Oxidation behaviour of silicon nitride under cyclic and static conditions

Ceramics International 15 (1989) 247-253

Oxidation Behaviour of Silicon Nitride under Cyclic and Static Conditions M i n o r u M a e d a , Kazuo N a k a m u r a , T s u t o m u O h k u b o , * M a s a r u Ito & Eiichi Ishii Government Industrial Research Institute, Nagoya, Hirate-cho, Kita-ku, Nagoya 462, Japan (Received 27 June 1988; accepted 29 August 1988)

Abstract: Silicon nitride, hot-pressed with alumina and yttria additives, was oxidized cyclically at 1200°C in flowing dry air for a total oxidation time of 3000 h. Static oxidation was also carried out for comparison. The cyclic oxidation apparently proceeded in two stages with different reaction rates, but both stages approximately followed the parabolic rate law. This difference ofreaction rates may well be related to the change with time of the properties of the oxide layer and sublayer formed during oxidation. The main crystalline phases formed during the gradual oxidation on the surface were cristobalite, yttrium disilicate (Y203 • 2SIO2) and tridymite. The corresponding oxide layers were porous and contained many cracks and channels. The surface roughness increased gradually with time, and reached a maximum value after 750 h oxidation. Then it decreased by degrees up until the end of the runs (t = 3000h). A consecutive degradation of the roomtemperature flexural strength was observed. The static oxidation resulted in the same weight gain, the formation of different relative quantities of cristobalite, tridymite and Y.2S, approximately the same surface roughness and a slightly higher room-temperature flexural strength compared with the cyclic oxidation.

1 INTRODUCTION Silicon nitride ceramics have been studied extensively in recent years on account of their potential application as high-temperature engineering materials. In these applications, oxidation resistance is of major importance. However, most published works ~-2° on the oxidation behaviour of silicon nitride have been concerned only with static oxidation for short times and there have been few studies devoted to long-term or cyclic oxidation. Weaver and Lucek 2~ investigated the oxidation behaviour of hot-pressed Si3N 4, containing 8 wt% Y 2 0 3 , at temperatures varying from 1000°C to 1300°C for times up to 300 h. The weight-gain data were compared with those relative to specimens that had been removed from the furnace at 24 h intervals * Former staff of the Institute.

to determine the effects of cyclic oxidation. No significant difference was observed. Warburton et al. 22 studied the oxidation of reaction-sintered silicon nitride with a density of 2-10-2.39g/cm 3 in water-saturated air between 700°C and ll00°C for times u p to 8000h. They observed that in 1 atm of damp air at 900°C and l l00°C, 15% of nitride converted to silica with exposure times of 1000-1600 h and 50 h respectively. By contrast, the reaction was slight at 700-800°C. Sheehan 2~ oxidized hot-pressed Si3N 4 materials containing 1 and 5 w t % MgO for 1000h at 1000, 1100 and 1200°C in helium at 0-4-0.8Pa total oxidants, and showed that the amounts of both passive and active oxidation were greater for the material containing 5 wt% MgO. The oxidation at 1200°C resulted in high weight losses for both specimens. The purpose of the present paper is to study the

247 Ceramics International 0272-8842/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

Minoru Maeda, Kazuo Nakamura, Tsutomu Ohkubo, Masaru Ito, Eiichi Ishii

248

long-term oxidation behaviour, in cyclic and static thermal exposures, of hot-pressed silicon nitride containing Y203 and Al20 a and to evaluate the oxidation effect on the strength.

2 EXPERIMENTAL PROCEDURES 2. 1 Materials Hot-pressed silicon nitride containing alumina and yttria as densifying aids was used for the oxidation tests. The specimens were cut with diamond tools from large billets (93 x 93 x 14 mm a) into rectangular bars and the final size of the specimens was 3 x 4 x 40mm 3. Their surface was ground in the longitudinal direction with a 400-grit diamond wheel and ultrasonically cleaned in acetone and alcohol. The edges were bevelled at 45 ° by 0.2 mm in accordance with Japanese Industrial Standards, JIS R 1601 (testing method for flexural strength of highperformance ceramics). The bulk density, apparent density, apparent porosity and water absorption were measured by the displacement method and the amount of major elements other than silicon was determined by chemical analysis. They are shown in Table 1. 2.2 Oxidation Oxidation experiments were done at 1200°C in flowing dry air (1.071itres/min) after passage through silica gel and dispersed phosphorus pentoxide drying towers. The specimens were placed in a high-alumina holder and heated in a high-alumina muffle in a boxtype electric furnace. The cyclic oxidation was performed for different exposure times (30-3000 h) successively by removing the specimens from the furnace, cooling, weighing, and then reinserting them into the furnace. The static oxidation was carried out in the same furnace for 1470 h and 3000 h without interruption of heating. After oxidation, the weight changes of each specimen were measured. Table 1.

