Characterization and Analysis of Micromechanical Properties on DNTF and CL-20 Explosive Crystals

Abstract

To study the crystal mechanical properties of 3,4-dinitrofurazanofuroxan (DNTF) and hexanitrohexaazaisowurtzitane (CL-20) deeply, the crystals of DNTF and CL-20 were prepared by the solvent evaporation method. The crystal micromechanical loading procedure was characterized by the nanoindentation method, and then obtained the mechanical parameters. In addition, the crystal fracture behaviors were investigated with scanning probe microscopy (SPM). The results show that the hardness for DNTF and CL-20 was 0.57 GPa and 0.84 GPa, and the elastic modulus was 10.34 GPa and 20.30 GPa, respectively. CL-20 obviously exhibits a higher hardness, elastic modulus and local energy-dissipation and a smaller elastic recovery ability of crystals than those of DNTF. CL-20 crystals are more prone to cracking and have a lower fracture toughness value than DNTF. Compared to DNTF crystals, CL-20 is a kind of brittle material with higher modulus, hardness and sensitivity than that of DNTF, making the ignition response more likely to happen.

1. Introduction

An energetic crystal is a key composition for the explosive formulation designation and its application. In particular, its sensitivity has an important impact on the safety performance of explosive mixtures. Material mechanical performance plays a crucial role on the response behavior of crystals under external mechanical load (such as the impact, friction, impact, etc.), which could result in the formation of “hot spots”, and also relate to impact sensitivity. It is reported that for most chemical compounds, sensitivity increases with an energy content rise, although this is not a strict rule [1]. It is of great significance to fully grasp the micromechanical properties of crystals for further understanding the safety properties of materials and analyzing the response mechanism.

The traditional mechanical test method is only suitable for samples with a large size, and struggles to tests smaller ones, especially in the nanometer dimension. Additionally, this problem can be resolved effectively by nanoindentation technology. As a new testing method invented in the early 1990s, nanoindentation technology has been extensively applied to all kinds of materials in the nano/micro dimension, such as ceramics, metals, alloys, energetic materials, etc. [2,3,4,5,6]. Nanoindentation technology uses a computer-controlled load to push a rigid indenter of a specific shape into the surface of the material being tested. At the same time, a high-resolution displacement sensor is used to collect the depth of pressure on the surface of the measured material, and the load–displacement curve of the material surface is obtained. This can effectively measure some mechanical behaviors of materials at the micro/nanoscale, such as hardness, elastic modulus, fracture toughness, strain hardening effect, creep behavior, etc. [7]. Nanoindentation technology is becoming the primary choice for the mechanical property testing of micro/nanoscale materials and structures due to its advantages of simple test operation, high measurement efficiency and wide application range [8]. At present, the research on nanoindentation mainly focuses on the scale effect of indentation experiments. Early researchers [9,10] found that indentation hardness increased with the decrease in indentation depth through experimental studies. Gerberich [11] studied and found the size effect of indenter shape on indentation hardness. Swadender et al. [12] found that the hardness value decreases with the decrease in the radius of the contact area. Scholars mainly focus on vertical loading and unloading, and study the scale effect of mechanical properties of materials by fitting load–displacement curve and hardness-displacement curve.

With the development of nanoindentation technology, it is gradually applied to the characterization of energetic materials. Ramos et al. analyzed the deformation mechanism of brittle material with the nanoindentation test of cyclotetramethylene tetranitramine (HMX) simulative material [13] and the surface testing about monocrystalline Cyclotrimethylene trinitramine (RDX) [14]. Hudson et al. obtained the micromechanical properties of the different crystal RDX, which demonstrated a potential relationship with the degree of crystal internal defects [15]. Mathew and Sewell [16] studied the crystal micromechanical properties of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and carried out its molecular dynamic simulation. Matthew et al. [17] tested and analyzed the elastic and plastic characteristics for FOX-7, HMX and ADAAF. Zhai et al. explored the yield behavior of PETN and found that the indentation modulus decreases with the increase in indentation depth [18]. According to the investigation on the regular jump phenomenon of RDX (210) [19], Li et al. computed the yield stress and hardness values, and analyzed the elastic modulus of crystal β-HMX [20]. Zhu et al. [21] found that DNAN had worse ability to resist deformation than TNT, but more obvious slow recovery elasticity and stronger impact energy absorption ability. Meanwhile, they found that HATO was harder and more brittle compared with RDX when impacted by external shock [22]. Moreover, Ekaterina et al. found that surface dynamics influence a material’s ability to dissipate excess energy, acting as a buffer to mechanical initiation [23]. For the materials with less hardness, such as picric acid and 3,4-dinitropyrazole, the surfaces could be rearranged in response to mechanical deformation. DNTF and CL-20 are typical highly energetic materials with superior crystal density and energy compared to RDX, HMX, TNT, which are crucial parameters to improve the explosive properties. However, few studies have focused on the micromechanical properties of DNTF and CL-20. This paper analyzed parameters such as the elastic modulus and hardness, characterized the break behavior and explored the relationship between crystal properties and sensitivity by means of nanoindentation technology.

