Chemical processes that degrade composites of alumina with SiC whiskers

Materials Science and Engineering, A 112 (1989) 225-231

225

Chemical Processes that Degrade Composites of Alumina with SiC Whiskers* S. KARUNANITHY

National Research Council of Canada, Atlantic Research Laboratory, 1411 Oxford Street, Halifax, Nova Scotia, B3H 3Z1 (Canada) (Received September 12, 1988; in revised form December 7, 1988)

Abstract

*NRCC No. 3 0 1 4 8 .

and the whisker-matrix interface exhibits a strong influence on the fracture toughness of these materials [4, 7]. The role of a surface layer of silica (SiO2), sometimes present on the surface of SiC whiskers, is particularly important. The successful reinforcement of a sintered matrix by the whiskers would, however, depend on the chemical changes in the whiskers and the whisker-matrix interface, under the sintering conditions. The silica layer on the whiskers functions as a protective coating in oxidative environments and therefore plays an important part in the practical applications of this composite. Studies on the fracture toughness and the strength of alumina-SiC whisker composites [16, 17] have demonstrated the significance of the chemical reactions on the whiskers in high-temperature applications of these composites. The increased toughness and drastic loss of strength in air at temperatures above l l00°C of the whisker-reinforced alumina have been attributed to a substantial increase in the amount of amorphous silica and mullite crystals at the whisker-matrix interface. As alumina itself is stable to 2000°C in air [18], the degradation of mechanical properties should be directly related to the whiskers. A uniform distribution of the whiskers or fibers in the ceramic matrix is essential in fabricating toughened composite materials. One way in which this is achieved is mixing them in suitable liquids to form a homogeneous slurry. This slurry is then either flocculated and dried to form a solid mixture of the matrix and the whiskers, or converted to a stabilized slip and cast into the required shapes. The amount of S i O 2 which may be present on the surface of the SiC whiskers plays a fundamental role in the dispersion behaviour and stability of the slips because it influences the electrostatic repulsion which is required

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The interface between SiC and Al,_O~ in alumina reinforced by whiskers of sificon carbide plays an important role in determining the properties of the composite. Chemical degradation of this composite, however, can result in oxidation of the whiskers with formation of silica and loss of material from the whiskers in the form of gaseous byproducts. When a sintered composite was heated in air at 1000 °C for 1 h, the whiskers became hollow and showed an increase in the amount of silica. Both of these effects will contribute to the degradation of this composite in the service environments. 1. Introduction

A ceramic matrix may be reinforced with fibers or whiskers in order to improve such mechanical properties as fracture stress and toughness. In this respect, silicon carbide (SIC) in the form of fibers and whiskers is being actively explored as reinforcement for ceramic matrices such as alumina [1-9], mullite [2, 10], silicon carbide [11, 12] and silicon nitride [13-15]. This activity in the field of alumina-SiC whisker composites is attributable, at least in part, to the high-temperature stability and erosion resistance of alumina. The whiskers themselves typically have strengths in excess of 7.0 GPa [7] and are used to prevent catastrophic failure of the matrix by "pulling out" during fracture and dissipating the energy of fracture. The extent of energy dissipation, however, depends on the nature of the interface material, which should be strong enough to transfer the load from the matrix to the whisker, but weak enough to allow the pull-out of the whiskers during fracture. Therefore, the chemistry of the whisker-surface

226

to prevent agglomeration of the particles and whiskers. Recent studies [9, 19] on the formation of stable suspensions of alumina-SiC whisker mixtures have reported that the best dispersion of the whiskers could be obtained in weakly acidic (pH of 4) slurries. In addition, whiskers with higher amounts of silica have been observed to sediment more slowly and, therefore, may be expected to be better candidates for forming stable suspensions [19]. All of these effects are especially important in slip casting where a stabilized, well-dispersed slip is essential in forming homogeneous green casts. The study reported here was directed towards identifying chemical reactions which may take place at the whisker-matrix interface. SiC-reinforced composites were fabricated using different amounts of the whiskers. Slip casting was used as a simple technique for preparing solid composites from liquid slurries of the components to ensure a relatively uniform distribution of the whiskers throughout the samples. The samples were sintered under identical conditions, after which scanning electron microscopy (SEM) and Fourier transform (FT)-IR techniques were used to identify chemical changes in the surface and bulk

oo01

0

o 0

5

10

15

20

$heor Role (min-1)

Fig. 1. Viscosity of alumina-SiC whisker (30 vol.%) slurries.

