Thermal diffusivity measurement of ceramic materials used in spraying of TBC systems
- 174 Downloads
The basic goal of this article was thermal diffusivity characterization of ceramic materials used in thermal barrier coating (TBC) systems for depositions of the insulation layer and characterization of the materials’ morphology and remanufacturing process. The base material was oxide 8YSZ (ZrO2 × 8Y2O3), which is usually dedicated to deposition of an insulating top layer in TBC systems. The data related to thermal properties such as thermal diffusivity and thermal conductivity are widely presented in the literature, but there is lack of information about the morphological form of investigated materials, and the presented results vary widely. Data on thermal properties based on the literature sources are inadequate for the real morphological form of materials used in the experiment (e.g., massive or single crystalline material vs. plasma-sprayed coatings), which consequently gives an unsatisfactory accuracy of the obtained numerical simulations by MES methods. This article presents the characterization of thermal diffusivity of the commercial 8YSZ ceramic material synthesized or remanufactured by different routes, which is investigated in the forms of pressed powder pellet (two commercial nano-sized powders with different morphologies), sintered pellets (one commercial powder, solid-state co-precipitated reacted powder of 8YSZ type), and a two-layered coating system of In625 + NiCrAlY/8YSZ type. The range of analysis included morphological investigations of different types of powders in initial conditions and after remanufacturing (sintering, thermal spraying) as well as the thermal diffusivity analysis by the laser flash method. The obtained data were corrected by porosity factor and compared to each other. The best similarity for obtained thermal diffusivity data was found for commercial powers of HOSPTM type after pressing and sintering processes and calculated (2-layered model) value of thermal diffusivity for two-layered system of In625/8YSZ TBS system. The results showed that there are significant differences in thermal diffusivity values for materials with different morphological forms.
KeywordsTBC LFA Thermal diffusivity 8YSZ Insulation properties Morphology
The basic role of ceramic top-coats in thermal barrier coatings systems is thermal insulation of the metallic substrate during operations at high temperature with additional assistance of a strong aggressive environment. The typical temperature of the analyzed ceramic layer at its surface is up to 1200–1400 °C. The temperature of Ni(Co)CrAlY or NiAl(Pt) bond-coats, localized below the top-coat, should not be higher than 900 °C [1, 2]. The function of the top-coat is to decrease the temperature to at least 300 °C, under the assumption that the thickness of the ceramic insulation is generally no higher than 300 µm. This assumption requires the knowledge of the thermal properties of the ceramic materials used in TBC deposition, especially the thermal conductivity or diffusivity in a wide temperature range [3, 4, 5]. Thermal conductivity and diffusivity are important in the numerical modeling of the temperature and the stress distribution in the ceramic layer. However, the major problem is related to the precision of the obtained thermal data, as well as the morphological accordance of the tested ceramic materials with that of the finally obtained materials during the deposition processes of ceramic coatings using different plasma methods. Different processing routes of the same feedstock material allow us to produce materials having the same chemical and phase composition, but totally different morphologies and, in consequence, thermal properties [6, 7, 8, 9, 10, 11, 12, 13, 14].
The main goal of this article is the overall characterization of the ZrO2 × 8 mass% Y2O3 ceramic material, which is typically used as a feedstock powder for deposition of the insulation layer in TBC systems and characterization of thermal diffusivity of these materials according to their morphological forms created with different routes of synthesis or remanufacturing.
The first step of investigations was related to the characterization of morphology and thermal diffusivity measurement of commercial powders of 8YSZ type obtained by three different methods: plasma remelting process (commercially available powder of HOSPTM type was used), spray drying, and crushing and milling. The range of investigations in this part included phase analysis of powders (Jeol JDX-7S diffractometer), their chemical composition (ICP-OES—inductively coupled plasma–optical emission spectrometer—Ultima 2 ICP OES spectrometer—basic elements and additives, HFIR—high-frequency infrared—Coulomat 702 by Strohlein—carbon and sulfur, THE—high-temperature extraction—ON-mat 8500 by Strohlein—oxygen and nitrogen) as well as their morphology (Hitachi 3400-N scanning electron microscopy), powder size by laser diffraction method, (Mastersizer–Hydro 2000S Malvern Inc.) and thermal diffusivity measured by laser flash method (LFA 427 by Netzsch).
The thermal diffusivity measurement was made on the pressed and pressed/sintered specimens with known thickness, as well as on the TBC system built from three layers: substrate material, bond-coat and insulating ceramic layer. In analyzed case only data for ceramic layer were showed, as a result of calculation based on Proteus software (Netzsch) and 2 layered model with heat loss. As the first layer, the data obtained for the substrate material with bond-coat (IN625/NiCrAlY) were used. As the second layer, the data for only the ceramic sublayer were calculated on the basis of thermal diffusivity of the IN625/NiCrAlY first layer and data obtained for all TBC systems (IN625/NiCrAlY/8YSZ).
