Ball Mill Shaft- Journal Bearing Area HVOF Coating Issue

[tobjung]

Dear forum

We've serviced Ball Mill's shaft bearing area with HVOF coating and we failed. After 1-2 days of running, the coating was peel off and had many blisters all over the shaft area. (See picture)
Now, with hydrodynamic bronze journal bearing system, it should not be any impact on coating during operation. But, it is an ideal situation. When we disassembled the journal bearing from the shaft, we saw bronze color adhered on the shaft surface as in the pictures. Anyone know what's going on ? In the end we have to grind HVOF out and undersized the shaft. Under HVOF coating we have arc spray coating with Ni5Al with a thickness of 0.2mm on 3 different locations along with the shaft material itself (See final result). And that's the final surface that works just fine. Now the customer have been operating the Ball mill for months with no problem. Here are some details.

Ball Mill specifications:
Type: Overflow Ball Mill
Weight: Ball Mill = 50 Tons, Ball steels and products = 50 Tons. Total = 100 Tons
Journal Bearing: Bronze hydrodynamic journal bearing
Repairing area: Dia. 1070mm x L 460mm

Thermal Spray specifications:
Coating process: HVOF Coating
Coating Material: Ni20Cr as bond coating + SS420 for top coat (0.2mm)
Grinding and polishing
(Underneath coating on 3 different locations: Arc spray with Ni5Al 0.2mm) 

Please advice

Best regards