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Hi, Thank for the article. The problem which i face is : within a ceramic coating, i have 3 different layers, i need to evaluate the toughness of a particular layer. These methods you have in the articles estimate the toughness of the entire coating system, instead of the individual layer which im looking for.

You could cross section the sample and mount it and then use indentation techniques to measure the fracture toughness of each layer.  You need a micro-indentor with a load cell to accomplish the test.  There are numerous articles published on fracture toughness measurements using indentation testing.  Let me know if you have trouble loading those and I will send you some links.

@rajendra, thanks for the suggestion. i tried it once in the past but got a big standard deviation due to the non uniformity in my coating (Vertical cracks, porosity etc). How do you think i need to proceed with such a non uniform coating using this method to avoid the big scatter in E mod values.

Unfortunately with ceramics a standard deviation of upto 30% can be expected.  Try the smallest load possible and the lowest speed on the indentation machine.  We used indentation successfully with Alumina and Molybdenum Disilicide coatings of varying thicknesses and porosities.  These were plasma sprayed coatings.  We managed to get readings within a standard deviation of 20%.  Give it a try.

@rajendra, i will try and get back to you if i need further help. 

@ Guang, nano-indentation seems to give a highly localized value of the coating and i think it would not help me estimate the bulk E mod values. However, i would consider it if the micro indentation doesn't work with the smaller loads. 

Thanks again

Hello, sometimes you may have to think outside the box for your particular situation.  I have worked with 47 layers of coatings in the past for projection lamps of old days for slide projectors.  It depends o what characteristic you are looking for either for development, application or otherwise, such as duration, longevity.

Is it some idea of residual stresses of the coating as result of fabrication, for example?  Such stresses could potentially be different for different layers, giving opportunity for multiple coatings not to adhere well or de-layer on subsequent use, etc.

Using micro/nano hardness can provide some information, but are practically useless due to several limitations cited by others.

Has anyone looked into the application of focused sonics/ultrasonics, which can be mapped by using modern techniques, and could potentially succeed in characterizing different layers?  Such technique can be totally nondestructive, relatively fast, subject to fast scanning of vast areas and automatic interpretation and display of results in real time.  Cheers!

@Matukeshwara, Thanks for the suggestion.

Hello Satyapal,

We tried "traditional" fracture toughness measurements as well, and came to the conclusion that they are somewhat unreliable. The cracks from the ends of the indent were very rarely straight, often the whole indent had collapsed etc. Hence we pretty much discarded this method.

I have approached the problem in a more macro scale: so far we have tested some impact- and bending methods for the whole coating+substrate -system which have their own limitations as to accuracy but are more tangible when transferred into real applications. Problem with these is that we don't get any data comparable with other tests so the validity will only be revealed once an actual component is tested :)

Regards, Jarkko

Hi,

we tried indentation fracture toughness as well. we good pleasant results for hardmetall TS-coatings. as it comes to ceramics, things become more difficult...

- with porous coatings you end up with crashed indents of irregular shape and mostly no crack propagation

- even if you have cracks, it is hard to tell, where they are ending (or is it a splat to splat interface?)

- crack visibility depends highly on cross section preparation and the the eyes of the researcher

- sometime you have cracks in 4 directions (e.g. suspension coatings), sometimes only in 2. there is calculation models for both cases, which one to chose?

- there is a minimum load to create cracking, which leads to big indents. to avoid edge effects, you need thick coatings (400-600µm)

- at least once you should check the cracking mode (half-penny, palmquist, lateral or cone), we used step by step grinding and polishing

- after all, you still have a high variation/standard deviation

conclusion: we did not publish results of indentation fracture toughness for ceramic coatings

good luck! please keep me posted, I am still very interested to find a good solution for this!

It just came into my mind, there is a bending method with acoustical detection of the cracking.

@Richard, Thanks for the suggestions.

Do you recommend the bending method for coatings which have vertical cracks as well or only for bulk ceramics only?