An investigation into the effect of substrate on the load-bearing capacity of thin hard coatings
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TiN and diamond-like carbon (DLC) coatings were deposited on Ti substrates with and without processing by high-pressure torsion (HPT). The HPT processing refined the grain size of titanium from the coarse-grained (CG) size of ~8.6 µm to the ultra-fine grained (UFG) size of ~130 nm and increased the hardness from ~1.83 to ~3.05 GPa. Scratch test results revealed that all the thin hard coatings had a higher critical load when deposited on the harder UFG Ti substrate compared to those deposited on the softer CG Ti. A three-dimensional finite element model (3D FEM) revealed that the improved load-carrying property of the thin hard coatings on the harder titanium substrates was related to the higher stresses generated within the substrate and its deformation behaviour which reduced the strain at the coating/substrate interface. A model based on the hardness of a two-layered composite was used to explain this effect, and it is shown that the model is reasonably successful in predicting the critical load of a wide range of coating–substrate systems. The analytical model and the results of the 3D FEM modelling emphasize the contribution of the substrate to the load-bearing capacity of thin coatings.
KeywordsCritical Load Indentation Depth Physical Vapour Deposition Scratch Test Track Width
Miss A. Escudeiro and Prof A. Cavaleiro of University of Coimbra, Portugal, and Dr T. Polcar of University of Southampton, U.K., are gratefully appreciated for their assistance with the DLC-Zr coatings. This work was partially supported by the EPSRC under Grant No. EP/D00313X/1, the National Science Foundation of the United States under Grant No. DMR-1160966, the Fundamental Research Funds for the Central Universities No. 30915118812 (CTW and TGL) and the VTT Technical Research Centre of Finland, Ltd.
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Conflict of Interest
The authors declare that they have no conflict of interest.
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