Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Mechanical properties and ballistic behavior of LiF-added Al2O3–4wt%Nb2O5 ceramics
Jheison Lopes dos Santosa,, , Rubens Lincoln Santana Blazutti Marçala, Paulo Roberto Rodrigues de Jesusa, Alaelson Vieira Gomesa, Édio Pereira Limaa, Daniel Navarro da Rochaa, Maria Aparecida Pinheiro dos Santosb, Lucio Fabio Cassiano Nascimentoa, Sergio Neves Monteiroa, Luis Henrique Leme Louroa
a Military Institute of Engineering – IME, Department of Materials Science, Praça General Tibúrcio 80, Urca, 22290-270 Rio de Janeiro, RJ, Brazil
b Brazilian Navy Research Institute – IPqM, Division of Strucural and Electronic Ceramics, Rua Ipiru, 02, Cacuia, 21931-095 Rio de Janeiro, RJ, Brazil
Received 27 February 2018, Accepted 07 September 2018
Abstract

For the first time, the behavior of LiF-added alumina-niobia ceramics was quasi-static and dynamically analyzed in ballistic tests. Different compositions were investigated for the compound Al2O3–4wt%Nb2O5nLiF, with n ranging from 0 to 1.50wt%, processed at three different sintering temperatures (1300, 1350 and 1400°C). Elastic properties, elastic wave velocities, Vickers microhardness and ballistic tests were performed according to standards. The results showed that the LiF addition benefits the Al2O3–4wt%Nb2O5 ceramic properties, allowing to reduce by approximately 100°C the sintering temperature without impairing the ballistic performance. The Al2O3–4wt%Nb2O5–0.5wt%LiF ceramics exhibit comparable properties to the commonly used Al2O3–4wt%Nb2O5 ceramics.

Keywords
Alumina, Niobia, Lithium fluoride, Mechanical properties, Elastic properties, Ballistic test
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1Introduction

Aluminum oxide, or alumina (Al2O3), one important and most used engineering ceramics is fragile due to both low fracture strength and low tenacity [1–4]. The introduction of a second-phase reinforcement combined with sintering additives and annealing induced phase transformation, significantly improve the fracture strength and toughness of alumina-based ceramics, as well as alumina composites [5–9].

Aiming to improve the processing of alumina-based ceramics, Gomes et al. [10] reduced the sintering temperature by adding different fractions of niobium oxide, or niobia (Nb2O5). Their results indicated that even an addition of 4wt% of Nb2O5 in the alumina allowed the sintering temperature of the ceramic to be reduced from 1600°C to 1450°C, still maintaining its dynamic behavior, such as the ballistic protective capacity.

Lithium fluoride (LiF) has also been shown to be a potential sintering additive [11–14]. Indeed, highly dense MgO/Al2O3 spinels, with small additions of LiF, have been sintered as transparent ceramics [11,12]. Investigations [11–13] indicated that the LiF contribution is due to particle rearrangement, by formation of a transient liquid phase. Moreover, partial reactions yield oxygen lattice vacancies, promoting diffusion enhanced sintering [13]. Santos et al. [14] added LiF to the Al2O3–4wt%Nb2O5 ceramic compound, to further reduce the sintering temperature. They optimized the addition of LiF by 0.5wt%, obtaining about 90% of theoretical densification at just 1300°C.

Due to its high compressive strength and hardness, an alumina-based ceramics may also be used as the front component in ballistic multilayered armor systems [15–18]. Its main function is to dissipate part of the projectile impact energy through the fragmentation of both the projectile tip as well as the front ceramic layer [15–19]. The second layer, just behind the ceramic, may be a high strength synthetic polymer fiber, such as aramid, or composites of polymeric matrix and natural fibers. The function of this layer is to retain fragments generated from the first layer of brittle material, as well as to assist in the partial dissipation of projectile energy [20–28].

