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Microstructural analysis of reaction-bonded silicon nitride
M E T A L L O G R A P H Y 9~ 427-446 (1976)
427
Microstructural Analysis of Reaction-Bonded Silicon Nitride
H. M. JENNINGS,* S. C. DANFORTH, AND M. H. RICHMAN Division of Engineering, Brown University, Providence, Rhode Island
Reaction-bonded silicon nitride contains at least two different crystallographic forms of Si3N4 and several different microconstituents and morphologies. Application of a comprehensive study using x-ray and electron diffraction, electron microprobe analysis, optical metallography, and scanning and transmission electron microscopy has allowed a complete microstructural characterization to be achieved.
Introduction With the recent a d v e n t of worldwide fuel shortages, it is increasingly evident t h a t more efficient use m u s t be made of our available fuel reserves. One way of achieving this goal is to increase the efficiency of turbine engines for propulsion as well as for power generation. This increased efficiency m a y be gained b y increasing the operating t e m p e r a t u r e b y several hundred degrees Celsius. Unfortunately this requires replacement materials for the superalloys currently in use and those of the next generation. Ceramic materials h a v e become v e r y i m p o r t a n t as replacements for superalloys in rotors, stators, heat exchangers, combustion chambers, etc. of gas turbine engines. T h e y have the ability to withstand high stresses at high temperatures, thermal shock, and the corrosive and erosive effect of the combustion gases. T w o ceramics stand out in this regard--silicon carbide and silicon nitride. Silicon carbide and silicon nitride are available in the hot-pressed form, but the silicon nitride is more easily fabricated and has better high-temperature properties if it is reaction bonded. This paper reports on the microstructures and morphologies developed in SigN4 fabricated b y reacting a This work was supported by the U.S. Army Research Office under Grant DA-ARO-D31-124-72-G183 (Project No. 10618-MC). * Presently at University of Cape Town, South Africa. © American Elsevier Publishing Company, Inc., 1976
428
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green compact of silicon at high temperature in a nitrogen atmosphere. While other researchers have attacked various aspects of the problem, this study represents a comprehensive investigation utilizing the combination of x-ray and electron diffraction, electron microbeam probe analysis, optical metallography, and scanning and transmission electron microscopy to characterize fully the microstructural and morphological development of silicon nitride reaction bonded under a variety of processing parameters. Review of Previous Work
There are several analytical techniques available for the microstructural characterization of reaction bonded Si3N4: optical microscopy, transmission and scanning electron microscopy, x-ray and electron diffraction, and electron microbeam probe analysis. Messier and Wong [-1, 2], Atkinson et al. [-3J, and Evans and Sharpe [-4] have all presented optical work that has characterized the microstructure of Si3N4 as a function of processing parameters. Unreacted silicon appears as large angular white grains. The reacted Si3N4 forms as a grey matrix which reacts with the finer particles first and then with the edges of the larger particles of unreacted silicon growing in toward their centers. While this type of characterization is well documented, there is no clear distinction between the morphologically different a and ~ forms and no previous work traces their formation from early to later stages. Evans and Sharpe [-4, 5] have conducted some transmission electron microscopy of ion beam milled samples of reaction-bonded material and using the Harwell million-volt electron microscope have studied the dislocation structure in the ~ grains and the nature of the a needles. Danforth and Richman [-6] have also employed transmission electron microscopy but with a 100-kV electron microscope. They have confirmed the findings of Evans and Sharpe relative to the size (1-5 ~m) of the ~ grains and the high dislocation density common to them. They have also corroborated the picture of some a needles consisting of a crystalline core surrounded by an amorphous sheath. In addition, Danforth and Richman have found that certain a needles exhibit helical markings which can be characterized as impurity bands and that the a grains which comprise the a-matte morphological form are relatively free of dislocations due to their small size and the mechanism of formation. Scanning electron microscopy by Barnby and Taylor [-7] has shown that the ~-SisN~ forms large angular grains which fracture intergranularly while a-SisN4 forms a very fine-grained matte which tends to fracture with a fairly even textured surface. Needles of a are also evident in pores which were once occupied by silicon which has melted out.
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Messier and Wong E2J have reported the utilization of electron microbeam probe analysis to determine the concentration and distribution of impurities in the reaction-bonded silicon nitride.
Experimental Procedure Reaction bonding of silicon nitride requires the preparation of a green compact of silicon which is then processed through a definite time-temperature cycle in an atmosphere containing nitrogen. Two types of silicon powder were used in this study: 200 mesh 99% purity and - 4 0 0 mesh 99.99% purity. Both were purchased from Electronic Space Products (Los Angeles, Calif). The powder was maintained and handled in dry boxes filled with a dry nitrogen atmosphere. A split steel die was used to form a green powder compact 2.54 cm in diameter. The powder and binder, if any (e.g., acetone), was subjected to 4300-6400 psi pressure. The green compact was then placed in a vacuum dissicator to draw off any volatile matter. An average green density was 1.5 g/cm 3. The reaction bonding or nitridation process was carried out in a vertical tube furnace heated with a silicon carbide element and utilizing a recrystallized alumina muffle. The sample was supported on a block of silicon nitride and the nitrogen atmosphere was maintained at a positive pressure inside the furnace. Nitridation temperatures ranged from 1200 to 1600°C for times from several hours to several days. The rate of heating and cooling were automatically controlled and varied as desired. The reaction occuring is 3 Si + 2 N2(g) --~ Si3N4 After nitridation, the density of the briquette was determined by weighing the sample and measuring its dimensions. If complete nitridation has occurred, an 8.00 g silicon compact of density 1.50 g/cm 3 would react to form a Si3N4 sample weighing 13.28 g with a density of 2.49 g/cm 3. This sample would contain porosity, of course, since the theoretical density of silicon nitride is 3.18 g/cm a. Samples are cut from the reacted briquette by use of a diamond saw on a micromatic wafering machine for the following tests: (1) X-ray diffraction to determine the relative amounts of a and Si3N4 unreacted silicon and any other impurity phases. (2) Mechanical testing and scanning electron microscopy of the fracture surfaces. (3) Transmission electron microscopy after ion beam milling. (4) Optical metallography.
