Sintering of a high-performance ceramic for ballistic applications

Abstract

The present invention relates to a method for manufacturing an item of ballistic protection equipment by sintering powder mainly composed of boron carbide B4C, the method being characterised 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 content by weight of between 5% and 15%, and preferably between 6% and 12%, of a sintering aid, - the sintering treatment is applied by two opposing uniaxially displaced pistons having curved front faces. The invention also relates to an item of ballistic protection equipment obtained by means of this method.

Description

Sintering of a high-performance ceramic for ballistic applications FIELD OF 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 ballistics for several distinct reasons that make it one of the most effective materials for manufacturing protective equipment, such as body armor, armor plates, and shields, due to its mechanical properties: 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 the impacts of high-velocity projectiles, 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, which increases 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 power: Compared to other ceramics, such as aluminum oxide (Al2O3) or silicon carbide (SiC), boron carbide offers superior ballistic stopping power for the same thickness or mass, making it a material of choice for applications requiring maximum protection with minimal weight. Compatibility with other materials: Boron carbide can be used in combination with other materials to enhance 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] The prior art is known to describe 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 with a diameter less than 30 micrometers.

[0004] French patent application FR1486894 relates to a process for forming ceramic parts, particularly boron carbide parts, but does not cover SPS or HP sintering, nor a direct sintering process. It describes a hot pressing process with a flexible jacket for forming parts from a malleable body.

[0005] The article "YE FENG ET AL: "Densification and Mechanical Properties of Spark Plasma Sintered B₄C with Si as a Sintering Aid", JOURNAL OF THE AMERICAN CERAMIC SOCIETY, vol. 93, no. 10, July 15, 2010, pp. 2956-2959, XP093277913" analyzes the effect of adding silicon as a sintering agent on the densification and mechanical properties of boron carbide (B₄C), a material known for its hardness and lightness, but difficult to densify by conventional sintering. The authors use the spark plasma sintering (SPS) process, which allows for rapid heating and improved densification. They show that the introduction of a small amount of silicon significantly improves the relative density of the resulting ceramics. The silicon promotes diffusion and reduces the required sintering temperature, while limiting grain growth.

[0006] ZEPEI DU ET AL: "Strengthening and toughening of CNTs-reinforced B4C-SiC ceramic composite material via spark plasma sintering", INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, vol. 20, no. 6, June 26, 2023, pp. 3691-3700, XP072508724 focuses on the development of a B4C-SiC ceramic composite reinforced with carbon nanotubes (CNTs) obtained by spark plasma sintering (SPS). The objective is to simultaneously improve mechanical strength and fracture toughness, two properties that are often antagonistic in ceramics.

[0007] The addition of CNTs acts as a reinforcing agent due to their exceptional rigidity and stress transfer capacity. The controlled integration of the nanotubes also promotes microstructural homogeneity and limits the growth of SiC and B₄C grains.

[0008] The energy absorption mechanism during crack propagation is multifaceted: bridging, deflection, and detachment of CNTs, which slows brittle fracture. The resulting samples exhibit high density and a compact microstructure. Experimental results demonstrate a significant improvement in Vickers hardness, flexural strength, and fracture toughness compared to conventional B₄C-SiC composites. This work confirms that the incorporation of CNTs via SPS is an effective approach for designing high-performance ceramics.

[0009] These two scientific articles focus on sintering or material characterization processes, but do not address the claimed combination of additives, tooling geometry, or even fine control of powder particle size.

[0010] US patent application 2007 / 010391 A 1 (MIKIJELJ BILJANA [US] ET AL) filed January 11, 2007, describes B₄C powders with optimized grain morphology and size, enabling more efficient densification during sintering. It highlights the use of adapted processes (notably hot pressing or flash sintering) to obtain dense ceramics with reduced porosity.

[0011] This document emphasizes the possible combination of B₄C with additives or secondary phases (e.g. silicon carbide, nitrides, oxides) to improve mechanical strength and fracture toughness. Disadvantages of prior art

[0012] Ballistic equipment manufactured according to prior art solutions has ballistic properties that can be improved.

[0013] For the manufacture of ballistic protection elements, prior art solutions involve complex post-machining treatments and do not allow the production of a curved shape in a single step. Solution provided by the invention

[0014] 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 between 3% and 15% and preferably from 5% to 15% and even more preferably from 6% to 12% of a sintering aid additive consisting of silicon metal (Si), silicon carbide SiC, 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.

