Coating failure analysis (thermal cycling/abrasion/oxidation)

[Arkzik]

I have following situation for the discussion:

In attachment you see the WC-Co HVOF coating images in cross-section. Or better to say the rest of the coating after exploitation.
This coating was used as a part of draw die, operating at contact temperatures up to 900°C. It should be outlined, that we have at thermal cycling component with high amplitude. There is also a lubricant used during process (glass powder).

I would like to discuss the coating failure reasons to get the whole picture.

Figure 1: Optical microscopy view on the coatings and substrate. Substrate is a typical hot working tool steel. It can be seen that after some exploitation time the cracks formation takes place. Cracks penetrate through the coating and also growth deep inside of the substrate. In some cases cracks will be closed with lubricating material, otherwise it is oxidising.
Figure 2: SEM image in cross-section. The cracking is also found to take place on the interface between coating and substrate, which also does accelerate the spallation of coating.
Figure 3: SEM image with higher magnitude, showing some oxidation of WC-Co thermal sprayed coating.

It is clear, that WC-Co was not the best coating choice for named exploitation conditions; however for me it is more important to understand the failure mechanisms.

• Is the thermal cycling the main reason of cracks propagation or coating failure?
• Does oxidation of coating accelerates the cracks propagation?
• What role plays lubricant by closing cracks?
• How can be explained so pronounced inside (inner) corrosion on the border between coating and substrate – is it due to cracking and further oxidation or is it due to corrosion of WC-Co coating?
• Can it be correlated to the not correct choice of deposition parameters?

Regards,

Arkadi
   
   
   

[Gordon]

Hi Arkzik

• Is the thermal cycling the main reason of cracks propagation or coating failure?

Possibly, but probably not the only one.

• Does oxidation of coating accelerates the cracks propagation?

Possibly by production of brittle phases.

• What role plays lubricant by closing cracks?

Possibly a good one by reducing flow of oxygen through coating.

• How can be explained so pronounced inside (inner) corrosion on the border between coating and substrate – is it due to cracking and further oxidation or is it due to corrosion of WC-Co coating?

I think mainly from cracking

• Can it be correlated to the not correct choice of deposition parameters?

I think probably not, although poor deposition parameters could certainly produce cracks before part got into service. It could be correlated to a poor choice of coating material though. These coatings are not normally recommended in service temperatures above 500 C

[Arkzik]

Hello Gordon!

Thank you for your reply and good comments!
As for cracking - we have observed them also by nitrided layers, actually in the similar way with deep penetration in to the base material. Same also with cladded coatings. Guess, the thermal shock is still a main influencing factor.
Best wishes,

Arkadi