Drop-hammer impact sensitivity and damage effect of two PTFE-matrix reactive materials

Reactive materials such as Aluminum-Polytetrafluoroethylene (Al-PTFE) have become an emerging research hotspot in the field of energetic materials in recent years. They have the characteristics of high energy density, good stability and easy preparation. Moreover, they can undergo violent chemical reactions and release a large amount of heat under impact compared with traditional energetic materials such as explosives and propellants.^1–3 In the field of military applications, the reactive materials can be made into an energetic warhead with Impact-Response double damage effect. They possess the kinetic energy penetrating characteristics of conventional metal penetrators, and also can penetrate the armor layer and enter the target interior to react violently, forming a comprehensive killing effect such as explosion shock, overpressure, and ignition (explosion), which produce a more significant damage effect than a conventional warhead.⁴ At the same time, they also have great application value and development prospects in the fields of space, petroleum industry and so on.^5–8 

An energetic warhead undergoes severe deformation, failure and chemical reactions in the process of penetration. It is difficult to explore its failure mode, reaction time and mechanism due to the short time and destructiveness of the penetration process. Some scholars have made a series of studies on the mechanisms and damage effect of energetic warhead. Xu et al. studied the impact damage effect of PTFE/Al/W reactive material projectiles on 2024-T3 aluminum plates at different velocities. Theoretical analysis and experimental data show that the damage of aluminum plates by reactive material projectiles impact not only varies with the kinetic energy, but is also significantly influenced by the chemical energy released in the penetration process.⁹ Peng et al. investigated the penetration ability of an energetic warhead on a steel target and the damage effect behind the target. Their experimental results showed that the warhead penetration of a the steel target can produce a certain number of burst fragments and form a strong shock wave overpressure in a certain area behind the target.⁴ Ames et al. studied the energy release characteristics of energetic projectiles in a ballistic impact experiment by using a vented chamber. They determined a correlation between the pressure in the chamber and the chemical energy released. They showed that the impact-induced initiation characteristics and the chemical reaction during penetration are significantly affected by impact velocity, target thickness and so forth.^10,11

Although the strength and density of Al/PTFE reactive materials are higher than those of traditional energetic materials, they are still inferior to metal components as warheads. In order to improve the mechanical properties and increase the energy release rate of Al/PTFE reactive materials, some scholars have introduced heavy metals (W),^12,13 metal oxides (Fe₂O₃, CuO₂),^14,15 metal hydrides to Al/PTFE (TiH₂)¹⁶ and so forth. Moreover, the mechanical properties, reaction characteristics and mechanisms of the above reactive materials have also been studied. Among them, W has been widely discussed in Al/PTFE applications due to its high density. Meanwhile, Nickel (Ni), a high-density (8.9 g/cm³) transition metal, can react with aluminum to form Ni-Al intermetallic compounds with a very high energy release rate.^17–19 It can also be introduced into Al/PTFE as an excellent energetic additive.

Therefore, multiple sets of PTFE-matrix specimens and liners were fabricated by molding-vacuum sintering. Drop-hammer tests were conducted to explore the impact sensitivity and energy release characteristics of two types of reactive materials. And the impact experiments of two energetic warheads on steel targets were then carried out to study the damage effects.

The raw powders adopted to prepare hemispherical liners had the following average sizes: PTFE: 25μm (from 3F, Shanghai, China); Al: 1∼2μm (from JT-4, Hunan, China); Ni: 2μm (from Naiou, Shanghai, China); W: 2μm (from Naiou, Shanghai, China). Six groups of Al/Ni/PTFE (ANP) and Al/W/PTFE (AWP) homogeneous mixtures with different Ni and W contents, as well as pure PTFE and the chemical equilibrium ratio (26.5 wt.%/73.5 wt.%) Al/PTFE reactive material were prepared as comparative groups. Eight groups of materials were labeled as 1^#-8^# for convenience of presentation. The sample formulations are shown in Table I.

The raw material powder was weighed according to the proportion and added to a beaker, suspended in anhydrous ethanol solution and then stirred using a motor-driven blender for 20 min. The suspension was dried in a vacuum oven for 48 h at a temperature of 60 °C. Finally, the completely dried mixtures was sieved (60 meshes) to produce uniform powders.

The eight groups of materials with different proportions were cold uniaxial pressed into cylindrical specimens (Φ10 mm × 3 mm) and hemispherical liners (2 mm wall thickness and 30 mm diameter) for drop-hammer tests and impact experiments using a molding die and a hydraulic press. Next, they were sintered at 360 °C for 4 h in a vacuum sintering oven with a heating rate of 90 °C.h^-1 and a cooling rate of 50 °C.h^-1. The sintered cylindrical specimens and hemispherical liners are shown in Figure 1.

