SiC-whisker-reinforced ceramics with modulated microstructures

Materials Science and Engineering, A 177 (1994) 277-281

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SiC-whisker-reinforced ceramics with modulated microstructures* M. Farkash, N. S h a f r y a n d D. G. B r a n d o n Department of Materials Engineering, Technion-lsrael Institute of Technology, Haifa 32000 (Israel) (Received May 26, 1993; in revised form July 15, 1993)

Abstract Randomly oriented extruded thread, produced from Si3N4 or Al203 ceramic slips containing 26 vol.% SiC whiskers (M. Farkash and D. G. Brandon, Mater. Sci. Eng. A, ?? (1994) ???), was filter pressed and then dried and hot pressed to form a dense ceramic body. The resultant microstructure was modulated, with the reinforcing whiskers locally aligned on the scale of the original extruded thread. The fracture toughness of the SiC,v-whisker-reinforced Si3N4 composites was significantly improved by the modulated microstructure but the bend strength was almost unaffected. In the alumina whisker-reinforced composites, the improvement in fracture toughness was marginal, but the composites with the modulated microstructure were found to be less sensitive to the deleterious effects of residual porosity than the composites with random SiC wwhisker reinforcement.

I. Introduction Whisker reinforcement is a promising strategy for improving the mechanical properties of a monolithic ceramic. Previous workers have demonstrated that, with suitable processing, both the bend strength and the fracture toughness of alumina and silicon nitride may be improved by additions of silicon carbide whiskers [1-5]. Increasing the whisker content decreased the rate of densification, since the whiskers inhibit shrinkage by forming a semirigid, three-dimensional scaffold within the matrix. In general, composites containing more than 20 vol.% whiskers are difficult to densify [1, 6, 7], and decreases in bend strength with increasing whisker content have been reported in the range 20-40 vol.% of whisker additions

{1,3,s1. The aspect ratio of the whiskers is expected to give rise to anisotropy of the material properties if the whiskers are partially aligned in the matrix. Anisotropy of the whisker orientation distribution is generally a result of unidirectional dimensional changes accompanying hot pressing. In the hot-pressed product, the whiskers are partially rotated into the plane perpendicular to the direction of hot-pressing, but are randomly orientated in this plane [2, 3, 6, 7]. The anisotropy of the whisker orientation distribution influences the mechanical properties. Becher and Wei [9] measured a higher fracture toughness for crack

*Paper presented at the Stein Conference, Drexel University, Philadelphia, PA, USA, October 21-23, 1992. 0921-5093/94/$7.00 $57)I (1921-5093(93)09415-F

propagation in a plane which contained the direction of hot pressing (perpendicular to the partially aligned whiskers) than in the plane perpendicular to the direction of hot pressing (the plane of partial alignment). These researchers also found that the bend strength was higher when the tensile axis lay in the plane of partial whisker alignment. Buljan and his coworkers [2] measured the fracture toughness of SiC-whiskerreinforced Si3N 4 from indentation cracking, and found that the highest toughness values corresonded to crack propagation parallel to the direction of hot pressing, while the lowest values were obtained for crack propagation in the plane of partial whisker alignment. Intermediate values of toughness were measured when the plane of partial alignment contained the direction of crack propagation, while the crack plane contained the direction of hot pressing. In the work reported here we attempt to improve the crack propagation resistance of whisker-containing ceramic matrix composites by producing dense ceramic bodies with modulated microstructures. These correspond to local alignment of the whiskers over distances of the order of 0.1-0.5 ram. The modulated microstructures were generated by the compaction of an extruded thread whose production is described elsewhere [10].

2. Experimental procedure In the present work we used the extruded green ceramic thread previously described [10] as an intermediate product in the processing of a dense whiskerreinforced ceramic body. © 1994 - Elsevier Sequoia. All rights reserved

