Due to its high hardness, low density, and cost-effectiveness, Al2O3 ceramic, known for its ballistic resistance, occupies a significant position in the field of protective armor. In the future, what areas will it further develop into?
Bulletproof alumina
- Transparent Al2O3 Ceramic
With the increasing demands for armored systems in modern warfare, not only is full-spectrum protection required, but also the preservation of soldiers’ mobility. Transforming from “passive” to “active” defense, the development of “active armor,” which can pre-identify targets and utilize bait-triggered and physically destructive methods to neutralize incoming threats, has become a significant advantage in combat. Transparent ceramic materials, due to their high strength and hardness, have been applied to meet this demand for protective armor, serving as alternatives to bullet-resistant glass in applications such as face shields, missile detection windows, and protective windows for ground combat vehicles. Currently, commonly used transparent ceramic materials include aluminum oxide (sapphire), aluminum oxynitride, and aluminum magnesium spinel.
Different addition amounts of MgO+Y0; Physical photos of alumina ceramics with composite sintering aids
Research on transparent Al2O3 ceramic primarily focuses on theoretical studies to improve light transmission and optimizing manufacturing processes. The main research elements include Al2O3 powder selection, choice of sintering additives, and sintering processes. The powder used for transparent Al2O3 ceramic requires a purity of not less than 99.9% and must be α-Al2O3 with particle sizes smaller than 0.3μm. Impurities in Al2O3 powder can easily form heterophase, increasing the scattering centers for light, thereby reducing the intensity of transmitted light in the incident direction and consequently decreasing the transparency of the product. Therefore, extremely high purity is required for Al2O3 powder. Additionally, the powder should be highly dispersed to prevent agglomeration into large particles, which would cause the loss of the advantages of the original small particles. During the sintering process of transparent Al2O3 ceramic, a small amount of sintering aids such as MgO, Y2O3, and La2O3 is added to prevent the formation of pores within the ceramic.
Microscopic morphology of transparent alumina ceramics with 0.075wt% MgO added
Researchers have found that adding (0.02% to 0.05%) composite sintering aids of MgO and Y2O3 to ultrafine Al2O3 powder and sintering for 2.5 hours can produce transparent Al2O3 ceramic with a total transmittance greater than 80%. This is exemplified by the sample labeled 2M+2Y in the second row of Figure 1. Zhang Xiao and colleagues investigated the influence of MgO on the optical properties of transparent Al2O3 ceramic. Microscopic observations of the Al2O3 ceramic with added MgO, as shown in Figure 2, combined with the EDS spectrum analysis in Table 1, indicate that Mg mainly distributes at the grain boundaries and near-grain boundary regions inside the Al2O3 grains. MgO, in the form of Mg-rich nanoparticles, effectively inhibits grain growth at grain boundaries. The fitting results show that when the added amount of MgO is 0.075wt%, the theoretical porosity is at its lowest, the equivalent porosity is moderate, and the transparency performance is optimal.
The sintering process for transparent Al2O3 ceramic is similar to that of conventional ceramics, including vacuum sintering, atmospheric pressure sintering combined with hot isostatic pressing, spark plasma sintering, and microwave rapid sintering.
Elements analysis by micro-area EDS
- Toughened Al2O3 Ceramic
Al2O3 ceramics are characterized by high brittleness and low fracture toughness. Therefore, research on toughening Al2O3 ceramics has been a major focus. Methods for toughening Al2O3 ceramics include multiphase ceramic composite systems, functionally graded ceramics, and layered structure design. In the ZrO2-Al2O3 composite ceramic, ZrO2 particles introduced as dispersed phases effectively suppress the growth of Al2O3 grains. As shown in Figure 3, within the range of 0 to 20wt% of added nano ZrO2, with the increase in ZrO2 content, the grain size decreases. Comparing the mechanical properties of different Al2O3-based composite ceramics, it is found that when the ZrO2 content is in the range of 20 to 30wt%, the composite ceramic exhibits optimal flexural strength and fracture toughness.
ZrO2-Al203cope potatoes 29943; SEM icons: a)Al0; (b)10wt%ZrO2-90wt%Al2O3; (c)20wt%ZrO2-80wt%Al2O3
Huang et al. demonstrated that Al2O3-ZrO2 functionally graded materials prepared by pressureless sintering exhibit optimal energy absorption and ballistic resistance when they possess the same surface density or thickness.
Layered ceramic interlayer structure
To further enhance the fracture toughness of ceramics, layered structure design of multiphase ceramics has been investigated. Typical layered structures, as shown in Figure 4, are designed to improve fracture toughness through the presence of numerous interfaces, which promote multiple crack deflection. Figure 5 illustrates SEM observations and schematic diagrams of the cross-section of layered structure ceramics. The accumulation of numerous small brittle fractures endows layered materials with higher energy absorption capacity than monolithic ceramics. Another direction for ceramic toughening is fiber-reinforced ceramic composites, where toughening mechanisms include crack deflection, microcrack toughening, fiber pull-out, fiber bridging, and fiber extraction.
SEM photos of crack propagation in layered ceramic composite materials
Fiber-reinforced ceramics are considered the optimal combination for achieving mass reduction and energy absorption. Glass fibers and carbon fibers are commonly used toughening fibers. For instance, S-2 glass fibers are frequently used in the panels of lightweight vehicles like Jeeps. Taking SiC ceramics as an example, the strain capacity of SiC fiber-reinforced/SiC composite ceramics can be increased by nine times compared to pure SiC ceramics. Moreover, the fracture toughness of Si3N4 ceramics can be enhanced by more than four times by incorporating continuous carbon fibers compared to pure Si3N4 ceramics.
Bending strength and fracture toughness of Al2O3-based multiphase ceramics
The selection of raw materials for Al2O3 ceramics is becoming more refined, with strict requirements for purity and particle size of Al2O3. The manufacturing processes have matured, and new forming techniques such as gel injection molding and powder micro-injection molding, as well as sintering processes like microwave sintering and spark plasma sintering, are gradually being adopted. Transparent Al2O3 ceramics have developed based on existing manufacturing processes, with toughening and strengthening of Al2O3 ceramics emerging as a major current trend.