The understanding of these mechanisms of fragmentation of the projectile and front ceramic, as well as capture of fragments by the second layer, relies on the comprehension of the dynamic behavior during the ballistic test. This behavior has been explained in terms of shock waves propagation inside a multilayered system [29]. The projectile impact energy dissipation depends on the resulting shock wave reflection at the interface between distinct layers. A greater energy dissipation occurs in case the second layer has a density lower than that of the front ceramic [23]. This is the case of aramid fiber, which is less dense than alumina-based ceramic investigated in a previous work [27]. The projectile-impacted compression shock wave is reflected at the interface as a tension shock wave, which contributes to an effective ceramic fragmentation associated with higher energy dissipation [27]

The present work investigates, for the first time, mechanical properties and ballistic behavior of LiF-added Al2O3–4wt%Nb2O5 ceramic compounds. Microhardness values were measured for the LiF additions by weight of 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50%. Subsequently, the elastic properties and wave velocities were determined for the addition of LiF optimized by Santos et al. [14]. In addition, this new 0.5wt% LiF ceramic as well as a control LiF-free ceramic were ballistic tested as front layer followed by a Kevlar™ as second layer and Al alloy as back layer against high impact class III [30] 7.62mm ammunition. The absorbed energy by the ceramic alone and the depth of penetration by the multilayered (ceramic/Kevlar™/Al alloy) system were evaluated in corresponding ballistic tests against 7.62 projectiles.

2Materials and methods

The precursor ceramic mixture, made in deionized water, was composed of 94.53wt% alumina (Treibacher Schleifmittel, Brazil) and 3.94wt% niobium oxide (CBMM, Brazil) together with polyethylene glycol, PEG, organic binder (1.53wt%, from VETEC, Brazil). After the binder burnout, the ratio becomes Al2O3–4wt%Nb2O5. Different additions of pro-analysis lithium fluoride (LiF, Vetec, Brazil) up to 1.50wt% were investigated together with LiF-free Al2O3–4wt%Nb2O5, for control. All ceramic compositions were sintered at 1300, 1350 and 1400°C for 3h under a pressure of 50MPa. The measurement of elastic properties was performed by the impulse excitation technique, using samples of 57mm diameter with 60g of mass. They were measured in a Sonelastic® equipment, from ATCP Engenharia, Brazil. For the Vickers microhardness, with 1kgf application load for 15s, using 20mm diameter and 5g specimens, tests were carried out in a model Micromet 5104 Buehler equipment. The samples used for the ballistic test to evaluate residual velocity were the same of the elastic properties analysis, fixed as shown in Fig. 1a. The multilayered armor system (MAS), studied in the present work (Fig. 1b) is composed of a hexagonal (3.8cm edge) ceramic front, followed by a single layer of aramid fabric to assist in containing the fragments. This second layer of aramid (Kevlar™, from LFJ Blindagens, Brazil), was composed of 18 plies with areal density of 0.460kg/m2, bonded with neoprene. A back layer of a 5052 H34 aluminum alloy is placed behind the aramid fabric layer, illustrated in Fig. 1b.

Fig. 1.
(0.41MB).

Fixation of the samples for the ballistic tests of (a) residual velocity and (b) plasticine indentation.

The residual velocity tests were performed as per ABNT NBR 15000 [31], with commercial 7.62mm M1 ammunition shot 15m from the target. The ballistic tests of indentation in the modeling clay witness were performed following the NIJ-0101.06 [31] body armor standard, to evaluate the protection level of the MAS. In this test, the MAS target, fixed in front of the clay witness (applied to simulate the consistency of the human body), were also 15m away from the shooting gun barrel. The projectile's velocity measuring device was a model B290 High Pressure Instrumentation (HPI) equipment. The ballistic tests were performed in the Brazilian Army Assessment Center (“Centro de Avaliações do Exército” – CAEx), Rio de Janeiro, Brazil. The fracture surfaces of the ceramic targets were observed using scanning electron microscopy (SEM), in a model TM3000 Hitachi equipment, available at the Nuclear Engineering Institute (IEN), Rio de Janeiro, Brazil.

