Only in rare cases have chemically deposited metallic coatings been so pure and have such a regular structure that they have the same properties as the corresponding chemically pure substances. Coatings containing non-metallic components - phosphorus or boron - can exhibit very different properties.
The coating is slightly less dense than the bulk metal. This is associated with a rather irregular coating structure: they contain more defects (porosity of impurities and inclusions). For example, electroless deposited copper typically has a large number of tiny voids 20 to 300 Å in diameter, formed by hydrogen occlusion of the coating. Nickel phosphorus and nickel boron coatings usually have a layered structure, which is due to the uneven distribution of phosphorus and boron in the coating.
The mechanical properties of the coating may vary widely, depending on the electroless plating conditions, bath composition and deposition rate.
For chemically deposited copper coatings, such as printed circuit boards, it is very important to have sufficient electrical resistance and ductility. Coatings with a tensile strength of about 40 to 50 kg/mm at temperatures between 2 May and 70°C are ductile with an ultimate elongation of 6 to 8%. Copper deposits obtained at room temperature are more brittle. High toughness coatings can only be obtained with solutions containing special additives. The ductility increases when the deposited layer is heated in an inert atmosphere at a temperature of 300–500 °C.
Nickel-phosphorus and nickel-boron coatings are relatively hard; after deposition, their hardness depends on the amount of P and B, which is 350-600 kg/mm2 (3400-5900 MPa) and 500-750 MPa for ni-p coatings. square millimeters. (4900 to 7400 MPa) for nickel-boron coating, while after heating at about 400°C it is 800 to 1000 kg/mm2 for nickel-phosphorus and 1000 to 1250 kg/mm2 for nickel-B. Therefore, this coating has the same hardness as chrome plating. The tensile strength of nickel-phosphorous coatings is between 40 and 80 kg/mm2.
The hardness of the nickel coating, if hardness is considered, is quite high: its ultimate elongation is less than 2%. This combination of hardness, wear resistance and ductility is unique.
Electroless deposited coatings are generally less conductive than the respective pure metals. The resistivity of thin copper plating (0.5 to 1 μM) is twice as large at room temperature as 3 to 4 × 1–8Ω⋅M, pure bulky copper. The surface resistance of this coating is 0.03 to 0.07 Ω/however, the ductile copper coating has a resistivity of 2×10–8Ω⋅m at a temperature of 50 to 70°C, which is close to that of pure copper.
Ni P and Ni–B coating resistivity depends on the amount of metal parts, it is usually in the range from 3 to 9×10–7Ω⋅m; that is much higher than pure nickel (0.69×10–7Ω⋅m). Heating results in a decrease in resistivity.
The magnetic properties of coatings of ferromagnetic materials such as nickel and cobalt vary over a wide range. As the content of phosphorus in the nickel coating increases, its ferromagnetism decreases, and the coating containing more than 8 mass percent of phosphorus or 6.5 mass percent of boron is non-magnetic.
Cobalt, phosphorus, cobalt, cobalt alloys and coatings of other metals have highly different magnetic properties. These depend on the composition, structure and thickness of the plating layer, which can be controlled by changing the composition, pH and temperature of the electroless plating solution. Typically, cobalt coatings have high coercivity (15 to 80 kcal/m); however, soft magnetic coatings (0.1 to 1 kcal/m) can also be deposited.
The optical properties of the coating do not change much, and are not much different from pure metals. Chemically deposited coatings are usually dull; when special additives are introduced, bright coatings are obtained. Since they are not used as finish decorative coatings, the characteristics of appearance and brightness are usually not necessary.
Silver and gold plating are often used as mirrors, but the reflective surface is usually the inner surface adjacent to the smooth glass surface. Thin films of chemically deposited gold are used as optical filters; they transmit visible light but reflect infrared and radio waves.
Electroless deposited layers are generally less porous than their electroplated counterparts; therefore, they provide a better basis for protecting the metal from corrosion. The corrosion resistance of the coating itself may vary depending on the structure and composition. –NiP and Ni–B are more corrosion-resistant than Ni plating coatings; this may be due to their fine-grained structure.
