Application of Tantalum in Military Industry and Aerospace

Many key components in the aerospace and military industries work under high temperature conditions and are subject to the interaction of thermal, mechanical and chemical processes, which lead to serious ablation and wear. Therefore, materials with good wear and ablation resistance at high temperatures are urgently needed in the cutting-edge industrial field.
1. Ablative wear in military industry and aerospace
Hazards caused by ablation and wear
Wear and ablation are the phenomena of material surface damage caused by the interaction of thermodynamic, mechanical and chemical processes, and are the main forms of material failure under extreme conditions of high temperature. Under the condition of wear and burning, the loss of material, the change of surface chemical composition, the formation of heat-affected layer and the cracking of the surface continue to occur on the surface of the workpiece. The local damage of the surface will eventually lead to the failure of the whole workpiece, resulting in huge waste of resources, economic losses and even security threats. A typical scenario for this situation is located in the barrel. If the workpiece failure problem occurs on the weapon equipment, the impact may be huge.
At present, the solution to the problem of wear resistance and ablation resistance is mainly the preparation of protective coatings, and chromium plating is the only coating technology that can be used on a large scale at home and abroad. However, under some extreme conditions, the temperature of the workpiece surface is as high as 2200 to 3500 degrees Celsius, of which about 1/5 of the heat is absorbed by the workpiece material, and the thermal action time is 10 microseconds. The surface heat of the workpiece is too late to transfer to the outside, there is a large temperature gradient, and the resulting thermal stress will cause the chromium coating to fall off.
In addition, the high temperature also intensifies the chemical interaction between the reactive gas and the metal on the surface of the workpiece, resulting in the formation of low melting point metal oxides, which further contributes to the melting of local areas of the surface of the workpiece. The melted area will be gradually stripped under the scouring of the gas and mechanical wear. This ablation wear is many times faster than normal wear. At the same time, chromium has high hardness, high brittleness, and low shear and tensile strength. Therefore, it is difficult for chromium coating to meet the existing requirements of wear and ablation, and it is urgent to develop higher performance coatings.
Requirements for new coatings
For new high-performance coatings, the following requirements need to be met: high melting point; good high-temperature strength; resistance to reactive gas burning; thermal mechanical properties matching with steel; good bonding with the substrate; and a certain thickness to protect the workpiece due to the reduction of mechanical strength caused by thermal action.
2. Material for solving the problem of ablation and wear-tantalum
Tantalum (Ta), a metal element, is dominated by body-centered cubic phase a, and its melting point is about 2996 ° C, second only to carbon, europium, rhenium and osmium. The elastic modulus of tantalum is similar to that of steel, and has good conductivity and plasticity. In addition, the metal activity of tantalum is low and has extremely high corrosion resistance. Various strong acids cannot be corroded at room temperature (except hydrogen fluoride and fuming sulfuric acid). At the same time, tantalum also has good biocompatibility, good wear resistance and so on. At present, tantalum metal has been used in many fields such as medical, aerospace and military industries, especially in aerospace and military fields.
The good high-temperature mechanical properties of tantalum meet the requirements of wear and ablation conditions, and the reasonable development and utilization of tantalum coating can improve the life of the workpiece under wear and burning conditions, save resources, and also obtain good economic benefits and safety guarantees. Lee et al. prepared tantalum coatings and studied the ablation wear resistance of tantalum coatings and chromium coatings, and the wear rate of chromium coatings increased significantly after 1200 cycles, while the tantalum coating remained stable.
3. Application Examples of Tantalum
Gun body inner covering material
When the gunpowder explodes, it produces a tail flame with a temperature of 2500~3500K and a pressure of 300 ~ 800MPa. The tail flame contains corrosive components such as H2S,CO,O2,H2,H2O,N2 and residual particles of gunpowder. Therefore, the gun barrel will be subjected to the physical and chemical effects of high-temperature and high-pressure gunpowder gas when the projectile is fired (the thermal effect of high-temperature gas, the erosion of high-speed airflow, the corrosion of the inner bore by the residue of gunpowder gas, and the wear of the inner wall by the high-speed moving projectile). Under this condition, the inner bore of the gun barrel will be subjected to severe ablation erosion and wear, resulting in changes in the geometry and size of the inner bore, which directly affects the firing accuracy of the gun and the life of the barrel.
