Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering （ LPS ）. The Si2 N2O phase was generated by an in-situ reaction 2 Si3 N4 （ s ） ＋ 1.5 02 （ g...Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering （ LPS ）. The Si2 N2O phase was generated by an in-situ reaction 2 Si3 N4 （ s ） ＋ 1.5 02 （ g ） = 3 Si2 N2O （ s ） ＋ N2 （ g ） . The content of Si2 N2 O phase up to 60% in the volume was obtained at a sintering temperature of 1 650℃ and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1 700 ℃. The sintering procedure contains a complex crystallization and a phase transition ： amorphous silicon nitride→equiaxial α- Si3 N4→ equiaxial β- Si3 N4→ rod- like Si2 N2O→ needle- like β- Si3N4 . Small round-shaped β→ Si3 N4 particles were entrapped in the Si2 N2O grains and a high density of staking faults was situated in the middle of Si2 N2O grains at a sintering temperature of 1 650 ℃. The toughness inereased from 3.5 MPa·m^1/2 at 1 600 ℃ to 7.2 MPa· m^1/2 at 1 800 ℃ . The hardness was as high as 21.5 GPa （Vickers） at 1 600 ℃ .展开更多
Polyimide(PI)composite films were synthesized incorporating amino modified silicon nitride(Si_(3)N_(4))nanoparticles into PI matrix via in situ polymerization technique.The mechanical and thermal performances as well ...Polyimide(PI)composite films were synthesized incorporating amino modified silicon nitride(Si_(3)N_(4))nanoparticles into PI matrix via in situ polymerization technique.The mechanical and thermal performances as well as the hydrophobic properties of the as prepared composite films were investigated with respect to the dosage of the filler in the PI matrix.According to Thermogravimetric(TGA)analysis,meaningful improvements were achieved in T5(5%weight loss temperature)and T10(10%weight loss temperature)up to 54.1℃ and 52.4℃,respectively when amino functionalized nano Si_(3)N_(4) particles were introduced into the PI matrix.The differential scanning calorimetry(DSC)results revealed that the glass transition temperature(Tg)of the composites was considerably enhanced up to 49.7℃ when amino functionalized Si_(3)N_(4) nanoparticles were incorporated in the PI matrix.Compared to the neat PI,the PI/Si_(3)N_(4) nanocomposites exhibited very high improvement in the tensile strength as well as Young’s modulus up to 105.4%and 138.3%,respectively.Compared to the neat PI,the composites demonstrated highly decreased water absorption behavior which showed about 68.1%enhancement as the content of the nanoparticles was increased to 10 wt%.The SEM(Scanning electron microscope)images confirmed that the enhanced thermal,mechanical and water proof properties are essentially attributed to the improved compatibility of the filler with the matrix and hence,enhanced distribution inside the matrix because of the amino groups on the surface of Si_(3)N_(4) nanoparticles obtained from surface functionalization.展开更多
Three-dimensional silica fiber reinforced silicon nitride based composites were fabricated by preceramic polymer infiltration and pyrolysis method using perhydropolysilazane as a precursor. The effects of precoating a...Three-dimensional silica fiber reinforced silicon nitride based composites were fabricated by preceramic polymer infiltration and pyrolysis method using perhydropolysilazane as a precursor. The effects of precoating and high temperature calcination on the microstructures of the composites were investigated by scanning electron microscopy. For the composite without a precoating, the fracture surface is plain, and the fiber/matrix interfaces become very unclear after calcination at 1 600 ℃ due to intense interfacial reactions. The composite with a precoating shows tough fracture surface with distinct fiber pull-outs, and the fiber/matrix interfaces are still clear after calcination at 1 600 ℃. It is the appropriate precoating process that contributes to the good interfacial microstructures for the composite.展开更多
Silicon nitride composite is joined to itself by heating interlayer of Y2 O3 -AL2O3 -SiO2 mixtures above their liquidus temperatures in flowing nitrogen. The joined specimens are tested in four point flexure from room...Silicon nitride composite is joined to itself by heating interlayer of Y2 O3 -AL2O3 -SiO2 mixtures above their liquidus temperatures in flowing nitrogen. The joined specimens are tested in four point flexure from room temperature to 1373 K. The interface microstruclure and fractured surfaces after testing are observed and analyzed by SEM, EPMA and XRD respectively. The results show that F2 O3 -A12 O3 -SiO2 glass reacts with Si3 N4 at interface, forming the Si3 N4/Si2 N2 O( Y-AlrSi-O-N glass/ Y-Al- Si-O glass gradient interface. With the increase of bonding temperature and holding time, the joint strength first increases, reaching a peak, and then decreases . According to interfacial analyses , the bonding strength depends on joint thickness .展开更多