The second part of analysis was related to the characterization of 8YSZ materials after different routes of synthesis. The pellet sintered at 1500 °C/24 h and made from plasma remelting of 8YSZ powder was used as a model specimen. The next specimen was synthesized by solid-state reaction (SSR) of ZrO2 and Y2O3 feedstock powders with an adequate ratio of 8YSZ formula. The input materials in SSR methods are oxide powders, which are mixed in appropriate proportions, homogenized and finally sintered. In the SSR route, powders of ZrO2 and Y2O3 were weighed stoichiometrically and mixed. The mixtures were homogenized by wet milling in ethanol for 15 min, then dried and finally sintered in a “Degussa” vacuum press at 1300 °C for 2 h with an additional 15 MPa load pressure. These methods are relatively simple, but the structure of the final product is often inhomogeneous. In the chemical methods (e.g. co-precipitation, sol–gel), which was used to prepare the last specimen (co-precipitation—C-P), the 8YSZ powder is obtained from liquid solutions. Precursors are dissolved and mixed at the molecular level, which enhances homogeneity. However, the reagents are often more expensive; therefore, these methods are preferred when there is high homogeneity but a large amount of material are needed [5, 6, 7, 8]. The starting materials were zirconium oxychloride (ZrOCl2·8H2O, 99.99%) and yttrium nitrate (Y(NO3)3·6H2O). They were weighed to obtain the equimolar cation ratio, dissolved in distilled water and magnetically stirred for several hours. Citric acid (CA) was added to form metal–citrate complex compounds at c.a. 80 °C, and finally, ethylene glycol was added to initiate polycondensation after evaporation of solvents. The powder, called organic precursor, was calcined at 1000 °C in the muffle furnace and then vacuum-sintered into pellets in the same condition as SSR powders (1300 °C/2 h/15 MPa). Both SSR and C-P synthesized samples were additionally sintered at similar conditions as plasma-remelted specimens.
The last part of the experiment consisted of characterization of plasma-sprayed ceramic coatings deposited from the spray-dried commercial powder of 8YSZ type. TBC coating was fabricated by atmospheric plasma spraying. Before spraying, the superalloy IN625 substrates were immersed in ethanol for ultrasonic cleaning. Next, the substrate was degreased and grit-blasted with corundum in order to increase the bonding strength. Feedstock powder was thermally sprayed using an Ar/H2 APS torch (F4 type). Thermal spray parameters were as follows: current 570A, voltage 55 V, flow rate of primary gas 100 SCFH (SCFH + 0.472Lmin−1), feedstock given rate 7 g/min, spray distance 90 mm, spray angle 90° and spray velocity 35 mm/s. Additionally, the microstructure of the obtained coating was shortly described with special attention to porosity.
Results and discussion
Chemical composition of the analyzed powders
6.10 ± 0.10
6.20 ± 0.10
6.20 ± 0.10
0.17 ± 0.008
0.21 ± 0.008
0.21 ± 0.008
< 0.067 ± 0.003
0.020 ± 0.003
0.020 ± 0.003
0.004 ± 0.0004
0.002 ± 0.0002
0.002 ± 0.0002
0.018 ± 0.002
0.033 ± 0.002
0.033 ± 0.002
12.9 ± 0.08
12.8 ± 0.08
12.8 ± 0.08
101 ppm ± 10
132 ppm ± 10
134 ppm ± 10
This accordance is especially good at lower temperatures. In the temperature range from 400 to 1000 °C, the observed differences are higher due to different levels in porosity of both materials and a stronger effect of radiative transport of heat in the case of sintered specimens.
The present investigations revealed that the morphology of the feedstock powders is related to the methods of their synthesis, and remanufacturing routes have very strong influence on the internal structure of the final material. In consequence different thermal properties are received for the same materials from chemical and phases constituent point of view but for example remanufactured by different routes.
This information is important especially in the case of numerical simulation of temperature and stress distribution, for example in the case of thermal barrier coatings. The differences in thermal properties of 8YSZ can generate mistakes in developed models. The present investigations reveal that the best accordance in the thermal diffusivity value, between real component such as the 8YSZ top-coat of the TBC system and other synthesized or remanufactured 8YSZ materials, was obtained for the pellets pressed and sintered at 1500 °C from the plasma-treated feedstock powder of 8YSZ type.
This work was supported by the National Science center, Poland, under Grant Number 2016/21/D/ST8/01687.
- 1.Soares C. Gas turbines: a handbook of air, land and sea applications. Amsterdam: Elsevier; 2011.Google Scholar
- 2.Han J-C, Dutta S, Ekkad S. Gas turbine heat transfer and cooling technology. Boca Raton: CRC Press; 2012.Google Scholar
- 3.Hillery R, Pilsner B, McKnight R, Cook T, Hartle M. Thermal barrier coating life prediction model development. NASA Contractor Report 180807. 1988. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890004250.pdf.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.