Tantalum (Ta) has good physical and chemical properties: it is a refractory metal with high melting point (melting point 2996 ℃), low thermal conductivity (57 W/m ℃), good chemical corrosion resistance (corrosion resistance to acid, salt and organic chemicals at high temperature), excellent ablation resistance and good plastic and toughness (bcc structure Ta). Therefore, tantalum or tantalum alloy coatings are considered to be an ideal coating system to replace ablation-resistant, brush-resistant electroplated Cr coatings. If the Ta layer is to be applied to the gun barrel and has the purpose of long-term fire retardant gas ablation, the sputtered Ta layer should be mainly composed of α-Ta, with a thickness of at least 75μm, and the coating and the substrate should have sufficient bonding force in all directions to resist the thermal shock and high shear stress during the firing process of the gun.
Lee et al. deposited a 50 ~ 125μm thick Ta layer in a 20mm inner diameter steel rifling liner using a test triode sputtering system. After 1500 live-fire target tests, the Ta layer was complete and played a good protective role on the substrate. At the same time, Lee et al. prepared an α-Ta layer in a steel rifling liner using molten salt at 800 ℃, and the coating was still dense and tightly bonded to the substrate after 5034 live target tests.
Application of penetrator
With the development of armor materials, modern anti-armor warheads have higher and higher requirements for explosive forming ammunition type cover materials. The formation of a longer and stable jet of the drug type cover requires the drug type cover material to have high density, high sound velocity, good thermal conductivity, high dynamic fracture elongation and other properties. In addition, the drug type cover material also requires a fine grain, low recrystallization temperature, a certain texture and other microstructure morphology.
Tantalum and depleted uranium have excellent comprehensive characteristics such as high density, high dynamic elongation and arson. In particular, tantalum has high density (16.6g/cm3) and good dynamic characteristics, which is a material mainly used in explosive forming ammunition cover in foreign research. Ta is widely used in U. S.-made TOW-2B, TOW-NG and other missiles as an explosive forming ammunition cover material. Ballistic experiments show that the affinity of Ta is 30% ~ 35% higher than that of Cu, which can reach 150mm.
Application of Tantalum in Spacecraft
Tantalum is a key additive for superalloys, especially nickel-based superalloys. Tantalum is added to various alloys such as nickel-based, cobalt-based and iron-based alloys to produce high-performance alloys such as superalloys, corrosion-resistant alloys, and wear-resistant alloys. Tantalum superalloy can work below 800~1000 ℃. The addition of tantalum is mainly to play a solid solution strengthening role, improve the ultimate strength of the alloy, especially high temperature creep strength, oxidation resistance and corrosion resistance. High-temperature alloys have excellent high-temperature strength, good oxidation resistance and hot corrosion resistance, good fatigue performance and fracture toughness, making them key materials for high-temperature components such as aero-engine turbine blades, guide vanes, and turbine disks.
At present, almost all high-performance military and civil aviation engines in foreign countries have the highest heat resistance and the largest stress load components, and they all use tantalum high-temperature alloys. The melting point of the recently developed third-generation tantalum-containing single crystal alloys has been further improved, and the single crystal turbine blades can be manufactured to operate at higher temperatures, save fuel, and have a longer life.
In order to withstand the test of high temperature thermal cycle (1300 ℃/20min), the surface of spacecraft parts must be coated to improve its oxidation resistance. Therefore, the study of economically feasible and stable and reliable high-temperature protective coatings is of great significance for the high-temperature application of tantalum in the aerospace industry. Due to the high melting point of tantalum, it is mainly used as heating furnace parts and jet engine parts, and it occupies an extremely important position in aerospace and missile technology.