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Characterization of the adherence of plasma-sprayed ZrO/sub 2/ coatings

Description: Analysis of the microstructure and adherence of ZrO/sub 2/ coatings revealed that the adherence decreased with increasing coating thickness and could be increased by incorporating MgO as a second phase as well as by including noncubic ZrO/sub 2/ in the cubic ZrO/sub 2/ coating. Residual stresses from the plasma spraying process limit adherence (hence the coating thickness dependence) but these can be relieved by plastic flow in the MgO phase during post-spray cooling. Some degree of strength/toughening is also derived by the presence of microcracks and/or from transformation associated with the presence of tetragonal ZrO/sub 2/.
Date: January 1, 1980
Creator: Becher, P.F. & Rice, R.W.
Partner: UNT Libraries Government Documents Department

Analyses of residual thermal stresses in ceramic matrix composites

Description: Residual thermal stresses in ceramic matrix composites containing either ellipsoidal inclusions or short fibers (i.e., fibers of finite length) are considered. First, the residual stresses in ellipsoidal inclusions are uniform, and they are analyzed using a modified Eshelby model. Although closed-form analytical solutions are obtained, their formulations are formidable. When the aspect ratio of the ellipsoid is 0, 1, or infinity, simple analytical solutions can be obtained using different models, and they are in excellent agreement with those obtained from the modified Eshelby model. Second, residual stresses in short fibers are nonuniform, and they are analyzed using a modified shear lag model, in which imaginary fibers are introduced to satisfy the continuity condition at the fiber ends. The analytical solutions are compared to the experimental results.
Date: December 31, 1995
Creator: Hsueh, C.H. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Strong, Tough Ceramics Containing Microscopic Reinforcements: Tailoring In-Situ Reinforced Silicon Nitride Ceramics

Description: Ceramics with their hardness, chemical stability, and refractoriness could be used to design more efficient energy generation and conversion systems as well as numerous other applications. However, we have needed to develop a fundamental understanding of how to tailor ceramics to improve their performance, especially to overcome their brittle nature. One of the advances in this respect was the incorporation of very strong microscopic rod-like reinforcements in the form of whiskers that serve to hold the ceramic together making it tougher and resistant to fracture. This microscopic reinforcement approach has a number of features that are similar to continuous fiber-reinforced ceramics; however, some of the details are modified. For instance, the strengths of the microscopic reinforcements must be higher as they typically have much stronger interfaces. For instance, single crystal silicon carbide whiskers can have tensile strengths in excess of {ge}7 GPa or >2 times that of continuous fibers. Furthermore, reinforcement pullout is limited to lengths of a few microns in the case of microscopic reinforcement due as much to the higher interfacial shear resistance as to the limit of the reinforcement lengths. On the other hand, the microscopic reinforcement approach can be generated in-situ during the processing of ceramics. A remarkable example of this is found in silicon nitride ceramics where elongated rod-like shape grains can be formed when the ceramic is fired at elevated temperatures to form a dense component.
Date: June 27, 1999
Creator: Becher, P.F.
Partner: UNT Libraries Government Documents Department

R-curve behavior of whisker-reinforced ceramic composites

Description: Ceramics have attracted great interest for advanced heat engines and industrial applications as a result of their stiffness, light weight, corrosion resistance, and superior performance at high temperatures. However, their utilization in engineering applications is severely limited by their brittleness. One approach toward substantially reducing this brittleness is by incorporating strong whiskers into the ceramic matrix. Studies of whisker-reinforced ceramic composites have shown both rising fracture resistance with crack extension (i.e. R-curve behavior) and a three- to four-fold increase in steady-state toughness. The primary toughening mechanism is bridging of the crack surface by whiskers. This paper discusses the bridging mechanism in detail and presents R-curves for SiC whisker-reinforced Al{sub 2}O{sub 3} composites.
Date: June 1, 1995
Creator: Hsueh, Chun-Hway & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Stress-temperature-lifetime response of nicalon fiber-reinforced SiC composites in air

