ICMCTF2012 Session F3-1: New Boron, Boride and Boron Nitride Based Coatings

Quantum-mechanically guided materials design and selection enjoys increasing attention due to significantly reduced material development time compared to the conventional “trial-and-error” approach. This trend is enabled by a significant increase in computer performance allowing faster and increasingly complex electronic descriptions of structure and properties that are useful for materials selection and design. In the following this will be illustrated on boron-based hard coatings, especially on the nanolaminate Mo₂BC as well as on boride materials composed of complex B-icosahedra, e.g. MXB₁₄ (M, X = usually metals).

Boron-based hard coatings, due their outstanding properties of high stiffness and hardness, have been of interest for many years. Typical examples are TiB₂ and cubic BN (the second hardest phase known). Recently Mo₂BC has attracted attention: Electronic structure calculations of Mo₂BC predicted very high stiffness and moderate ductility [1]. This property combination is attractive for protective coatings on cutting tools. Mo₂BC thin films were synthesized using DC magnetron sputtering. Nanoindentation experiments determined a high Young’s modulus of 470 GPa. Topographical imaging of the residual indent did not reveal any crack formation but pile-up was measured confirming the combination of high stiffness and moderate ductility of Mo₂BC predicted by ab initio calculations.

MXB₁₄ (M, X = usually metals) is a class of materials with the crystallographic structure based on a framework of B-icosahedra and bestows these phases with excellent mechanical and wear properties. However, a serious challenge is to synthesize crystalline MXB₁₄ coatings. A quantum-mechanical description of XMgB₁₄ (X = Al, Ge, Si, C, Mg, Sc, Ti, V, Zr, Nb, Ta, Hf) [2] and a detailed charge analysis based on Bader decomposition revealed that these phases are stabilized by the transfer of electrons from the X-element to the B-icosahedron, reflected by the effective charge of B-icosahedron. Not only the element but also the configurations of the phases, investigated for Al_xY_yB₁₄ (x, y = 0.25, 0.5, 0.75, 1), influences the phase stability through the effective charge of the B-icosahedron [3]. Generally, the maximum phase stability was identified by approximately two electrons transferred and seems to be connected to electronegativity and ionization potential.

1. J. Emmerlich et al., J. Phys. D-Appl. Phys. 42 (2009) (18), p. 6.

2. H. Kolpin et al., Phys. Rev. B 78 (2008) (5), p. 6.

3. H. Kolpin et al., J. Phys.-Condes. Matter 21 (2009) (35), p. 6.

Characteristics of microstructure of cubic boron nitride film, deposited with hydrogen containing Ar-nitrogen mixed gas were investigated. The films were deposited by UBM (unbalanced magnetron sputtering) method. A boron nitride target of 3” diameter was used as a sputtering source, which was connected with 13.56 MHz RF (radio-frequency) electric power supply. The substrate holder was placed at 7.5 cm under the target and 200 KHz electric power supply was used as a substrate bias power supply. Either Si or Si wafer with nanocrystalline diamond thin film on it was used as a substrate. The chamber was evacuated down to 10^-6 torr and a mixed gas of Ar-10% nitrogen was flowed into the chamber during deposition. The hydrogen was added to the mixed gas up to 5 sccm while maintaining the total gas flow rate at 20 sccm. The deposition pressure was maintained at 2 or 4 mtorr. The electric power of the target was 500 W and the substrate self bias voltage was -60 V. FTIR, SEM, as well as TEM were used to analyze the phase and microstructure.

TEM observation has shown three layered structure of a-BN, t-BN and cBN on Si substrate, which was little affected with the addition of hydrogen. The high resolution TEM structure of t-BN, however, was shown to vary with the addition of hydrogen. The alignment of t-BN laminate was broken for samples with hydrogen, which is believed to correlate the residual stress formation. The microstructure variation of the cBN layer itself was also shown to be affected by the presence of the hydrogen in the reaction gas. The possibility of the codeposition of hBN and cBN phase was shown in the microstructure of the existence of the hBN phase within the cBN layer. The role of hydrogen in the formation of such a microstructure as well as the relation with the variation with the FTIR spectra was also discussed.

