Influence of alloying elements on the mechanical properties, especially fracture toughness, of the WB2-z base system

Transition metal diborides are an emerging class of thin film materials with promising properties ranging from ultra-low compressibility, high thermal stability, super hardness to superconductivity. These properties allow an application as protective coating in harsh environments. Our recent ab initio calculations suggest an attractive combination of both, high hardness and relatively high fracture toughness, for WB2. This is enabled by a stabilization of the α-structure (space group 191, AlB2-prototype, P6/mmm) over the intrinsic more stable ω-structure due to omnipresent point defects in physical vapor deposited coatings (i.e. boron and metal vacancies) [1]. However, those point defects in turn lower the thermal stability as the are affected by recovery events, leading to phase transformation into the ω-type. Further calculations point towards a stabilization of the α-type with the addition of Ta (which diboride is stabilized in the α-structure without the need of vacancies) at—compared to other transition metals investigated—low cost on ductility. Within this study we deposited various W1-xMxB2-z solid solution coatings with different alloying element contents and examined them for mechanical properties and thermal stability. It was found for M=Ta that the hardness increases ~4 GPa (from 40.8±1.5 to 45.0±2.0 GPa) together with an improvement of the thermal stability (a change of the phase transformation temperature from ~800-1000 °C to over 1400 °C was observed) [2,3]. Besides these characteristics, in various applications a certain amount of damage tolerance (crack initiation and propagation) is required to prevent premature failure. To assess this behavior, we determined the fracture toughness of these coatings by performing micromechanical experiments by means of single cantilever bending tests within the framework of specifications given by Matoy et al. and Brinckmann et al. [4–6]. At the same time of the increase in hardness and thermal stability, we observe a decrease (in agreement with our DFT calculations) in fracture toughness (from 3.7±0.3 MPaÖm for to 3.0±0.2 MPaÖm) with the addition of tantalum up to a maximum content of 26 at% on the metal sublattice. [1] V. Moraes, H. Riedl, C. Fuger, P. Polcik, H. Bolvardi, D. Holec, P.H. Mayrhofer, Sci. Rep. (2018). [2] V. Moraes, C. Fuger, V. Paneta, D. Primetzhofer, P. Polcik, H. Bolvardi, M. Arndt, H. Riedl, P.H. Mayrhofer, Scr. Mater. 155 (2018) 5–10. [3] C. Fuger, V. Moraes, R. Hahn, H. Bolvardi, P. Polcik, H. Riedl, P.H. Mayrhofer, MRS Commun. (2019) 1–6. [4] K. Matoy, H. Schönherr, T. Detzel, T. Schöberl, R. Pippan, C. Motz, G. Dehm, Thin Solid Films 518 (2009) 247–256. [5] S. Brinckmann, C. Kirchlechner, G. Dehm, Scr. Mater. 127 (2017) 76–78. [6] S. Brinckmann, K. Matoy, C. Kirchlechner, G. Dehm, Acta Mater. 136 (2017) 281–287

Fracture properties of CrN hard coatings: Influence of the microstructure, alloying elements, and coating architecture

Transition metal nitrides are well known and applied as protective coating materials based on their unique refractory characteristics, such as high hardness or Young’s modulus. However, for long-term applications, the fracture toughness KIC is an essential factor as the integrity of the coating-substrate interface is impaired by cracking and subsequent environmental attacks. Please click Download on the upper right corner to see the full abstract

First GOCE gravity field models derived by three different approaches

Three gravity field models, parameterized in terms of spherical harmonic coefficients, have been computed from 71 days of GOCE (Gravity field and steady-state Ocean Circulation Explorer) orbit and gradiometer data by applying independent gravity field processing methods. These gravity models are one major output of the European Space Agency (ESA) project GOCE High-level Processing Facility (HPF). The processing philosophies and architectures of these three complementary methods are presented and discussed, emphasizing the specific features of the three approaches. The resulting GOCE gravity field models, representing the first models containing the novel measurement type of gravity gradiometry ever computed, are analysed and assessed in detail. Together with the coefficient estimates, full variance-covariance matrices provide error information about the coefficient solutions. A comparison with state-of-the-art GRACE and combined gravity field models reveals the additional contribution of GOCE based on only 71 days of data. Compared with combined gravity field models, large deviations appear in regions where the terrestrial gravity data are known to be of low accuracy. The GOCE performance, assessed against the GRACE-only model ITG-Grace2010s, becomes superior at degree 150, and beyond. GOCE provides significant additional information of the global Earth gravity field, with an accuracy of the 2-month GOCE gravity field models of 10 cm in terms of geoid heights, and 3 mGal in terms of gravity anomalies, globally at a resolution of 100 km (degree/order 200)