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THE INTERFACIAL MICROSTRUCTURE OF SiC-WHISKER REINFORCED Si3N4
A. Swan, E. Olsson, R. Lundberg, G. Dunlop
To cite this version:
A. Swan, E. Olsson, R. Lundberg, G. Dunlop. THE INTERFACIAL MICROSTRUCTURE OF
SiC-WHISKER REINFORCED Si3N4. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-1043-
C1-1048. �10.1051/jphyscol:19901162�. �jpa-00230268�
COLLOQUE DE PHYSIQUE
Colloque Cl, supplement au n o l , Tome 51, janvier 1990
THE INTERFACIAL MICROSTRUCTURE OF Sic-WHISKER REINFORCED Si,N,
A.H. SWAN, E. OLSSON"), R. LUNDBERG' and G.L. DUN LOP'^'
Department of Physics, Chalmers University of Technology, S-412 96
$bteborg, Sweden
Swedish Ceramic Institute, Box 5403, S-402 29 Gbteborg, Sweden
Abstract
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The fine scale interfacial microstructure of S i c whisker reinforced Si3N4 composites has been studied using analytical electron microscopy. The composites have been prepared in two different ways, by hot pressing and by nitrided pressureless sintering. Mechanical tests were performed at room temperature as well a% at elevated temperatures. Debonded whiskers were found in heat treated materials and microcracks were found in samples strained at elevated temperatures. This suggests that a different mechanism was responsible for the fracture process at elevated temperatures. The fracture surfaces of samples tested at room temperature showed signs of whisker pull-out in a limited zone surrounding the fracture origin.Ceramic matrix composites have gained considerable interest due to their potential for application at high temperatures /l/. A promising candidate is S i c whisker reinforced Si3N4. One critical factor in determining the mechanical properties of these composites is the nature of the interface between the whiskers and the surrounding matrix material. If the whiskers are strongly bonded, cracks can propagate through the whiskers without deflection and the composite fails in a brittle manner. However, if the interface is weakly bonded, several fracture toughening mechanisms can lead to a non-linear stress-strain curve and an increase in fracture toughness. The fracture toughening mechanisms include crack deflection, whisker pull-out, crack bridging and microcracking /Z/.These mechanisms predict a different dependence of toughness upon microstructure.
S i c whisker reinforced Si3N4 fabricated by both the hot pressing (HP) and the nitrided pressureless sintering (NPS) techniques have been examined by transmission electron microscopy (TEM), analytical electron microscopy (STEMIEDS), X-ray diffractometry (XRD) and SEM in order to determine the interfacial microstructure of the materials, the characteristics of the whiskerlmamx interface and the prevalent fracture toughening mechanism.
(')Present address : IBM Research Division, T J Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA
(2)Present address : Department of Mining and Metallurgical Engineering, University of Queensland, St Lucia,
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901162
COLLOQUE DE PHYSIQUE
2 - EXPERIMENTAL
2.1
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Fabrication, The composite materials were prepared using two different techniques: if hot pressing(HP) and ii) nitrided pressureless sintering(NPS) of the green bodies.i) The hot pressed materials were prepared with 6 wt-% Y2O3 and 2 wt-% A1203 as sintering aids. The S i c whiskers were coated with aluminiumisopropylate to yield an A1203 surface layer after calcination. This surface modification technique has been described elsewhere 131. The coating step was carried out in order to control the microstructure of the whisker/mamx interface and to obtain a loose bonding. The materials were hot pressed in a graphite die at 40 MPa and at temperatures of 1750°C or 1800°C. Materials containing 0 and 25 vol-% S i c whiskers were produced. Thus two parameters were varied, the hot pressing temperature and the whisker content.
ii) The nimded pressureless sintered materials contained 10 wt-% Y2O3 and 6 wt-% A1203 as sintering aids. The NPS technique involves the nitridation and subsequent pressureless sintering of compacts of submicron Si and Si3N4 powders together with the sintering aids. The method of fabrication 14.51 and the development of microstructure during fabrication of the matrix material has been described elsewhere in detail 16.71. The starting submicron powder compacts were nimded to 100% conversion of Si to Si3N4 at 1350°C. The nitrided specimens were embedded in a protective Si3N, powder bed and pressureless sintered in a nitrogen atmosphere at 1820°C.