Description: Time-to-failure tests were conducted in four-point flexure and in air as a function of stress levels and temperatures to study the lifetime response of various Nicalon fiber-reinforced SiC (designated as Nic/SiC) composites with a graphitic interfacial coating. The results indicated that all of the Nic/SiC composites exhibit a similar stress-dependent failure at applied stress greater than a threshold value. In this case, the lifetimes of the composites increased with decrease in both stress level and test temperature. The lifetime of the composites appeared to be relatively insensitive to the thickness of graphitic interface layer and was enhanced somewhat by the addition of oxidation inhibitors. Electron microscopy and oxidation studies indicated that the life of the Nic/SiC composites was governed by the oxidation of the graphitic interfaces and the on of glass(es) in composites due to the oxidation of the fiber and matrix, inhibitor phases.
Date: February 1996
Creator: Lin, Hua-Tay & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Reinforced ceramics employing discontinuous phases

Description: The fracture toughness of ceramics can be improved by the incorporation of a variety of discontinuous reinforcing phases and microstructures. Observations of crack paths in these systems indicate that these reinforcing phases bridge the crack tip wake region. Recent developments in micromechanics toughening models applicable to such systems are discussed and compared with experimental observations. Because material parameters and microstructural characteristics are considered, the crack bridging models provide a means to optimize the toughening effects. 18 refs., 2 figs.
Date: January 1, 1990
Creator: Becher, P.F.
Partner: UNT Libraries Government Documents Department

Creep behavior in SiC whisker-reinforced alumina composite

Description: Grain boundary sliding (often accompanied by cavitation) is a major contributor to compressive and tensile creep deformation in fine-grained aluminas, both with and without whisker-reinforcement. Studies indicate that the creep response of alumina composites reinforced with SiC whiskers can be tailored by controlling the composite microstructure and composition. The addition of SiC whiskers (< 30 vol%) significantly increases the creep resistance of fine-grained (1--2 {mu}m) alumina in air at temperatures of 1,200 and 1,300 C. However, at higher whisker contents (30 and 50 vol%), the creep resistance is degraded due to enhanced surface oxidation reactions accompanied by extensive creep cavitation. Densification aids (i.e., Y{sub 2}O{sub 3}), which facilitate silica glass formation and thus liquid phase densification of the composites, can also result in degradation of creep resistance. On the other hand, increasing the matrix grain size or decreasing the whisker aspect ratio (increased whisker number density) results in raising the creep resistance of the composites. These observations not only explain the variability in the creep response of various SiC whisker-reinforced alumina composites but also indicate factors that can be used to enhance the elevated temperature performance.
Date: October 1, 1994
Creator: Lin, H. T. & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Initial results of a high-power microwave sintering experiment at ORNL

Description: Experiments have recently begun at Oak Ridge National Laboratory to develop microwave sintering techniques suitable for large ceramic parts. Microwave sintering offers the advantages of faster heating rates, more uniform heating, and greater energy efficiency than conventional sintering techniques. We are using 28-GHz, 200-kW cw gyrotrons as the heating source. An untuned cavity is used as the applicator to eliminate geometry sensitivity in coupling efficiency.
Date: January 1, 1986
Creator: Kimrey, H.D.; White, T.L.; Bigelow, T.S. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

The R-curve response of ceramics with microscopic reinforcements: Reinforcement and additive effects

Description: Using direct observations with the scanning electron and optical microscopes, simultaneous measurements of fracture resistance versus crack length (R-curve behavior) and crack interactions with microstructural features at the crack tip and in its wake were made. Selecting whisker-reinforced aluminas and self-reinforced silicon nitrides, one can examine the effects of systematic modifications of microstructure and composition on the R-curve response and the mechanisms giving rise to it. Specifically, increases in whisker content and size can increase the R-Curve response, even for short crack lengths. In the self-reinforced silicon nitrides, changes in alumina: yttria additive ratios also modify the R-curve. Modeling of the R-curve response allows one to verify toughening mechanisms and, with experimental studies, to optimize the R-curve behavior in ceramics containing microscopic reinforcements, e.g., whiskers and elongated grain structures.
Date: May 1, 1996
Creator: Becher, P.F.; Sun, E.Y.; Plucknett, K,P. & Hsueh, C.H.
Partner: UNT Libraries Government Documents Department

