great for dimensional restoration, they will not restore strength.

This technique is obviously not only suitable for HVOF carbide coatings, but also many thermal spray coatings that suffer from limited coating thickness capability. Also intermediate layers can be used to redistribute residual stresses away from weak areas as in gradated thermal barrier coating systems. Intermediate layers of say Hasteloy or Inconel type alloys under ceramics can apart from increasing coating system thickness limitation add an important additional corrosion barrier.

[Gordon]

Moving thread to main Surface Engineering Forum section.

[hvofhamid]

Basic Question
What process, steps, materials, and limitations for thickness are best for building up under HVOF Carbide coatings?

Background:
I have tried to anticipate several questions in this message, sorry it is so long. In our experience, as well as common experience in the field, HVOF Gas Fuel coatings of Carbides are limited to thickness of 300 microns (radial thickness after grinding of the coating assumption being this is a cylindrical component).

Since we are doing repair, often components are required for repair with thicknesses exceeding 300 microns, often up to 1500 micron or more. Obviously no TS process adds mechanical strength to the component so in most cases components with wear exceeding 1500 micron radial we would scrap or possibly PTAW weld .

In a situation where the build up is more than the 300 micron of HVOF Carbide, which process do our members recommend from experience as effective, efficient, strong, and practical, and technically correct.

In our own shop we have the HVOF Top Gun and the DJ2700 equipment and have used the following variety of processes, procedeures:

1. NiAl HVOF with top laver HVOF Carbide
   
2. Arc Spray NiAl bond, build up arc AISI 410, machining, blasting on AISI 410, HVOF top layer
   
3. NiAl Arc Spray thick, machining, blasting, HVOF top layer
   
4. NiAl Arc buildup thick, as sprayed, with top layer HVOF Carbide
   

We have also used PTAW, GTAW, etc, etc. looking forward to practical aspects of this. I prefer NiAl Arc build up as sprayed, or NiAl HVOF build up as sprayed as the base layer for the HVOF Carbide

hello
once ,I was confronted to the same problem to repair some pistons,my technical responsible was affraid to coat more than 300 micron carbide.
then I started to think about what are the raisons which limite carbid thikness.
conclusion:mechanical carasteristiques change with heat,if your substrat bear heat of thikness of 300 micron ,you coat 300 micron and stop.let the heat going down and recommence,
I made a test with 3 mm carbide, of course on several times respecting the temperature and dillatation.
if you want ,make a test and you will see.
important:never depass the temperature controlled with 300 micron coating.
good luck

[Stuart Milton]

Hi,

Great post and question.

To my knowledge, when HVOF spraying carbides with gas fuelled systems, the coatings are stressed in tension due to the higher heat/lower velocity characteristics of gas fuelled HVOF systems (big generallisation and others may have different views and it is not quite as simple as mentioned). With liquid fuel HVOF systems, the coatings are normally compressively stressed due to the lower heat/higher velocity characteristics (again, a generallisation but this is our experience). We have acheived coatings of circa 2mm thick with a liquid fuelled system.

Having said that, Gordon has made a very valid point regarding the commercial aspect of applying such a thick carbide layer and it is potentially more sense to apply an intermdiate layer of lower cost materials.

[Gordon]

Hi All

I was trying to think of an application that actually uses thick HVOF carbide coatings ~2mm. I didn't come up with any that I was sure of . I can visualise situations where a component only requires wear resistance, with geometry and surface finish not being important i.e. to make use of a full 2mm of wear limit. But can't think of anything that's been put into practice.

[jasonpbond]

Hi All

I was trying to think of an application that actually uses thick HVOF carbide coatings ~2mm. I didn't come up with any that I was sure of . I can visualise situations where a component only requires wear resistance, with geometry and surface finish not being important i.e. to make use of a full 2mm of wear limit. But can't think of anything that's been put into practice.

I think there are many applications where 2mm thick of carbide is required, however at that thickness the question becomes about economics, as mentioned previously. 2mm thick carbide coatings are extremely common with PTA welding. In various situations, the geometry and surface finish are unimportant, and PTA coatings can be left as welded. I'm thinking about dewatering screws in the MDF industry, various types of crushers/hammers/rotors etc. But unless HVOF is all you have, a 2mm thick HVOF carbide coating does seem to be a bit excessive!

[shantanu]

This is a very common problem for probably all job shops catering to almost any segment / industry

I would think (to also make it economically viable);
A base coat of NiAl with ARC
An internediate coat with a Metcoloy II (cheap and easy to build)
Top coat of a carbide sprayed with HVOF (GF / LF) ensuring that a max of 300-400 microns on radius is sprayed that can be ground.
Keeps your cost in control and typically should solve your purpose. The Base coat of 95 Ni-5 Al is very important for thr bonding though.
Regards

Shantanu
Eastern Metallizing
www.emcpl.in

[djewell]

We frequently spray 1mm thick WC coatings with HVAF without a bond layer, but we have sprayed up to 13mm thick. The real trick to spraying thick coatings is to match the contraction of the coating to the substrate. Since WC has such a low CTE, it does not contract much when it cools. Thus the key is to spray the part as cool as possible.

[shantanu]

We frequently spray 1mm thick WC coatings with HVAF without a bond layer, but we have sprayed up to 13mm thick. The real trick to spraying thick coatings is to match the contraction of the coating to the substrate. Since WC has such a low CTE, it does not contract much when it cools. Thus the key is to spray the part as cool as possible.

13mm...wow! can you give us idea of the DE of the powder at such high thickness coatings.

[ServiceTech]

One of the keys to remember when building up the underlayer is the tensile and/or bond strength of the underlayer versus the carbide top coat.

The coating system will only be as good as the weakest link. Most HVOF aerospace specs require the >8,000 psi of tensile strength HVOF coatings offer. In this case, the thermal spray solution would be an HVOF underlayer........providing you can get approval of course.

[djewell]

We frequently spray 1mm thick WC coatings with HVAF without a bond layer, but we have sprayed up to 13mm thick. The real trick to spraying thick coatings is to match the contraction of the coating to the substrate. Since WC has such a low CTE, it does not contract much when it cools. Thus the key is to spray the part as cool as possible.

13mm...wow! can you give us idea of the DE of the powder at such high thickness coatings.

The DE was 50%-60%.

[Met.Eng.]

Can I ask smtg..
for ex.
the base material is 316 L. and There are 2 mm deformation wear on the surface . Can I coat 2 mm metco 41C with 6P-II (Fe Cr Ni (AISI Type 316 Stainless Steel) after than 200 micron WC Co with HVOF ??????????????