High Temperature Properties and Microstructures of Si3N4 with AZ-Type Non-Toxic, Non-Oxide Additives

D0I:10.13374/j.issn1001-053x.1991.s2.007 北京科技大学学报 第15岳5(I)期 Vol.13 No.5(I) 1991年9月 Journal of University of Science and Technology Beijing Sept.1991 High Temperature Properties and Microstructures of SiN,with AZ- Type Non-Toxic,Non-Oxide Additives+ Ge Changchun·Xia Yuanluo'Chen Limin.Yuan Vi… ABSTRACT:A new type of Sia Ns ceramics (ZAN)is developed in our labora- tory.Densification of ZAN is promoted by non-toxic,non-oxide AZ-type additives.In this work high temperature (HT)properties and microstructures of ZAN are investigated. KEY WORDS:SisN ceramics,non-oxide,additives,high temperature properties Silicon nitride is one of the most important candidate materials for various high temperature (HT),high-stressed components of engines.Due to the low self-diffusivity,SisN.ceramics are generally densified by the oxide additives such as Al2O3,MgO,YaOs etc.which react with silica on the surface of SisN powders to form liquid phase during sintering and promote densification. But the glassy phase in grain boundaries formed after sintering usually causes series degradation of HT strength, The amount and characteristics of the intergranular glassy phase are closely related to the oxide content of the SisN4-based ceramics.Utilization of non- oxide additives instead of oxide additives can significantly reduce the amount of glassy phase and raise its softening temperature,that favour the improve- ment of its HT strength but make it difficult to be sintered.Prochazka and Greskovich used BeSiNz as additive(1),which led to high densification and 1991一06一13收稿 +本文属国家自然科学基金项目、国家数委博士点基金项目、中国发明协会基金项目 .Former research fellow of Alexander von Humboldt Foundation ·特种陶绕粉未治金研究室(Laboratory of Special Ceramics and Powder Metallurgy) ,,·材料科学与工程系(Department of Materi4 Is Science and Engincering) 35

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good HT properties.But BeSiNz is toxic and the fracture toughness of the ceramic is low.Highly dense Si,N4 ceramics with additives of metal nitrides were fabricated by expensive high pressure hot-pressing under 3 GPa at 1500~ 1800C or HIP at 1650~1800C under 150GPa by Shimada,N.Uchida and M.Koizumi,with no HT bending strength reported(2).Recently O,Abe reported relatively high strength was maintained below 1300C on hot-pressed SisN,with alkaline-earth nitrides.One problem for practical application is the chemical unstability of alkaline-earth nitrides in airt3). A new type.of SisN,ceramics (ZAN)is developed in our laboratory, densification of which is promoted by non-toxic,non-oxide proprietary AZ- type additives either with gas-pressure sintering or conventional hot-pressing. In this work HT properties and microstructures of ZAN are investigated in comparison with a conventional SisNs ceramic with spinel as oxide additive, 1 Materials and Methods Commercial SisNs powders (Shanghai Eiectroceramics,Factory BET-specific surface area Sg 5.8 m2/g)and self-made AZ-type powders (Sg 5.4 m2/g)are used as raw materials.The oxygen contents of Si,N,powders and AZ-type powders are 2.25%and 2.45%respectively.The metallic impurities content of Si,N.powders is<4590ppm and that of AZ-type powder is<1285ppm. SisN,with MgO.Al2Oa as oxide additive (MAO)is prepared as refere- nce material for comparison. SisNa powders with different additives,are mixed and ball-milled with ethanol as medium for 96h,vacuum dried,sieved through-200 mesh with Sg of 12.5 m2/g and then hot-pressed in a small hot-pressing furnace under N2 atmo- sphere with 10 MPa at different temperatures, Dimensions of specimens for 3-pt bending strength are 3mm x 4mm x 40mm with span of 30mm;loading rate is 0.5mm/min;specimens (5 replicates)are tested with the loading direction coincident with that of hot pressing. The microstructures are observed with SEM,TEM with EDX and HREM, 2 Results and Discussions 2.1 Oxidation Behavior at High Temperature Specimens are oxidized at 1400C for 110h.The'weight increase-time rela- tions are shown in Fig.1.The oxidation behavior in the first stage of oxi- dation can be expressed as: 36