H.M. Jennings et al.
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For the x-ray diffraction analysis, the slice was ground in a mortar and pestle and examined in a General Electric x-ray diffraction unit using nickel filtered Cu K , radiation and in a Philips x-ray diffraction unit using a graphite crystal monochromator. By comparing a and t~ peaks, the relative amounts of the two phases could be determined. The existence of silicon or impurity lines would indicate unreacted silicon or impurity phases in the sample. Specimens were broken in three-point bending and the fracture surfaces coated with 100-200 /~ of gold. They were then examined in an ARLEMX-SEM. Each sample for transmission electron microscopy was hand thinned and finally ion-beam milled following the technique described by Danforth and Richman [-6] to a thickness of 500/~. These specimens were then examined on a JEM-7 electron microscope at 100 kV using a tilting stage. Specimens for optical metallography were prepared by mounting in bakelite and hand polishing through a sequence of 320, 600, and 600 soft papers. Two stages of vibromet polishing followed: 6 hr using a napless lap and 0.3 ~m alumina followed by nearly 4 hr using a microcloth lap and 0.06 t~m alumina. Care had to be exercised during the final stage because if the samples were polished too long, a brown film due to differences in electropotential tended to form on the surface. Once polished the samples were ultrasonically cleaned and examined in bright field illumination (no etching was required.). Microstructural Characterization
Reaction-bonded silicon nitride manifests itself in two crystallographic forms, a and f~ Si3N4, and in three basic morphologies, a needles, a matte, and ~ spikes. There are also ladders and ~ grains as well as a wurzite form of silicon which have been observed. This section on microstructural characterization is subdivided as follows: (1) formation of a needles, (2) early stages of ~ formation, (3) t~ spikes (4) early formation of a matte, (5) late stages of f~ formation, (6) late stages of a matte formation, (7) formation of and/~ together, and (8) effect of processing parameters on microstructure. 1.
FORMATION OF a N E E D L E S
Long thin needles formed during the very early low-temperature stages of reaction. Samples that were reacted only slightly--with small percentages of a---exhibited high aspect ratio needles. The needles always grew into pores and so no optical micrographs were possible. A scanning electron
Study of Silicon Nitride
431
micrograph showing these high aspect ratio needles growing into a pore is shown in Fig. 1. On some of the needles there is a small bead at the free end indicating the possibility of growth by a VLS mechanism. A transmission electron micrograph (Fig. 2) shows the various types of needles found. At the lower left are two needles with beads at the free ends. The center two needles exhibit the crystalline core and amorphous sheath first detected by Evans and Sharpe [-4, 57 and the needle in the upper right exhibits helical markings felt to be impurity bands [-6J that segregate as the vapor-solid interface progresses helically down the axis of the needle.
2. EARLY STAGES OF ~ FORMATION The f~ phase of Si3N4 forms as very prismatic but low aspect ratio spikes that grow from the surface of a grain of unreacted silicon in towards the center at a very high rate of growth. (The high rate is due to the ease of diffusion of nitrogen down the tunnels which exist in the B structure.) A typical optical micrograph of this stage in ~ formation is presented in
FIG. 1. Scanning electron micrograph: High aspect ratio a-needles growing into a pore from grains of unreacted silicon.
C
B
8 .5"2" FIG. 2. Transmission electron micrograph: A a-needle with beads, B a-needles with cores, C ~needle with impurity bands.
FIG. 3. Optical micrograph: Early stages of fi -- SisN4, A fi, B unreacted silicon, C porosity, L "ladders." 432
Study of Silicon Nitride
433
Fig. 3 which reveals the/~ as the dark grey regions inside a large white grain of silicon. The reason that there appears to be non-spike-like grains of/3 in the interior is due to the plane of section of the metallographic sample. The black region at the left in Fig. 3 is a pore which was either originally present in the green compact or resulted from the melting out of silicon due to the high heating rates and temperatures used to promote this type of structure. Also included in this micrograph are several ladders. These are the alternating layers of /3 silicon nitride and unreacted silicon which are thought to arise due to the large compressive stresses generated ahead of the growing ~ spike due to the 22% volume increase on nitridation. This compression has been postulated by Danforth, Jennings, and Richman [-8] to cause the transformation of the silicon to a more dense wurzite form reported by Jennings and Richman [-9-]. 3.
