Steven M. Goodrich

Thermal, structural, and optical properties of Cleartran® multispectral zinc sulfide

Thermal, mechanical, and optical properties of Cleartran® multispectral zinc sulfide were measured. Heat capacity, thermal conductivity, and thermal expansion were measured over the temperature range 213 to 643 K. Youngs modulus and shear modulus were measured at 289 to 473 K. The characteristic four-point flexure strength for a 1-cm2 area was 75.4±0.5 MPa at 289 K and 89.5±0.4 MPa at 473 K, with Weibull moduli of 11.0±0.7 and 19.3±1.5, respectively. All 24 specimens tested at 289 K conformed to the Weibull curve. However, 2 of 23 specimens tested at 473 K had low, outlying strengths. The fracture origins of the outliers were surface flaws that were not detected by microscopic examination prior to mechanical testing. The only reliable means that we are aware of to identify weak material is a mechanical proof test. Infrared transmittance and reflectance at 297, 373, and 473 K were measured with an integrating sphere. The transmittance of s- and p-polarized infrared light was measured at angles of incidence of 0 to 60 deg. By fitting the transmittance with the Fresnel equations, the absorption coefficient was computed for wavelengths of 8 to 14 µm. Bidirectional transmittance and reflectance distribution functions (optical scatter) were measured at 3.39 and 10.6 µm.

Material Characterization of Candidate Silicon Based Ceramics for Stationary Gas Turbine Applications

The Ceramic Stationary Gas Turbine (CSGT) Program is evaluating the potential of using monolithic and composite ceramics in the hot section of industrial gas turbines. Solar Turbine’s Centaur 50 engine is being used as the test bed for ceramic components. The first stage blade, first stage nozzle and the combustor have been selected to develop designs with retrofit potential, which will result in improved performance and lowered emissions. As part of this DOE sponsored initiative a design and life prediction database under relevant conditions is being generated. This paper covers experiments conducted to date on the evaluation of monolithic silicon based ceramics. Mechanical property characterizations have included dynamic fatigue testing of tensile as well as flexural specimens at the temperatures representative of the blade root, the blade airfoil and the nozzle airfoil. Data from subcomponent testing of blade attachment concepts are also included.Copyright

How edge finish effects the strength of sapphire

Exotic Electro-Optics (EEO) has completed a study of how the edge finish of an A-plane sapphire sample affects its flexural strength when tested using the 4-point bend test method. Flexural bar samples were fabricated out of a sapphire panel that was polished to production quality using EEOs standard production methods. All samples were configured to meet the requirements for a C-size sample as defined by ASTM C-1161. The only difference between the three sample groups was the edge finish applied to the sample - conventionally ground, fine ground or a commercial polish edge finish. The edge finish on each sample was quantitatively characterized prior to strength testing. All samples were visually inspected prior to testing to identify any potential fracture initiation points. The samples were then tested using an Instron Universal tester per ASTM C-1161 in the UDRI Ceramics and Glasses Laboratory. After testing, a visual inspection was performed to identify the fracture initiation surface and location. Observations confirmed that all sample data was valid (all fractures initiated inside the two inner load dowels), no fractures were initiated on the edges, and no fractures initiated at any of the suspect sites noted in the pre-test visual inspection. The data was post processed using standard statistical and Weibull analysis methodologies. The results showed no significant difference when comparing the flexural strength of the three edge finish groups. The data suggest that the surface quality of the planar surfaces and the bevels is more critical than the finish of the full edge.

Electron Irradiation of Transparent and Ceramics Window Materials

The use of energetic electrons to modify the optical and mechanical properties of several window materials was examined. The materials were exposed to fields of high-energy electrons (5 MeV at a dose of 1,000 MRad). In this paper, we will report on the electron irradiation effects on the following materials: alumina, ALON, ZnSe and ZnS. Alumina irradiated under these conditions revealed little if any changes in flexure strength at room temperature. Irradiation changes in ALON hardness were measured. The hardness fracture toughness of electron beam irradiated ZnS and ZnSe was examined by both indentation and known flaw methods. Toughness measured by both methods were then compared and contrasted to ascertain the effects induced by the irradiation. The electron irradiation produced changes in the fracture toughness of both the ZnS and the ZnSe. The optical properties of the ZnSe and ZnS were measured by FTIR indicated minor changes in the absorption spectra.

Sapphire statistical characterization and risk reduction program

The Sapphire Statistical Characterization and Risk Reduction Program tested 1400 4-point flexure bars with different crystal orientations at different temperatures to establish a mechanical strength database for engineering design. Sapphire coupons were selected to represent surfaces on two different missile windows and a missile dome. Sapphire was obtained from the same suppliers used for the windows or dome and, as much as possible, coupons were fabricated in the same manner as the corresponding part of the window or dome. For one missile window, sapphire from one fabricator was 50% stronger than sapphire made to the same specifications from the same blanks by another fabricator. In laser thermal shock tests, sapphire performed better than predicted from flexure tests. Of several nondestructive methods evaluated for their ability to identify mechanically weak specimens, only x-ray topography was correlated with strength for a limited set of specimens.

Slow crack growth study of polycrystalline alumina and multispectral zinc sulfide

Samples of fine-grain, transparent polycrystalline alumina (CeraNova Corp) and multispectral zinc sulfide (Cleartran) were tested to determine mechanical strength and slow crack growth parameters. Mechanical strength measurements of coupons were fit to a Weibull equation that describes the material strength and its distribution. Slow crack growth parameters were calculated using the procedure set forth by Weiderhorn.1 This paper describes the derivation of Weibull and slow crack growth parameters from strength measurements over a range of stress rates and how these parameters are used to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

The Thermal-Mechanical Behavior of Selected Si3N4 and SiC Ceramics

This paper presents the results of a long term program initiated in December of 1984 to investigate the effects of environment (temperature, atmosphere, and stress) on the mechanical behavior of eight Si3N4 and three SiC ceramics being considered for heat engine applications. Microstructure, chemistry, and physical properties were determined. The mechanical behavior of these materials was investigated from room temperature to 1400°C by employing tests for flexural and tensile strength, dynamic, static and cyclic fatigue, and fracture toughness. The results obtained from these evaluations showed that the thermal mechanical behavior was quite varied, depending on the composition and processing methods employed. Batch to batch differences were also found to cause variances in the property values measured. Insights gained from this work about the failure mechanisms and potential service life are also discussed.Copyright