TABLE 1

of the whiskers caused by the processing. In addition, some of the sintered samples were heattreated in air to observe possible oxidation effects.

2. Experimental details

2.1. Slip casting and sintering The properties of the materials that were used are listed in Table 1. Stabilized dispersed slips were prepared containing alumina and SiC whiskers with a total solids content of 60 wt.%. The whisker content was in the range 15-30 vol.% ( 12-26 wt.%). In preliminary experiments the viscosity of such slurries was measured in the pH range 2-7. As shown by Fig. 1, the viscosity was adequately low at a pH of 4, an acidity which is not deleterious to plaster moulds. In addition, the stability of slurries with respect to sedimentation was excellent at pH 4; after standing for 5 h a 10 ml column of slurry developed only 0.2 ml of clear supernatant liquid. For these reasons slip-casting slurries, made by the following method, were always adjusted to a pH of 4. The SiC whiskers were cleaned by washing with dilute HCI to remove metallic oxide impurities, the excess acid was removed by washing with distilled water, and the whiskers were dried for several days in a desiccator. Alumina powder and SiC whiskers, in the required proportions, were shaken in a Turbula mixer for 2 h. A water slip of this mixture was mixed in a Waring blender for 5 min, and then in an ultrasonic mixer (Heat Systems Ultrasonic Sonicator) for 2 min, with adjustment of the pH. Slip casting was carried out in conventional plaster moulds. Samples of cylindrical (2 cm height and 1 cm diameter), rectangular (1 cm × 1 cm × 3 cm) and cone shapes were slip cast and allowed to dry at room temperature for several days. These samples were sintered at 1600 °C for

Propertiesofmaterials

Material Alumina Reynolds RC-HP-DBM SiC whiskers Tateho SCW- 1-S- 1 Made by Tateho Chem. Corp., Japan Supplied by ICD Group, New York, U.S.A.

Properties Average particle size (/~m)

0.5

Crystalline phases Diameter (/~m) Length (/~m) Density (g cm 3)

95% Beta SiC 0.5-1.5 5-20 3.18

3"3 _-.7

TABLE 2

Sinteringconditions ALUm

Time (rain)

Temperature (°C)

Power 0 15 35 40 60 75 1(/0 135 175 205

on

25 3911 785 10(J0 1500 1600 1600 1600 1600 1600

55 250 225 300 1500 1000 750 500 500 450

Power 210 225 235 300 390

off 1521) 1271) 1035 510 150

40 40 40 40 40

SiC

Chamber pressure (mTorr)

A L U M I N A - SiC W H I S K E R

( 3 0 % v)

2 h, under the same conditions, in a vacuum furnace resistively heated by carbon elements. The heating-cooling cycle used and the chamber pressure observed, are shown in Table 2. Samples with 25-30 vol.% SiC in the matrix showed blackening of the surface (Fig. 2) after sintering. However, samples with a lower whisker content did not show any colour change on the surface.

2.2. Characterization Samples were broken with a hammer to obtain fracture surfaces which were examined by SEM. The electron micrographs and energy-dispersive X-ray (EDX) analyses were obtained using a JEOL JXA-35 scanning electron microscope equipped with a Kevex X-ray microanalysis system. The use of E D X analysis in following the whisker decomposition in these composites is limited. The EDX spectra obtained on the surface of a SiC whisker (oriented side-on) and the matrix in an alumina-SiC whisker composite are shown in Fig. 3. The element carbon falls on the lower detection limit of the system and therefore cannot be observed in this analysis. In addition, the material itself is coated with carbon to prevent electrical charging during the analysis. The spectrum obtained for the whisker showed a peak due to aluminum from the matrix and that obtained for the matrix adjacent to the whisker showed a peak due to silicon from the whiskers. This effect is unavoidable since the area of excitation, even at a low electron accelerating voltage such as 5 keV, has a radius (about 5/~m) larger

Fig. 2. Surface degradation (blackening) of the alumina-SiC whisker composites with higher whisker content: (a) alumina-10vol. oYoSiC;(b) alumina-30vol. oYoSiC.

SURFACE OF WHISKER Si AI

"~-'-0.OOO

Jj

RANGE= 10.230 keV

2.520

MATRI X AI

o *'-0.0OO

RANGE = 10.230 keY

2 . 5 2 0 "-~

Fig. 3. EDX analysis on sintered alumina-SiC whisker com-

posite.

