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

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.

Moving thread to main Surface Engineering Forum section.

Stephen Booth Wrote: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

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.

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.