Diffusion Barrier Coating Research Articles

Advances in Diffusion Barrier Coatings for High-Temperature Applications

Diffusion barrier coating (DBC) systems on heat resistant alloys consist of a multi-layer structure: an outer Al-reservoir layer and an inner diffusion barrier layer (DBL). The outer Al-reservoir layer forms a protective Al2O3 scale and DBL acts as a barrier layer against alloy interdiffusion. Three kinds of DBL were developed: Re on Ni-based superalloys; W on stainless steels; and Cr on Ni–Cr alloys. It was found that DBC systems have excellent mechanical properties (creep and fatigue), improving alloy substrate performance, and enhance the anti-exfoliation properties of YSZ in thermal barrier coatings (TBC) in addition to providing excellent oxidation resistance. At temperatures higher than 1300 °C, however, the DBC design based on kinetics (diffusion) is insufficient to form and maintain a protective Al2O3 scale. In this case a self-maintaining coating (SMC) system designed on the basis of thermodynamics (phase stability) is required. The SMC system formed on Nb–Hf (C-103) alloy consists of a multi-layer structure: an outer Re (Al, Si)1.8 and inner NbSi2 layers, plus a transient Nb5Si3 layers formed during oxidation. At temperatures higher than 1300 °C the Al2O3 can be formed by changing the Al/Si ratio in the Re (Al, Si)1.8 in which Si was supplied from the decomposition reaction of NbSi2 to Si + Nb5Si3 during selective oxidation of Al. It is proposed that coating alloys should be designed for considering not only high temperature oxidation, but also alloy substrate mechanical properties and anti-exfoliation of oxide scales, based on both kinetic principles (DBC system) and thermodynamics (SMC system).

Influence of talc and hydrogenated castor oil on the dissolution behavior of metformin-loaded pellets with acrylic-based sustained release coating

Multi-unit pellet system (MUPS) is of great interest as it is amenable to customization. MUPS comprises multi-particulates, usually as pellets or spheroids, which can be coated with diffusion barrier coatings. One commonly used diffusion barrier coating is the methacrylic acid copolymer, which can be used as a taste masking, enteric or sustained release polymer. While the versatility of methacrylic acid copolymers makes them pliable for pellet coating, there are impediments associated with their use. Additives commonly required with this polymer, including plasticizer and anti-adherent, have been shown to weaken the film strength. The objective of this study was to investigate the impact of osmotic pressure within the core on the sustained release coat integrity and functionality. Hydrogenated castor oil (HCO) was chosen as the additive to be studied. Metformin-loaded pellets, prepared via extrusion-spheronization, were coated with ethyl acrylate and methyl methacrylate copolymer (Eudragit RS 30 D) containing talc, talc-HCO, or HCO to different coat thicknesses. Drug release was investigated using the USP dissolution apparatus 2 and an ultraviolet imager. The swelling of the pellets when wetted was monitored by video imaging through a microscope. When coated to 7.5 % coat weight gain, coats with HCO slowed down drug release more than the other pellets. The pellets also swelled the most, which suggests that they were more resistant to the osmotic pressure exerted by metformin. For drugs which exert high osmotic pressure, HCO can serve as an efficient alternative to talc in the preparation of methacrylic acid copolymer coatings.

Formation of a Diffusion Barrier Coating on Ni-Based Alloy and Thermal Cycle Oxidation Property

Formation of a Diffusion Barrier Coating on Ni-Based Alloy and Thermal Cycle Oxidation Property

Nanomechanical behaviour of Ni – YSZ nanocomposite coatings on superalloy 690 as diffusion barrier coatings for nuclear applications

Ni-YSZ nanocomposite coatings with different Ni content in the range of 0–50 wt.% were developed on Inconel 690 substrates using an electron beam physical vapour deposition method to optimise the Ni concentration in order to enhance the durability of coatings for nuclear applications. X ray diffraction confirmed the formation of cubic phases of Ni and YSZ in the Ni-YSZ nanocomposite coatings. Increased addition of Ni was found to increase the crystallite size of Ni which resulted in lower strain. FESEM analysis of cross-sectional view of the compositionally graded Ni-YSZ coating with low concentration of Ni showed dense columnar structure and exhibited increased porosity along the columnar boundaries with higher concentration of Ni. FESEM and XRD analyses suggest that the grains of the columnar growth could be along (111) plane of the YSZ phase. The elemental composition of individual layers constituting the compositionally graded Ni-YSZ was confirmed by Energy Dispersive Spectroscopy analysis. The nanoindentation analysis of the as deposited coatings showed an increase in the hardness from 1.7 to 9.1 GPa, reduced Young’s modulus from 48 to 168 GPa, elastic recovery from 15.03% to 32.78% and resistance to plastic deformation from 0.0021 to 0.027 with the increased Ni content. Scratch test confirmed superior adhesion of the Ni-YSZ (50 wt.%: 50 wt.%) nanocomposite coating with the substrate. Also, investigation on the scratch track of Ni-YSZ (50 wt.%:50 wt.%) coating did not reveal chipping or spallation of the coating throughout the scratch track indicating a good adherence of coating with the substrate. The structural and the nanomechanical properties of Ni-YSZ (50 wt.%:50 wt.%) nanocomposite coating suggest that it could be used as diffusion barrier coating in the components of nuclear vitrification furnaces which are operated at higher temperatures.

