Researches on the processing method of ceramic bearing ball,the formation and propagation of defects in the manufacturing and the nondestructive evaluation(NDE) are summarized in this paper.The key for successful proc...Researches on the processing method of ceramic bearing ball,the formation and propagation of defects in the manufacturing and the nondestructive evaluation(NDE) are summarized in this paper.The key for successful processing of high strength ceramic balls is to avoid producing related defects.Many investigations show that the material microstructures,defects as well as mechanical processing parameters influence the final surface quality significantly.Most of NDE technologies,such as radiation,ultrasonic,dye-penetration and laser scatter,have been studied for ceramic bearing ball surface inspection around the world.So far,the difficulties to develop the perfect NDE system for ceramic bearing balls,which are caused by the defect variety and surface unfolding,have not been overcome yet.展开更多
By employing sintering additives of Li2CO3 and Y2O3,porous Si3N4 ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550°C,respectively.The experimental...By employing sintering additives of Li2CO3 and Y2O3,porous Si3N4 ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550°C,respectively.The experimental results demonstrate the completed phase transformation fromαtoβ-Si3N4 in Si3N4 ceramic samples with a amount of 1.60 wt%Li2CO3(0.65 wt%Li2O)and 0.33 wt%Y2O3 additives.The as-synthesized porous Si3N4 ceramics exhibit high flexural strength((126.7±2.7)MPa)and high open porosity of 50.4%at elevated temperature(1200°C).These results are attributed to the significant role of added Li2CO3 as sintering additive,where the volatilization of intergranular glassy phase occurs during sintering process.Therefore,porous Si3N4 ceramics with desired mechanical property prepared by altering the addition of sintering additives demonstrate their great potential as a promising candidate for high temperature applications.展开更多
Combining sintering additive with field assisted sintering,stereolithographical dense Si3N4 ceramics was successfully fabricated.Owing to a large amount of polymer during the stereolithography,the green parts have the...Combining sintering additive with field assisted sintering,stereolithographical dense Si3N4 ceramics was successfully fabricated.Owing to a large amount of polymer during the stereolithography,the green parts have the characteristics of low powder loading and high porosity.Adjusting the process parameters such as sintering temperature and soaking time can effectively improve the density of the specimens.The stress exponent n of all specimens is in a range of 1 and 2,which is derived from a modified sintering kinetics model.The apparent activation energy Qd of stereolithographic Si_(3)N_(4) ceramics sintered with applied pressures of 30 MPa,40 MPa,and 50 MPa is 384.75,276.61 and 193.95 kJ/mol,respectively,suggesting that the densification dynamic process is strengthened by raising applied pressure.The grain boundary slipping plays a dominating role in the densification of stereolithographic Si3N4 ceramics.The Vickers hardness and fracture toughness of stereolithographic Si3N4 ceramics are HV10/10(1347.9±2.4)and(6.57±0.07)MPaAbstract:Combining sintering additive with field assisted sintering,stereolithographical dense Si3N4 ceramics was successfully fabricated.Owing to a large amount of polymer during the stereolithography,the green parts have the characteristics of low powder loading and high porosity.Adjusting the process parameters such as sintering temperature and soaking time can effectively improve the density of the specimens.The stress exponent n of all specimens is in a range of 1 and 2,which is derived from a modified sintering kinetics model.The apparent activation energy Qd of stereolithographic Si3N4 ceramics sintered with applied pressures of 30 MPa,40 MPa,and 50 MPa is 384.75,276.61 and 193.95 kJ/mol,respectively,suggesting that the densification dynamic process is strengthened by raising applied pressure.The grain boundary slipping plays a dominating role in the densification of stereolithographic Si3N4 ceramics.The Vickers hardness and fracture toughness of stereolithographic Si3N4 ceramics are HV10/10(1347.9±2.4)and(6.57±0.07)MPa·m^(1/2),respectively.展开更多
Aiming at developing novel microwave-transparent ceramics with low dielectric loss, high thermal conductivity and high strength, Si3Na-AIN （30%, mass fraction） composite ceramics with La203 as sintering additive wer...Aiming at developing novel microwave-transparent ceramics with low dielectric loss, high thermal conductivity and high strength, Si3Na-AIN （30%, mass fraction） composite ceramics with La203 as sintering additive were prepared by hot-pressing at 1 800 ℃ and subsequently annealed at 1 450 ℃ and 1 850 ℃ for 2 h and 4 h, respectively. The materials were characterized by XRD and SEM. The effect of annealing process on the phase composition, sintering performance, microstructure, bending strength, dielectric loss and thermal conductivity of the materials was investigated. The results showed that both annealing at 1 850 ℃ and 1 450 ℃ promoted the phase transformation of α-Si3N4 to β-Si3N4. After annealing at 1 850 ℃, grain growth to a certain extent occurred in the materials. Especially, the elongated β-Si3N4 grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio. As a result, as the annealing time increased to 4 h, the bending strength declined from 456 MPa to 390 MPa, whereas the dielectric loss decreased to 2.15× 10^-3 and the thermal conductivity increased to 16.3 W/（m.K） gradually. When annealed at 1 450 ℃, increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La2Si6N803 phase in the materials, which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/（m·K）, respectively, and simultaneously the decrease in dielectric loss to 1.53× 10^-3.展开更多
基金the National Nature Science Foundation of China (50275031)
文摘Researches on the processing method of ceramic bearing ball,the formation and propagation of defects in the manufacturing and the nondestructive evaluation(NDE) are summarized in this paper.The key for successful processing of high strength ceramic balls is to avoid producing related defects.Many investigations show that the material microstructures,defects as well as mechanical processing parameters influence the final surface quality significantly.Most of NDE technologies,such as radiation,ultrasonic,dye-penetration and laser scatter,have been studied for ceramic bearing ball surface inspection around the world.So far,the difficulties to develop the perfect NDE system for ceramic bearing balls,which are caused by the defect variety and surface unfolding,have not been overcome yet.
