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FIGURE 8.2 Mechanism of bond rupture. (From Ref. 8.21.)

White et al. [8.24] reported that Li+ ions in solution negated the enhanced rates noted in high pH solutions, where OH- controls the rate of bond breaking by readily associating with the OH-, and not allowing OH- to react with the Si–O–Si bond at the crack tip. This type of reaction is not exhibited by other alkalies, because they do not readily react with OH- ions.

A more recent review of this topic of stress corrosion in silicate glasses was given by Gy [8.25].

8.3 DEGRADATION OF SPECIFIC MATERIALS

8.3.1 Degradation by Oxidation

Carbides and Nitrides

When evaluating the effects of corrosion, one must be alert to the changes that occur if samples are corroded, then cooled to room temperature for mechanical testing. McCullum et al. [8.26] found that the room-temperature flexural strength of SiC increased with exposure time to an air environment at 1300°C, whereas it decreased for Si3N4. They attributed this

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increased strength for SiC to the formation of a thin silica surface layer that healed surface flaws. The decreased strength for Si3N4 was attributed to the formation of a much thicker silica surface layer that cracked upon cooling. This cracking of the surface oxide layer was caused by stresses arising from the volume difference between the nitride and the oxide, and to the polymorphic transformation of either cristobalite or tridymite. Exposure times beyond 100 hr did not yield continued lower strengths for the Si3N4, because the layer thickness remained essentially constant for exposure times greater than 100 hr. Hench et al. [8.27] reported that if Si3N4 contained 8 wt.% ZrO2 as a hot-pressing additive, an amorphous silicate film formed (≤18 µm thick) that provided an effective diffusion barrier to oxygen when heated to about 1430°C in air for 100hr. The room-temperature MOR of this material decreased about 40% after oxidation, whereas the MOR of a material containing 3 wt.% MgO decreased by more than 60%. The difference was attributed to the oxidized layer formed on the zirconiacontaining material being essentially amorphous, and that of the magnesia-containing material being totally crystalline and much thicker (~3×).

When tested at temperature, McCullum et al. [8.26] found the flexural strength for SiC remained constant with increasing exposure times to oxidation, with values less than when tested at room temperature. This lower strength obtained when tested at temperature when compared to room temperature strength, was attributed to the formation of a compressive layer on the surface when cooled. In contrast, the Si3N4 exhibited a slight increase in strength with exposure time when tested at temperature with values greater than the room temperature values. This was attributed to the integrity of the surface layer at temperature. These data are generalized in Fig. 8.3.

McCullum et al. [8.26] gave the following equation to evaluate the dynamic fatigue of several different SiC and Si3N4 samples:

(8.5)

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FIGURE 8.3 Fracture strength vs. time for Si3N4 and SiC. (From Ref. 8.26.)

where:

Sf=fracture strength A’=material constant s=loading rate

n=stress corrosion susceptibility constant

Although they showed no strength vs. loading rate plots, one can obtain the value of n from the slopes of these plots. The values for the stress corrosion susceptibility constant, obtained in this manner for several different materials, ranged from infinity to about 8, over temperatures ranging from 20 to 1400°C, respectively.

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Because of a larger quantity of sintering aids for the Si3N4 as compared to the SiC samples, the nitride samples were more susceptible to strength loss due to increased temperatures and decreased loading rates than the carbide. Variations in the amount and chemistry of the sintering aids in Si3N4 caused a variation in the mechanical behavior. Tensile test results followed the same general trends as flexural test results. In general, the tensile strength values were lower than the flexural values.

In the evaluation of a sintered and a hot-pressed Si3N4 under pure oxidation and oxidation under a static load, Easler et al. [8.28] found that, when exposed to air at 1370°C for times ranging from 0.5 to 50 hr, fracture data indicated that the range in flaw sizes decreased, whereas it increased after exposure for 1 hr under a static load. For the sintered material, strengths increased for oxidation under a static load of 23 or 45 MPa; however, the higher load condition resulted in a wider range of flaw sizes. In contrast, the hot-pressed material exhibited lower strengths for static loads of 45 and 160 MPa during oxidation. Under pure oxidation, the strengths of both materials increased for short (0.5 hr) oxidation times, and then decreased at longer times. The increased strengths were attributed to flaw tip blunting. The reduced strengths under static loading conditions were attributed to subcritical crack growth. Easler et al. concluded that the strength-controlling mechanisms, at least for silicon nitride, were dynamic in nature and very material-specific.

