1 Research status of metal coating
Thermal spraying metal coating is a kind of wear-resistant coating that has been studied and applied earlier. The commonly used coatings are metal (Mo, Ni), carbon steel and low alloy steel, stainless steel and Ni Cr alloy series coatings. Generally, flame spraying, arc spraying, plasma spraying, HVOF and explosive spraying processes are used. The coating has the advantages of high bonding strength with the substrate, good wear resistance and corrosion resistance, and is used for repairing worn parts and machining out of tolerance parts.
When the piston ring, synchronous ring, cylinder and other parts are sprayed with aluminum alloy plasma spraying technology, the coating has good wear resistance, high bonding strength and excellent adhesive wear resistance, and has good anti seizing and anti pulling properties under the condition of lubricating oil. High carbon steel wire and stainless steel (Crl3, 18-8, etc.) alloy wire are commonly used wear-resistant and corrosion resistant spraying materials. It has the characteristics of high strength, good wear resistance, wide source and low price. NiCr coating has good heat resistance, corrosion resistance and erosion wear resistance, which can be used as protective coating for superheater tube and reheater tube of power plant boiler. NiCr metal wear-resistant coating with different structures can be prepared by flame and plasma spraying, and the porosity and oxide content in the coating are high.
Research status of ceramic coatings
Thermal sprayed ceramic powders include oxides, carbides, borides, nitrides and silicides, which are crystalline or amorphous compounds composed of metal elements and non-metallic elements. Ceramic coating has the characteristics of high melting point, high hardness, good wear resistance, corrosion resistance and high temperature stability. However, the ceramic coating process is complex, the cost is high, and the surface of the coating is prone to cracks, and the thermal fatigue resistance is not as good as the metal coating; Moreover, the toughness of the coating is poor, and it can not be used to withstand large impact loads. At present, commonly used ceramic coatings include A12O3, TiO2, Cr2O3, ZrO2, WC, TiC, Cr3C2, TiB2, etc., which are generally prepared by plasma spraying, flame spraying, HVOF and explosive spraying technologies.
The sliding friction and wear characteristics of plasma sprayed A12O3-40% TiO2 and Cr2O3 ceramic powder coatings indicate that the wear resistance of Cr2O3 coating is higher than that of A12O3-40% TiO2 coating. The wear mechanism of Cr2O3 coating is mainly abrasive wear. Under large load, the wear of Cr2O3 coating presents brittle fracture characteristics. The wear mechanism of A12O3-40% TiO2 coating is mainly plastic deformation and delamination. The A12O3+TiO2+NiCrAlY composite ceramic coating researched by Chen Chuanzhong and others can reduce the porosity of the coating and further improve the strength, toughness and wear resistance of the coating due to the mutual solubility of molten TiO2 and A12O3 to a certain extent.
Lin et al. studied the sliding friction and wear characteristics of plasma sprayed multilayer metal and ceramic coatings. The spraying sequence is as follows: first, NiCr is sprayed on the substrate to form the bottom layer, then NiCr-Cr2O3 transition layer with different proportions is sprayed, and the surface is 100% Cr2O3. It is found that the wear resistance of the coating can be improved by the transition layer with proper metal to ceramic ratio. The main wear mechanisms of the coatings are brittle fracture, abrasive wear, adhesion and oxidation wear.
3 Research status of cermet coating
Metal and ceramic materials have their own unique excellent properties and obvious performance weaknesses. How to combine the advantages and properties of metal and ceramic materials has always been the research direction of materials science and engineering. The cermet composite coating technology, that is, ceramic phases with appropriate particle shape and size are evenly distributed on the plastic substrate, which successfully combines the advantages of metal and ceramics, and prepares composite materials that have both the strength and toughness of metal and the advantages of high temperature resistance, wear resistance, corrosion resistance, etc. of ceramics, greatly expanding the application scope of metal materials and ceramic materials, It has been successfully applied in aviation, aerospace, chemical industry, machinery, power and other industrial fields. The most widely used cermet coatings in industry mainly include: Cr3C2 NiCr, WC Co. Most of them adopt HVOF, plasma and explosive spraying processes.
Cr3C2-NiCr cermet coating is composed of refractory chromium carbide hard phase and nickel chromium alloy phase with good toughness. It has high high temperature hardness, excellent high temperature wear resistance, corrosion resistance, oxidation resistance and high bonding strength. It is widely used for parts working under high temperature (530-9000 ℃) abrasive wear, corrosion wear and erosion wear conditions, such as furnace roll of continuous annealing line Core roll, cylinder piston ring, cylinder liner, etc. on continuous production line of rolling mill. TiB2 based cermet coating has high melting point, high hardness, good electrical and magnetic properties and high corrosion resistance, and is a potential alternative to Cr3C2 for high-temperature and wear-resistant cermet. It has higher wear resistance than A12O3, Cr3C2 NiCr and WC Co.
WC based cermet coatings are commonly used in abrasive wear and erosion wear conditions below 450 ℃. Xu Xiangyang et al. studied the fretting wear mechanism of plasma sprayed WC/18Co coatings. The results show that the fretting wear of the coating is mainly adhesive wear at the beginning, and the coating has high hardness, strong adhesion resistance and slight wear; The stable stage is dominated by fatigue delamination and brittle cracking and spalling. The coating is brittle, the bonding strength between spraying particles is low, and it is easy to wear. The oxide inclusion in the spray coating is the main reason for the lack of fretting wear resistance of the coating.
4 Research Status of Amorphous Coatings
Amorphous is a kind of material with long range disorder and short range order. The physical and chemical properties of amorphous materials are often better than those of corresponding crystalline materials, with high strength, high toughness, high hardness, high corrosion resistance, soft magnetic properties, etc. It is a new type of metal materials with great development prospects. Thermal spraying amorphous alloy coating is a new field of extensive research in material science in recent years. As one of the preparation methods of large-area amorphous coating, thermal spraying technology has begun to attract widespread attention. The commonly used methods are plasma spraying, HVOF and explosive spraying.
Fe based amorphous alloy coating (including Si, B, Cr, Ni, etc.) is prepared by plasma spraying. The microstructure of the coating is uniform in all areas, the coating density is high, the porosity is low, the oxide content is small, and the hardness is very high. The microhardness is within the range of 530~790HV0.1, and the coating is well combined with the substrate.
Jin et al. studied the microstructure and wear resistance of explosive sprayed Fe-Cr-B alloy coating. The results show that the coating has good wear resistance and corrosion resistance. During sliding wear, the amorphous surface film is produced dynamically, which significantly improves the wear resistance of the coating, while the friction coefficient decreases significantly.
To sum up, the wear-resistant coatings of metal, ceramics, cermets and amorphous can be effectively improved by HVOF, plasma spraying, arc spraying, explosive spraying and other methods. The wear mechanism of thermally sprayed coatings and the influence of coating structure on wear characteristics are studied in depth to provide theoretical basis for improving coating structure, optimizing spraying process and developing new wear-resistant coatings.
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