Elevated temperature static fatigue of a Nicalon fiber-reinforced SiC composite

Description: Static fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425 to 1,150 C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 {mu}m graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of {approximately} 100 MPa at temperatures < 950 C which decreased to {approximately} 70 MPa at 1,150 C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures {le} 700 C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures {ge} 950 C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425 C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 C.
Date: December 31, 1994
Creator: Lin, H.T.; Becher, P.F. & Tortorelli, P.F.
Partner: UNT Libraries Government Documents Department

Interfacial debonding versus fiber fracture in fiber-reinforced ceramic composites

Description: Toughening of fiber-reinforced ceramic composites by fiber pullout relies on debonding at the fiber/matrix interface prior to fiber fracture when composites are subjected to tensile loading. The criterion of interfacial debonding versus crack penetration has been analyzed for two semi-infinite elastic plates bonded at their interface. When a crack reaches the interface, the crack either deflects along the interface or penetrates into the next layer depending upon the ratio of the energy release rate for debonding versus that for crack penetration. This criterion has been used extensively to predict interfacial debonding versus fiber fracture for a crack propagating in a fiber-reinforced ceramic composite. Two modifications were considered in the present study to address the debonding/fracture problem. First, the authors derived the analysis for a strip of fiber, which had a finite width and was sandwiched between two semi-infinite plates of matrix. It was found that the criterion of interfacial debonding versus fiber fracture depended on the fiber width. Second, a bridging fiber behind the crack tip was considered where the crack tip initially circumvented the fiber. Subsequent to this, either the interface debonded or the fiber fractured. In this case, the authors have considered a bridging-fiber geometry to establish a new criterion.
Date: November 1, 1998
Creator: Hsueh, C.H.; Becher, P.F. & He, M.Y.
Partner: UNT Libraries Government Documents Department

Analyses of interfacial shear debonding in fiber-reinforced ceramic composites

Description: An important toughening mechanism in fiber-reinforced ceramic composites is pullout of fibers from the matrix during matrix cracking. This relies on mode II (i.e., shear) debonding at the fiber/matrix interface which can be analyzed using either the strength-based or the energy-based criterion. In the strength-based approach, debonding occurs when the maximum interfacial shear stress induced by the applied load reaches the interfacial shear strength, {tau}{sub s}. In the energy-based approach, a mode II crack propagating along the interface is considered, and debonding occurs when the energy release rate due to crack propagation reaches the interface debond energy, {Gamma}{sub 1}. Based on the above two criteria, the applied stress on the fiber to initiate debonding (i.e., the initial debond stress), {sigma}{sub d}, can be derived. The first issue considered in the present study is the relation between {tau}{sub s} and {Gamma}{sub 1}. Also, for a monolithic ceramic, the tensile strength can be related to its defect size based on the Griffith theory. A question is hence raised as to whether the initial debond stress for fiber pullout in a fiber-reinforced ceramic composite can be related to any defect at the interface.
Date: February 1, 1998
Creator: Hsueh, C.H. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Theoretical and experimental analysis of the toughening behavior of whisker reinforcement in ceramic matrix composites

Description: Analytical solutions are presented describing the experimentally verified toughening of whisker reinforced ceramics. Clear insights are provided into the interrelationships of whiskers, matrices, and interfaces in the case of strong interfaces with minimized whisker pullout.
Date: January 1, 1988
Creator: Becher, P.F.; Hsueh, C.H.; Angelini, P. & Tiegs, T.N.
Partner: UNT Libraries Government Documents Department