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The wet, extruded and gelled thread (consisting of Si3N4 or Al203 powders and appropriate sintering additions, together with 26vo1.% of SiC whiskers aligned in the extrusion direction) was collected, with no attempt at thread alignment, and filter pressed to remove excess fluid. The stainless steel filter press consisted of a standard filter paper on a cloth base with a stainless steel mesh support (Fig. 1). Excess liquid was first removed by vacuum suction, after which pressure was slowly applied up to 70 MPa. The cold-compacted samples were 30 mm in diameter and up to 20 mm thick. The cold-compacted samples were stacked in a graphite mould for hot pressing, using carbon black insulation in an otherwise unprotected atmosphere. The maximum hot press temperature was 1820 °C for whisker-reinforced Si3N 4 and 1750°C for whiskerreinforced AI203. The maximum pressure applied for both materials was 35-38 MPa. The rate of heating was controlled and kept to 4-6 °C min -1 until the maximum temperature was reached. Temperature was monitored by either a thermocouple, mounted in the graphite piston immediately above the specimen, or a pyrometer, sited on a cavity in the graphite susceptor sleeve. All temperatures quoted are those measured by the pyrometer. In order to evaluate the influence of the extrusion process on the microstructure and the mechanical properties, reference sample groups were prepared for each material. In the case of Si3N 4 the reference sample groups were monolithic Si3N4 and whisker-reinforced Si3N4 prepared without slip extrusion, while in the case of AI203 the reference sample group was whiskerreinforced AIzO 3 prepared without slip extrusion. Sections of the cold-compacted samples were dried and vacuum impregnated with epoxy resin. They were then polished for optical micrographic evaluation. Micrographic sections of the dense, hot-pressed samples were also prepared by standard grinding and

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polishing. Crystalline phase content analysis was made by X-ray diffraction. Scanning electron microscopy was used for fractographic evaluation. Hot-pressed samples were sectioned into test bars with a 3 mm x 4 mm cross-section. The bend strength was determined in three-point bending for samples in which the tensile surface was either parallel or perpendicular to the direction of hot pressing. The fracture toughness was measured using the single edge-notched beam test. In all cases the crack plane contained the direction of hot pressing, but with the direction of crack propagation either parallel or perpendicular to the direction of hot pressing.

3. Results and discussion

3.1. Densification

The heating rate during hot pressing determined the rate of sintering, which reached maximum values of up to 1 vol.% min- 1 (equal to the rate of linear shrinkage). Shrinkage of the reinforced Si3N4 started at 1300 °C, began to slow down in the temperature range 1650-1700°C and reached rates of 0.2vo1.% min -1 o & e 2000

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after 15-20 min at 1800-1820°C (Fig. 2(a)). The onset of sintering corresponds well to the first formation of a liquid phase in the AI203-Y203-SiO2 phase diagram [11], which has a eutectic point at 1380°C. The relatively low shrinkage rate appears consistent with the constrains imposed on viscoelastic stress relaxation by the interlocking network of constraining whiskers [12, 13]. The reinforced A1203 started to sinter at about 1100 °C and the sintering rate slowed in the temperature range 1500-1600°C, reaching 0.1-0.2 vol.% min -~ (equal to the rate of linear shrinkage) at 1700-1750 °C (Fig. 2(b)). Increasing the temperature above 1800-1850 °C resulted in a further increase in shrinkage and was accompanied by the formation of a glassy phase which coated the graphite mould. The onset of shrinkage is associated with the first formation of liquid in the SiO2-AI203-CaO system [14]. The presence of some calcium was detected by energy-dispersive analysis and is associated with the ion exchange process used to gel the thread in the ion exchange reaction [10]. The formation of additional liquid phase above 1800°C is presumed to be associated with partial reduction of the A1203 by carbon. No evidence for the formation of A14C3 was found. The final density of the reinforced Si3N4 samples was 98%-99% of theoretical, while that of the reinforced A1203 was only 95%-98% of theoretical. There is no clear explanation for the comparatively low density of the alumina samples. In all cases X-ray diffraction analysis failed to detect the presence of crystalline phases other than the primary constituents (fl-Si3N 4 and (a+fl)-SiC in the case of the Si3N4 composites, and ~-AI203 and (a + fl)-SiC in the case of the A1203 composites). 3.2. Microstructure

Whisker-reinforced specimens prepared without slip extrusion exhibited random whisker distributions in the plane normal to the hot pressing direction (Fig. 3(a)) and partial alignment in the planes containing the hot pressing direction (Fig. 3(b)), consistent with previous work [2, 3, 6, 7]. Samples prepared from extruded thread, on the contrary, exhibited a modulated structure in the green bodies which was preserved after hot pressing. In the modulated microstructures, whisker alignment in the extrusion direction of the thread is maintained over domains of the order of the thread diameter. The basic morphology of the compacted thread is readily visible at low magnifications (Fig. 4). The domains are randomly oriented in the plane perpendicular to the hot pressing direction (Fig. 5(a)) and partially aligned at 0°-30 ° to this plane in the sections which contain the hot pressing direction (Fig. 5(b)). There is no evidence for any reduction in whisker

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Fig. 3. The microstructure of unextruded whisker-reinforced Si3N4: (a) Section perpendicular to the direction of hot pressing; (b) section parallel to the direction of hot pressing. content between the aligned domains, suggesting that the homogenous dispersion of whiskers in the original slip is preserved throughout the subsequent extrusion, cold compaction and hot pressing processing steps. That is, the geometrical changes in the microstructure accompanying processing were confined to whisker rotation, during extrusion, and shear between the whiskers, during cold compaction and hot pressing. 3.3. Mechanical properties

