Zirconia (ZrO₂) materials have been widely applied in fuel cell, refractory materials, and construction materials due to the prominent mechanical and chemical properties such as high hardness, strength, good wear-resistant, fine oxygen ionic conductivity, excellent heat-insulating and corrosion protective^[1⇓-3]. With the rapid advances of engineering applications, ceramic materials with higher strength and reliability are greatly needed. Therefore, significant improvements are essential to fabricate zirconia components with higher strength and excellent damage tolerance to improve the lifetime and safety of ceramic components. Many efforts including reinforcement strengthening (such as the incorporation of fibers, particles, and whiskers) and transformation toughening were applied to improve the strength and toughness of ZrO₂^[4⇓-6]. Among them, the way to enhance the strength and damage tolerance of brittle ceramics by introducing residual compressive stress in the surface layer was more attractive because of its remarkable strengthening effect and convenience^[7-8]. It is well-known that the fracture of ceramics is mainly caused by crack propagation in the surface layer. Thus, the key to improve the strength and damage tolerance of ceramics is to enhance the resistance to crack propagation in the surface^[9]. By covering a coating with lower coefficient of thermal expansion (CTE) than the substrate, compressive stress generated in the coating layer after sintering. Then the compressive stress hinders the crack extension, thereby improve the strength and damage tolerance of ceramics.

In order to illuminate the strengthening mechanism, it is important to understand the effect of residual stress on crack propagation. As it is well known that indentation deformation strongly links to the crack initiation and crack propagation. For the crack propagation behavior investigation, Vickers indentation proposed by Palmquvist in the late 1950s has been widely used because of its simplicity and efficiency^[10]. By comparing the growth of crack from the indentations made in unstressed and stressed materials, the effect of residual stress on crack propagation could be studied.

In this work, hot-pressed sintering (HP) was used to fabricate ZrO₂ ceramics covered by Al₂O₃ coating with strong interface bonding and high density. For comparison, monolithic ZrO₂ ceramics were also prepared by the HP process using the same technical parameters. The effects of different ratios of cross-sectional area of substrate to coating (marked as S_s/S_c) on residual stresses and the crack resistance were further investigated.

1 Experimental

1.1 Material synthesis

Commercial zirconia with an average particle size of D₅₀=220 nm (3% Y₂O₃ (molar percent), G3Y-020OO, Shangdong Sinocera Functional Materials Co. Ltd., China) and alumina with an average particle size of D₅₀=220 nm (SAO-020A-M, Shangdong Sinocera Functional Materials Co. Ltd., China) were used as the raw materials in this research. Alumina powders, deionized water, and dispersant (Isobam104 with a molecular weight of 55,000- 60,000, provided by Kuraray Co., Ltd., Osaka, Japan) were added to produce alumina slurry, and their mass contents were 30%, 69.2%, and 0.8%, respectively. The mixture was obtained using a planetary mill (YXQM-1L, MITR, China) for 24 h in zirconia containers with the ball-to-powder weight ratio of 10:1 and rotational speed of 150 r/min.

To fabricate the A_CZ_S pre-stressed ceramics by hot- pressed sintering, graphite die with a diameter of 50 mm was uniformly sprayed with the above alumina slurry. And the thickness of the Al₂O₃ coating was about 40 μm. Then zirconia powder was put into the Al₂O₃-coated graphite die followed by applying a constant pressure of 10 MPa. The specimens were heated up to 1450 ℃ at a heating rate of 15 ℃/min, and then a constant pressure of 30 MPa was applied for 60 min. For comparison, a monolithic ZrO₂ specimen was prepared by a hot-pressing sintering at 1450 ℃ with the pressure of 30 MPa for 60 min. Both the ZrO₂ specimens with a dimension of 3 mm×4 mm×36 mm and the A_CZ_S pre-stressed ceramics with the size of 3.6 mm×4 mm×36 mm were machined (the original thickness of A_CZ_S pre-stressed ceramics prepared by hot-pressed sintering was 3.6 mm).

