Threshold Stress for Crack-Healing of Si3N4/SiC and Resultant Cyclic Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were hot-pressed in order to investigate the crack-healing behavior under stress. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant bending stress, and the resultant bending strength and cyclic fatigue strength were studied. The threshold stress for crack-healing was investigated at healing temperatures of 1000° and 1200°C. The cyclic fatigue strengths of crack-healed specimens were also investigated at healing temperatures of 900° and 1000°C. The main conclusions are as follows: (1) The threshold cyclic and constant stresses for crack-healing, below which pre-cracked specimens recovered their bending strength, were 300 MPa, which was 75% of the bending strength of the pre-cracked specimens and (2) the crack-healed specimens exhibited quite high cyclic fatigue strength at crack-healing temperatures of 900° and 1000°C.     

Crack-healing behavior and static fatigue strength of Si3N4/SiC ceramics held under stress at temperature (800, 900, 1000 °C)

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm in surface length was introduced on each specimen. The pre-crack was healed under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Applied stress of 210 MPa is ∼70% of the bending strength of pre-cracked specimen. Bending strength and static fatigue strength of crack-healed specimens were systematically investigated at each crack-healing temperature. The bending strength of crack healed specimen showed almost the same value as smooth specimen. Thus, Si₃N₄/SiC composite ceramics could heal a crack even under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Moreover, crack-healed zone had quite high static fatigue limit at each crack-healing temperature. These conclusions indicate that Si₃N₄/SiC composite ceramics has an ability to heal a crack under service condition, i.e. high temperature and applied stress.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

Effect of crack-healing and proof-testing procedures on fatigue strength and reliability of Si3N4/SiC composites

A Si₃N₄/SiC composite was hot-pressed. Using this material, fatigue tests on crack-healed and proof-tested specimens were conducted at 1000–1400 °C. A surface elliptical-crack of about 110 μm in diameter was introduced on the specimens using a Vickers hardness indenter. The crack-healing was performed at 1300 °C for 1 h in air, mainly. The fatigue limit of the crack-healed and proof-tested specimen (C.P specimen) decreased slightly with increasing test temperature. However, the crack-healed specimen is not sensitive to low-cycle fatigue up to 1400 °C, and the fatigue limit is almost equal to the minimum bending strength at each temperature. To investigate the reason, the crack-healing behavior under cyclic stress was carried out systematically at 1200 °C in air. A 110 μm surface crack could be healed perfectly at 1200 °C in air under cyclic stress with a frequency of 0.001–5 Hz. From this, it can be concluded that [crack-healing+proof test] and crack-healing during service are useful techniques for maintaining structural integrity of these ceramic components.     

Crack-healing behavior and resultant strength properties of silicon carbide ceramic

Silicon carbide (SiC) has good high temperature strength and resistance to radioactivity. However, it has poor fracture toughness. To overcome this weakness, a crack-healing ability is very desirable. This study focuses on the crack-healing behavior of commercial SiC ceramic. The crack-healing behaviors of SiC ceramic were investigated systematically, as a function of crack-healing temperature, time, crack size and temperature dependence of the resultant bending strength. Three-point bending specimens were made and a semi-elliptical crack was introduced on the specimen by a Vickers indenter. Pre-cracked specimens were healed at various conditions. All fracture tests were performed on a three-point loading system with a 16 mm bending span. The main conclusions obtained were: (1) optimized crack-healing condition is; temperature: 1773 K, 1 h in air. (2) The maximum crack size that can be healed completely under the optimized condition was semi-elliptical surface crack of 450 μm in diameter. (3) Limiting temperature for bending strength of crack-healed zone for bending strength was about 873 K.     

