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failure occurred upon loading the samples. Swab and Leatherman also reported a 30% decrease in the roomtemperature strength of Y-TZP after 500 hr at 1000°C in the presence of Na2SO4. This lowered strength for Y-TZP was probably a result of leaching of the yttria from the surface, which caused the transformation of the tetragonal phase to the monoclinic phase.

8.3.5 Degradation by Molten Metals

The strength degradation of sintered α-silicon carbide was evaluated in both an as-received and as-ground (600 grit) condition after exposure to molten lithium by Cree and Amateau [8.51]. Transgranular fracture was exhibited for all samples when treated at temperatures below 600°C. At temperatures above 600°C, both transgranular and intergranular fracture occurred. The transgranular fracture strengths were generally greater than 200 MPa, whereas the intergranular strengths were less than 200 MPa. The lowstrength intergranular failure was attributed to lithium penetration along grain boundaries beyond the depth of the uniform surface layer that formed on all samples. Grain boundary degradation was caused by the formation of Li2SiO3, from the reaction of oxidized lithium and silica. The formation of lithium silicate was accompanied by an increase in volume by as much as 25%, depending upon the temperature of exposure. The localized stresses caused by this expansion promoted intergranular crack propagation.

8.3.6 Degradation by Aqueous Solutions

Bioactive Materials

Bioactive ceramics include those materials that rapidly react with human tissue to form direct chemical bonds across the interface. Poor bonding across this interface and a sensitivity to stress corrosion cracking has limited the use of some

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materials. Alumina is one material that has received a reasonable amount of study. Porous alumina has been shown to lose 35% of its strength in vivo after 12 weeks [8.52]. Seidelmann et al. [8.53] have shown that alumina loses about 15% of its strength after exposure to deionized water or blood when subjected to a constant stress. They also concluded that the service life of a hip endoprosthesis was dependent upon the density of the alumina. Ritter et al. [8.54] studied the effects of coating alumina with a bioactive glass that retarded the fatigue process.

Bioactive glasses, although bonding well to bone and soft tissue, generally lack good mechanical properties. Bioactive glasses are especially sensitive to stress corrosion cracking. Barry and Nicholson [8.55] reported that a soda-lime phosphosilicate bioactive glass was unsuitable for prosthetic use at stresses above 15 MPa, thus limiting its use to tooth prostheses. This glass sustained a tensile stress of 17 MPa for only 10 years in a pH=7.4 environment. Troczynski and Nicholson [8.56] then studied the fatigue behavior of particulate and fiber-reinforced bioactive glass of the same composition. The reinforcement materials were either -325 mesh silver powder or silicon carbide whiskers. These materials were mixed with powdered glass and hot-pressed at

700°C and 30 MPa for 30 min. The composite containing the silver particulates exhibited a decreased sensitivity to stress corrosion cracking, while the composite containing the silicon carbide whiskers exhibited a sensitivity similar to that of the pure glass. Comparison of the 10-year lifetimes of the two composites indicated that the particulate-containing material survived a static stress of 22 MPa, and the whisker-containing material survived a static stress of 34 MPa. Fractography results indicated agglomerate-initiated failure for the composites as opposed to surface machining defects for the pure bioactive glass.

Nitrides

In the evaluation of several hot isostatically pressed silicon nitrides, Sato et al. [8.57] found that the dissolution in HCl of the sintering aids (Y2O3 and Al2O3) from the grain boundaries

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decreased the three-point flexural strength. Their test variables included acid concentration, temperature, duration of dissolution, and crystallinity of the grain boundary phase. In general, the flexural strength decreased with increasing dissolution of Y3+ and Al3+ cations. Strengths were decreased by at least 50% after being exposed to 1 M HCl solution for 240 hr at 70°C. As expected, the grain boundary phase, having the highest degree of crystallinity, exhibited the highest strength (i.e., it is easier to leach cations from a glass than from a crystal). A control composition containing no sintering aids exhibited little, if any, strength degradation after the HCl treatment, although the strengths were considerably below those materials containing sintering aids (initially 240 vs. 600 MPa).

Glassy Materials

In their investigation of silica optical fibers, Dabbs and Lawn [8.58] presented data that questioned the acceptance of the Griffith flaw concept, which assumed that the flaws were exclusively cracklike and were free of preexisting influences. The real problem lies in predicting fatigue parameters for ultrasmall flaws from macroscopic crack velocity data. Abrupt changes in lifetime characteristics can occur as a result of evolution of flaws long after their inception. To conduct experiments with well-defined flaws, many investigators are now using microindentation techniques. It has been reported by Lawn and Evans [8.59] that the formation of radial cracks from indentations is dependent upon the applied load. There exists a threshold load below which no radial cracks are generated; however, radial cracks may spontaneously form at the corners of subthreshold indentations long after the initial indent has been implanted if the surface is exposed to water [8.60]. Dabbs and Lawn reported data for silica optical fibers showing an abrupt increase in strength under low load conditions below the threshold for formation of radial cracks. They attributed this behavior to a transition from crack propagation-controlled failure to one of crack initiationcontrolled failure. Although the subthreshold indents had no

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well-developed radial cracks, they were still the preferred site for fracture origin and, therefore, must overcome crack initiation first. This crack initiation step, being close to the sample’s free surface, was thus sensitive to environmental interactions. This low load region exhibited three general features when compared to the high load region where failure was controlled by crack propagation: an increase in strength, an increase in fatigue susceptibility, and an increase in scatter of the data.

