Jonathan Levin

Purdue University

Advisor: Professor Rodney Trice
Project Title: In-Situ Observation of Crack Behavior in Plasma Sprayed 7 wt% Yttria-Stabilized Zirconia

Introduction

Yttria-stabilized zirconia (YSZ) is a commonly used thermal barrier coating in applications such as protecting turbine blades in engines from excessive heat. This allows the engines to run hotter and more efficiently. This experiment tested a stand-alone YSZ tube under an incremental compressive stress inside of an environmental scanning electron microscope (ESEM). This was done because it mimics the thermal stresses experienced in service caused by a hot thermal barrier coating on a cooler superalloy substrate. Previous experiments have shown the modulus of the material to increase with an increase in stress. It has been believed that this was caused by cracks closing in. This experiment sought to prove that as well as observing how cracks propagate through the sample.

Objectives

  • Obtain a stress-strain curve from the sample
  • See if cracks perpendicular to the applied load close in
  • See if cracks parallel to the applied load open
  • Observe how cracks propagate through the sample

Experimental Approach

Samples were prepared by plasma spraying aluminum onto an alumina rod and then plasma spraying a 200-300 _m layer of YSZ on top of the aluminum. The rod was then cut into smaller sections with a diamond lathe and the smaller sections were placed in HCl to dissolve the aluminum, releasing the YSZ tubes. Two strain gauges were then placed onto opposite sides of one of the tubes in the axial direction to balance the sample in the load frame and to obtain a stress-strain curve. The sample was then placed into the load frame which slides into the ESEM. Once in the ESEM, the sample was loaded in 8-12 MPa increments. At each increment images were taken at multiple magnifications and strain values were recorded. The experiment was run until critical failure.

Results

The micrographs taken using the ESEM showed that cracks oriented perpendicularly to the applied load closed, and cracks oriented parallel to the applied load opened. Also, microcracks started forming and propagating at approximately half of the failure stress which agrees with previous research on the failure of brittle solids. Figure 1 shows the stress strain curve for the sample showing an increasing modulus with increasing stress and two distinct regions of modulus. Figure 2 shows a plot of crack width vs. stress for five crack locations in the micrographs.

Figure 1. Stress-strain curve for YSZ tube showing two distinct regions of differing modulus.
Figure 2. Crack Width vs. Stress plot for five crack locations in the sample. The vertical cracks run parallel to the applied load and open and the horizontal cracks run perpendicular to the applied load and close.
Final Presentation