[0015] Preferably, said powder is a composite of B4C and silicon carbide SiC.

[0016] Advantageously, the sintering cycle time is 1 hour 15 minutes ± 15 minutes and the sintering temperature is 2000°C ± 7% and preferably 1950°C ± 1%.

[0017] Preferably, the B4C and sintering aid additive composite is produced by energetic grinding using a planetary mill or by triaxial mixing called turbula.

[0018] Advantageously, sintering is achieved by tooling comprising two uniaxial displacement pistons moving in a tubular matrix, with sheets of flexible graphite being arranged between said composite powder and respectively between said pistons and said matrix.

[0019] 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.

[0020] The sintering treatment is applied by two opposing pistons with uniaxial displacement and curved front surfaces.

[0021] Detailed description of a non-limiting example of implementation

[0022] 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:

[0023] Lare represents a schematic view of a tool for implementing the process according to the invention. General principle of the invention

[0024] 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. To reduce the complexity of manufacturing curved shapes, the invention proposes: To use a B₄C powder in which 90% of the grains have a size ≤ 50 µm, guaranteeing a good compromise between densification and mechanical performance; To add a controlled percentage (5–15%) of a sintering additive chosen from Si, SiC, Y₂O₃, TiB₂, Al₂O₃ or c-BN, allowing either to reduce the sintering temperature (Si, Y₂O₃), or to increase the toughness (c-BN); To employ a tooling composed of two pistons with curved surfaces, allowing to directly sinter a product of doubly curved shape (infantry breastplate) – Near Net Shape (NNS) process.

[0025] The best performance is achieved with the specific combination of particle size, additive range, and curved tooling, which would allow for obtaining a curved part in a single step, with optimized density and superior ballistic properties.

[0026] 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 (SiC), yttrium trioxide (Y2O3), titanium diboride (TiB2), alumina (Al2O3), or cubic boron nitride (c-BN). According to one embodiment, the sintering aid additive is chosen from the following constituents: silicon nitride (Si) x N y, titanium diboride TiB2, yttrium aluminium YAG, Graphene, carbon fibres, silicon carbide fibres SiC, fibre verre, 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

[0027] The target product in this example is an infantryman's breastplate (approximately 345x245mm). 2 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).

[0028] The goal is to achieve strong and lightweight ballistic protection. Numerous materials have been explored in this area to reduce the weight of the equipment. These materials must possess excellent ballistic characteristics while also having sufficient ductility to absorb the energy released during ballistic impact. The ceramic must be able to erode the projectile and dissipate the kinetic energy of the ballistic impact to stop the projectile.

[0029] The selected powdered material is boron carbide (B4C), a non-oxide ceramic with a melting point of 2427°C and a density of 2.43 to 2.52 g / cm³. 3 and preferably 2.43 g.cm 3Boron carbide is a high-melting-point ceramic that requires a large energy input. The molecular bonds involved in this material are covalent (strong and directional), thus explaining the need for a significant 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 its flexural strength ranges from 400 to 828 MPa.

[0030] 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.

[0031] The sintering of boron carbide (B4C) powder is influenced by the need for 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 affects the sintering process, with a transition to a partially conductive state at approximately 1700°C, requiring a high current density to initiate inter-grain bonding and promote densification.

[0032] 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, necessitating precise temperature control to promote uniform densification while minimizing these phenomena. The exothermic reaction between boron and carbon promotes B4C sintering, while the transition from nanometer-sized to micrometer-sized powders can lead to an increase in mechanical properties due to the grain size of the resulting microstructure.Despite the potential advantages, handling nanopowders 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

[0033] The particle size distribution of the selected B4C powder is quite narrow, 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 being significantly smaller.

[0034] 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 bulk 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. 3 . Merging aid add-on

[0035] The objectives of these additives are therefore to maintain the mass gain (density of B4C), and to maintain or improve ballistic properties by allowing partial passages into the liquid phase to lower the sintering temperature.

[0036] The additives used to aid sintering are: metallic silicon (Si), alumina (Al2O3) and cubic boron nitride (c-BN).

