Sintering of a high-performance ceramic for ballistic applications

Abstract

The present invention relates to a method for manufacturing ballistic protective equipment by sintering powder composed primarily of boron carbide (B4C), characterized in that: the average grain size of said B4C powder is less than 50 µm for 90% of the grains, and said powder contains a mass percentage of between 5% and 15%, and preferably between 6% and 12%, of a sintering aid additive; the sintering treatment is applied by two opposing pistons with uniaxial displacement having curved front surfaces. It also relates to ballistic protective equipment obtained according to this method. Abstract figure: Figure 1

Description

Title of the invention: Sintering of a high-performance ceramic for ballistic applications. FIELD OF THE INVENTION

[0001] The present invention relates to the field of manufacturing lightweight and resistant ballistic protection, for example an infantryman's breastplate produced according to dimensions close to the final shapes (in English "Near Net Shape - NNS"), by spark plasma sintering (in English "Spark Plasma Sintering - SPS") of advanced materials such as boron carbide (B4C) or by hot-pressed sintering (in English "hot-pressed sintering - HP").

[0002] Boron carbide (B4C) is a prized material in the field of ballistics for several distinct reasons that make it one of the most effective materials for the manufacture of protective equipment, such as bulletproof vests, armor plates and shields, due to its mechanical qualities: • Exceptional lightness: Boron carbide is one of the lightest ceramic materials, with a density of approximately 2.52 g / cm³. This lightness is essential in the design of wearable ballistic protection, as it reduces the load the user must carry while still providing high protection. • Extreme hardness: With a Mohs hardness of 9.5, boron carbide is one of the hardest materials after diamond and cubic boron nitride. This hardness allows it to effectively withstand high-velocity projectile impacts, dispersing the energy over a larger area and thus minimizing penetration. • High impact resistance: Due to its crystalline structure and therefore its improved mechanical properties resulting from sintering, boron carbide can absorb and disperse the energy of ballistic impacts very effectively. It excels particularly at stopping high-velocity projectiles, such as armor-piercing bullets. • Wear and corrosion resistance: Boron carbide is highly wear-resistant, which is an advantage in conditions where armor plates may be exposed to abrasive environments. Furthermore, it does not oxidize easily, increasing the durability of ballistic equipment in harsh environmental conditions. • Thermal resistance: Boron carbide retains its mechanical properties even at high temperatures, which is beneficial for military or industrial applications where equipment may be exposed to extreme thermal conditions. • Superior ballistic stopping capacity: Compared to other ceramics, such as aluminum oxide (Al2O3) or silicon carbide (SiC), boron carbide offers superior ballistic stopping capacity for the same thickness or mass, making it a material of choice for applications requiring maximum protection with minimum weight. • Compatibility with other materials: Boron carbide can be used in combination with other materials to improve ballistic performance. For example, it is often combined with fiber composites (such as Kevlar) to create multi-layered protective systems, combining the advantages of both types of materials. STATE OF THE ART

[0003] Prior art is known patent application WO2006069050 which describes a process for preparing a boron carbide article comprising:

[0004] formation of a green boron carbide body from undoped boron carbide particles coated with boron oxide; • to expel the said boron oxide from the said green boron carbide body at an removal temperature greater than 1100° C. and less than 1400° C. to obtain a reduced boron carbide body; • pressureless sintering of said reduced boron carbide body at a pressureless quenching temperature selected in the range of 2200°C–2317°C for a quenching time not longer than that required for said reduced boron carbide body to reach a shrinkage rate of 0.005% per minute, followed by cooling of said reduced boron carbide body to obtain a pressureless sintered boron carbide body having a relative density of at least 93%; and • hot isostatic pressing of said sintered boron carbide body without pressure at a temperature of at least 2000° C.

[0005] Patent application FR3120073 describes a ballistic armor element, comprising a ceramic body comprising a sintered material made up of ceramic grains with Vickers hardnesses greater than 5 GPa, the total pore volume of said material being between 0.5 and 10%, said ceramic body being characterized in that the cumulative volume of pores with a diameter between 30 and 100 micrometers represents between 0.2 and 2.5% of the volume of said material, the cumulative volume of pores with a diameter greater than 100 micrometers is less than 0.2% of the volume of said material, the remainder of said total pore volume being made up of pores whose diameter is less than 30 micrometers.

