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Nanostructure defects in Si3N4
Volume 4, number
IO
MATERIALS
NANOSTRUCTURE
LETTERS
September
I986
DEFECTS IN Si3N,
Shulin WEN and Jingwei FENG Shunghar
Institute
qf Ceratnm.
:lcadetniu
Sinicu,
865 Chang-ning
Road, Shanghai
200050,
China
Received 28 July 1986
Microstructure defect studies of Si3N, have contributed much to the development of SiiNl ceramics material. In this paper we present some results of nanostructure defects by using high-resolution electron microscopy (HREM ). showing new structural phenomena at the atomic level
1. Introduction
3. Results and discussion
The investigations of the microstructure and their defects of Si,NJ ceramics are very impo~ant for their close relationships with high-temperature engineering applications. The structure defects such as grain boundaries and interfaces have been studied a great deal [ l-61. Among these studies, it was shown that the crystallization of the glassy phase in the grain boundaries improved mechanical properties [ 51. Moreover, during the observation of structure defects by HREM we have shown recently that a new kind of structure defects (nanocrack, amorphous nanodomain, superstructure nanodomain, nanodomain boundary and spot defects) exists in the Si,NJ grains. We suggested that these defects may be named as nanostructure defects as their sizes are usually in the nano-scale.
3.1. Nanocrack Some kinds of cracks in a-Si,N, grain as shown at ab and cd in fig. 1 were found. As these cracks were as narrow as in the order of a nanometer. and their lengths were only several nanometers, we suggest the crack to be named as nanocrack. In fig. 1 the nanocrack ab seems to be in irregular steps along the (00 1) zone in the grain; in contrast to ab, nanocrack cd has much bigger steps. The darker image contrast around the cracks ab and cd was caused by the stress in these areas. The deformation of the unit cell at the area bc could be easily seen.
2. Experimental The samples* were prepared by hot pressing at 1650°C for 1 min with MgO as additive. The specimens for EM observations were prepared by ion milling after they had been cut and polished properly at right thicknesses and sizes. A JEM JEOL 200CX electron microscope was used for the HREM observations. Fig. cd.
* Kindly prepared by Dr. H.R. Zhuang and Dr. W.L. Li. 420
I Lattice image of a-SilN, grain showing nanocracks
ab and
167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0
Volume
4.
numberIO
MATEKI.?LS
F1g.2. Lattice image of a-Si,N, domains.&,. A, and A,.
grain showing
amorphous
I986
nano-
Under certain conditions. the nanocracks may widen and extend and become microcracks under stress. And these kinds of microcracks probably lead to trans-crystalline fracture. Structurally these nanocracks are caused by special structural defects. In the nanocrack cd, four Si-N layers along the (00 1) plane near area c are missing ; when eight 5-N layers are missed, the crack obtained a step. Therefore the nanocrack is not straight. 3.2. Amorphous
Scmmb~r
LE.TTEKS
Fig.3. Lattice image of a-Si,N, nodomain S.
grain showmg superstructure
na-
structure nanodomain with d, spacing being 3 times the d, ,~,~spacing. The reason of forming a superstructure domain in a-SiiN, is not clear at the moment. However, if the
nanodomain
During the imaging of a-Si,N, grains along the (00 1) zone, amorphous domains as shown at A,, A2 and A, in fig. 2 were observed. The amorphous nanodomains were usually very small. about the size of 10 nm in diameter. And the image contrast of amorphous nanodomains was brighter than that of the normal crystalline phase marked by C. It seems to be difficult to think that some isolated amorphous domains exist in a continuing crystalline phase. However. if the radiation effects both from electrons and Ar ions are considered [ 61, its existence is plausible. 3.3. Superstructure
nanodomain
Fig. 3 shows a superstructure nanodomain. Here N presents the normal area of Si3N, grain with ad,,,,, spacing of 0.658 nm: and S represents the super-
Fig.4. Structure ary AB.