Formation of a Diffusion Barrier Coating on Ni-Based Alloy and Thermal Cycle Oxidation Property

Formation of a Diffusion Barrier Coating on Ni-Based Alloy and Thermal Cycle Oxidation Property

High temperature behavior of a diffusion barrier coating evolved from ZrO2 precursor layer

The zirconia (ZrO2) as a precursor layer for the diffusion barrier coating (DBC) was deposited by electron beam physical vapor deposition (EB-PVD) between the René N5 superalloy substrate and NiCrAl coating. The high temperature exposure for the N5/ZrO2/NiCrAl system was carried out at 1000 °C for 5 h, 100 h and 200 h in atmosphere to investigate the structure evolution of the ZrO2 precursor layer and the elements interdiffusion of the system compared with the N5/NiCrAl system. The results showed that a sandwich structural DBC (α-Al2O3/rich-Zr/α-Al2O3) was formed through redox reaction between zirconia and aluminum under high temperature, leading to the transformation of N5/ZrO2/NiCrAl system into the N5/DBC/NiCrAl system. It was found that the sandwiched DBC could effectively suppress the interdiffusion of elements compared to the N5/NiCrAl system.

Extreme Temperature Coatings for Future Gas Turbine Engines

The National Energy Technology Laboratory-Regional University Alliance (NETL-RUA) has been developing extreme temperature coating systems that consist of a diffusion barrier coating (DBC), a low-cost wet slurry bond coat, a commercial yttria stabilized zirconia (YSZ) thermal barrier coating (TBC), and an extreme temperature external coating that are deposited along the surface of nickel-based superalloys and single crystal metal substrates. Thermal cyclic testing of these multilayer coatings was conducted in steam-containing environments at temperatures ranging between 1100 and 1550 °C. This paper discusses the response of these materials during bench-scale testing, and their potential use in advanced H- and J-class land-based gas turbine engines.

Development of Yttrium Stabilized Zirconia (YSZ) diffusion barrier coatings for mitigation of Fuel–Cladding Chemical Interactions

Fuel–Cladding Chemical Interactions (FCCIs) in a nuclear reactor occurs due to thermal and radiation enhanced inter-diffusion between the cladding and fuel materials. This can have the detrimental effects of reducing the effective cladding wall thickness and the formation of low melting point eutectic compounds. Deposition of thin diffusion barrier coatings in the inner surface of the cladding can potentially reduce or delay the onset of FCCI. This study examines the feasibility of using nanofluid-based electrophoretic deposition (EPD) process to deposit coatings of Yttrium Stabilized Zirconia (YSZ) as the diffusion barrier coating. The deposition parameters, including the nanofluid solvent, additive, particle size, current, and voltage were optimized using test flat substrates of T91 ferritic–martensitic steel. A post deposition sintering step was also conducted and optimized to improve the bonding and mechanical integrity of the coating. Diffusion characteristics of the coatings were investigated by diffusion couple experiments using cerium as a fuel fission product responsible for solid state FCCI. These diffusion couple studies performed at 575°C for 100h showed that the YSZ coatings significantly reduced the solid state inter-diffusion between cerium and steel. A heat transfer model was developed to simulate the changes in temperature profile inside the fuel cladding by addition of YSZ coating. It was found that even though the temperature can increase in the coated cladding, the temperature falls below the melting point of uranium and eutectic temperature in Fe–U phase diagram. Using a co-axial configuration in conjunction with the EPD process, YSZ was successfully deposited uniformly on the inner surfaces of 12″ length sections of cladding with 4mm inner diameter. Such a coating is extremely hard to make by conventional coating technologies like thermal spray or vapor deposition.

Development of titanium diffusion barrier coatings for mitigation of fuel–cladding chemical interactions

High temperature and radiation in nuclear reactors result in inter-diffusion and reaction between fuel and cladding. This phenomenon is called fuel–cladding chemical interactions (FCCI). It reduces the effective cladding wall thickness and might lead to eutectic liquation. Having a thin diffusion barrier coating inside the cladding can potentially reduce or postpone the onset of FCCI. This study examines the feasibility of using a nanofluid-based electrophoretic deposition (EPD) process to deposit titanium metallic coating as the diffusion barrier. The deposition parameters, including the nanofluid solvent, additive, particle size, current, and voltage were optimized using test flat substrates of T91 ferritic–martensitic steel. Postdeposition sintering was also conducted and optimized to achieve the best bonding and mechanical integrity. These diffusion couple studies were performed at 575°C for 100h between cladding and cerium as the fuel surrogate. Results showed that titanium coatings significantly reduced the solid state inter-diffusion between cerium and steel. Using a co-axial EPD, titanium was successfully deposited uniformly on the inner surfaces of 12″ length of cladding with 4mm inner diameter. Such a coating is extremely difficult to manufacture by conventional coating technologies like thermal spray or vapor deposition.

