Evolution of microstructure during the growth of thermal barrier coatings by electron-beam physical vapor deposition

The mechanisms responsible for the formation of porosity and crystallographic orientation (texture) in the microstructure of thermal barrier coatings (TBCs) grown by electron-beam physical vapor deposition (EB-PVD) are investigated. A matrix of 7 wt.% Y₂O₃-ZrO₂ TBC specimens was generated by independently varying two processing parameters: substrate temperature (T_s) and pattern of vapor incidence.; TBCs deposited on stationary substrates oriented normal to the vapor source yielded columnar microstructures possessing fiber textures. Growth directions changed from ⟨111⟩ to ⟨111⟩ + ⟨110⟩ to ⟨110⟩ + ⟨113⟩ as T_s increased from 900–1100°C. Increasing the angle of vapor incidence to 45° favored biaxially aligned columnar growth in the ⟨110⟩ direction, while rotating the substrates produced biaxially aligned ⟨100⟩ columns. The texture orientation is correlated with the observed column tip morphologies by considering the growth directions defined by symmetric arrangements of {lcub}111{rcub} preferred growth planes about a column axis. The change in texture orientation with increasing T_s under normal incidence on stationary substrates is linked to changes in the mechanism of crystal growth. The pattern of vapor incidence on stationary oblique and rotated substrates has a stronger influence on texture than T_s. Here, the requirement that faces composing a column tip receive equal amounts of vapor flux determines the outcome of a competitive growth process yielding the observed biaxial orientations.; The formation of porosity is in general attributed to shadowing of the incident vapor by geometric features of the TBC surface. These features are crystallographic in nature such that the formation of porosity is intimately tied to the crystallographic texture of the coating. Intercolumnar gaps are generated by the interaction between the pattern of vapor incidence and the column tip morphology, whereas the feather-like shape of the open intracolumnar porosity evolves from shadows cast by steps on the column tip faces. Closed porosity can result from morphological breakdown of the feather-like pores or from the shadowing of small surface features having a symmetric profile with respect to the pattern of vapor incidence. Shadowing at macroscopic length scales is shown to cause variability in the TBC microstructure with implications for its strain-tolerance and thermal resistivity.