Combined pre-annealing and pre-oxidation treatment for the processing of thermal barrier coatings on NiCoCrAlY bond coatings

A combined pre-annealing and pre-oxidation treatment was developed for the processing of partially yttria stabilized (PYSZ) thermal barrier coatings (TBC) on top of NiCoCrAlY bond coatings (BC). To develop this pre-treatment, the influence of the oxygen potential during pre-annealing and pre-oxidation on the life span and failure mechanisms of the entire high temperature coating system upon thermal cycling was investigated. The results of this study showed that the service life of the coating system depended strongly on the composition and microstructure of the thermally grown oxide (TGO) after pre-oxidation. The longer life spans were obtained if the TGO thickened very slowly during thermal cycling due to a large α-Al₂O₃ grain size. Such a slow-growing TGO corresponded with a pre-treatment for which θ-Al₂O₃ was formed during pre-oxidation and for which the yttrium was located within a high density of pegs along the TGO/BC interface after pre-oxidation. If the yttrium was present on top of the TGO after pre-oxidation, a thick mixed alumina-zirconia layer formed upon thermal cycling. This mixed oxide layer contributed significantly to the total oxide layer thickness, resulting in short life spans. The formation of NiAl₂O₄ spinel in between the TBC and the α-Al₂O₃ should be avoided, since this can lead to premature failure along the spinel/α-Al₂O₃ interface.     

常压等离子体渗氮工艺的研究

利用介质阻挡放电原理，在自行研制的设备上进行了常压非平衡等离子体渗氮。研究表明，在等离子环境中，试样表面能迅速获得很深的渗层和白亮层，而电场外试样几乎无渗层。说明电场内气体电离率高，活性粒子浓度高。整个工艺过程操作简便，是一种很有发展前景的新工艺。     

Technical note: Plasma-sprayed ceramic thermal barrier coatings for smooth intermetallic alloys

A durable ceramic thermal barrier coating is applied directly to a smooth, highly oxidation resistant intermetallic alloy in two layers. The first layer of ceramic is applied by low pressure plasma spraying and the second layer is applied by conventional atmospheric pressure plasma spraying. This approach would allow the use of plasma sprayed ceramic coatings in applications where a metallic bond coat is not desirable.     

Graded plasma spraying of premixed metalceramic powders on metallic substrates

The mismatch between the thermal expansion coefficients of ceramics and metals and the differential stresses it causes at the interface create problems in metal to ceramic joining. Research has been con-ducted to solve this problem in thermal barrier coating technology. Previous studies have considered met-al-ceramic multilayers or graded-coatings, which include a metallic bond coat. In this study, a graded plasma-sprayed metal-ceramic coating is developed using the deposition of premixed metal and ceramic powders without the conventional metallic bond coat. Influences of thickness variations, number, and composition of the layers are investigated. Coatings are prepared by atmospheric plasma-spraying on In-conel 718 superalloy substrates. Ni-Cr-Al and ZrO₂ -8 % Y₂O₃ powders are used for plasma spraying. Ad-hesive and cohesive strength of the coatings are determined. The concentration profile of the elements is determined by x-ray energy-dispersive analysis. The microstructure and morphology of the coatings are investigated by optical and scanning electron microscopy (SEM). Results show that the mixed metal-ce-ramic coating obtained with the deposition of premixed powders is homogeneous. The morphology and microstructure of the coatings are considered satisfactory.     

Gas barrier properties and periodically fractured surface of thin DLC films coated on flexible polymer substrates

Thin DLC films coated on polymer surfaces are attracting considerable attention due to their wide applications and their interesting surface properties. When DLC films were coated on polymers, the resulting DLC-polymer composites are highly functionalized materials, some of which presenting dramatically improved gas barrier properties.In this paper, we will introduce several commonly used polymers including polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP) for semi-crystalline polymers, and polymethyl methacyrlate (PMMA) for an amorphous polymer. The polymers were coated with thin DLC films and the gas barrier properties of the resulting DLC-polymer composites were investigated. Some of the DLC-polymer composites dramatically improved their gas barrier properties while they presented horizontal crack lines and vertical micro-buckling lines on the DLC surface when stretched to a certain strain. The gas barrier properties of the DLC-polymer composites with fractured DLC surface were also studied. It was found that the gas barrier property of the polymer substrates with lower residual strains was less damaged, when the substrates were mechanically deformed, than that of the polymer substrates with higher residual strains. When the number of the cracks increases, the strain imposed on each crack decreases, since the overall deformation is almost equally distributed to each crack while the crack spacing of each crack becomes shorter. Thus, it was found that the degradation of the gas barrier property after mechanical deformation is dependent on the residual strain of the polymers and the number of cracks on DLC films.     

Life time dependency on the pre-coating treatment of a thermal barrier coating under thermal cycling

A thermal barrier coating system consisting of the single crystalline Ni-based superalloy CMSX 4, a Pt aluminide bond coat and an EB-PVD processed ceramic top coat was thermally cycled in order to study the influence of three different treatments prior the deposition of the ceramic top coat. Besides the standard treatment, one type of treatment was annealing in vacuum, while the other was annealing in an O containing ArH atmosphere; in both cases for 4 h at 1080 °C.Compared to the standard treatment, annealing in vacuum almost doubled and annealing in ArH atmosphere almost tripled the cyclic life time of the ceramic coating. The improvement was related to the creation of a defined alumina scale before and during TBC deposition.     

TBC experience in land- based gas turbines

This paper summarizes prior and on-going machine evaluations of thermal barrier coatings (TBC) for power generation, that is large industrial gas turbine applications. Rainbow testing of TBCs on turbine nozzles, shrouds, and buckets are described along with a test of combustor liners. General Electric Power Generation has conducted more than IS machine tests on TBC turbine nozzles with various coatings. TBC performance has been quite good, and additional testing, including TBCs on shrouds and buckets, is continuing. Included is a brief comparison of TBC requirements for power generation and aircraft turbines.     

Monitoring delamination of plasma-sprayed thermal barrier coatings by reflectance-enhanced luminescence

Plasma-sprayed thermal barrier coatings (TBCs) present a challenge for optical diagnostic methods to monitor TBC delamination, because the strong scattering exhibited by plasma-sprayed TBCs severely attenuates light transmitted through the TBC. This paper presents a new approach that indicates delamination in plasma-sprayed TBCs by utilizing a luminescent sublayer that produces significantly greater luminescence intensity from delaminated regions of the TBC. Freestanding coatings were produced with either a Eu-doped or Er-doped yttria-stabilized zirconia (YSZ) luminescent layer below a plasma-sprayed undoped YSZ layer. A NiCr backing layer was added to represent an attached substrate in some sections. For specimens with a Eu-doped YSZ luminescent sublayer, luminescence intensity maps showed excellent contrast between unbacked and NiCr-backed sections. Discernable contrast between unbacked and NiCr-backed sections was not observed for specimens with a Er-doped YSZ luminescent sublayer, because luminescence from Er impurities in the undoped YSZ layer overwhelmed luminescence originating from the Er-doped YSZ sublayer.     

Properties and Performance of High-Purity Thermal Barrier Coatings

It has been found that reducing the level of impurity oxides (particularly SiO₂ and Al₂O₃) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Isothermal oxidation of physical vapor deposited partially stabilized zirconia thermal barrier coatings

Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt.%Y₂O₃) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of αAl₂O₃ and rutile TiO₂ or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.