Definition, principle and characteristics of cold spray
Cold spraying is a solid-state coating preparation technology. Its core feature is the use of high-speed airflow to accelerate metal powder particles (typically 1-50μm) into the substrate surface at speeds of 300-1200m/s. Through intense plastic deformation, solid-state bonding is achieved without melting the powder during the entire process. This technology was first proposed by Soviet scientists in the 1980s and was initially used to prepare anti-corrosion coatings for spacecraft. It has now developed into a multifunctional technology system encompassing coating deposition, additive manufacturing, and part repair. Compared with traditional thermal spraying, cold spray avoids high-temperature oxidation, phase transformation, and grain growth of the material. It is particularly suitable for the preparation of nanostructured coatings and heat-treated hardened alloy coatings, and has irreplaceable advantages in high-end fields such as aerospace and nuclear energy.
The technical principles of cold spraying involve two key processes: gas dynamics and material plastic deformation. First, high-pressure gas (usually nitrogen or helium) expands and accelerates within the spray gun nozzle, forming a supersonic flow. This accelerates the powder particles fed into the nozzle to above a critical velocity—this critical velocity varies depending on the material, for example, approximately 500 m/s for aluminum powder and 800 m/s for copper powder. When the high-speed particles impact the substrate, their kinetic energy is converted into plastic deformation energy, generating local stresses up to 1 GPa and a transient temperature rise at the point of impact (typically below 0.6 Tm, where Tm is the melting point of the material). This ruptures the oxide films on the particle and substrate surfaces, exposing the fresh metal surface and forming a coating through metallic bonding. Research by Li Wenya’s team at Northwestern Polytechnical University has discovered that cold-sprayed copper coatings undergo grain rotation and merging during stretching, while twinning facilitates plastic deformation. This mechanism enables the copper deposits produced to achieve an ultimate tensile strength of 270 MPa and a plasticity of nearly 30%, breaking through the bottleneck of cold-sprayed coatings’ high strength without plasticity.
One of the core features of cold spraying is the material performance advantages afforded by low-temperature solid-state deposition. By avoiding high-temperature processes, the coating retains the microstructure and properties of the original material. For example, nanocrystalline powders retain their nanoscale grains after spraying, and age-hardened aluminum alloy coatings do not soften due to high temperatures. A study showed that a cold-sprayed 7075 aluminum alloy coating achieved a 36% increase in hardness (150 HV) compared to a thermally sprayed coating (110 HV), while maintaining corrosion resistance comparable to that of the substrate. Furthermore, cold spraying has minimal thermal impact on the substrate, with substrate temperatures typically below 100°C during coating deposition. This makes it suitable for surface metallization of heat-sensitive substrates such as plastics and composites. This characteristic is particularly important in the repair field: engine blades repaired using cold spray can achieve thermal deformation within 0.01 mm, far less than the 0.1 mm achieved with welding, ensuring the dimensional accuracy of precision components.
Another notable feature of cold spraying is its wide material adaptability and process flexibility. Regarding metal materials, coatings can be applied not only to highly ductile metals like aluminum, copper, and zinc, but also, through process optimization, to high-strength materials like titanium alloys and nickel-based superalloys. Research from the University of Tenaga Nasional in Malaysia has demonstrated that cold-sprayed IN625 nickel-based alloy coatings on cast iron components can achieve a bond strength exceeding 50 MPa, successfully used in power plant equipment repair. In the field of composite materials, cold spraying can produce functional coatings such as metal-ceramic and metal-intermetallic compounds. For example, nickel-alumina composite coatings can achieve a hardness of 300 HV and boast wear resistance twice that of pure nickel coatings. This process flexibility is reflected in its ability to produce functional coatings with thicknesses ranging from a few microns to additive manufacturing of several centimeters. An aviation company has used cold spray technology to directly fabricate complex titanium alloy components, increasing material utilization from 10% forging to 80%.
The application characteristics of cold spray technology give it unique advantages in specific areas. In the power industry, cold spray has become the preferred technology for turbine blade repair. A thermal power plant in Italy used high-pressure cold spray to repair corroded turbine blades, increasing erosion resistance by 2-3 times and extending service life by three times. In the nuclear energy sector, 304L stainless steel coatings have successfully repaired stress-corroded stainless steel parts, achieving corrosion resistance comparable to that of the parent material. Compared with other spray technologies, cold spraying suffers from high equipment investment (approximately 2 million yuan for a high-pressure system) and stringent powder flowability requirements (typically requiring spherical powders), which increases material costs by approximately 30%. However, with the development of low-cost processes such as nitrogen replacing helium (Li Wenya’s team has demonstrated copper coatings produced using nitrogen with performance comparable to helium), as well as the use of automated spray guns, the economics of cold spraying are gradually improving, and its application in civilian industrial applications is expected to grow by over 50% over the next five years.