228 than the average diameter of the whiskers. In this respect, the F T - I R spectrometry was found to be more informative for these materials. The F T - I R spectra were obtained using a B O M E M Model DA 3.02 spectrometer. The samples were ground and KBr pellets were made for F T - I R analysis. After sintering, thermal decomposition studies were carried out in air. Samples of the composite were heated in a porcelain crucible using a Lucifer Furnace at a rate of 20 °C per minute and held at 1000 °C for 1 h, before being allowed to cool to room temperature. These samples were also studied using SEM and FT-IR. 3. Results and discussion

3.1. Processing and characterization The SiC whiskers used in this study were "Tateho-SCW-lS"-type, which have no surface silica on them. Previous studies [7] on the surface composition of these whiskers by Si 2p electron spectroscopy for chemical analysis techniques have identified only SiC in the whiskers, while the silica on other types of SiC whiskers (e.g. SilarSC-9) was easily detectable by the same technique [7]. A large increase in the chamber pressure was observed in the temperature range 1500-1600 °C during sintering, indicating a gasforming chemical reaction. In addition, samples with 20-30 vol.% of SiC whiskers exhibited blackening of the surface after the sintering process. Densification measurements showed that the samples with higher whisker content were less densified than those with lower whisker content. The composites with 10 vol.% of the whiskers showed densification of 75% of the theoretical density (TD), while those with 30 vol.% of the whiskers attained 6 0 % - 6 5 % TD. SEM analysis of the fracture surface (Fig. 4) of the composite showed that the whiskers were well distributed in the final product. In addition, SEM did not detect any interracial layer which could have been formed by reaction between the matrix and the whiskers. Earlier studies [7] using transmission electron microscopy (TEM) indicated the absence of such an interracial layer, but could not rule out all reaction between the matrix and the whiskers due to the high value of the interfacial shear stress observed with composites containing Tateho-type SiC whiskers. However, this observed increase in the interfacial shear stress could also be caused by the roughening of the whisker surface, due to the loss of material,

Fig. 4. Fracture surface of alumina-SiC whisker composite sintered at 1600 °C in vacuum. during the sintering process. Because the samples used in this study were sintered in a vacuum furnace, the evolution of gaseous material(s) from the sample at temperatures above 1500°C was clearly evident, as shown by the increased chamber pressure. Considering the stability of the alumina matrix in the temperature range involved in the sintering process, one could assume that the gaseous material released came from the whiskers. Similar material loss was detected previously by following the weight loss in alumina composites with Tateho-SiC whiskers, fabricated by hot pressing at 1850°C [20]. However, the fracture surface of the latter composite showed whiskers completely decomposed to form a smooth layer, possibly of silica. From this work, it is clear that the decomposition reactions start at about 1500°C but do not affect the interface of the composite to a significant extent even at 1600°C, allowing the whiskers to retain both their morphology and the mechanical bonding to the matrix. The work using FT-IR spectrometry, the details of which are given elsewhere [21], showed clear evidence for the formation of silica in the composites during the sintering process. Quantitative evaluation of the amount of silica before and after the sintering process showed that the extent of oxidation of the whiskers during the sintering process was about 42 wt.%. In addition, the decomposition of SiC whiskers to form carbon is clearly visible (Fig. 2) on the composites with

229

higher whisker content. Chemical analysis, by the pyrolysis method, indicated an increase of about 1% in free carbon content in the darkened areas. A recent study [22] by T E M detected the formation of graphitic carbon at the whisker-matrix interface, during thermal oxidation of alumina-SiC whisker composites in air. The observed pressure increase during the sintering, and the IR spectral observation of the increase in silica content of the material, indicate that any of the following oxidation reactions (1)-(4) could have taken place. The free energy differences A G given below were calculated using the computer program FACT [23] SiC~s~+ ~O2,.g,= SiO2il/+ CO/g i A G1873r¢= - 797.7 kJ mol-i

(1)

SiC(s I -I- Ozig ) ~- SiO{g) + CO(g) AG1873

K =

- 4 7 3 . 1 kJ mo1-1

(2)

SiC!s )+ 02(g ) = SiO2¢~i+ C~ i A Glsv3 K= -- 522.3 kJ tool-1

(3)

SiC(s)+ 202(g ) = SiO2(l)+ CO2(g ) A G1873 K =

--

918.3 kJ mol- 1

(4)

All of the above reactions are thermodynamically possible and have been observed with SiC under similar conditions [24]. The observation of carbon formation on the composites with higher whisker content supports the occurrence of reaction (3) during the sintering process. The formation of silica during the sintering process is not attributable to a reaction between the matrix and the whiskers according to

SiC which may occur in the interior of the composite where the oxygen partial pressure is lower. In addition, no protective SiO 2 layer will form in this reaction.