[0006] The article Braun Dresch, Alexander & Venturini, Janio & Arcaro, Sabrina & Montedo, Oscar & Bergmann, Carlos. (2020). Ballistic Ceramics and Analysis of their Mechanical Properties for Armour Applications: A Review. Ceramics International. 47. 10.1016 / j.ceramint.2020.12.095. presents a comparison of the most commonly used ballistic ceramics (alumina, silicon carbide, and boron carbide) by examining their performance through different mechanisms. The objective of this study is to analyze the mechanical properties of ceramics and their role in dynamic ballistic performance, addressing the controversy surrounding the feasibility of comparing the static properties with the dynamic performance of ceramics. The results show that silicon carbide offers the best ballistic performance against 7.62 AP caliber projectiles, followed by alumina and boron carbide.The correlation between hardness and flexural strength with ceramic performance in penetration depth tests has been verified. The ballistic performance of ceramics depends on several properties, including microstructure, which directly influences all mechanical properties. Hardness is crucial for fracturing and eroding projectiles, while fracture toughness and flexural strength allow the ceramic to withstand multiple impacts. The modulus of elasticity is related to stress wave propagation, and the fracture mode is associated with the amount of energy absorbed by the ceramic.

[0007] The article Zeng, X., Liu, W. Enhanced Sintering of Boron Carbide-Silicon Composites by Silicon. J. of Materi E tre ng and Perform 25, 5014-5019 (2016). https: / / doi.org / 10.1007 / sll665-016-2313-z is also known. This article proposes a low-temperature sintering route for B4C ceramics with improved properties. The results indicate that the addition of silicon powder can act as a sintering agent and contribute to the densification of the sintered material. The addition of silicon powder can also act as a second phase and contribute to the strengthening of the composites. The relative density of the B4C-Si composite samples with the addition of 10 wt% Si powder, prepared by SPS at 1600 °C and 50 MPa for 8 minutes, reaches 98.3%. The flexural strength, fracture toughness and Vickers hardness of the sintered samples are 518.5 MPa, 5.87 MPa ml / 2 and 38.9 GPa respectively.High-temperature flexural strength and fracture toughness depend on the test temperature, with a maximum value at 1350 °C. The study states that B4C-Si composites prepared at 1600, 1650 and 1700 °C exhibit good high-temperature mechanical properties.

[0008] The thesis of Mathieu Dutto, "Microwave process for the development of B4C-SiC composites by infiltration and reaction of silicon, for ballistic applications," Mines Saint Etienne, Saint-Etienne, 2017 focuses on the development of boron carbide (B4C) and silicon carbide (SiC) composites by silicon infiltration using a microwave process, intended for ballistic applications. The author explores the effects of microwave heating on the manufacturing parameters, microstructure, and mechanical properties of the B4C-SiC composites. Disadvantages of prior art

[0009] Ballistic equipment manufactured according to prior art solutions has ballistic properties that can be improved. Solution provided by the invention

[0010] In order to overcome these drawbacks, the present invention relates to a method for manufacturing ballistic protection equipment by sintering, in particular by SPS or HP sintering, of powder composed mainly of boron carbide B4C, characterized in that: - the average grain size of said B4C powder is less than 50 µm for 90% of the grains and - said powder contains a mass percentage of between 3% and 15% and preferably of 5% to 15% and even more preferably of 6% to 12% of a sintering aid additive consisting of metallic silicon (Si), silicon carbide, yttrium trioxide (Y2O3), titanium diboride (TiB2), alumina (Al2O3) or cubic boron nitride (c-BN) - the sintering treatment is applied by two opposing uniaxial displacement pistons having curved front surfaces.

[0011] Preferably, said powder is a composite of B4C and silicon carbide SiC.

[0012] Advantageously, the sintering cycle time is 1 hour 15 minutes ± 15 minutes and in that the The sintering temperature is 2000 °C±7% and preferably 1950°C ±1%.

[0013] Preferably, the B4C and sintering aid additive composite is produced by energetic grinding by planetary mill or by triaxial mixing called turbula.

[0014] Advantageously, the sintering is ensured by a tooling comprising two uniaxial displacement pistons moving in a tubular matrix, sheets of flexible graphite being arranged between said composite powder and respectively between said pistons and said matrix.

