Browsing by Author "Tiwari, Rajesh Kumar"

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    Relaxation mechanism in methyl stearate monolayer films at the air/water interface
    Tiwari, Rajesh Kumar (Virginia Tech, 1994)
    A monolayer film of methyl stearate was compressed until catastrophic film collapse took place. Surface pressure relaxation was then followed as a function of time. Investigation involving the effects of film compression beyond the collapse pressure revealed an important process involved in the surface pressure relaxation mechanism. When the monolayer is compressed beyond the collapse pressure and then held at a constant area, the surface pressure relaxation, in a plot of surface pressure vs time, was delayed during the initial stage of the process. A similar delay in the surface pressure relaxation was also observed for a monolayer film of methyl stearate when it was compressed and held at 40 mN/m, below the collapse pressure, for some time before allowing it to relax under a constant area condition. A relaxation mechanism has been proposed to explain the delay phenomenon observed during the surface pressure relaxation at constant area: At collapse, the monolayer film buckles and folds over to form bilayer molecular channels (ridges and ribbons). The ridges and ribbons act as a reservoir for monolayer material to make up for lost molecules at the air/water interface due to the growth of a bulk (crystalline) phase under a constant area condition. The results from temperature dependence studies as well as from the area-relaxation experiments strongly support the proposed relaxation mechanism. The Langmuir-Blodgett films of methyl stearate, deposited before and after the catastrophic film collapse, revealed interesting structural features of the collapsed film. The experimental results from the pressure-time, area-time, and pressure-area isotherms strongly suggest that the methyl stearate monolayer film undergoes an organized film collapse. This work helps to better understand the relaxation mechanism in monolayer films at the air/water interface.
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    The Thermal Stability of Anodic Oxide Coatings - Strength and Durability of Adhesively Bonded Ti-6Al-4V Alloy
    Tiwari, Rajesh Kumar (Virginia Tech, 2002-07-18)
    The lap shear strength of chromic acid anodized, primed, Ti-6Al-4V alloy bonded with a high performance FM-5 polyimide adhesive has been investigated as a function of thermal treatment for selected times at various temperatures in air. The research findings indicate that the lap shear strength decreases with the increase in duration of the thermal treatment at constant temperature and with the increase in temperature at constant time. The bond fails increasingly in the oxide coating with increasing treatment temperature and time of treatment. Surface analysis results for debonded specimens suggest that the process leading to failure is the formation of fluorine-containing materials within the oxide, which weakens the adherend-adhesive bond. The formation of the fluorine components is facilitated by treatment at elevated temperatures. This study suggests that the presence of fluoride ions in the anodic oxide coating, prior to bonding, is detrimental to the bond strength of adhesively bonded Ti-alloy when exposed to high temperatures. The wedge test configuration was used to investigate the influence of temperature on the bond durability of adhesively bonded chromic acid anodized Ti-6Al-4V alloy in air. Based on the average crack length vs. exposure time data, the bond durability varied in the order -25°C > 24°C > 177°C. In each case, the bonded joint failed cohesively within the adhesive, irrespective of the temperature of exposure. XPS analysis and scanning electron photomicrographs of failure surfaces revealed that the failure occurred at the scrim cloth/adhesive interface. The influence of thermal treatment history on the bond durability of adhesively bonded chromic acid anodized Ti-6Al-4V alloy immersed in boiling water was also investigated. The average crack length vs. immersion time indicated no significant differences for specimens that were thermally treated and then bonded compared to the non-thermally treated specimens. In addition, the failure mode was cohesive within the adhesive for specimens prepared using various thermal treatment conditions. The crack growths for samples treated for 0.5 hour and 1.0 hour and for non-thermally treated specimens for any given exposure time were equivalent. In addition, cohesive failure (failure within adhesive) was observed for each specimen under each treatment condition. The specimens that were bonded and then thermally treated for 3 hours, failed in the oxide coating immediately upon insertion of the wedge. Surface analysis results for debonded specimens suggest that the process leading to failure is the formation of fluorine-containing materials within the oxide. The measured average activation energy for the formation of aluminum fluoride species is 149 kJ/mol. The high activation energy suggests that the rate of aluminum fluoride formation is substantial only at high temperatures. In summary, the presence of fluorides in the anodic oxide coatings prior to bonding is detrimental to the overall strength and durability of adhesively bonded chromic acid anodized Ti-6Al-4V joints which have been exposed to high temperatures (350°C-399°C).