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Self-crack-healing behavior under a combustion gas atmosphere with a low oxygen partial pressure, , is important for actualizing ceramic gas turbines, but to date only self-crack-healing behavior in air has been investigated. In this study, we investigated crack-healing behaviors at 1273–1773 K under several levels of . Crack-healing in atmospheres with gave rise to the complete strength recovery of cracked specimens, resulting from passive oxidation. Based on the obtained results, the kinetics for strength recovery by self-crack-healing was expressed as a function of healing temperature, TH (K), and (Pa). The strength recovery rate for complete crack-healing, vH (s−1), could be expressed as Using this rate equation, one can evaluate the healing time for complete strength recovery under combustion gas in a gas turbine. [ABSTRACT FROM AUTHOR]

Self-crack-healing by oxidation of silicon carbide (SiC) is the most effective method to improve the reliability of ceramics, as this eliminates surface cracks completely. Furthermore, if the service condition corresponds to the state under which self-crack-healing can occur, cracks introduced during service can be completely healed during service. However, the ceramic composites having crack-healing ability exhibit only a small efficient temperature region for crack-healing during service. Therefore, the authors have tried to decrease the temperature at which crack-healing reaction becomes active, as well as to increase the temperature limit for bending strength by size reduction in the contained SiC particles. The present study adapts the following self-propagating high temperature synthesis (SHS) to fabricate alumina composite containing nanosized SiC particles: 3(3A12O32SiO2) + 8A1 + 6C = 13A12O3 + 6SiC. The formed nanosized SiC particles have a particle size of 10-30 nm and exist in alumina grain. As a result, the temperature at which self-crack-healing becomes active can be decreased 200°C by using SHS. Furthermore, the crack-healed specimen has no large strength decrease up to 1300°C. Therefore, new alumina! SiC nanocomposite is found to exhibit the in situ self-crack-healing efficient temperature region of 300°C. [ABSTRACT FROM AUTHOR]

The article presents a study which investigated the strength recovery of aluminum oxide/silicon carbide specimen with a semicircular groove by crack-healing for improving the structural integrity of machined alumina and reducing machining costs. The study proposed using the crack-healing method to guarantee the reliability of ceramic components before service. It was concluded that 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.

The article presents a study on the improvement in the self-crack-healing ability of alumina containing nanometer-sized silicon carbide (SiC) particles composite fabricated by self-propagating high-temperature synthesis. The study found that the limit temperature for the bending strength of the alumina composite is equal to that of the alumina containing commercial SiC particles composite. It was concluded that the alumina/nanometer-sized SiC particle composite can be applied to structural components with self-crack-healing from 1223 kelvin to 1573 K.

Al[sub 2]O[sub 3] reinforced by SiC whiskers (Al[sub 2]O[sub 3]/SiC-W) was hotpressed to investigate the crack-healing behavior. Semiellipticai 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 threepoint bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al[sub 2]O[sub 3]/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2c = 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 Al[sub 2]O[sub 3] tougher. [ABSTRACT FROM AUTHOR]

Crack-healing behavior of liquid-phase-sintered (LPS) SiC ceramics has been studied as functions of heat-treatment temperature and crack size. Results showed that heat treatment in air could significantly increase the indentation strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the softening temperature of an intergranular phase in each material. After heat treatment at the optimum temperature in air, the crack morphology almost entirely disappeared and the indentation strength recovered to the value of the smooth specimens at room temperature for the investigated crack sizes up to ∼200 μm. In addition, a simple heat treatment of SiC ceramics sintered with Al[sub 2]O[sub 3]-Y[sub 2]O[sub 3]CaO at ll00°C for 1 h in air resulted in even further improvement of the strength, to a value of 1054 MPa (∼150% of the value of the unindented strength). Crack closure and rebonding of the crack wake due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in LPS-SiC ceramics. [ABSTRACT FROM AUTHOR]

Examines the crack-healing behavior of mullite or silicon carbide (SiC) ceramics. Ability of mullite or SiC composite ceramics to heal following cracking incidents; Analysis on the crack-healing behavior and cyclic fatigue properties; Effect of crack size and crack-healing time on strength recovery behavior.

Abstract: 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 8h 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 250MPa, and the threshold cyclic stress was found to be 300MPa. Considering that the crack growth is time-dependent, the threshold stress of every condition during crack-healing of alumina(w) was found to be 250MPa. The results showed that the threshold stress intensity factor during crack-healing was 3.8MPam1/2. The same experiment conditions were applied to specimens cracked and annealed at 1300°C for 1h 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 3.2MPam1/2 for the specimens crack-healed with annealing. The threshold stress intensity factor during crack-healing of alumina(w) was chosen to be 3.2MPam1/2 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. [Copyright &y& Elsevier]

Abstract: High-strength mullite/SiC/Y2O3 composite ceramics with a great crack-healing ability were developed and subjected to three-point bending. A semicircular surface crack 100μm in diameter was made in each specimen. Crack-healing behaviour was systematically studied as a function of crack-healing temperature and healing time, and the bending strength of the specimens crack-healed up to 1573K was investigated. The effects of the crack-healing treatment on the structural integrity and reliability of machined components were studied systematically. Four main conclusions were drawn from the present study: (1) mullite/SiC/Y2O3 composite ceramics have the ability to heal after cracking from 1423K to 1673K in air, (2) the heat-resistance limit temperature for strength of the crack-healed specimen is ≅1473K, while at 68% of the specimens fractured from outside the crack-healed zone in the tested-temperature range of 300–1573K, (3) due to the crack-healing treatment, the strength of the machined specimen increased by 40–200%. Local fracture strength was assumed to recover completely, and the cracks formed by machining were healed completely, and (4) a large self-crack-healing ability is desirable for obtaining a higher structural integrity in ceramic components. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]