Can high strength and moderate ductility be combined in wear resistant coatings? A fundamental plasticity study of X2BC nanolaminates (X=Hf, Mo)
Final Report Abstract
The major objective of this project was to identify the fundamental mechanisms that govern the plastic behavior of X2BC nanolaminates (X=Hf, Mo) and compare these results to previously published and newly obtained quantummechanical predictions. To this end the following questions linking microstructural characteristics with the mechanical properties were answered: 1. Does the microstructure of X2BC nanolaminate (X=Hf, Mo) coatings influence their mechanical behavior? The microstructure of the nanolaminates had only a marginal influence on the fracture properties investigated. This is rationalized by failure along transgranular and/-or along amorphous regions of the microstructure. It is interesting that at ultrafine grain sizes, the nanolaminated structure may not have a strong influence in the fracture of the hard coatings. 2. What deformation mechanisms are active in X2BC nanolaminates (X=Hf, Mo)? Columnar grain morphology and presence of amorphous regions controls fracture in these materials. Brittle fracture predominates in these hard coatings. Linear elastic fracture mechanics considerations reveal that crack tip stresses are not high enough to activate global plasticity; however, it might be speculated that they are sufficient to initiate dislocation activity along specific crystallographic planes with relative low lattice resistance stress, thus acting as energy dissipating mechanism. Observations of crack deflection and crack kinking in micro cantilevers may point to the presence of a crack growth resistance behaviour. However, further investigations are necessary before definite conclusions can be made. 3. Do the B/G ratio and the Cauchy pressure serve as predictors for the plastic behavior of X2BC nanolaminates (X=Hf, Mo)? Improved deformation and fracture response of Mo2BC nanolaminated hard coatings definitely attest to a good correlation between high B/G ratio and positive Cauchy pressure as a good predictor for the plastic response of hard materials. However, the inability to synthesize and stabilize the Hf 2BC counterpart coatings with low B/G ratio and a negative Cauchy pressure deters us from making an unambiguous assertion about these factors as the key determinant of mechanical properties of the nanolaminates.
Publications
- Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography, Journal of Applied Physics 122, 075305 (2017)
S. Gleich, H. Fager, H. Bolvardi, J.-O. Achenbach, R. Soler, K.G. Pradeep, J.M. Schneider, G. Dehm, C. Scheu
(See online at https://doi.org/10.1063/1.4999304) - Fracture toughness of Mo2BC thin films: Intrinsic toughness versus system toughening, Materials & Design, 154, 20-27 (2018)
R. Soler, S. Gleich, C. Kirchlechner, C. Scheu, J.M. Schneider, G. Dehm
(See online at https://doi.org/10.1016/j.surfcoat.2018.06.006) - Modifying the nanostructure and the mechanical properties of Mo2BC hard coatings: Influence of substrate temperature during magnetron sputtering, Materials & Design 142, 203-211 (2018)
S. Gleich, R. Soler, H. Fager, H. Bolvardi, J.-O. Achenbach, M. Hans, D. Primetzhofer, J.M. Schneider, G. Dehm, C. Scheu
(See online at https://doi.org/10.1016/j.matdes.2018.01.029) - Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering, Surface and Coatings Technology 349, 378-383 (2018)
S. Gleich, B. Breitbach, N.J. Peter, R. Soler, H. Bolvardi, J.M. Schneider, G. Dehm, C. Scheu
(See online at https://doi.org/10.1016/j.surfcoat.2018.06.006) - Correlative experimental and theoretical investigation of the angle-resolved composition evolution of thin films sputtered from a compound Mo 2BC target, Coatings 9 (3), 206 (2019)
J.-O. Achenbach, S. Mráz, D. Primetzhofer, J.M. Schneider
(See online at https://doi.org/10.3390/coatings9030206)