In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toug...In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toughness measures 9-10 MPa·m<sup>1/2</sup>.The static fa-tigue behavior at 1370℃ of this material is also encouraging.Besides,two another α′/β′sialon composites doped with rare-earth oxides are also described.The effects of processingparameters on the microstructure and the properties of the materials are discussed in somedetails.展开更多
Composite SiNx/DLC films were deposited on Si substrate by RF magnetron sputtering of silicon nitride (Si3N4) target simultaneously with filtered cathode arc (FCA) of graphite. The RF power was fixed at 100 W whereas ...Composite SiNx/DLC films were deposited on Si substrate by RF magnetron sputtering of silicon nitride (Si3N4) target simultaneously with filtered cathode arc (FCA) of graphite. The RF power was fixed at 100 W whereas the arc currents of FCA were 20, 40, 60 and 80 A. The effects of arc current on the structure, surface roughness, density and mechanical properties of SiNx/DLC films were investigated. The results show that the arc current in the studied range has effect on the structure, surface roughness, density and mechanical properties of composite SiNx/DLC films. The composite SiNx/DLC films show the sp3 content between 53.5% and 66.7%, density between 2.54 and2.98 g/cm3, stress between 1.7 and 2.2 GPa, and hardness between 35 and 51 GPa. Furthermore, it was found that the density, stress and hardness correlate linearly with the sp3 content for composite SiNx/DLC films.展开更多
Zircon （ ≤44 μm） and carbon black （≤30μm） were used as starting materials and mixed for 24 h using anhydrous ethanol as medium with the mass ratio of 100：40, dried fully at 60 ℃ and then dry mixed for 10 h. ...Zircon （ ≤44 μm） and carbon black （≤30μm） were used as starting materials and mixed for 24 h using anhydrous ethanol as medium with the mass ratio of 100：40, dried fully at 60 ℃ and then dry mixed for 10 h. Specimens with size of Ф20 mm × 5 mm were pressed under 60 MPa, then dried fully at 120 ℃ , put into a furnace with 1. 0 L ·min^-1 nitrogen gas and fired at 1 400, 1 450, 1 480 and 1 500℃ for 6, 9 and 12 h, respectively. The phase composition and microstructure of the specimens were studied by XRD and SEM, and the carbothermal reduction and nitridation reaction process was thermodynamically analyzed. The results show that using zircon and carbon black as starting materials, ZrN - Si3N4 composite is synthesized by carbothermal reduction and nitridation reaction in nitrogen atmosphere. The composites with different compositions are obtained by controlling the firing temperature and partial pressure of CO gas. The proper firing temperature and holding time to synthesize ZrN - Si3N4 composite are 1 500 ℃ for 12 h.展开更多
In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a se...In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.展开更多
The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been in...The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the a to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3＋Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The flexural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa.m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the flexural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3＋Al2O3 as additives, making it to possess better mechanical properties.展开更多
The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making applica...The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making application . The four kinds of admixtures can be used as anti-oxi-dants for Si3N4, but the mixture of Al and Si achieved the best result. The mixture can not only play the role as anti-oxidant, but also assist the sintering process and help form dense sintering layer, improving the property of the composite.展开更多