No whisker coating step was carried out for the NPS material. One parameter, the whisker content, was varied (0 and 30 vol-%).
2.2 - Mechanical test in^. The fracture strength was measured at room temperature using a Cpoint bend test. The HP material was also subjected to a series of other tests including bend testing at 1300°C in air. The main objective of the high temperature testing was to evaluate the influence of whisker reinforcement on creep resistance. This was done using a low loading rate and recording the applied load as a function of crosshead deflection. The elongation was calculated according to a method used for ceramics 181. The room temperature fracture strength was measured for the NPS materials. 4-point bend tests at 1200°C in air were also performed.
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2.3
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The microstructure of the samples was studied using analytical electron microscopy (SEM, TEM, STEM and EDS). Specimens for electron microscopy were prepared from the as- received material as well as from material which had been subjected to bend testing at elevated temperatures.Samples were taken both from the strained region and the unstrained regions of the specimens. The latter were considered as pure heat treated specimens. The TEM specimens were prepared by cutting, grinding, dimpling and ion beam thinning of the samples. The major crystalline phases were identified by X-ray diffractornetry of polished sections.
3.1
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The HP materiAThe mechanical properties of the hot pressed materials are summarized in Table 1. The addition of whiskers increased both the hardness and fracture toughness of the materials, however, the material hot pressed at 1750°C exhibited more promising mechanical properties than the material hot pressed at 1 800°C.The room temperature strength of the whisker reinforced material was about 100 MPa lower than that of unreinforced Si3N4. This can be explained by processing related defects as it was found that the whisker coating step tended to create whisker agglomerates which were often found to be the fracture origin. However the high temperature mechanical properties improved with the whisker addition. It has previously been suggested 191 that the major factors that determine the difference in high temperature properties of these materials are the amounts and compositions of the intergranular glass phase.
Table 1. Results of mechanical testing of HP non-reinforced Si3N4 and S i c whisker reinforced Si3N4
Hot pressing temperature ("C) 1750°C 1 800°C
Whisker content (vol-%) 0 25 0 25
Fracture toughness (MPamln) 6.4 7.4 6.7 7.3
Hardness (GPa) (Hv ,o) 14.4 16.8 14.2 16.7
Room temperature strength (MPa) 710 570 662 556
1300°C strength (MPa) 192 311 178 265
Elongation (%) at a stress of 170 MPa
(1 300°C bend test) 0.009 0.004 0.012 0.006
The results of analysis of the glass phase compositions by STEMIEDS show that the compositional changes are small, with a slightly larger fraction of additive cations in the HP 1800°C intergranular glass phase (see l'able 2).
The amount of intergranular glass varied with depth below the specimen surface and was thus hard to quantify.
However no significant difference in the amount of the glass phase was observed between the two HP materials.
X-ray diffraction analysis detected minor amounts of secondary crystalline phases in the HP 1 800°C material whereas no minor crystalline phases were detected in the HP 1750°C material.
Table 2. Average compositions of the intergranular glass phase of HP S i c whisker reinforced Si3N4 as determined by STEMEDS. Error limits are one standard deviation of a number of measurements.
The whiskertmamx interface appeared to be smooth and no sign of corrosive attack was observed. The whiskers were always surrounded by a thin amorphous film about 2 nm in thickness and glassy pockets were often present at the triple grain junctions of the as received material, as shown in Fig. la. No glassy pockets were observed in the material tested at 1300°C. IIowever, extensive crystallisation of the glassy phase and growth of the P-Si3N4 grains was evident (see Fig. lb). A thin glass film was however always present at the interfaces between P-Si3N4 grains and between the S i c whiskers and P-Si3N4 grains. TEM studies of the HP heat treatcd material showed that a number of whiskers had debonded, as shown in Fig. 2, which implies that the whisker matrix/interface bond is quite weak.