3Results and discussion

Fig. 2 presents microhardness results for the investigated ceramics. In this figure, for each sintering temperature, the addition of LiF causes a higher microhardness as compared to the corresponding one for the control LiF-free Al2O3–4wt%Nb2O5. A tendency for superior values of microhardness at 1400°C is observed in Fig. 2, in comparison with ceramics sintered at 1300 and 1350°C. This is an expected result, since higher sintering temperatures promote better structure consolidation, which is associated with improved mechanical properties [32]. It is also important to notice in Fig. 2 that a maximum in microhardness is reached for 0.5wt% LiF at both 1300 and 1350°C sintering temperatures. Due to the densification and microhardness values obtained for the LiF addition of 0.5wt%, it was considered that this was the optimized composition. Indeed, the densification of the investigated ceramic compositions was previously studied and presented elsewhere [14]. It was then found best results for the 0.5wt% addition of LiF. Therefore, the Al2O3–4wt%Nb2O5–0.5wt%LiF had its properties and behavior analyzed in comparison with the control Al2O3–4wt%Nb2O5, as reference material.

Fig. 2.
(0.08MB).

Vickers microhardness vs. composition for the different temperatures.

Table 1 shows the elastic properties: Young and shear moduli as well as Poisson coefficient, and Lamé constant of the sintered samples. At the temperature of 1300°C, samples with LiF addition obtained elastic and shear moduli comparable to those reported in the literature [19], in plain Al2O3–4wt%Nb2O5 at the sintering temperature of 1400°C. Samples containing LiF sintered at 1350°C disclosed Young and shear moduli comparable to those obtained by Trindade et al. [19] without the addition of LiF and sintering temperature of 1400°C for 8h. In practice, such small difference is fully justified in terms of the reduction of MAS ceramic component production cost. This indicates that the addition of 0.5wt% LiF allowed a reduction of 100°C in the sintering temperature, without causing a significant reduction of the elastic properties.

Table 1.

Elastic properties of the samples used in the ballistic tests.

Samples [Al2O3–4%Nb2O5]–nLiF  E (GPa)  G (GPa)  ν  λ (GPa) 
0.5%LiF – 1300°C  232.935±5.052  93.90±2.08  0.24  86.90±1.59 
0.5%LiF – 1350°C  265.414±4.630  106.99±1.89  0.24  99.03±1.61 
0.5%LiF – 1400°C  289.754±4.908  116.81±1.99  0.24  108.11±1.87 
0.0%LiF – 1400°C  257.010±20.093  103.61±8.13  0.24  95.87±7.22 

From the elastic properties, the velocities of the elastic waves (C0) of the samples could be determined. Table 2 shows the velocities of elastic waves acting on the samples used in the ballistic test. The C0 values for samples with 0.5wt% LiF sintered at 1300°C, and those without the additive and sintered at 1400°C, exhibited a relative small discrepancy of the order of 3%. This small reduction is fully rewarded by the significant reduction in sintering temperature (100°C) in terms of performance and cost. The graph of Fig. 3 shows a comparison between the values of microhardness and modulus of elasticity obtained, and the densification of samples without LiF and with 0.5wt% of this additive [14], in the three temperatures studied.

Table 2.

Elastic waves velocities of the samples used in the ballistic tests.

Samples [Al2O3–4%Nb2O5]-nLiF  C0 (km/s)  CR (km/s)  CL (km/s)  CS (km/s) 
0.5%LiF – 1300°C  8.19±0.09  7.16±0.08  8.90±0.10  5.20±0.06 
0.5%LiF – 1350°C  8.52±0.09  7.44±0.07  9.26±0.08  5.41±0.05 
0.5%LiF – 1400°C  8.81±0.07  7.69±0.06  9.57±0.08  5.59±0.05 
0.0%LiF – 1400°C  8.47±0.22  7.40±0.19  9.20±0.24  5.38±0.14 
Fig. 3.
(0.05MB).

Comparison between the microhardness, Young modulus and the densification values.

