The Effects of Particle Size on the Properties of Thermal Barrier Coatings Charlie Weiss Advisor: Prof. Trice

TBC’s Definition: TBC’s are any material with a low thermal conductivity that coats a substrate of less thermal resistance in order to protect it from damaging affects of heat Type: (for my project) Yttria Stabilized Zirconia (YSZ) 7% wt. yttria k th from 2.2 to 5.4 W/m/K Plasma sprayed

Material Problems Loss of insulating efficiency with use at high temperatures Loss of insulating efficiency with use at high temperatures  Densification of initially porous microstructure  Crystal phase change

Small Powders = Small LamellaeLarge Powders = Large Lamellae Effects of Particle SizeTypeSize Density (g/cm 3 ) Large um 3.58 Medium um 3.87 Small um 3.76

Goals Testing effects of particle size on the densification of plasma sprayed YSZ Testing effects of particle size on the densification of plasma sprayed YSZ Examine how phases change in relationship to particle size Examine how phases change in relationship to particle size Developing an effective etching procedure for YSZ Developing an effective etching procedure for YSZ

Sample Preparation  Cut   Dissolve Al in HCl Removing alumina core

Dilatometry Goal is to examine when and how much densification occurs Goal is to examine when and how much densification occurs Running samples in dilatometer at 3 o C/min from 25 to 1450 degree Celsius Running samples in dilatometer at 3 o C/min from 25 to 1450 degree Celsius

Data Calculations Raw Data TE (disks) TE (YSZ) _________ Densification of YSZ - - = (this includes TE (YSZ) + TE (disks) + densification of YSZ)

Linear Shrinkage

Comparison

Repeated Heating

Change in Density after Heating SampleInitialFinalChange Large % Small %

Dilatometry Conclusions Pre-densification anomalies are likely a material response Pre-densification anomalies are likely a material response  Possibly due to irregularities in structure Temperature at which densification begins varies with density but not particle size Temperature at which densification begins varies with density but not particle size  Lower densities begins to densify first

Dilatometry Conclusions (cont.) Small particle size exhibits more linear shrinkage* Small particle size exhibits more linear shrinkage*  Large/ Medium  -0.8%  Small  -1% Linear shrinkage decreases with repeated heating* Linear shrinkage decreases with repeated heating*   -1% during first heating   -0.1% during second heating Higher sintering temperatures observed during repeated heatings Higher sintering temperatures observed during repeated heatings  950 o o C for first run  o C for second run * Conclusion based on dilametry data

Phases Three Phases  Tetragonal  Cubic  Monoclinic Phase m-ZrO 2 t-ZrO 2 c-ZrO 2 Thermal o C, W/m/K

Crystal Phase Change Three samples: one of each particle range Three samples: one of each particle range Cut in half Cut in half  Heated half at 1400 o C for 50 hours  Left other half unheated Examined each sample using XRD from degrees and degrees Examined each sample using XRD from degrees and degrees

6  m Want to see: Want to see:  Micro cracks  Lamella boundaries  Columnar grain structures (if possible) Goal of Etching Accentuate microstructures to allow examination Accentuate microstructures to allow examination

240 Seconds in 4-wt% HF

Further Etching Experiments Two new tests: Two new tests:  Etched samples for 8 and 12 minutes  Tested NaOH as a supplement to HF etching Problem: Unable to examine etched microstructures Unable to examine etched microstructures Problem with the SEM Problem with the SEM

Sulfuric Acid Decided to try H 2 SO 4 Decided to try H 2 SO 4 Tested mounting compounds reactivity in H 2 SO 4 Tested mounting compounds reactivity in H 2 SO 4  Bakelite  Cold mount  Epoxy Tested with half-strength acid at 23 o C and 60 o C Tested with half-strength acid at 23 o C and 60 o C Negligible loss of mass Negligible loss of mass

Etching Summary Succeeded in accentuating: Succeeded in accentuating:  Grain boundaries  Micro cracks Lacks accentuation of: Lacks accentuation of:  Columnar grains Current best is 4-5 minutes in 4 wt-% HF Current best is 4-5 minutes in 4 wt-% HF

Project Conclusions Particle size may effect the magnitude of linear shrinkage Particle size may effect the magnitude of linear shrinkage Sintering start temperatures is a factor of density, which is not a directly related to particle size Sintering start temperatures is a factor of density, which is not a directly related to particle size Repeated heatings result in less densification Repeated heatings result in less densification Four minutes in 4-wt% HF is an effective etchant but fails to accentuate columnar grains Four minutes in 4-wt% HF is an effective etchant but fails to accentuate columnar grains