Hexagonal boron nitride(h-BN)ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles,owing to their superior thermal stability and excellent dielectric properties.However,their ...Hexagonal boron nitride(h-BN)ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles,owing to their superior thermal stability and excellent dielectric properties.However,their densification during sintering still poses challenges for researchers,and their mechanical properties are rather unsatisfactory.In this study,SrAl_(2)Si_(2)O_(8)(SAS),with low melting point and high strength,was introduced into the h-BN ceramics to facilitate the sintering and reinforce the strength and toughness.Then,BN-SAS ceramic composites were fabricated via hot press sintering using h-BN,SrCO_(3),Al_(2)O_(3),and SiO_(2) as raw materials,and effects of sintering pressure on their microstructure,mechanical property,and thermal property were investigated.The thermal shock resistance of BN-SAS ceramic composites was evaluated.Results show that phases of as-preparedBN-SAS ceramic composites are h-BN and h-SrAl_(2)Si_(2)O_(8).With the increase of sintering pressure,the composites’densities increase,and the mechanical properties shew a rising trend followed by a slight decline.At a sintering pressure of 20 MPa,their bending strength and fracture toughness are(138±4)MPa and(1.84±0.05)MPa·m^(1/2),respectively.Composites sintered at 10 MPa exhibit a low coefficient of thermal expansion,with an average of 2.96×10^(-6) K^(-1) in the temperature range from 200 to 1200℃.The BN-SAS ceramic composites prepared at 20 MPa display higher thermal conductivity from 12.42 to 28.42 W·m^(-1)·K^(-1) within the temperature range from room temperature to 1000℃.Notably,BN-SAS composites exhibit remarkable thermal shock resistance,with residual bending strength peaking and subsequently declining sharply under a thermal shock temperature difference ranging from 600 to 1400℃.The maximum residual bending strength is recorded at a temperature difference of 800℃,with a residual strength retention rate of 101%.As the thermal shock temperature difference increase,the degree of oxidation on the ceramic surface and cracks due to thermal stress are also increased gradually.展开更多
To further improve the thermal shock resistance and ablation resistance of SiBCN system ceramics targeted much harsh environment,both ultra-high temperature ceramic phases and carbon fibers are expected to be incorpor...To further improve the thermal shock resistance and ablation resistance of SiBCN system ceramics targeted much harsh environment,both ultra-high temperature ceramic phases and carbon fibers are expected to be incorporated simultaneously as the reinforcements.However,tough problems like difficulty in densification as well as degradation of C fibers(Cf)due to interfacial reaction usually cannot be avoided.Thus,in this study,Zr as well as chopped Cf with BN coating were introduced into SiBCN ceramics by mechanical alloying and after hot-pressing sintering to prepare Cf/SiBCNZr composites.The structure and phase component development have been characterized in detail by XRD,SEM,TEM and XPS etc.,and mechanical properties,fracture behavior and toughening mechanisms were also investigated.Zr-B and Zr-O bonds are detected besides Si-C,N-B and N-B-C bonds when pre-alloyed ZrB2 amorphous powder was incorporated into SiBCN system by further mechanical-alloying treatment.After hot-pressing,the ultra-high temperature phases of ZrB2 and ZrN were formed as expected besides the original matrix phases of BN(C)and SiC.Excellent mechanical properties can be obtained for SiBCNZr ceramic.The BN coating on carbon fibers successfully avoids the severe fiber degradation and makes it possible for moderating strong interface bonding,which ensuring the debonding,crack deflection,fiber pulling-out and bridging during the fracture.As a result,the Cf/SiBCNZr composites retain good mechanical properties and fractures in a pseudoplastic fracture manner,showing better potential applications in harsh environments.Optimization of the BN coating on Cf and Zr incorporation by pre-mechanical alloying can be a good route to achieve Cf/SiBCNZr composites with a better balance between thermal shock resistance,ablation resistance and mechanical properties.展开更多
基金funded by the National Postdoctoral Program for Innovative Talents (Grant No. BX20190095)National Key Research and Development Program (Grant No. 2017YFB0310400)+1 种基金Financial support from the National Natural Science Foundation of China (Grant No. 51621091, 51472059)China’s Postdoctoral Science Fund (Grant No. 2019M660072, LBH-Z19141) were also appreciated