The results of the absorbed energy by the distinct ceramic components subjected to the ballistic impact are presented in Table 3. Comparatively, it was observed that the energy absorbed by the ceramics without the addition of LiF at 1400°C was close to that obtained with the LiF addition sintered at 1300°C. Therefore, the inclusion of LiF contributed to improve the energy absorption of the ceramic, even sintering at a temperature 100°C lower, as compared to the Al2O3–4wt%Nb2O5 used in previous works [10,19]. In addition, by analyzing the behavior of the ceramic compound in terms of energy absorption, it is possible to observe that higher values of densification, reported by Santos et al. [14], and modulus of elasticity, Table 1, did not necessarily result in a higher percentage of absorbed energy. There is a marked difference between the sonic impedance of the porosity and the solid part of the ceramic. In fact, a pore acts like a hot spot, accumulating energy [29]. This evidences the role of porosity as an important parameter in the absorption of energy. A feature of a ceramic, which performs well for energy absorption, is the intergranular propagation of cracks. SEM micrographs showed in Fig. 4 that fractured samples after ballistic impacts revealed such behavior, reinforcing the results of Table 4.

Table 3.

Mean values of impact and residual velocities and absorbed energy.

Samples [Al2O3–4%Nb2O5]–nLiF  Vi (m/s)  VR (m/s)  Eabs (%) 
0.5%LiF – 1300°C  848.99  726.66  26.71±3.40 
0.5%LiF – 1350°C  845.35  722.48  26.87±5.07 
0.5%LiF – 1400°C  848.58  704.60  30.75±9.06 
0.0%LiF – 1400°C  844.17  711.01  28.96±6.54 
Fig. 4.
(0.23MB).

SEM micrograph of crack propagation in samples: with 0.5% LiF sintered at (a) 1300°C, (b) 1350°C, (c) 1400°C, and (d) without LiF sintered at 1400°C.

Table 4.

Mean values of indentation in the clay witness.

Samples [Al2O3–4%Nb2O5]–nLiF  Vi (m/s)  Penetration (mm) 
0.5%LiF – 1300°C  846.75  37.20±0.89 
0.5%LiF – 1350°C  842.72  37.44±1.11 
0.5%LiF – 1400°C  848.47  34.16±2.00 
0.0%LiF – 1400°C  847.86  34.52±2.23 

Table 4 presents the results of indentation (trauma) in the witness clay. All compositions met the requirement of the standard [30], with trauma less than 44mm. The trauma left in the witness clay in the ceramic with 0.5wt% LiF sintered at 1300°C was only 7% greater than that produced using the control ceramic without the addition of LiF sintered at 1400°C. This confirms the efficiency of the new LiF-added ceramic compound studied and suggests it to be applied as a novel ceramic component in MAS, sintered at lower temperatures, 1300°C, which saves time and processing energy.

4Conclusions

  • All additions of LiF increased the microhardness of the Al2O3–4wt%Nb2O5 ceramics, especially the addition of 0.5wt%, associated with maximum values at 1300 and 1350°C.

  • The results obtained from the mechanical properties of the ceramic composition with LiF, sintered at different temperatures, showed that it is possible to reduce the sintering temperature by 100°C (from 1400 to 1300°C) without significant reduction in the elastic properties.

  • High Young's modulus, hardness and densification (low porosity) do not necessarily represent a more efficient ballistic armor. This evidences the role played by the porosity and bond force roles in ballistic impact energy absorption.

  • The results of the ballistic tests also indicated an amazing performance of the new ceramic composite, even sintered at lower temperatures.

  • By the significant decrease in the sintering temperature, there is a corresponding reduction in the processing cost of the material presented in this work, which showed interesting results for potential ballistic application, as in MAS.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors of the present work thank the Brazilian supporting agency CAPES, and the CAEx, for performing the ballistic tests.

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Paper was part of technical contributions presented in the events part the part of the ABM Week 2017, October 2nd to 6th, 2017, São Paulo, SP, Brazil.

Corresponding author. (Jheison Lopes dos Santos lopesjheison@gmail.com)
Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
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