Rapid oxidation in air of Y-doped, sintered reaction bonded Si3N4 at 1000°C was reported by Govila et al. [8.29] to lower the strength and cause early failure. The fracture origins were determined to be β-Si3N4 needles. An excessive weight gain was reported to occur at 1000°C that was attributed to oxygen and nitrogen absorption of the matrix and secondary phases, one of which was reported to be YSiO2N. The oxidation of YSiO2N to Y2Si2O7 is accompanied by a 12% molar volume change. This anomalously high weight gain was accompanied by a 15% loss in the room-temperature strength. Stress rupture

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tests indicated the presence of stress-enhanced oxidation at 1000°C, with failure times ranging from 19 to 93 hr at an applied load of 138 MPa, and from 14 to 31 hr at an applied load of 276 MPa. Losses in strength at temperatures greater than 1200°C were attributed to the softening of the glassy grain boundary phase, which leads to creep by grain boundary sliding. Samples exposed to oxidation at 1200°C at an applied load of 344 MPa, did not fail, even after 260 hr, although some slight deformation had occurred.

In an effort to determine the effects of oxidation upon the flexural strength of Si3N4, Kim and Moorhead [8.30] evaluated the room-temperature four-point bend strength of HIP-SN (with 6 wt.% Y2O3 and 1.5 wt.% Al2O3) after exposure in either H2/H2O or Ar/O2 at 1400°C for 10 hr. In both atmospheres, the strength was dependent on the amount of oxidant present. However, the actual variation in strength was different, depending upon the alteration of the surface layers formed and their characteristics. In the H2/H2O atmosphere at low pH2O, a nonprotective and not well-attached glass-like layer containing crystalline Y2Si2O7 formed. Because this layer was relatively uniform with no new strength-limiting flaws being formed (although some large bubbles were found at the surface/substrate interface), the maximum reduction in strength was limited to about 20% at a pH2O of 2×10-5 MPa. A significant strength increase occurred as the pH2O was increased, which the authors attributed to blunting of preexisting cracks by the interfacial silicate phase. This silicate phase was a continuous dense layer of Y2Si2O7 containing small isolated bubbles believed to be formed by nitrogen generation during oxidation of the Si3N4. In the Ar/O2 atmosphere, a similar reduction and subsequent increase in strength was not found. Instead, at low pO2, an increase in strength occurred with increasing pO2. The maximum strength occurred at pO2 (10-5 MPa) that yielded the greatest weight loss. Even at low pO2, a surface reaction product of Y2Si2O7 formed in isolated pockets at grain junctions, presumably by the reaction of Y2O3 solid with SiO gas. Kim and Moorhead attributed the increased

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strengths observed to the formation of more Y2Si2O7 as the pO2 increased. At approximately a pO2 of 10-5 MPa, where the maximum strength was observed, the Y2Si2O7 layer became interconnected and, although not continuous, blunted strength limiting flaws. At higher pO2, where weight gains were observed and a continuous layer containing Y2Si2O7 and cristobalite formed, the increase in strength was not as significant. In this region, competition between crack blunting and formation of new flaws (cracks and bubbles) was suggested as the reason for the slightly lower strengths. This particular study by Kim and Moorhead pointed out very well the effects that the surface layer characteristics have upon the mechanical properties. Similar strength increases were found by Wang et al. [8.31] for two silicon nitride materials, one containing 13.9% Y2O3 plus 4.5% Al2O3 and the other containing 15% Y2O3 plus 5% Al2O3, when exposed to air at 1200°C for 1000 hr prior to strength testing at 1300°C. Strength increases as high as 87% were reported when compared to the unoxidized 1300°C strength, although the preoxidized 1300°C strength was slightly less than the unoxidized room temperature strength. Wang et al. attributed these strength increases to healing of surface flaws and crack blunting during oxidation, along with purification of the grain boundaries that raised the viscosity of the glassy boundary phase. These beneficial effects were not present when oxidation was conducted at 900°C.

Lange and Davis [8.32] have suggested that oxidation can lead to surface compressive stresses that, if optimum, may lead to increased apparent strengths. If the compressive stresses become too severe, then spalling may occur leading to lowered strengths. They demonstrated this concept with Si3N4 doped with 15% and 20% CeO2 exposed to oxidation in air, at temperatures ranging from 400 to 900°C. The apparent critical stress intensity factor (Ka) increased for short exposure times at 400, 500, and 600°C. This increase in Ka was attributed to oxidation of the Ce-apatite secondary phase and subsequent development of a surface compressive layer. At longer times ( ~ 8 hr) and the two higher temperatures, surface spalling caused a decrease in Ka. At higher

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