基金National Natural Science Foundation of China (52072088, 52072089)Natural Science Foundation of Heilongjiang Province (LH2023E061)+1 种基金Scientific and Technological Innovation Leading Talent of Harbin Manufacturing (2022CXRCCG001)Fundamental Research Funds for the Central Universities (3072023CFJ1003)。
文摘Hexagonal boron nitride(h-BN)ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles,owing to their superior thermal stability and excellent dielectric properties.However,their densification during sintering still poses challenges for researchers,and their mechanical properties are rather unsatisfactory.In this study,SrAl_(2)Si_(2)O_(8)(SAS),with low melting point and high strength,was introduced into the h-BN ceramics to facilitate the sintering and reinforce the strength and toughness.Then,BN-SAS ceramic composites were fabricated via hot press sintering using h-BN,SrCO_(3),Al_(2)O_(3),and SiO_(2) as raw materials,and effects of sintering pressure on their microstructure,mechanical property,and thermal property were investigated.The thermal shock resistance of BN-SAS ceramic composites was evaluated.Results show that phases of as-preparedBN-SAS ceramic composites are h-BN and h-SrAl_(2)Si_(2)O_(8).With the increase of sintering pressure,the composites’densities increase,and the mechanical properties shew a rising trend followed by a slight decline.At a sintering pressure of 20 MPa,their bending strength and fracture toughness are(138±4)MPa and(1.84±0.05)MPa·m^(1/2),respectively.Composites sintered at 10 MPa exhibit a low coefficient of thermal expansion,with an average of 2.96×10^(-6) K^(-1) in the temperature range from 200 to 1200℃.The BN-SAS ceramic composites prepared at 20 MPa display higher thermal conductivity from 12.42 to 28.42 W·m^(-1)·K^(-1) within the temperature range from room temperature to 1000℃.Notably,BN-SAS composites exhibit remarkable thermal shock resistance,with residual bending strength peaking and subsequently declining sharply under a thermal shock temperature difference ranging from 600 to 1400℃.The maximum residual bending strength is recorded at a temperature difference of 800℃,with a residual strength retention rate of 101%.As the thermal shock temperature difference increase,the degree of oxidation on the ceramic surface and cracks due to thermal stress are also increased gradually.
基金supported by the National Key Research and Development Program of China(Grant Nos.2017YFB0703200,2017YFB0310400)the National Natural Science Foundation of China(Grant Nos.51621091,51472059,51225203&51272300)the Postdoctoral Innovative Talents Support Program(Grant No.BX20190095)。
文摘To further improve the thermal shock resistance and ablation resistance of SiBCN system ceramics targeted much harsh environment,both ultra-high temperature ceramic phases and carbon fibers are expected to be incorporated simultaneously as the reinforcements.However,tough problems like difficulty in densification as well as degradation of C fibers(Cf)due to interfacial reaction usually cannot be avoided.Thus,in this study,Zr as well as chopped Cf with BN coating were introduced into SiBCN ceramics by mechanical alloying and after hot-pressing sintering to prepare Cf/SiBCNZr composites.The structure and phase component development have been characterized in detail by XRD,SEM,TEM and XPS etc.,and mechanical properties,fracture behavior and toughening mechanisms were also investigated.Zr-B and Zr-O bonds are detected besides Si-C,N-B and N-B-C bonds when pre-alloyed ZrB2 amorphous powder was incorporated into SiBCN system by further mechanical-alloying treatment.After hot-pressing,the ultra-high temperature phases of ZrB2 and ZrN were formed as expected besides the original matrix phases of BN(C)and SiC.Excellent mechanical properties can be obtained for SiBCNZr ceramic.The BN coating on carbon fibers successfully avoids the severe fiber degradation and makes it possible for moderating strong interface bonding,which ensuring the debonding,crack deflection,fiber pulling-out and bridging during the fracture.As a result,the Cf/SiBCNZr composites retain good mechanical properties and fractures in a pseudoplastic fracture manner,showing better potential applications in harsh environments.Optimization of the BN coating on Cf and Zr incorporation by pre-mechanical alloying can be a good route to achieve Cf/SiBCNZr composites with a better balance between thermal shock resistance,ablation resistance and mechanical properties.
