فایل ورد کامل مقاومت خزشی فیلم های نازک هیبریدی PI/SiO2 تحت بارگذاری خستگی و ثابت

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آزمایشات خزشی فیلم های نانوکامپوزیت PI/SiO2 تحت بارگذاری استاتیک و خستگی انجام می شود. کشش خزشی، سرعت خزشی و وادادگی خزشی تحت سطوح مختلف و الکوهای مختلف فشار حاصل می شوند.نتایج آزمایشی سخت شدن دوره ای معنی دار را در فیلم های کامپوزیت های خالص پلی آمید PI/SiO2 تحت بارگذاری خستگی نشان می دهد. پدیده های انتشار انرژی، خزش دوره ای و سخت شدن چرخه ای در مرحله اولیه سخت شده و عدد چرخه ای آن ها کم تر از ۰۱ عمر خستکی مواد است.
در رابطه با دفورماسیون خزشی، نتایج نشان می دهد که مرحله اولیه خزش ارتباط نزدیکی با جریان ویسکوز زنجیره های بی شکل دارد و الگوی فشار مهم ترین نقش را در میان عوامل دیگر ایفا می کند. با ورود خزش به مرحله بعدی، این سطح فشار به جای الگوی فشار است که نقش مهمی ایفا می کند زیرا سطح فشار لازم باید به آسیب نهایی بار زنجیره های بی شکل و متبلور برسد.
به علاوه، نتایج حاکی از آن است که محتوی نانواکسید سیلسیم می تواند موجب بهبود موثر مقاومت خزش و حیات خستگی فیلم های کامپوزیت PI/SiO2 شود. با این حال، با افزایش سطح دوپینگ نانو اکسید سیلیسیم به ۸ درصد وزنی، عمر خستگی مواد به شدت و به طور معنی داری کاهش می یابد و این در حالی است که مقاومت خزش افزایش می یابد. از این روی، به منظور بهبود عملکرد این دسته از نانوکامپوزیت ها و مناسب سازی آن ها برای کاربرد های بارگذاری بلند مدت در شرایط بارگذاری استاتیک و دینامیک، عوامل دیگر( ظرفیت دفورماسیون پلاستیک) باید مد نظر قرار گیرند.

عنوان انگلیسی:Creep resistance of PI/SiO2 hybrid thin films under constant and fatigue loading~~en~~

