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Study of Metal Whiskers Growth and Mitigation Technique Using Additive Manufacturing

Description: For years, the alloy of choice for electroplating electronic components has been tin-lead (Sn-Pb) alloy. However, the legislation established in Europe on July 1, 2006, required significant lead (Pb) content reductions from electronic hardware due to its toxic nature. A popular alternative for coating electronic components is pure tin (Sn). However, pure tin has the tendency to spontaneously grow electrically conductive Sn whisker during storage. Sn whisker is usually a pure single crystal tin with filament or hair-like structures grown directly from the electroplated surfaces. Sn whisker is highly conductive, and can cause short circuits in electronic components, which is a very significant reliability problem. The damages caused by Sn whisker growth are reported in very critical applications such as aircraft, spacecraft, satellites, and military weapons systems. They are also naturally very strong and are believed to grow from compressive stresses developed in the Sn coating during deposition or over time. The new directive, even though environmentally friendly, has placed all lead-free electronic devices at risk because of whisker growth in pure tin. Additionally, interest has occurred about studying the nature of other metal whiskers such as zinc (Zn) whiskers and comparing their behavior to that of Sn whiskers. Zn whiskers can be found in flooring of data centers which can get inside electronic systems during equipment reorganization and movement and can also cause systems failure.Even though the topic of metal whiskers as reliability failure has been around for several decades to date, there is no successful method that can eliminate their growth. This thesis will give further insights towards the nature and behavior of Sn and Zn whiskers growth, and recommend a novel manufacturing technique that has potential to mitigate metal whiskers growth and extend life of many electronic devices.
Date: August 2015
Creator: Gullapalli, Vikranth
Partner: UNT Libraries

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

Early Stages of Pulsed-Laser Growth of Silicon Microcolumns and Microcones in Air and SF<sub>6</sub>

Description: Dense arrays of high-aspect-ratio silicon microcolumns and microcones are formed by cumulative nanosecond pulsed excimer laser irradiation of single-crystal silicon in oxidizing atmospheres such as air and SF<sub>6</sub>. Growth of such surface microstructures requires a redeposition model and also involves elements of self-organization. The shape of the microstructures, i.e. straight columns vs steeply sloping cones and connecting walls, is governed by the type and concentration of the oxidizing species, e.g. oxygen vs fluorine. Growth is believed to occur by a �catalyst-free� VLS (vapor-liquid-solid) mechanism that involves repetitive melting of the tips of the columns/cones and deposition there of the ablated flux of Si-containing vapor. Results are presented of a new investigation of how such different final microstructures as microcolumns or microcones joined by walls nucleate and develop. The changes in silicon surface morphology were systematically determined and compared as the number of pulsed KrF (248 nm) laser shots was increased from 25 to several thousand in both air and SF<sub>6</sub>. The experiments in air and SF<sub>6</sub> reveal significant differences in initial surface cracking and pattern formation. Consequently, local protrusions are first produced and column or cone/wall growth is initiated by different processes and at different rates. Differences in the spatial organization of column or cone/wall growth also are apparent.
Date: July 29, 1999
Creator: Lowndes, D.H.; Fowlkes, J.D. & Pedraza, A.J.
Partner: UNT Libraries Government Documents Department

Whisker weaving: Invalid connectivity resolution and primal construction algorithm

Description: This paper describes the techniques used to resolve invalid connectivity created as a natural part of the whisker weaving algorithm. These techniques rely on the detection of {open_quotes}repeated hexes{close_quotes} in the STC data, which indicate face pairs which share two edges. The {open_quotes}repeated hex{close_quotes} case is described in detail, including the resolution technique by which a self-intersecting whisker sheet with two independent face loops are created. The algorithm used to construct the primal of an all-hexahedral mesh (i.e. the actual nodes and hex elements) from the connectivity data contained in the STC is also described. The primal is constructed using a {open_quotes}gift-wrapping{close_quotes} algorithm, where all the mesh edges and hexes containing a particular node are found by traversing between hexes already known to share the node. This algorithm is implemented inside the CUBIT code and is used to generate meshes for several example problems.
Date: November 1, 1995
Creator: Tautgas, T.J. & Mitchell, S.A.
Partner: UNT Libraries Government Documents Department