~ SPIKES
A region with many angular /~ spikes growing into a large grain of unreacted silicon is shown in Fig. 4 and the growth and coalescence of these
FIG. 4. Optical micrograph: Gray angular /~-spikes growing into unreacted silicon grain (white).
FIG. 5. Optical micrograph: Gray E-spikes coalesce to form dense t~-matrix.
FIG. 6. Scanning electron micrograph: B-spikes growing into what was previously a silicon grain. 434
Study of Silicon Nitride
435
spikes into a dense ~ matrix is illustrated in Fig. 5. A scanning electron micrograph of these large/~ spikes growing into what was formerly a grain of silicon is presented in Figs. 6 and 7 while Fig. 8 shows a spike protruding out of a grain of unreacted silicon. 4.
E A R L Y S T A G E S OF a F O R M A T I O N
The a-Si3N4 forms with a very different morphology from the ~. The early stages of a formation are represented in Fig. 9. The a has a fine texture because of which it is called matte. This microconstituent forms a relatively even boundary around the unreacted silicon grains and grows inwards toward their centers. In the early stages, the smallest grains of silicon are the ones to react first and the p r o d u c t - - t h e m a t t e - - i s often found around the larger grains particularly in areas where the smaller and larger grains were contiguous. As the reaction proceeds, the smaller grains are transformed and as the front of a matte advances inwards toward the centers of the larger grains a great deal of fine porosity can be seen (Fig. 9) ahead of the advancing front. This was observed in all specimens of high a content and tends to support
FIG. 7. Scanning electron micrograph: t~-spikes growing into what was previously a silicon grain.
FIG. 8. Scanning electron micrograph: ~-spike Marked A, protruding out of an unreacted silicon grain.
FIG. 9. Optical micrograph: Textured gray areas are a-matte, white areas are tmreacted silicon particles, and black areas are porosity. 436
Study of Silicon Nitride
437
the mechanism proposed by Atkinson et al [-3]. This mechanism involves the formation of a layer of nitride around a silicon grain and the subsequent diffusion of silicon through the layer to form the nitride (probably by volatilization) in the spaces in the compact originally void of solid material. As a consequence, vacancies are left behind in the unreacted silicon and these combine to form pores which fill with new nitrogen. New nitride can then form in the pores as the surrounding silicon volatilizes. Scanning electron microscopy of the boundary between the a matte and the unreacted silicon reveal the presence of this porosity very clearly (Fig. 10). In this micrograph the a matte is at the bottom and the unreacted silicon at the top. This is in marked contrast to the lack of porosity ahead of the growing ~ spikes and grains. 5.
LATE STAGES OF f~ FORMATION
As the reaction to form ~-Si3N4 goes to completion, the ~ spikes penetrate further into the unreacted silicon grains and eventually they meet to form
FIG. 10. Scanning electron micrograph: Boundary between regions of a-matte A, and unreacted silicon B; note dark porosity at boundary.
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438
grain boundaries and, thereby, a dense matrix of f~. As the spikes meet they will tend, depending on their relative orientations, to isolate small regions of unreacted silicon. This sequence is illustrated in the optical micrographs of Figs. 11, 12, and 13. A scanning electron micrograph of these large ~ spikes growing together to form the granular matrix is shown in Fig. 14. This view is made possible by raising the temperature during the reaction bonding process well above the temperature of fusion of silicon ( > 1500°C) causing it to melt out and form cavities into which the ~ spikes have penetrated. Transmission electron microscopy of the ~ microconstituent shows grains that are quite large (1-5 ~m), many of which contain dislocations. This is illustrated in Figs. 15 and 16. The first of these contains the start of a dislocation tangle while the second shows a large set of dislocations moving from lower left to upper right on parallel slip planes. 6.
L A T E R STAGES OF a FORMATION
When the nitridation conditions are optimized to produce a-Si3N4, the reaction can be taken to over 99% completion without the addition of any
FIG. 11. Optical micrograph: Gray B-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity.
FIG. 12. Optical micrograph: Gray E-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity.
FIG. 13. Optical micrograph: Gray E-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity. 439
FIG. 14. Scanning electron micrograph: Large B-spikes growing together to form a granular matrix. f
s t I n
R
FIG. 15. Transmission electron micrograph: Large E-grains with start of dislocation tangle. 440
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441
z'
~8
Fro. 16. density.
Transmission electron micrograph: Large /~-grains with high dislocation
nitridation aids. As was shown earlier in Section 4, the a matte grows with a regular boundary and fine porosity in the unreacted silicon grain ahead of the advancing front. Like the f~, the a tends to leave behind small isolated regions of unreacted silicon as the reaction nears completion; with the a,, however, there is a large amount of porosity in these remaining silicon grains. A sample reacted 99% to a is shown in Fig. 17. A scanning electron micrograph of the fracture surface of this sample shows the a matte with its very fine grain size (Fig. 18) and this is further revealed in the transmission electron micrograph of Fig. 19. Here the a grains are <0.5 gm and are mostly free of dislocations. This is again a result of the formation mechanism which would predict fine, relatively unstressed grains and many grain boundaries as sinks for dislocations as opposed to the highly stressed large grains of the fl phase. 7.
FORMATION OF a AND /~ TOGETHER
Under all but the most extreme nitridation conditions, a and ~ will form in the same sample. This may be seen in Fig. 20. The ~ tends to surround
FIG. 17. Optical micrograph: Late stages of a-matte, white areas are unreacted silicon and dark areas are residual porosity.