2. Materials and Methods

2.1. Materials

DNTF and CL-20 crystals were prepared by volatilization using acetone as a solvent at room temperature, where the CL-20 was from Qingyang Special Chemical Industry Co., Ltd. (Qingyang, China), and the DNTF was synthesized by Xi’an Modern Chemistry Research Institute. In addition, the nanomechanical analyzer, TI950, made by the Hysitron company in America, was used to employ the nanoindentation experiments to obtain the mechanical characteristics of crystalline CL-20 and DNTF. The mechanical characteristics included the testing of material hardness, elastic modulus, and fracture toughness, where the indenters were both kinds of Berkovich and the parameters of scanning probe image were 2 μN contact force and 15 μm × 15 μm size, respectively.

2.2. Methods

In the process of nanoindentation testing, the indenter was pressed into the surface of the samples with a certain load, and when the load reached a designed value, the external force was unloaded.

During the test loading, the indenter displacement (H) and load (P) were recorded by means of the high-precision load–displacement testing technique. At the contact with the indenter, in the direction of the load, the material had a certain degree of elastic recovery. Figure 1 shows the typical curves of displacement and load in the process of loading and unloading. The key parameters included the maximum load (P_max), maximum displacement (h_max), the final indentation depth after complete unloading (h_f) and the top slope S of the unloading curve.

In the experiments, the crystalline DNTF and CL-20 explosives were loaded with forces of 500 μN to 5000 μN with the same conditions of 5 s loading, 2 s pressure maintaining and 5 s unloading.

3. Results and Discussion

3.1. Indentation Curve of Crystalline Material

According to the nanoindentation testing, the curves of loading and unloading for DNTF and CL-20 were obtained as shown in Figure 2.

The curves obtained exhibited similarity to the theory curve above. With the increase in loading, the maximum depth of the indenter (h_max) and the final indentation depth (h_f) increase continuously gradually. During the loading, the displacement on both crystals showed a sudden increase, which was mainly caused by internal microdefects such as microcracks, micropores, etc. When the contact surface of the indenter is close to the defect, the elasticity and hardness of the local material will decrease sharply, resulting in the sudden increase in the indenter displacement. Therefore, when the existing defects are sensed by the indenter, the indentation displacement increases. In order to clearly compare the loading characteristics of the two kinds of crystal mechanics, three loading–displacement curves at 1000, 3000 and 5000 μN were compared, and the results are shown in Figure 3.

It can be seen from Figure 3 that the two kinds of material showed different mechanical behaviors, in that the indentation depth on the crystal surface was quite different under the same load. Figure 4 shows that the maximum displacement (h_max) of DNTF was higher than that of CL-20 under the same load. In addition, when the pressure was completely unloaded, the final indentation depth h_max of the two crystals was basically the same. Therefore, the DNTF was more prone to deformation. Furthermore, the top slope S of the unloading curve was also named contact stiffness and increased with the load. The S_CL-20 was obviously higher than S_DNTF, which indicated that CL-20 had a harder contact stiffness, as shown in Figure 5.

3.2. Crystalline Elastic Modulus and Hardness

The hardness and elastic modulus of crystalline materials were calculated by the theory of Oliver–Pharr, and the formulas of hardness H and elastic modulus E are shown below:

$$H=\frac{P_{\max }}{A}$$

(1)

$$\frac{1}{E_r}=\frac{1-v^2}{E}+\frac{1-v_i^2}{E_i}$$

(2)

$$E_r=\frac{\sqrt{\pi }}{2\beta }\frac{S}{\sqrt{A}}$$

(3)

where E_r is the equivalent modulus. E_i is the modulus of indenter. A is the contact area. β is a constant related with indenter shape. ν_i is the Poisson ratio of the indenter and ν is the Poisson ratio of samples. According to the formulas above, the values of crystalline elastic modulus and hardness were obtained and the variation with load, as can be seen in Figure 6 and Figure 7.

As shown in Figure 6 and Figure 7, with the increase in H_max, the hardness and elastic modulus of DNTF and CL-20 decreased first and then trended toward a fixed value. That is, when the compression depth is small, the mechanical parameters of the material are larger. As the depth of compression increases, the mechanical parameters of the material approach a constant value, which is called the “scale effect”, and this effect is related to the plastic strain and the plastic strain gradient of the material [15]. The elastic modulus of the crystal is mainly determined by the strength of the intermolecular binding force. The stronger the intermolecular binding force is, the less easy it is to deform, and the higher the elastic modulus is. With the increase in indentation depth, the elastic modulus changes little but decreases slightly.