Properties of silicon nitrida

Chemical analysis (wt%)

Bulk density (g/cm =) Apparent density (g/cm 3) Apparent porosity (%) Water absorption (%)

AI Y Mg

2.3 Samp/e characterization Detailed microstructural analyses of the oxidized specimens were performed with a metallurgical microscope, scanning electron microscope (SEM), electron probe X-ray microanalyzer (EPMA) and Xray diffractometer (XRD). The surface roughness of specimens before and after oxidation was measured by the contact profile method. The bend strength was determined using three specimens for each oxidation test in a four-point bend test geometry over an outer span of 3"00 cm with an inner span of 1.00cm (one-third-point loading). All flexural strength tests were carried out at a cross-head speed of 0.5mm/min at room temperature and without any preparation of the specimen surface after oxidation.

3 RESULTS A N D D I S C U S S I O N 3. 1 Oxidation kinetics The oxidation kinetics for times up to 3000h at 1200°C are shown in Fig. 1. The weight gain is plotted against (time) t/2 on the assumption that the oxidation behaviour is governed by a simple diffusion-controlled process. The time intervals between different experiments were chosen in such a way that experimental values at approximately equal intervals on the square-root axis of time could be obtained. In the beginning all the samples were heated, and just after the furnace had reached the test temperature of 1200°C, it was cooled, and some of the samples were removed from the furnace and weighed. The remaining samples in the furnace were again heated to 1200°C and kept for 30h at this temperature, and some of the samples were removed from the furnace after cooling, and were weighed. This was repeated at 1200°C for different time intervals. Therefore the last samples, after the eleventh oxidation cycle, were oxidized for 3000 h by i

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249

Oxidation behaviour o f SiaN 4 under cycfic and static conditions

repeatedly heating and cooling during 0, 30, 90, 150, 210, 270, 330, 390, 450, 510 and 570h successively. The oxidation reaction followed approximately the parabolic rate law. However, the parabolic plot exhibits a break after 270 h of reaction time (Fig. 1). According to Davidge et al., 2 the oxidation of reaction-bonded silicon nitride (containing 20% open porosity) occurs in two well-defined stages; at internal (stage I) and external (stage II) surfaces. And they stated that the oxidation could occur at both surfaces provided that the internal pores were not completely filled or that a continuous film was not formed at the surface. Because the internal surface area is greater than the external area, the oxidation in stage I is much more rapid than in stage II. Hasegawa et aL 24 also reported almost the same reaction mechanism for the oxidation of a Sialon material. However, the specimens tested in the present work were dense bodies having a low porosity (0-11%). Then, the role of open porosity, which is important in RBSN materials (20%), is presumed to be a secondary factor in this oxidation test. It is well known that HPSN materials densified with additives contain a continuous intergranular glass phase. This phase may act as a fast, or shortcircuit, diffusion path through the microstructure, enabling rapid transport of ions from the interior of the material to the surface and vice versa. Hence, parabolic oxidation is produced, not by a protective layer, but by a sublayer compositional change. Therefore, the break observed on the parabolic plot (at 1200°C) could be correlated to this phenomena. Indeed no drastic change of the morphology of the oxide layer was observed before and after 270h oxidation. The specimen surface first began to be covered with silicate glassy material. After 30 h oxidation accicular crystals of Y203.2SIO 2 (hereafter, referred to as Y.2S) appeared as well as small cracks on the oxide layer surface. With an increase in oxidation time, Y. 2S crystals grew, the oxide layer became more severely cracked, and the oxide layer surface became more uneven and knobby. For longer oxidation times, the surface roughness gradually became smooth, because of the sealing of the rugged surface by silicate glassy materials formed in large quantities. From a 170 h oxidation run of hot-pressed silicon nitride at 1450°C, Tripp and Graham 5 observed (like us) a break in the parabolic plot (t ,-, 16 h). For longer times ( > 100h) their kinetics became somewhat erratic, and the rates were more rapid than parabolic. They proposed several possible explan-

ations for a change in reaction mechanism including (1) a crystalline transformation, (2) a change from an amorphous to a crystalline scale, (3) a scale-doping effect of impurity and (4) melting of the crystalline silicate phase. An additional explanation was given by Singhal: 25 a small change in the viscosity of the surface oxide could alter the rates at which additives and impurities diffuse through this layer. In the present test, the oxidation behaviour was not simple and some dispersion of weight-gain values was found in both stages and this dispersion was attributed presumably to the changes mentioned above observed by Tripp and Graham, and Singhal. The aim of the static oxidation tests was to determine the effects of continuous oxidation for durations (1470h and 3000h) identical to those of the cyclic tests. Almost the same values of weight gain were observed. Weaver et al. 21 also performed both cyclic and static oxidation tests on hot-pressed Si3N4-8% Y 2 0 3 materials and obtained identical results independently on the nature of the tests at 1000, 1100, 1200 and 1300°C for times up to 96h,

