Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia
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- Paul, S., Cipitria, A., Golosnoy, I. et al. J Therm Spray Tech (2007) 16: 798. doi:10.1007/s11666-007-9097-5
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.
Keywordsapparent Young’s modulusplasma spray coatingsporous materialsinteringthermal conductivity
There is concern that such sintering-induced changes can occur under service conditions and may adversely affect the protective function of the coating. Of course, sintering is not the only potential source of degradation. Stover and Funke (Ref 18), in reviewing the future of TBCs in power generation gas turbines, identify residual stress generation, thermo-mechanical loading, cyclic strain loads, creep, sintering and interface oxidation as the key phenomena likely to cause coating failure and spallation. In general, other authors (Ref 3, 19) are in agreement with this analysis (for both plasma sprayed and PVD coatings). It is important to appreciate, however, that most of these effects are likely to be exacerbated by the substantial stiffening commonly associated (Ref 14, 17, 20-22) with sintering phenomena, so it is not only the shrinkage caused by sintering that is likely to be deleterious. Moreover, sintering can also cause significant increases in the through-thickness thermal conductivity (Ref 6, 20, 23-25), reducing the thermal protection offered to the substrate. Finally, it is well established (Ref 26) that the erosion resistance of these coatings tends to be impaired by sintering effects.
There is relatively little information available about the influence of composition on the sintering behavior of these coatings. There have been a few studies (Ref 23, 25) covering the effects of stabilizers (dopants), but in general little is known about whether significant improvements in sintering resistance can be achieved by this type of compositional control. There have also been some reports (Ref 27-29) concerned with the effects of impurities, notably the presence of silica and/or alumina, which are the contaminants most likely to be present in these powders at significant levels. In general, these studies have indicated that sintering tends to be accelerated by the presence of these species. For example, Chen et al. (Ref 27) found that 3 wt.%SiO2 significantly enhanced the sintering rate, and this was attributed to liquid sintering—i.e., to the formation of a vitreous, low viscosity phase, which would be able to effect mass redistribution via capillarity forces, rather than just by (surface, interface, or lattice) diffusion. This is consistent with the work of Stemmer et al. (Ref 30), who used high-resolution TEM and EELS to report that samples with ∼0.1 wt.%SiO2 exhibited an amorphous silicate phase at triple grain junctions. However, there has been little or no systematic work on whether worthwhile reductions in sintering rates can be achieved via impurity control. The present article addresses both this issue and the question whether coating properties are significantly affected by impurity content. Both dilatometry during heat treatment and post-treatment measurement of properties are used to investigate this.
Production of Plasma-Sprayed Coatings
Plasma spray parameters (Sulzer Metco 9MB plasma system, Sulzer Metco, Westbury, New York, USA)
Chemical composition of powders (wt.%)
Heat Treatment and Dilatometry
Detached coatings were isothermally heat treated in air at 1400 °C. A heating rate of 20 °C min−1 was used, while cooling after the heat treatment was carried out by simply removing the sample from the furnace. Phase constitution after heat treatment was checked using x-ray diffraction and it was concluded that the phase changes are expected to make only relatively small contributions to shrinkage (Ref 31). Dimensional changes during heat treatment were monitored using a DIL 402C Netzsch dilatometer (Selb, Germany). Dilatometry was performed in both in-plane and through-thickness directions.
Microstructural examinations were carried out using a JEOL 6340F FEG-SEM (Tokyo, Japan). To prevent charging, coatings were sputter coated with gold. A typical microstructure is shown in Fig. 1. Heat treatment results in grain growth, often bridging interfaces between splats, as shown in Fig. 2. There is also evidence of healing of microcracks. However, large voids remain relatively unaffected.
Mercury Intrusion Porosimetry
Surface Area Measurement by Gas Adsorption
A MicroMeritics TriStar 3000 (Norcross, GA 30093-1877, USA) was used to measure the specific surface area of coatings. Coating fragments of known total mass (∼10 g), with dimensions ∼5 mm × 5 mm × 2 mm, were used. They were thoroughly dried (∼250 °C overnight) before measurement. The sample chambers were then cooled to liquid N2 temperature and evacuated. Nitrogen was introduced, in controlled pressure increments, and the equilibrated pressures measured and compared with the saturation pressure, to determine the quantities of adsorbed gas. The Brunauer-Emmett-Teller (BET) adsorption isotherm was then used to determine the specimen surface area.
Apparent Young’s moduli in the in-plane direction were measured before and after heat treatment, using specimens with dimensions 100 mm × 10 mm × 1 mm. These measurements were carried out using a customized four-point bending rig. The load was applied via weights on a counter-balanced pan, while displacements were measured using a scanning laser extensimeter. It was confirmed that only elastic deformation was taking place by checking the linearity and reversibility of the load-displacement plots. Stiffness data were also obtained by measurement of the resonant vibration frequency, using RFDA MF 23 equipment (IMCE, Diepenbeek, Belgium).
Thermal Conductivity Measurement
Through-thickness thermal conductivities of coatings were measured using specimens with dimensions 34 mm × 28 mm × 2 mm. The HotDisk® method (Ref 33) was used.
Shrinkage, Surface Area, and Pore Size Distributions
It should first be emphasized that, while dilatometry is a useful technique for monitoring of the shrinkage associated with sintering, it does not give complete information about sintering effects. It is well established (Ref 34) that some sintering mechanisms lead to densification (shrinkage), while others do not. The important point is whether material reaching the growing necks (high curvature regions) has come from regions located between the centers (of the dense particles), in which case the centers approach each other and densification occurs, or from other regions (e.g., regions already on the free surface), in which case it does not. In general, volume and grain boundary diffusion tend to contribute to densification, while surface diffusion does not. All types of diffusion, on the other hand, can contribute to surface area reduction. It can therefore be useful to obtain both shrinkage and surface area data.
Stiffness and Conductivity
Reducing the contents of alumina and silica, from ∼0.1-0.2 wt.% down to ∼0.01-0.05 wt.%, effects a significant reduction in the sintering rates, as monitored by dilatometry, porosimetry, surface area measurement, stiffness, and thermal conductivity.
This reduction in sintering rates is attributed to slower diffusion, with lattice, grain boundary and surface diffusion probably all affected. The fact that rates of surface area reduction reflect the rates of linear contraction suggests that lattice and/or grain boundary diffusion contribute strongly to the sintering mechanism, so it is unlikely that only surface diffusion is involved. It is possible that liquid phase sintering is taking place, particularly with the higher impurity levels, but it is not a dominant mechanism under the conditions studied.
The sintering effects significant changes in pore architecture, with healing of intra-splat microcracks and enhanced inter-splat bonding. These are accompanied by pronounced grain growth. These changes lead to significant increases in both in-plane stiffness and through-thickness thermal conductivity. The latter is sensitive to increased inter-splat area, while the former is affected by both microcrack healing and improved inter-splat bonding.
Funding for this work has been provided by an EPSRC Platform Grant (IOG), the Gates Foundation (SP), and the Basque Government (AC).