Nano SiC - BN composite powders were prepared by dissolving analytically pure H3BO3 and CO（ NH2 ）2 with the mole ratio of 1：2.5 in the absolute alcohol, adding 80% E-SiC with 0. 2 μm average grain size while stirr...Nano SiC - BN composite powders were prepared by dissolving analytically pure H3BO3 and CO（ NH2 ）2 with the mole ratio of 1：2.5 in the absolute alcohol, adding 80% E-SiC with 0. 2 μm average grain size while stirring, firing at 850 ℃ in nitrogen （purity： 99. 99%, pressure： O. 92 -0. 93 MPa） for 15 h. Nano SiC -BN composite specimens were prepared by hot-pressed sintering the nano SiC - BN composite powder in N2 atmosphere with 0. 92 - 0. 93 MPa and at 30 MPa axial pressure for 0. 5 - 1 h at 1 750 - 1 800 ℃. The thermal shock resistance of nano SiC -BN composites was studied by three-point bending, TEM and SEM. The results show that, adding BN can decrease the modulus of elasticity of SiC materials, which improves thermal shock resistance;furthermore, because of the large difference of thermal expansion coefficient between matrix SiC and second phase hexa-BN, thermal mismatch effect results in intercrystalline delamination of h-BN grains and forming many micropores in composite ceramic, which can relax the thermal expansion caused by high tempera- ture effectively, and improve the thermal shock resistance significantly.展开更多
Si3N4/BN nanocomposite powders with the mi-crostructure of the micro-sized a-Si3N4 particles coated with nano-sized BN particles were synthesized via the chemical reaction of boric acid, urea, and a-Si3N4 powder in a ...Si3N4/BN nanocomposite powders with the mi-crostructure of the micro-sized a-Si3N4 particles coated with nano-sized BN particles were synthesized via the chemical reaction of boric acid, urea, and a-Si3N4 powder in a hydro-gen gas. The results of XRD, TEM, and selected area elec-tron diffraction showed that amorphous BN and a little amount of turbostratic BN(t-BN) were coated on Si3N4 parti-cles as the second phase after reaction at 1100℃. After re-heating the composite powders at 1450℃ in a nitrogen gas, the amorphous and turbostratic BN is transformed into h-BN. These nanocomposite powders can be used to prepare Si3N4/BN ceramic composites by hot-pressing at 1800℃, which have perfect machinability and can be drilled with normal metal tools.展开更多
Extensive attention has been drawn to the development of carbon-matrix composites for application in the aerospace and military industry,where a combination of high mechanical strength and excellent frictional propert...Extensive attention has been drawn to the development of carbon-matrix composites for application in the aerospace and military industry,where a combination of high mechanical strength and excellent frictional properties are required.Herein,carbon-matrix composites reinforced by Si_(3)N_(4)nanowires@pyrolytic carbon nanolayers(Si_(3)N_(4nws)@PyCnls)coupled with hydroxyapatite nanosheets is reported.The Si_(3)N_(4nws)@PyCnls(SP)with coaxial structure could increase the surface roughness of Si_(3)N_(4nws)and promote the stress transfer to the carbon matrix,whereas the porous hydroxyapatite nanosheets favor the infiltration of the carbon matrix and promote the interfacial bonding between the SP and carbon matrix.The carbon matrix composites reinforced by SP coupled with hydroxyapatite nanosheets(Si_(3)N_(4nws)@PyCnls-HA-C)exhibit excellent mechanical strength.Compare with the conventional Si_(3)N_(4nws)reinforced carbon composites,Si_(3)N_(4nws)@PyCnls-HA-C(SPHC)have 162%and 249%improvement in flexural strength and elastic modulus,respectively.Moreover,the friction coefficient and wear rate decreased by 53%and 23%,respectively.This study provides a co-reinforcement strategy generated by SP coupled with hydroxyapatite nanosheets for effective improvement of mechanical and frictional properties of carbon matrix composites that are used for aerospace and military industry applications.展开更多
基金Funded by the National Science Foundation of China ( No.50375037)
文摘Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering （ LPS ）. The Si2 N2O phase was generated by an in-situ reaction 2 Si3 N4 （ s ） ＋ 1.5 02 （ g ） = 3 Si2 N2O （ s ） ＋ N2 （ g ） . The content of Si2 N2 O phase up to 60% in the volume was obtained at a sintering temperature of 1 650℃ and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1 700 ℃. The sintering procedure contains a complex crystallization and a phase transition ： amorphous silicon nitride→equiaxial α- Si3 N4→ equiaxial β- Si3 N4→ rod- like Si2 N2O→ needle- like β- Si3N4 . Small round-shaped β→ Si3 N4 particles were entrapped in the Si2 N2O grains and a high density of staking faults was situated in the middle of Si2 N2O grains at a sintering temperature of 1 650 ℃. The toughness inereased from 3.5 MPa·m^1/2 at 1 600 ℃ to 7.2 MPa· m^1/2 at 1 800 ℃ . The hardness was as high as 21.5 GPa （Vickers） at 1 600 ℃ .