The average values of hardness for DNTF and CL-20 is 0.57 GPa and 0.84 GPa, and the elastic modulus is 10.34 GPa and 20.30 GPa, respectively. The average deviations of hardness for DNTF and CL-20 are 0.07 GPa and 0.06 GPa, and those of the elastic modulus are 0.54 GPa and 0.74 GPa, respectively. The elastic modulus and hardness of CL-20 are about 47% and 96% higher than those of DNTF, respectively, which indicates that CL-20 has a high stiffness and is difficult to deform. On the contrary, DNTF experiences easier indentation—namely, CL-20 is “hard” and DNTF is “soft”.

In addition, the elastic modulus of a material is not directly proportional to its hardness. The elastic–plastic local deformation in the loading process determines the hardness of the material and the work conducted by external forces, and the elastic recovery in the unloading process reflects the local energy dissipation and elastic modulus of the material. Based on elastic contact theory, the relationship between the elastic modulus and hardness of solid materials depends on the energy dissipation capacity of materials. Additionally, the local energy dissipation R_S of the material is inversely proportional to the ratio of H/E [19]. The ratio of CL-20 and DNTF is calculated to be 0.041 and 0.055, respectively, so the local energy dissipation of CL-20 crystals is greater than that of DNTF, which will lead to a lower elastic recovery capacity around the indentation of CL-20 than that of DNTF.

The hardness and elastic modulus of crystals are closely related to the intermolecular binding force. Figure 8 and Figure 9 show the molecular structure of CL-20 and DNTF.

CL-20 is a caged nitroamine explosive with molecular formula C₆H₆O₁₂N₁₂, and there are van der Waals forces and hydrogen bond interactions between molecules. Pampuram et al. [24] found a novel synthesis method of hexaazaisowurtzitane cages to access CL-20, where CL-20 with a yield of 25% was successfully obtained.

By contrast, DNTF is a typical furazan compound, which is composed of an oxidized furazan ring, a furazan ring, a nitro group and other groups. The molecular formula is C₆O₈N₈, and there is no hydrogen element in the molecule, so there is no hydrogen bond between molecules, which is mainly dominated by van der Waals forces. Therefore, the intermolecular binding force of DNTF is weaker than that of CL-20. Due to its strong intermolecular binding force, CL-20 is not easy to deform, resulting in its mechanical properties differing from those of DNTF.

3.3. Crystalline Elastic Property

In the testing of nanoindentation, pure elastic deformation is almost impossible. Due to the high local stress concentration, local plastic deformation inevitably occurs to some extent, so the variation of crystal depth mainly includes elastic and plastic deformation. In the process of pressing, the total work transforms to the sum of elastic and plastic work of materials. In addition, after unloading, only part of the elastic work is released. From the curve of loading and unloading, the total deformative work A_t and elastic work A_e are obtained, and accordingly, the plastic work A_p is calculated. The plasticity of crystalline materials can be expressed by dimensionless δ_A as follows [25,26,27]:

$${\delta }_A=\frac{A_p}{A_t}=\frac{A_t-A_e}{A_t}$$

(4)

$$A_t={\displaystyle {\int }_0^{h_{\max }}Pdh}$$

(5)

$$A_e={\displaystyle {\int }_{h_p}^{h_{\max }}Pdh}$$

(6)

It can be seen in

Table 1. Calculated elastic–plastic work of DNTF and CL-20.

Pmax/μN		500	700	1000	2000	3000	4000	5000
DNTF	At × 10−10/J	0.38	0.67	1.1	3.52	6.65	9.95	14.32
	Ae × 10−10/J	0.15	0.24	0.43	1.2	2.21	3.45	4.72
	δA	0.61	0.64	0.61	0.66	0.67	0.65	0.67
CL-20	At × 10−10/J	0.31	0.49	0.89	2.55	4.67	7.41	9.95
	Ae × 10−10/J	0.09	0.14	0.24	0.67	1.25	1.94	2.67
	δA	0.71	0.7	0.73	0.74	0.73	0.74	0.73

that the total deformation work and elastic deformation work of the two materials increase with the increase in load. Additionally, under the same load, the total deformation work and elastic deformation work of DNTF crystal are significantly greater than that of CL-20. However, the values of δ_A of the two materials are basically the same, which shows that when the crystal is stimulated by external load, the ratio of elastic deformation to plastic deformation of DNTF and CL-20 remains unchanged, and most of the total deformation work is converted into plastic deformation work.