3.2 Characterization of oxidation products Oxidation effects on the microstructure of oxide products were determined by optical microscope, SEM, EPMA and XRD. The gradual superficial oxidation with reaction time was confirmed by microscopic examination. First, the surface of specimens started to lose their smoothness and to roughen. Small crystals of Y. 2S and cristobalite appeared in the surface oxide layer and grew very slowly as a function of oxidation time. i

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250

Minoru Maeda, Kazuo Nakamura, Tsutomu Ohkubo, Masaru Ito, Eiichi Ishii

Cracks appeared on the surface due to the transformation of the oxide layer. For longer times, up to 3000 h, the gradual oxidation proceeded with slow crystal growth of Y. 2S and cristobalite but no drastic morphological change of the oxidized surface was observed. Between the samples oxidized by cyclic and static

thermal exposures for the corresponding oxidation times, no definite morphological differences of the surface oxide layers were noticed. Figure 2 shows the change in surface roughness of the specimens before and after oxidation measured by the contact profile method. This change corresponded well to that of the microscopic observation.

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251

Oxidation behaviour of SiaN 4 under cyclic and static conditions

The surface roughness increased gradually with oxidation time, and reached its maximum value after 750 h oxidation. By further oxidation, it decreases up to 3000 h. Fig. 3(a) shows a SEM micrograph of a polished cross-section of the sample oxidized at 1200°C for a total time of 3000 h by cyclic thermal exposures. Figures 3(b), (c), (d), (e) and (f) show respectively the elemental distribution of nitrogen (N), oxygen (O), aluminium (A1), silicon (Si) and yttrium (Y). N is evenly distributed throughout the substrate and not found within the oxide layer. Na, K, Ca and Mg are predominantly concentrated in the oxide layer. SEM and EPMA examinations showed that the oxide layer had a porous texture and contained many cracks and channels. In addition, Y.2S crystals were dispersed in the oxide layer not only on the outer-surface oxide layer but also in the inner oxide layer. The only crystalline phases detected by X R D within the oxide layer are cristobalite and tridymite. The thickness of the oxide layer increased with the increase in oxidation time and was 6-8 tim thick after 3000h cyclic oxidation at 1200°C. However, no drastic modification of the microstructure of oxide layer with time was observed. Between the cyclic and static oxidation, no definite microstructure differences of oxide layer were noticed. By XRD, the oxide layers were analysed and cristobalite, Y. 2S and tridymite were detected as crystalline products. Up to 1080h oxidation, the amount of cristobalite increased with reaction time and by further oxidation, it decreased gradually for longer times (t < 3000 h). The amount of Y. 2S was the same at all oxidation times. Tridymite appeared only after 1080 h oxidation and increased gradually in quantity, at the expense of cristobalite. Figure 4 illustrates the effect of oxidation on the relative intensities of the X-ray diffraction peaks. The amount of cristobalite formed during the cyclic oxidation was more than that associated to the static oxidation. On the contrary, the amount of tridymite corresponding to the cyclic oxidation was less than that grown by static oxidation after 1470h and 3000 h of oxidation.

3.3 Strength after oxidation The effects of oxidation on the room-temperature flexural strength of hot-pressed silicon nitride are shown in Fig. 5. The curve joining the mean values has to be considered as a guide for the evaluation of the strength of oxidized specimens. However, it

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diffraction peaks. clearly reveals continuous losses in strength as oxidation proceeded. Wu et al. 9 have studied the oxidation behaviour and strength degradation, due to long-term hightemperature exposure in air, of SiaN 4 with MgO, Z r O 2 or Y203, as densification aids. They found that the most significant effect on strength was a large (30-60%) reduction for almost all compositions except for the specimen with 8 wt% Y203. Govila et al. 16 observed small decreases (less than 10%) in flexural strength at room temperature for the yttriadoped sintered reaction-bonded silicon nitride, oxidized in air at 1200°C and 1400°C for 300h. The structure of the oxide layer formed on the specimen