1.2 Characterization

Morphology of the specimens was observed by using a scanning electron microscope (SEM) (Merlin, Zeiss, Germany). The flexural strength was measured by the three-point bending tests with a span length of 30 mm and a crosshead speed of 0.5 mm/min (according to ISO 14704:2000). The residual strengths were investigated to evaluate the thermal shock resistance after quenching the specimens from different temperatures (200, 250, 300, 350, 400 ℃) to 20 ℃ water (according to ASTM C1525-18). The measured value of strength is the average of 5 separate measurements. In order to study the effect of residual stress on crack propagation behavior, the specimens with Al₂O₃ coatings of different thicknesses (40, 30, 20 μm) were used, because the residual stresses change with the coating thickness. The Vickers indenter (Tukon2500B, Wilson, America) was used to generate the indentation and indentation cracks under different loadings.

2 Results and discussion

The A_CZ_S pre-stressed ceramics were designed as the schematic diagram for plates in Fig. 1(a). To generate compressive stresses in the surface layer, Al₂O₃ was applied as the coating material, because it has a lower coefficient of thermal expansion (CTE) than ZrO₂. Due to CTE difference between Al₂O₃ coating and ZrO₂ substrate, residual compressive stress was generated in the Al₂O₃ coating during the cooling process of the A_CZ_S pre-stressed ceramics. The way to fabricate pre-stressed ceramics is also available for any ceramic components of various shapes, including cylinders, tubes and spheres, as illustrated in Fig. 1(a). As for structural ceramics, the interface bonding generates significant impacts on mechanical properties^[11]. Fine interface bonding manner was investigated from the continuous and tight interface illustrated in Fig. 1(b). The cross-sectional SEM image of the A_CZ_S pre-stressed ceramics indicates that the thickness of Al₂O₃ coating is near 40 μm (while the thickness for ZrO₂ substrate is 3.6 mm). Therefore, by employing HP sintering, A_CZ_S pre-stressed ceramics with a good interface bond between Al₂O₃ coating and ZrO₂ substrate can be successfully fabricated.

图 1.

Fig. 1. (a) Schematic diagram of the A_CZ_S pre-stressed ceramics; (b) SEM image of interface between ZrO₂ substrate and Al₂O₃ coating

下载图片 查看所有图片

To investigate the effect of residual stress on crack propagation behavior, Vickers indentation tests were carried out on the ZrO₂ ceramics and the A_CZ_S pre-stressed ceramics with different ratios of sectional area (S_s/S_c is the ratio of substrate to coating in the schematic diagram of plates shown in Fig.1(a)). By comparing the length and expanded direction of crack in Fig. 2(b) and Fig. 3, the indentation diagonal half-length a and the crack length c (illustrated in Fig. 2(a)) can be easily obtained through the observation of indentation morphology. On this basis, the indentation fracture resistance (K_{I, IFR}) used to describe the resistance to indentation cracking^[12] can be calculated by Eq. (1)^[13]. Since the residual stresses are distributed anisotropically, the test conditions for determine the crack resistance does not satisfy all the application requirements of Eq. (1) mentioned in ISO 14627. It is effective to evaluate the crack resistance at different directions in the surface layer and substrate by comparing the measured values of K_{I, IFR}.

Where, K_{I, IFR}: the indentation fracture resistance value (MPa·m^1/2); E: modulus of elasticity (GPa); H_V: Vickers hardness (GPa);

P: pressing in load (N); c: half of average of crack length (mm); a: half of average of diagonal line length of indentation (mm);

图 2.

Fig. 2. (a) Typical Vickers indentation of ZrO₂ ceramics; (b) Optical photograph of the indentation morphology for ZrO₂ ceramics without residual stress

下载图片 查看所有图片

图 3.