Comparison of short and longer term loading on the creep behaviour of alumina-silicon carbide whisker composites

Description: The incorporation of SiC whiskers into alumina ceramics has resulted in several improved room temperature properties of the composite such as fracture toughness, strength, erosion resistance and fatigue. The creep resistance has also been generally observed to increase with the addition of whiskers. However, significant scatter in measured secondary creep rates still exist, depending on the material and the test conditions that were utilized. The objective of this investigation was to compare two separate creep tests performed in compression using a SiC{sub w}/Al{sub 2}O{sub 3} material obtained from the same process batch. The differences in creep behavior between the tests could thus be ascribed to the difference in creep conditions solely. In one case the creep behavior was studied under short term loading (<3 h) conditions in air, while longer term loading (<100 h) conditions in argon were applied in the other case. The creep results and the microstructure of the samples were compared and related to each other, and to previous results in the literature.
Date: November 1, 1992
Creator: Swan, H.; Arellano-Lopez, A.R. de; Dominguez-Rodriguez, A.; Routbort, J.L. & Swain, M.V.
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

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

Degradation of structural ceramics by erosion

Description: Materials wastage by solid-particle erosion can be severe and can limit lifetimes. This paper will review the theoretical description of solid-particle erosion in brittle materials, which is well-developed for monolithic ceramics. The models can usually account for effects from the principal projectile properties of size, impact velocity, and impact angle. Materials parameters such as fracture toughness and hardness can be included. Steady-state erosion measurements on a wide variety of ceramics, ranging from Si single crystals to SiC-whisker-reinforced Al{sub 2}O{sub 3}, are reviewed and compared with the models. It is believed that R-curve behavior and/or particle fragmentation is responsible for discrepancies between theory and experimental results for composite ceramics. In addition, the theories make no attempt to describe threshold or incubation effects.
Date: December 1, 1994
Creator: Routbort, J.L.
Partner: UNT Libraries Government Documents Department

Whisker reinforced glass ceramic. Quarterly progress report No. 3, March 28, 1995--June 27, 1995

Description: Goal is to to investigate physical properties of an in-situ whisker- reinforced glass ceramic material and to produce prototype heat exchanger tubes for testing in a coal combustion environment. Problems in casting and cooling the glass were encountered and dealt with. The slow cooled glass melt powder will be used to prepare the bar samples for mechanical tests.
Date: July 25, 1995
Creator: Pickrell, G.R. & Brown, J.J. Jr.
Partner: UNT Libraries Government Documents Department

Plastic deformation of alumina reinforced with SiC whiskers

Description: Addition of small amounts of stiff reinforcement (SiC whiskers) to a polycrystalline AL{sub 2}O{sub 3} matrix partially inhibits grain boundary sliding because of an increase in threshold stress. When the concentration of whiskers is high enough, a pure diffusional mechanism takes over the control of plastic deformation of the composites. For higher whisker loadings, the materials creep properties depend on a microstructural feature different from the nominal grain size. A tentative correlation of this effective microstructural parameter with the spacing between the whiskers was established through a model.
Date: June 1, 1993
Creator: DeArellano-Lopez, A.R.; Dominguez-Rodriguez, A.; Goretta, K.C. & Routbort, J.L.
Partner: UNT Libraries Government Documents Department

GABAᴀ Receptors in Rat Whisker Barrel Cortex: Effects of Sensory Deprivation

Description: The GABAergic system in adult sensory cortex is affected by sensory deprivation, but little is known about how this predominant inhibitory system is affected during ontogeny. The present study investigates developmental effects of whisker trimming on GABAa receptors in rat barrel cortex. Rats trimmed for 6 wk beginning at birth and adulthood showed similar decreases in [3H]muscimol binding in deprived relative to non-deprived barrels, suggesting absence of a critical period.
Date: August 1995
Creator: Salazar, Eduardo, 1962-
Partner: UNT Libraries


Description: Evolution of local strain on electrodeposited tin films upon aging has been monitored by digital image correlation (DIC) for the first time. Maps of principal strains adjacent to whisker locations were constructed via comparing pre- and post-growth scanning electron microscopy (SEM) images. Results showed that the magnitude of the strain gradient plays an important role in whisker growth. DIC visualized the dynamic growth process in which the alteration of strain field has been identified to cause growth of subsequent whiskers.
Date: May 11, 2011
Creator: Hoffman, E. & Lam, P.
Partner: UNT Libraries Government Documents Department

Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments

Description: The slow crack growth properties of six structural ceramics were measured by dynamic fatigue in air and inert atmospheres over a range of elevated temperatures. The material response varied from no strength degradation as a function of stress and environment to significant strength degradation by slow crack growth (SCG) and by a combination of SCG and creep. The fractographic investigation showed that SCG was evidenced by growth of isolated cracks and often by an intergranular fracture mode, while creep was evidenced by accumulated damage such as void formation and opening of the microstructure at grain boundaries and triple junctions. For the materials in which the strength was unaffected by the stress and environment, the fracture surfaces were essentially indistinguishable from the inert fracture surfaces.
Date: December 31, 1995
Creator: Breder, K.; Wereszczak, A.A.; Tennery, V.J. & Mroz, T.J.
Partner: UNT Libraries Government Documents Department

High-temperature deformation of ZrO{sub 2}-Al{sub 2}O{sub 3}/SiC whisker composites fabricated by two techniques

Description: ZrO{sub 2} {minus} Al{sub 2}0{sub 3}/SiC whisker-reinforced composites, with whisker volume fractions of 0 to 28%, fabricated both by powder and precursor processing, have been deformed at temperatures of 1300--1500 C under constant compression rates of 1.7 {times} 10{sup {minus}5} to 6.8 {times} 10{sup {minus}5} s{sup {minus}1}. Above 1400 C, a stress at which the work-hardening rate became zero could be measured and correlated with whisker content. On the other hand, at 1300 C all samples broke within the elastic.regime. At 1350 C, increased whisker content appeared to inhibit fracture, so plastic behavior was obtained for samples containing 28% SiC. In the range of temperatures and compression rates, noted above, stress exponents were determined and tentatively correlated with microstructural features and their evolution during plastic deformation.
Date: March 1, 1995
Creator: Calderon Moreno, J.M.; DeArellano-Lopez, A.R.; Dominguez-Rodriguez, A. & Routbort, J.L.
Partner: UNT Libraries Government Documents Department

Friction and wear behavior of in-situ reinforced silicon nitride. Final report

Description: Specimens of in-situ-reinforced silicon nitride (ISRSH) have been wear tested in lubricated, reciprocating, sliding motion against a silicon nitride counterface. Only mild wear of the ISRSN was observed at contact pressures up to 4.8 GPa at an average sliding velocity of 0.3 m/s. At 0.6 m/s, a wear mode transition was observed in ISRSN at 4.2 - 4.4 GPa. In comparison, the wear mode transition in silicon carbide whisker reinforced silicon nitride at both velocities was evident at about 2.2 - 2.4 GPa. Scanning electron microscopy of the ISRSN wear surfaces revealed the presence of a 40 pm thick debris layer on the mild wear tracks. The ISRSN wear mode transition response indicated a potential for an improved wear resistance in this material as compared to whisker reinforced silicon nitride.
Date: October 1, 1995
Creator: Yust, C.S.
Partner: UNT Libraries Government Documents Department

Microstructure / mechanical-property relationships and R-curve behavior in Si{sub 3}N{sub 4}/Si{sub 3}N{sub 4}(w) composites

Description: Fracture toughness of Si{sub 3}N{sub 4}-whisker-reinforced Si{sub 3}N{sub 4}-matrix composites were evaluated with various whisker contents. Toughness was observed to increase with increasing whisker content, reaching a maximum value of 8.8 MPa{radical}4 at 5 vol.% whisker content. Additional whisker content caused reduced toughness. A model based on microstructural features indicates that the observed dependence of toughness on whisker content is due to both whisker-toughening and matrix-grain-size effects. Composites with 5 vol.% whisker content showed a rising crack growth resistance (R-curve) behavior. This is believed to be a combined effect of crack bridging and crack deflection due to both elongated grains and reinforcing whiskers.
Date: June 1, 1992
Creator: Singh, J. P.; Chu, C. Y.; Murphy, C. & Singh, D.
Partner: UNT Libraries Government Documents Department