[0037] c-BN is a very high-melting-point ceramic with higher hardness and toughness than B4C. Its use is advantageous for the cubic-to-hexagonal transition, which triggers an exothermic reaction and thus provides a very high energy input over a short period, allowing for a lower sintering temperature. The drawbacks of this ceramic are related to the difficulty in controlling the phase change at 1400°C and the material cost.

[0038] Silicon has a melting point of 1414°C, a low fusion energy (50.55 kJ / mol), 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 reduction of approximately 40% and can react with B4C to form SiC (a ballistic ceramic). Because silicon offers numerous advantages over previously considered systems, it is currently the best candidate. It is a preferred melting aid additive. The grain size of 90% of this silicon powder is less than 10 µm, ranging from 1 to 5 µm.

[0039] The role of the additive is to lower the sintering temperature without compromising the ceramic's properties, i.e., by creating any unwanted phases. Silicon offers numerous 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

[0040] The key to creating this composite is the mixing method, which is essential for achieving a homogeneous blend 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

[0041] Dry mixing methods were used for different compositions such as: B4C with a Si mass content of 2.5; 5; 10 and 20% as well as the composition test B4C – 5%w. c-BN and B4C – 5%w. Y2O3.

[0042] The entire mixture was adapted to meet the challenge of scaling up to industrial production. The initial conditions chosen were:

[0043] (i) For the use of planetary milling: a powder mass to ball mass ratio (BPR) of 2.5, a rotation speed of -250 / 250 rpm for the jar and tray speeds respectively, and a milling time of 4 hours. These conditions should allow for the evaluation of the reduction in sintering temperature resulting from the use of mechanically activated mixtures.

[0044] (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.

[0045] The powder from the mixing stage has a homogeneous appearance, free of agglomerates. It remains to be verified that the experimental conditions meet the established 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

[0046] The goal is to achieve the highest possible densification rate. Dilatometric tests of the B4C powder have allowed for the adjustment of the SPS cycle conditions to be applied, depending on the additive used and the behavior of the ceramic. 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 properties similar to, or even improved upon, those reported for B4C in the literature.

[0047] 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 to the three-dimensional configuration of the equipment to be produced.

[0048] These pistons (10, 20) move in a tubular matrix (30) also made of graphite, surrounded by a felt (40).

[0049] Sheets of flexible graphite (6, 7, 8) are placed between the composite powder (5) and the pistons (10, 20) and the matrix (30), respectively. These sheets of flexible graphite (6, 7, 8), for example PAPYEX™, have a reflective quality that helps reduce energy loss. The anisotropy of its thermal conductivity allows for better temperature homogenization throughout the large-dimensional composite powder mass.

[0050] 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 initial stage of sintering and adjust the temperature accordingly.

[0051] 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. Tests were initially performed 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 silicon mass fraction is a parameter of interest because, depending on the amount of silicon, grain wetting will be partial or optimal. Diffusion phenomena between the silicon 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 silicon will sublime in order to locally increase the stress experienced by the grains and initiate the sintering process.Amorphous phases weaken ceramics because they are sensitive to sudden temperature changes, leading to cracking. Therefore, precise control of these parameters is crucial for achieving optimal sintering of different powder mixtures.

[0052] The densification results show a densification rate close to 100%, hardnesses exceeding 4000 HV1, and toughnesses of at least 4 MPa.m -0,5 The hardness-toughness combination is important for achieving the desired ballistic performance.

Claims

– A method for 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 µm 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. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that said powder is a composite of B4C and silicon carbide SiC. - Method for manufacturing ballistic protection 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 %. - 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. - A method for manufacturing ballistic protection equipment according to claim 1 characterized in that said sintering aid additive consists of metallic silicon (Si), or silicon carbide SiC, yttrium trioxide (Y2O3), or alumina (Al2O3) or cubic boron nitride (c-BN) - A method for manufacturing ballistic protective equipment according to claim 1, characterized in that said sintering aid additive is selected from the following constituents: silicon nitride Si x N y, titanium diboride TiB2, yttrium aluminium YAG, Graphene, carbon fibres, silicon carbide fibres SiC, fibre verre, titanium carbide TiC, nano-diamond, tantalum carbide TaC, titanium nitride TiN, zirconium dioxide ZrO2. - Method of manufacturing ballistic protection equipment according to claim 1 characterized in that the composite of B4C and sintering aid additive is produced by triaxial turbula mixing. - 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). - 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 µm 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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