SINTERING BEHAVIOUR

As preliminary tests, a mixture of SiC+10 vol% MoSi2 was hot pressed at 1900°C, without any of sintering additive. However, the final relative density was lower than 60%. An improvement of the final relative density, from 60 to 86%, was obtained with the addition of 2 wt% of Y2O3. Actually, only the simultaneous addition of alumina and yttria, or aluminum nitride and yttria, allowed to achieve nearly fully dense composites (Tab. 6.I).

Alumina and yttria are the most common sintering additives used for the densification of silicon carbide.^1-7 The Al2O3/Y2O3 weight ratio (3/2) was the same of the SiC reference material (SAYM0).⁴ The densification of these compositions occurred through liquid-phase sintering, owing to the liquid formed by reaction between the sintering aids and the SiO2 present as oxidation product in the starting SiC and MoSi2

powders. Al2O3, SiO2 and Y2O3 have eutectic temperatures in the 1400-1800°C range,

depending on their relative amounts.¹⁷ Analogously, Y2O3-AlN, in the molar ratio 6/4, form an eutectic point at around 1840°C.¹⁸

Tab. 6.I: Sintering conditions, experimental density and microstructural parameters of the hot pressed materials (see also Tab. 5.II).

Sample Hot pressing

Fig. 6.1: Densification curves for a) SAYM0, b) SAYM10, c) SAYM30, d) SYNM30 samples.

Relative density is plotted as a function of temperature and dwell time.

The shrinkage curves recorded during the hot pressing runs, and shown in Fig. 6.1, highlighted that the addition of MoSi2 particles did not significantly alter the densification behavior of the SiC-based composites with respect to the reference SAYM0 material. The starting shrinkage temperatures of SAYM0, SAYM10, SAYM30 and SYNM30 ceramics (1490°C, 1530°C, 1480°C and 1690°C, respectively) were close and in agreement with the eutectic temperatures reported in

0 10 20 30 40

the literature.¹⁷ The maximum sintering temperatures and the dwell times needed to obtain the full densification were very similar as well (Tab. 6.I).

Of the three MoSi2-containing samples, the one that moves away from the standard behavior is, as expected, SYNM30, for which a delay in the starting densification temperature was recorded, due to a higher refractoriness of the liquid phase deriving from Y2O3-AlN. However, during the isothermal stage the densification was faster compared to the composites with Al2O3-Y2O3.

It has to be pointed out that the final relative density in Tab. 6.I was calculated on the basis of the starting composition and thus does not consider the eventual presence of SiO2 coming from SiC and MoSi2 powders or the formation of glassy phases. In other words, a higher final relative density does not always imply a more dense material. A more reliable indication of the residual porosity can be disclosed by image analysis (see Section below).

6.3 MICROSTRUCTURE

A comparison among the XRD patterns of the materials containing Al2O3-Y2O3 as sintering additives is reported in Fig. 6.2.

The crystalline phases detected by X-ray diffraction in the MoSi2-containing materials, with Al2O3-Y2O3 as sintering additives, were mainly β-SiC, MoSi2 and traces of crystalline YAG (3Y2O3·5Al2O3). Additional peaks with very low intensity were attributed to the Mo5Si3 and Mo4.6Si3C0.6 phases, which are commonly formed in SiC-MoSi2 or C-MoSi2 compositions.^{11, 19}

The X-ray diffraction pattern of the material containing Y2O3-AlN as sintering additives shows the presence of β-SiC, tetragonal MoSi2 and traces of YAG, Mo4.8Si3C0.6 and residual AlN, Fig. 6.3.

All the materials were nearly full dense after the hot pressing routes, (Tab. 6.I). An estimation of the residual porosity by image analysis indicated the SYNM30 composite as the most dense.

The development of microstructure during sintering involved partial solution of the original SiC grains, precipitation around undissolved nuclei and subsequent grain

growth. Compared to silicon nitride, SiC solubility in the melt is lower of about 10%.

Intergranular crystalline phases, corresponding to YAG, were observed in amounts around 5 vol%. This value, semi-quantitatively evaluated from the peaks intensities in X-ray diffractograms, was in agreement with calculations based on the starting compositions. Beside these crystalline phases, an amount of amorphous phase was supposed to form in all materials owing to reaction of additives with the silica, despite no confirmation was found in the XRD patterns. By image analysis, the estimated amounts of amorphous Al-silicates is about 4%. Typical microstructural features of the reference hot pressed material are shown in Fig. 6.4.

Fig. 6.2: X-ray diffraction pattern for the SAYM0, SAYM10 AND SAYM30 samples.

Fig. 6.3: X-ray diffraction pattern for the SYNM30 sample.

