Zirconates as New Materials for Thermal Barrier Coatings

Abstract

Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO 3 , BaZrO 3 , and La 2 Zr 2 O 7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low-sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y 2 O 3 -stabilized ZrO 2 (YSZ); thermal conductivities of SrZrO 3 and BaZrO 3 were comparable or slightly higher than those of YSZ. La 2 Zr 2 O 7 had a lower thermal conductivity. SrZrO 3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700°and 800°C. Mechanical properties (hardness, fracture toughness, and Young's modulus) of dense BaZrO 3 and La 2 Zr 2 O 7 samples were determined by indentation techniques and showed lower hardness and Young's modulus compared to YSZ. BaZrO 3 and La 2 Zr 2 O 7 powders were optimized for application as powders for plasma spraying. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO 3 coating, with loss of BaO. On the other hand, the La 2 Zr 2 O 7 coating showed excellent thermal stability and thermal shock behavior.

Microstructure design for blended feedstock and its thermal durability in lanthanum zirconate based thermal barrier coatings

“…The superior thermal insulation ability of LZO can be explained by two 4 reasons. LZO has a pyrochlore crystal structure which contains more oxygen vacancies than fluorite YSZ material, resulting in disturbance of lattice scattering which is the dominant mechanism of thermal conduction in ceramic material [6,13]. The lattice scattering is also interrupted by the atomic weight difference between lanthanum and zirconium which are 138 and 91 g•mol -1 , respectively.…”

“…LZO has lower coefficient of thermal expansions (CTEs, 9.1-9.7×10 -6 •K -1 for LZO and 10.5-11.5×10 -6 •K -1 for 8YSZ) and inferior fracture toughness than YSZ (~ 1.1 MPa·m 1/2 for LZO and ~ 2.0 MPa· 1/2 for 8YSZ) [6,15]. The former results in severe CTE mismatch with typical bond coat materials.…”

“…At higher operating temperatures, YSZ based TBCs have a degradation by phase transition and microstructure densification. YSZ transforms from t' phase to the tetragonal and cubic phase (t and c phases, respectively) during cooling process after higher operating temperatures above 1200 o C, and then to monoclinic (m) phase with a volume expansion of about 3-5 vol.%, resulting in the spallation or delamination of TBCs [6]. In addition, the poor sintering resistance of YSZ at higher temperatures reduces porosity in the top coat, deteriorating the thermal insulation ability of YSZ coating.…”

“…There were many trials to overcome the limitation of current YSZ material, such as zirconates, perovskites, and hexaaluminates [6,[10][11][12]. Among them, zirconates which contain rare-earth elements have been investigated as a new material for advanced TBCs in higher operating temperatures above 1200 o C. Especially, lanthanum zirconate (La2Zr2O7: LZO) has significantly lower thermal conductivities than YSZ.…”

Improving Atmospheric Plasma Spraying of Zirconate Thermal Barrier Coatings Based on Particle Diagnostics

“…These shortcomings regarding such elevated turbine gas inlet temperatures have initiated many research activities seeking even better ceramics than YSZ (Ref [7][8][9]. Apart from perovskites, also hexaaluminates, doped zirconia, and ceramics with pyrochlore structure offer very attractive characteristics.…”