How to Improve the Thermal Shock Resistance of Alumina Ceramics
Alumina-based ceramics, known for their high melting point, high strength, wear resistance, and corrosion resistance, are widely used in metallurgy, machinery, aviation, aerospace, and other fields. However, the thermal shock resistance of Al₂O₃ ceramics is relatively poor, with the strength retention rate after a single thermal shock of 300℃ temperature difference being only about 22%. This limitation restricts their application fields. The thermal shock resistance of Al₂O₃ ceramic materials plays a decisive role in practical applications. The editor, drawing on domestic research literature, briefly describes the methods to enhance the thermal shock resistance of Al₂O₃ ceramics.
Factors Affecting the Thermal Shock Resistance of Al₂O₃ Ceramics
The thermal shock resistance of Al₂O₃ ceramics is mainly influenced by microstructural characteristics, surface conditions, and geometric dimensions.
1. Influence of Microstructure of Al₂O₃ Ceramics
Microstructural characteristics of Al₂O₃ ceramics, such as grain size, microcracks, porosity, and pore distribution, significantly affect their thermal shock resistance. For highly dense Al₂O₃ ceramics, fine-grained ceramics exhibit better thermal shock resistance within the small grain size range, while coarse-grained ceramics show better thermal shock resistance within the large grain size range. Researchers currently consider 10μm as the boundary between coarse and fine grains in Al₂O₃ ceramics.
Figure 1 SEM image of the microstructure of hot-pressed sintered fine-grained Al₂O₃ ceramics
In addition, non-uniform pore distribution in alumina ceramics has a more significant effect on reducing the strength and Young's modulus of ceramics compared to uniform pore distribution. Microcracks inherent in ceramics do not always immediately lead to material fracture under thermal shock conditions. This is often due to the suppression of thermal shock crack nuclei by pores. The presence of an appropriate amount of microcracks can enhance the toughness of ceramics through a microcrack toughening mechanism, thereby improving the thermal shock resistance of Al₂O₃ ceramics.
2. Influence of Surface Conditions
Before use, Al₂O₃ ceramics typically undergo mechanical surface treatment methods such as grinding, polishing, and milling, which alter the surface roughness. The thermal shock resistance of Al₂O₃ ceramics after grinding and polishing treatments was investigated, and it was found that Al₂O₃ ceramics subjected to grinding exhibit better thermal shock resistance. The critical thermal shock temperature differences for Al₂O₃ ceramics after grinding and polishing were 235℃ and 185℃, respectively. This is because the higher initial defect density on the ground surface allows the elastic energy generated by thermal shocks to be distributed across more cracks, resulting in relatively smaller crack extensions.
3. Influence of Geometric Dimensions
The thermal shock resistance of Al₂O₃ ceramics is also affected by their geometric dimensions. Generally, reducing the thickness of ceramics can enhance the critical thermal shock temperature difference. This is primarily attributed to the internal bending moments generated by bending stresses.
For instance, different testing methods were employed to evaluate the thermal shock resistance of Al₂O₃ ceramics with varying thicknesses. One surface of the ceramics was heated using an oxyhydrogen flame while the other surface was cooled in a quenching medium. It was observed that as the ceramic thickness increased from 2mm to 6mm, the failure temperature rose from 342℃ to 700℃, indicating that thicker ceramics exhibit better thermal shock resistance. From the perspective of ceramic thickness influence on thermal shock resistance, the overall tensile stress decreases with increasing ceramic thickness, which necessitates a higher thermal shock temperature difference to induce failure.
Figure 2 Al₂O₃ ceramics with different geometric dimensions
Measures to Improve the Thermal Shock Resistance of Al₂O₃ Ceramics
1. Second Phase Method to Enhance the Thermal Shock Resistance of Al₂O₃ Ceramics
Introducing appropriate and controlled amounts of a second phase into the Al₂O₃ ceramic matrix is an effective way to improve the thermal shock resistance of Al₂O₃ ceramics. The second phase can be non-metallic or metallic materials and can be introduced in the form of particles, whiskers, fibers, or sols.
