WC-Co Hardness

[Sati]

Good morning 

Could anyone share with us how to manipulate hardness in the carbides coatings especially in WC-Co sprayed by HVOF (for gasses and liquid version) . I am interested  general rule/s not specific setup which parameters shall be adjusted to achieve harder or softer coating ? Preferable  given  answer  based on your own experiance .

[Vadim Verlotski]

Hi Sati,
The faster the spray particles move, the harder the coatings in the WC-Co or WC-Co-Cr system. This means that for maximum hardness, the spray parameters must achieve maximum gas velocity, and the spray nozzle must be as long as possible. Of course, the particle temperature and, consequently, the coating efficiency will drop to a minimum under such conditions, but this is the price for increased hardness. In short, higher hardness requires "fast and cold" spray parameters and leads to reduced coating process efficiency.

[Sati]

Thank You very much Vadim for Your response . Sound logic but please advice how You doing that ? My first idea is to decrase the distance but it not work in that particular example .Second is to increase Kerosine or Kerosine and Oxygen but it simultanoesly increasing heat and You said "fast and cool" . I have the longest avaible nozle so it is also out. So what is the method to do that what You recomend ?

[Vadim Verlotski]

Hi Sati,
I don't know what kind of HVOF torch you have. There are torches, such as the CJS from Thermico, which I used to work with, that have a connection for nitrogen. In this case, you can introduce nitrogen alongside the oxygen in the combustion chamber (ignition occurs with pure oxygen, and nitrogen is added afterward). Nitrogen cools the flame and allows you to increase the pressure in the combustion chamber. This way, you get "fast and cold" spray parameters.

If you only have connections for oxygen and kerosene, it's more difficult, but not entirely impossible. In this case, you can add nitrogen to the oxygen in an external gas mixing system.

[Sati]

Vadim thank You again that you sharing with Your knowledge . I am using thermico CJS as well but our configuration is unfortunatly with Hydrogen ... By the way how do You think what is the main role in your opinion with Hydrogen in that mixture ?

[Vadim Verlotski]

Hydrogen has two functions:
1. Smooth start of the torch (without a detonation),
2. Fine adjustment of the flame stoichiometry (lambda value +- 0.001).
If you have a CJS torch, it must have both hydrogen and nitrogen connections. 150 mm and 400 mm nozzles are also available. With the CJS torch, hardnesses of up to 1700 HV0.2 can be achieved for WC-Co and WC-Co-Cr.

[Sati]

400 mm wow ? i have heard about 200 mm so i will check it ! How You managing with Carbide contents ? I mean when you would like to decrease carbide content ? As far i know the most people increasing flame temp to "evaporate" them or decreasing feed rate to change flame/powder volume ratio but it more influence to APS than HVOF.

[Vadim Verlotski]

Normally, the carbide content in the coating remains the same as the carbide content in the powder. Only with a very hot oxidizing flame can some of the carbides burn, but these are completely incorrect spraying parameters. With "fast and cold" spraying parameters, the particle temperature remains well below 1500°C, so no melt can form.