Crack-Healing Behavior Under Stress of Mullite/Silicon Carbide Ceramics and the Resultant Fatigue Strength

Mullite/SiC composite ceramics were sintered and subjected to three-point bending of specimens made according to the appropriate JIS standard. A semicircular surface crack 100 to 250 μm in diameter was made on each specimen. We systematically studied crack-healing behavior and cyclic- and static-fatigue strengths at room temperature and 1000°C (crack-healing temperature) by using three types of specimens (smooth, cracked, and crack-healed). The main conclusions are as follows: (i) mullite/SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens exhibited higher static and fatigue strengths than those of smooth specimens, which was caused by crack-healing; (iii) a sample crack-healed at 1000°C had a high fatigue strength at 1000°C; and (iv) mullite/SiC ceramics can heal a crack under stress at 1000°C, and this behavior was considered using crack-driving force and crack-healing force, qualitatively.     

High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the fatigue strength of crack-healed specimens at high temperature. Semielliptical surface cracks of 100 μm surface length were introduced on each specimen surface. These specimens were crack-healed at 1300°C for 1 h in air, and static and cyclic fatigue strengths were systematically investigated at room temperature, 900° and 1100°C by three-point bending. The static and cyclic fatigue limits of the crack-healed specimens were more than 70% of the average bending strength at each testing temperature. Crack-healed specimens of Al₂O₃/SiC-W were not sensitive to static and cyclic fatigue at room temperature and high temperatures. Therefore, the combination of crack-healing and whisker reinforcement can play an important role in increasing static and cyclic fatigue strengths at high temperature.     

Crack-healing behavior of Si3N4/SiC ceramics under stress and fatigue strength at the temperature of healing (1000 °C)

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending specimens made according to the appropriate JIS standard. A semi-circular surface crack of 100 μm in diameter was made on each specimen. We systematically studied crack-healing behavior, and cyclic and static fatigue strengths at the service temperature (1000 °C) by using three kinds of specimens (smooth, cracked and crack-healed). The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have the excellent ability to heal a crack at 1000 °C; (2) this sample could heal a crack even under cyclic stress at 1000 °C; (3) a new crack-healing process was proposed. The sample crack-healed at 1000 °C by the process exhibited a sufficient static and cyclic fatigue strength at 1000 °C.     

Critical crack-healing condition for SiC whisker reinforced alumina under stress

Alumina reinforced by SiC whisker, called here “alumina(w)” was developed with the objective of improving fracture toughness and crack-healing ability. The composites were crack-healed at 1200 °C for 8 h in air under elevated static and cyclic stresses. The bending strength at 1200 °C of the crack-healed composites were investigated. The threshold static stress during crack-healing of alumina(w) has been determined to be 250 MPa, and the threshold cyclic stress was found to be 300 MPa. Considering that the crack growth is time-dependent, the threshold stress of every condition during crack-healing of alumina(w) was found to be 250 MPa. The results showed that the threshold stress intensity factor during crack-healing was 3.8 MPa m^1/2. The same experiment conditions were applied to specimens cracked and annealed at 1300 °C for 1 h in Ar, to remove the tensile residual stress at a tip of the crack. Thus, the threshold stress intensity factor during crack-healing was found to be 3.2 MPa m^1/2 for the specimens crack-healed with annealing. The threshold stress intensity factor during crack-healing of alumina(w) was chosen to be 3.2 MPa m^1/2 to facilitate comparison with the values of the threshold stress intensity factor during crack-healing. The residual stress was slightly larger than the intrinsic value.     

Crack Healing of Machining Cracks Introduced by Wheel Grinding and Resultant High-Temperature Mechanical Properties in a Si3N4/SiC Composite

The crack-healing behavior of machining cracks in a Si₃N₄/SiC composite containing Y₂O₃ and Al₂O₃ as a sintering additive was investigated. The machining cracks were introduced by a wheel grinding process, which is the most common method for finishing ceramic components. A semicircular groove was made at the center of small bending specimens by the machining. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimen was found to be a temperature of 1300°C and a time of 1 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens that underwent the healing process. Moreover, the bending strength and fatigue limit of the machined and healed specimens were systematically investigated at temperatures ranging from room temperature to 1300°C. The heat-resistance temperature has been determined to be approximately 1000°C. Also, the specimens exhibited high static and cyclic fatigue limits at temperatures of 800° and 1000°C. These results demonstrate that crack healing could be an effective method for improving the structural integrity and reducing the manufacturing costs of a Si₃N₄/SiC composite ceramic.     