Fro. 18. Scanning electron rmcrograph: Fine grained a-matte. 442
,1
T~
i ~i~¸
/ 4~
............
~
f
FIG. 19. Transmission electron micrograph: a - m a t t e , note the small and almost dislocation-free grains of a.
FIG. 20. Optical micrograph: a a n d B -- Si3N4 together, note t h a t the gray angular B-grains s u r r o u n d the unreacted silicon while the a s u r r o u n d s the B. 443
444
H.M. Jennings et al.
FIG. 21. Scanning electron micrograph: a-matte in lower left corner and/3-spikes in upper right portion growing together. the grain of unreacted silicon and grow inward in spike form. The a tends to form a layer outside the ~, completely reacting the finer particles first. There is, to date, no definitive evidence t h a t a phase transformation is possible whereby /~ transforms to a or vice versa during the nitridation process. Samples t h a t are well reacted with both a and ~ have microstructures consisting of large grains of fl with a fine dispersion of unreacted silicon grains with a layer of a at the edges of the/~. If the a --* ~ transformation were proceeding, one would expect to see the a-fl boundary advance into the center of the grains as the reaction proceeds to completion. No such evidence has been observed. A scanning electron micrograph of f~ spikes and a matte growing together is presented in Fig. 21 and a transmission electron micrograph of large ~ and small a grains is shown in Fig. 22. 8.
E F F E C T OF P R O C E S S I N G P A R A M E T E R S ON MICROSTRUCTURE
I t has been possible to isolate some of the conditions favorable to the formation of the various microstructural and morphological forms of
Study of Silicon Nitride
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FIG. 22. Transmission electron micrograph: Large fl-grains in center and fine a-grains in upper left,. silicon nitride. P o w d e r size, t e m p e r a t u r e cycle, r a t e of h e a t i n g , gas c o m p o s i tion, an d gas flow r a t e e x e r t a s t r o n g influence on t h e m i c r o s t r u c t u r e . T h o s e t r e n d s a n d c o n d i t i o n s w h i c h e n c o u r a g e c e r t a i n r e a c t i o n s w h i c h lead t o a p a r t i c u l a r m o r p h o l o g y are o u t l i n e d in T a b l e 1. TABLE 1 Processing Parameters Encouraging Specific Morphologies Microconstituent a needles
Time
Temperature
Heating rate Gas flow rate
<24 hr
< 1400°C
300°C/hr
a matte
--
< 1450°C
slow
none
/~ spikes
--
> 1450°C
rapid
fast
> 1450°C
rapid
fast
grains
>5 hr
- - : Evidence indicates variable is not important.
--
Comment primarily less pure samples purer and finer powder less pure and coarser powder
446
H.M. Jennings et al.
Conclusions B y t h e c o m b i n e d a p p l i c a t i o n of diffraction a n d optical a n d electron m e t a l l o g r a p h i c techniques, t h e v a r i o u s c r y s t a l l o g r a p h i c forms a n d microc o n s t i t u e n t s of r e a c t i o n b o n d e d silicon n i t r i d e were characterized.
The authors would like to thank the U.S. Army Research O.l~ce (Durham) for financial support of this research. Particular appreciation is due to Mr. Joseph Fogarty for his technical assistance during this study. References 1. D. R. Messier and P. Wong, Kinetics of formation and mechanical properties of reaction-sintered Si3N~, in Ceramicsfor High Performance Applications, (J. J. Burke, A. E. Gorum, and R. N. Katz, Eds.), Brook Hill, Chestnut Hill, Mass. (1974), Chap. VIII, pp. 181-194. 2. D. R. Messier and P. Wong, Duplex ceramic structures--Interim report No. 1: Kinetics of fabrication of silicon nitride by reaction sintering, AMMRC TR 72-10, Army Materials and Mechanics Research Center, March 1972. 3. A. Atkinson, P. J. Leatt, A. J. Moulson, and E. W. Roberts, A mechanism for the nitridation of silicon powder compacts, J. Mat. Sci. 9~ 981-984 (1974). 4. A.G. Evans and J. V. Sharpe, Microstructural studies on silicon nitride, J. Mat. Sci. 6~ 1292-1302 (1971). 5. A. G. Evans and J. V. Sharpe, Transmission electron microscopy of silicon nitride, in Electron Microscopy and Structure of Materials (G. Thomas, R. M. Fulrath, and R. M. Fisher, Eds.), U. of Calif. Press, Berkeley (1972), pp. 1141-1154. 6. S. C. Danforth and M. H. Richman, Transmission electron microscopy of reactionbonded silicon nitride, Metallography 9(4), 321-332 (1976). 7. J. T. Barnby and R. A. Taylor, The fracture resistance of reaction-sintered silicon nitride, Special Ceramics No. 5 (P. Popper, Ed.), British Ceramic Res. Assoc., London (1973), pp. 311-328. 8. S. C. Danforth, H. M. Jennings, and M. H. Richman, The ladder microconstituent of silicon nitride, MetaUography 9(4), 361-365 (1976). 9. H. M. Jennings and M. H. Richman, A hexagonal (wurzite) form of silicon submitted to Science (to be published).