基金the National Natural Science Foundation of China(51373044)Natural Science Foundation of Heilongjiang Province of China(E2017018).
文摘Polyimide(PI)composite films were synthesized incorporating amino modified silicon nitride(Si_(3)N_(4))nanoparticles into PI matrix via in situ polymerization technique.The mechanical and thermal performances as well as the hydrophobic properties of the as prepared composite films were investigated with respect to the dosage of the filler in the PI matrix.According to Thermogravimetric(TGA)analysis,meaningful improvements were achieved in T5(5%weight loss temperature)and T10(10%weight loss temperature)up to 54.1℃ and 52.4℃,respectively when amino functionalized nano Si_(3)N_(4) particles were introduced into the PI matrix.The differential scanning calorimetry(DSC)results revealed that the glass transition temperature(Tg)of the composites was considerably enhanced up to 49.7℃ when amino functionalized Si_(3)N_(4) nanoparticles were incorporated in the PI matrix.Compared to the neat PI,the PI/Si_(3)N_(4) nanocomposites exhibited very high improvement in the tensile strength as well as Young’s modulus up to 105.4%and 138.3%,respectively.Compared to the neat PI,the composites demonstrated highly decreased water absorption behavior which showed about 68.1%enhancement as the content of the nanoparticles was increased to 10 wt%.The SEM(Scanning electron microscope)images confirmed that the enhanced thermal,mechanical and water proof properties are essentially attributed to the improved compatibility of the filler with the matrix and hence,enhanced distribution inside the matrix because of the amino groups on the surface of Si_(3)N_(4) nanoparticles obtained from surface functionalization.
文摘Three-dimensional silica fiber reinforced silicon nitride based composites were fabricated by preceramic polymer infiltration and pyrolysis method using perhydropolysilazane as a precursor. The effects of precoating and high temperature calcination on the microstructures of the composites were investigated by scanning electron microscopy. For the composite without a precoating, the fracture surface is plain, and the fiber/matrix interfaces become very unclear after calcination at 1 600 ℃ due to intense interfacial reactions. The composite with a precoating shows tough fracture surface with distinct fiber pull-outs, and the fiber/matrix interfaces are still clear after calcination at 1 600 ℃. It is the appropriate precoating process that contributes to the good interfacial microstructures for the composite.
文摘Silicon nitride composite is joined to itself by heating interlayer of Y2 O3 -AL2O3 -SiO2 mixtures above their liquidus temperatures in flowing nitrogen. The joined specimens are tested in four point flexure from room temperature to 1373 K. The interface microstruclure and fractured surfaces after testing are observed and analyzed by SEM, EPMA and XRD respectively. The results show that F2 O3 -A12 O3 -SiO2 glass reacts with Si3 N4 at interface, forming the Si3 N4/Si2 N2 O( Y-AlrSi-O-N glass/ Y-Al- Si-O glass gradient interface. With the increase of bonding temperature and holding time, the joint strength first increases, reaching a peak, and then decreases . According to interfacial analyses , the bonding strength depends on joint thickness .