Fracture surface analysis of the same material tested at 1300°C in air showed occasional protruding whiskers indicating the presence of limited whisker pullout in the region of crack initiation. The extent and the length of the protruding whiskers were difficult to determine due to oxidation of the fracture surface.
3.2 - The NPS Material. The room temperature fracture strength of the whisker reinforced NPS material was about 400 MPa, a lower value than that obtained for the HP material. Processing defects proved to be detrimental to the mechanical properties of the NPS material. The granules did not compact sufficiently in the whisker reinforced material when pressed prior to sintering. This resulted in large crack-like porosity of the sintered composites. Possibilities exist for improvement of the processing conditions and therein the mechanical properties. Studies of the fracture mechanisms for this material are thus of interest.
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Fig. 1. Interfacial microstructure at S i c whiskers in the hot pressed material. Bright field TEM.
a) Whisker interface in material hot pressed at 1800°C. Note the intergranular glass pockets (G) mple grain junctions. b) Whisker interface in material hot pressed at 1750°C and heat treated f min at 1300°C. A thin glass film was always present at the interfaces.
Fig. 2. Bright field TEM micrograph of material hot Fig. 3. TEM image of NPS material showinl pressed at 1800°C and heat treated at 1300°C the secondary crystalline phases (Y) showing debonding at the interface between a surrounding the P-Si3N4 grains.
whisker and adjacent phases. The image is slightly The glass film is -5 nm thick.
overfocussed in order to reveal the microcrack (dark contrast) along the interface (arrowed).
The large volume fraction of sintering additives resulted in a large amount of liquid phase during sintt During cooling from the sintering temperature most of the sintering liquid crystallised into secondary k phases leaving only thin films of intergranular glass surrounding the grains, as shown in Fig. 3. A specimen of the material containing O % whiskers taken from the region of maximum strain during 4- bending at 1200°C showed a substantial amount of contrast due to internal deformation of the Si3N4 grains specimen contained numerous microcracks running mainly along interfaces between the different grains (se~
4). The calculated 181 maximum strain reached was approximately 1.1% with a creep exponent of n=l microcracks were found in the whisker containing NPS material that had been deformed. This was probabl:
to negligible strain deformation before failure ( = 0.3% strain with n=l). No formation of cavities d, deformation was observed for the temperature and loads that were used.
Fracture surface analysis showed the presence of limited whisker pullout in the as-received whisker containi
Fig. 4. Bright field TEM micrograph of NPS material Fig. 5. SEM micrograph of whisker reinforced containing 0 vol-% whiskers.Specimen taken NPS Si3N4 showing fracture surface with from the region of max strain in 4-point bending limited whisker pullout.
at 1200°C.Note the connast and the microcracks.
material in the region of crack initiation, Fig. 5. The lengths of the protruding whiskers were approximately 1-2 p m and only those whiskers perpendicular to the crack plane showed signs of pullout. The crack has then propagated in a brittle manner.
4 - PISCUSSJON
A thin glassy film was always found to surround the whiskers as well as the matrix grains. Extensive heat treatment did not affect the film thickness. This observation is in agreement with the theory of Clarke /10/ who proposes that an equilibrium thickness of grain boundary films will eventually be reached since amorphous films wetting grain boundaries reduce their interfacial energy. The composition of the intergranular glass in the hot pressed materials were found to vary with the HP temperature. This could possibly be due to supersaturation of the liquid upon cooling from 1800°C thus increasing the driving force for nucleation of intergranular crystalline phases.The X-ray diffraction results are in agreement with this explanation and although it has not been verified experimentally, this would result in a larger volume fraction of intergranular glass in the material hot pressed at
1750°C.