Received August, 1975
427
Microstructural Analysis of Reaction-Bonded Silicon Nitride
H. M. JENNINGS,* S. C. DANFORTH, AND M. H. RICHMAN Division of Engineering, Brown University, Providence, Rhode Island
Reaction-bonded silicon nitride contains at least two different crystallographic forms of Si3N4 and several different microconstituents and morphologies. Application of a comprehensive study using x-ray and electron diffraction, electron microprobe analysis, optical metallography, and scanning and transmission electron microscopy has allowed a complete microstructural characterization to be achieved.
Introduction With the recent a d v e n t of worldwide fuel shortages, it is increasingly evident t h a t more efficient use m u s t be made of our available fuel reserves. One way of achieving this goal is to increase the efficiency of turbine engines for propulsion as well as for power generation. This increased efficiency m a y be gained b y increasing the operating t e m p e r a t u r e b y several hundred degrees Celsius. Unfortunately this requires replacement materials for the superalloys currently in use and those of the next generation. Ceramic materials h a v e become v e r y i m p o r t a n t as replacements for superalloys in rotors, stators, heat exchangers, combustion chambers, etc. of gas turbine engines. T h e y have the ability to withstand high stresses at high temperatures, thermal shock, and the corrosive and erosive effect of the combustion gases. T w o ceramics stand out in this regard--silicon carbide and silicon nitride. Silicon carbide and silicon nitride are available in the hot-pressed form, but the silicon nitride is more easily fabricated and has better high-temperature properties if it is reaction bonded. This paper reports on the microstructures and morphologies developed in SigN4 fabricated b y reacting a This work was supported by the U.S. Army Research Office under Grant DA-ARO-D31-124-72-G183 (Project No. 10618-MC). * Presently at University of Cape Town, South Africa. © American Elsevier Publishing Company, Inc., 1976
428
H.M. Jennings et al.
green compact of silicon at high temperature in a nitrogen atmosphere. While other researchers have attacked various aspects of the problem, this study represents a comprehensive investigation utilizing the combination of x-ray and electron diffraction, electron microbeam probe analysis, optical metallography, and scanning and transmission electron microscopy to characterize fully the microstructural and morphological development of silicon nitride reaction bonded under a variety of processing parameters. Review of Previous Work
There are several analytical techniques available for the microstructural characterization of reaction bonded Si3N4: optical microscopy, transmission and scanning electron microscopy, x-ray and electron diffraction, and electron microbeam probe analysis. Messier and Wong [-1, 2], Atkinson et al. [-3J, and Evans and Sharpe [-4] have all presented optical work that has characterized the microstructure of Si3N4 as a function of processing parameters. Unreacted silicon appears as large angular white grains. The reacted Si3N4 forms as a grey matrix which reacts with the finer particles first and then with the edges of the larger particles of unreacted silicon growing in toward their centers. While this type of characterization is well documented, there is no clear distinction between the morphologically different a and ~ forms and no previous work traces their formation from early to later stages. Evans and Sharpe [-4, 5] have conducted some transmission electron microscopy of ion beam milled samples of reaction-bonded material and using the Harwell million-volt electron microscope have studied the dislocation structure in the ~ grains and the nature of the a needles. Danforth and Richman [-6] have also employed transmission electron microscopy but with a 100-kV electron microscope. They have confirmed the findings of Evans and Sharpe relative to the size (1-5 ~m) of the ~ grains and the high dislocation density common to them. They have also corroborated the picture of some a needles consisting of a crystalline core surrounded by an amorphous sheath. In addition, Danforth and Richman have found that certain a needles exhibit helical markings which can be characterized as impurity bands and that the a grains which comprise the a-matte morphological form are relatively free of dislocations due to their small size and the mechanism of formation. Scanning electron microscopy by Barnby and Taylor [-7] has shown that the ~-SisN~ forms large angular grains which fracture intergranularly while a-SisN4 forms a very fine-grained matte which tends to fracture with a fairly even textured surface. Needles of a are also evident in pores which were once occupied by silicon which has melted out.
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429
Messier and Wong E2J have reported the utilization of electron microbeam probe analysis to determine the concentration and distribution of impurities in the reaction-bonded silicon nitride.
Experimental Procedure Reaction bonding of silicon nitride requires the preparation of a green compact of silicon which is then processed through a definite time-temperature cycle in an atmosphere containing nitrogen. Two types of silicon powder were used in this study: 200 mesh 99% purity and - 4 0 0 mesh 99.99% purity. Both were purchased from Electronic Space Products (Los Angeles, Calif). The powder was maintained and handled in dry boxes filled with a dry nitrogen atmosphere. A split steel die was used to form a green powder compact 2.54 cm in diameter. The powder and binder, if any (e.g., acetone), was subjected to 4300-6400 psi pressure. The green compact was then placed in a vacuum dissicator to draw off any volatile matter. An average green density was 1.5 g/cm 3. The reaction bonding or nitridation process was carried out in a vertical tube furnace heated with a silicon carbide element and utilizing a recrystallized alumina muffle. The sample was supported on a block of silicon nitride and the nitrogen atmosphere was maintained at a positive pressure inside the furnace. Nitridation temperatures ranged from 1200 to 1600°C for times from several hours to several days. The rate of heating and cooling were automatically controlled and varied as desired. The reaction occuring is 3 Si + 2 N2(g) --~ Si3N4 After nitridation, the density of the briquette was determined by weighing the sample and measuring its dimensions. If complete nitridation has occurred, an 8.00 g silicon compact of density 1.50 g/cm 3 would react to form a Si3N4 sample weighing 13.28 g with a density of 2.49 g/cm 3. This sample would contain porosity, of course, since the theoretical density of silicon nitride is 3.18 g/cm a. Samples are cut from the reacted briquette by use of a diamond saw on a micromatic wafering machine for the following tests: (1) X-ray diffraction to determine the relative amounts of a and Si3N4 unreacted silicon and any other impurity phases. (2) Mechanical testing and scanning electron microscopy of the fracture surfaces. (3) Transmission electron microscopy after ion beam milling. (4) Optical metallography.
