Study On Crack Self-Healing Properties Of SiC-Al₂O₃ Ceramic Composites At Low Temperature

Ceramics and ceramic matrix composites are widely used in the fields of life,production and aerospace.However,it is subjected to thermal and mechanical stress during use,and the resulting residual stress can form micro-cracks in the material and on the surface,which can lead to component failure,thus limiting its use as a structural engineering material.Many people try to imitate the self-healing mechanism of biological system to design materials and coatings,so as to make ceramic materials have the function of crack self-healing,reduce the cost of maintenance,inspection,processing and polishing of ceramic parts,and improve the reliability and service life of parts in use.In order to meet the application requirements of crack healing of ceramic composites at relatively low temperature,the self-healing ceramic composites with B₄C and TiB₂ as healing agents were studied respectively.The main work and results are as follows:The SiC-Al₂O₃-B₄C ceramic composites with B₄C as the healing agent were prepared by spark plasma sintering technology,and the effects of B₄C content on the flexural strength,fracture toughness and microstructure of the composites were studied.The research shows that the addition of B₄C helps to improve the flexural strength and fracture toughness of ceramic composites.When the content is 15 wt%,the performance is the best.The flexural strength and fracture toughness are 380 MPa and4.71 MPa·m^1/2 respectively,which are 44.5%and 23.9%higher than those without B₄C.The fracture morphology analysis shows that there are two fracture modes:intergranular fracture and transgranular fracture at the same time.The indentation method was used to prefabricate cracks on the surface of ceramic composites.After the cracks were introduced,the flexural strength was reduced from 380 MPa to 120 MPa,which was reduced to 31.6%of the original.The effects of healing temperature and healing time on the self-healing properties of ceramic composites were explored.The results showed that the healing effect was better when the healing temperature was700°C;After holding at 700°C for 30 minutes,the flexural strength recovered to 358MPa,and the highest healing efficiency reached 94.2%.By analyzing the micro morphology of the crack,it was found that the crack was completely healed,and the purpose of rapid healing at low temperature was achieved.The changes of element composition and micro morphology in the crack area of SiC-Al₂O₃-B₄C ceramic composite before and after self-healing are studied.The healing mechanism is as follows:under the appropriate healing temperature and healing time,the healing agent B₄C is oxidized to form B₂O₃ and CO₂,B₂O₃ flows into the crack in liquid form,expands in volume after cooling,fills the crack,and the performance is restored.However,when a large amount of B₄C is oxidized,CO₂ will escape and leave pores on the substrate surface.When B₄C is used as healing agent,there is gas in the process of healing and oxidation,which will have an adverse impact on the material.TiB₂ is further selected as healing agent,and SiC-Al₂O₃-TiB₂ ceramic composite with TiB₂ as healing agent is prepared by spark plasma sintering technology.The research shows that the addition of TiB₂ plays a reinforcing role to the matrix,increases the phase interface of the material,makes the fracture path more tortuous,consumes more fracture energy when the material breaks,and improves the flexural strength and fracture toughness.When the content of TiB₂ is 15 wt%,the maximum flexural strength reaches 408MPa,which is45.7%higher than that without TiB₂.After the crack is introduced,the flexural strength decreases with the increase of crack size.The relationship between flexural strength and healing time and healing temperature shows that SiC-Al₂O₃-TiB₂ ceramic composite has the best healing effect after healing at 700°C for 90 min,and the prefabricated crack is completely healed.The flexural strength of the sample with prefabricated 350μm crack reached 388 MPa after healing,and the healing efficiency was 95.1%;the sample with prefabricated 260μm crack reached 394 MPa after healing,and the healing efficiency was 96.6%;Micro-cracks of different sizes have the same healing law,the smaller the crack length,the lower the degree of damage,and the higher the healing efficiency in a short time.The phase and chemical composition of the healed surface were analyzed and the oxidation ability was tested.The mechanism of crack self-healing of SiC-Al₂O₃-TiB₂ ceramic composite was obtained.The healing agent TiB₂ undergoes oxidation reaction to generate Ti O₂ and B₂O₃ to fill the cracks,thereby healing the cracks.The advantage of TiB₂ as a healing agent is that no gas is generated during the oxidation reaction,which is beneficial to improve the mechanical properties and surface quality of ceramic composites and protect the matrix from further oxidation.