As shown in Figure 2, the structure of the energetic damage element is mainly composed of hemispherical liners, pressed plastic explosive, shell and other accessories. The shell, end cap and buffer layer were made of a nylon material, which was machined using a lathe. The purpose of adding a buffer layer is to avoid the direct impact of explosive on the hemispherical liner. The charge was evenly pressed and its quality was uniform to 15 g. There were design threads on the outside of the retaining ring and inside the housing for fixing the liner. The assembled damage element and the physical diagram of the component are shown in Figure 3.

The impact sensitivity of the specimens was measured using a drop-hammer machine (HGZ-1, TD, Xiangfan, China) with reference to GJB772A-1997 method 601.2. The experimental settings are shown in Figure 4. The instrument has a drop weight of 10 kg, a range of 0-156 cm, and a maximum output energy of 152.9 J. The free falling drop mass impacted the cylindrical specimens placed on the anvil of the tester, and a high-speed camera with a frame rate up to 20000 frames/s (Vision Research, Inc., Wayne, NJ, USA) was used to observe and record the reaction condition of the specimens. The impact sensitivity of the materials was measured by the characteristic drop height (H₅₀) when the specimens had a 50% probability to react. The test was conducted at an ambient temperature of 21°C.

The damage experiment settings are depicted in Figure 5. The target plate used in the experiment was plain carbon steel, with a length and width of 500×500 mm, and a thickness of 3 mm and 10 mm. The distance between the damage element and the target plate was 6 cm. An electric detonator was used to detonate an explosive change which drove the liner to impact the steel plate. Moreover, high-speed photography (FASTCAM SA-Z, Photron, Tokyo, Japan) was also used to record the reaction process to observe the damage effect of the energetic liner.

The characteristic drop height (H₅₀) was obtained by the method called “up-and-down technique.”^20–22 Since the drop hammer was free to fall, the impact time was very short and so the impact process can be regarded as instantly completed. It is considered that the energy absorbed by the impact specimens was approximately equal to the potential energy of the drop mass. The potential energy of the drop mass at the characteristic drop height was the energy required for the reaction of specimens. The calculation formulas of the characteristic drop height and the ignition energy of specimens are shown in Formula (1) and (2).²³ 

Where A is the lowest height in the test, B is the increment of the height, i is the order of the drop height beginning from 0, N is the total number of reaction events in the test, N_i is the number of reaction events at a certain height, E_i is the ignition energy of specimens, E_p is the potential energy of the drop mass, and m is the mass of the drop hammer.

Based on the test method of “up-and-down technique,” the characteristic drop height and ignition energy of seven groups (2^#-8^#) of reactive materials are shown in Table II. It can be seen from the table that the characteristic drop height of ANP and AWP was larger than that of Al/PTFE and increased with increasing Ni and W content, indicating that both additives contributed to reducing the sensitivity of Al/PTFE reactive material, but to a different degree. The variation trend of the characteristic drop height with the increase of metal addition is depicted in Figure 6. The effect of Ni on the impact sensitivity of Al/PTFE reaction materials was more obvious than that of W. Especially, the characteristic drop height of ANP with 5% Ni content increased 20.53 cm than that of Al/PTFE and the effect is most remarkable.

Wu et al. found that the reaction site of Al/PTFE reactive material in the drop impact was circumferential open crack,²⁴ which was consistent with the reaction phenomenon of Al/PTFE in quasi-static compression test found by Feng et al.²⁵ The metal W cannot participate in the whole reaction process. Moreover, Al reacts with PTFE first in ANP ternary system, and when there is excessive Al, metal Ni may participate in the reaction.²⁶ Therefore, the proportion of Al and PTFE which can react reduced due to the addition of metal additives. The area where can generate “hot spots” decreased so that the possibility of reactive materials reacting and the impact sensitivity reduced in the drop hammer impact test.

The reaction phenomenon recorded by high-speed camera in drop-hammer test suggests that for the two materials with the same metal content, the energy release level of ANP was significantly different from that of AWP at different drop heights. Taking ANP and AWP with 10% metal content as an example, the energy release level of ANP was lower than that of AWP when the drop height was 80 cm, while it obviously rose as the fall height was increased to 140 cm. The ignition reaction phenomenon of two materials with 10% metal content at different drop heights are shown in Figures 7 and 8. The analysis results show that the energy release level of the Al/PTFE reactive material was related to the initial excitation energy. The reaction between Al and PTFE is insufficient when the excitation energy was low so that the released energy cannot trigger Al-Ni intermetallic reaction because of its higher activation energy. In the case of the same content of Ni and W, the sensitivity of AWP reactive material was higher than that of ANP, which showed a larger flame area. The energy release level of ANP was significantly higher than that of AWP when the impact height of drop hammer increased and the excitation energy was sufficient to initiate the Al-Ni reaction.

