Microcrack Behavior of Ceramic Coatings:
Phase Transition and Residual Stress Effects
MSN-484
GURUR UNAN
BERKAY DEMİR
What Are Ceramic
Coatings ?
• Ceramic coatings are hard, heat-
resistant materials applied to
metal surfaces. They protect
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against high temperatures,
corrosion, and wear. But
ceramics are brittle, which
means they can crack easily,
especially under stress or heat.
Figure 1. Microcrack Behavior
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Why Do Microcracks Form?
Table 1. Microcracks
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How Do Microcracks
Grow?
Start: Cracks begin at weak spots or
where stress is highest.
Grow: They grow in the direction of
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stress.
Change Direction: In layered materials,
cracks may turn instead of going straight.
Join Together: Small cracks can merge
and become big ones.
Peel Off: In some cases, the whole Figure 2 Growing
coating may fall off.
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Real-Life Examples
Thermal Barrier Coatings (TBCs)
• Material: Yttria-stabilized zirconia (YSZ)
• Problem: Cracks from repeated heating in engines
• Good News: Some cracks are slowed by internal compressive stress
Layered Ceramics (Al₂O₃/ZrO₂)
• Layers stop or change the path of cracks
• Zirconia changes phase and slows cracks down
Biomedical Coatings (e.g., dental or bone implants)
• Temperature changes in the mouth can start cracks
• Compression stress helps stop them
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How to Reduce Microcracks?
Table 2. Reducing Microcracks
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Damage Mechanism of Ceramic
Coatings Under Thermal Cycling
Microcracks, Phase Transformations, and Stresses
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Thermal Barrier Coatings (TBC)
TBCs are used to protect components operating at high
temperatures.
• They are commonly applied to hot-section parts such
as gas turbine blades in the aerospace industry.
• Their primary purposes are:
• To increase the thrust of aircraft engines
• To improve the thermal efficiency of fuel
YSZ (Yttria-stabilized Zirconia) coatings are the most
widely used type, but they no longer meet modern
performance demands due to:
• High thermal conductivity Thermal barrier coating (colored white) on
a turbine guide vane in a V2500 turbine engine
• Low thermal stability
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Al₂O₃-Doped YSZ Coatings: Advantages and Recent Research
Properties and Potential of Al₂O₃-YSZ Coatings
• Developed as an alternative to conventional YSZ, Al₂O₃-doped YSZ coatings offer:
• Lower thermal conductivity (~0.91–0.99 W/m·K)
• Improved thermal stability at high temperatures
• Enhanced mechanical properties and corrosion resistance
Focus of Recent Research
• Nano-Al₂O₃ reinforcement limits zirconia grain growth and enhances high-temperature durability.
• Literature shows that with a double-layer design, Al₂O₃-YSZ coatings can achieve thermal cycle lifespans
comparable to those of YSZ coatings.
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Critical Factors Affecting Thermal Cycling Lifetime
Microstructure, Phase Transformation, and Residual Stress
• Most conventional studies focus on macro-level factors such as thermal expansion coefficient and
residual stress.
• However, the importance of microstructure and micro-stresses is often overlooked:
• Microstructural changes (e.g., grain orientation, phase transformations) directly influence crack formation
and propagation in the coating.
• Finite element analyses indicate that micro-stresses can reach magnitudes in the MPa range, and in some
cases, even up to the GPa level.
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Key Factors Influencing Thermal Cycling Lifetime
In this study:
• The microstructural evolution of YSZ and Al₂O₃-
YSZ coatings produced by atmospheric plasma
spraying (APS) was investigated.
• Microstructural changes occurring during thermal
cycling were analyzed using the SEM.
• The effects of microscopic strain and phase
transformations on the coating damage mechanism
were evaluated.
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Experimental Method and Material Preparation
Coating System and Materials:
• Substrate: Ni-based superalloy (20 mm × 10 mm × 2 mm)
• Bond Coat: Produced by Vacuum Plasma Spraying (VPS) (100 µm thickness)
Ceramic Top Coats:
• YSZ (7.5 wt% Y₂O₃)
• Al₂O₃-YSZ (10 wt% Al₂O₃, 7.5 wt% Y₂O₃)
• Applied using Atmospheric Plasma Spraying (APS) (200 µm thickness)
Thermal Cycling Tests:
• Samples were held at 950 °C for 15 minutes, then cooled to room temperature in air for
25 minutes.
