Impact-Initiation Sensitivity of High-Temperature PTFE-Al-W Reactive Materials

Abstract

Drop-weight tests were conducted to investigate the impact-initiation sensitivity of high-temperature PTFE-Al-W reactive materials. The test results show that the impact-initiation sensitivity of the materials more than doubles with increasing the sample temperature from 25 to 350 °C. Combined with the impact-induced initiation process recorded by high-speed video and the difference between reacted and unreacted residues, the crack-induced initiation mechanism was revealed. The rapid propagation of crack provides a high-temperature and aerobic environment where Al reacts violently to PTFE, which induces the initiation. Moreover, the influence of sample temperature on the sensitivity was discussed and analyzed. The analysis results indicate that the sensitivity shows a temperature interval effect, and 127 and 327 °C are the interval boundaries where the sensitivity changes significantly. The sensitivity may leaps at 127 °C and increases more rapidly in the temperature interval from 127 to 327 °C, but hardly changes after the temperature reaches 327 °C.

1. Introduction

Reactive materials are classified as a kind of energetic structural materials. Such material stays inert and insensitive under ambient conditions but reacts violently with a large amount of chemical energy released upon dynamic loading. Due to the distinctive properties, reactive materials have potential applications in both military and civilian fields [1,2]. PTFE-Al composites are a typical reactive material. Many studies on formulations and fabrications [3], mechanical properties [4], impact initiation [5], and energy release characteristics [6] have been conducted based on theoretical analysis, numerical simulation, and experiments. However, PTFE-Al materials have limitations in application due to the low material strength and density [7].

PTFE-Al-W reactive materials, fabricated by adding the high-density tungsten powder into PTFE-Al mixtures and then pressing and sintering the metal-polymer powder mixtures, show a higher strength [8,9]. The related ballistic impact experiments also indicate that the PTFE-Al-W reactive material projectile has not only a similar penetration capability but also has explosive-like explosion characteristics compared to the steel projectile of the same size [10,11]. As a result, much attention has turned to the chemical response of the materials. The reaction of the reactive materials is actually the redox reaction of Al and PTFE, in which Al is oxidized by fluorine from the PTFE. The reaction equation can be expressed as:

$$4\mathrm{Al}+3(-{\mathrm{C}}_2{\mathrm{F}}_4-)\to 4{\mathrm{AlF}}_3+6\mathrm{C}+8678\mathrm{J}/\mathrm{g}$$

(1)

Through high-speed photography, Mock [12] found that the stored chemical potential of Al-PTFE composites was released after the material deformed and fractured. Feng [13] observed that the deflagration of reactive materials appeared at crack and revealed the initiation mechanism. He thought that the initiation of Al-PTFE composites was a mechanochemical process induced by the propagation of fast-moving cracks in the glassy polymer. The W particles did not participate in the reaction between the PTFE matrix and Al particles, but the addition of W particles changed the fracture toughness of the materials, which promoted or restrained the reaction of Al-PTFE composites. Hunt [14] demonstrated that the extrusion and friction caused by W particles during loading would damage the oxide shell of Al particles, which exposed the aluminum core to surrounding oxidizers and thus promoted the reaction of the materials. Wang’s [15] research demonstrated that with the increased W content, the absorbed critical energy before reaction and the insensitivity to impact loading both exhibited an obvious increasing tendency. However, Ge [16] obtained an opposite conclusion, where the impact-initiation sensitivity increased with the increased W content, for brittle PTFE-Al-W reactive materials fabricated under a much lower sintering temperature and a much shorter duration. Herbold’s [17] research indicated that the increase of the size of W particles promoted the reaction of Al-PTFE composites. Using a sealed chamber, Xu [18] conducted a series of ballistic experiments to investigate the influence of impact velocity on the energy release characteristics of a PTFE-Al-W reactive material projectile. The results show that the reaction efficiency increases with increasing impact velocity. However, the influence of material temperature on the chemical response has been less known.

A related study indicated that space debris shield structures made from reactive materials have been observed to possess a better protection ability than those made from inert materials [19]. When used as a space debris shield structure, reactive materials could be exposed to extreme or unexpected environmental conditions. It is necessary to understand the initiation behaviors of high-temperature reactive materials. In this paper, a series of drop-weight tests were conducted to study the impact-initiation sensitivities of high-temperature PTFE/Al/W reactive materials. Combined with the impact process recorded by high-speed video and the difference between reacted and unreacted residues, the crack-induced initiation mechanism was revealed. Moreover, the influence of temperature on the sensitivity of the materials was discussed and analyzed. The temperature interval effect of the sensitivity was found.

2. Experimental

2.1. Sample Preparation

Raw materials were: PTFE powder (average particle size: 100 nm, density: 2.2 g·cm⁻³, from 3F, Shanghai, China), Al powder (average particle size: 24 μm, density: 2.78 g·cm⁻³, from XRY, Beijing, China), and W powder (average particle size: 44 μm, density: 19.2 g·cm⁻³, from XRY, Beijing, China).