Effect of whisker surface treatments and processing conditions on the SiC/Al sub 2 O sub 3 interface

Description: Various electron microscopy techniques have been used to evaluate the microstructural and interfacial characteristics of silicon carbide whisker-reinforced alumina composites. The effects of subjecting whiskers to oxidizing and reducing treatments prior to composite fabrication were examined. Whisker/matrix interfaces in which carbon coatings were applied to whiskers prior to specimen fabrication were also examined. Physical and thermodynamic arguments can be used to explain why it is difficult to greatly modify the physical nature of the whisker/matrix interface. 7 refs., 7 figs.
Date: January 1, 1990
Creator: Alexander, K.B.; Angelini, P. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Fabrication and properties of alumina matrix composites containing nickel aluminide reinforcements

Description: Ductile nickel-aluminide intermetallic alloys have been successfully used to toughen ceramic materials intended for use at a wide range of temperatures. Traditional ceramic processing procedures have been used to produce a variety of microstructures. The fracture toughness increases with increasing particle aspect ratio, however, the flexural strength decreases with increasing particle size. Fracture toughnesses up to 7.6 MPa m{sup 1/2} and flexural strengths up to 550 MPa were observed in an alumina composite containing 10 vol.% nickel aluminide.
Date: September 1, 1994
Creator: Alexander, K. B.; Lin, H. T.; Schneibel, J. H. & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Processing and properties of FeAl-bonded composites

Description: Iron aluminides are thermodynamically compatible with a wide range of ceramics such as carbides, borides, oxides, and nitrides, which makes them suitable as the matrix in composites or cermets containing fine ceramic particulates. For ceramic contents varying from 30 to 60 vol.%, composites of Fe-40 at. % Al with WC, TiC, TiB{sub 2}, and ZrB{sub 2} were fabricated by conventional liquid phase sintering of powder mixtures. For ceramic contents from 70 to 85 vol.%, pressureless melt infiltration was found to be a more suitable processing technique. In FeAl-60 vol.% WC, flexure strengths of up to 1.8 GPa were obtained, even though processing defects consisting of small oxide clusters were present. Room temperature fracture toughnesses were determined by flexure testing of chevron-notched specimens. FeAl/WC and FeAl/TiC composites containing 60 vol.% carbide particles exhibited K{sub Q} values around 20 MPa m{sup 1/2}. Slow crack growth measurements carried out in water and in dry oxygen suggest a relatively small influence of water-vapor embrittlement. It appears therefore that the mechanical properties of iron aluminides in the form of fine ligaments are quite different from their bulk properties. Measurements of the oxidation resistance, dry wear resistance, and thermal expansion of iron aluminide composites suggest many potential applications for these new materials.
Date: December 31, 1996
Creator: Schneibel, J.H.; Subramanian, R.; Alexander, K.B. & Becher, P.F.
Partner: UNT Libraries Government Documents Department

Ductile intermetallic toughened carbide matrix composites

Description: Ductile Ni{sub 3}Al alloys have been used as binder phase for fabrication of TiC and WC matrix composites. Ni{sub 3}Al has good corrosion resistance to aqueous acidic environments, and its yield strength increases with temperature to a max at 700-800 C; this combined with high tensile ductilities (up to 50% strain) make Ni{sub 3}Al attractive for replacing Co in cemented carbides. Materials have been fabricated by both hot pressing and vacuum sintering, with Ni{sub 3}Al contents of 15 to 95 vol%. Vacuum sintering cycles, similar to those used for WC/Co and TiC/Ni (1450-1600 C), resulted in sintered densities >95% theoretical. WC/Ni{sub 3}Al materials showed an order of magnitude improvement in corrosion resistance over WC/Co, in sulfuric/nitric acid. These materials also had improved high temperature strength retention compared to WC/Co cermets, though initial RT strengths were lower. Fracture toughness varied between 8 and 25 MPa.m{sup 1/2} and depended primarily on Ni{sub 3}Al content and composition.
Date: August 1, 1996
Creator: Plucknett, K.P.; Tiegs, T.N.; Becher, P.F.; Waters, S.B. & Menchhofer, P.A.
Partner: UNT Libraries Government Documents Department