基金the High Technology Research and Development Programme of China.
文摘In the present paper,a silicon nitride-based composite processed with rare-earth oxidesadditives is presented.Its bend strength can be maintained at a value as high as 1000 MPafrom 1000 to 1370℃ and the fracture toughness measures 9-10 MPa·m<sup>1/2</sup>.The static fa-tigue behavior at 1370℃ of this material is also encouraging.Besides,two another α′/β′sialon composites doped with rare-earth oxides are also described.The effects of processingparameters on the microstructure and the properties of the materials are discussed in somedetails.
文摘Composite SiNx/DLC films were deposited on Si substrate by RF magnetron sputtering of silicon nitride (Si3N4) target simultaneously with filtered cathode arc (FCA) of graphite. The RF power was fixed at 100 W whereas the arc currents of FCA were 20, 40, 60 and 80 A. The effects of arc current on the structure, surface roughness, density and mechanical properties of SiNx/DLC films were investigated. The results show that the arc current in the studied range has effect on the structure, surface roughness, density and mechanical properties of composite SiNx/DLC films. The composite SiNx/DLC films show the sp3 content between 53.5% and 66.7%, density between 2.54 and2.98 g/cm3, stress between 1.7 and 2.2 GPa, and hardness between 35 and 51 GPa. Furthermore, it was found that the density, stress and hardness correlate linearly with the sp3 content for composite SiNx/DLC films.
文摘Zircon （ ≤44 μm） and carbon black （≤30μm） were used as starting materials and mixed for 24 h using anhydrous ethanol as medium with the mass ratio of 100：40, dried fully at 60 ℃ and then dry mixed for 10 h. Specimens with size of Ф20 mm × 5 mm were pressed under 60 MPa, then dried fully at 120 ℃ , put into a furnace with 1. 0 L ·min^-1 nitrogen gas and fired at 1 400, 1 450, 1 480 and 1 500℃ for 6, 9 and 12 h, respectively. The phase composition and microstructure of the specimens were studied by XRD and SEM, and the carbothermal reduction and nitridation reaction process was thermodynamically analyzed. The results show that using zircon and carbon black as starting materials, ZrN - Si3N4 composite is synthesized by carbothermal reduction and nitridation reaction in nitrogen atmosphere. The composites with different compositions are obtained by controlling the firing temperature and partial pressure of CO gas. The proper firing temperature and holding time to synthesize ZrN - Si3N4 composite are 1 500 ℃ for 12 h.
基金financially supported by the International S&T Cooperation Program of China(No.2010DFR50360)
文摘In this study, we have investigated how the dielectric loss tangent and permittivity of AlN ceramics are affected by factors such as powder mixing methods, milling time, sintering temperature, and the addition of a second conductive phase. All ceramic samples were pre-pared by spark plasma sintering （SPS） under a pressure of 30 MPa. AlN composite ceramics sintered with 30wt%-40wt%SiC at 1600℃ for 5 min exhibited the best dielectric loss tangent, which is greater than 0.3. In addition to AlN and β-SiC, the samples also contained 2H-SiC and Fe5Si3, as detected by X-ray difraction （XRD）. The relative densities of the sintered ceramics were higher than 93%. Experimental results indicate that nano-SiC has a strong capability of absorbing electromagnetic waves. The dielectric constant and dielectric loss of AlN-SiC ce-ramics with the same content of SiC decreased as the frequency of electromagnetic waves increased from 1 kHz to 1 MHz.
基金Founded by the National Natural Science Foundation of China(Nos.90916019 and 50902150)Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province and Aid Program for Innovative Group of National University of Defense Technology
文摘The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the a to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3＋Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The flexural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa.m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the flexural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3＋Al2O3 as additives, making it to possess better mechanical properties.
文摘The effect of the admixtures of Al and Si metals and B4 C and MgAlON compounds on the oxidation of MgO/ Si3N4 composite refractory has been studied, which is a promising carbon free refractory for steel-making application . The four kinds of admixtures can be used as anti-oxi-dants for Si3N4, but the mixture of Al and Si achieved the best result. The mixture can not only play the role as anti-oxidant, but also assist the sintering process and help form dense sintering layer, improving the property of the composite.