(1) Added ZrO₂
Adding ZrO₂ to Al₂O₃ ceramics can enhance the thermal shock resistance, flexural strength, and fracture toughness of ceramics. The primary mechanisms for improving the mechanical properties of Al₂O₃ ceramics are second-phase particle dispersion strengthening, tetragonal ZrO₂ stress-induced phase transformation toughening, and microcrack toughening.
Figure 3 SEM images of Al₂O₃ thermal shock-resistant ceramics with different ZrO₂ contents
The following table shows the thermal shock resistance of Al₂O₃ ceramics with different ZrO₂ additions.
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Second Phase
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Experimental Conditions
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Experimental Results
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None
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Cooled in water (100℃) after being kept for 20 minutes in an electric furnace at 300, 500, 700, 900, and 1100℃
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Bending strength before thermal shock: 189 MPa. After one thermal shock, the bending strength decreased to about 10 MPa with increasing thermal shock temperature
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3YSZ (15% by volume)
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Cooled in water (100℃) after being kept for 20 minutes in an electric furnace at 300, 500, 700, 900, and 1100℃
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Bending strength before thermal shock: 641 MPa. After one thermal shock, the bending strength decreased to about 480 MPa with increasing thermal shock temperature
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None
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Thermal shock temperature difference at 600℃ (water cooling)
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Thermal shock resistance index: 28
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ZrO₂ (30% by mass)
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Thermal shock temperature difference at 600℃ (water cooling)
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Thermal shock resistance index: 42
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None
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Cooled in air after being kept for 15 minutes in an electric furnace at 1400℃
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Thermal shock resistance cycles: 10 times
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ZrO₂ fibers (15% by mass)
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Cooled in air after being kept for 15 minutes in an electric furnace at 1400℃
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Thermal shock resistance cycles: 30 times
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None
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Thermal shock temperature differences at 200, 300, 400, 500, and 600℃ (water cooling)
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Critical thermal shock temperature difference: 200℃
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ZrO₂ fibers (20% by mass)
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Thermal shock temperature differences at 200, 300, 400, 500, and 600℃ (water cooling)
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Critical thermal shock temperature difference: 500℃
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None
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Cooled in water at room temperature after being kept for 30 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 2 times
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ZrO₂ (4% by mass)
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Cooled in water at room temperature after being kept for 30 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 3 times
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TiO₂ (4% by mass)
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Cooled in water at room temperature after being kept for 30 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 3 times
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ZrO₂+TiO₂ (each 2% by mass)
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Cooled in water at room temperature after being kept for 30 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 10 times
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(2) Added Rare Earth Compounds
Rare earth compounds, particularly rare earth oxides, are commonly used as additives in Al₂O₃ ceramics, followed by rare earth halides. Due to their unique physical and chemical properties, rare earth oxides have been utilized to enhance the properties of Al₂O₃ ceramics.
Taking the addition of CeO₂ as an example, when the mass fraction of CeO₂ is 1%, the thermal shock resistance of Al₂O₃ ceramics after thermal shocks of 200-600℃ temperature differences (water-cooled) is optimal. On the one hand, CeO₂ addition increases the fracture toughness of Al₂O₃ ceramics. On the other hand, when the CeO₂ addition is 1.5%, the porosity of the ceramics is 1.16%, while at 1% CeO₂ addition, the porosity is 4.18%. An appropriate porosity can blunt crack tips, reduce stress concentration, and provide thermal insulation, thereby improving the thermal shock resistance of ceramics. Thus, the improvement in thermal shock resistance of Al₂O₃ ceramics by adding CeO₂ can be attributed to the synergistic effects of toughening and appropriate porosity, with toughening being the primary contributor, although porosity also affects thermal shock resistance. The following table shows the thermal shock resistance of Al₂O₃ ceramics with different rare earth compound additions.
