T. P. Kirkland

Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y2O3-doped -Si3N4

Abstract The uniaxial tensile and compressive creep rates of an yttria-containing hot-isostatically-pressed silicon nitride were examined at several temperatures between 1316 and 1399°C and found to have different stress dependencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both tensile and compressive stresses. The model also depicted the asymmetric creep deformation using exponential and linear dependence on tensile and compressive stress, respectively. Unlike other models which represent either tensile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deformation rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model’s regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavitation accounted for the creep strain asymmetry between them, and that cavitation initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0·1%.

Rolling Contact Fatigue of Ceramics

High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Silicon nitride (Si{sub 3}N{sub 4}) has been found to have a good combination of properties suitable for these applications. However, much is still not known about rolling contact fatigue (RCF) behavior, which is fundamental information to assess the lifetime of the material. Additionally, there are several test techniques that are employed internationally whose measured RCF performances are often irreconcilable. Due to the lack of such information, some concern for the reliability of ceramic bearings still remains. This report surveys a variety of topics pertaining to RCF. Surface defects (cracks) in Si{sub 3}N{sub 4} and their propagation during RCF are discussed. Five methods to measure RCF are then briefly overviewed. Spalling, delamination, and rolling contact wear are discussed. Lastly, methods to destructively (e.g., C-sphere flexure strength testing) and non-destructively identify potential RCF-limiting flaws in Si{sub 3}N{sub 4} balls are described.

The effects of residual α phase on the 1370 °C creep performance of yttria-doped HIPed silicon nitride

The creep behaviour at 1370°C (2500°F) of yttria-doped, hot isostatically pressed silicon nitride was examined as a function of residual α phase content. The pre-test silicon nitride materials had either 30% or 40% α phase content. The creep resistance was found to increase as the residual α phase content decreased. For equivalent times and stresses, the higher α-containing silicon nitride accumulated more creep strain and exhibited faster creep rates. The residual α phase decreased as a function of time at 1370°C and converted to β phase; it was also found that the α to β phase transformation rate was enhanced by stress. In the absence of stress, the kinetics of the α to β phase transformation at 1370°C followed a first-order reaction. If a first-order reaction was assumed for the α to β phase transformation in the presence of stress at 1370°C, then the magnitude of the reaction rate constant for this transformation was twice as large for tensile stresses equal to or greater than 130 MPa than for the reaction rate constant describing the transformation with no applied stress.

Strength and dynamic fatigue of silicon nitride at intermediate temperatures

The flexure strength distributions and dynamic fatigue of GS44, NT551, and NT154 silicon nitrides were determined at intermediate temperatures (≤850°C) in ambient air. GS44 and NT551 were fabricated by gas pressure sintering and had a larger volume fraction of secondary phase than the hot-isostatically pressed NT154. The inert characteristic strength for the GS44 and NT551 decreased with temperature while that for the NT154 was statistically independent of temperature. The slow crack growth exponent, N, for the GS44 and NT551 exhibited a 70–80% decrease between 700 and 850°C while the strength of NT154 was comparatively independent of stressing rate at 850°C. Associated with those changes in mechanical performances, the coefficient of thermal expansion (CTE) for the GS44 and NT551 sharply increased between 700 and 850°C while that for the NT154 did not. Although the observed change in CTE (and the temperature range where it occurred) did not have any obvious correlation with a change in inert strength at these intermediate temperatures, its presence was an indicator of a threshold temperature where dynamic fatigue resistance was compromised if the temperature exceeded it. The change in material state (and the associated temperature region for which it occurs) is linked to the equilibrium state of the secondary phase in the silicon nitride.

Stress relaxation behavior and dimensional stability of INCONEL® alloy 783

Abstract The stress relaxation behavior of a recently developed, low coefficient-of-thermal expansion superalloy, INCONEL ® alloy 783, 1 has been studied in the temperature range from 482 to 649°C and at two different strain levels of 0.1 and 0.2%. These conditions are representative of bolting applications in steam turbines. The initial stress increase, observed at the lower temperatures and stresses, is correlated with dimensional instabilities upon extended exposures at the same temperatures, i.e. the volumetric shrinkage measured in situ in a dilatometer. The experimental results are discussed in the context of creep and transformation strains.

Creep of CaO/SiO2-containing MgO refractories

Compressive creep of five commercially-available brands of CaO/SiO2-containing MgO refractories was measured over a temperature range of 1400-1550°C and compressive stresses of 0.10–0.30 MPa. All brands had a MgO content greater than 96 wt%, a CaO/SiO2 wt% ratio equal to or greater than 1.9, and a firing temperature greater than 1535°C. The more creep resistant brands were observed to have a combination of: (1) a larger average grain size and wider grain size distribution, (2) a low iron content, and (3) an absence of CaO-MgO-SiO2 compounds. Creep-stress exponents for three of the five brands indicated their creep was rate-controlled by diffusion, and their activation energy values indicated that creep was accommodated by grain boundary sliding through viscous flow of the calcium silicate grain-boundary phase. Two brands exhibited dramatic time-hardening behavior which resulted in their creep not being well-represented by the power-law creep formulation. The observed attributes among the brands were combined and a hypothetical CaO/SiO2-containing MgO refractory is proposed.

