Diffusion studies in magnetron sputter deposited silicon nitride films

In this work, silicon nitride coatings were deposited by magnetron sputtering onto float glass substrates and post-deposition annealed at 650°C for 5min. The structures and compositions of the coatings were investigated by X-ray diffraction, X-ray reflectometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Samples were then over-coated with silver and subjected to a second annealing process to initiate the diffusion of silver through the adjacent coating layers. Additional silicon nitride coatings were then deposited on selected samples to produce Si3N4/Ag/Si3N4/glass stacks, which were annealed at temperatures in the range 100-600°C. Ag and Na diffusion coefficients were then calculated from compositional profiles obtained from time of flight secondary ion mass spectrometry analysis. The coatings deposited in this study were found to have stoichiometric Si3N4 compositions and were amorphous after annealing. The diffusion rate of silver through these coatings was found to depend on annealing temperature and coating density and roughness, which in turn can be related to the deposition conditions. © 2013 Elsevier B.V

Investigation of silver diffusion in TiO2/Ag/TiO2 coatings

Low emissivity (low-E) coatings consisting of dielectric/silver/dielectric multi-layer stacks are applied to large-area architectural glazing to reduce heat losses from buildings. In this work TiO2/Ag/TiO2 stacks were deposited onto soda-lime glass by pulsed DC reactive magnetron sputtering. The coatings were annealed in the range 100-600 C to study silver diffusion through neighbouring layers. Depth-profiling analysis was performed on these samples using time-of-flight secondary ion mass spectrometry and selected samples were also analysed by X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Fick's second diffusion law was used to find diffusion coefficient values and to investigate the temperature dependence of silver diffusion. To investigate film morphology and composition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed. The purpose of this study is the requirement for the understanding of the issue of silver diffusion during annealing treatments used in glass fabrication and the results obtained show that silver diffuses through the adjacent layers in a stack during heat treatment. However, in the temperature range investigated, the diffusion rates did not follow an Arrhenius dependence. At higher temperatures and longer annealing times sodium also diffuses from the glass into the coating, replacing the silver between the titania layers. © 2013 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved

Thermo – mechanical properties of SPS produced self-healing thermal barrier coatings containing pure and alloyed MoSi2 particles

Yttria – partially stabilised zirconia (YPSZ) MoSi2 composites have been designed to prolong the lifetime of the matrix by self – healing cracks during thermal cycling. The healing reaction at high temperatures is based on the decomposition of MoSi2, leading to a volumetrically expanding reaction product, which seals the crack. In this work, coefficient of thermal expansion (CTE) and the fracture toughness of composites containing MoSi2 particles, produced by spark plasma sintering (SPS) have been compared to conventional YPSZ. The CTE mismatch between YPSZ and MoSi2 was found to be small, implying that thermally induced mismatch stresses will be small and the composites have a similar CTE to conventional YPSZ. Fracture toughness was found not to be affected by the particles and showed similar values to unreinforced YPSZ. Cracks introduced by indentation have been shown neither to prefer, or avoid, the particles suggesting that such a composite system is capable of autonomously activating the self – healing reaction

A Conformable High Temperature Nitride Coating for Ti Alloys

There are many applications including aeroengine design where one would like to operate Ti or its alloys at higher temperatures, but the threat of oxidation or fire remains a longstanding challenge. Here, we have designed a bilayer nitride coating for Ti and its alloys produced by magnetron sputter deposition of a SiAlN coating (1.2 μm thick) with a Mo interlayer. We have taken advantage of interdiffusion and inter-reaction at the interface during cyclic oxidation at 800°C to form a layered nitride coating system comprising: a SiAlN top layer, a TiN0.26 and Ti5Si3 mixed phase interlayer, and a Ti-Mo solid solution. The novel TiN0.26 interlayer exhibits adaptive conformability via mechanical twinning, thereby accommodating the thermal mismatch strain between the coating and substrate. This, along with high adhesion, confers excellent thermal cycling life with no cracking, spallation and oxidation of the coating evident after hundreds of hours of cyclic oxidation (>40 cycles) in air at 800°C. This work provides a design pathway for a new family of coatings displaying excellent adhesion, adaptive conformability and superior environmental protection for Ti alloys at high temperature

Comparison of the oxidation behavior of a zirconium nitride coating in water vapor and air at high temperature

The oxidation behavior of zirconium nitride coating in high-temperature water vapor and air environments was studied. The parabolic rate constant of ZrN oxidizing in the water vapor environment at 600 ℃ was approximately 100 times faster than that in air, due to the larger pores and greater number of cracks that were formed across ZrO2 oxide layer formed during the water vapor oxidation process than during the air oxidation process. A bilayer-structure ZrO2 with tetragonal ZrO2 near the ZrN/ZrO2 interface and monoclinic ZrO2 approaching the outer ZrO2 surface were formed in both cases. The lateral cracks across the ZrO2 scale were caused by volume expansion from the tetragonal ZrO2 phase to the monoclinic ZrO2 phase transition

Influence of embedded MoSi2 particles on the high temperature thermal conductivity of SPS produced yttria-stabilised zirconia model thermal barrier coatings