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Second Phase
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Experimental Conditions
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Experimental Results
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None
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Thermal shock temperature difference: 200–600℃ (water cooling) Thermal shock cycles: 2 times
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Critical thermal shock temperature difference: 200℃
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CeO₂ (mass fraction 1%)
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Thermal shock temperature difference: 200–600℃ (water cooling) Thermal shock cycles: 2 times
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Critical thermal shock temperature difference: 300℃
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None
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Thermal shock temperature difference: 200–600℃ (water cooling) Thermal shock cycles: 1 time
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Critical thermal shock temperature difference: 200℃ Bending strength loss rate under thermal shock temperature differences of 200, 300, and 400℃: 24.14%, 28%, 43%
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Y₂O₃+MgO (mass fraction 0.5%)
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Thermal shock temperature difference: 200–600℃ (water cooling) Thermal shock cycles: 1 time
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Critical thermal shock temperature difference: 300℃ Bending strength loss rate under thermal shock temperature differences of 200, 300, and 400℃: 6.59%, 12.37%, 37.99%
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None
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Cooled in water at room temperature after being kept for 20 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 6 times
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LaF₃ (mass fraction 0.2%)
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Cooled in water at room temperature after being kept for 20 minutes in an electric furnace at 1100℃
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Thermal shock resistance cycles: 16 times
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2. Addition of Low Thermal Expansion Coefficient Constituents
Incorporating constituents with low or negative thermal expansion coefficients, such as cordierite, mullite, andalusite, aluminum titanate, and lithium feldspar, into Al₂O₃ ceramics can reduce the thermal expansion coefficient of ceramics, thereby improving their thermal shock resistance.
Cordierite, a mineral material with a low thermal expansion coefficient, can be synthesized using various methods, including high-temperature solid-state reactions with natural minerals, high-purity oxides, industrial or agricultural waste materials, as well as sol-gel and low-temperature combustion synthesis methods. It has extensive applications in structural ceramics, refractory materials, porous materials, dielectric materials, infrared materials, and electronic packaging materials.
Figure 4 SEM image of Al₂O₃ thermal shock-resistant ceramics containing 10% cordierite
The addition of cordierite to Al₂O₃ ceramics reduces the material's thermal expansion coefficient, thereby enhancing the thermal shock resistance of Al₂O₃ ceramics. Studies have shown that within the sintering temperature range of 1230-1280℃, the thermal conductivity of Al₂O₃-cordierite ceramics increases with rising sintering temperature, which is related to the increased density of ceramics at higher sintering temperatures. The heat capacity of gases is lower than that of Al₂O₃ crystals, and gases can cause phonon scattering. As sintering temperature increases, pores decrease, and thermal conductivity improves. After undergoing 10 thermal shocks of 800℃ temperature differences (water-cooled) and 40 thermal shocks of 600℃ temperature differences (water-cooled), Al₂O₃-cordierite ceramics remain uncracked.
The following table shows the thermal shock resistance of Al₂O₃ ceramics with different low thermal expansion coefficient constituents.
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Second Phase
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Experimental Conditions
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Experimental Results
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Kyanite (mass fraction 10% -40%)
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Cooled in air after being kept for 10 minutes in an electric furnace at 1500℃
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Ceramics remained intact after one thermal shock cycle
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Original kyanite (mass fraction 10% -40%)
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Cooled in air after being kept for 10 minutes in an electric furnace at 1500℃
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Ceramics remained intact after one thermal shock cycle
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Mullite (mass fraction 5%)
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Cooled in air after being kept for 20 minutes in an electric furnace at 1200℃
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Bending strength before thermal shock:247.49 MPa Bending strength after thermal shock:218.52 MPa
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None
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Thermal shock temperature difference:1100℃
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Thermal shock resistance cycles:1 time
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Corundum (mass fraction 35%)
