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.     

Threshold Stress for Crack Healing of Mullite Reinforced by SiC Whiskers and SiC Particles and Resultant Fatigue Strength at the Healing Temperature

A mullite/SiC whisker/SiC particle multi-composite, having excellent crack-healing ability and mechanical properties, was hot pressed in order to investigate the crack-healing behavior under stress and the resultant fatigue strength at the temperature of healing. A semi-elliptical surface crack 100 μm in surface length was introduced on each specimen. The pre-cracked specimens were crack healed under cyclic or constant stress by using a three-point bending stress at 1473 K, and the resultant bending strength and cyclic fatigue strength were measured at 1473 K. The pre-crack on the surface of the specimens could be healed even under stress. The threshold stresses for crack healing, as determined by evaluating the strengths of crack-healed specimens at a healing temperature of 1473 K, were 170 MPa for both constant and cyclic stresses, corresponding to 77% of the bending strength of the pre-cracked specimens. The static and cyclic fatigue behaviors of crack-healed specimens were also investigated at a healing temperature of 1473 K.     

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.     

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 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.     

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 and strength recovery in SiC/spinel nanocomposite

The crack healing behavior and the strength recovery of the newly introduced SiC/spinel nanocomposite were investigated. SiC/spinel nanocomposite containing 27.26 wt% SiC was prepared by the ball milling of talc, graphite and aluminum powders with subsequent annealing at 1200 °C for 1 h in a vacuum. The SEM results showed that the surface cracks produced by Vickers indenter on the prepared SiC/spinel pellets can be completely healed after sintering at 1545 °C for 1 min in air atmosphere. Furthermore, an almost complete strength recovery of the specimens can be obtained in those samples heat treated at 1550 °C for 1 min, as evaluated by diametral tensile strength (DTS) test. The formations of mullite and aluminosilicate glassy phases are the major factors which are responsible for the crack healing and strength recovery in the structure. It was found that the healing efficiency of those specimens healed at 1550 °C for 1 min is 99%.     

Crack healing in liquid-phase-pressureless-sintered silicon carbide–aluminum nitride composites

Crack healing in liquid-phase-pressureless-sintered SiC–AlN composites was investigated by introducing cracks into specimens and subsequently heat-treating the specimens. It was observed that cracks were healed and the strength was recovered. Cracks were filled with silica or mullite produced by the oxidation of the composites. It was shown that the healing temperature could be fixed in the range 1100–1300 °C and that large cracks up to about 300 μm could be healed completely. Our results imply that a simple oxidation heat-treatment can improve the reliability of silicon carbide–aluminum nitride components.     

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.     

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.     

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.     

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%.     

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.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

High-temperature flexural strength of SiC ceramics prepared by additive manufacturing

Silicon carbide (SiC) ceramics, as a kind of candidate material for aero-engine, its high-temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high-temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high-temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.     

Effect of SiC whiskers on the anti-oxidation properties of Yb2Si2O7/mullite/SiC tri-layer environmental barrier coating for CMCs

The mullite and ytterbium disilicate (β-Yb₂Si₂O₇) powders as starting materials for the Yb₂Si₂O₇/mullite/SiC tri-layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti-oxidation properties of Yb₂Si₂O₇/mullite/SiC tri-layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb₂SiO₅, α-Yb₂Si₂O₇, and β-Yb₂Si₂O₇ were gradually formed between 800 and 1000°C, and Yb₂SiO₅ and α-Yb₂Si₂O₇ were completely transformed into β-Yb₂Si₂O₇ at a temperature above 1200°C. The weight loss of Yb₂Si₂O₇/(SiC_w–mullite)/SiC tri-layer coating coated specimens was 0.15 × 10⁻³ g cm⁻² after 200 h oxidation at 1400°C, which is lower than that of Yb₂Si₂O₇/mullite/SiC tri-layer coating (2.84 × 10⁻³ g cm⁻²). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β-Yb₂Si₂O₇ outer coating, but also improve the self-healing performance of the mullite middle coating owing to the self-healing aluminosilicate glass phase formed by the reaction between SiO₂ (oxidation of SiC whiskers) and mullite particles.     

Effect of DC and AC electric fields on crack healing behavior in 8 mol% yttria-stabilized cubic zirconia polycrystal

The effect of the flash event (FE) on microcrack healing behavior in 8 mol% yttria-stabilized zirconia was examined at healing temperatures of 1040 and 1230°C under the direct and alternating (DC and AC) electric fields. The crack healing behavior changed depending on the factors of the electric field, healing temperature, and crack length. Although the crack healing proceeded with the temperature, the healing rate increased with the crack length, suggesting that the external energy stored as crack surface energy would provide a driving force for the crack healing. Although the crack healing occurs even under the static annealing without the electric field, the healing rate was accelerated by FE significantly more under the AC field than under the DC field. The microcracks with a length of ≈20 μm were fully healed at 1230°C only for 10 min by the FE treatment under the AC field, and the flash healing behavior was four times faster than that of the static annealing. These results suggest that the enhanced healing behavior cannot be explained only by thermal effects, and the accelerated diffusivity caused additionally by nonthermal effect under FE might contribute to the enhanced healing behavior, especially in the AC electric field.     

Temperature-dependent mechanical and long crack behavior of zirconium diboride–silicon carbide composite

Hot pressed ZrB2–20 vol.% SiC ultra-high temperature ceramic composites have been prepared for strength and fracture investigations. Two composites fabricated under differing hot pressing temperatures with (ZSB) and without (ZS) B4C sintering aids were selected for room temperature modulus of rupture (MOR) strength and single-edge-notch bend (SENB) fracture toughness experiments. Structure property relationships were examined for both composites. MOR and stiffness temperature dependence was also investigated up to 1500 °C. Long crack propagation studies were conducted up to 1400 °C using the double cantilevered beam geometry with half-chevron-notch initiation zones. Residual Boron-rich carbide maximum particle sizes were found to be strength limiting in ZSB billets while SiC controlled strength in ZS billets. Flexure strength decreased linearly with temperature from 1000 to 1500 °C with no visible plastic deformation prior to fracture. Similar stiffness decreases were observed with a transition temperature range of 1100–1200 °C. Long crack studies produced R-curves that show no significant toughening behavior at room temperature with some modest rising R-curve behavior appearing at higher temperatures. These studies also show the plateau toughness increases with temperature up to 1200 °C. This is supported by an observed transition from primarily transgranular fracture at room temperature to primarily intergranular fracture at high temperatures. Wake zone toughening is evident up to 1000 °C with K_R rise from 0.1 to 0.5 MPa√m. Beyond 1000 °C fracture mechanism transitions to include creep zone development ahead of crack tip with wake zone toughening vanishing.     

Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.