Ram Kossowsky

Tensile and creep strengths of hot-pressed Si3N4

Tensile, creep, and stress-rupture data for Norton HS-110 and HS-130 hot-pressed Si3N4 are presented. It is shown that the strength of the material is controlled by the grain-boundary glass phase. At elevated temperatures, >1000‡ C, deformation is controlled by grain-boundary sliding. A model based on the concept of geometrically necessary wedge cracks is then developed which accounts for the observed effects of strain rate, stress, temperature, environment, and impurity content. It is concluded that the currently available hot-pressed Si3N4 is creep strain limited.

Measurement of viscosity of the grain-boundary phase in hot-pressed silicon nitride

An internal friction technique has been used to measure the viscosity of the grain-boundary amorphous phase in commercial hot-pressed silicon nitride. The viscosity in the region of the glass transition (850 to 900° C) was approximately 5×1015 P per unit thickness (cm) of the grain boundary, with an apparant activation energy of 163 kcal mol−1.

The microstructure of hot-pressed silicon-nitride

Grain morphology, distribution of impurities and inclusions, phases and dislocation structures in two grades of hot-pressed Si3N4 were investigated by means of replica and thin foil transmission microscopy, and by X-ray diffraction, microprobe and Auger analyses. High concentrations of impurities, specifically Ca, were detected at the grain boundaries. Fe-W-Si particles were seen within the grain. Non-densified Si3N4 inclusions were found to be detrimental to the strength. Possible correlations among strength, densification data and distribution of elements and phases are discussed.

Aneurysm Clip Motion during Magnetic Resonance Imaging: In Vivo Experimental Study with Metallurgical Factor Analysis

Because of various mechanical, metallurgical, and commercial constraints, aneurysm clips are manufactured from different alloys, including several stainless steel and cobalt alloys. Some of the steels contain volume fractions of the crystal phase known as martensite. Martensitic alloys have body-centered cubic structure, are prone to stress corrosion failure, and are ferromagnetic. Martensitic steel can be displaced like a compass needle when exposed to a magnetic field such as that generated during magnetic resonance imaging (MRI). The force exerted by the magnetic field is proportional to the volume fraction of the magnetic phase. We investigated the martensitic content and magnetic field-induced displacement of 12 common aneurysm clips. Four clips of each of the following types were examined: Sugita, Sundt-Kees Multi-Angle, Heifetz (two types), Vari-Angle McFadden, Yasargil (two types), Scoville, Mayfield, Vari-Angle, Pivot, and Kapp. Phase homogeneity and crystal structure were analyzed by x-ray diffraction using a Phillips x-ray diffractometer. Clip deflection in an Oxford Research Systems MRI spectrometer was measured in our in vivo rat abdominal aortic aneurysm model. Results showed that the volume fraction of the martensitic phase in the various clips correlated with the magnitude of the deflection. Among the clips examined, the Yasargil, Sugita, Heifetz Elgiloy, and Vari-Angle McFadden had a nonmartensitic composition and did not deflect in the magnetic field. The Scoville contained 5% martensite and deflected only marginally. Martensite comprised 35% of the Mayfield clip, which deflected 45 degrees, and 90% of the Heifetz, Vari-Angle, Pivot, and Sundt-Kees Multi-Angle clips, which deflected approximately 70 degrees or slipped off the aneurysm.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracranial Clips: An Examination of the Devices Used for Aneurysm Surgery

A properly functioning aneurysm clip is of paramount importance in the surgical ablation of intracranial aneurysms. An appropriate match between the closing force needed to ablate an aneurysm and the force exerted by the aneurysm clip must be achieved. In this study, the opening and closing forces exerted by several commonly used aneurysm clips were tested. There was a strong correlation between the classification of the clips, according to the Dujovny system, and their mechanical behavior. Minimal variability among different lots of the same clip type and minimal hysteresis were generally exhibited by the alpha mobile fulcrum class clips, such as the Yasargil, Vari-Angle McFadden, Scoville, and Mayfield clips. Significant exceptions included several types of Mayfield and Drake clips. In contrast, pivot class clips generally showed twice as much lot variability among different lots, as well as a significant amount of hysteresis. Clips of this class include the Vari-Angle, Heifetz, and Pivot clips. Because significant variations in clip force exist and because several other factors can compound these differences, it is suggested that aneurysm clips be individually tested for their closing forces before they are used in an operation.

Cellular precipitation in Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys

The phases present in directionally solidified Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys are identified. These are the nickel-and chromium-rich lamellae, a Widmanstätten precipitation of nickel in the chromium-rich lamellae, grain boundary precipitation of chromium, and scattered discrete particles of chromium oxide. Upon annealing, the amount of the nickel precipitates increases drastically and a well defined cellular precipitation appears in the nickel-rich lamellae. The cellular precipitation conforms in almost every way to typical cellular reaction such as in the Pb-Sn system. Cellular (linear) growth rate, G, and interlamellar spacing, S, were measured on specimens annealed for times ranging from 1/2 to 100 hr at 625°, 700°, 760°, and 850°C. G increased from (average for both Ni-Cr alloys) 2.3 × 10−8 cm per sec at 625°C to 7.7 × 10−7 cm per sec at 760°C and decreased again at 850°C. S varied from 2 to 10 × 10−5 cm as the annealing temperature was increased. The calculated grain boundary diffusivity, DB, representing the diffusion of chromium in fcc Ni-Cr solid solution, increased from 6.7 × 10−11 sq cm per sec at 625°C to 8.6 × 10−8 sq cm per sec at 850°C. The activation energy 64 kcal per mole, is of the order of that obtained for self-diffusion.