• Analyzed after different numbers of thermal cycles (0, 5, 10, 15, 20, 25, 30, 40 cycles).
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Characterization Methods
Ex-situ Characterization:
• A fixed region in the cross-sections of the coatings was analyzed.
• A SEM equipped with Electron Backscatter Diffraction was used.
• Grain orientation was measured using Euler angles, and orientation changes were
calculated.
Analyzed Properties:
• Microstructural changes (SEM images)
• Phase analysis (XRD results)
• Grain orientation variation and micro-stresses (EBSD measurements)
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Initial Microstructure of the Coatings
YSZ Coating:
• Dominated by the tetragonal phase (crystallinity close to 100%)
• Small crack sizes (~2.5 µm²)
Al₂O₃-YSZ Coating:
• Dominated by the cubic phase, lower crystallinity (~78.56%)
• Large crack sizes (~5.6 µm²)
• Formation of amorphous phase around cracks (promotes crack growth)
SEM images from cross-sections of (a) YSZ coatings and (b) Al2O3-YSZ coatings.
Low crystallinity promotes the formation of larger cracks, reducing the durability of the coating.
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Grain Orientation and Micro-Stresses
Post-Thermal Cycling Changes (After 5 Cycles):
• Small orientation change in YSZ coating (1.49°)
• Large orientation change in Al₂O₃-YSZ coating (5.42°)
Relationship Between Orientation Change and Micro-
Stress:
• A larger orientation change indicates higher micro- (a) SEM image and (b) EBSD image of Al2O3-YSZ coatings.
stress (in the MPa range).
• Higher micro-stress accelerates crack propagation and
reduces the coating’s service life.
SEM images of equiaxed grains in Al2O3-YSZ coatings (a) before
thermal cycles; (b) after thermal cycles.
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Effect of Grain Morphology on Crack Propagation
• Equiaxed Grains:
• Cracks propagate easily along grain boundaries (intergranular).
• High energy and atomic instability make crack formation
easier.
Columnar Grains:
• Cracks propagate through the grains (transgranular), which is
more difficult.
• Requires higher fracture energy, slowing down crack growth.
Conclusion: SEM images of polished cross-sections of different coatings
before and after thermal cycles: (a)(b)(d)(e) before thermal
cycles; (c)(f) after five thermal cycles.
• Columnar grains help block crack propagation and enhance the
durability of the coating.
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Effect of Phase Transformation
Critical Phase Transformation (Between 5–10 Cycles):
• Transformation occurs from cubic phase to tetragonal
phase.
• A 2% volume shrinkage takes place during the
transformation.
• When combined with existing cracks, this transformation
leads to coating spalling.
Advantages of the Tetragonal Phase:
• Higher strength and toughness.
• After the phase transformation, micro-stress is reduced and
damage progression slows down.
Microscopic strain of Al2O3-YSZ coating during thermal cycles.
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Conclusion
General Conclusions:
• Crystallinity: Low crystallinity supports the formation of larger cracks and shortens the coating’s
lifespan.
• Micro-Stresses: High micro-stresses accelerate crack propagation, reducing thermal cycling durability.
• Phase Transformations: Cubic-to-tetragonal phase transformation leads to crack coalescence and
spalling, but the tetragonal phase improves durability.
• Grain Morphology: Columnar grain structures hinder crack growth and enhance coating resilience.
Practical Recommendation:
• To extend the service life of thermal barrier coatings, prefer coatings with high crystallinity, columnar
grain structure, and an optimized proportion of the tetragonal phase.
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REFERENCES
• Dai, M., Song, X., Lin, C., Liu, Z., Zheng, W., & Zeng, Y. (2022). Investigation of microstructure
changes in Al₂O₃–YSZ coatings and YSZ coatings and their effect on thermal cycle life. Journal of
Advanced Ceramics, 11(2), 345–353. https://doi.org/10.1007/s40145-021-0538-2
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THANKS
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