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

Diamond has the nearest lattice parameter to cubic boron nitride and been considered as an adequate substrate to deposit cubic boron nitride thin film without any t-BN like intrinsic buffer layer. In this study, the behavior of the intrinsic t-BN like buffer layer was investigated when the cBN films were deposited on nanocrystalline diamond (NCD) film at various temperatures. The films were deposited by UBM (unbalanced magnetron sputtering) method. A boron nitride target of 3” diameter was used as a sputtering source, which was connected with 13.56 MHz RF (radio-frequency) electric power supply. The substrate holder was placed at 7.5 cm under the target and 200 KHz electric power supply was used as a substrate bias power supply. Either Si or Si wafer with nanocrystalline diamond thin film on it was used as a substrate. The chamber was evacuated down to 10 -6 Torr and a mixed gas of Ar-10% nitrogen was flowed at 20 sccm into the chamber during deposition. The deposition pressure was maintained at 2 or 4 mTorr. The electric power of the target was 500 W and the substrate selfbias voltage was -60 V. The deposition temperature was varied in the range between room temperature and 1000℃. FTIR, SEM and TEM RBS were used to analyze the phase and structure.

With increasing the deposition temperature, the hBN intensity of the FTIR spectrum decreased. No visible hBN peak was observed for the films deposited around 800℃ under the above deposition condition. The high resolution TEM microscopy has shown very thin discontinuous hBN layers between the NCD and the cBN layer and epitaxial growth of the cBN on the NCD was also found. The films deposited at room temperature, however, showed typical three layered structure of a-BN, t-BN and cBN. It is thus believed that the hBN peak of the FTIR spectra was originated from the interfacial layer. Other behaviors such as stress variation was also discussed.

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea

A microwave assisted thermo-chemical surface treatment of highly porous carbon foams was developed to synthesize boron-carbon-nitrogen foams for thermal energy storage and release using adsorption/desorption cycle with lightweight hydrocarbons. Carbon foams provide a combined advantage of large surface area and high thermal conductivity critical for thermal energy storage, but they are prone to oxidation and have a low adsorption enthalpy for lightweight hydrocarbons. This report describes carbon foam surface modification to synthesize oxidation resistant and high thermal sorage capacity B-C-N foams. Boron and nitrogen were incorporated in graphitic carbon foam through microwave-assisted thermo-chemical synthesis using boric acid and urea as boron and nitrogen sources respectively. A 400 W microwave treatment for 5-30 minutes was used to accelerate foam surface modification, which was followed by high temperature annealing in an inert atmosphere to complete carbon foam surface conversion to B-C-N and to remove excess oxygen content. The resultant B-C-N foams were characterized by XPS, XRD, FESEM and Raman measurements to quantify their stoichiometry, structure, and morphology. The results reveal the formation of hexagonal B-C-N on the surface of graphitic carbon foams, where B-N and C-N bonding arrangements were dominant and indicate a direct substitution of carbon atoms in graphite lattice with boron and nitrogen atoms. The boron and nitrogen content can be increased with the higher annealing temperature and saturate at approximately BC₄N stoichiometry at 1100⁰C. Foam thermal conductivity was measured by transient plane source and laser flash techniques. Methanol adsorption experiments on the B-C-N foam surface were done by a BET method. The adsorption-desorption enthalpy of methanol molecules on the B-C-N foam surface was measured by differential scanning calorimetry (DSC). The adsorption enthalpy was found to increase with a decrease of the residual oxygen content within the B-C-N foam. An enhancement of adsorption enthalpy was found for B-C-N foam in comparison to pure carbon foam, confirming the B-C-N foam benefits for adsorption cooling applications and waste heat storage and utilization. The advantages of the microwave assisted B-C-N foam synthesis and surface modification for thermal storage enhancement are discussed.

Due to the very limited availability of ³He, neutron detectors based on other elements are urgently needed. Here we present a method to produce thin films of ¹⁰B₄C, with a maximized detection efficiency, intended to be part of a new generation of large area detectors for neutron scattering instrumentation. A full-scale detector could be in total ~1000 m² of two-side coated Al-blades with ~1 mm thick ¹⁰B₄C films. B₄C thin films have been deposited onto Al blade and Si wafer substrates by DC magnetron sputtering from ^natB₄C and boron-10 enriched ¹⁰B₄C targets in an Ar discharge, using an industrial deposition system. The films were characterized with scanning electron microscopy, elastic recoil detection analysis, X-ray reflectivity, and neutron radiography. We show that the film-substrate adhesion and film purity are improved by increased substrate temperature and deposition rate. A substrate temperature of 400 °C results in films with a density close to bulk values, good adhesion to film thickness above 3 mm, and a boron-10 content of close to 80 atomic %. Neutron absorption measurements agree with Monte Carlo simulations and show that the layer thickness, number of layers, neutron wavelength, and amount of impurities are determining factors. Initial prototype performance measurements yield an efficiency of ca. 50%, which is in general agreement with the theoretical predictions.