Tantaalin sovellukset sotilas- ja avaruusteollisuudessa
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, resulting in serious ablation and wear. Therefore, materials with good wear and ablation resistance at high temperatures are an urgent need in cutting-edge industrial fields.
Ablative wear problems in military industry and aerospace
Hazards caused by ablative wear
Wear and ablation are material surface damage phenomena caused by the interaction of thermodynamics, mechanics, and chemistry. They are the main forms of material failure under extreme high-temperature conditions. Friction and wear at high temperatures are more severe and the mechanism is more complex. Under wear and burn conditions, material loss, surface chemical composition changes, heat-affected layer formation, and surface cracking will continue to occur on the surface of the workpiece. Local damage to the surface will eventually lead to the failure of the entire workpiece, resulting in huge waste of resources and economic losses. It even poses a security threat. A typical scenario where this happens is in the gun barrel. If a workpiece failure occurs on the weapon equipment, the impact may be huge.
Solution
The current solution to the problems of wear resistance and ablation resistance is mainly to prepare protective coatings, and chromium plating is currently the only coating technology that can be used on a large scale at home and abroad. However, under some extreme working conditions, the temperature of the workpiece surface is as high as 2200-3500 degrees Celsius, of which about one-fifth of the heat is absorbed by the workpiece material, and the heat action time is 10 microseconds. The heat on the surface of the workpiece has no time to be transferred outward, and there is a large temperature gradient. The resulting thermal stress will cause the chromium coating to fall off.
In addition, high temperature also intensifies the chemical interaction between the reactive gas and the metal on the workpiece surface, causing the formation of low-melting-point metal oxides, which further promotes the melting of local areas on the workpiece surface. The melted area will be gradually peeled off by gas erosion and mechanical wear. This ablation wear is many times faster than normal wear. At the same time, chromium also has high hardness, high brittleness, and low shear and tensile strength. Therefore, it is difficult for chromium coatings to meet the existing wear and ablation requirements, 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; resistant to burning of reactive gases; thermomechanical properties matching those of steel; good bonding with the substrate; and a certain thickness for protection The mechanical strength of the workpiece is reduced due to thermal effects.
Material to solve ablative wear problem——tantalum
Tantalum (Ta), a metal element, has a body-centered cubic a phase as its dominant structure. 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 it has good conductivity and easy plasticity. In addition, tantalum has low metal mobility and extremely high corrosion resistance. It cannot be corroded by various strong acids at room temperature (except hydrogen fluoride and fuming sulfuric acid). At the same time, tantalum also has good biocompatibility and good wear resistance. At present, tantalum metal has been used in many fields such as medical, aerospace, military industry, etc., especially in the aerospace and military fields.
Tantalum's good high-temperature mechanical properties meet the requirements of wear and ablation conditions. The reasonable development and utilization of tantalum coatings can improve the life of workpieces under wear and burning conditions, save resources, and at the same time obtain good economic benefits and safety guarantees. Lee et al. prepared a tantalum coating and studied the ablative wear resistance of tantalum coating and chromium coating. As a result, the wear rate of the chromium coating increased significantly after 1200 cycles of experiments, while the tantalum coating remained stable.
Tantalum application examples
Cannon body inner covering material
When 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 artillery barrel will undergo the physical and chemical effects of high-temperature and high-pressure gunpowder gas when the projectile is launched (the thermal effect of high-temperature gas, the erosion of high-speed airflow, the corrosion of the inner bore by gunpowder gas residue, and the wear of the inner wall by high-speed moving projectiles). Under this working condition, the inner bore of the artillery barrel will undergo severe ablation erosion and wear, resulting in changes in the geometry and size of the inner bore, which directly affects the shooting accuracy of the artillery and the life of the barrel.
Tantalum (Ta) has good physical and chemical properties: it is a refractory metal with a high melting point (melting point 2996°C), low thermal conductivity (57W/m°C), and good resistance to chemical corrosion (can resist acid, Corrosion by salt and organic chemicals), excellent ablation resistance and good plasticity and toughness (bcc structure Ta). Therefore, tantalum or tantalum alloy coatings are considered to be an ideal coating system to replace electroplated Cr coatings for ablation resistance and erosion resistance. If the Ta layer is to be applied to the artillery barrel and have the purpose of long-term protection against fire gas ablation, the sputtered Ta layer should mainly consist of α-Ta, with a thickness of at least 75 μm, and sufficient thickness in all directions between the coating and the substrate. Bonding force to withstand thermal shock and high shear stress during artillery firing.
Lee et al. used an experimental triode sputtering system to deposit a 50~125μm thick Ta layer in a 20mm inner diameter steel rifled liner. After 1500 live target tests, the Ta layer was complete and had a good protective effect on the substrate. At the same time, Lee et al. used 800°C molten salt to prepare an α-Ta layer in a steel rifled liner. After 5034 live target tests, the coating was still dense and tightly bonded with the matrix.
With the development of armor materials, modern anti-armor warheads have increasingly higher requirements for explosively formed ammunition cover materials. The formation of a longer and stable jet by the medicine cover requires the medicine cover material to have high density, high sound velocity, good thermal conductivity, high dynamic fracture elongation and other properties. In addition, the medicine cover material is also required to have fine grains, low recrystallization temperature, certain texture and other microstructure morphology.
Tantalum, depleted uranium, etc. have excellent comprehensive properties such as high density, high dynamic elongation and arson. In particular, tantalum has a high density (16.6g/cm3) and good dynamic characteristics. It is a material mainly used for explosively formed ammunition covers in foreign research. As an explosively formed ammunition cover material, Ta is widely used in US-made TOW-2B, TOW-NG and other missiles. Ballistic experiments show that its penetration is 30% to 35% higher than that of Cu, and can reach 150mm.
Tantalum in spacecraft applications
Tantalum is a key additive in high-temperature alloys, especially nickel-based high-temperature alloys. 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 high temperature alloy can work below 800~1000℃. The addition of tantalum mainly plays a solid solution strengthening role and improves the ultimate strength of the alloy, especially high-temperature creep resistance, oxidation resistance and corrosion resistance. The excellent high-temperature strength, good oxidation resistance and hot corrosion resistance, good fatigue performance and fracture toughness of superalloys make it a key material for high-temperature components such as aeroengine turbine blades, guide vanes, and turbine disks.
At present, almost all high-performance military and civil aviation engines in foreign countries use tantalum high-temperature alloys as the components with the highest heat resistance and the greatest stress load. The melting point of the recently developed third-generation tantalum-containing single-crystal alloy has been further improved, allowing the single-crystal turbine blades to operate at higher temperatures, save fuel and have a longer life.
In order to withstand the test of high-temperature thermal cycling (1300°C/20min), the surface of spacecraft parts must be coated with a protective coating to improve its oxidation resistance. Therefore, the study of economically feasible, stable and reliable high-temperature protective coatings is of great significance for the high-temperature application of tantalum in the aerospace industry. Due to its high melting point, tantalum is mainly used as heating furnace parts and jet engine parts, and it plays an extremely important role in aerospace and missile technology.