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(AW/A)2=K·t AW-weight increase during oxidation (kg); 1-oxidation duration (s); A-surface area of specimen (m2); K一oxidation rate constant(kg2·m-4·s-l)y K(ZAN)=3.5×10-11(kg2·m-4·s1),is two orders lower than K(MA0)=1.15×10-9(kg2·m4·s-1). 07、 3o0 e 2!4」 .'h Fig.1 Dependence of weight increase on oxidation time Fig.2 SEM micrographs of oxidized surfaces of (a)ZAN and (b)MAO SEM micrographs of oxidized specimens (Fig.2)show that many protrusions appear on the porous oxidation films on specimens MAO,while rather smooth oxidation films adhere on specimens ZAN,indicating better oxidation resistance, Passive oxidation of SisN4 procceds according to the reaction: SigN4+302=3Si02+2N2 It is controlled by diffusion of oxygen through surface SiO2.Greskovich et al.estimated that the oxidation rate constant of CVD-Sia Na at 1400C should be less than 5x 10-13 kg?,m-4.s-1 c). The oxidation resistance of MAO is significantly reduced because of the 37

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considerable amount of intergranular glassy phase,which is formed through reac -tion of the oxide additive with SiOz on the surface of SiaN powder particles. While the oxidation resistance of ZAN is much better than MAO through the use of non-oxide AZ-type additives instead of oxide additives,due to much less amount of intergranular glassy phase with much higher N content and viscosity. 2.2 Bending Strength and Strength Retention at HT The RT and HT bending strength with the strength retention at 1250C and 1400C of specimens are listed in Table 1.It is evident that the bending strength at RT,1250C and 1400C for ZAN are much higher than MAO.The more important fact is that the strength retentions at 1250C and 1400C of ZAN reach 90.2%and 71.0%,while those of MAO are only 61%and 41,5%. Table1 The bending strength and strength retention at1250℃，1400℃. ORT 01250 01400 01250 01400 Specimen % % MPa MPa MPa GRT ORTi ZAN 686 619 484 90,2 71.0 MAO 544 332 226 61,0 41.5 SEM fractographs of specimens after bending strength tests at 1400C shown in Fig.3,it further proved the different characteristics of ZAN and MAO.Many small“spheres”appeared on the fraeture,surface of MAO at 140o'℃，which formed as liquid phase at 1400C under the action of surface tension. (a)ZAN, (b),(c)MAO Fig.3 SEM fractographs after bending strength test at 1400C Cracks between "spheres"and matrix developed after a certain amount of plastic deformation through sliding along the surfaces of these "spheres". While the fracture surface of the bending strength specimen ZAN at 1400C kelpt the brittle characteristics like that of specimen broken at room tempe- 38

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rature. 2.3 Microstructure Features of SisNa with AZ-Type Non-Oxide Additive Superior RT and HT mechanical properties and oxidation behavior of ZAN are closely related to the microstructures.Hexagonal of elongated B/-SS grains of <1um are showh in specimen hot pressed at 1800C under.TEM,Many ZrN particles dispersed in matrix and secondary crystalline phase distributed in triple regions of grain boundaries are observed.(Fig.4,5) Fig.4 TEM of ZAN showing secondary crystalline phase in triple regions. (a)ZrN particles dispersed in matrix (b)EDX analysis of the compositions of secondary crystalline phase:atom ratio of Zr/Si=1/3 Fig.5 Intergranular glassy phase in the triple regions of Be-SiaN4 matrix Zr and Si are identified in this crystalline phase with EDX which might be oxynitrides of.Si and Zr precipitated from liquid phase during cooling after hot-pressing. Minor intergranular glassy phase is observed at few triple regions,but is not found in most of grain boundaries as shown in lattice images in Fig.6. In specimens ZAN hot-pressed at 1650C,different superlattice structures in 39

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