Mattison K. Ferber

Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal René N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.

Oxidation and degradation of a plasma-sprayed thermal barrier coating system

The isothermal oxidation behavior of thermal barrier coating (TBC) specimens consisting of single-crystal superalloy substrates, vacuum plasma-sprayed Ni-22Cr-10Al-1Y bond coatings and air plasma-sprayed 7.5 wt.% yttria stabilized zirconia top coatings was evaluated by thermogravimetric analysis at 1150{degrees}C for up to 200 hours. Coating durability was assessed by furnace cycling at 1150{degrees}C. Coatings and reaction products were identified by x-ray diffraction, field-emission scanning electron microscopy and energy dispersive spectroscopy.

Mechanical reliability evaluation of silicon nitride ceramic components after exposure in industrial gas turbines

Several studies have recently been undertaken to examine the mechanical reliability and thermal stability of silicon nitride ceramic components that are currently being considered for structural application in industrial gas turbines. Specifically, ceramic components evaluated included a bow-shaped silicon nitride nozzle evaluated in an engine test rig, silicon nitride vanes exposed in an engine field test, and an air-cooled silicon nitride vane that is currently under development. Despite the differences in field test conditions all of the exposed silicon nitride ceramic components exhibited a significant material recession arising from the oxidation of silicon nitride and subsequent volatilization of the oxide (i.e., silica). The fracture strength of exposed airfoils was also decreased due to the formation of a subsurface damage zone induced by the turbine environments. In addition, studies indicated that the properties of as-processed ceramic components, especially in airfoil regions, were not always comparable to those generated from the standard specimens with machined surface extracted from production billets. The component characterization efforts provided an important insight into the effect of gas turbine environments on the material recession and mechanical reliability of materials as functions of exposure time and conditions, which were very difficult to obtain from a laboratory scale test.

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%.

Metastable tetragonal zirconia formation and transformation in reactively sputter deposited zirconia coatings

Zirconia coatings were produced by reactive d.c. magnetron sputter deposition using a system with multiple sputter sources and a biased substrate stage. Crystal structure and phase stability of the coatings were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tetragonal zirconia with either a random orientation or a highly (111) preferred orientation was deposited when a substrate bias was applied, whereas coatings grown with no substrate bias had the equilibrium monoclinic structure. It was revealed that bias sputtering effectively decreased crystallite size in the as-deposited coatings, which resulted in room temperature stabilization of the metastable tetragonal phase. XRD analysis of annealed coatings showed that the volume fraction and stability of the tetragonal phase was strongly dependent on substrate bias and annealing temperature.

Exposure of Ceramics and Ceramic Matrix Composites in Simulated and Actual Combustor Environments

A high-temperature, high-pressure, tube furnace has been used to evaluate the long term stability of different monolithic ceramic and ceramic matrix composite materials in a simulated combustor environment. All of the tests have been run at 150 psia, 1204 degrees C, and 15% steam in incremental 500 h runs. The major advantage of this system is the high sample throughput; >20 samples can be exposed in each tube at the same time under similar exposure conditions. Microstructural evaluations of the samples were conducted after each 500 h exposure to characterize the extent of surface damage, to calculate surface recession rates, and to determine degradation mechanisms for the different materials. The validity of this exposure rig for simulating real combustor environments was established by comparing materials exposed in the test rig and combustor liner materials exposed for similar times in an actual gas turbine combustor under commercial operating conditions.

The interlaminar tensile and shear behavior of a unidirectional CC composite

Abstract The interlaminar shear and tensile strengths of a unidirectional carbon-carbon composite were determined in air at room temperature and in argon at 1000°C. It was found that the room temperature interlaminar tensile strength was 2.53±0.23 MPa, whereas the interlaminar shear strength was found to be 11.35 ± 2.03 MPa at room temperature and 9.32 ± 2.59 MPa at 1000°C. Novel experimental procedures are described for the determination of the interlaminar shear strength of 1-D and 2-D composites by the compression of double-notched specimens both at room and elevated temperatures. Attempts to determine the interlaminar shear strength of this material by the Iosipescu test were unsuccessful.

Apparent coefficient of thermal expansion and residual stresses in multilayer capacitors

The thermal expansion behavior and residual stresses in multilayer capacitor (MLC) systems are analyzed in the present study. An MLC consists of a laminate of multiple alternating electrode layers and dielectric layers sandwiched between two ceramic cover layers. An analytical model is developed to derive simple closed-form solutions for the apparent coefficients of thermal expansion (CTEs) of the laminate. Plasticity of electrodes is included in the analysis. The predicted apparent CTEs are compared with measurements of some laminated ceramic composites. The effects of plasticity on apparent CTEs and residual stresses in MLC systems are discussed.

Tensile dynamic and static fatigue relations for a HIPed silicon nitride at elevated temperatures

Abstract Dynamic and static fatigue behavior of a hotisostatically pressed (HIPed) silicon nitride was investigated at 1150, 1260 and 1370°C. Uniaxial tensile tests were conducted over ranges of constant stresses and constant stress rates. Correlation of stress-life relations between static and dynamic fatigue results was evaluated and failure modes were determined as functions of temperature, stress and stress rate. At 1150°C the static and dynamic fatigue failures were controlled by a slow crack growth mechanism for all stresses and stress rates. Creep rupture was the dominant failure mechanism in static loading at 1260 and 1370°C. A transition in the dominant failure mechanism in dynamic fatigue at 1260 and 1370°C occurred at a stress rate of 10 −2 M Pa/s. Slow crack growth was the dominant failure mechanism with stress rates > 10 −2 M Pa/s while creep rupture was the governing mechanism for the failure at stress rates ≤10 −2 M Pa/s.

Shear Strength of Continuous Fiber Ceramic Composites

The applicability of ASTM Test Method for Shear Strength of Continuous Fiber-Reinforced Advanced Ceramics at Ambient Temperatures (C 1292) was investigated to determine the shear strength of one-dimensional and two-dimensional continuous fiber-reinforced ceramic composites. The advantages and disadvantages of the test method are addressed, and the effect of notch separation on the measured interlaminar shear strength by the compression of double-notched specimens are discussed. Experimental results are presented for the interlaminar and in-plane shear behavior of two-dimensional Nicalon/SiC and the interlaminar shear strength of one-dimensional carbon/carbon composites both at room temperature and 1000°C. The effect of fiber coating thickness on the interlaminar shear strength of two-dimensional Nicalon/SiC is also discussed.