The results of the bend strength and fracture toughness measurements on Si3N4 matrix materials are summarized in Table 1. The bend strengths of monolithic and reinforced Si3N4 were found to be similar, that is no statistically significant differences between the sample groups were detected. The fracture toughness, on the contrary, was found to be markedly improved (34%) by random whisker reinforcement, and even more so (50%) in the case of the modulated microstructure. In all cases the toughness was greatest when crack propagation was parallel to the direction of hot pressing (Fig. 6). An increase in toughness with no appreciable change in

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bend strength implies an improvement in the damage tolerance. The improvement in flaw tolerance associated with the modulated microstructure in whisker-reinforced silicon nitride may be associated with an increase in bridging length due to the presence of the domains of aligned whiskers. Similarly, the higher toughness for crack propagation parallel to the direction of hot pressing can be associated with the smaller whisker spacing in this direction due to the uniaxial contraction accompanying hot pressing. This explanation is certainly incomplete, since it would imply that the obstacle spacing along the crack front is of less significance than the obstacle spacing in the direction of crack propagation. No attempt was made to evaluate the relative importance of the operative toughening mechanisms in the two test directions. As noted above, the whisker-reinforced alumina failed to achieve full density. The sample results for both the extruded whisker-containing material and the randomly dispersed whisker-containing alumina were divided into two groups corresponding to a high resi-

Fig. 4. Low magnificationimage of an Si3N 4 matrix composite parallel to the hot pressing direction, showingretention of thread structure in locally aligned domains of the dense, hot-pressed ceramic.

SiCw-reinforced ceramics

dual porosity and a low residual porosity. The corresponding results of the mechanical property measurements are given in Table 2. While the high residual porosity obscures some of the effect of whisker reinforcement on the properties, the best mechanical properties are clearly associated with the higher density (low porosity) extruded material tested for crack propagation parallel to the hot pressing direction, while the poorest properties were for the high porosity, randomly reinforced material

Fig. 5. The microstructureof extruded whisker-reinforcedSi3N4: (a) section perpendicular to the direction of hot pressing; (b) section parallel to the direction of hot pressing.

TABLE 1. Mechanicalproperties of monolithicand whisker-reinforcedSi3N 4 Sample group

Monolithic Reinforced Reinforced and extruded

Bend strength (three point)(MPa)

Fracture toughness single edge notch beam (SENB) (MPa m~n),

Parallel

Perpendicular

Parallel

Perpendicular

690 + 89 676 + 88 705 + 131

678 + 159 664 + 96 641 + 75

5.5 + 0.6 7.7 + 1.2 9.0 + 0.9

-7.0 + 0.5 7.5 + 0.8

Parallel and perpendicularindicate the direction of load with respect to the hot pressing direction.

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TABLE 2. Mechanical properties of whisker-reinforced AleO3 Sample group

Porosity (%)

Reinforced Reinforced and extruded

94.6 97.9 95.3 97.1

Bend strength (three point)(MPa)

Fracture toughness (SENB) (MPa m 1'e)

Parallel

Perpendicular

Parallel

Perpendicular

489 _+43 530+ 136 451 _+46 592_+87

462 _+91 506_+ 135 484 _+36 565_+87

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5.2_+ 1.1 6.2 _+0.8 5.7 _+0.5 5.8_+0.3

Parallel and perpendicular indicate the direction of load with respect to the hot pressing direction.

nology. They are also grateful for the use of scientific facilities belonging to the Georg Sachs Center for Processing Research and Materials Characterization at the Technion.

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Fig. 6. Fracture toughness of monolithic si3N4, unextruded whisker-reinforced Si3N4 and extruded whisker-reinforced Si3N4, measured in two test directions. tested perpendicular to the direction of hot pressing. In effect, the extruded material exhibit an improvement in flaw tolerance and hence a reduced sensitivity to residual porosity, although the results are less clear cut than those obtained with the silicon nitride samples.

4. C o n c l u s i o n s

T h e local alignment of SiC whisker reinforcement in A120 3 and Si3N 4 ceramics into domains, characteristic of a modulated microstructure, can be achieved without affecting the homogeneity of the whisker distribution by cold compaction of an extruded thread containing aligned whiskers followed by hot pressing. T h e modulated microstructure improves the fracture toughness of reinforced silicon nitride and both the fracture toughness and bend strength of reinforced alumina.

Acknowledgments

The authors would like to acknowledge the financial support received from the E u r o p e a n C o m m u n i t y (DGXII) and the Israel Ministry of Science and Tech-

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