基金Project(202045007)supported by the Start-up Funds for Outstanding Talents in Central South University,China。
文摘By employing sintering additives of Li2CO3 and Y2O3,porous Si3N4 ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550°C,respectively.The experimental results demonstrate the completed phase transformation fromαtoβ-Si3N4 in Si3N4 ceramic samples with a amount of 1.60 wt%Li2CO3(0.65 wt%Li2O)and 0.33 wt%Y2O3 additives.The as-synthesized porous Si3N4 ceramics exhibit high flexural strength((126.7±2.7)MPa)and high open porosity of 50.4%at elevated temperature(1200°C).These results are attributed to the significant role of added Li2CO3 as sintering additive,where the volatilization of intergranular glassy phase occurs during sintering process.Therefore,porous Si3N4 ceramics with desired mechanical property prepared by altering the addition of sintering additives demonstrate their great potential as a promising candidate for high temperature applications.
基金Project(20170410221235842)supported by Shenzhen Technical Innovation and Tackling Program,ChinaProject(2019zzts859)supported by the Fundamental Research Funds for the Central Universities,ChinaProject(20203BBE53053)supported by Key R&D Project of Jiangxi Provincial Department of Science and Technology,China。
文摘Combining sintering additive with field assisted sintering,stereolithographical dense Si3N4 ceramics was successfully fabricated.Owing to a large amount of polymer during the stereolithography,the green parts have the characteristics of low powder loading and high porosity.Adjusting the process parameters such as sintering temperature and soaking time can effectively improve the density of the specimens.The stress exponent n of all specimens is in a range of 1 and 2,which is derived from a modified sintering kinetics model.The apparent activation energy Qd of stereolithographic Si_(3)N_(4) ceramics sintered with applied pressures of 30 MPa,40 MPa,and 50 MPa is 384.75,276.61 and 193.95 kJ/mol,respectively,suggesting that the densification dynamic process is strengthened by raising applied pressure.The grain boundary slipping plays a dominating role in the densification of stereolithographic Si3N4 ceramics.The Vickers hardness and fracture toughness of stereolithographic Si3N4 ceramics are HV10/10(1347.9±2.4)and(6.57±0.07)MPaAbstract:Combining sintering additive with field assisted sintering,stereolithographical dense Si3N4 ceramics was successfully fabricated.Owing to a large amount of polymer during the stereolithography,the green parts have the characteristics of low powder loading and high porosity.Adjusting the process parameters such as sintering temperature and soaking time can effectively improve the density of the specimens.The stress exponent n of all specimens is in a range of 1 and 2,which is derived from a modified sintering kinetics model.The apparent activation energy Qd of stereolithographic Si3N4 ceramics sintered with applied pressures of 30 MPa,40 MPa,and 50 MPa is 384.75,276.61 and 193.95 kJ/mol,respectively,suggesting that the densification dynamic process is strengthened by raising applied pressure.The grain boundary slipping plays a dominating role in the densification of stereolithographic Si3N4 ceramics.The Vickers hardness and fracture toughness of stereolithographic Si3N4 ceramics are HV10/10(1347.9±2.4)and(6.57±0.07)MPa·m^(1/2),respectively.
基金Project(50872052) supported by the National Natural Science Foundation of ChinaProject(2009AA05Z313) supported by the National High Technology Research and Development Program of ChinaProject supported by the Commission of Science,Technology and Industry for National Defence,China
文摘Aiming at developing novel microwave-transparent ceramics with low dielectric loss, high thermal conductivity and high strength, Si3Na-AIN （30%, mass fraction） composite ceramics with La203 as sintering additive were prepared by hot-pressing at 1 800 ℃ and subsequently annealed at 1 450 ℃ and 1 850 ℃ for 2 h and 4 h, respectively. The materials were characterized by XRD and SEM. The effect of annealing process on the phase composition, sintering performance, microstructure, bending strength, dielectric loss and thermal conductivity of the materials was investigated. The results showed that both annealing at 1 850 ℃ and 1 450 ℃ promoted the phase transformation of α-Si3N4 to β-Si3N4. After annealing at 1 850 ℃, grain growth to a certain extent occurred in the materials. Especially, the elongated β-Si3N4 grains showed a slight increase in diameter from 0.2 μm to 0.6 μm approximately and a decrease in aspect ratio. As a result, as the annealing time increased to 4 h, the bending strength declined from 456 MPa to 390 MPa, whereas the dielectric loss decreased to 2.15× 10^-3 and the thermal conductivity increased to 16.3 W/（m.K） gradually. When annealed at 1 450 ℃, increasing the annealing time to 4 h significantly promoted the crystallization of glassy phase to La2Si6N803 phase in the materials, which led to the increase in bending strength to 619 MPa and thermal conductivity to 15.9 W/（m·K）, respectively, and simultaneously the decrease in dielectric loss to 1.53× 10^-3.
