Abstract

ABSTRACT
Laser Remelting of Yttria Stabilized Zirconia Coatings Deposited by
Suspension Plasma Spraying
ASHKAN BAADI
CONCORDIA UNIVERSITY, 2020
Thermal barrier coatings (TBCs) are applied as a protective layer in a range of applications,
mainly in the aero-engine and power generation industries to protect the metallic parts from high
operating temperatures, especially in gas turbine-engines. One way to improve the efficiency of
the engines is to increase the combustion temperature; in order to reduce potential damage to the
metallic parts, TBCs are commonly applied to these components. The TBC comprises a bond coat
and a top coat. Since Yttria Stabilize Zirconia (YSZ) has the best combination of properties among
the various options, this material is most commonly used as the top coat on commercial TBCs.
The top coat in TBCs can be applied by different methods, including Electron Beam -
Physical Vapour Deposition (EB-PVD), Atmospheric Plasma Spraying (APS) and the recently
developed Suspension Plasma Spraying (SPS) which is one of the newest methods in applying top
coat layers. SPS has the potential to generate columnar microstructures with a beneficial range of
porosity: these columns reduce thermal stresses in the TBCs and at the same time provide an
acceptable range of porosity which reduces the thermal conductivity of the coated layers. The
columnar structure of this type of coating, despite having a potential to increase the life cycle of
the top coat in terms of thermal stresses, can be a way of penetration for calcium–magnesium–
aluminosilicates (CMAS) into the TBC structure, which will result in deterioration of the TBC.IV
In this thesis, the formation of a variety of top coats using Suspension Plasma Spraying SPS
method is used in order to obtain the desired columnar microstructure. Subsequently, these layers
are laser treated to study the possibility of creating a remelted layer across the TBC surface which
should reduce the CMAS penetration. In this regard, advantages and disadvantages of the major
laser parameters such as scanning speed, output power, power density and energy density were
observed. Based on the experimental tests on columnar structures, it was found that increasing
scanning speed and power does not have a linear relation and that increasing the laser travel speed
above 2 m/min will cause nonuniform melting and create different phases on the surface of the
substrate. At the same time, decreasing power below a certain amount will not cause significant
changes to the substrate. A specific range of energy and power density need to be considered in
order to obtain a uniform melted layer over the substrate.