Critical review of claims for ultra-hardness in nanocomposite coatings

Abstract

Claims for ultra-hardness (H ≈ 100 GPa) in nanocomposite coatings are critically examined in terms of the experimental evidence first presented in 1999 and theoretical support published over the past 10 years. It is shown that the results of experimental work cannot be validated, and that there are many unresolved issues associated with the supporting theoretical arguments. Using the methods outlined by the authors, whose work is reviewed here, but with more precise application of the equations involved, and reading directly from their reported relationships between Y and H, the best estimate of the hardness for the materials under consideration appears to be of the order of ≈55 GPa. This estimate is validated by actual measurements on a diamond sample and super-hard coatings, and finite element computations in comparison with experimental results for ultra-hard coatings. It is shown that the conclusions of the work being reviewed do not stand up to scrutiny and that the hardness of the ultra-hard coatings is most likely over-estimated by a factor of ≈2.

Keywords:

• nanoindentation
• thin film
• super-hardness
• ultra-hardness
• elastic moduli
• nanocomposite
• indentation hardness

Acknowledgements

The authors have great pleasure in acknowledging the very detailed and helpful discussions with W.J. Clegg, University of Cambridge, during the course of this work. We thank M. Fuchs of Technical University of Chemnitz for the initial suggestion for this review, and several anonymous referees for useful comments.