The debonding observed in the HP heat treated material is due to thermal expansion mismatch between thc whisker and the matrix resulting in a residual tensile stress at the interface of about 300 MPa. No debonding was observed in the as-received material, suggesting that the interfacial properties had changed during heat treatment.
The NPS material was shown to have a notably low volume fraction of intergranular glass, and a high degree of secondary crystallisation. Studies of the region of maximum deformation indicate that the strain imposed can be attributed partially to deformation of the P-Si3N4 grains and the secondary crystalline phases surrounding the
P-
Si3N4 grains, and partly to the formation of microcracks, predominantly along the grain boundaries. It can be noted that the observed strain contrast was partially due to thermal expansion mismatch between the different phases since this type of contrast was found on unstrained reference samples as well. The contrast was however much less on the latter samples. No microcrdcking was observed in the unstrained samples.
The fracture surface analysis performed on the samples showed that the toughening contribution due to whisker pullout is negligible. However, if the pullout region could be extended over the whole fracture surface the
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toughness of the composite would improve. The fracture studies described in this article indicate that a coml theoretical model of the fracture toughness of these materials at room temperature would have to inc:
debonding, crack bridging and whisker pullout. The stored strain energy in the whiskers, dissipated at fail would also have to be included. The effect of whiskers upon crack deflection has not been determined, but c microstructural effects such as cracks between the granules, porosity and impurities seem to have a la influence upon the room temperature fracture behaviour of these materials. A model for the creep frac process at elevated temperatures would have to include microcracking.
5 - CONCLUSION
1. The interfaces between the S i c whiskers and surrounding phases generally contained a thin glassy film.
2. In hot pressed materials, a significant number of these interfaces were debonded after heat treatmen 1300°C.
3. In the NPS material containing 0 vol-% whiskers, substantial microcracking occurred mainly along interfaces in the region of max deformation during Cpoint bending at 12W°C. Because of negligible stl deformation before failure, no rnicrwracks were observed in the whisker reinforced NPS material.
4. Whisker pullout was observed, but only in a zone extending around 200 Km from the point of cr;
initiation. The contribution to toughening from whisker pullout was thus negligible.
5 . The major contributions to the increased Eracture toughness of the whisker reinforced material are belie to be debonding at the whisker interface, especially in the HP material, and the strain energy stored in whisker over the debonded length.
REFERENCES
111 Phillips, D.C., "9th Ris0 Int. Symp. on Met. and Mater. Sci." ed. by S I Andersen and 11 Lilholr C Pedersen ( Riso National Laboratory, Roskilde, 1988) 183
I21 Tiegs, T.N., Becher, P.F., and Harris, L.A., "Ceramic Microstructures %G", (Plenum Press, New York London, 1986) 91 1.
131 Persson, M., Bostedt, E., Carlsson, R., "Proc. Int. Conf. ECerS-l". (1989) to be published.
141 Pompe, R., Hermansson, L. and Carlsson, R., Sprechsaal, (1982) 1098
151 Pornpe, R., Hermansson, L. and Carlsson, R.,"Engineering with Ceramics", ed. by R.W. Davidge (Bri~
Ceramic Society, Stoke-on-Trent, 1982) 65
161 Falk, L.K.L., Pornpe, R. and Dunlop, G.L., J. Mater. Sci.
211
(1985) 3545nl
Falk, L.K.L., Pompe, R. and Dunlop, G.L., Mater. Sci. Eng. 71 (1985) 123181 Hollenberg, G.W., Terwillinger, G.R., arld Gordon, R.S., J. Am. Ccram. Soc. 54 (1971) 196
191 Swan, A-I-I., Olsson, E., Linde, K.M., and Lundberg, R., "New Materials and Processes", ed. by 1.L 1-Iansson and H. Lilholt (Danish Society for Materials Testing and Research, Copenhagen, Denmark, 191 655
I101 Clarke, D.R., Ann. Rev. Mater. Sci. (1987) 57
