Posted by Gordon England (62.252.0.4) on July 06, 2004 at 01:32:13:

In Reply to: HV hardness of plasma sprayed coatings posted by Daniël Sellekaerts on July 05, 2004 at 18:53:05:

: Hi,
: I have an additional question on the RE:HV hardness The plasma spraying can be up to...
: When measuring HV of an AL2O3-ceramic coating,whish load to you use? Our testresults are approx.900 HV with 25g.load.Greater loads results in coatingcracks.
: Can we link the max.load to the interparticles cohesian and the erosion resistence(a sort of Vickers cohesian test)?Can we convert the HV 25g.to HV 300g?
: I hope to learn something more about HV quality control of Ceramics.Thank you for reply.

Hi Daniël

Generally, the industry standard for cross-section microhardness on thermal spray coatings is HV/0.3 (are yours cross-section or surface?).

I prefer to use a range of loads, usually 0.3 and 1 kgf where conditions permit and lower if problems such as cracking occurs with the larger loads. Also lighter loads for distinguishing between different phases or particles, although this is where the Knoop hardness test can be of benefit.

It is interesting when comparing the results obtained using different loads. I feel confident when say 0.3 and 1 kgf test results are similar and no significant cracking is observed. To me this indicates good coating quality. Also, higher loads give better accuracy. On many coatings though, the 0.3 kgf load results in higher but more scattered results than 1 kgf. The larger the discrepancy the more I worry.

I think there is a link between hardness difference/onset of cracking using different loads and the cohesive strength/density/brittleness of coatings, but I feel this will only be at best a rough qualitative relationship rather than quantitative.

Cracking with ceramics on hardness testing can be a problem, though I would be concerned if you have to drop to 0.025 kgf to avoid cracking. How severe is the cracking at 0.3 kgf? Very minor cracking at the corners of the impression (assuming a good square impression) generally seem not to effect the result significantly. Usually, for good reporting I would take the mean of at least 10 readings and initially disregarding any readings that are severely cracked or not square, but making note of these as they are an important observation, particularly if you have many.

There is no conversion of Vickers hardness between different loads. There is a trend that smaller loads yield higher values, but this is very specific to the nature of the material under test. Example chromium oxide coating 1800 HV/0.3 and 1750 HV/1 while another chromium oxide coating gives 1450 HV/0.3 and 1050 HV/1.

Hope this helps.

Regards Gordon

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