This investigation highlights the rationale of adhesive bonding of atmospheric pressure plasma treated high temperature resistant polymeric sheet such as polyimide sheet (Meldin 7001), with titanium sheets. The surface of polyimide (PI) sheet was treated with atmospheric pressure plasma for different exposure times. The surface energy was found to increase with increase in exposure time. However, longer exposure time of plasma, results in deterioration of the surface layer of PI resulting in degradation and embrittlement.

Contact angle measurements with sessile drop technique were carried out for estimation of surface energy. SEM (EDS) and AFM analyses of treated and untreated specimens were carried out to examine the surface characteristics and understanding morphological changes following surface treatment. Untreated samples and atmospheric pressure plasma treated samples of polyimide Meldin 7001 sheet were bonded together as well as with titanium substrates to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to measure bond strength and to investigate the effect of surface treatment on adhesive bond strength. An optimized plasma treatment time results in maximum adhesive bond strength and consequently, this technology is highly acceptable for aviation and space applications.
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In this investigation surface treatment of titanium is carried out by plasma ion implantation under atmospheric pressure plasma in order to increase the adhesive bond strength. Prior to the plasma treatment, titanium surfaces were mechanically treated by sand blasting. It is observed that the contact angle of de-ionized water decreases with the grit blast treatment time. Optical microscopy and scanning electron microscopic (SEM) analysis of untreated and atmospheric plasma treated titanium are carried out to examine the surface characteristics. A substantial improvement in the surface energy of titanium is observed after the atmospheric pressure plasma treatment. The surface energy increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength of the titanium. Unmodified and surface modified titanium sheets by atmospheric pressure plasma were adhesively bonded by high temperature resistant polyimide adhesive. The glass transition temperature of this adhesive is 310 1C and these adhesively bonded joints were cured at high temperature. A substantial improvement in adhesive bond strength was observed after atmospheric pressure plasma treatment.@en

The high temperature resistant polymers and metal composites are used widely in aviation, space, automotive and electronics industry. The high temperature resistant polymers and metals are joined together using high temperature adhesives. Polyimide and epoxy adhesives that can withstand high temperature (200 °C-300 °C) are commonly used for joining high temperature metals and polymers. The performance of adhesively bonded metals and polymers depends upon physical properties of these high temperature adhesives. The physical properties like modulus, Tg. coefficient of thermal expansion (CTE) etc., are affected by external factors such as force, temperature, humidity etc. The external factors play a vital role in the adhesive bond strength and the durability of bond between metal and polymer. In this investigation moisture absorption analysis of polyimide adhesive is performed using Q5000 moisture absorption analyzer. The moisture absorption data of polyimide at different temperatures and humidity level is obtained. Further, the moisture absorption data is fitted to well known Fickian-fit model to determine the diffusion coefficient (D) and saturated moisture gain Msat. Diffusion coefficient (D) and Msat of polyimide and epoxy adhesive are calculated at different temperature and different humidity level. It is observed that diffusion coefficient changes with the change in temperature and humidity level. The diffusion coefficient (D) and M sat data are used in Fick¿s second law of diffusion to estimate the time needed for preconditioning of the adhesively bonded titanium samples in humidity chamber at elevated temperature and higher moisture level. After preconditioning of adhesively bonded Titanium samples in moisture chamber for estimated time, samples will be subjected to lap shear tensile test to study the effect of these elevated conditions on adhesive bond strength.@en

Titanium is one of the most effective materials for structural application of space craft and aviation. Titanium alloys are widely used in solid rocket booster cases, guidance control pressure vessel and other different applications demanding light weight and reliability. Aerospace industry is also a larger market for titanium products and adhesive bonding is advantageous in terms of its fabrication. However, surface treatment of titanium alloy is critical in improving the adhesive bond strength and long term durability of the adhesive joint. In this investigation surface treatment of titanium is carried out by plasma ion implantation in order to increase adhesive bond strength and durability. Optical microscopic and SEM analysis of untreated and atmospheric plasma treated specimens is carried out to examine the surface characteristics. A substantial improvement in the surface energy of Titanium is observed after the atmospheric plasma treatment. The Treated surface was basically characterized by contact angle analyzer for the activation property on the surface. The surface energy of titanium surface increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results maximum adhesive bond strength with polyimide adhesive and consequently, this technology is highly acceptable for aviation and space applications. Key Words: Titanium, Surface modification, Atmospheric pressure plasma and Surface Energy, Adhesive bonding@en

It is noted that in search of long term and efficient service performance in the context of future generation of aerospace materials, there is increasing need of metal-high performance polymer composite. Based on these considerations, high temperature resistant polymeric sheet such as Polyimide Meldin7001 sheet, is joined with Titanium sheet by employing ultra high temperature resistant Polyimide adhesive. In order to increase surface energy of Polyimide surface, atmospheric pressure plasma treatment is used to modify the Polyimide surface. Atmospheric pressure plasma treatment creates physical and chemical changes such as cross linking, formation of free radicals and oxygen functionalization in the form of polar groups on polymer surface resulting in improvement of wetting and adhesion characteristics. Surface of Polyimide (PI) sheet is treated with atmospheric pressure plasma for different exposure periods. Surface energy of PI sheet increases with increase in exposure time. However, after a certain exposure time of plasma, deterioration of surface layer of PI substrate results in degradation and embitterment of PI which is not suitable for adhesive bonding. Optical microscopic, SEM (EDS), analysis of treated and untreated specimen is carried out to examine the surface characteristics. Treated samples and untreated samples of Polyimide are bonded together with overlap joints. Lap shear bond strength of treated and untreated samples was measured by tensile test to study the effect of treatment on adhesive bond strength. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength and consequently, this technology is highly acceptable for aviation and space applications.@en