H.M. Jennings et al.
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For the x-ray diffraction analysis, the slice was ground in a mortar and pestle and examined in a General Electric x-ray diffraction unit using nickel filtered Cu K , radiation and in a Philips x-ray diffraction unit using a graphite crystal monochromator. By comparing a and t~ peaks, the relative amounts of the two phases could be determined. The existence of silicon or impurity lines would indicate unreacted silicon or impurity phases in the sample. Specimens were broken in three-point bending and the fracture surfaces coated with 100-200 /~ of gold. They were then examined in an ARLEMX-SEM. Each sample for transmission electron microscopy was hand thinned and finally ion-beam milled following the technique described by Danforth and Richman [-6] to a thickness of 500/~. These specimens were then examined on a JEM-7 electron microscope at 100 kV using a tilting stage. Specimens for optical metallography were prepared by mounting in bakelite and hand polishing through a sequence of 320, 600, and 600 soft papers. Two stages of vibromet polishing followed: 6 hr using a napless lap and 0.3 ~m alumina followed by nearly 4 hr using a microcloth lap and 0.06 t~m alumina. Care had to be exercised during the final stage because if the samples were polished too long, a brown film due to differences in electropotential tended to form on the surface. Once polished the samples were ultrasonically cleaned and examined in bright field illumination (no etching was required.). Microstructural Characterization
Reaction-bonded silicon nitride manifests itself in two crystallographic forms, a and f~ Si3N4, and in three basic morphologies, a needles, a matte, and ~ spikes. There are also ladders and ~ grains as well as a wurzite form of silicon which have been observed. This section on microstructural characterization is subdivided as follows: (1) formation of a needles, (2) early stages of ~ formation, (3) t~ spikes (4) early formation of a matte, (5) late stages of f~ formation, (6) late stages of a matte formation, (7) formation of and/~ together, and (8) effect of processing parameters on microstructure. 1.
FORMATION OF a N E E D L E S
Long thin needles formed during the very early low-temperature stages of reaction. Samples that were reacted only slightly--with small percentages of a---exhibited high aspect ratio needles. The needles always grew into pores and so no optical micrographs were possible. A scanning electron
Study of Silicon Nitride
431
micrograph showing these high aspect ratio needles growing into a pore is shown in Fig. 1. On some of the needles there is a small bead at the free end indicating the possibility of growth by a VLS mechanism. A transmission electron micrograph (Fig. 2) shows the various types of needles found. At the lower left are two needles with beads at the free ends. The center two needles exhibit the crystalline core and amorphous sheath first detected by Evans and Sharpe [-4, 57 and the needle in the upper right exhibits helical markings felt to be impurity bands [-6J that segregate as the vapor-solid interface progresses helically down the axis of the needle.
2. EARLY STAGES OF ~ FORMATION The f~ phase of Si3N4 forms as very prismatic but low aspect ratio spikes that grow from the surface of a grain of unreacted silicon in towards the center at a very high rate of growth. (The high rate is due to the ease of diffusion of nitrogen down the tunnels which exist in the B structure.) A typical optical micrograph of this stage in ~ formation is presented in
FIG. 1. Scanning electron micrograph: High aspect ratio a-needles growing into a pore from grains of unreacted silicon.
C
B
8 .5"2" FIG. 2. Transmission electron micrograph: A a-needle with beads, B a-needles with cores, C ~needle with impurity bands.
FIG. 3. Optical micrograph: Early stages of fi -- SisN4, A fi, B unreacted silicon, C porosity, L "ladders." 432
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Fig. 3 which reveals the/~ as the dark grey regions inside a large white grain of silicon. The reason that there appears to be non-spike-like grains of/3 in the interior is due to the plane of section of the metallographic sample. The black region at the left in Fig. 3 is a pore which was either originally present in the green compact or resulted from the melting out of silicon due to the high heating rates and temperatures used to promote this type of structure. Also included in this micrograph are several ladders. These are the alternating layers of /3 silicon nitride and unreacted silicon which are thought to arise due to the large compressive stresses generated ahead of the growing ~ spike due to the 22% volume increase on nitridation. This compression has been postulated by Danforth, Jennings, and Richman [-8] to cause the transformation of the silicon to a more dense wurzite form reported by Jennings and Richman [-9-]. 3.
~ SPIKES
A region with many angular /~ spikes growing into a large grain of unreacted silicon is shown in Fig. 4 and the growth and coalescence of these
FIG. 4. Optical micrograph: Gray angular /~-spikes growing into unreacted silicon grain (white).