文摘Nano SiC - BN composite powders were prepared by dissolving analytically pure H3BO3 and CO（ NH2 ）2 with the mole ratio of 1：2.5 in the absolute alcohol, adding 80% E-SiC with 0. 2 μm average grain size while stirring, firing at 850 ℃ in nitrogen （purity： 99. 99%, pressure： O. 92 -0. 93 MPa） for 15 h. Nano SiC -BN composite specimens were prepared by hot-pressed sintering the nano SiC - BN composite powder in N2 atmosphere with 0. 92 - 0. 93 MPa and at 30 MPa axial pressure for 0. 5 - 1 h at 1 750 - 1 800 ℃. The thermal shock resistance of nano SiC -BN composites was studied by three-point bending, TEM and SEM. The results show that, adding BN can decrease the modulus of elasticity of SiC materials, which improves thermal shock resistance;furthermore, because of the large difference of thermal expansion coefficient between matrix SiC and second phase hexa-BN, thermal mismatch effect results in intercrystalline delamination of h-BN grains and forming many micropores in composite ceramic, which can relax the thermal expansion caused by high tempera- ture effectively, and improve the thermal shock resistance significantly.
基金the National Natural Science Foundation of China (Grant No. 50072017)
文摘Si3N4/BN nanocomposite powders with the mi-crostructure of the micro-sized a-Si3N4 particles coated with nano-sized BN particles were synthesized via the chemical reaction of boric acid, urea, and a-Si3N4 powder in a hydro-gen gas. The results of XRD, TEM, and selected area elec-tron diffraction showed that amorphous BN and a little amount of turbostratic BN(t-BN) were coated on Si3N4 parti-cles as the second phase after reaction at 1100℃. After re-heating the composite powders at 1450℃ in a nitrogen gas, the amorphous and turbostratic BN is transformed into h-BN. These nanocomposite powders can be used to prepare Si3N4/BN ceramic composites by hot-pressing at 1800℃, which have perfect machinability and can be drilled with normal metal tools.
基金his work was supported by the National Natural Science Foundation of China under Grant Nos.51872232the Research Fund of the State Key Laboratory of Solidification Processing(NWPU),China(Grant No.136-QP-2015),the“111”project of China(B08040)+1 种基金the National Training Program of Innovation and Entrepreneurship for Undergraduates(Grand No.S202010699336)Project supported by the Joint Funds of the National Natural Science Foundation of China(Grant No.U21B2067).
文摘Extensive attention has been drawn to the development of carbon-matrix composites for application in the aerospace and military industry,where a combination of high mechanical strength and excellent frictional properties are required.Herein,carbon-matrix composites reinforced by Si_(3)N_(4)nanowires@pyrolytic carbon nanolayers(Si_(3)N_(4nws)@PyCnls)coupled with hydroxyapatite nanosheets is reported.The Si_(3)N_(4nws)@PyCnls(SP)with coaxial structure could increase the surface roughness of Si_(3)N_(4nws)and promote the stress transfer to the carbon matrix,whereas the porous hydroxyapatite nanosheets favor the infiltration of the carbon matrix and promote the interfacial bonding between the SP and carbon matrix.The carbon matrix composites reinforced by SP coupled with hydroxyapatite nanosheets(Si_(3)N_(4nws)@PyCnls-HA-C)exhibit excellent mechanical strength.Compare with the conventional Si_(3)N_(4nws)reinforced carbon composites,Si_(3)N_(4nws)@PyCnls-HA-C(SPHC)have 162%and 249%improvement in flexural strength and elastic modulus,respectively.Moreover,the friction coefficient and wear rate decreased by 53%and 23%,respectively.This study provides a co-reinforcement strategy generated by SP coupled with hydroxyapatite nanosheets for effective improvement of mechanical and frictional properties of carbon matrix composites that are used for aerospace and military industry applications.
