ICMCTF2015 Session F3: New Boron, Boride and Boron Nitride Based Coatings

Previous studies in the field of PVD processed low friction coatings pointed out the possibilities of architectural designs, such as multilayer or nanocomposite coatings (e.g. TiAlN/VN, TiC-C), to combine particular properties and hence gain superior tribological performance. To follow this concept we investigated the mechanical and tribological properties of arc evaporated Ti_1-xAl_xN and Mo_1-x-ySi_xB_y multilayer coatings. The combination of Ti_1-xAl_xN - as a well-established thin film showing high hardness and thermal stability - and Mo_1-x-ySi_xB_y - as a Mo based coating system with excellent thermal stability and the ability to form lubricious Magnéli phase oxides such as Mo_nO_3n-1​ - aims for excellent wear behavior at elevated temperatures. A chemical variation of the Mo_1-x-ySi_xB_y layers allows designing a high thermal stability and high oxidation resistance by forming stable borosilica scales and the necessary oxidation to form Mo_nO_3n-1 Magnéli phase oxides.

Low temperature growth strategies for Mo₂BC coatings are reviewed and initial theoretical and experimental data pertaining to the applicability of these coatings during forming of Al based alloys are discussed. A Mo₂BC(040) surface was exposed to O₂. The gas interaction was investigated using ab initio molecular dynamics and x-ray photoelectron spectroscopy (XPS) of air exposed surfaces. The calculations suggest that the most dominating physical mechanism is dissociative O₂ adsorption whereby Mo – O, O – Mo – O and Mo₂ – C – O bond formation is observed. To validate these results, Mo₂BC thin films were synthesised utilizing high power pulsed magnetron sputtering and air exposed surfaces were probed by XPS. MoO₂ and MoO₃ bond formation is observed and is consistent with here obtained ab initio data. Additionally, the interfacial interactions of O₂ exposed Mo₂BC(040) surface with an Al nonamer is studied with ab initio molecular dynamics to describe on the atomic scale the interaction between this surface and Al to mimic the interface present during cold forming processes of Al based alloys. The Al nonamer was disrupted and Al forms chemical bonds with oxygen contained in the O₂ exposed Mo₂BC(040) surface. Based on the comparison of here calculated adsorption energy with literature data, Al – Al bonds are shown to be significantly weaker than the Al – O bonds formed across the interface. Hence, Al-Al bond rupture is expected for a mechanically loaded interface. Therefore the adhesion of a residual Al on the native oxide layer is predicted. This is consistent with experimental observations. The data presented here may also be relevant for other oxygen containing surfaces in a contact with Al or Al based alloys for example during forming operations.

Computational materials science has proven to be a reliable tool for developing high performance materials, tailor made for specific applications. Recent studies show, that first principle calculations allow reliably predicting crystal structure, phase stability or elastic properties of ceramic-like hard coatings.

In a computational feasibility study, we have investigated three different structural modification of B_xAl_1-xN for their respective stability, showing that solid solutions are likely to be accessible by physical vapour deposition.

Based on the computational studies, B_xAl_1-xN thin films were deposited using pulsed DC magnetron sputtering of a boron and an aluminium target under nitrogen and argon atmosphere at different substrate temperatures . The coatings, typically 1 - 2 µm thin, were studied in detail by X-ray diffraction, secondary ion mass spectroscopy, nanoindentation, and cross sectional transmission electron microscopy. Selected samples, especially the B-rich samples, were additionally investigated after vacuum annealing treatments up to 1000 °C.

In thin film form, zirconium diboride (ZrB₂) may be an invaluable material for use in high temperature sensors and other electronic components, thanks to its high electrical and thermal conductivity, and remarkably strong bonding. In this work, a series of thin films with controlled composition ranging from pure Zr to pure B have been grown on sapphire substrates by electron beam co-evaporation from individual elemental sources. Films grown at temperatures below 400°C are predominantly amorphous, whereas for higher growth temperatures, films with an excess of Zr show nano-sized ZrB₂ crystallites and those with excess B remain amorphous. No morphological changes are observed by scanning electron microscopy (SEM) upon annealing the films in ultra-high vacuum up to 1000°C, however x-ray diffraction (XRD) reveals ZrB₂ grain growth in films with excess Zr. The as-deposited nanocrystalline films exhibit a ZrB₂ <100> texture, and the degree of texture increases, particularly for Zr-rich film compositions. Regardless of composition, XRD shows no Zr-B phases other than ZrB₂ crystallites in an amorphous matrix. However, x-ray absorption spectroscopy measurements near the Zr K edge indicate distinct changes in the local bonding environment around the Zr atoms with changes in composition, independent of crystallinity. Analysis of x-ray absorption fine structure (XAFS) spectra confirms that the nearest–neighbor distances are dependent on both composition and annealing temperature. The electrical conductivity of the Zr-B thin film series ranges from 0.01 to 0.6 MS/m, with increased ordering and grain growth leading to increased conductivity.

