S. Liet al. Journal of the European Ceramic Society 21(2001)841-845 exponent for the material is 1.69. It is usually considered The microstructure of the materials observed by that a number of deformation mechanisms, depending scanning electron microscopy(SEM) was illustrated in on the presence of grain boundaries, account for the creep Fig. 4. It is observed that in the less glassy phase in behavior. I These include diffusion creep processes, grain Si3 n4 grains, the cavities and voids were less on the boundary sliding controlled by the viscosity of the grain boundary in Si3 N4/BN fibrous monoliths, which impurity phases at crystal boundaries, dissolution and could ascribe to the bn layers. The Bn layers have redeposition of material and transfer of the viscous pha- capacity to absorb the glass and improve the creep ses from boundaries under compression to those under resistance of the materials, but, as is well known, BN is tension. But the structure of Si3 N4 fibrous monoliths was unstable above 900 C, so the conclusion could be made unique and obviously different from Si3 N4 monoliths. that bN will lose that capacity. In order to examine the Fibrous monoliths consisted of"soft"and"hard"pha- change in BN, we analyzed the XRD results obtained In Si3 N,/BN fibrous monolithic ceramic, the"soft" from the specimens after creep tests at high phases were BN and the "hard"phases were Si3 N4. The under 400 MPa. It was found that Bn stayed intact glassy phase was present in the boron nitride and the less until 1200oC, so the bn interface improved the resis glassy phase was present in the silicon nitride grains tance to oxidation in the fibrous monoliths. At 1200oC, within the cells of fibrous monoliths as compared to sill n began to oxidise and formed B2O3. This reacted con nitride grains in a monolithic specimen. 2, 13 There- with Al2O3 and resulted in 2Al2O3B2O fore, the Si3 N4/BN fibrous monoliths possess a low stress ponent and high creep resistance. 4BN(s)+302→2B2O3()+2N2(g) 2AlO3+B2O3- 2Al2O3.B,O 3. 2. Microstructure observations and analysis The 2Al2O3 B2O3 formed strengthened the combina- There are differences between the Si3N4/BN fibrous tion between BN and Si3 N4. So with the temperature monolithic ceramic and monolithic Si,N4 ceramic in increased, although the creep rate was increased rapidly, structure. The microstructure of Si3 N4/BN fibrous the strength at 1200C did not rapidly decrease monolithic ceramic is shown in Fig. 2(a) and (b). The The fracture surface of composite samples is illu polycrystalline Si3N4 cells and Bn cell boundaries are strated in Fig. 5. Whisker pullout and crack deflection viewed in the hot-pressing direction and normal to the along the interface during fracture were indicated and hot-pressing direction. the traces after the whisker pulled-out were observed. In Si3N4/BN fibrous monolith, a major crack propagated The Si3N4/BN fibrous monolithic ceramic was made by major crack progressed through fibrous cells to the next became aligned such that the major axes were pre interface. Sometimes it extended along the interface ferentially oriented perpendicular to the direction of between fibrous cells. a branch-like crack could be pressing. Whisker orientation affects the strength and observed in Fig. 3. The crack path was tortuous instead fracture toughness of this material; therefore, its effect of line shaped. The branch-like crack makes the on creep deformation vestigated. 4 In the case of surface irregular and the surface area is increased. whisker-oriented alignment, most of the whiskers were Therefore, it consumes more energy, which benefits perpendicular to the crack plane, i.e. parallel to the creep resistance. direction of stress, so that whiskers can effectively 3456 85 8s° ig. 2. The microstructure of Si3 N4/BN fibrous monolith (a) Viewed in hot-pressing direction.(b) Viewed normal to the hot-pressing1       (73B7 F  8            !  #     #    !       7(( /   :    ! #   8   #    8     8    8   8   !                                    7                  8  :        7 ;           LL MM  LLMM  7 F           !  LL MM     LLMM   7 / #8              #8             #                         #          7(+!( /  !               1    #   7 /-,-     "    /   :                          7 /                    ; #7 +<=  <=7 /  88                  #           #   7 F        !  H   #  #       7     H   #  #      1 7     1  #       7 .         ; #7 7 /           7 /       #      7 / !     #8!      7 /         8  #      8 <94=    ; #7 7 F       #8     # !           #   8         !          87 /  8   8     #           ! !     !      B,,

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