2-Theta

20 25 30 35 40 45 50 55 60 65 70

β-SiC MoSi2

Mo4.8Si3C0.6

Mo5Si3

YAG Al2O3

SAYM0 SAYM10 SAYM30

20 30 40 2-Theta 50 60 70

β-SiC MoSi₂ Mo_4.8Si₃C_0.6

YAG AlN

Fig. 6.4: Polished section of the silicon carbide reference material, SAYM0, after plasma etching. a) Lower magnification and b) higher magnification showing the core-rim structure and the grain boundaries phases.

a

core rim

b

YAG

Si-Al-O

Silicon carbide grains were etched away by a CF4 plasma, thus the microstructure was delineated by the grain boundary phases. Grain morphology was mainly equiaxed with grain size distribution ranging from 0.1 to 2 μm. Due to the presence of large particles in SiC powder, a small number of larger grains (up to 10 μm) with irregular shape was also observed. Adjacent SiC grains were mainly separated by a thin grain boundary film, which was the residue of liquid phase sintering medium. Intergranular phases appeared mainly at three- and four-grain pockets: these areas were supposed to be crystalline,²⁰ while the grain boundary phase between SiC grains was presumed to be amorphous.

The etched surfaces of the samples produced with the addition of Al2O3+Y2O3 revealed a core/rim structure in SiC grains. EDS analyses confirmed variations in the core and rim composition. In the core region, only Si and C could be detected. In the outer rim, traces of Al and O were also observed, in agreement with previous results which claimed the presence of Al, O and Y.²¹ This provided evidence that liquid phase sintering of SiC proceeded via a classical solution-reprecipitation mechanism. As the sintering process started, small SiC grains dissolved into the oxide melts until the solubility limit was reached. Then, SiC reprecipitated on favourably oriented facets of large undissolved SiC grains, which acted as nucleation sites. Undissolved grains constituted the core and the precipitating liquid formed the rim. The difference in chemical composition between core and rim suggested that SiC with small amounts of Al and O in solid solution was more stable in contact with a Al-Y-O-Si liquid than pure SiC. Smaller SiC grains with rounded shapes were observed in larger pockets of grain boundary phase, while most of the SiC grains developed a faceted interface with the grain boundary phase. This confirmed that, during densification and grain growth, a reactive Al-Y-O rich liquid phase was present and that surface silica on the starting SiC particles participated in this reaction, as previously observed.²¹

The microstructure of the MoSi2-containing composites is shown in the fracture and polished surfaces of Fig. 6.5. The bright contrast inclusions are MoSi2 particles dispersed in the dark contrasting SiC matrix. The fracture mode (Fig. 6.5a,c,e) was mainly intergranular in the SiC matrix. On the other hand, for MoSi2 particles, the fracture was both intergranular and transgranular. Despite the differences in thermal

expansion coefficient between the two phases, no microcracks were observed at the boundaries between the inclusions and the matrix.

Fig. 6.5: BSE images of fractured and polished surfaces for a) and b) SAYM10, c) and d) SAYM30, e) and f) SYNM30.

b

d

f a

c

e

MoSi

MoSi

MoSi

Fig. 6.6: Polished surface for a) SAYM10 and b) SAYM30. In the insets the EDS spectra of the secondary phases: YAG, brighter, and Si-Al-O, darker.

a

O C

Y

Al Y Si

KeV

C O Al Si

KeV

b

The microstructural features of the SiC matrix, for SAYM10 and SAYM30 samples, resembled those of typical liquid-phase sintered SiC, such as SAYM0, and consisted of preferentially rounded SiC grains separated by intergranular silicate films and of a secondary phase at triple junctions (see Figs. 6.6). The intergranular phase among adjacent SiC grains was based on Si-Al-O or Y-Al-O elements (Fig. 6.6), likewise the baseline SAYM0 material.⁴ The SiC mean grain size was in the range of 0.5-1.0 μm.

All the materials containing MoSi2 showed the presence of Mo5Si3 which appeared ad brighter phase, Fig. 6.7.

Fig. 6.7: SEM micrograph of SAYM30 sample showing the two Mo-Si-based phases and the corresponding EDS spectra.

2 μm

Si

Mo Mo Mo

Mo MoSi2

Mo Si

Mo Mo

Mo

KeV

2

KeV

Mo5Si3

MoSi2

MoSi2

MoSi2

Mo5Si3

In the SYNM30 sample, some abnormal grain growth occurred, probably due to the presence of nitrogen in the sintering additives which formed a more viscous liquid phase, favouring an inhomogeneous grain growth.²² In the grain boundary phase, the nitrogen peak was not detected by EDS analysis. In pressureless sintering, the mechanism of volatilization of gaseous species is well known,²² whilst in pressure-assisted sintering this phenomenon was not noticed yet.

The MoSi2 monolithic material, shown in Fig 6.8, was fully dense, with mean grain size of about 2-3 μm. Pockets of silica, about 5 vol%, were found at the triple grain junctions and derived from silica contamination in the starting powder, as reported by several authors.^8,19 Besides the silica phase, a white phase was detected and assessed to be Mo5Si3 by EDS analysis.

Fig. 6.8: Fracture surface of the monolithic MoSi2 sample.