image of ~-SI,N,
showing
nanodomain
bound-
421
Volume 4 number
IO
MATERIALS
September
LETTERS
1986
fact that impurity ions, such as Mg’+ and Cal+ can enter the a-Si3N, structure during a-Si3N, sintering [6] is considered, the formation of some kind of superstructure in the processing may be possible. 3.4. Nanodomain
boundary
In some P-Si3N4 grains, the nanodomain boundary as shown at AB of fig. 4 has been observed. We suggest a structure model of the boundary to match the structure image along the (00 1) zone, showing the atomic arrangement around the boundary area. In this model, big circles represent Si atoms and small ones represent N atoms; the black circles show first layer and open circles show second layer. Normally there are 6 Si atoms around the structure hole (bright spot) on (00 1) projection in P-Si3N4. However, in the boundary area there are only 5 Si atoms around the structure hole. Besides, in the boundary structure holes seem to be smaller and become very near in ( 100) direction. 3.5. Spot defects A spot defect as shown by the arrow in fig. 5 has been observed. In fig. 5, a structure image of P-SijN4 on (001) direction has been shown. In the normal area of the structure image, 6 Si atoms distribute regularly along a structure hole (bright spot) as shown in the right part of fig. 5. However, in the left part of fig. 5, the fact that 6 Si atoms distribute irregularly along a structure hole was revealed (see arrow). This defect may be probably caused by deformation of Si-N tetrahedra in Si3N4, as sometimes such a spot defect could be moved by electron beam irradiation according to our observation.
422
Fig.5. Structure
image of B-SilN, showing spot defect (arrow).
Acknowledgement We are grateful for the support of the Chinese Science Foundation and the Science Foundation of the Chinese Academy of Science.
References [ I ] D.R. Clarke, J. Am. Ceram. Sot. 62 (1979) 236. [2] O.L. Krivanek, T.M. Shaw and G. Thomas, J. Am. Ceram. Sot. 62 (1979) 585. [ 31 K. Hiraga, K. Tsuno and D. Shindo, Phil. Mag. A4 (1983) 483. [4] S. Wen, D.A. Jefferson and J.M. Thomas, J. Chinese Silicate Sot. I(1982) I. [ 51 S. Wen and J. Feng, Sci. Bull. 3 (1985) 184. [ 61 S. Wen, J. Feng and D. Yan. Mat. Letters 3 (1984) 15.
IO
MATERIALS
NANOSTRUCTURE
LETTERS
September
I986
DEFECTS IN Si3N,
Shulin WEN and Jingwei FENG Shunghar
Institute
qf Ceratnm.
:lcadetniu
Sinicu,
865 Chang-ning
Road, Shanghai
200050,
China
Received 28 July 1986
Microstructure defect studies of Si3N, have contributed much to the development of SiiNl ceramics material. In this paper we present some results of nanostructure defects by using high-resolution electron microscopy (HREM ). showing new structural phenomena at the atomic level
1. Introduction
3. Results and discussion
The investigations of the microstructure and their defects of Si,NJ ceramics are very impo~ant for their close relationships with high-temperature engineering applications. The structure defects such as grain boundaries and interfaces have been studied a great deal [ l-61. Among these studies, it was shown that the crystallization of the glassy phase in the grain boundaries improved mechanical properties [ 51. Moreover, during the observation of structure defects by HREM we have shown recently that a new kind of structure defects (nanocrack, amorphous nanodomain, superstructure nanodomain, nanodomain boundary and spot defects) exists in the Si,NJ grains. We suggested that these defects may be named as nanostructure defects as their sizes are usually in the nano-scale.
3.1. Nanocrack Some kinds of cracks in a-Si,N, grain as shown at ab and cd in fig. 1 were found. As these cracks were as narrow as in the order of a nanometer. and their lengths were only several nanometers, we suggest the crack to be named as nanocrack. In fig. 1 the nanocrack ab seems to be in irregular steps along the (00 1) zone in the grain; in contrast to ab, nanocrack cd has much bigger steps. The darker image contrast around the cracks ab and cd was caused by the stress in these areas. The deformation of the unit cell at the area bc could be easily seen.
2. Experimental The samples* were prepared by hot pressing at 1650°C for 1 min with MgO as additive. The specimens for EM observations were prepared by ion milling after they had been cut and polished properly at right thicknesses and sizes. A JEM JEOL 200CX electron microscope was used for the HREM observations. Fig. cd.
