ICMCTF2012 Session PDP: Post Deadline Discoveries and Innovations

Refractory metals have low coefficients of thermal expansion. The ability of refractory metals to withstand extreme temperatures without significantly expanding or softening makes them useful as nozzle inserts in solid or liquid rocket-motor environments. However, the oxidation resistance for refractory metals is poor. Iridium (Ir) has a high melting temperature, excellent chemical compatibility and stability, low oxygen permeability and good oxidation resistance. Many of features of Ir make itself suitable for protecting the refractory metals from higher temperature damage and then have a longer service life. However, Ir does not form a condensed oxide due to the vapor species. In order to provide enhanced high temperature protection over a wide range of operating conditions, the refractory metals could be improved by depositing a graded coating of Ir-Zr. In this article, the Ir-Zr and Ir coatings were produced on molybdenum substrate by double glow plasma technology. The structure and composition of the Ir-Zr and Ir coatings were conformed by SEM, AFM, XRD and EDS. The adhesion between the coating and the substrate was evaluated by a scratch tester. Thermal stability and oxidation resistance of Ir-Zr and Ir coatings were evaluated at high temperature.

Hot corrosion is one of the principal destructive factors in thermal barrier coatings (TBCs) at high temperatures. Low quality fuels usually consist of impurities such as Na and V which can form Na₂SO₄ and V₂O₅ salts onto the turbine blades. Hence, hot corrosion resistance of three types of plasma sprayed TBCs was investigated: (a) normal YSZ (yttria-stabilized zirconia), (b) layer composite of (YSZ / normal Al₂O₃ as an outer layer) and (c) layer composite of (YSZ/ nano Al₂O₃ as an outer layer). Hot corrosion tests were done onto the coatings in molten salts (45%Na₂SO₄ +55%V₂O₅) at 1000⁰C for 52 h. The cracking and the premature spallation were observed in normal YSZ coating. The formation of monoclinic ZrO₂ and YVO₄ large crystals as hot corrosion products caused the degradation of the mentioned TBC. Although monoclinic ZrO₂ and YVO₄ crystals had been considerably reduced in (YSZ / normal Al₂O₃) in comparison with normal YSZ, YVO₄ crystals had main role in creation of micro-cracks in (YSZ / normal Al₂O₃) coating. Nano alumina coating as an outer layer in YSZ/ nano Al₂O₃ significantly reduced the penetration of molten salts into the YSZ layer and resulted in the further resistance of TBC against hot corrosion, because the hot corrosion products had been substantially decreased in YSZ/ nano Al₂O₃ coating in comparison with YSZ/normal Al₂O₃ and usual YSZ coatings.

Key words: Hot corrosion; YSZ; TBC; nano Al₂O₃ layer; YVO₄ crystals; normal Al₂O₃;monoclinic ZrO₂

It is a common technique to enhance the adhesion of hydrogenated amorphous carbon (a-C:H) coatings on steel substrates by applying adhesion layers based on different elements like Cr or Si. They frequently show a complex assembly with distinct chemical and mechanical gradients on a length scale in the submicron range. Therefore a correlation between the local mechanical properties and the corresponding chemical composition of these layers is difficult. In this work a Si-based adhesion layer for a-C:H coatings with a thickness of about 1 µm was investigated. The coating system was deposited by PECVD and the adhesion layer consists of a silicon rich layer followed by an adjacent ramp layer with a graded chemical composition. The adhesion layer was characterized in terms of microstructure, chemical composition and local mechanical properties by means of focused ion beam (FIB), auger electron spectroscopy and nanoindentations. Using the small-angle cross-section method detailed information on the local mechanical properties as hardness and Young`s modulus as well as on the chemical composition of the adhesion layer was obtained. It was found that the mechanical properties are strongly influenced by the chemistry of the adhesion layer. In addition, residual stresses in the a-C:H coating were determined by means of FIB and digital image correlation (DIC). For this a H-bar was FIB milled in the a-C:H coating which causes the residual stresses to relax locally. By determining the resulting displacements with DIC and correlating them to an appropriate finite element analysis the residual stresses can be quantified. For the a-C:H coating residual compressive stresses of about – 2 GPa were found.

Clarifying the correlations among the structures, mechanical properties and deformation and fracture behaviors of hierarchical bone tissue at a nanometer scale will improve the research in bone nanomechanics and the development of biomedical coatings and implants. Thus in this study, the nanostructures of normal and osteoporotic mouse bone were characterized; nanoscopic deformation and fracture were examined by the in-situ observations of bone nanopillars under indentations in transmission electron microscopy. Normal mouse bone comprised densely-packed hydroxyapatite crystallites and plied collagen fibers, and presented a ductile fracture toughened by microcracking, crack deflections and ligament bridging. Lattice distortions and a large number of dislocations that formed in mineral crystals consumed applied strain energy additionally. Upon osteoporosis, mouse bone changed to a loose structure with dispersed mineral crystals in a matrix of ground substance. The loss of collagen fibers, the sliding and rotations of dispersed crystals and also the intergranular fracture along the weak matrix led to the brittleness of the osteoporotic bone tissue.

We have deposited and characterized coordination preference in B-C-N films as a function of deposition parameters using a new multi-chamber system which allows for the of deposition of coatings in a chamber not only kept under UHV (~ 10^-10 mbar) conditions, but also enables in situ characterization. With the help of this system, contamination of freshly prepared surfaces due to exposure to ambient is eliminated prior to analysis with a monochromated XPS system (energy resolution < 0.7 eV). Furthermore, the need for ion-etching for the removal of the contaminated layers prior to XPS analysis is also eliminated allowing for the direct access to pristine film surface.

Our results regarding B-C-N films indicated findings contrary to our earlier ex situ findings. XPS, Raman and FTIR data from depositions carried out under ex situ conditions suggested increasing the substrate bias and/or increasing N₂ flow rate during depositions triggered formation of an hexagonal network of well defined separate B-N and C-C dominated domains. This is generally considered as an evidence for phase segregation. On the other hand, in situ characterization of B-C-N films deposited with comparable deposition parameters in the multi-chamber system suggested the evidence of a significant C-N bonding component. In some cases, nitrogen coordinated carbon amount is even found to be more than carbon-carbon coordination.