Fig. 3. Optical photographs of the indentation morphologies for A_CZ_S pre-stressed ceramics with different ratios of the cross-sectional areas, showing the effects of the compressive stresses in the coating and tensile stresses in the substrate

下载图片 查看所有图片

As for the indentation on the polished surface of ZrO₂ shown in Fig. 2(b), the cracks propagate outward along the diagonal direction. According to Eq. (1), the calculated K_{I, IFR} of ZrO₂ ceramics was 9.10 MPa·m^1/2 (as shown in Table 1). However, different indentation morphology was presented at the A_CZ_S pre-stressed ceramics with different thicknesses of Al₂O₃ coating in Fig. 3. As shown in Fig. 3(a-c), the crack in Al₂O₃ coating layer propagates along the direction parallel to the interface, while the crack in ZrO₂ substrate propagates along the direction perpendicular to the interface. The above phenomenon might be attributed to the compressive stress existed in the Al₂O₃ coating, while the tensile stress existed in the ZrO₂ substrate, as shown in Fig. 4. And it is well known that the compressive stresses could restrain crack extension and the tensile stress could promote crack initiation and growth^[7-8]. Therefore, the cracks perpendicular to the interface were restrained in Al₂O₃ coating layer, while those were promoted in ZrO₂ substrates. Moreover, it was found that the length of crack in Al₂O₃ surface layer decreases with the increased value of S_s/S_c. The measured K_{I, IFR} of Al₂O₃ coatings and ZrO₂ substrates vary with the coating thickness, as shown in Table 1. The result suggests that the variations in the length of crack is the consequences of varied residual stress in the A_CZ_S pre-stressed ceramics with different values of S_s/S_c. Combined Eqs. (3) and (4), the residual stress in the Al₂O₃ coatings and ZrO₂ substrates could be calculated.

图 4.

Fig. 4. Stress state and Vickers indentation of A_CZ_S pre-stressed ceramics h_s and h_c: the thicknesses of substrate and coating

下载图片 查看所有图片

In any cross-section of the sample, the tensile stresses in the substrate and the compressive stresses in the coating should keep balance,

where S_s, E_s, α_s are the values of the cross-section areas, elastic modulus and CTE of the substrate, S_c, E_c and α_c are those of the coating, respectively. ΔT, the temperature difference, in this test: ΔT=T_sintering-T_room=1425 ℃.

Depending on the calculations, the values of residual stress in the coatings (σ_c) and substrates (σ_s) were plotted in Fig. 5. As is shown, σ_c increased while σ_s decreased with the value of S_s/S_c increasing. It can be deduced that, with the value of S_s/S_c increasing, the compressive stress in Al₂O₃ coatings increased, while the tensile stress in ZrO₂ substrates presented opposite tendency. As compressive stresses could restrain crack extension and the tensile stress could promote crack initiation and growth, the crack resistance of A_CZ_S pre-stressed ceramics was enhanced as the value of S_s/S_c increasing. This is consistent with the crack propagation behavior shown in Fig. 2(b) and Fig. 3.

图 5.

Fig. 5. Calculated residual stress in the Al₂O₃ coating and ZrO₂ substrate of A_CZ_S pre-stressed ceramics, as the function of the ratio of cross-sectional area of ZrO₂ substrate to Al₂O₃ coating

下载图片 查看所有图片

Fractographic features of ZrO₂ ceramics with and without Al₂O₃ coating (40 μm thick) were observed through SEM after bending tests. As shown in Fig. 6(a, c), the fracture morphology is smooth without ductile ridge marks. This means that the fracture mode of both ZrO₂ ceramics and A_CZ_S pre-stressed ceramics is brittle fracture. Grain sizes of uncoated ZrO₂ ceramic and ZrO₂ substrate of A_CZ_S pre-stressed ceramics displayed in Fig. 6(b, d) are in the range of 400 to 600 nm. Moreover, few pores can be found from the microstructure, indicating that the specimens prepared by the hot-pressing sintering method have high density.