Verification of creep performance of a ceramic gas turbine blade

Description: Tensile creep tests were carried out on a Norton NT164 silicon nitride ceramic turbine blade containing 4 wt. % Y{sub 2}O{sub 3} sintering additive at 1,370 C in air under selected stress levels. The objective of this study was to measure the creep properties of test specimens extracted from a complex shaped ceramic gas turbine blade to verify the response of actual components. The creep results indicated that specimens from both the airfoil and dovetail sections exhibited creep rates that were about 4 to 100 times higher than those obtained from both the buttonhead and dogbone creep specimens machined from the developmental billets fabricated with the same composition and processing procedures. Electron microscopy analyses suggested that high creep rates and short lifetimes observed in specimens extracted from the turbine blade resulted from a higher glassy phase(s) content and smaller number density of elongated grain microstructure. Silicon nitride ceramics with an in-situ reinforced elongated microstructure have been the primary candidates for both advanced automotive and land-based gas turbine engine applications.
Date: March 1, 1998
Creator: Lin, H.T.; Becher, P.F.; Ferber, M.K. & Parthasarathy, V.
Partner: UNT Libraries Government Documents Department

Lifetime Response of a Hi-Nicalon Fiber-Reinforced Melt-Infiltrated SiC Matrix Composites

Description: Lifetime studies in four-point flexure were performed on a Hi-Nicalon<sup>TM</sup> fiber-reinforced SiC matrix composite over a temperature range of 700 degrees to 1150 degrees C in air. The composite consisted of ~40 vol. % Hi-Nicalon<sup>TM</sup> fiber (8-harness weave) with a 0.5 Mu-m BN fiber coating and a melt-infiltration SiC matrix wand was tested with as-machined surfaces. Lifetime results indicated that the composite exhibited a stress-dependent lifetime at stress levels above an apparent fatigue limit, similar to the trend observed in CG-Nicalon<sup>TM</sup> fiber reinforced CVI SiC matrix composites. At less than or equal to 950 degrees C, the lifetimes of Hi-Nicalon/MI SiC composites decreased with increasing applied stress level and test temperature. However, the lifetimes were extended as test temperature increased from 950 degees to 1150 degrees C as a result of surface crack sealing due to glass formation by the oxidation of Mi SiC matrix. The lifetime governing processes were, in general, attributed to the progressive oxidation of BN fiber coating and formation of glassy phase, which formed a strong bond between fiber and matrix, resulting in embrittlement of the composite with time.
Date: April 25, 1999
Creator: Becher, P.F.; Lin, H.T. & Singh, M.
Partner: UNT Libraries Government Documents Department

R-curve response of silicon carbide whisker-reinforced alumina: Microstructural influence

Description: Rising fracture resistance with crack extension (R-curve response) can lead to improvements in the mechanical reliability of ceramics. To understand how microstructures influence the R-curve behavior, direct observations of crack interactions with microstructural features were conducted on SiC whisker-reinforced alumina. The contribution of the dominant toughening mechanisms to the R-curve behavior of these composites is discussed using experimental and theoretical studies.
Date: December 31, 1995
Creator: Sun, E. Y.; Hsueh, C. H. & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Transformation behavior in Al{sub 2}O{sub 3}-ZrO{sub 2} ceramic composites

Description: Neutron powder diffraction was used to investigate the tetragonal to monoclinic transformation of ZrO{sub 2} in a A1{sub 2}O{sub 3}-ZrO{sub 2} ceramic composite containing 40 vol % tetragonal ZrO{sub 2}. The neutron diffraction data were analyzed using the Rietveld refinement technique, which allowed to determine the extent of the transformation as a function of temperature. The onset transformation temperature determined for this sample was 130 K. Below this temperature, the fraction of the monoclinic phase continued to increase to about 9 vol % at 80 K and remained constant for temperatures below 80 K. The calculated thermal expansion, using the refined lattice parameters, was found in excellent agreement with dilatometry data, confirming that the sharp increase in the thermal expansion upon cooling resulted from the tetragonal to monoclinic phase transformation in ZrO{sub 2}.
Date: August 15, 1994
Creator: Wang, X. L.; Fernandez-Baca, J. A.; Hubbard, C. R.; Alexander, K. B. & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Environmental aging degradation in continuous fiber ceramic composites