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Thermal shock temperature difference:1100℃
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Thermal shock resistance cycles:46 times
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Kyanite (mass fraction 15%)
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Cooled in air after being kept for 20 minutes in an electric furnace at 800℃
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No cracks after 30 thermal shock cycles
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None
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Cooled in air after being kept for about 10-15 minutes in an electric furnace at 1520℃
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Ceramics deformed after one thermal shock cycle
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Titanium aluminate (mass fraction 20%)
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Cooled in air after being kept for about 10-15 minutes in an electric furnace at 1520℃
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Ceramics remained intact after one thermal shock cycle
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None
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Cooled in water (10℃) after being kept for 2 hours in an electric furnace at 750℃
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Bending strength before thermal shock:200 MPa Bending strength after thermal shock:45 MPa
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Titanium aluminate (mass fraction 10%)
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Cooled in water (10℃) after being kept for 2 hours in an electric furnace at 750℃
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Bending strength before thermal shock:200 MPa Bending strength after thermal shock:65 MPa
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None
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Cooled in air after being kept for 30-60 seconds in steel at 1500-1580℃
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Ceramics pulverized or severely deformed after one thermal shock
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Spodumene (mass fraction 20%)
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Cooled in air after being kept for 30-60 seconds in steel at 1500-1580℃
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Ceramics remained intact after one thermal shock
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3. Addition of High Thermal Conductivity Constituents
The addition of high thermal conductivity constituents such as SiC and metal particles to Al₂O₃ ceramics can enhance the thermal conductivity of ceramics. This reduces the instantaneous temperature gradient during quenching, thereby decreasing the thermal stress experienced by ceramics after thermal shock cycles and improving their thermal shock resistance.
(1) SiC
SiC has a higher thermal conductivity than Al₂O₃ and a lower thermal expansion coefficient. Theoretically, adding SiC to Al₂O₃ ceramics can reduce the thermal expansion coefficient and increase the thermal conductivity of ceramics. SiC is typically added to Al₂O₃ ceramics in the form of nanoparticles or whiskers. Studies have shown that adding 5% volume fraction of SiC can significantly improve the thermal shock resistance of ceramics. The critical thermal shock temperature difference of Al₂O₃ ceramics increases from 70℃ to 185℃.
(2) Metals
Metals generally possess high thermal conductivity and toughness. When introduced into Al₂O₃ ceramics, they can not only alleviate thermal stress by enhancing the thermal conductivity of ceramics but also improve mechanical properties by hindering crack propagation through various mechanisms, such as crack deflection, blunting, pinning, and metal particle拔out. For example, the effects of Mo particle addition (volume fractions of 10% and 20%) and particle size (0.56 and 10μm) on the thermal shock resistance of Al₂O₃ ceramics were studied. It was found that adding 20% volume fraction of 10μm Mo particles can increase the flexural strength, fracture toughness, and thermal conductivity of ceramics. It also reduces the elastic modulus, thermal expansion coefficient, and thermal stress intensity factor of ceramics and refines ceramic grains. Metal Mo particle bridging is the primary reason for enhancing the toughness and thermal shock resistance of Al₂O₃ ceramics. The introduction of Mo particles can increase the critical thermal shock temperature difference of Al₂O₃ ceramics from 200℃ to 450℃.
Figure 5 Crack propagation in Al₂O₃ ceramics containing Mo particles
4. Fabrication of Porous Structures
In addition to the characteristics of Al₂O₃ ceramics, porous Al₂O₃ ceramics also exhibit high open porosity and can be used as catalyst carriers, filtration materials, heat exchangers, thermal insulators, and biomedical implants.
Studies have shown that as porosity increases, the room temperature flexural strength of ceramics gradually decreases. However, the critical thermal shock temperature difference and residual flexural strength of porous ceramics increase with porosity. When porosity is 6%, the critical thermal shock temperature difference and residual flexural strength of porous Al₂O₃ ceramics are 200℃ and 15MPa (ΔT=700℃), respectively. When porosity exceeds 43%, the critical thermal shock temperature difference and residual flexural strength of porous ceramics are 400℃ and 21MPa (ΔT=800℃), respectively. The improvement in thermal shock resistance of porous Al₂O₃-ZrO₂ ceramics is attributed to the presence of pores, which effectively alleviate thermal shock stress and prevent microcrack propagation.
Figure 6 SEM images of porous Al₂O₃ ceramics before and after thermal shock (left: before thermal shock; right: after thermal shock)