[0015] The invention also relates to ballistic protection equipment, in particular a chest plate, consisting of a curved monolithic piece produced by sintering powder composed mainly of boron carbide B4C, characterized in that: - the average grain size of said B4C powder is less than 50 µm for 90% of the grains and - said powder contains a mass percentage of between 5% and 15% and preferably between 6% and 12% of a sintering aid additive

[0016] The sintering treatment is applied by two opposing pistons with uniaxial displacement having curved front surfaces

[0017] Detailed description of a non-limiting example of embodiment

[0018] The present invention will be better understood upon reading the following description, concerning a non-limiting example of an embodiment illustrated by the attached figure where:

[0019] Fig. 1 represents a schematic view of a tool for implementing the process according to the invention. General principle of the invention

[0020] The invention relates to a method for manufacturing ballistic equipment by sintering boron carbide-based powder and the equipment thus produced. The equipment includes, for example, ballistic breastplates forming an insert for a bulletproof vest.

[0021] The invention relates to sintering parameters, and in particular: - the average grain size of said B4C powder: less than 50 µm for 90% of the grains - the composition of the B4C powder: it contains a mass percentage between 6% and 12% of a sintering aid additive consisting of metallic silicon (Si), silicon carbide, yttrium trioxide (Y2O3), titanium diboride (TiB2), alumina (Al2O3) or cubic boron nitride (c-BN). - According to one variant, the sintering aid additive is chosen from the following constituents: silicon nitride SixNy, titanium diboride TiB2, yttrium aluminium YAG, graphene, carbon fibres, silicon carbide fibres SiC, fibre glass, titanium carbide TiC, nano-diamond, satellites, tantalum carbide TaC, titanium nitride TiN, zirconium dioxide ZrO2. - the application of compression by two antagonistic pistons with uniaxial displacement having curved front surfaces. Example of how to make a breastplate

[0022] The target product according to this example is an infantryman's breastplate (of the order of 345x245mm2 and of the thinnest possible thickness while retaining the desired properties), the design of this breastplate is more complex than a disc because it has a relatively complex shape due to the presence of a double curvature, and the objective is to produce it as close as possible to the final dimensions, i.e. Near Net Shape (NNS).

[0023] The aim is to obtain strong and lightweight ballistic protection. Many materials have been explored in this area in order to reduce the weight of the equipment. The materials must possess excellent ballistic characteristics while also having sufficient ductility to absorb the energy released during the ballistic impact. The ceramic must be capable of eroding the projectile and dissipating the kinetic energy imparted by the ballistic impact to bring the projectile to a stop.

[0024] The selected powder material is boron carbide (B4C), a non-oxide ceramic with a melting point of 2427°C, a density of 2.43 to 2.52 g / cm³, and preferably 2.43 g / cm³. Boron carbide is a high-melting-point ceramic that requires a large energy input. The molecular bonds involved in this material are covalent bonds (strong and directional), thus explaining the need for a substantial energy input to sinter this type of powder. The hardness of this material is extremely high (on the order of 3000-4000 HV), second only to cubic boron nitride (BN) and diamond, and a flexural strength between 400 and 828 MPa.

[0025] Boron carbide requires high sintering temperatures, i.e. between 1700°C-2200°C, as well as the application of a significant stress between 50-100MPa in order to obtain the finest possible microstructure.

[0026] The sintering of boron carbide (B4C) powder is influenced by the need to supply a significant amount of energy due to its strong covalent bonds, which are essential for initiating the granular rearrangement, neck formation, and densification steps. The amount of energy required is related to the material's breakdown voltage, which activates the diffusion processes necessary (i.e., mass transport) for sintering. Using powders with a larger specific surface area requires less energy, and the addition of low-melting-point metal binders can reduce sintering temperatures. The semiconducting behavior of B4C at elevated temperatures influences the sintering process, with a transition to a partially conductive state at approximately 1700°C, requiring a high current density to initiate the formation of bonds between the grains and promote densification.

[0027] During the sintering of boron carbide (B4C) powder, the presence of impurities such as free boron, free carbon, silicon, and iron can lead to the formation of melt zones at grain boundaries, thus promoting the sintering process by accelerating densification. The semiconducting nature of boron carbide can result in a densification gradient during the sintering process, with localized overheating zones, requiring precise temperature control to promote uniform densification while limiting these phenomena. The exothermic reaction between boron and carbon promotes the sintering of B4C, while the transition from nanometric to micrometric powders can lead to an increase in mechanical properties due to the grain size of the resulting microstructure. Despite Despite the potential advantages, handling nanometric powders requires careful consideration of occupational health and safety aspects due to their varying behavior in industrial environments. Because of the diverse implementation conditions, different processes must be developed to meet cost and quality requirements. Characteristics of boron carbide powder

[0028] The particle size distribution of the selected B4C powder is quite close, with the presence of angular grains and agglomerates. The average grain size is less than 50 µm for 90% of the grains, with the majority having a much smaller size.