Crack healing behaviour and high-temperature strength of mullite/SiC composite ceramics

Mechanical properties of sintered mullite/SiC ceramics related to its significant crack healing behavior are discussed in this paper. This investigation was made on four kinds of specimens such as as-received smooth, heat treated smooth, pre-cracked and pre-crack healed. Pre-crack sizes were 100 and 200 μm and they were semi-elliptical in shape. The main conclusions were obtained as follows: (a) mullite/SiC composite ceramic has ability to heal crack, (b) the best healing condition was found to be 1300 °C in air for 1 h, (c) maximum crack size able to be healed is semi-elliptical crack of 200 μm in diameter, (d) crack-healed zone has enough strength up to 1200 °C and most specimens failed outside the crack-healed zone.     

Crack Healing Behavior of Silicon Carbide Ceramics

This study focuses on the crack healing behavior of three kinds of commercial SiC ceramics. Specimens with and without cracks were subjected to thermal treatment at different temperatures, and their strengths were measured by a three-point bending test in accordance with JIS standards. The tests were performed in air at both room temperature and elevated temperatures between 600° and 1500°C. The healed specimens showed a complete recovery of strength at room temperature for the investigated crack sizes of 2 c ≅ 100 μm and 2 c ≅ 200 μm, and their strength increased in accordance with the healing temperature. The behavior of the healed specimens at elevated temperatures was influenced by the material used and the test temperature. Generally, the strength decreased at a high temperature, but the degree of strength reduction was determined by the kind of ceramic. The most important difference between the healed and smooth specimens was exhibited in material A. It was observed that at 1400°C, the bending strength of the healed specimens made from this ceramic was about 37% of the value for specimens in an as-received state. Static fatigue tests were also performed for ceramic B at 900° and 1000°C. The experiment demonstrated that the static fatigue limit of a healed specimen is about 75% of the monotonic bending strength at the same temperature.     

Improvement of strength and reliability of ceramics by shot peening and crack healing

In this study we propose a new method for improving the surface strength and reliability of ceramics that combines shot peening with crack-healing ability. We used Si₃N₄/SiC composite ceramics with high crack-healing ability and subjected the specimens to shot peening and crack healing. To evaluate the effect of our method, we investigated the residual stress after shot peening and crack healing and examined the specimens’ mechanical properties, including apparent fracture resistance, contact strength and bending strength. We found that shot peening and crack healing is effective to increase apparent fracture resistance, contact strength and bending strength.     

Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al₂O₃/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al₂O₃ tougher.     

Effect of Heat Treatments on the Crack-Healing and Static Fatigue Behavior of Silicon Carbide Sintered with Sc2O3 and AlN

Crack-healing behavior of silicon carbide ceramics sintered with AlN and Sc₂O₃ has been studied as a function of heat-treatment temperature and applied stress. Results showed that heat treatment in air could significantly increase the indentation strength whether a stress is applied or not. After heat treatment with no applied stress at 1300°C for 1 h in air, the indentation strength of the specimen with an indentation crack of ∼100 μm (≈2c) recovered its strength fully at room temperature. In addition, a simple heat treatment at 1200°C for 5 h under an applied stress of 200 MPa in air resulted in a complete recovery of the unindented strength at the healing temperature. However, higher applied stress led to fracture of the specimens during heat treatment. The static fatigue limit of the specimens crack healed at 1200°C for 5 h under 200 MPa was ∼450 MPa at the healing temperature. The ratio of the static fatigue limit of the crack-healed specimen to the unindented strength was ∼80%.     