Sample preparation included sample fabrication and sample heat treatment. In the sample fabrication, the powders of PTFE, Al, and W were firstly mixed uniformly for about 40 min in a vacuum. The mass ratios of PTFE, Al, and W were 17.2%, 6.2%, and 76.5%, respectively. Then, the uniform mixtures were isostatically pressed at a pressure of 200 MPa in a steel mold with an inner diameter of 10 mm. Finally, the pressed samples experienced a sintering cycle in a vacuum oven under the protection of argon atmosphere. Briefly, the sintering cycle included heating the pressed samples to 380 °C at a rate of about 50 °C/h, holding at 380 °C for 6 h, then cooling to 310 °C at a rate of approximately 50 °C/h and holding for 4 h, and further cooling to the ambient temperature at an average rate of about 50 °C/h, as depicted in Figure 1a. The fabricated samples have a size of about Φ10 × 3 mm and a density of about 7 g/cm³, as presented in Figure 1b. Note that restricted by the fabrication technology, there were inevitably slight size differences between the fabricated specimens. To improve the test accuracy as much as possible, the specimens with height or diameter errors of more than 5% were removed. In the sample heat treatment, the fabricated samples were heated again for about 10 min in a crucible furnace kept at constant temperature. The temperatures of the crucible furnace were set at 25, 100, 200, 300, and 350 °C.

2.2. Drop-Weight Tests

As shown in Figure 2, a standard drop-weight apparatus was employed to investigate the impact-induced initiation behavior of the high-temperature PTFE-Al-W reactive materials. The drop mass used in the tests was 10 kg, and it was controlled to rise and release by switch. The maximum drop height of the drop mass was 200 cm. The prepared sample without constraint was placed on the anvil. When the release key is pressed, the drop mass separates from the electromagnet and falls freely from a certain height to impact the sample. To reduce the heat loss, the time between taking the sample out of the crucible furnace and starting the test was controlled within 5 s. Moreover, high-speed video was used to record the time sequences of the impact process. In this study, the frame rate was set to 100,000 per second, where a frame was taken every 10 μs.

The impact sensitivity of the materials was characterized by a characteristic drop height, at which the material had a 50% probability of reaction. The well-known “up-and-down” test method [20] was adopted in this study to obtain the characteristic drop height, which can be expressed as:

$$H_{50}=A+B\left(\frac{{\displaystyle \sum i{C}_i}}{D}-\frac{1}{2}\right)$$

(2)

where H₅₀ is the characteristic drop height, A is the lowest drop height in the test, B is the increment of drop height, i is the order of the drop height starting from 0, C_i is the number of reaction events under a certain drop height, and D is the number of reaction events among the tests. In this study, twenty tests were conducted for the samples at each temperature and B = 5 cm. According to the conservation of energy, impact energy to initiate and corresponding impact velocity can be approximately written as:

$$E_{\mathrm{c}}=Mg{H}_{50}=\frac{1}{2}M{v}_{\mathrm{c}}^2$$

(3)

where E_c is the impact energy to initiate, M is the drop mass, g is the acceleration of gravity, and v_c is the impact velocity.

3. Results and Discussion

3.1. Crack-Induced Initiation Mechanism

Typical impact-induced initiation phenomena of the PTFE-Al-W reactive materials at elevated temperatures under dynamic compression are shown in Figure 3. Starting from the contact of the drop mass with the sample, a light of fire was observed after a delay time, during which the sample underwent severely plastic deformation. Then, a violent reaction lasting about 700~900 μs occurred, accompanied by a bright flame and explosion sound. Numerous small particles were ejected from the reaction zone under the gas products.

Table 1. Typical unreacted and reacted sample residues.

Temperature (°C)	25	100	200	300	350
Unreacted
Reacted

exhibited the typical unreacted and reacted sample residues. A significant difference between them could be observed, which was that there was an approximate triangular fracture notch in the reacted samples while the unreacted samples only underwent a plastic deformation.

Based on the results in Figure 3 and Table 1, the crack-induced initiation mechanism could be revealed. Firstly, the sample experiences severely plastic deformation under dynamic compression, during which axial compression causes radial expansion and axial compressive stress is turned into tangential tensile stress, mainly concentrated on PTFE, resulting in the orientation of PTFE molecular chains [21]. As the materials are further compressed, the tangential tensile stress gradually increases, and some orientated PTFE molecular chains are broken. As a result, micro-defects appear. Moreover, the orientated PTFE molecular chains cannot compensate for the development of micro-defects, and cracks start to propagate with accelerating speed [22]. On the one hand, an extremely high temperature rise is produced due to the fast-moving crack tip, which accounts for the decomposition of PTFE. On the other hand, the orientated PTFE molecular chains away from the crack tip are broken, which allows external air to flow into the crack. In other words, the crack propagation provides a high-temperature and aerobic environment in the crack. As a result, the fine-sized PTFE particles react violently with Al particles in the crack, thus resulting in the initiation. Finally, a large amount of gas products produced due to the reaction converge and expand in the crack and then work outward. Numerous unreacted particles are ejected outward under the reaction gas products.

