Diffusion Studies in Toughenable Low-E Coatings

Low emissivity (Low-E) coatings are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region. To achieve this combination of properties, Low-E coatings generally consist of dielectric-silver-dielectric multi-layer systems or stacks, where the thin (~10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti-reflectance layers. The dielectric layers are commonly TiO2, SnO2 or ZnO, and all the layers are usually deposited by magnetron sputtering. The market for Low-E coatings has grown considerably in recent years due to environmental legislation and increased energy costs. To further expand the market, the next generation of Low-E coatings are increasingly being deposited onto toughenable glass, which is post-deposition annealed at temperatures of up to 650oC. However, under these conditions, silver atoms are highly mobile and can rapidly diffuse through the other constituent layers of the coating stack, which can have a detrimental impact on the performance of the coating. Diffusion in polycrystalline films occurs much faster than in bulk samples and by different mechanisms. This is caused by the physical properties of thin films, which may contain a high density of defects such as dislocations, vacancies and grain boundaries that can act as pathways for diffusion processes. The aim of this project therefore is to carry out a detailed study of diffusion processes in dielectric-silver coating systems deposited under industrially relevant conditions (i.e. using commercially available magnetron designs and power deliver modes). TiO2 coatings have been deposited onto float glass substrates by reactive pulsed magnetron sputtering and characterised using Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy and X-ray diffraction. The coatings have been annealed at temperatures in the range of 100oC to 800oC and re-analysed to determine the effect of annealing on the film structures. An interesting transition from a weakly crystalline rutile-like structure with very small grain sizes to a strongly crystalline anatase structure or mixed-phase structure with much larger grains was observed as annealing temperature was increased. Selected coatings were over coated with silver and annealed for a second time. These coatings were analysed by X-ray photoelectron spectroscopy and secondary ion mass spectrometry to determine the diffusion profiles of silver through the titania layer and the reverse diffusion of sodium from the glass substrates. Little difference in the diffusion rate of silver was observed with annealing temperature, but sodium was observed to diffuse significantly faster through samples annealed at higher temperature range. Similar studies have been performed for Al-doped ZnO, Zn2SnO4 and Si3N4 coatings. These films have been post-deposition annealed at 650oC then over coated with silver and re-annealed at 250oC. Diffusion profiles for both Ag and Na atoms were measured using secondary ion mass spectrometry. Finally dielectric/Ag/dielectric layers were deposited to investigate the behaviour of silver and sodium after annealing at 250oC. The basic models of diffusion mechanisms in thin films have been developed using Fick’s second diffusion law. Analytical modelling was used to fit the experimental data into a concentration dependent curve that represents the solution to Fick’s second law. Moreover selected dielectric/Ag/dielectric stacks were subjected to temperature dependency of silver diffusion studies using Arrhenius diffusion principle. Samples were post-deposition annealed at the temperature range of 200-650oC for 5 minutes to investigate silver diffusion at different heat treatment conditions and diffusivity values were used to find activation energies and frequency factors from Arrhenius plot. Overall findings from the diffusion studies are that from dielectric materials investigated in this work Al-doped ZnO coatings have the best barrier properties for silver atoms diffusion and show relatively low values for sodium diffusion, when not annealed at relatively high temperatures. Zinc stannate, on the other hand, was found to be the material through which atoms investigated here diffuse fairly easily. Both silver and sodium atoms were found to have the highest diffusion rates through zinc stannate films relative to the other coatings investigated in this work

Diffusion studies in toughenable low-E coatings

Low emissivity (Low-E) coatings are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region. To achieve this combination of properties, Low-E coatings generally consist of dielectric-silver-dielectric multi-layer systems or stacks, where the thin (~10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti-reflectance layers. The dielectric layers are commonly TiO2, SnO2 or ZnO, and all the layers are usually deposited by magnetron sputtering. The market for Low-E coatings has grown considerably in recent years due to environmental legislation and increased energy costs. To further expand the market, the next generation of Low-E coatings are increasingly being deposited onto toughenable glass, which is post-deposition annealed at temperatures of up to 650oC. However, under these conditions, silver atoms are highly mobile and can rapidly diffuse through the other constituent layers of the coating stack, which can have a detrimental impact on the performance of the coating. Diffusion in polycrystalline films occurs much faster than in bulk samples and by different mechanisms. This is caused by the physical properties of thin films, which may contain a high density of defects such as dislocations, vacancies and grain boundaries that can act as pathways for diffusion processes. The aim of this project therefore is to carry out a detailed study of diffusion processes in dielectric-silver coating systems deposited under industrially relevant conditions (i.e. using commercially available magnetron designs and power deliver modes). TiO2 coatings have been deposited onto float glass substrates by reactive pulsed magnetron sputtering and characterised using Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy and X-ray diffraction. The coatings have been annealed at temperatures in the range of 100oC to 800oC and re-analysed to determine the effect of annealing on the film structures. An interesting transition from a weakly crystalline rutile-like structure with very small grain sizes to a strongly crystalline anatase structure or mixed-phase structure with much larger grains was observed as annealing temperature was increased. Selected coatings were over coated with silver and annealed for a second time. These coatings were analysed by X-ray photoelectron spectroscopy and secondary ion mass spectrometry to determine the diffusion profiles of silver through the titania layer and the reverse IV diffusion of sodium from the glass substrates. Little difference in the diffusion rate of silver was observed with annealing temperature, but sodium was observed to diffuse significantly faster through samples annealed at higher temperature range. Similar studies have been performed for Al-doped ZnO, Zn2SnO4 and Si3N4 coatings. These films have been post-deposition annealed at 650oC then over coated with silver and re-annealed at 250oC. Diffusion profiles for both Ag and Na atoms were measured using secondary ion mass spectrometry. Finally dielectric/Ag/dielectric layers were deposited to investigate the behaviour of silver and sodium after annealing at 250oC. The basic models of diffusion mechanisms in thin films have been developed using Fick’s second diffusion law. Analytical modelling was used to fit the experimental data into a concentration dependent curve that represents the solution to Fick’s second law. Moreover selected dielectric/Ag/dielectric stacks were subjected to temperature dependency of silver diffusion studies using Arrhenius diffusion principle. Samples were post-deposition annealed at the temperature range of 200-650oC for 5 minutes to investigate silver diffusion at different heat treatment conditions and diffusivity values were used to find activation energies and frequency factors from Arrhenius plot. Overall findings from the diffusion studies are that from dielectric materials investigated in this work Al-doped ZnO coatings have the best barrier properties for silver atoms diffusion and show relatively low values for sodium diffusion, when not annealed at relatively high temperatures. Zinc stannate, on the other hand, was found to be the material through which atoms investigated here diffuse fairly easily. Both silver and sodium atoms were found to have the highest diffusion rates through zinc stannate films relative to the other coatings investigated in this work