As a member of the III-nitride wide bandgap semiconductor family, boron nitride (BN) has received much less attention in comparison with other nitride semiconductors. The stable phase of BN synthesized at any temperature and under normal pressure is hexagonal. Hexagonal BN (hBN) possesses extraordinary physical properties including wide bandgap (E_g ~ 6 eV), high temperature stability and corrosion resistance, and large optical absorption and neutron capture cross section. Due to its similar lattice constants to graphene, hBN is also an ideal template and dielectric layer for graphene devices. Furthermore, hBN represents an ideal platform for probing fundamental 2D properties in semiconductors. The synthesis of wafer-scale hBN epilayers by MOCVD has been demonstrated [1-6]. It was shown that the unique 2D structure of hBN induces high density of states and large exciton binding energy, which result in high optical absorption and emission intensity [3,4,7]. P-type conduction and diode behaviors in the p-n structures consisting of p-hBN/n-Al_xGa_1-xN (x~0.62) have been demonstrated [2,5]. Carrier mobility-lifetime products and diffusion lengths of hBN thin film detectors have been characterized. As deep UV photodetectors, hBN detectors exhibit a peak responsivity at 217 nm and a cut-off wavelength at around 230 nm with virtually no responses for below bandgap excitation. As thermal neutron detectors, hBN detectors exhibit an effective conversion efficiency approaching ~80% for the absorbed thermal neutrons and the measured pulse height spectra exhibit unprecedented narrow peaks registered by the product energies of ¹⁰B and thermal neutron reaction [8,9].

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[2] R. Dahal, J. Li, S. Majety, B.N. Pantha, X. K. Cao, J. Y. Lin, and H.X. Jiang, APL 98, 211110 (2011).

[3] J . Li, S. Majety, R. Dahal, W. P. Zhao, J. Y. Lin, & H. X. Jiang, Appl. Phys. Lett. 101, 171112 (2012).

[4] B. Huang, X. K. Cao, H. X. Jiang, J. Y. Lin, and S. H. Wei, Physical Review B 86, 155202 (2012).

[5] S. Majety, J. Li, X. K. Cao, R. Dahal, B. N. Pantha, J. Y. Lin, and H. X. Jiang, APL 100, 061121 (2012).

[6] S. Majety, J. Li, W. P. Zhao, B. Huang, S. H. Wei, J. Y. Lin, and H. X. Jiang, APL 102, 213505(2013)

[7] X. K. Cao, B. Clubine, J. H. Edgar, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 103, 191106 (2013).

[8] J. Li, R. Dahal, S. Majety, J.Y. Lin, and H.X. Jiang, Nuclear Inst. and Methods in Physics Research Section A 654, 417 (2011).

[9] T. C. Doan, S. Majety, S. Grendadier, J. Li, J. Y. Lin, and H. X. Jiang, Nuclear Inst. and Methods in Physics Research Section A 748, 84 (2014).

The growth kinetics of the boride coatings (FeB and Fe₂B) at the surface of AISI M2 high speed steels were studied in this work. Boriding thermochemical treatment was carried out by dehydrated paste pack at three different temperatures 1172, 1227 and 1273 K and four exposure times 1, 3, 5 and 7 h, respectively. The presence of FeB and Fe₂B phases were identified by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction method. In order to obtain the boron diffusion coefficients at the FeB/Fe₂B boride coatings, a mathematical model based on the mass balance at the growing interfaces was proposed under certain assumptions. Likewise the boride coatings thickness (FeB and Fe₂B) were established as a function of the parameters η(T) and ε(T), which are related to the temperature and the effect of the incubation times for the boride formation. The activation energy values estimated for the FeB and Fe₂B layers were 233.42 and 211.89 kJ mol⁻¹ respectively. A good agreement was obtained between the simulated values of boride layers thicknesses and the experimental conditions used in this work. Finally, empirical relationships of boride coatings thickness as a function of boriding temperature and time are presented.