An energetic hemispherical liner driven by detonation pressure of plastic explosive after initiation impacts the target plate at a high speed. The impact process of the inert and reactive damage elements on the steel plate recorded by the high-speed camera, is shown in Figure 9. As can be seen, an intense fire was produced at the moment of impact, but a obvious reaction zone (Fig. 9(b)) appeared after the energetic damage element impacted the steel plate, indicating that the reactive liner underwent a chemical reaction during the collision process, which coincided with the numerical simulation result in Section III A.

Figures 10 and 11 present the damage effect of energetic liners against targets. Black burning marks and carbon deposits were found in the impact area of the target plate, which indicates that the energetic liner underwent chemical reaction during the impact process. The product produced by the reaction had a better radial expansion effect, and all the perforations were petalling damage pattern. The corresponding damage effect data are tabulated in Table III. It can be seen that all seven energetic liners can penetrate the 3 mm thick steel target except the inert damage element prepared by PTFE. The 6^# liner had the strongest reaming ability, and the hole diameter formed after penetrating the target is 9.4 cm. It can be seen from Table IV that when impacting the 10 mm thick steel plate, only the 5^# liner can penetrate it with a hole diameter of 1.5 cm. The impact of 4^# liner on steel plate did not completely perforate, and a hole with a diameter of 1.2 cm and a depth of 0.6 cm was formed on the front side the plate. The 8^# liner did not cause significant damage to the target plate, while the other liners failed to break through the target plate, causing deformation pits only at the impact site.

In the process of high-speed impact of PTFE-based energetic liners on steel targets, violent chemical reactions occured and a large amount of heat was released. The chemical reaction in this process may occur as follows:

The results of penetration on a 3 mm thick steel target suggests that 6^# liner had the strongest reaming ability, and the penetration holes formed by the 3^#, 4^# and 5^# were smaller than that of the 2^# without metal additives. The reason is that 6^# possessed higher strength and density than 2^#, which made more reactive fragments release energy through the target plate so that it was better at expanding holes. Because of the low sensitivity of ANP reactive material system, the 3 mm steel plate failed in advance during the impact process due to its low strength. The impact energy can not initiate Ni-Al intermetallic reaction.^27,28 Therefore, the energy release level of ANP reactive material was lower than that of AWP, which manifested smaller aperture. When the thickness of the steel target is 10 mm, only the 5^# liner can penetrate it on account of its higher strength and density with the increase of Ni content.¹⁹ Meanwhile, compared with the 8^# liner, the energy released by Al/PTFE reaction triggered Ni-Al intermetallic reaction, which greatly improved the energy release level of reactive material and the damage effect. In summary, the damage effect of reactive materials on the target plate was determined by its strength, density and energy release level.

In this study, PTFE-matrix specimens and liners were prepared by molding-vacuum sintering. Drop-hammer tests and impact damage experiments of 3 mm and 10 mm thick steel targets were carried out. The main conclusions are as follows:

1. The characteristic drop height (H₅₀) of ANP and AWP improved with the increase of Ni and W content from 0 to 15%. The two additives contributed to reducing the sensitivity of Al/PTFE reactive material because the addition of Ni and W reduced the content of Al and PTFE which can react to generate “hot spots,” but the effect of Ni on the impact sensitivity of Al/PTFE was more obvious than that of W. The energy release level of ANP and AWP with the same metal content was significantly different at different drop heights. When the energy released by insufficient reaction of Al and PTFE cannot trigger Al-Ni intermetallic reaction with high activation energy, the impact sensitivity of AWP was higher than that of ANP. While the energy release level of ANP was significantly higher than that of AWP when the excitation energy was sufficient to initiate the Al-Ni reaction.

2. The impact process recorded by the high-speed camera, black burning marks and carbon deposition at the front impact site of the target plate show that the energetic liner reacted during the impact process. All the perforations were petal-shaped holes due to the radial expansion effect of the reaction product. Seven groups of energetic damage elements can penetrate 3 mm steel target and 6^# had the strongest reaming ability. Only 5^# can penetrate 10 mm thick steel plate, while the rest of liners failed to break through it and caused certain unperforated holes or deformed pits at the impact site.

3. The 6^# liner had higher strength and density than 2^#, and exhibited stronger reaming ability when penetrating 3 mm steel target. However, in the impact process of three groups of low sensitivity ANP reactive material systems with 3 mm thick steel target, the target plate failed early because of its low strength and the impact energy cannot induce the Ni-Al intermetallic reaction. The energy release level of the ANP energetic material was lower than that of the AWP, which showed smaller aperture. Only the 5^# liner with higher strength and higher density can penetrate the steel plate with thickness of 10 mm. Compared with the AWP reactive material system, the energy released by the Al/PTFE reaction triggered Ni-Al intermetallic reaction, which greatly increased the energy release level of energetic materials and made the damage effect better. The intensity, density and energy release level all affected the damage effect of the reactive materials on target.

The financial support from the National Natural Science Foundation of China (General Program. Grant No. 51673213) is gratefully acknowledged.

The authors declare no conflicts of interest.

This research was funded by the National Natural Science Foundation of China (No. 51673213).