FIG. 5. Optical micrograph: Gray E-spikes coalesce to form dense t~-matrix.
FIG. 6. Scanning electron micrograph: B-spikes growing into what was previously a silicon grain. 434
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spikes into a dense ~ matrix is illustrated in Fig. 5. A scanning electron micrograph of these large/~ spikes growing into what was formerly a grain of silicon is presented in Figs. 6 and 7 while Fig. 8 shows a spike protruding out of a grain of unreacted silicon. 4.
E A R L Y S T A G E S OF a F O R M A T I O N
The a-Si3N4 forms with a very different morphology from the ~. The early stages of a formation are represented in Fig. 9. The a has a fine texture because of which it is called matte. This microconstituent forms a relatively even boundary around the unreacted silicon grains and grows inwards toward their centers. In the early stages, the smallest grains of silicon are the ones to react first and the p r o d u c t - - t h e m a t t e - - i s often found around the larger grains particularly in areas where the smaller and larger grains were contiguous. As the reaction proceeds, the smaller grains are transformed and as the front of a matte advances inwards toward the centers of the larger grains a great deal of fine porosity can be seen (Fig. 9) ahead of the advancing front. This was observed in all specimens of high a content and tends to support
FIG. 7. Scanning electron micrograph: t~-spikes growing into what was previously a silicon grain.
FIG. 8. Scanning electron micrograph: ~-spike Marked A, protruding out of an unreacted silicon grain.
FIG. 9. Optical micrograph: Textured gray areas are a-matte, white areas are tmreacted silicon particles, and black areas are porosity. 436
Study of Silicon Nitride
437
the mechanism proposed by Atkinson et al [-3]. This mechanism involves the formation of a layer of nitride around a silicon grain and the subsequent diffusion of silicon through the layer to form the nitride (probably by volatilization) in the spaces in the compact originally void of solid material. As a consequence, vacancies are left behind in the unreacted silicon and these combine to form pores which fill with new nitrogen. New nitride can then form in the pores as the surrounding silicon volatilizes. Scanning electron microscopy of the boundary between the a matte and the unreacted silicon reveal the presence of this porosity very clearly (Fig. 10). In this micrograph the a matte is at the bottom and the unreacted silicon at the top. This is in marked contrast to the lack of porosity ahead of the growing ~ spikes and grains. 5.
LATE STAGES OF f~ FORMATION
As the reaction to form ~-Si3N4 goes to completion, the ~ spikes penetrate further into the unreacted silicon grains and eventually they meet to form
FIG. 10. Scanning electron micrograph: Boundary between regions of a-matte A, and unreacted silicon B; note dark porosity at boundary.
H.M. Jennings et al.
438
grain boundaries and, thereby, a dense matrix of f~. As the spikes meet they will tend, depending on their relative orientations, to isolate small regions of unreacted silicon. This sequence is illustrated in the optical micrographs of Figs. 11, 12, and 13. A scanning electron micrograph of these large ~ spikes growing together to form the granular matrix is shown in Fig. 14. This view is made possible by raising the temperature during the reaction bonding process well above the temperature of fusion of silicon ( > 1500°C) causing it to melt out and form cavities into which the ~ spikes have penetrated. Transmission electron microscopy of the ~ microconstituent shows grains that are quite large (1-5 ~m), many of which contain dislocations. This is illustrated in Figs. 15 and 16. The first of these contains the start of a dislocation tangle while the second shows a large set of dislocations moving from lower left to upper right on parallel slip planes. 6.
L A T E R STAGES OF a FORMATION
When the nitridation conditions are optimized to produce a-Si3N4, the reaction can be taken to over 99% completion without the addition of any
FIG. 11. Optical micrograph: Gray B-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity.
FIG. 12. Optical micrograph: Gray E-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity.
FIG. 13. Optical micrograph: Gray E-spikes coalescing, leaving behind white areas of unreacted silicon, black areas are porosity. 439
FIG. 14. Scanning electron micrograph: Large B-spikes growing together to form a granular matrix. f
s t I n
R
FIG. 15. Transmission electron micrograph: Large E-grains with start of dislocation tangle. 440
Study of Silicon Nitride
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z'
~8
Fro. 16. density.
Transmission electron micrograph: Large /~-grains with high dislocation
nitridation aids. As was shown earlier in Section 4, the a matte grows with a regular boundary and fine porosity in the unreacted silicon grain ahead of the advancing front. Like the f~, the a tends to leave behind small isolated regions of unreacted silicon as the reaction nears completion; with the a,, however, there is a large amount of porosity in these remaining silicon grains. A sample reacted 99% to a is shown in Fig. 17. A scanning electron micrograph of the fracture surface of this sample shows the a matte with its very fine grain size (Fig. 18) and this is further revealed in the transmission electron micrograph of Fig. 19. Here the a grains are <0.5 gm and are mostly free of dislocations. This is again a result of the formation mechanism which would predict fine, relatively unstressed grains and many grain boundaries as sinks for dislocations as opposed to the highly stressed large grains of the fl phase. 7.
FORMATION OF a AND /~ TOGETHER
Under all but the most extreme nitridation conditions, a and ~ will form in the same sample. This may be seen in Fig. 20. The ~ tends to surround
FIG. 17. Optical micrograph: Late stages of a-matte, white areas are unreacted silicon and dark areas are residual porosity.