3.2. Thermal decomposition studies Alumina-SiC whisker composites are being used as inserts in cutting tools [25]. In high-speed machining, the efficiency of the cutting tool is limited by the ability of the insert to survive the higher stress at the elevated temperatures of the tool-workpiece interface. Under actual cutting conditions the temperature at this interface could exceed 1000°C and therefore the chemical degradation of the composite in air at this temperature would determine the life of these cutting tool inserts. In this respect, a composite fabricated during this work was heat-treated in air at 1000 °C for 1 h. The fracture surface of the heattreated material (Fig. 5) showed that the whiskers remained in the composite after the heat treatment. However, a major difference was observed in the morphology of the whiskers in the fracture surfaces before and after the heat treatment in air: a significant proportion of the whiskers in the heat-treated material appeared hollow in the core. A comparison of the fracture surfaces before (Fig. 4) and after (Fig. 5) the heat treatment shows that this "hole formation" is an important step in the whisker degradation. Even though some fine pores were observed in some of the whiskers before the heat treatment, they can

3SiC + 2A1203 = AI4C 3 + 3SiO 2 AGj~73 K= + 474.6 kJ mol-J

(5)

because it is not thermodynamically favourable. However, it is possible that a small amount of oxygen present in the system either as residual O~ in the gas phase or absorbed on the powder could contribute to the oxidation of the whiskers. Reaction (3), however, describes a passive oxidation of SiC. This results in the formation of a protective silica coating on the whiskers, which prevents further degradation by preventing oxygen diffusion through the interface. Therefore the whiskers are retained in the matrix after the sintering with the fracture toughening still possible. Reaction (2), on the other hand, describes an active oxidation of

Fig. 5. Fracture surface of alumina-SiC whisker composite, heat treated in air at 1000 °C for 1 h.

230

be attributed to partial decomposition during the sintering process, as confirmed by the increase in the chamber pressure. This observation is rather intriguing as one would expect material loss, during the thermal decomposition of the whiskers, to take place at the whisker-matrix interface and not at the core. As described before, the whiskers used in this study were derived from rice hulls. Several studies [26-28] have been carried out to characterize them. The core of these whiskers was found to contain cavities 1-20 nm in diameter usually confined to a region 100 nm wide [27]. A recent study [28], using high-resolution images, established the presence of oxygen along with the metallic impurities in the places earlier identified as cavities. Based on the microanalysis results, it was concluded that the impurities present in the core of the whiskers may include a complex S i - O - C phase, the identity of which could not be deduced. Similar oxycarbides of silicon are, however, known to exist in compositions such as Si2C20, which decompose easily to form SiO(g) and C [29]. When heated in air, the latter would be removed a s CO(g) o r CO2(g). The size of the holes observed in the core of the whiskers after the heat treatment ranged from 200 to 500 nm, which is much larger than the size (20 nm) of the cavities in the original whiskers. Therefore, it is possible that the thermal decomposition of the whiskers resulted in the loss of material from the cavities which increased in size and coalesced to form the hollow core of the whiskers. These observations suggest that the degradation of the properties of the composite in air at elevated temperatures would begin not only at the whisker-matrix interface as suggested by previous workers [22], but also at the core of the whiskers. Because this effect takes place at about the temperature usually reached at the interface of the alumina-SiC whisker cutting tool and the workpiece, a practical concern of this phenomenon is the weakening of the composite due to loss of material from the core of the whiskers.

4. Conclusions The degradation of the properties of SiC whisker-reinforced alumina is caused by chemical reactions of the whiskers. These chemical reactions contribute to the increase in silica, and in the case of the composites with more than 25

vol.% of the whiskers, deposition of free carbon on the surface. In addition, the whiskers become hollow due to loss of material by chemical degradation at elevated temperatures in air.

Acknowledgments The author is grateful to Dr. S. G. Whiteway for stimulating discussions and comments about the manuscript, Dr. P. Odense and Mr. D. O'Neill for taking electron micrographs, and Dr. T. A. Wheat of C A N M E T for the use of the vacuum furnace for sintering of the samples.

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