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252

Minoru Maeda, Kazuo Nakamura, Tsutomu Ohkubo, Masaru Ito, Eiichi Ishii

surface is one of the main factors influencing the mechanical properties of the materials. 26 Ziegler et al. 27 reported that the oxidation products and microcracks formed during cooling from the oxidation temperature, lead to strong variations in the properties of RBSN materials. However, very different behaviours have been reported. Indeed, depending on the experimental conditions, strength degradation, increase in strength and unaffected strength results have been observed. At the present time, an unequivocal explanation cannot be given for the variations in strength caused by oxidation effects. Several opposing factors might be effective, depending on the oxidation conditions and on the microstructure of the materials. Weaver et al. 21 measured the room-temperature flexural strength of heat-treated SiaN4-8%Y20 a samples at temperatures varying from 1000°C to 1300°C for 300 h and found a noticeable decrease in strength at 1100°C. According to Wu e t al., 9 the strength degradation that occurred was due to the formation of oxidation pits. The size of pits being largest in most MgO additive bodies, hence they suffered the most severe strength loss. Previous investigators 12'2a'29 have related that surface pits and bubbles formed during oxidation appeared to be responsible for the degradation, e.g. pits and bubbles are common fracture origins. In the present work, the oxidized surface was covered with silicate glass materials containing Y. 2S crystals and cracks, pits or bubbles were very few on the surface, and the texture of oxide layer was porous with cracks and channels. Then, the observed decrease in strength after long-term oxidation seemed to be related to an increasing coarsening of oxide layer texture. However, there were some contradictions concerning the strength and surface roughness or morphology of oxide layer. The surface roughness increased with time and after reaching the maximum value for 750h oxidation, started to decrease gradually, and this propensity lasted up to 3000h oxidation. The strength decreased steeply with time up to about 750 h oxidation but did not show any appreciable increase with further oxidation. In addition, the specimens exposed to static oxidation showed a higher strength than those tested by cyclic oxidation, although they had almost the same surface roughness. The reasons for these discrepancies in the relation between the strength and the surface roughness or morphology of oxide layer are presently unexplained. Considering the

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Fig. 6. SEM micrograph of the fracture surface of SiaN4 (cyclicoxidation, 1920hi. complexity of the system, the fact that the specimens exhibiting the maximum surface roughness for 750 h oxidation showed the minimum strength may be quite fortuitous. In another study, 3° it was found that long-term oxidation was making SiaN 4 porous. This is believed to be one of the main factors responsible for the observed decreasing strength. Figure 6 illustrates a fracture surface of the specimen oxidized by cyclic exposure for 1920 h. The failure occurred at a cavity existing originally on the surface and the mirror region is seen around it. 4 CONCLUSIONS The cyclic oxidation of hot-pressed silicon nitride, with alumina and yttria as sintering additives, was performed at 1200°C in flowing dry air for times up to 3000h and compared with the static oxidation behaviour. There are apparently two stages, with different reaction rates in the oxidation process, nearly following the parabolic rate law. This difference of reaction rates may well be related to a change in the properties of the oxide layer and sublayer formed during oxidation with time. However, no drastic morphological change was observed before and after the break point corresponding to an exposure of 270h. The gradual oxidation of the surface with reaction time was confirmed by microscopic examination. The main crystalline phases formed during oxidation were cristobalite, Y. 2S and tridymite. Up to 1080h oxidation, the amount of cristobalite increased with oxidation time. With further oxidation, tridymite appeared first, increased in amount; on the contrary cristobalite started to decrease in

Oxidation behaviour o f S i 3 N 4 under cyclic and static conditions

amount up to 3000h. The amount of Y . 2 S was constant during the process. The textures of the oxide layers were porous, contained many cracks and channels and changed gradually with time. The surface roughness also increased with time, reached the maximum value after 750 h oxidation and then started to decrease up to 3000 h. The room-temperature flexural strength distribution indicated clearly that there were consecutive losses in strength as oxidation proceeded. Prolonged oxidation seemed to result in a decrease in the strength associated with an increasing coarsening of the oxide layer texture, though there were some contradictions concerning the correlation between strength and surface roughness or morphology of oxide layer. The fact that the Si3N ¢ was made porous by prolonged oxidation is also considered to be one of the main factors responsible for the observed strength degradation. The static oxidation runs showed that by comparison with the cyclic tests (1) the weight gains were almost the same, (2) a smaller amount of cristobalite, a larger amount of tridymite and almost the same amount of Y. 2S were formed in the oxide layer, (3) the surface roughness was almost identical and (4) the flexural strength was slightly higher.

20.

REFERENCES

21.

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