10 research outputs found
Mechanical properties of silicon nitride using RUS & C-Sphere methodology
Silicon Nitride is a type of engineering ceramics which has been used in ball bearing and other rolling contact applications due to its good fatigue life, high temperature strength and tribological performance. In this paper, the mechanical properties of Hot Isostatically Pressed (HIPed) and Sintered and Reaction Bonded Silion Nitride (SRBSN) have been studied. The elastic modulus and poisson’s ratio of three type of commerical grade HIPed silicon nitride, and groudn SRBSN with three surface condidtions were measured using a Resonance Ultrasound Spectroscopy (RUS). The RUS measurement reveals the variation of elastic properties across different types of HIPed silicon nitride specimens. The surface strength of silicon nitride are studied using a C-Sphere specimen, and the results show that different commercial grade HIPed silicon nitride show varying surface strength. The surface conditions of ground SRBSN have an effect on the surface strength of the specimens. The RUS and C-Sphere techniques can potentially be used to sample the quality and consistency of ball bearing elements
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Creep performance of candidate SiC and Si{sub 3}N{sub 4} materials for land-based, gas turbine engine components
Tensile creep-rupture of a commercial gas pressure sintered Si3N4 and a sintered SiC is examined at 1038, 1150, and 1350 C. These 2 ceramics are candidates for nozzles and combustor tiles that are to be retrofitted in land-based gas turbine engines, and there is interest in their high temperature performance over service times {ge} 10,000 h (14 months). For this long lifetime, a static tensile stress of 300 MPa at 1038/1150 C and 125 Mpa at 1350 C cannot be exceeded for Si3N4; for SiC, the corresponding numbers are 300 Mpa at 1038 C, 250 MPa at 1150 C, and 180 MPa at 1350 C. Creep-stress exponents for Si3N4 are 33, 17, and 8 for 1038, 1150, 1350 C; fatigue- stress exponents are equivalent to creep exponents, suggesting that the fatigue mechanism causing fracture is related to the creep mechanism. Little success was obtained in producing failure in SiC after several decades of time through exposure to appropriate tensile stress; if failure did not occur on loading, then the SiC specimens most often did not creep-rupture. Creep-stress exponents for the SiC were determined to be 57, 27, and 11 for 1038, 1150, and 1350 C. For SiC, the fatigue-stress exponents did not correlate as well with creep-stress exponents. Failures that occurred in the SiC were a result of slow crack growth that initiated from the surface
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Prediction of the Inert Strength Distribution of Si3N4 Diesel Valves
Censored Weibull strength distributions were generated with NT551 silicon nitride four-point flexure data using the ASTM C1161-B and 5.0 mm diameter cylindrical specimens. Utilizing finite element models and AlliedSignal's life prediction codes, the inert or fast fracture strength failure probability of a ceramic diesel valve was estimated from these data sets. The failure probability prediction derived from each data set were found to be more conservative than valve strength data. Fractographic analysis of the test specimens and valves showed that the cylindrical specimens failed from a different flaw population than the prismatic flexure bars and the valves. The study emphasizes the prerequisite of having coincident flaw populations homogeneously distributed in both the test specimen and the ceramic component. Lastly, it suggests that unless material homogeneity exists, that any meaningful life prediction or reliability analysis of a component may not be possible
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Differences in creep performance of a HIPed silicon nitride in ambient air and inert environments
High temperature tensile creep studies of a commercially available hot isostatically pressed (HIPed) silicon nitride were conducted in ambient air and argon environments. The creep performance of this HIPed silicon nitride was found to be different in these environments. The material crept faster (and had a consequential shorter lifetime) in argon than in ambient air at 1370{degrees}C at tensile stresses between 110-140 MPa. The stress dependence of the minimum creep rate was found to be {approx} 6 in argon and {approx} 3.5 in air, while the minimum creep rates were almost an order of magnitude faster in argon than in air at equivalent tensile stresses. Differences in the creep performance are explained with reference to the presence or absence of oxygen in the two environments
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Effect of cyclic loading on the creep performance of silicon nitride
Tension-tension cyclic fatigue tests (triangular waveform, {sigma}{sub max} = 100 MPa, R = 0.1) were conducted on hot isostatically pressed (HIPed) silicon nitride at frequencies spanning several orders of magnitude (5.6 {times} 10{sup {minus}6} to 0.1 Hz or 10{sup {minus}3} MPa/s to 18 MPa/s) at 1,370 C in air. The amount of cyclic creep strain was found to be a function of the frequency or stressing rate with greater strains to failure observed as the frequency or stressing rate decreased. The total strain was viewed as the sum of elastic, anelastic (or transient recoverable), and plastic (viscous or non-recoverable) strain contributions, after the empirical Pao and Marin model. The plastic strain was found to be the dominant component of the total creep and was unsatisfactorily represented by the Pao and Marin model. To circumvent this, a time exponent was introduced in the plastic strain term in the Pao and Marin model. This modification resulted in good correlation between model and experiment at the slower frequencies examined but over-predicted the cyclic creep strain at the faster frequencies. The utility of using the modified Pao and Marin model to predict cyclic creep response from static creep and strain relaxation tests is described
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Specimen Size Effect on the Creep of Si3N4
The effect of specimen size on the measured tensile creep behavior of a commercially available gas pressure sintered Si3N4 was examined. Button-head tensile test specimens were used for the testing, and were machined to a variety of different gage section diameters (ranging from 2.5 to 6.35 mm) or different surface-area-to-volume ratios. The specimens were then creep tested at 1350 Degrees C and 200 MPa with tensile creep strain continuously measured as a function of time. The steady-state creep rate increased and the lifetime decreased with an increase in diameter (or decrease in the ratio of gage section surface area to volume). The time and specimen size dependence of transformation of a secondary phase correlated with the observed creep rate and lifetime dependence
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Strength and Fatigue of NT551 Silicon Nitride and NT551 Diesel Exhaust Valves
The content of this report is excerpted from Mark Andrew's Ph.D. Thesis (Andrews, 1999), which was funded by a DOEYOTT High Temperature Materials Laboratory Graduate Fellowship. It involves the characterization of NT551 and valves fabricated with it. Greater detail of the described issues may be found in that reference or through communications with Andrew Wereszczak
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Stress relaxation of silicon nitride at elevated temperatures
The stress relaxation behavior of SN88, SN253, and NCX-5102 silicon nitride materials were experimentally determined in tension at 1300{degrees}C using buttonhead specimens. Specimens were held at constant strain after being loaded at 10 MPa/s to an initial stress of 276 MPa (40 ksi) or 414 MPa (60 ksi). The subsequent decay in tensile stress was measured as a function of time. A non-negative least squares algorithm used in conjunction with a generalized Maxwell model proved to be an efficient means to define characteristic relaxation modulus spectra and stress relaxation behavior. In the last part of this study, the utility of using short-term stress relaxation testing to predict long-term creep performance was examined