Erosion of Composite Ceramics

Description: The theoretical basis to describe solid-particle erosion of monolithic ceramics is well developed. In many cases, the models can account for the impact velocity, impact angle and erodent-size dependencies of the steady-state erosion rate. In addition, the models account for effects of materials parameters such as fracture toughness and hardness. Steady-state erosion measurements on a wide variety of composite ceramics, including SiC whisker-reinforced Al{sub 2}O{sub 3}, Si{sub 3}N{sub 4} containing Si{sub 3}N{sub 4} or SiC whiskers, Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} reinforced with SiC whiskers, and duplex-microstructure Si{sub 3}N{sub 4} have been reported. The theories developed for monolithic ceramics are, however, less successful in describing the results for composites.
Date: August 1992
Creator: Routbort, J. L.
Partner: UNT Libraries Government Documents Department

Application of neutron diffraction technology to the determination of residual strain in engineering composites

Description: Knowledge of fabrication induced residual stresses in the fiber and matrix of advanced engineering composites is important as these stresses can greatly influence the mechanical properties of these composites. In this paper, the application of neutron diffraction technology to the determination of thermal residual strains in the constituents of composites (from which stresses can be calculated) is discussed. Experimental determination of temperature dependent strain in the fiber and matrix of three composites compare favorably with the results of analytical and finite element methods used to predict strain. These composites (two ceramic matrix and one metal matrix) are materials of interest to a variety of industries. In this paper, the benefit of applying a National Laboratory developed technology to a problem of interest to industry, is shown.
Date: December 1, 1992
Creator: Kupperman, D. S.
Partner: UNT Libraries Government Documents Department

Thermally induced micromechanical stresses in ceramic/ceramic composites

Description: The internal micromechanical stresses which develop in ceramic-ceramic composites as a consequence of temperature changes and thermoelastic property differences between the reinforcing and matrix phases are addressed by the Eshelby method. Results for two whisker reinforced ceramic matrix composites and for quartz particles in porcelain are discussed. It is concluded that the stresses which develop in the second phase reinforcing inclusions are quite substantial (GPa-levels) and may be highly anisotropic in character. These stresses are additive to the macroscopic thermal stresses from temperature gradients which are encountered during heating and cooling, and also to externally apphed mechanical stresses (loads). These micromechanical stresses are expected to be highly significant for thermal cycling fatigue and other failure processes.
Date: November 1, 1992
Creator: Li, Zhuang & Bradt, R. C.
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

A model of toughening effects in whisker-reinforced composites

Description: A numerical approach is presented that lends itself to modeling the screening or antiscreening effects due solely to modulus differences of a discrete array of whiskers in an elastic matrix. The method is applied to single whiskers, and to examining the issue of whisker orientation of the toughness of ceramic composites. The model results indicate that crack-tip shielding due to modulus defect interactions occurs when the reinforcement has a higher modulus than the matrix material, and that anti-shielding occurs for the opposite case. Results for a single whisker located at the crack-tip (maximum effect) indicate that the crack-tip stress intensity is reduced by about 10% when a modulus ratio of four is assumed. Calculations performed with whisker arrays demonstrate pronounced effects of whisker orientation on the crack-tip screening, being larger for whiskers oriented perpendicular to the crack plane, as expected. Ordered whisker arrays produce larger and more uniform screening than do random whisker arrays. 16 refs., 10 figs.
Date: January 1, 1991
Creator: Hoagland, R.G. (Washington State Univ., Pullman, WA (United States). Dept. of Mechanical and Materials Engineering) & Henager, C.H. Jr. (Pacific Northwest Lab., Richland, WA (United States))
Partner: UNT Libraries Government Documents Department

Performance of commercial and research grade SiC whiskers in a borosilicate glass matrix

Description: Dry blending of the whiskers and glass powder yielded composites of satisfactory, but not optimum, homogeneity. The whiskers inhibited composite densification, but this could be remedied by modifying the hot-pressing conditions. All types of whiskers produced improvements in the flexural strength of the composites, with the highest value achieved with the smallest diameter whiskers. The composites displayed a 4- to 5-fold increase in fracture toughness compared to that displayed by the unreinforced glass, but definitive trends could not be related to the whisker size characteristics.
Date: January 1, 1986
Creator: Gac, F.D.; Petrovic, J.J.; Milewski, J.V. & Shalek, P.D.
Partner: UNT Libraries Government Documents Department