Microstructures,Thermal Shock Resistance and Ablation Resistance of BN-MAS Composite Ceramics

Author:Cai De Long

Supervisor:jia de chang


Degree Year:2017





Hexagonal boron nitride(h-BN)has a series of unique combination of properties,for instance chemical inertness,extremely high sublimation temperature of about 3000 °C(under non-oxidizing atmosphere),low dielectric coefficient,low loss tangent,excellent thermal shock resistance and good machinability.Thus,h-BN ceramics has potential applications in the field of new generation hypersonic aircraft wave transparent radome and antenna windows that should also possess excellent high-temperature and thermal protective performance.However,there exist some problems that limit the application of single-phase h-BN material,such as low strength,poor sintering properties,poor rain and particle erosion resistance,etc.Therefore,it is highly desirable to introduce a high strength and dense secondary phase to assist sintering by forming liquid-phase and thus,improve the mechanical properties of h-BN matrix composites.Magnesium aluminum silicate(Mg-Al-Si-O)is one of the most important glass ceramic systems that can be used for high-temperature wave transparent fields.The main phases of the MAS system include Indialite(α-Cordierite,Mg2Al4Si5O18),μ-Cordierite(Mg2Al4Si5O18),Mullite(Al6Si2O13)and Spinel(Mg Al2O4).The MAS ceramics have a high strength,excellent thermal and chemical stabilities,proper dielectric properties,low thermal expansion coefficient(12×10-6/K),and a melting point of 1400 °C1465 °C which can intensify the liquid phase sintering of h-BN properties.Therefore,it gives the chance to combine h-BN and MAS together,so as to innovate a novel wave transparent composites with excellent properties.In this study,BN-MAS composite ceramics were fabricated by hot-press sintering at relatively low temperature and pressure.The microstructures,mechanical properties,densification behavior,strengthening mechanism,dielectric properties,thermal properties,thermal shock resistance and ablation resistance of h-BN combined with MAS(Mg2Al4Si5O18)have been studied by XRD,TEM,SEM,mechnical properties test and vector network analyzer,etc.Phase transformations,microstructural development and crystallization inhibiting mechanism of the BN matrix composites were systematically investigated.And the influence mechanism of MAS on thermal properties and dielectric properties of the material was also analyzed and discussed.The results showed that h-BN can inhibit the crystallization of MAS,and with the increase of sintering temperature,the inhibition effect of crystallization became more obvious.The B-N bond is partial ionic,in which the B ion is electrophilic while the N ion is nucleophilic.h-BN and MAS are considered to be co-enhanced by each other,resulting in better sintering ability and mechanical properties of composite ceramics than that of single-phase h-BN or MAS.In this system,MAS is an effective liquid-phase sintering aid during the hot-pressing,thus strengthening the h-BN composites.At the same time,h-BN flakes with layered structure can play a role in promoting the toughness of the MAS matrix.When sintered at 1450 °C and 10 MPa,the composite ceramics exhibits the highest bending strength and fracture toughness of 213 ± 25 MPa and 2.49 ± 0.35 MPa·m1/2,respectively.Sintering pressure also has a profitable influence on the mechanical properties of composite ceramics.U nder the given sintering pressure of 30 MPa,the fracture toughness of composites was greatly improved 66%(from 2.49 to 4.12 MPa·m1/2,with similar densities).The main strengthening and toughening mechanism is the pull-out and crack deflection of lamellar h-BN.The dielectric constant and dielectric loss tangent value of the BN-MAS composites increased with the increase of MAS content at room temperature.The dielectric constant and loss tangent value of the BN-MAS composites sintered at high temperature are lower than the composites prepared at lower temperature.The dielectric constant of the composites ranges from 3.5 to 4.3 and the dielectric loss tangent value is in the range of(1.37.7)× 10-3.The dielectric properties of the composites show a stable performance on test frequency.The dielectric constant and dielectric loss tangent value of the BN-MAS composites increased with the increasing test temperature.The hot-press sintering temperature showed a significant effect on the high-temperature dielectric loss properties of the composites.All kinds of composite ceramics in this research had a relative low thermal expansion coefficient.With the increase of MAS content,the average thermal expansion coefficient ranged from 2.73 x 10-6 /K to 6.70 x 10-6 /K and showed an increasing trend.The variation of thermal expansion coefficient is mainly related to the density,MAS content and phase composition of the composite ceramics.The effect of magnesium aluminum silicate(MAS)glass on the thermal shock resistance and the oxidation behavior of h-BN matrix composites were systematically investigated by water quenching at temperature differences from 600 °C up to 1400°C.The residual strength rate of the composites rose with increasing addition of MAS and exhibited a maximum value when MAS is 60 wt.%.Compared with the original strength,the residual strength of the specimen after thermal shock increased 77 %(ΔT = 1000 °C).In the process of thermal shock,MAS can react with B2O3 that generated by the oxidation of h-BN on the surface of the composite ceramics,forming a dense layer of Mg-Al-Si-B-O oxide glass.The oxide layer can not only improve the thermal shock resistance by bridging the surface defects and producing stress strengthening,but also help to prevent the rapid diffusion of oxygen into the composites.Ablation behavior and properties of BN-MAS composites impinged with an oxyacetylene flame were investigated.The results suggested that with the increase of MAS content,the mass and linear ablation rate of the composite ceramics showed an increasing trend.Meanwhile,with the increase of ablation time,both the mass and linear ablation rate showed an increasing trend as well.When the ablation time was 30 s,the mass ablation rate and linear ablation rate of the composite ceramics were up to 0.0254 g/s and 0.087 mm/s,respectively.During the ablation process,the solid/liquid/gas multiphase high-temperature chemical reaction will occur in the composite ceramics,including both thermal chemical ablation and melting loss simultaneously.