The high temperature compressive strength of non-oxide ceramic foams

Abstract The compression strengths of five coated vitreous carbon foams under development were measured at 25, 1000, 1200 and 1400°C. Only qualitative trends in the measured strengths were obtainable due to a numerical lack of specimens. One developmental foam was a pyrolytic carbon (PC)-coated reticulated vitreous carbon (RVC) foam, and it was tested in argon at the three elevated temperatures. Four developmental RVC foams had chemical vapor infiltration (CVI)-SiC coatings on them, each with different coating thicknesses and consequential different bulk densities; these SiC-RVC foams were tested in ambient air at the elevated temperatures. The strength of the PC-RVC foam was independent of temperature up to 1400°C in argon. The compressive strengths of the SiC-RVC foams having the two thinnest coatings (or the two smallest bulk densities) were equivalent in ambient air to those of the PC-RVC foam in argon up to 1400°C, while the SiC-RVC foams having the two thickest coatings (or the two greatest bulk densities) were consistently stronger. These results show thin-SiC layered SiC-RVC foams grant oxidation protection that PC-RVC foams do not possess to 1400°C, and that thicker SiC layers on SiC-RVC foams are required for added strength.

Evolution of Oxidation and Creep Damage Mechanisms in HIPed Silicon Nitride Materials

Several yttria-fluxed, hot-isostatically pressed (HIPed) silicon nitrides have been tensile creep tested at temperatures representative of those present in expected gas turbine engines. Creep and oxidation assisted damage mechanisms concurrently evolve when these materials are tested at relatively high temperatures and low stresses (i. e., long exposure times at temperature). Atmospheric creep testing results in the creation of oxygen and yttrium gradients across the radial dimension of the specimens. High concentrations of oxygen and yttrium coincide with dense populations of lenticular-shaped cavities near the surface of the crept specimens. The center of the tensile specimens was devoid of oxygen or yttrium; in addition, lenticular cavities were rarely observed. The gradient in lenticular-cavity concentration is coincident with the oxygen and yttrium gradients. Stress corrosion cracking (SCC) also occurs in these HIPed silicon nitrides when they are subjected to stress at high temperatures in ambient air. The size of this damage zone increases when the temperature is higher and/or the applied stress is lower. Stress-corrosion cracking initiates at the surface of the tensile specimen and advances radially inwards. What nucleates SCC has not yet been identified, but it is believed to result from a stress-concentrator (e. g., machining damage) at the specimen’s surface and its growth is a result of the coalescence of microcracks and cavities. The higher concentration of oxygen and yttrium in the grain boundaries near the specimen’s surface lessens the local high temperature mechanical integrity; this is believed to be associated with the growth of the SCC zone. This SCC zone continues to grow in size during tensile loading until it reaches a critical size which causes fracture.

Size-scaling of tensile failure stress in boron carbide

Abstract Abstract Weibull strength size scaling in a rotary ground, hot pressed boron carbide is described when strength test coupons sampled effective areas from very small (∼0·001 mm2) to very large (∼40 000 mm2). The testing of this ceramic is relevant because it is a candidate material for use in personnel armour. Equibiaxial flexure and Hertzian testing were used for the strength testing. Characteristic strengths for several different specimen geometries are analysed as a function of effective area. Characteristic strength was found to substantially increase with decreased effective area and exhibited a bilinear relationship. Machining damage limited strength as measured with equibiaxial flexure testing for effective areas greater than ∼1 mm2, and microstructural scale flaws limited strength for effective areas <0·1 mm2 for the Hertzian testing. The selections of a ceramic strength to account for ballistically induced tile deflection and expanding cavity modelling are uniquely considered in context with the measured strength size scaling.

Strength Distribution Changes in a Silicon Nitride as a Function of Stressing Rate and Temperature

Machining damage (a surface flaw) and porous-region-flaw (a volume flaw) populations limited the flexure strengths of a commercially available silicon nitride at 25°C, while these same flaws, along with inclusions, limited flexure strengths at 850°C. The machining damage and porous region flaws were the primary interest in the present study because they caused failure at both temperatures. Censoring revealed that the two-parameter Weibull strength distributions representing each flaw population changed as a function of stressing rate (i.e., dynamic fatigue) and temperature. A decrease in the Weibull scaling parameter is recognized as an indication of slow crack growth or time-dependent strength reduction in monolithic ceramics. Available life prediction codes used for reliability predictions of structural ceramic components consider the slow crack growth phenomenon. However, changes in the Weibull modulus are infrequently observed or reported, and typically are not accounted for in these life prediction codes. In the present study, changes in both Weibull parameters for the strength distributions provided motivation to the authors to survey what factors (e.g., residual stress, slow crack growth, and changes in failure mechanisms) could provide partial or full explanation of the observed distribution changes in this silicon nitride. Lastly, exercises were performed to examine the effects of strength distribution changes on the failure probability prediction of a diesel exhaust valve. Because the surface area and volume of this valve were substantially larger than those of the tested bend bars, it was found that the valve’s failure probability analysis amplified some slight or inconclusive distribution changes which were not evident from the interpretation of the censored bend bar strength data.© 1998 ASME