Fro. 18. Scanning electron rmcrograph: Fine grained a-matte. 442
,1
T~
i ~i~¸
/ 4~
............
~
f
FIG. 19. Transmission electron micrograph: a - m a t t e , note the small and almost dislocation-free grains of a.
FIG. 20. Optical micrograph: a a n d B -- Si3N4 together, note t h a t the gray angular B-grains s u r r o u n d the unreacted silicon while the a s u r r o u n d s the B. 443
444
H.M. Jennings et al.
FIG. 21. Scanning electron micrograph: a-matte in lower left corner and/3-spikes in upper right portion growing together. the grain of unreacted silicon and grow inward in spike form. The a tends to form a layer outside the ~, completely reacting the finer particles first. There is, to date, no definitive evidence t h a t a phase transformation is possible whereby /~ transforms to a or vice versa during the nitridation process. Samples t h a t are well reacted with both a and ~ have microstructures consisting of large grains of fl with a fine dispersion of unreacted silicon grains with a layer of a at the edges of the/~. If the a --* ~ transformation were proceeding, one would expect to see the a-fl boundary advance into the center of the grains as the reaction proceeds to completion. No such evidence has been observed. A scanning electron micrograph of f~ spikes and a matte growing together is presented in Fig. 21 and a transmission electron micrograph of large ~ and small a grains is shown in Fig. 22. 8.
E F F E C T OF P R O C E S S I N G P A R A M E T E R S ON MICROSTRUCTURE
I t has been possible to isolate some of the conditions favorable to the formation of the various microstructural and morphological forms of
Study of Silicon Nitride
445
FIG. 22. Transmission electron micrograph: Large fl-grains in center and fine a-grains in upper left,. silicon nitride. P o w d e r size, t e m p e r a t u r e cycle, r a t e of h e a t i n g , gas c o m p o s i tion, an d gas flow r a t e e x e r t a s t r o n g influence on t h e m i c r o s t r u c t u r e . T h o s e t r e n d s a n d c o n d i t i o n s w h i c h e n c o u r a g e c e r t a i n r e a c t i o n s w h i c h lead t o a p a r t i c u l a r m o r p h o l o g y are o u t l i n e d in T a b l e 1. TABLE 1 Processing Parameters Encouraging Specific Morphologies Microconstituent a needles
Time
Temperature
Heating rate Gas flow rate
<24 hr
< 1400°C
300°C/hr
a matte
--
< 1450°C
slow
none
/~ spikes
--
> 1450°C
rapid
fast
> 1450°C
rapid
fast
grains
>5 hr
- - : Evidence indicates variable is not important.
--
Comment primarily less pure samples purer and finer powder less pure and coarser powder
446
H.M. Jennings et al.
Conclusions B y t h e c o m b i n e d a p p l i c a t i o n of diffraction a n d optical a n d electron m e t a l l o g r a p h i c techniques, t h e v a r i o u s c r y s t a l l o g r a p h i c forms a n d microc o n s t i t u e n t s of r e a c t i o n b o n d e d silicon n i t r i d e were characterized.
The authors would like to thank the U.S. Army Research O.l~ce (Durham) for financial support of this research. Particular appreciation is due to Mr. Joseph Fogarty for his technical assistance during this study. References 1. D. R. Messier and P. Wong, Kinetics of formation and mechanical properties of reaction-sintered Si3N~, in Ceramicsfor High Performance Applications, (J. J. Burke, A. E. Gorum, and R. N. Katz, Eds.), Brook Hill, Chestnut Hill, Mass. (1974), Chap. VIII, pp. 181-194. 2. D. R. Messier and P. Wong, Duplex ceramic structures--Interim report No. 1: Kinetics of fabrication of silicon nitride by reaction sintering, AMMRC TR 72-10, Army Materials and Mechanics Research Center, March 1972. 3. A. Atkinson, P. J. Leatt, A. J. Moulson, and E. W. Roberts, A mechanism for the nitridation of silicon powder compacts, J. Mat. Sci. 9~ 981-984 (1974). 4. A.G. Evans and J. V. Sharpe, Microstructural studies on silicon nitride, J. Mat. Sci. 6~ 1292-1302 (1971). 5. A. G. Evans and J. V. Sharpe, Transmission electron microscopy of silicon nitride, in Electron Microscopy and Structure of Materials (G. Thomas, R. M. Fulrath, and R. M. Fisher, Eds.), U. of Calif. Press, Berkeley (1972), pp. 1141-1154. 6. S. C. Danforth and M. H. Richman, Transmission electron microscopy of reactionbonded silicon nitride, Metallography 9(4), 321-332 (1976). 7. J. T. Barnby and R. A. Taylor, The fracture resistance of reaction-sintered silicon nitride, Special Ceramics No. 5 (P. Popper, Ed.), British Ceramic Res. Assoc., London (1973), pp. 311-328. 8. S. C. Danforth, H. M. Jennings, and M. H. Richman, The ladder microconstituent of silicon nitride, MetaUography 9(4), 361-365 (1976). 9. H. M. Jennings and M. H. Richman, A hexagonal (wurzite) form of silicon submitted to Science (to be published).
Received August, 1975