The effect of temperature-induced phase transition of PTFE on the dynamic mechanical behavior and impact-induced initiation characteristics of Al/PTFE

Highlights

• The mechanical behavior of Al/PTFE took on strong bilinear temperature dependence.

• Crack propagation is related to brittle-to-ductile transition near room temperature.

• The constitutive model can well characterize the mechanical response of Al/PTFE.

• Explosion flare enhanced and reaction time prolonged as the temperature evaluated.

• Fracture-induced reaction dominates the ignition of Al/PTFE at low strain rate.

Abstract

PTFE is a semi-crystalline polymer which can undergo two phase transitions at 19°Ϲ and 30°Ϲ. To investigate influence of temperature-induced phase transition of PTFE on the mechanical behavior and impact-induced initiation characteristics of Al/PTFE, the dynamic mechanical analysis, split Hopkinson pressure bar test and drop-weight test were conducted at different temperatures. The correlation between fracture modes and ignition mechanism were analyzed associated with microstructures and reaction phenomena. The results show that with the temperature elevated, the mechanical behavior and reactive characteristics of Al/PTFE manifested a bilinear temperature dependence. Al/PTFE went through a gradual transition from brittleness to ductility. The ignition energy declined, explosion flare enhanced and reaction time prolonged consistent with the change of mechanical properties. The shock pressure and ignition time under drop-weight tests were far from the requirement of shock-induced reaction, indicating that it is fracture-induced reaction dominates the ignition of Al/PTFE at low strain rate.

Introduction

Reactive materials are also known as impact induced energetic material, which have the characteristics of rapid energy release of single-molecule energetic materials and high energy density of metal fuels. Fluoropolymer/metal reactive materials, represented by aluminum/polytetrafluoroethylene (Al/PTFE), have great application value and development prospect in the field of new warhead damage due to their excellent impact energy release characteristics [[1], [2], [3]]. It can be made into the shaped charge with energetic liner, reactive fragments and high energy additive, which can result in multiple damage effects including direct kinetic energy strike, over temperature, overpressure and arson effect [4,5].

The fluoropolymer based reactive materials are composed of two or more non-explosive solid materials, the research of them always aim at high density composites, excellent mechanical properties and high energy output [6,7]. Lan et al. [8] studied the effect of multi-oxidants as additives on reactive characteristics of Al/PTFE. Ding et al. [9] designed a new energy release testing device to assess the energy release ability of PTFE/Al/CuO. He et al. [10] made use of a synthesized polydopamine binding layer to adjust the reactivity of nanoscale Al/PTFE. The results manifests that the composites of PTFE and nanoscale Al coated with polydopamine took on improved energy release and dropped sensitivity, and more crucially tunable reactivity.

Because the tunable reactivity of Al/PTFE is outstanding, by controlling the initial material parameters, preparation process and application environment, the mechanical properties and reactive characteristics of the materials can be significantly changed, but this adjustable property also brings challenges for its practical application [11,12]. Feng et al. [13] found a violent reaction phenomenon under quasi-static compression of Al/PTFE reactive materials fabricated through a specific sintering process for the first time, which makes the production and service safety of this kind of materials more concerned. In light of the discovery, the effect of sintering temperature, composition proportion, Al particle size and crystallinity on the quasi-static reaction of Al/PTFE were studied [14,15], while the influence of temperature was not considered. Joyce et al. [16,17] tested the fracture toughness of PTFE and Al/PTFE according to ASTM E1820 standard, obtained the fracture toughness of two kinds of materials in different temperature and loading strain rate range. They found that for temperatures slightly below ambient temperature, the fracture resistance would rapidly degrade and the crack emerged in a ductile even nearly brittle manner. Wang et al. [18] conducted quasi-static compression test to study the impact of temperature on the mechanical behavior of Al/PTFE. Scanning electron microscope findings suggest that the temperature played an essential role in the fracture mechanisms of PTFE, and then affected its quasi-static compression reaction characteristics. Huang et al. [19] established the constitutive model of PTFE/Al/MoO₃ ignoring the temperature effect. These work only took into account the influence of temperature on quasi-static mechanical properties and reactive characteristics. From the perspective of practical application, Al/PTFE is mainly applied in high overload environment and high strain rate. Consequently, it is necessary to establish the constitutive model of Al/PTFE considering temperature effect and investigate the influence of temperature on the impact-induced initiation characteristics of Al/PTFE.

PTFE is a semi-crystalline polymer composed of crystalline and amorphous regions, and its crystalline region will undergo two phase transitions at 19°Ϲ and 30°Ϲ [20]. Under normal pressure, as the temperature increases, the crystalline phase of the PTFE crystal region transform from well-ordered triclinic crystal (<19°Ϲ) with 13 atoms/180° turn to a partially ordered hexagonal crystal (19°Ϲ ~ 30°Ϲ) with 15 atoms/180° turn [21]. When the temperature continues to rise above 30 °C, the molecular chains in the PTFE crystal region are further unhelixed to form pseudo-hexagonal crystal. Under high pressure, its crystal region will show a planar zig-zag crystal [22]. Based on the works of Brown, the molecular structures and phase transition diagram of PTFE crystallization zone at different temperatures and pressures are shown in Fig. 1 [[23], [24], [25], [26]].

In the present work, the dynamic mechanical analysis (DMA), split Hopkinson pressure bar (SHPB) system test and drop-weight test were carried out at different temperatures to encompass the three crystalline phase of the PTFE matrix with transitions at 19 °C and 30 °C. The correlation between fracture modes and ignition mechanism were analyzed associated with microstructures and reaction phenomena captured by the high-speed camera. The influence of temperature-induced phase transition of PTFE on the mechanical behavior and impact-induced initiation characteristics of Al/PTFE were ascertained, and the constitutive model of Al/PTFE considering temperature effect was established.