Effect of Arcing on the Microstructure and Morphology of Ag-CdO Contacts

The changes in the microstructure, the composition, and the morphology of internally oxidized Ag-CdO contacts after high current arcing were investigated. The contacts were always opened on the same half cycle (60 Hz ac); thus one electrode was always the cathode, the other always the anode. Qualitative spectra of the arc at its midpoint were also taken, The results indicate that during arcing, there is a much higher concentration of Cd vapor than of Ag vapor in the interelectrode region. It is also found that the erosion of the cathode is more severe, and that the metal in the interior of the cathode craters is depleted of CdO, while a layer of Ag-Cdo is built up in the anode craters.

The Use of Ceramic Matrices Infiltrated with Silver for Electric Contact Applications

Results are presented of experiments that compare the performance of three ceramic-Ag contact materials (TiC-Ag, TiB 2 -Ag and NbC-Ag) with the well known WAg contact material. The conductivities of the ceramic-Ag contacts were found to be comparable to that of W-Ag. Erosion measurements in an 880 A 440 V rms circuit showed that the rate of erosion was not a simple function of the ceramic materials heat of evaporation. The erosion mainly occurred through evaporation with some particle ejection in case of the W-Ag and NbC-Ag materials. The ceramic-Ag materials performed well when subjected to testing in a 100 A low voltage circuit breaker. A microstructural evaluation revealed that the contacts could have been improved by lowering the surface tension between the ceramic and silver to achieve better wet- ting.

Creep behavior of Ni-Cr lamellar eutectic alloy

The structural changes that occur during creep deformation at 625° and 760°C, with creep and creep rupture data of a directionally solidified Ni-Cr lamellar eutectic alloy are presented and discussed. It is shown that the characteristic features of stage I deformation are the formation of dislocation tangles in the nickel-rich phase and shearing of the cellular structure; these features are then carried into stage II without any additional changes. The onset of accelerated creep is associated with the fracture of the chromium-rich lamellae. During this stage well-defined dislocation cells are formed. More than an order of magnitude increase in lifetime over cast specimens is obtained in the lamellar material with intermediate results for partially dendritic specimens. The activation enthalpy for creep is strain dependent, increasing from about 40 kcal per mole at low strains to a constant value of 80 kcal per mole at about 2 pct plastic strain. Stress dependence of steady-state creep for both test temperatures conforms to the expression έ =Aσn witha − 7 for the lamellar eutectic anda − 5 for cast specimens.

Advances in Materials Science and Implant Orthopedic Surgery

Preface. Introduction: Biomaterials and Implant Surgery J. E. Lemons. Particles of Biomaterials Recruit Macrophages which can Differentiate into Bone Resorbing Cells R. Pandet et al. Biological Consequences of Polymeric Materials - Silicones, PMMA, UHMWPE N. Kossovsky. The Characterization of Biologically Important Apatite Surfaces Using the Dual Composition and FTIR Microsocopy Techniques E. P. Paschalis et al. Activity of Plasma Proteins Regarding Biomaterials Corrosion - pH Effects H. Thomas et al. Zespol Stabilizer in the Treatment of Open Fractures of the Zeugopodium Bones in Dogs and Cats Z. Klos et al. R&D and Manufacturing of Biomaterials and Implants in the Socio-Political Context C. A. Homsy. Materials and Reliability Issues in Total Hip Arthroplasty E. M. Lenoe. Mechanical Failure of Cemented Femoral Total Hip Replacement N. Verdonschot, R. Huiskes. Modelling and Finite Element Analysis of a Ceramic Femoral Knee Prosthesis W. M. Payten et al. The Influence of Processing Conditions on the Mechanical Behaviour of UHMWPE/HA and PMMA/HA Composites A. M. Cunha et al. Behaviour of HA Coated Ti-6A1-4V under Cyclic Bending in Hanks and Isotonic Saline Solutions: An Electrochemical and Structural Study R. L. Reis, F. J. Monteiro. Improvement of Ion Implantation of Prosthetic Femoral Heads. Testing of the Friction Behaviour with a Physiological Hip Joint Simulator F. Bernard et al. Wear Behaviour of Ion Implanted TI6AL4V Against UHMWPE H. Schmidt et al. Biological Resurfacing Using Synthetic Materials: A Review of Surgical Philosophy and Clinical Experience Using a Carbon Fibre R. J. Minns. Processing Biodegradable Natural Polyesters for Porous Soft Materials C. Chaput et al. The Role of Acrylic Polyelectrolytes and Acrylic Gels as CrystallizationInhibitors and Promotors of Calcium Oxalate M. Oner, P. Calvert. Sol-Gel Derived Bioactive Hydroxyapatite Coatings B. Ben-Nissan, C. Chai. Brazing of Al2O3 to AISI 316L Stainless Steel and Ti c.p. for Packaging Telemetric Systems for Orthopaedic Applications O. C. Paiva, M. A. Barbosa. Summary Discussion. Index.