Thermal conductivity of binary ceramic composites made of insulating and conducting materials comprising full composition range – applied to yttria partially stabilized zirconia and molybdenum disilicide

The thermal diffusivity and conductivity of dense and porous binary composites having an insulating and conducting phase were studied across its entire composition range. Experimental evaluation has been performed with MoSi2 particles embedded into yttria partially stabilized zirconia (YPSZ) as prepared by spark plasma sintering (SPS). The thermal diffusivity of the composites was measured with Flash Thermography (FT) and Laser Flash Analysis (LFA) techniques. Subsequently, the thermal conductivity was determined with the measured heat capacity and density of the composites. The actual volume fraction of the conducting phase of the composites was determined with image analysis of X-ray maps recorded with scanning electron microscopy (SEM). The phases present and their density were determined with X-ray diffractometry (XRD) using Rietveld refinement. The thermal diffusivity increases with increasing volume fraction of MoSi2. Porosity reduces the thermal diffusivity, but the effect diminishes with high volume fractions MoSi2. The thermal diffusivity as a function of the MoSi2 volume fraction of the YPSZ composites is captured by modelling, which includes the porosity effect and the high conductivity paths due to the percolation of the conductive phase

Sputter-deposited nitrides for oxidation protection in a steam environment at high temperatures

The oxidation behaviours of ZrN, TiN and TiSiN in a steam environment in the high temperature range of 600 – 900ºC have been studied and compared. Nitride coatings were deposited by reactive magnetron sputtering onto Zirc-alloy and silicon wafer substrates. The steam oxidation test was performed in order to investigate oxidation resistance in the Loss–of–Coolant Accident (LOCA) scenario in Light Water Reactor applications. It was found that TiSiN showed better oxidation resistance in a steam environment than ZrN and TiN. Coatings in the as-deposited state and after thermal exposure were characterised using focused ion beam, transmission electron microscopy and X-ray diffraction to evaluate microstructure and phases present in the coatings

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

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

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

Characterisation Studies of the Structure and Properties of As-Deposited and Annealed Pulsed Magnetron Sputtered Titania Coatings

Titanium dioxide thin films are durable, chemically stable, have a high refractive index and good electro/photochemical proprieties. Consequently, they are widely used as anti-reflective layers in optical devices and large area glazing products, dielectric layers in microelectronic devices and photo catalytic layers in self-cleaning surfaces. Titania coatings may have amorphous or crystalline structures, where three crystalline phases of TiO2 can be obtained: anatase, rutile and brookite, although the latter is rarely found. It is known, however, that the structure of TiO2 coatings is sensitive to deposition conditions and can also be modified by post-deposition heat treatments. In this study, titania coatings have been deposited onto soda-lime glass substrates by reactive sputtering from a metallic target. The magnetron was driven in mid-frequency pulsed DC mode. The as-deposited coatings were analysed by micro Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Selected coatings were annealed at temperatures in the range 200–700 °C and re-analysed. Whilst there was weak evidence of a nanocrystallinity in the as-deposited films, it was observed that these largely amorphous low temperature structures converted into strongly crystalline structures at annealing temperatures above 400 °C

Characterisation Studies of the Structure and Properties of As-Deposited and Annealed Pulsed Magnetron Sputtered Titania Coatings

Titanium dioxide thin films are durable, chemically stable, have a high refractive index and good electro/photochemical proprieties. Consequently, they are widely used as anti-reflective layers in optical devices and large area glazing products, dielectric layers in microelectronic devices and photo catalytic layers in self-cleaning surfaces. Titania coatings may have amorphous or crystalline structures, where three crystalline phases of TiO2 can be obtained: anatase, rutile and brookite, although the latter is rarely found. It is known, however, that the structure of TiO2 coatings is sensitive to deposition conditions and can also be modified by post-deposition heat treatments. In this study, titania coatings have been deposited onto soda-lime glass substrates by reactive sputtering from a metallic target. The magnetron was driven in mid-frequency pulsed DC mode. The as-deposited coatings were analysed by micro Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Selected coatings were annealed at temperatures in the range 200–700 °C and re-analysed. Whilst there was weak evidence of a nanocrystallinity in the as-deposited films, it was observed that these largely amorphous low temperature structures converted into strongly crystalline structures at annealing temperatures above 400 °C