* Kindly prepared by Dr. H.R. Zhuang and Dr. W.L. Li. 420
I Lattice image of a-SilN, grain showing nanocracks
ab and
167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
0
Volume
4.
numberIO
MATEKI.?LS
F1g.2. Lattice image of a-Si,N, domains.&,. A, and A,.
grain showing
amorphous
I986
nano-
Under certain conditions. the nanocracks may widen and extend and become microcracks under stress. And these kinds of microcracks probably lead to trans-crystalline fracture. Structurally these nanocracks are caused by special structural defects. In the nanocrack cd, four Si-N layers along the (00 1) plane near area c are missing ; when eight 5-N layers are missed, the crack obtained a step. Therefore the nanocrack is not straight. 3.2. Amorphous
Scmmb~r
LE.TTEKS
Fig.3. Lattice image of a-Si,N, nodomain S.
grain showmg superstructure
na-
structure nanodomain with d, spacing being 3 times the d, ,~,~spacing. The reason of forming a superstructure domain in a-SiiN, is not clear at the moment. However, if the
nanodomain
During the imaging of a-Si,N, grains along the (00 1) zone, amorphous domains as shown at A,, A2 and A, in fig. 2 were observed. The amorphous nanodomains were usually very small. about the size of 10 nm in diameter. And the image contrast of amorphous nanodomains was brighter than that of the normal crystalline phase marked by C. It seems to be difficult to think that some isolated amorphous domains exist in a continuing crystalline phase. However. if the radiation effects both from electrons and Ar ions are considered [ 61, its existence is plausible. 3.3. Superstructure
nanodomain
Fig. 3 shows a superstructure nanodomain. Here N presents the normal area of Si3N, grain with ad,,,,, spacing of 0.658 nm: and S represents the super-
Fig.4. Structure ary AB.
image of ~-SI,N,
showing
nanodomain
bound-
421
Volume 4 number
IO
MATERIALS
September
LETTERS
1986
fact that impurity ions, such as Mg’+ and Cal+ can enter the a-Si3N, structure during a-Si3N, sintering [6] is considered, the formation of some kind of superstructure in the processing may be possible. 3.4. Nanodomain
boundary
In some P-Si3N4 grains, the nanodomain boundary as shown at AB of fig. 4 has been observed. We suggest a structure model of the boundary to match the structure image along the (00 1) zone, showing the atomic arrangement around the boundary area. In this model, big circles represent Si atoms and small ones represent N atoms; the black circles show first layer and open circles show second layer. Normally there are 6 Si atoms around the structure hole (bright spot) on (00 1) projection in P-Si3N4. However, in the boundary area there are only 5 Si atoms around the structure hole. Besides, in the boundary structure holes seem to be smaller and become very near in ( 100) direction. 3.5. Spot defects A spot defect as shown by the arrow in fig. 5 has been observed. In fig. 5, a structure image of P-SijN4 on (001) direction has been shown. In the normal area of the structure image, 6 Si atoms distribute regularly along a structure hole (bright spot) as shown in the right part of fig. 5. However, in the left part of fig. 5, the fact that 6 Si atoms distribute irregularly along a structure hole was revealed (see arrow). This defect may be probably caused by deformation of Si-N tetrahedra in Si3N4, as sometimes such a spot defect could be moved by electron beam irradiation according to our observation.
422
Fig.5. Structure
image of B-SilN, showing spot defect (arrow).
Acknowledgement We are grateful for the support of the Chinese Science Foundation and the Science Foundation of the Chinese Academy of Science.
References [ I ] D.R. Clarke, J. Am. Ceram. Sot. 62 (1979) 236. [2] O.L. Krivanek, T.M. Shaw and G. Thomas, J. Am. Ceram. Sot. 62 (1979) 585. [ 31 K. Hiraga, K. Tsuno and D. Shindo, Phil. Mag. A4 (1983) 483. [4] S. Wen, D.A. Jefferson and J.M. Thomas, J. Chinese Silicate Sot. I(1982) I. [ 51 S. Wen and J. Feng, Sci. Bull. 3 (1985) 184. [ 61 S. Wen, J. Feng and D. Yan. Mat. Letters 3 (1984) 15.