The Effects of Particle Size on the Properties of Thermal Barrier Coatings Charles J. Weiss Carleton College Advising Professor: Rodney Trice

Introduction

Thermal barrier coatings (TBC's) are any thermally insulating material used to protect a substrate from intense heat. This technology has a variety of applications, but its development is mainly driven by its use in protecting parts inside turbine engines. By protecting critical areas of the engines with TBC's, the engines are able to run at higher, more efficient temperatures or require less air-intake for cooling. A common TBC employed today in turbine engines is yttria-stabilized zirconia (YSZ). One concern with these materials is their loss of insulating properties with use due to densification of the initially porous structure and phase changes to more thermally conductive crystal phases.

Project Objectives

• Testing the effects of particle size on the densification and phase changes of plasma-sprayed YSZ
• Develop an effective chemical etching procedure for YSZ

Approach:

• Prepared cylindrical YSZ samples by cutting alumina rods with plasma-sprayed coating on them. After soaking in acid, the coatings were removed from the substrate to provide stand-alone coatings.
• Heated samples of varying particle size in dilatometer up to 1450^oC and subtracted out thermal expansion of the samples and apparatus. This left only the linear shrinkage of the sample due to densification.
• Heated samples in oven at 1400^oC for 50 hours and observed crystal phase changes in XRD.
• To determine the optimal etching times, mounted and polished samples were etched in 4-wt% hydrofluoric acid at varying times and examined under the SEM.
• Mounted various samples in mounting compound and ground and polished the surface. Then the samples where etched in 4-wt% hydrofluoric acid and examined under an SEM for changed in pore structures and grain boundaries.

A plot of YSZ undergoing linear shrinkage due to densification as the sample is heated in the dilatometer.

Scanning electron micrograph at 6000x magnification showing the grain boundary and micro crack structure of YSZ.