3.2. Influence of Temperature on Impact Sensitivity

The data of each group of drop-weight tests are shown in Figure 4. The calculated characteristic drop heights, impact energy to initiate, and impact velocity by Equations (2) and (3) are listed in

Table 2. The calculated results of drop-weight tests.

T (°C)	25	100	200	300	350
H50 (cm)	153.6	145	97	79.3	74
Ec (J)	150.5	142.1	95	77.7	72.5
vc (m/s)	5.5	5.3	4.4	3.9	3.8

. T is the initial temperature of the tested sample. It could be found from Table 2 that as the initial temperature increased from 25 to 350 °C, the characteristic drop height, H₅₀, decreased from 153.6 to 74 cm and the impact energy to initiate, E_c, decreased from 150.5 to 72.5 J, indicating that the impact sensitivity almost doubled. This should be attributed to the thermal softening effect of the materials. The material is a polymer matrix composite and shows a typical elastic-plastic property. In addition, the temperature plays a vital role for the mechanical property of the materials. The material characteristic parameters, including the yield strength, the tangent modulus, the ultimate strength, and the critical failure strain, all show a decreasing trend with the elevated temperatures. On the one hand, the absorbed energy of the materials during deformation to failure decreases due to the thermal softening. On the other hand, less energy is required for crack propagation. As a result, the total energy inducing the materials to initiate decreases.

3.3. Thermal Sensitivity

It is notable that the decreased rate of characteristic drop height in each temperature interval is significantly different, as shown in Figure 5. It was found that the decreased rate in the temperature interval from 100 to 200 °C was about four times of that in the temperature interval from 25 to 100 °C. This indicates that the impact-initiation sensitivities of the materials display a temperature interval effect.

Based on the crack-induced initiation mechanism, it is reasonable to believe that the break of PTFE-oriented molecular chains is necessary for the initiation of materials under dynamic compression. The more easily the orientation is broken, the greater the impact-initiation sensitivity is. PTFE is a complicated semi-crystalline polymer treated as a composite that is composed of a “rigid” crystalline phase and a “softer” amorphous phase, where the entanglement of molecular chains in amorphous regions hinders the orientation [23,24]. The entanglement leads to the break of the orientated chains under tensile stress. Ascribing to molecular thermal motion, the break of the orientated chains exhibits significant dependence on temperature. With the increased temperature, the capability of the entanglement hindering the orientation becomes prominent due to intensified molecular thermal motion, promoting the break of the orientated chains and thus improving the impact-initiation sensitivities of the materials.

However, the PTFE molecular thermal motion, which determines the impact-initiation sensitivity of the materials, displays a temperature interval effect [25,26]. When the temperature is between 25 and 127 °C, PTFE is in a glassy state, where the molecular chains are frozen. Only the local vibration of small units such as side groups, links, and short branches occurs. As the temperature rises, the impact-initiation sensitivity of the materials increases slightly. At 127 °C, PTFE turns into a highly elastic state from the glassy state, where the molecular chains begin to thaw. The molecular chain segments are activated to move freely. The impact-initiation sensitivity of the materials significantly increases at this temperature. As the temperature continues to rise, the impact-initiation sensitivity of the materials increases more than that when PTFE is in the glassy state due to the predominant movement of the molecular chain segments. Furthermore, the synergistic effect of the molecular chain segments gradually causes the movement of molecular chains with the increasing temperature. When the temperature rises to 327 °C, the melting point of PTFE is reached, at which the molecular chains can move freely. This indicates that the capability of the entanglement hindering the orientation almost approaches a maximum. Therefore, further temperature rises hardly improve the impact-initiation sensitivity of the materials. Therefore, 127 and 327 °C are the turning temperatures of the impact-initiation sensitivity of the materials with elevated temperature.

Figure 6 presents the temperature interval effect on the impact-initiation sensitivity of the materials. It could be observed from Figure 6 that the impact-initiation sensitivity of the materials may leap at 127 °C and hardly increases after the temperature reaches 327 °C. The increased rate of the sensitivity with rising temperature when PTFE is in a highly elastic state is 1.6 times that when PTFE is in a glassy state.

4. Conclusions

A series of drop-weight tests were conducted to investigate the impact-initiation sensitivity of high-temperature PTFE/Al/W reactive materials. The following conclusions were drawn:

(a)

The crack of the materials subjected to dynamic loading provided a high-temperature and aerobic environment, in which Al reacted violently with PTFE, thus inducing the initiation.

(b)

The impact-initiation sensitivity of the materials significantly depends on temperature. The sensitivity more than doubled with increasing the sample temperature from 25 to 350 °C.

(c)

The impact-initiation sensitivity of the materials showed a temperature interval effect. The sensitivity increased more rapidly in the temperature interval from 127 to 327 °C, but hardly changed after the temperature reached 327 °C.