Description: The thermal stability of two-continuous fiber ceramic composites (CFCC`s) has been assessed. A Nicalon/CaO-Al{sub 2}O{sub 3}-SiO{sub 2} (CAS) glass-ceramic composite has been subjected to unstressed, oxidation heat treatments between 375 and 1200{degrees}C, after which the material was tested in flexure at room temperature. The static fatigue behavior of a chemical vapor infiltrated (CVI) Nicalon/SiC ceramic matrix composite has been assessed in air, between 425 and 1150{degrees}C, both with and without protective seal coating. Severe property degradation was observed due to oxidation of the graphite fiber/matrix interlayer in both CFCC`s.
Date: December 31, 1995
Creator: Plucknett, K. P.; Lin, H. T.; Braski, D. N. & Becher, P. F.
Partner: UNT Libraries Government Documents Department

Control of interface fracture in silicon nitride ceramics: influence of different rare earth elements

Description: The toughness of self-reinforced silicon nitride ceramics is improved by enhancing crack deflection and crack bridging mechanisms. Both mechanisms rely on the interfacial debonding process between the elongated {Beta}-Si{sub 3}N{sub 4} grains and the intergranular amorphous phases. The various sintering additives used for densification may influence the interfacial debonding process by modifying the thermal and mechanical properties of the intergranular glasses, which will result in different residual thermal expansion mismatch stresses; and the atomic bonding structure across the {Beta}-Si{sub 3}N{sub 4} glass interface. Earlier studies indicated that self-reinforced silicon nitrides sintered with different rare earth additives and/or different Y{sub 2}O{sub 3}:AI{sub 2}0{sub 3} ratios could exhibit different fracture behavior that varied from intergranular to transgranular fracture. No studies have been conducted to investigate the influence of sintering additives on the interfacial fracture in silicon nitride ceramics. Because of the complexity of the material system and the extremely small scale, it is difficult to conduct quantitative analyses on the chemistry and stress states of the intergranular glass phases and to relate the results to the bulk properties. The influence of different sintering additives on the interfacial fracture behavior is assessed using model systems in which {Beta}-Si{sub 3}N{sub 4}whiskers are embedded in SIAIRE (RE: rare-earth) oxynitride glasses. By systematically varying the glass composition, the role of various rare-earth additives on interfacial fracture has been examined. Specifically, four different additives were investigated: Al{sub 2}0{sub 3}, Y{sub 2}0{sub 3}, La{sub 2}O{sub 3}, and Yb{sub 2}O{sub 3}. In addition, applying the results from the model systems, the R- curve behavior of self-reinforced silicon nitride ceramics sintered with different Y{sub 2}0{sub 3}:AI{sub 2}0{sub 3} ratios was characterized.
Date: October 1, 1996
Creator: Sun, E. Y.; Becher, P. F.; Waters, S. B.; Hsueh, Chun-Hway; Plucknett, K. P. & Hoffmann, M. J.
Partner: UNT Libraries Government Documents Department

Interfacial composition analysis of silicon nitride whisker reinforced oxynitride glass systems

Description: Scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy and parallel-detection electron energy-loss spectroscopy was used to analyze the composition at interfaces in {beta}-Si{sub 3}N{sub 4} whisker reinforced oxynitride glass systems.
Date: June 1, 1995
Creator: Sun, E. Y.; Alexander, K. B.; Becher, P. F. & Hwang, S. L.
Partner: UNT Libraries Government Documents Department