Since the δ_A average value of DNTF (0.64) is smaller than that of CL-20 (0.73), it can be concluded that CL-20 crystals have a higher plastic deformation and transformation ability. However, the elastic transformation ability of DNTF crystal is stronger, and the elastic recovery ability of DNTF is higher than that of CL-20, which further reflects the characteristics of the large local energy dissipation of CL-20. This also means that under the same loading conditions, the structural integrity of CL-20 crystals is weaker than that of DNTF crystals, so it is more likely to be damaged under impact.

3.4. Fracture Toughness Property

Crack formation is an important form of crystal mechanics, and the fracture toughness (K_IC) parameter is generally used to quantify and measure the nanoindentation. K_IC reflects the energy required for crystal fracture, which means the ability of crystals to prevent crack propagation. In general, the higher the fracture toughness value of the material, the higher the critical stress required for crack instability propagation and the stronger the crack resistance. According to the theory of fracture mechanics and the analysis of the angular radial crack traces in the nanoindentation test, the mathematical relationship between the fracture toughness value and the indentation crack length is as follows:

$$K_{IC}=\alpha \sqrt{\frac{E}{H}}\left(\frac{P_m}{C^{3/2}}\right)$$

(7)

where P_m is the load, C is the radial crack length, and α is the empirical parameter related to the indenter (α of the cubic angle indenter is 0.036).

A cubic angle indenter was used to test the radial crack on the surface of DNTF and CL-20 crystals at 3000 μN, and the results are shown in Figure 10. It can be seen that obvious cracks appear in CL-20, while no cracks appear in DNTF. The radial cracks of DNTF and CL-20 and the average radial crack lengths were obtained under 4000 μN and 5000 μN loads by means of the same loading method. The elastic modulus and hardness obtained were substituted into Equation (7) to obtain the fracture toughness values under different loads, as shown in

Table 2. Crack length and fracture toughness values at different loads.

Pmax/μN	DNTF		CL-20
	L/μm	KIC/MPa	L/μm	KIC/MPa
3000	--	--	1.89	92.36
4000	2.63	114.37	2.42	85.22
5000	3.25	114.80	3.37	64.50

.

With the increase in loading from 3000 to 5000 μN, the crack length on the surface of the two crystals shows an increasing trend, and the crack length of CL-20 is more significant. With the increase in loading, the fracture toughness value of CL-20 decreases continuously, showing a typical material brittle fracture behavior. In addition, the fracture toughness value of CL-20 crystals is lower than that of DNTF crystals, and it is more prone to cracking than DNTF crystals. The experimental results show that CL-20 exhibits brittleness. Although the compressive strength is high under quasi-static conditions, the impact resistance is weak. In contrast, DNTF shows toughness—that is, under the same impact load, it will absorb more energy and undergo large deformation without sudden failure, and DNTF crystal has strong anti-failure ability.

3.5. Crystal Mechanics and Crystal Sensitivity

Under impact conditions, when materials with different elastic moduli deform, the higher the elastic modulus is, the higher the strain rate will be, and the greater the impact stress will be [28]. Therefore, under the dual action of stress and strain rate, stress concentration is more likely to lead to crystal fracture and the formation of “hot spots” for the high elastic modulus material. From the perspective of the mechanical properties of materials, by comparing the mechanical properties of CL-20 and DNTF crystals, it can be seen that CL-20 crystal has a high elastic modulus and hardness, and CL-20 is brittle and prone to cracking. Consequently, CL-20 is more likely to lead to an ignition response than DNTF. In addition, DNTF crystal is a typical high-energy melting and casting carrier. In addition to low modulus and hardness, it also has the property of endothermic melting, which is beneficial to inhibit the formation of “hot spots”. Understandably, DNTF is less likely to react than CL-20.

As the typical high energy density materials, both DNTF and CL-20 have high energy and high sensitivity. According to the mechanical sensitivity test method of GJB772A-97, the impact sensitivity of CL-20 and DNTF is 100% and 88%, respectively [29,30], and the friction sensitivity is 100% and 84% [31,32], which means that DNTF mechanical sensitivity explosion probability is lower than CL-20. It can be concluded that although DNTF and CL-20 are both highly sensitive materials, DNTF is relatively safer than CL-20 according to the crystal mechanical properties and sensitivity performance data.

4. Conclusions

The average hardness of DNTF and CL-20 is 0.57 GPa and 0.84 GPa, and the average elastic modulus is 10.34 GPa and 20.30 GPa. The hardness, elastic modulus and local energy dissipation of CL-20 are significantly higher than those of DNTF.

Most of the pressing work of CL-20 crystal is converted to plastic work, and its elastic recovery ability is less than that of DNTF. The indentation morphology shows that CL-20 crystal is more prone to cracking than DNTF crystals, and the fracture toughness value is lower than that of DNTF crystals. Compared with DNTF crystals, CL-20 is a brittle crystal material with high modulus and high hardness.

Based on crystal micromechanics, CL-20 crystals have a lower sensitivity than DNTF and are more prone to an ignition response.