I have done Vicker's hardness analysis with titanium and thin-film coated titanium and I'm trying to show the hardness of the coated titanium. I plotted the graph of hardness versus load from 0.1 to 30Kgf, however, for the titanium - the graph was declined as the load increased, and for the coated titanium - the graph went up as the load increased.
I'm not understanding how to conclude from the graph?
Thank you in advance.
Dear Eliza Ranjit,
The Vikers hardness tester you are using will not be suitable for measuring thin filmas hardness:
1.An ultra-low load hardness tester (miligramų or micrograms) must be used. Thiswill reduce thedepth of needle insertion and help prevent the substrate damage.
2.in your CASE it is nscessary to use not a Vikers but a Knoop nedle with 172 degree prism, compared to 136 degree of Vicker. This allows to reduce the depth of embedding and increase the geometrical parameters of themeasured mark.
3.In to day, there are devices that automatically perform nano - hardness measurements.
Best regards
Dr. Gedvidas Bikulcius
Dear Eliza Ranjit,
I completely agree with Dr. Gedvidas Bikulcius, that conventional Vickers hardness testers cannot be used for thin films, since thin films are simply pressed under heavy loads on the indenter.
In addition, I still want to explain why in your experiment you observe a drop in hardness with an increase in the load on the indenter in the case of uncoated titanium and an increase in hardness in the case of coated titanium.
1. Uncoated titanium. Almost all titanium alloys consist of a relatively soft matrix and harder dispersed phases. When measuring hardness at low loads, it is quite likely that the indenter will hit the hard phase, and the measurement will show its high hardness. When the loads on the indenter are high and the footprint is large, then the actual hardness of the soft matrix is measured, which is minimal.
2. Titanium with thin film coating. The greater the load on the indenter, the larger the footprint and the larger the coating area that the indenter has to destroy. The coating film prevents the indenter from penetrating at high loads more than at low loads, so higher hardness is measured. Of course, this measured hardness has nothing to do with the actual hardness of the titanium substrate.
Firstly, to measure the hardness of a coating by the Vickers method, one should use a section of the cross section of the coated sample, and not a longitudinal section of the coating from above. Secondly, the size of the imprint from the indenter should be several times smaller than the coating thickness.
In practice, using a minimum load on the indenter of 10-15 grams, it is possible to measure the hardness of highly hard coatings with a thickness of 20 micrometers.
Thin films are very accurate and sensitive. It can not carry as much weight as those used by the Vicker hardness . I agree with Dr. Gedvidas Bikulcius that you find another way to measure the hardness, milligram or microgram (hardness tester ).
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