Microstructure and Mechanical Properties of SiC Platelet/Cordierite Glass-Ceramic Composites

Cordierite glass-ceramics reinforced with different volume fractions (0, 10, 20, and 30%) of SiC platelets were fabricated by mechanically mixing appropriate powders and hot pressing above the glass transition temperature. Some of the specimens were subjected to postsintering crystallization treatments. The resultant sintered microstructure was textured, and within it the SiC platelets tended to be oriented with their basal planes perpendicular to the hot-pressing direction. The bending strength of the hot-pressed specimens was constant, while the fracture toughness increased with the SiC content. Crystallization heat treatment in argon for 30 min at 1200°C caused a significant decrease in the bending strength, but improved the fracture toughness. Fractography has confirmed the crack deflection to be a dominant mechanism in the crystallized specimens, which contributes to the improvement in the fracture toughness. The overall changes in the mechanical properties are discussed with respect to the change in the residual stresses and oxidation characteristics of the SiC platelets.     

Crack-healing ability of structural ceramics and a new methodology to guarantee the structural integrity using the ability and proof-test

Recently, the authors developed Si₃N₄, Al₂O₃ and mullite ceramics with good self-crack-healing abilities. It was shown that the optimized crack-healing condition to get high temperature strength was: 1573 K, 1 h, in air, and the healed zone exhibited the same strength as the base material up to about 1573 K (Si₃N₄ and Al₂O₃) and 1473 K (mullite), respectively. Using this good crack-healing ability, a new methodology to guarantee the reliability of ceramic components [crack-healing + proof test] was proposed. It was shown that reliability could be guaranteed before service by this technique, using about 200 samples. However, if a crack initiated during service, reliability would be severely impaired. Therefore, if a material can heal a crack during service, and if the healed zone has enough strength at the temperature of healing, it would be very desirable for structural integrity. From the above points of view, a new methodology to guarantee the structural integrity of ceramic components using in situ crack-healing ability was proposed and the usefulness is discussed using the test results in terms of crack-healing behavior and proof test theory by the authors.     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Electrochemically assisted room-temperature crack healing of ceramic-based composites

We demonstrated for the first time the room-temperature crack healing in ceramic-based composites. For this purpose, 20 and 30 vol% of fine titanium metal (Ti) were homogeneously dispersed in electrically insulating alumina (Al₂O₃) ceramic to obtain composites that exhibited excellent electrical conductivity. Electrochemical anodization at room temperature was used to successfully heal cracks induced in the Al₂O₃/Ti composites and recover their fracture strength and reliability. The bending strength of as-prepared, crack-induced, and electrochemically healed composites was measured to evaluate the crack-healing ability. Moreover, the effects of the anodization current density, crack size (including length and crack open distance), and the conductivity of the composites on their crack-healing behavior were investigated and discussed. The results indicate that the bending strength of crack-induced composites, which was approximately 61% of the crack-free composite strength, was completely recovered after the crack-healing procedure at room temperature under appropriate anodization conditions. The titanium oxides obtained after anodization formed bridges that healed the crack; this was considered to be the main strength recovery mechanism. By anodizing Al₂O₃/Ti composites, we developed a new and convenient approach to heal cracks and recover the fracture strength of cracked ceramics at room temperature.     

Influence of tensile pre-fatigue stress on residual strength of SiC/(PyC/SiC)2/SiC minicomposites

The tensile-tension fatigue behavior of minicomposite SiC/(PyC/SiC)2/SiC at room temperature was studied, and the residual mechanical properties of specimens were tested after 10⁶ pre-fatigue cycles under different levels of stress. The results show that the residual strength first increases owing to the release of stress concentration and then decreases owing to excessive fiber wear. In addition, it is worth noting that the tensile curve of pre-fatigue specimens deflects twice; the first occurrence correlates with matrix crack reopening, and the second occurs when the uniaxial tensile load exceeds the pre-fatigue stress, and the degree of deflection also gradually decreases or almost disappears.