The Effect of a High Thermal Gradient on Sintering and Stiffening in the Top Coat of a Thermal Barrier Coating (TBC) System

Superalloy substrates coated with plasma sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats have been subjected to a high heat flux in a controlled atmosphere chamber. The sintering exhibited by the top coat under these conditions has been studied and compared with the behavior observed during isothermal heating, both when attached to the substrate and when detached. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical heat flow model has been used to explore the stress state under isothermal and thermal gradient conditions. Sintering proceeds faster at higher temperature, but is retarded by the presence of tensile stresses (from differential thermal expansion between coating and substrate) within the top coat. Sintering occurs preferentially near the free surface of the top coat under gradient conditions, not only because of the higher temperature, but also because the in-plane stress is more compressive in that region

A steady-state Bi-substrate technique for measurement of the thermal conductivity of ceramic coatings

This paper presents a steady-state, bi-substrate technique for measurement of the through-thickness thermal conductivity of ceramic coatings, with a range of specimen thickness and porosity content. The technique is based on establishing unidirectional steady-state heat flow through the sample, sandwiched between a pair of (metallic) substrates with known thermal properties. Comparison between the heat fluxes passing through the two substrates allows a check to be made about the accuracy of the assumption of unidirectional heat flow. The interfacial conductances must be known and these can be estimated by testing samples of different thickness. Measured conductivities are likely to be more accurate if the interfacial conductance is relatively high. This is assisted by the introduction of a thin interfacial layer of a viscous, thermally conductive compound, or thermal pads of some sort, and by maintaining a suitable pressure across the setup. However, if such compounds (pastes) are used, then care must be taken to ensure that it does not enter the specimen via surface-connected pores, since this could significantly affect the measured conductivity. The reliability of the technique has been confirmed by testing fused silica samples of known thermal conductivity. It has also been applied to sprayed zirconia and plasma electrolytic oxide (PEO) alumina coatings. The values obtained were 1.05±0.10 W m?1 K?1 and 1.63±0.35 W m?1 K?1, respectively

Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia

Yttria-stabilized zirconia powders, containing different levels of SiO2 and Al2O3, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<_50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes

Interactions between wear and corrosion on cast and sintered Ti-12Nb alloy in comparison with the commercial Ti-6Al-4V alloy

This work investigates the corrosion and tribocorrosion behavior of alpha + beta Ti-12Nb alloy processed by casting and sintering, in 9 g/l NaCl solution at body temperature, and compares the results with the commercial Ti-6Al-4 V alloy. Different electrochemical techniques were used to access the corrosion behavior. Tribocorrosion behavior was studied at open circuit potential under continuous and intermittent sliding, and at anodic potentiostatic condition under continuous sliding. Results revealed that Ti-12Nb alloys presented similar tribocorrosion behavior although the sintered one exhibited a better corrosion behavior. Nevertheless, Ti-6Al-4 V presented better overall degradation resistance than both Ti-12Nb alloys.- This work was supported by FCT national funds, under the national support to R&D units grant, through the reference project UIDB/04436/2020 and UIDP/04436/2020, together with M-ERA-NET/0001/2015, and co-supported by Brazilian agencies CNPq [grants #308.204/2017-4 and #125.954/2018-2] and FAPESP M-ERA-NET [grant #2015/50.280-5], also by MINECO (Spain) through the program PCIN-2016-123 and the Ramon y Cajal project RYC-2014-15014. I. Caha is grateful for a PhD grant under the NORTE-08-5369-FSE-000012 project

An analytical model for simulation of heat flow in plasma-sprayed thermal barrier coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model

The effect of a high thermal gradient on sintering and stiffening in the top coat of a thermal barrier coating (TBC) system

Superalloy substrates coated with plasma-sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats (TCs) have been subjected to a high heat flux under a controlled atmosphere. The sintering exhibited by the TC under these conditions has been studied and compared with the behavior observed during isothermal heating. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both the in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical model has been used to explore the stress state under isothermal and thermal gradient conditions. Dilatometry data indicate significant linear contractions during holding at elevated temperatures, particularly in the through-thickness direction. This is largely attributed to microstructural changes associated with sintering, with any volume changes due to phase transformations making relatively small contributions. Sintering proceeds faster at higher temperatures but is retarded by the presence of tensile stresses (from differential thermal expansion between the coating and substrate) within the TC. Thus, it occurs preferentially near the free surface of the TC under gradient conditions, not only due to the higher temperature, but also because the in-plane stress is more compressive in that region

Ti2AlC and Ti3SiCi2 MAX phase foams: Processing, porosity characterization and connection between processing parameters and porosity

MAX phases Ti2AlC and Ti3SiCi2 foams with controlled porosity and pore size were produced using the space holder method. The foams were processed using water-leachable crystalline carbohydrate as space holder that involves: mixing, cold isostatic pressing, dissolution and sintering. Three combinations of volume percentage (20%-60%) and size distribution (250-1000 µm) of space holder were introduced during mixing. The foams were characterized and compared with the material without space holder. The characterization included: morphology (overall, open and closed porosity by Archimedes method) and gas permeability. Foams with porosity up to about 60 vol% and pore size distribution ranging from about 250 to 1000 μm were produced. Experimental porosity was compared to the theoretical expected porosity. The results show a bimodal porosity that can be customized by the sintering and the space holder. This study connects the processing parameters to the porosity created and allows control of porosity and pore size to produce tailor-made properties

Effect of substrate temperature on the microstructure and properties of thick plasma-sprayed YSZ TBCs

Thick (~1.2 mm) thermal barrier coatings (TBCs) consisting of YSZ were deposited by plasma spraying. Spraying parameters were varied in a controlled manner to produce different microstructures. The effect of substrate temperature on the microstructural features and subsequently on the Young’s modulus was investigated. In addition, the residual stresses in the coatings were estimated using a numerical model and they were related to the microstructural features observed. Results showed that crack segmentation density, residual stresses in the coatings and thus coating properties are strongly affected not only by the average substrate temperature during spraying but also the variations between the minimum and maximum substrate temperature

Reducing the layer number of AB stacked multilayer graphene grown on nickel by annealing at low temperature

Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method uses make it a good candidate for controlling the layer number of already grown graphene in processes with a low thermal budget. © 2015 IOP Publishing Ltd