[0029] There are no intragranular pores. Intragranular porosity, characterized by voids within the grains of a powdered material, plays a crucial role in its properties. Its main impacts include a reduction in apparent density, essential for applications requiring a high densification rate for optimal mechanical properties. This porosity induces a decrease in mechanical properties such as tensile strength, hardness, and toughness. Furthermore, the thermal conductivity of materials exhibiting intragranular porosity can be reduced due to voids restricting the efficient transmission of heat through the material. The optimal density is 2.433 g / cm³. Merging aid add-on

[0030] The objectives of these additives are therefore to maintain the mass gain (density of B4C), and to maintain or improve the ballistic properties by allowing partial passages into the liquid phase to lower the sintering temperature.

[0031] The sintering aid additives are: metallic silicon (Si), alumina (Al2O3) and cubic boron nitride (c-BN).

[0032] c-BN is a very high melting point ceramic with higher hardness and toughness than B4C. This ceramic is of interest for the cubic-to-hexagonal transition, which induces an exothermic reaction and thus a very high energy input over a short period, allowing the sintering temperature to be lowered. The disadvantages of this ceramic are related to the difficulty in controlling the phase change at 1400°C and the material cost.

[0033] Silicon has a melting point of 1414°C, a low fusion energy (50.55 kJ.mol-l), and a vaporization point of 1850°C. It has a lower density than B4C (2.33) and a diamond-like crystal structure. It can provide a mass gain of approximately 40% and can react with B4C to form SiC (a ballistic ceramic). Since silicon has many advantages over the systems considered previously, this element is currently the best candidate. It constitutes a preferential melting aid additive. The size of 90% of the grains in this Si powder is less than 1 µm, on the order of 1 to 5 µm.

[0034] The role of the additive is to lower the sintering temperature without causing a decrease in the properties of the ceramic, i.e., by creating any unwanted phases. Silicon has many advantages such as low density, a melting point of 1414°C, and chemical compatibility with B4C (coefficient of thermal expansion, etc.). Preparation of the B4C-Si composite

[0035] The key to creating this composite is the mixing method, which is essential for obtaining a homogeneous mixture and a homogeneous sintered ceramic. The powders to be mixed are B4C and Si, with the aim of producing a lightweight and high-performance ballistic ceramic. The two suitable mixing methods are energetic grinding using a planetary mill and triaxial mixing, also known as turbula mixing. Mixing method

[0036] Dry mixing methods were used for different compositions such as: B4C with a mass percentage of Si of 2.5; 5; 10 and 20% as well as the composition test B4C - 5%w. c-BN and B4C - 5%w. Y2O3.

[0037] The entire set of mixtures has been adapted to meet the challenge of scaling up to an industrial level. The initial conditions chosen were:

[0038] (i) For use with planetary milling: a powder mass to mass ratio The chosen conditions are based on the use of 2.5 mm diameter balls (BPR), a rotation speed of -250 / 250 rpm for the jars and the tray respectively, and a grinding time of 4 hours. These conditions should allow for the evaluation of the reduction in sintering temperature achieved by using mechanically activated mixtures.

[0039] (ii) For use with the Turbula mixer: an identical BPR, a rotation speed of 50 rpm, two grinding times (12h and 24h) to match the conditions of the planetary mill.

[0040] The powder from the mixing step has a homogeneous appearance and is free of agglomerates. It remains to be validated that the experimental conditions are consistent with the stated objectives. Specifically, a sintering temperature below 2000°C, a stress not exceeding 40 MPa, and ultimately, properties that meet the specifications for the ballistic field. The transfer from planetary to turbula grinding was successfully implemented under the following conditions: BPR 2.5, 50 rpm, 24 h. SPS sintering

[0041] The objective is to achieve the highest possible densification rate. Dilatometric tests of the B4C powder made it possible to adjust the conditions of the SPS cycle to be applied, depending on the additive used and the behavior of the ceramic. The tests on small diameters (D30H5) aim to rapidly densify a range of mixtures to obtain the lowest possible sintering temperature while maintaining moderate stress (35 MPa) and similar, or even improved, properties compared to the known properties of B4C in the literature.

