Evaluation On Mechanical And Thermal Properties Of Mg_0.27Al_2.58O_3.73N_0.27 Transparent Ceramics

Transparent structural ceramics have the characteristics of high temperature,good corrosion and wear resistance,high impact resistance,high hardness and strength,as well as good electrical insulation.In addition,they also posess excellent optical performance,outstanding optical transmittance in the ultraviolet to mid-infrared light,which has been used as some important optical components,such as guided missile domes,high temperature optical windows,transparent armors,cell phone screen and advanced camera lens,etc.In previous study,our research group has successfully prepared Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramics with high optical transmittance.Compared with AlON,Mg_0.27Al_2.58O_3.73N_0.27 showed a wider optical transmission range.However,the study of mechanical properties and thermal properties of Mg_0.27Al_2.58O_3.73N_0.27.27 is still insufficient.Comprehensive evaluation of material properties is the basis for the application of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramics.In this dissertation,transparent Mg_0.27Al_2.58O_3.73N_0.27 ceramic with high transparency was obtained by pressureless sintering and subsequent hot isostatic pressing.Based on testing techniques for MgAl₂O₄ and AlON transparent ceramics,comprehensive evaluation of optical,thermal and mechanical properties were conducted for Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic.The flexural strength and Vicker’s hardness of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic were also investigated under different loading condition,while the flexural strength under high temperature was measured.The regularities of mechanical properties were revealed.Based on the results of measured linear thermal expansion coefficient and thermal conductivity,different figures of merit for thermal shock were calculated.The obtained results were compared with those of MgAl₂O₄ and AlON.Through the research,we found that the optical transmittance of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic is 80%in the wavelength ranged from 0.38to 4.34μm,with the maximum in-line transmittance of 84%at 3.4μm,while the refractive index decreases with the increasing light wavelength and lies between those of MgAl₂O₄ and AlON all the way.The Vicker’s hardness,flexural strength,fracture toughness,and Young’s modulus of the Mg_0.27Al_2.58O_3.73N_0.27.27 transparent ceramics were measured as 15.1±0.4 GPa,255±12 MPa,2.56±0.11 MPa/m^1/2,and 312±15GPa,respectively.While the flexural strength of sample at 1200°C was 125 MPa.Based on these results,the Weibull modulus of specimen was determined to be 4.5,and the parameters for slow cracks growth were n¹1,D²70 MPa.The thermal expansion coefficient of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramics was measured as5.8×10^-6 K^-1,and the thermal conductivity was 12.6[W/（m·k）].The thermal shock resistance of the material was calculated to be R=1.36,R’=0.11.Because of the indentation size effect,the test hardness value of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic decreased gradually with the increase of applying load,which showed a similar behavior as those of AlON and MgAl₂O₄.Due to the slow cracks growth,the fracture strength of Mg_0.27Al_2.58O_3.73N_0.27.27 transparent ceramic also gradually decreased with the decreasing loading rate.The fracture mechanism of Mg_0.27Al_2.58O_3.73N_0.27 is a mixed transcrystalline and intercrystalline fractures,which is a typical characteristic for brittle solids.With the rising environmental temperature,the fracture strength of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramics decreased.The descent velocity of fracture strength is lower than AlON when temperature is higher than 1000oC.Owning to the similar crystal structure and chemical component,the infrared wavelength cutoff of Mg_0.27Al_2.58O_3.73N_0.27transparent ceramic is longer than that of AlON but shorter than MgAl₂O₄,which is determined by the sequence of chemical bonding strength of the three crystals.The study showed that many properties of Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic lied between those of AlON and MgAl2O4,which demonstrated that Mg_0.27Al_2.58O_3.73N_0.27 transparent ceramic is an optical material with outstanding mechanical and thermal properties.