Diffusion barrier coating system concept for high temperature applications

The diffusion barrier coating (DBC) system was designed to suppress the interdiffusion between alloy substrate and coatings in addition to ensuring slow oxide growth at high temperatures. In this report, the concept of the DBC system will be introduced, and the results obtained for various heat resistant alloys are presented. In this DBC system, the σ-phase of the Re–Cr–Ni system was selected as the barrier and was formed by electroplating Re–Ni and Ni films from aqueous solutions, followed by Cr and/or Al pack cementations. A typical DBC system consists of Ni based superalloy/triple σ-Re based alloy layer/γ′-Ni3Al and/or β-NiAl (with or without Pt addition)/α-Al2O3. It was found that the DBC system maintained its structure for long time exposures because the σ-phase has tielines with the γ and γ′ alloy substrate and the γ′ and/or β coating. The σ-Re based alloy can suppress both the inward Al diffusion and the outward diffusion of alloying elements. The DBC system was successfully applied to various heat resistant alloys such as Ni–Mo and Fe–Cr–Ni based alloys, and these alloys with the DBC system were creep deformed slowly under external applied stresses. A simultaneous diffusion coating process for Al and Hf was developed using Hf+NH4Cl+Al2O3 pack cementation. To obtain reliable and long life coatings, the combination of alloy substrate, coating and external scale should be considered as a total system, to optimise oxidation resistance, structural stability and mechanical properties.Le système de revêtement à barrière de diffusion (système DBC) a été conçu pour supprimer la diffusion entre le matériau de base de l’alliage et les revêtements, en plus d’assurer une croissance lente de l’oxyde à hautes températures. Dans ce rapport, on introduit le concept du système DBC et l’on présente les résultats obtenus pour divers alliages résistants à la chaleur. Dans ce système DBC, on a choisi la phase s du système Re-Cr-Ni comme barrière et celle-ci est formée par électrogalvanisation de films de Re-Ni et de Ni à partir de solutions aqueuses, suivie par cémentation solide de Cr et/ou d’Al. Un système DBC typique consiste en un superalliage à base de Ni / une couche triple d’alliage à base de Re-s / Ni3Al-g’ et/ou NiAl-b (avec ou sans addition de Pt) / Al2O3-a. On a trouvé que le système DBC maintenait sa structure pendant de longues périodes d’exposition grâce aux lignes de jonction de la phase s avec le g et le g’ du matériau de base de l’alliage et avec le g’ et/ou le b du revêtement. L’alliage à base de Re-s peut supprimer tant la diffusion de l’Al vers l’intérieur que la diffusion vers l’extérieur des éléments d’alliage. On a appliqué avec succès le système DBC à divers alliages résistants à la chaleur comme les alliages à base de Ni-Mo et de Fe-Cr-Ni. Ces alliages avec le système DBC ont été lentement déformés par fluage sous des contraintes externes. On a développé un procédé de revêtement par diffusion simultanée de l’Al et du HF en utilisant la cémentation solide avec le Hf+NH4Cl+Al2O3. Afin d’obtenir des revêtements fiables et de longue durée, on doit considérer comme un système total la combinaison du matériau de base de l’alliage, du revêtement et de la couche externe, afin d’optimiser la résistance à l’oxydation, la stabilité structurelle et les propriétés mécaniques.

Diffusion Barrier Coating System and Oxidation Behavior of Coated Alloys

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Thermal Stability of a Rhenium-Based Diffusion Barrier Coating Layer on a Ni-Based Superalloy

The thermal stability of coatings containing a Re-based diffusion barrier layer was investigated by surface and cross-sectional analysis methods. A Re-based barrier layer accompanied by an outer Ni-Cr-Al layer was prepared by electrolytic plating onto a Ni-based superalloy, followed by Cr-pack cementation in vacuum at 1523 K. Vacuum annealing was carried out at 1423 K for 25 h. Another type of coating specimen, with an additional Al reservoir layer on the Re-based barrier layer, was oxidized in air for 25 one-hour cycles at 1423 K. EDXRF, XRD, SEM, EDS and EPMA were used for analysis to evaluate the effects of the heat treatments. It was found that the barrier layer decomposed at high temperature when it was coated with a low-Al Ni-Cr-Al phase, but had good stability when it was adjacent to a high-Al Ni-Cr-Al phase.