图 6.

Fig. 6. SEM images of the fracture surfaces for monolithic ZrO₂ ceramics (a, b) and ZrO₂ substrate of A_CZ_S pre-stressed ceramics (c, d) fabricated by the hot-pressing sintering method

下载图片 查看所有图片

The flexure strength of A_CZ_S pre-stressed ceramics with 40 μm thickness of Al₂O₃ coating is (1207±20) MPa, which is 32% higher than that of ZrO₂ ceramics ((821± 15) MPa) fabricated by the same process and about 3-fold of the flexural strength of Al₂O₃^[14]. According to the previous reports^[15⇓-17], the Al₂O₃/ZrO₂ composites fabricated by the conventional methods, such as ZTA or ATZ, or laminated Al₂O₃/ZrO₂, mostly possess a strength with the value between those of Al₂O₃ and ZrO₂. Generally, the strength of ceramic composites may be between the reinforcement material and matrix. However, the pre- stressing design endows the A_CZ_S ceramics with a higher strength than Al₂O₃ and ZrO₂. The results of the mechanical properties clearly show that the increase in strength is mainly attributed to the residual compressive stress in Al₂O₃ coating, thereby more energy was required for crack extension^[18]. In addition, the robust interface between substrate and coating material also makes restrictions against crack growth, which has a significant effect on the improvement of the strength and toughness^[19].

Another important mechanical property observed for ceramics is the damage tolerance, which represents the ability to inhibit crack extension. The residual strength of two materials was tested after thermal shock with various thermal shock temperatures, as illustrated in Fig. 7. The residual strength of both ZrO₂ and A_CZ_S pre-stressed ceramics with 40 μm thickness Al₂O₃ coating were reduced with the quenching temperature increasing. Especially, the residual strength of A_CZ_S pre-stressed ceramics rapidly dropped from 1040 MPa to 317 MPa when the thermal shock temperature raised from 350 ℃ to 400 ℃, while the residual strength of ZrO₂ quenched at 350 ℃ was only 100 MPa. The result could be ascribed to that the compressive stresses existing in the coating layer can hinder the crack extension. Consequently, it is significant to introduce residual compressive stress in the surface layer of ceramic components to prevent crack propagation.

图 7.

Fig. 7. Residual strength of A_CZ_S pre-stressed ceramics and ZrO₂ ceramics, after quenching at different thermal shock temperatures

下载图片 查看所有图片

Owing to the high density and residual compressive stresses in the surface layer, A_CZ_S pre-stressed ceramics were endowed with high flexural strength and excellent thermal shock resistance. Moreover, considering the feasibility in economy and simplification in the fabrication, the pre-stressing design to improve mechanical properties has great practical value and prospect in the fields of structural ceramics and domestic ceramics. Besides, unlike the pre-stressed concrete and tempered glass, the pre-stressed ceramics can be machined to specific dimensions, which is more suitable for industrialized production.

3 Conclusions

Pre-stressing design was used to enhance the strength and damage tolerance of ZrO₂ ceramics by coating a thin Al₂O₃ layer. The magnitude and direction of the residual stress were examined by Vickers indentation tests, and the crack resistance varied with the residual stress was further verified. Experiments indicated that the crack resistance of A_CZ_S pre-stressed ceramics was enhanced with increasing ratio of the cross-sectional area of ZrO₂ substrate to Al₂O₃ coating.

On account of the residual compressive stress in the coating, the bending strength of ZrO₂ specimens coated with 40 μm thickness of Al₂O₃ was (1207±20) MPa which is much higher than those of ZrO₂ (850 MPa) and Al₂O₃ (400 MPa). After quenching from 350 ℃ to water, the residual strength of the A_CZ_S pre-stressed ceramics was measured as 1040 MPa, while the measured residual strength of the ZrO₂ was only 100 MPa.