To prolong the lifetime of thermal barrier coatings (TBCs) recently a new method of microcrack healing has been developed, which relies on damage initiated thermal decomposition of embedded molybdenum disilicide (MoSi2) particles within the TBC matrix. While these MoSi2 particles have a beneficial effect on the structural stability of the TBC, the high thermal conductivity of MoSi2 may have an unfavourable but as yet unquantified impact on the thermal conductivity of the TBCs. In this work the thermal conductivity of spark plasma sintering (SPS) produced yttria-stabilised zirconia (YSZ) model thermal barrier coatings containing 10 or 20 vol.% of MoSi2 healing particles was investigated using the laser flash method. Measurements were performed on free-standing composite material over a temperature range from room temperature up to 1000 °C. Microstructural analysis was carried out by SEM combined with image analysis to determine the size, distribution and area fraction of healing particles. The measurements were compared with the results from microstructure-based multi-physics finite element (FE) models and analytical models (the asymmetric Bruggeman model and the Nielsen model) in order to study the effects of the addition of MoSi2 particles as well as the presence of micro-pores on the apparent thermal conductivity. The results show a strongly non-linear increase in the thermal conductivity of the composite material with the MoSi2 volume fraction and a dependence on the aspect ratio of MoSi2 particles. Interparticle connectivity is shown to play a big role too

Effects of interfacial depletion on the degradation of SiAlN coating

Thermally stable 0.4 µm, 0.8 µm, and 1.6 µm thick SiAlN coatings with Mo interlayers have been deposited on Ti substrates to serve as a protective barrier layer for aeroengine applications. After 50 h (10 cycles) of exposure at 800 °C in air, the 0.4 µm, 0.8 µm thick SiAlN coatings were depleted to tens of nanometers in thickness and then formed oxide scales, whereas 1.6 µm thick SiAlN coating retained a 0.5 µm thick remnant SiAlN layer without any observable oxidation. The depletion of the SiAlN coating is induced by purely interfacial diffusion/reactions with the underlying substrate. Once depleted to a few tens of nanometers thick, i.e., close to being fully depleted, the SiAlN coating starts to oxidise, along with an elemental composition change in the remnant SiAlN. The degradation mechanism of the SiAlN coating is determined by its depletion, as opposed to the interfacial reaction induced microstructural change of the remnant coating

Methane oxidation over supported Pd catalysts prepared by magnetron sputtering

While magnetron sputtering has been used to deposit metals onto a range of solid substrates, its application to produce porous heterogeneous catalysts in powder form is relatively unstudied. Here, magnetron sputtered Pd heterogeneous catalyst powders were prepared using a shaker operated at optimal experimental settings to give uniform coverage of the powders, and tested in the abatement of exhaust emissions in natural gas fuelled engines via the oxidation of methane. Pd nanoparticles were deposited onto alumina, titania and zeolite supports, in powder form. X-ray diffraction confirmed that the characteristic structure of each support was maintained following sputtering. The quantity of Pd increased (a) with deposition time and (b) as a function of support in the order alumina < zeolite < titania. The methane oxidation activity, measured as the temperatures at which 10% and 50% conversions were observed, T and T , increased with Pd content for each support and was most active over zeolite catalysts despite a greater amount of Pd present on titania. Overall, the findings demonstrate that magnetron sputtering is a viable method to prepare active precious metal based catalyst powders. Furthermore, this rapid one-step process is complete after 10–20 min deposition time and avoids any metal salt impurities or the need for solvent as required in traditional synthesis methods. 10 5

Effect of Nb and V doped elements on the mechanical and tribological properties of CrYN coatings

One of the most promising approaches to enhancing the tribological properties of engineering coatings is to add transition elements to the structure. In this study, Nb-doped CrYN and V-doped CrYN thin films were deposited by pulsed DC reactive sputtering in a closed-field unbalanced magnetron sputtering (CFUBMS) system. The deposition parameters examined were target current (1, 1.5 and 2 A), deposition pressure (0.15, 0.25 and 0.35 Pa), pulse frequency (100, 200 and 350 kHz) and duty cycle (85 %, 70 % and 50 %). A Taguchi L9 orthogonal design was used to define the deposition process parameters for each doped film. The Nb and V-doped CrYN thin films were characterized in terms of their microstructure, thickness, composition, hardness and tribological properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), nanohardness and pin-on-disc testing, respectively. The bond strength between the substrate and the films (adhesion) was analyzed by scratch testing. For the Nb-doped thin films, a maximum hardness value of 21.4 GPa and the lowest friction coefficient of 0.36 were obtained. On the other hand, in the V-doped thin films, the maximum hardness value was 16.1 GPa, while the lowest friction coefficient obtained was 0.11. In addition, Nb-doped and V-doped CrYN thin films exhibited extraordinary adhesion properties. The effect of the selected deposition parameters (target current, pulse frequency, and duty cycle) in relation to the film thickness, hardness, and coefficient of friction properties of the Nb and V-doped CrYN thin films were investigated using the Taguchi approach and optimum operating conditions were identified and confirmed

An amorphous SiAlN barrier coating against NaCl attack in humid air at elevated temperature

Amorphous magnetron sputtered SiAlN films have been designed to prevent NaCl attack of Ti alloy substrates in high temperature humid air environments. Uncoated Ti alloys suffer severe degradation after exposure to NaCl deposits in humid air at 700 °C for 100 h, whereas SiAlN coating forms protective scale mainly consisting of sodium silicate in identical conditions. Amorphous sodium silicate exhibits fast Na+ diffusion rate and extremely slow O2- diffusion rate, but degradation of SiAlN takes place in presence of both O2- and Na+ species. Sodium silicate scale provides good protection for SiAlN due to slow diffusion of O2- through sodium silicate