[0042] The tooling consists of two uniaxial displacement pistons (10, 20) made of graphite to ensure good electrical and thermal conductivity, each presenting a curved bearing surface, complementary with the three-dimensional configuration of the equipment to be produced.

[0043] These pistons (10, 20) move in a tubular matrix (30) also made of graphite, surrounded by a felt (40).

[0044] Sheets of flexible graphite (6, 7, 8) are arranged between the composite powder (5) and, respectively, the pistons (10, 20) and the matrix (30). These sheets of flexible graphite (6, 7, 8), for example PAPYEX™, exhibit reflective properties that help reduce energy losses. The anisotropy of its thermal conductivity allows for better temperature homogenization throughout the large-dimensional composite powder mass.

[0045] The crucial parameters to determine are the sintering temperature and the timing of stress application to densify these powder mixtures. The sintering rate in the temperature range of 1700-2150°C will be reduced to observe the first stage of sintering and adjust the temperature accordingly.

[0046] These three parameters (stress application time, sintering temperature, and cooling) are important in the SPS cycle to obtain an intact part with a high densification rate. The tests were initially carried out on B4C powder with a silicon additive. Silicon melts at 1414°C, and the induced liquid phase between 1300 and 1400°C facilitates the wetting of the B4C grains, thus activating diffusion phenomena at lower temperatures. The Si mass fraction is a parameter of interest because, depending on the amount of Si, grain wetting will be partial or optimal. Diffusion phenomena between the Si and the B4C will therefore be less significant. It will thus be necessary to increase the sintering temperature to reach the temperature range at which the silicon will sublime in order to locally increase the stress experienced by the grains and initiate the sintering steps.Amorphous phases weaken ceramics because they are sensitive to sudden temperature changes, generating cracks in the ceramic. Therefore, precise control of these parameters is important to achieve optimal sintering of different powder mixtures.

[0047] The densification results show a densification rate close to 100%, hardnesses exceeding 4000 HV1, and toughnesses at least equal to at 4 MPa.m °'5. The hardness-toughness combination is interesting with regard to the desired ballistic performance.

Claims

Demands

1. - Method of manufacturing ballistic protection equipment by sintering powder composed mainly of boron carbide B4C characterized in that: - the average grain size of said B4C powder is less than 50 pm for 90% of the grains and - said powder contains a mass percentage between 5% and 15% and preferably between 6% and 12% of a sintering aid additive - the sintering treatment is applied by two opposing uniaxial displacement pistons having curved front surfaces.

2. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that said powder is a composite of B4C and silicon carbide SiC.

3. - Method of manufacturing ballistic protective equipment according to claim 1 characterized in that the sintering cycle time is 1h15 ± 15 min and in that the sintering temperature is 2000 °C±7 %.

4. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that the composite of B4C and sintering aid additive is produced by energetic grinding by planetary mill.

5. - A method for manufacturing ballistic protective equipment according to claim 1 characterized in that said sintering aid additive is made of metallic silicon (Si), or silicon carbide, or alumina (Al2O3) or cubic boron nitride (c-BN)

6. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that said sintering aid additive is selected from the following constituents: silicon nitride SixNy, titanium diboride TiB2, yttrium aluminium YAG, graphene, carbon fibres, silicon carbide fibres SiC, fibre glass, titanium carbide TiC, nano-diamond, satellites, tantalum carbide TaC, titanium nitride TiN, zirconium dioxide ZrO2.

7. - Method of manufacturing ballistic protective equipment according to claim 1 characterized in that the composite of B4C and sintering aid additive is made by triaxial turbula mixing.

8. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that the sintering is ensured by a tool comprising two uniaxial displacement pistons (10, 20) moving in a tubular matrix (30), sheets of flexible graphite (6, 7, 8) being arranged between said composite powder (5) and respectively between said pistons (10, 20) and said matrix (30).

9. - Ballistic protection equipment consisting of a curved monolithic piece produced by sintering powder composed mainly of boron carbide B4C characterized in that: - the average grain size of said B4C powder is less than 50 pm for 90% of the grains and - said powder contains a mass percentage between 5% and 15% and preferably between 6% and 12% of a sintering aid additive - the sintering treatment is applied by two opposing uniaxial displacement pistons having curved front surfaces.

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