Using MD we are constructing deformation-mechanism maps based on fundamental physical processes in the deformation of nanostructured materials. The knowledge for these processes comes from modelling the nanostructures at the atomic level by using atomistic simulations. The power of our approach is that it integrates the most important physics associated with deformation phenomena, in a self-consistent description of the effects of stress, interfaces and grain size on the mechanical properties in nanocrystalline structures, thereby avoiding the empiricism of other methods.

We have prepared different Zr/Nb layered systems (with different Nb/Zr geometries and/or structures) and indentation hardness close to the theoretical limits was measured for special cases. Then, our theoretical calculations were compared to the experimental results. Special attention was devoted to the effect of interlayers, grain boundaries, dislocation reactions as well as the effect of phase transitions in Zr.

The dominant deformation mechanism of metallic multilayered systems is the confined volume dislocation reactions, however, we also showed that there is a high possibility to control the deformation by tunning phase transformations at the nanoscale. This knowledge will open new doors to shed light into hidden features of the deformation and strengthening mechanisms guiding the design the next generation of metallic coatings and alloys.

Such a combination of seemingly mutually exclusive properties was recently predicted by ab-initio calculations in inherently nanolaminated X₂BC crystalline coatings, where X is a metallic element [1]. W₂BC system was found the most promising regarding its mechanical properties with predicted Young’s modulus of ~ 470 GPa, B/G ratio of 1.91 and Cauchy pressure of 71 GPa. However, only a near zero negative value of its formation enthalpy of -0.16 eV/atom predicts problems with crystallization of this phase [1]. Indeed, previous works using mid-frequency pulsed DC sputtering did not report the formation of the crystalline W₂BC phase [2].

This contribution reports on the synthesis and comparison of W-B-C coatings sputtered from a compound W₂BC target using DCMS and HiPIMS, respectively. Different substrate temperatures up to 700°C were used in each case. The differences in the DCMS and HiPIMS deposition processes will be discussed. The microstructure of the coatings is investigated and is correlated to their mechanical properties. The fracture resistance and the deformation behaviour of the coatings will be discussed. Simultaneous achievement of high hardness and fracture resistance will be shown as well as the means of its achievement will be discussed.

This research has been supported by project LO1411 (NPU I) funded by the Ministry of Education, Youth and Sports of the Czech Republic.

[1] H. Bolvardi, J. Emmerlich, M. to Baben, D. Music, J. von Appen, R. Dronskowski, J.M. Schneider, J. Phys.: Condens. Matter 25 (2013) 045501

[2] M. Alishahi, S. Mirzaei, P. Soucek, L. Zabransky, V. Bursikova, M. Stupavska, V. Perina, K. Balazsi, Zs. Czigany, P. Vasina, Surf. Coat. Technol. 340 (2018) 103-111

Coherently grown multilayer coatings with a bilayer period in the nanometer range exhibit a superlattice (SL) effect in mechanical properties like hardness and fracture toughness [1, 2]. While the superlattice effect in hardness is well described by the model after Chu and Barnett [3] based on dislocation plasticity, the fracture toughness enhancement in ‘all-ceramic’ SL coatings is not primarily governed by plasticity and the underlying mechanisms are less well understood.

The aim of the present work is to study the effect of the residual stress state in SL coatings on the fracture toughness. In general, extrinsic residual stresses (due to a mismatch of the coefficients of thermal expansion) and intrinsic stresses (stemming from the film growth process) contribute to the overall stress state. A major part of the intrinsic stresses, in turn, are coherency stresses, which originate from the epitaxial growth and the lattice mismatch of the film forming phases and the substrate material. While the thermal mismatch stresses are almost independent of the individual layer thicknesses, coherency stresses show a strong dependency when considering the formation of misfit dislocations. Up to a critical thickness of the layers, the misfit in the lattice parameters is accommodated by elastic strain, thereafter misfit dislocations form, preferably at the interface to the substrate or the previously deposited layer [4, 5].

We have calculated the dislocation density as a function of the layer thickness by an energy balance considering the strain energy of the system and the dislocation energy. An analytical model - with layerwise deposition of the individual layers including misfit dislocations - was developed to predict the stress state of superlattice systems as a function of the bilayer period. Finally, the fracture toughness and its dependence on the bilayer period was calculated taking into account the stress state, the intrinsic fracture toughness of the phases and the spatial variation of elastic properties of respective SL architectures.

[1] U. Helmersson, S. Todorova, S.A. Barnett, J.-E. Sundgren, L.C. Markert, J.E. Greene, J. Appl. Phys. 62 (1987) 481.

[2] R. Hahn, M. Bartosik, R. Soler, C. Kirchlechner, G. Dehm, P.H. Mayrhofer, Scr. Mater. 124 (2016) 67.

[3] X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403.

[4] L.B. Freund, S. Suresh, Thin Film Materials: Stress, Defect Formation, and Surface Evolution, Cambridge University Press, Cambridge, 2003.

[5] D. Holec, P.M.F.J. Costa, M.J. Kappers, C.J. Humphreys, J. Crystal Growth 303 (2007) 314.

In this study, nanocrystallited carbon films were deposited on silica and N-type silicon substartes with plasma assisted PVD method. The nanocrystallited structure were observed through high resolution TEM and Raman spectra. The change of electrical resistances along with temperature and magnetic field strength were measured by using Quantum Design physical property measurement system(PPMS). The results showed that the nanocrystallied films showed large magnetoresistance at 300K, which is favorable for room temperature applications. The magnetic response behavior of the carbon films on different substrates were compared, which suggested that the n-silicon substrate can remarkably improved the magnetoresistacnce coefficient. The influences of gas flow, bias voltage, and deposition time on the structure, carrier density and magnetoresistance of the carbon films were invesitgated, and the transport mechanism of carriers in the films were discussed. We also found the large magnetoresistance of the film when using alternating current as stimulating source, when the largest rate of magnetoresistance can reach to 1000% per tesla at room temperature. This is ascribled to the inductive as well as spin-enhanced magnetoresistance effect due to the oriented graphene nanocrystalltes inside the semiconductive film matrix.

References:

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[2] M. Schneider, A. Bittner and U. Schmid, Improved piezoelectric constants of sputtered aluminium nitride thin films by pre-conditioning of the silicon surface, Journal of Physics D: Applied Physics 48 (2015) 405301.

[3] G. Pfusterschmied, M. Kucera, E. Wistrela, T. Manzaneque, V. Ruiz-Díez, J. L. Sánchez-Rojas, A. Bittner and U. Schmid, Temperature dependent performance of piezoelectric MEMS resonators for viscosity and density determination of liquids, Journal of Micromechanics and Microengineering 25 (2015).

[4] P. M. Mayrhofer, C. Rehlendt, M. Fischeneder, M. Kucera, E. Wistrela, A. Bittner and U. Schmid, ScAlN MEMS Cantilevers for Vibrational Energy Harvesting Purposes, Journal of Microelectromechanical Systems 26 (2017) 102-112.

[5] W. Wang, P. M. Mayrhofer, X. He, M. Gillinger, Z. Ye, X. Wang, A. Bittner, U. Schmid and J. K. Luo, High performance AlScN thin film based surface acoustic wave devices with large electromechanical coupling coefficient, Applied Physics Letters 105 (2014).