Matthewson and Kurkjian [8.61], however, have suggested that dissolution of high-strength silica fibers, with the subsequent formation of surface pits, was the cause of enhanced fatigue at low stress levels, and not the spontaneous crack “popin” as suggested by Dabbs and Lawn. “Pop-in” does occur for weaker fibers. Their dissolution theory of enhanced fatigue was supported by the data of Krause [8.62], who reported a twoto threefold reduction in strengths after exposure to water under zero stress. Because the time-to-failure was essentially linear with pH over the entire pH range, Matthewson and Kurkjian stated that the link between fatigue and dissolution was unclear. Matthewson et al. [8.63] showed that by incorporating colloidal silica into a polymer coating, substantial improvements in static fatigue and zero stress aging behavior could be obtained. This essentially delayed the onset of the fatigue knee (discussed below), leading to greater times-to- failure. The abrupt change of slope (or change in the fatigue parameter, n) in plots of applied stress vs. time-to-failure has been called the fatigue knee (see Fig. 8.5). If one were to extrapolate short-term data to longer times, a very much shorter fatigue life would be predicted. This fatigue knee, which has been well established for liquid environments, has also been recently established for vapor environments [8.64]. Matthewson et al. [8.63] have shown that the reduction in strength of silica fiber exposed to water under zero stress occurred at a time similar to that of the fatigue knee, and thus attributed both phenomena to the formation of surface pits by dissolution. These data all strongly suggested that enhanced

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FIGURE 8.5 Applied stress vs. time-to-failure; the so-called fatigue knee.

fatigue at low stress levels was caused by the initiation of new surface flaws from dissolution pits, and not by the propagation of cracks from preexisting defects. Thus it is best not to base lifetimes upon extrapolated data, but to study the behavior in the strength range of interest.

Ito and Tomozawa [8.11] investigated the effects of exposure to water and to Si(OH)4 aqueous solution at room temperature and 88°C upon the strength of high-silica glass rods. Mechanical strengths were determined at a constant stressing rate at room temperature in the two aqueous solutions and in liquid nitrogen. The room-temperature strength after exposure to Si(OH)4 at 88°C increased more rapidly during the first 250 hr than that of rods exposed to water. Strengths leveled off after 240 hr for the Si(OH)4 exposure, whereas strengths for rods exposed to water increased gradually throughout the entire range of exposure times approaching those of the Si(OH)4 exposed rods after 360 hr. Maximum obtained strengths were about 30% higher than for unexposed samples. The strengths of samples exposed to room temperature solutions were essentially unchanged. The weight loss at 88°C in water was much

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higher than in Si(OH)4 by a factor of about 10. As the strength increase was observed only when an observable weight loss was recorded, Ito and Tomozawa attributed the strength increase to a mechanism involving glass dissolution that increased the crack tip radii (i.e., crack blunting). If dissolution were the only phenomenon involved, strengths for water-exposed samples should be higher than those for Si(OH)4 exposed samples, because the dissolution was greater for samples exposed to water. Because solubility is a function of surface curvature, and if solubility and dissolution were proportional, the dissolution rate would decrease with decreasing crack tip radius. This leads to a variation in dissolution rate around the crack tip leading to diffusion of dissolved glass and the combined effect of dissolution and precipitation [8.65]. Ito and Tomozawa, therefore, attributed the strength increasing mechanism to one of crack tip blunting caused by dissolution and precipitation.

Crack tip blunting by a different mechanism was suggested by Hirao and Tomozawa [8.66] for soda-lime, borosilicate, and high-silica glasses that had been annealed at or near their transition temperatures for 1 hr in air or a vacuum. Diffusion of water vapor into the glasses as they were being annealed in air was confirmed by infrared spectroscopy. The more rapid strength increases for glasses annealed in air compared to those annealed in a vacuum were attributed to the faster rate of viscous flow (causing m ore rapid crack tip blunting) in the less-viscous water-containing glasses, indicating that the release of residual stresses by annealing was not the cause for the strength increase as suggested by Marshall and Lawn [8.67]. Hirao and Tomozawa thus suggested that the conventional idea of glass fatigue caused by crack propagation alone is not sufficient, and must include a cracksharpening step.

Environmentally enhanced crack growth was shown to be dependent upon composition in zirconia and barium fluoride glasses by Freiman and Baker [8.68]. They observed extended crack growth after 15 min in several

Copyright © 2004 by Marcel Dekker, Inc.