ICMCTF2014 Session F3: New Boron, Boride and Boron Nitride Based Coatings

Boron nitride nanomaterials (nanoparticles, nanotubes, nanofibers, and nanosheets) represent an innovative class of materials with intriguing prospects of their utilization for structural and medical applications. Boron nitride nanomaterials (BNNs) have long been in a shadow of their famous rivals - carbon nanotubes (CNTs) and graphenes. While both nano-systems revealed equally excellent mechanical properties, boron nitride nanotubes (BNNTs) possess chemical and thermal stability superior to those of CNTs. The large throughput fabrication of BN nanotubes and nanosheets, which was successfully developed in a number of scientific laboratories over the last years, allowed us to initiate advanced studies focused on their exploitation for the reinforcement of lightweight metallic matrices. In this presentation, the most recent achievements in the high-temperature CVD synthesis of BNNs are summarized. The mechanical properties of individual BNNTs are described. Different approaches concerning the surface functionalization of BNNs to improve their wettability and control interfacial interactions at the nano-BN metal matrix interface are considered. Available data in regards of the nature of BNNs binding with a variety of functional groups are highlighted. Fabrication and characterization of different metal/BNNs composite materials are presented. Application of BNNs as lubricants is demonstrated. The latest data concerning biological interaction between BNNs and living systems are also analyzed. Particular attention is paid to the fabrication of BNNs-based mesoporous structures for the transportation of medical agents. The range of confirmed and expected mechanical and functional properties makes BNNs an unprecedentedly interesting topic for the modern structural materials science and biotechnology.

The mechanical properties as well as the thickness of the borided layers obtained by boriding at the surface of the steel alloys are strongly related with three parameters: temperature of treatment, exposure time and boron potential.

In this study the effect of the boron potential in the thickness and in the mechanical properties of the borided layers was evaluated. The boron potential was established by means of the available atoms of boron, content in a control volume inside of a cylinder. The cylinders were manufactured of AISI 316L stainless steel and the boriding treatment was carried-out by using the powder pack technique at a temperature of 1273 K and 6 h of exposure. Four different internal diameters of the cylinders were evaluated (3.17, 4.76, 6.35 and 7.93 mm). The mechanical properties such as hardness, Young´s modulus and fracture toughness, were evaluated by the Berkovich instrumented indentation technic. The volume of the different diameters in the inner of the cylinders allowed controlling the available boron content for the formation of the layers with accuracy. The results showed a clear dependence of the mechanical properties of the borided layers in relation with the boron potential and also the layer thickness increased as the diameter of the cylinders were increased.

Finally, the influence of the boron potential in the constant of parabolic growth (k) was established as a function of the internal diameter of the cylinders.

For both missions, a main thermal protection system (TPS) will protect the payload within its umbra from the thermal radiations and solar winds. For this purpose, pyrolytic Boron Nitride (pBN) thin films deposited by CVD on carbon/carbon composites were studied as a potential candidate. Materials used as the outer layer of such TPS must show the following three mandatory features: 1- a low ratio of the solar absorptivity α to the total hemispherical emissivity ε in order to keep the equilibrium temperature as low as possible at a given distance from the Sun; 2- it must also be able to withstand simultaneously the high temperatures and two specific features of the Sun environment: the ion bombardment from the solar winds and the Vacuum Ultra Violet (VUV) radiation and 3- this material must present a low mass loss rate to avoid the pollution of the measurements of the scientific on-board instrumentation.

pBN coatings on C/C composites are currently under study at PROMES laboratory. To simulate the conditions that the materials will face near the Sun, the MEDIASE facility implemented at the focus of the 1 MW solar furnace at Odeillo has been developed. Samples can be heated up to 2500 K using the concentrated solar energy, in high vacuum (10-7 hPa), and with or without addition of proton bombardment and VUV radiation. This facility is also instrumented to perform several in-situ measurements: pressure, temperature via a pyro-reflectometer developed at PROMES, mass loss rate via a QCM, qualitative nature of the ejected species with an open source mass spectrometer, and directional total, spectral, or in narrow ranges emissivity via a radiometer. Experimental results of the degradation of the pBN films in this severe environment will be presented together with the evolution of its thermal radiative properties at high temperature. Different pBN coating thicknesses (100 to 300 microns) and also bulk pBN samples are studied to identify a possible thickness dependence of the thermal radiative properties. Our results have shown that a pyrolytic BN coating tends to reduce the α/ε ratio in comparison to uncoated C/C composites, which will help reduce the heat shield temperature.

Cubic boron nitride is a promising material for numerous applications due to its outstanding property profile. With its extreme hardness, excellent thermal conductivity, as well as chemical inertness at high temperatures, it is, for example, superior to diamond as a protective coating of tools for use in various high-temperature and ferrous-metal machining. Coatings based on cubic boron nitride can be produced nowadays already by almost every physical vapor deposition or plasma enhanced chemical vapor deposition method. However, such coatings usually exhibit unacceptably high compressive residual stresses resulting from the intense yet obligatory ion bombardment during the nucleation and growth process and, therefore, are still inadequate for the anticipated applications. In recent years many concepts and techniques have been actively explored in attempt to reduce the undesired coating stress, such as incorporation of a third element e.g. hydrogen, carbon, oxygen or silicon in the cubic boron nitride system, reduction of ion energy after the cubic phase nucleation, post-deposition thermal annealing, high-energy ex-situ ion implantation, fluorine-based surface chemistry, and composition-graded bond layer for enhanced adhesion. Different coating concepts and processes leading to low-stress, thick cubic boron nitride based coatings, results of modeling, microstructure and properties in relation to the process parameters as well as the challenges in industrial up-scaling will be discussed.

Vertical few-layer graphene petals are grown on macro-porous carbon foam surfaces having an intrinsic open porosity of 75%. This provides a hierarchical porous structure with a significant potential for surface adsorption/desorption or wetting/dewetting energy storage applications. Carbon foams have a combined advantage of large surface area and high thermal conductivity critical for thermal energy storage, but they are prone to oxidation and exhibit low adsorption enthalpies for lightweight hydrocarbons. Our previous work [Adv. Func. Mater, 22 (2012) 3682] on BN domain formation on carbon foam surfaces through microwave heating assisted chemical modification showed considerable increases in methanol wetting/dewetting enthalpy and thermal stability. Dewetting enthalpy of methanol on carbon foam increases from 460 to 780 J per gm of carbon foam and thermal stability against oxidation is found to increase from 570° to 780°C on BN modification. Here we report on graphene petal decoration of carbon foam surface and subsequent chemical modification through BN incorporation. The resulted hierarchical structure is characterized with XPS, XRD, TEM, FESEM and Raman measurements. Methanol wetting enthalpy and thermal stability of this three-dimensional hierarchical material is analyzed using a modified solution calorimeter and thermo-gravimetric analyzer, respectively. Influences of petal decoration on surface morphology of carbon foam, chemical modification and the stochiometry of the material surface, methanol wetting enthalpy and thermal stability against oxidation are discussed in details. The applicability of this hierarchical porous material for thermal and electrochemical energy applications is established.