WC-Co sprayed by APS and HVOF

[Zhao Yun]

Hello All,

Now I have some questions about WC-Co coating:
1. What's the difference of the coating sprayed by APS and HVOF?;
2. How to choose the process, based on the working conditions or something else?
3. Is the coating sprayed by APS sensitive to the environment? I worked out this coating with APS（F4 gun） and sometimes the crack occurs without any change, the preheat temperature is around 80℃。

[Vadim Verlotski]

You can get a good WC-Co coating using APS, but for that you need a high-speed plasma torch (HV-APS). “Normal” APS such as F4 are not suitable for such coatings. The reason is that good (dense and crack-free) WC-Co layers need very high particle speeds at particle temperatures below their melting point. These speeds and temperatures are achievable with HVOF/HVAF, but not with ordinary APS. Slow APS flame causes particles to melt, which leads to cracking as the coating cools.

[Zhao Yun]

You can get a good WC-Co coating using APS, but for that you need a high-speed plasma torch (HV-APS). “Normal” APS such as F4 are not suitable for such coatings. The reason is that good (dense and crack-free) WC-Co layers need very high particle speeds at particle temperatures below their melting point. These speeds and temperatures are achievable with HVOF/HVAF, but not with ordinary APS. Slow APS flame causes particles to melt, which leads to cracking as the coating cools.

Thanks for your reply, and pretty agree with your comments. And I have a confusion that we can see HVOF/HVAF is more suitable for WC-Co, why RR/GE have the requirement to spray this coating with APS?
And this is also why I raised this topic.

[Stephen Booth]

Good afternoon zhao yum, Vadim is the expert and what he says is accurate. HVOF, HVAF and D-Gun is optimized for WC coatings. However back before HVOF existed Aerospace companies and Gas Turbine OEMs had no choice but to use APS for coatings of WC. Aerospace specifications are slow to change, and often on older components it might not be feasible to change to the best coating at the end of the life cycle.

That would be my guess

[adam wintle]

Good afternoon zhao yum, Vadim is the expert and what he says is accurate.  HVOF, HVAF and D-Gun is optimized for WC coatings.  However back before HVOF existed Aerospace companies and Gas Turbine OEMs had no choice but to use APS for coatings of WC.  Aerospace specifications are slow to change, and often on older components it might not be feasible to change to the best coating at the end of the life cycle.

That would be my guess

Stephen's right. You can buy an anode, 6mm Nozzle, Type 257-1, 300029(TF) 1093141, for your F4. Metco made these for the PWA 257 Wc/Co coatings. Send me an email for parameters.

[Zhao Yun]

Good afternoon zhao yum, Vadim is the expert and what he says is accurate.  HVOF, HVAF and D-Gun is optimized for WC coatings.  However back before HVOF existed Aerospace companies and Gas Turbine OEMs had no choice but to use APS for coatings of WC.  Aerospace specifications are slow to change, and often on older components it might not be feasible to change to the best coating at the end of the life cycle.

That would be my guess

Thanks Stephen and Vadim, now my confusion is solved.