Many key components in aerospace and military industries work under high temperature conditions and are subject to the interaction of thermal, mechanical, and chemical processes, resulting in severe erosion and wear. Therefore, materials with good wear resistance and ablation performance at high temperatures are urgently needed in cutting-edge industrial fields.

1. Erosion and Wear Issues in Military Industry and Aerospace

one point one

The hazards caused by erosion and wear

Wear and erosion are surface damage phenomena caused by the interaction of thermodynamic, mechanical, and chemical processes, and 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 the condition of wear and burn, the surface of the workpiece continuously experiences material loss, changes in surface chemical composition, formation of heat affected layer, and surface cracking. Local damage to the surface will ultimately lead to the failure of the entire workpiece, causing huge resource waste, economic losses, and even posing a safety threat. A typical scenario where this occurs is within the barrel, and if there is a workpiece failure issue on the weapon equipment, the impact could be enormous.

one point two

terms of settlement

At present, the main solution to the problems of wear resistance and erosion resistance is to prepare protective coatings, and chrome plating is the only coating technology that can be widely used domestically and internationally. However, under certain extreme working conditions, the surface temperature of the workpiece can reach up to 2200-3500 degrees Celsius, with about one-fifth of the heat being absorbed by the workpiece material, and the thermal action time being 10 microseconds. The surface heat of the workpiece cannot be transferred outward in time, resulting in a large temperature gradient. The thermal stress caused by this can cause the chromium coating to peel off.

In addition, high temperature also intensifies the chemical reaction between reactive gases and the metal on the surface of the workpiece, leading to the formation of low melting point metal oxides, which further promotes the melting of local areas on the surface of the workpiece. The melted area will gradually peel off under the erosion of gas and mechanical wear, which is much faster than normal wear. At the same time, chromium has high hardness and brittleness, but low shear and tensile strength. Therefore, chromium coatings are no longer able to meet the existing requirements for wear and erosion, and the development of higher performance coatings is urgently needed.

one point three

Requirements for new coatings

For new high-performance coatings, the following requirements need to be met: high melting point; Good high-temperature strength; Can resist the burning of reactive gases; Match the thermal mechanical properties with the steel; Good adhesion with the substrate; And a certain thickness is required to protect the workpiece from the decrease in mechanical strength caused by thermal effects.

2. The solution material for erosion and wear problems - tantalum

Tantalum (Ta), a metallic element with a body centered cubic A-phase as the dominant structure, has a melting point of approximately 2996 ℃, second only to carbon, europium, rhenium, and osmium. The elastic modulus of tantalum is similar to that of steel, with good conductivity and plasticity. In addition, tantalum has low metal activity 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 wear resistance. At present, tantalum metal has been applied in various fields such as medical, aerospace, military industry, especially in the aerospace and military fields.

The excellent high-temperature mechanical properties of tantalum meet the requirements of wear and erosion conditions. The reasonable development and utilization of tantalum coatings can improve the service life of workpieces under wear and burn conditions, save resources, and also achieve good economic benefits and safety guarantees. Lee et al. prepared tantalum coatings and studied the erosion and wear resistance of tantalum coatings and chromium coatings. The results showed that the wear rate of chromium coatings increased significantly after 1200 cycles of experiments, while tantalum coatings remained stable.

3. Examples of Tantalum Applications

three point one

Material covering the barrel of the gun

Gunpowder produces a tail flame with a temperature of up to 2500-3500K and a pressure of up to 300-800MPa during explosion. The tail flame contains corrosive components such as H2S, CO, O2, H2, H2O, N2, and residual particles of gunpowder. Therefore, the gun barrel will undergo physical and chemical reactions of high-temperature and high-pressure gunpowder gas during projectile launch (thermal effects of high-temperature gas, erosion of high-speed airflow, corrosion of the inner chamber by gunpowder gas residue, and wear of the inner wall by high-speed moving projectiles). Under this operating condition, the inner bore of the artillery barrel will undergo severe erosion and wear, resulting in changes in the geometric shape and size of the barrel, directly affecting the firing accuracy of the artillery and the lifespan of the barrel.

Tantalum (Ta) has excellent physical and chemical properties: it is a high melting point refractory metal (melting point 2996 ℃), low thermal conductivity (57W/m ℃), good chemical corrosion resistance (able to resist acid, salt, and organic chemical corrosion at high temperatures), excellent ablation resistance, and good plasticity and toughness (bcc structure Ta). Therefore, tantalum or tantalum alloy coatings are considered an ideal coating system to replace electroplated Cr coatings for erosion and erosion resistance. If the Ta layer is to be applied to the barrel of a cannon and serve the purpose of long-term gas erosion of the fire retardant, the sputtered Ta layer should mainly consist of α - Ta, with a thickness of at least 75 μ m, and sufficient bonding force between the coating and the substrate in all directions to resist thermal shock and high shear stress during the firing process of the cannon.

Lee et al. used an experimental transistor sputtering system to deposit a 50-125 μ m thick Ta layer inside a 20mm inner diameter steel rifling liner. After 1500 live fire target tests, the Ta layer was intact and provided good protection to the substrate. At the same time, Lee et al. used 800 ℃ molten salt to prepare an alpha Ta layer inside the steel rifling liner. After 5034 live fire target tests, the coating remained dense and tightly bonded to the substrate.

three point two

Application in armor piercing bullets

With the development of armor materials, modern anti armor warheads have increasingly high requirements for explosive formed ammunition cover materials. The formation of a longer and more stable jet by the drug shaped cover requires the material of the drug shaped cover to have high density, high sound velocity, good thermal conductivity, high dynamic fracture elongation, and other properties. In addition, the medication mask material also requires a microstructure morphology such as fine grains, low recrystallization temperature, and certain texture.

Tantalum, depleted uranium, and other materials have excellent comprehensive characteristics such as high density, high dynamic elongation, and arson. Especially tantalum has a high density (16.6g/cm3) and good dynamic characteristics, making it a material mainly used for explosive forming ammunition covers in foreign research. Ta, as an explosive forming ammunition cover material, is widely used in American made missiles such as TOW-2B and TOW-NG. Ballistic experiments have shown that Ta has a 30% to 35% higher affinity than Cu, reaching up to 150mm.

three point three

Application of Tantalum in Spacecraft

Tantalum is a key additive in high-temperature alloys, especially nickel based high-temperature alloys. Tantalum can be 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 alloys can operate below 800-1000 ℃. The addition of tantalum is mainly for solid solution strengthening, improving the ultimate strength of the alloy, especially the high-temperature creep resistance, oxidation resistance, and corrosion resistance. High temperature alloys have excellent high-temperature strength, good oxidation and corrosion resistance, good fatigue performance, and fracture toughness, making them key materials for high-temperature components such as aircraft engine turbine blades, guide vanes, and turbine disks.

At present, almost all high-performance military and civil aviation engines abroad use tantalum high-temperature alloys as the components with the highest heat resistance and stress load. The melting point of the recently developed third-generation tantalum containing single crystal alloy has been further improved, resulting in single crystal turbine blades that can operate at higher temperatures, save fuel, and have a longer lifespan.

In order to withstand the test of high-temperature thermal cycling (1300 ℃/20min), protective coatings must be applied to the surfaces of spacecraft components to enhance their oxidation resistance. Therefore, researching economically feasible and stable 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 a heating furnace component and jet engine component, occupying an extremely important position in aerospace and missile technology.