Microhardness testing on polymers

[Davkoo]

Hi,

I am trying to perform microhardness tests either in terms of Vickers or Knoop on Nylon 6,6 samples which have been weathered for quite a long time from 1 to 5 years.

The samples are cylindrical in shape with a diameter of 30mm and a length of 17mm but the most important thing to note is that surface cracks (max crack width measured was about 80 microns) were present mostly on the circular surface of the samples due to weathering.

Consequently, the problem I am having right now is that I cannot perform indentation directly on the circular surface because it is difficult to distinguish and hence to measure the indentations due to the presence of cracks.

Will I need to perform the tests rather on the cross-section of the samples or are there any other better alternatives? I am asking this because if I perform the tests on the cross-sections, I will need to machine the cross-sectional surfaces such that they are both flat and parallel to the bottom surface and I am not sure if machining the surface will affect the hardness measured.

Regards.




Regards

 

[patprimmer]

The polymer goes hard as it is degraded by UV light. Your problem is that anything that removes the cracked surface will also remove the hardened polymer. I am presuming you are trying to measure the change in hardness due to the weathering, so removing the surface ruins that. Measuring other surfaces also ruins that.

Regards
Pat
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[Davkoo]

Yes you are right, I am trying to measure the change in hardness of my samples. It seems that I will need to study alternative test methods but thank you for replying!

 

[Demon3]

Why is the surface hardness relevant? What does the customer care about? It's probably one of two things.

1. Appearance. In that case measure gloss versus weathering time. The gloss decreases because the microcracks scatter light. This measurement is cheap, quick and meaningful.

2. Mechanicals. Specifically impact (and elongation) decrease as the cracks act as stress concentration sites. When stresses the cracks grow and the part fails.

Unless you were ordered to measure surface hardness I'd change my focus to another method.

Chris DeArmitt - PhD FRSC CChem
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