Introduction Polyimides (PIs) are considered to be one of the most important engineering materials because of their superior mechanical properties shown at different temperatures [1– ۳]. Since 1960s, essentially the beginning of the search for high temperature polymers, much attention has been paid on PIs than any other high performance polymers. Extensive application researches have demonstrated the possibilities to use PIs as buffer coatings, passivation layers, alpha particle barriers, interlayer dielectrics, water scale packages, etc. [4,5]. To further decrease their coefficient of thermal expansion (CTE) to match inorganic or metallic substrates and enhance their mechanical properties (e.g. fracture toughness), different kinds of nano-additives [6– ۸] have been introduced as reinforcements to construct nano-inorganic/PI composites. Similar as other viscoelastic materials, PIs tend to exhibit time-dependent deformations (even damage) over a wide range of temperatures, which are defined by creep for a constant load. If a sinusoidal force pattern (dynamic load) is applied, the strain increase can be referred to as ‘dynamic creep’. Creep deformation is a big barrier for thermoplastics to satisfy the requirements in long-term loading service because the accumulated strain might exceed the material’s deformation limitation and lead to creep fracture of the structures. Hence it is of great importance to know about the creep properties and improve the creep resistance for thermoplastics and their composites. In the past 10 years, a variety of inorganic nano-fillers have been considered as the reinforcements to improve the creep resistance of polymer matrix because they have high surface to volume ratio and the possibility of forming network structure by dispersing nano-fillers dispersed into polymer matrix to restrict the mobility of polymer chain [9]. Pegorettie et al. [10] researched the creep performance of recycled polyethylene terephthalate (PET) filled with layered silicate. They obtained a slight decrease of creep compliance and no-rising creep rate in composites than in pure matrix. Fornes and Paul [11] reported how to improve the glass transition temperature (Tg) of nylon 66 by introducing nano-particles to restrict the motion of the polymer chains. Ranade et al. [12] studied polyethylene/montmorillonite layered silicate (PE/MLS) films and found that the presence of rigid MLS contributed to the improved creep resistance of composites. Zhang and Yang [13,14] gave a general report about the creep resistance of polyamide 66 modified by different nano-fillers. In addition, some theoretical works were also carried out to predict the long-term creep behavior of nano-filler/polymer composites based on the short-term experimental data [15–۱۷]. However, the above researches about the creep behaviors of nano-filler/polymer composite have been focused on static loading, with few reports involved in the dynamic creep resistance, especially the lack of comparison about the creep resistance difference between static and dynamic loading. The aim of this work is to present a general understanding of the creep behaviors of PI/SiO2 nanocomposite thin films under constant and fatigue loading. The effects of silica doping levels, stress amplitude and stress pattern (constant or sinusoidal) are also considered. 2. Experimental procedure Four types of thin films: pure PI, PI/SiO2 hybrid thin films with 1, 3 and 8 wt.% silica doping levels, were prepared by an improved sol–gel technique. The average thickness of films is about 0.035 mm. Compared with the traditional processing technique, the new process can lead to a smaller particle size. More information about the processing technique and the static mechanical properties (tensile strength, Young’s modulus, elongation at break and CTE) has been reported in Ref. [18]. For the fatigue experiment, an INSTRON digital micro-electromagnetism testing system was used which has a dynamic capacity of 10 N, maximum displacement of 2.5 mm, and maximum frequency of 50 Hz. A sinusoidal force oscillation at 10 Hz was applied for the experiments with a stress ratio R = 0.1 (minimum stress divided by maximum stress). For the static creep experiment, a constant loading rate of 1 N/s was carried out until the load (or stress) reached a pre-setting value, and then the stress value was kept as constant. The testing temperature for static and fatigue loading was maintained at about 298 K. 3. Experimental result and discussion 3.1. Hysteresis loop In this study, the ‘dynamic creep’ was investigated by monitoring the change of hysteresis loops during fatigue process, by which different properties can be determined simultaneously, e.g. cyclic softening/hardening, stored and lost energies, material damping and cyclic creep behavior [19,20]. Fig. 1 presents typical hysteresis loops of pure PI and PI/SiO2 hybrid thin films in different cyclic stages. The fatigue life (Nf) and cyclic number (N) are also presented in the figures, and the stress amplitude can be referred in Y-axis. As a comparison, the corresponding parts of stress–strain curves under quasi-static tensile loading are also plotted in Fig. 1. Like most of polymers and matrix-dominated polymeric composite to be viscoelastic, a significant phase lag of the strain to stress was exhibited under fatigue loading, which caused the formation of hysteresis loops even if the applied stress was much lower than the yield stress of materials. The area enclosed in the hysteresis loop denoted the specific mechanic energy (in per volume) dissipated in every cycle, which is partly converted into the heat energy and partly stored as deformation energy leading to the final failure of materials [21]. Table 1 lists the values of the specific energy (wf) dissipated in the hystersis loops as shown in Fig. 1. According to Fig. 1 and Table 1, the specific cyclic energy dissipated in every cycle is decreased with increasing fatigue cycles, which indicates the expending of the viscous resistance during the fatigue process. Another noteworthy point in Fig. 1 is that the increase of the minimum strain in hysteresis loops is much quicker than that of the maximum strain. The phenomenon of cyclic hardening is exhibited for the thin films, although it is inferred in some earlier review papers [22–۲۴] that the mechanism of cyclic softening (e.g. large scale bulk flow of polymers under fatigue loading) commonly overwhelms that of cyclic hardening (e.g. the molecular orientation hardening and defect-hardening) for polymers under fatigue loading. The more prominent cyclic hardening in pure PI thin films compared to other hybrid films in Fig. 1 indicates that it is not caused by the presence of nano-silica particles.

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