Performance Testing of Suspension Plasma Sprayed Thermal Barrier Coatings Produced with Varied Suspension Parameters
2.1. Coating Production
|Suspension||Solids Loading||Particle D50 (nm)||Suspension Density (g/cc)||Deposition Rate (µm/pass)||Coating Thickness (µm)|
2.1.2. Suspension Atomization Parameters
2.1.3. SPS Sample Production
2.2. Porosity Measurement
2.3. Thermo-Cyclic Fatigue Testing
2.4. Thermal Diffusivity Analysis
3.1. Mercury Porosimetry
3.2. Microstructure Analysis
3.2.1. Influence of Solids Loading
- Reducing the suspension solids loading reduces the suspension viscosity as shown in Figure 2 for the 200 nm suspensions. The reduced viscosity will allow easier atomization and increase the production of the smallest suspension droplets during fragmentation according to Rampon et al.  and predicted by Equation (2).
- As a secondary effect, increasing the powder solids load within a suspension increases the mass of powder contained within each suspension droplet produced during the fragmentation process. A larger suspension droplet will also contain more powder particles, leading to a larger YSZ particle after solvent evaporation and subsequent melting of the ceramic.
3.2.2. Comparison of Sub-Micron Suspensions
3.2.3. Influence of Suspension Solvent
3.3. Thermal Conductivity Results
- The C1 coating was formed from YSZ powder with 14 wt % yttria, whereas all other suspensions contained YSZ powder with 8 wt % yttria. The incorporation of yttrium atoms into the zirconia lattice requires the generation of oxygen vacancies. These point defects produce scattering of the lattice waves (phonons) that transport the thermal energy through the coating [20,21,22]. Thus, compared to the other samples, the 75% increase in yttria within the C1 samples should have reduced the thermal diffusivity of these coatings.
- Additionally, interactions between phonons and pore boundaries can disrupt phonons transport through the coating. Hence, the porosity that most effectively reduces thermal diffusivity is that with the most area oriented perpendicular to the direction of heat transfer . Therefore, higher porosity level in the form of IPBs within the C1 coating, and which ran roughly perpendicular to the primary heat transfer direction, will have increased the effectiveness of the porosity within this coating at reducing thermal conductivity.
3.4. Thermo-Cyclic Fatigue
3.5. Failure Microstructures
- For ethanol-based suspensions, the tendency for columnar microstructure formation increases with reducing suspension viscosity due to stronger atomization of the suspension and resultant smaller particles in the plasma plume. Additionally, increasing suspension viscosity increased the coating thermal conductivity and tended to reduce the thermo-cyclic fatigue lifetimes.
- Changing the suspension solvent from ethanol to water produced a shift in the coating microstructure morphology from columnar to vertically-cracked. Higher suspension surface tension; due to moving from ethanol to water as a solvent, has the tendency to dramatically increase atomized droplet size and therefore transitions deposition to a more APS-like coating. This change also increased the coating thermal conductivity by roughly 3% and decreased the thermo-cyclic fatigue life by 54%.
- Median powder particle size in suspension has only an indirect impact on the size of the depositing droplets and consequently the coating deposition type. It is clear that the simple metric of powder size in suspension is not sufficient to help define parameters for suspension plasma spray coating.
Conflicts of Interest
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Curry, N.; VanEvery, K.; Snyder, T.; Susnjar, J.; Bjorklund, S. Performance Testing of Suspension Plasma Sprayed Thermal Barrier Coatings Produced with Varied Suspension Parameters. Coatings 2015, 5, 338-356. https://doi.org/10.3390/coatings5030338
Curry N, VanEvery K, Snyder T, Susnjar J, Bjorklund S. Performance Testing of Suspension Plasma Sprayed Thermal Barrier Coatings Produced with Varied Suspension Parameters. Coatings. 2015; 5(3):338-356. https://doi.org/10.3390/coatings5030338Chicago/Turabian Style
Curry, Nicholas, Kent VanEvery, Todd Snyder, Johann Susnjar, and Stefan Bjorklund. 2015. "Performance Testing of Suspension Plasma Sprayed Thermal Barrier Coatings Produced with Varied Suspension Parameters" Coatings 5, no. 3: 338-356. https://doi.org/10.3390/coatings5030338