Only in rare cases are chemically deposited metal coatings so pure and so structured that their properties are identical to those of the corresponding chemically pure substances. Coatings containing non-metallic components (phosphorus or boron) may exhibit very different properties.
The coating is slightly less dense than the bulk metal. This is associated with rather irregular coating structures: they contain more defects (porosity and foreign matter inclusions). For example, electroless deposited copper typically has a large number of microscopic voids with diameters ranging from 20 to 300 Å, formed by hydrogen trapped in the coating. Ni-P and Ni-B coatings usually have a layered structure, which is caused by the uneven distribution of phosphorus and boron in the coating.
Depending on the electroless plating conditions, bath composition and deposition rate, the mechanical properties of the coating can vary over a wide range.
For chemically deposited copper coatings, such as those on printed circuit boards, adequate electrical resistance and ductility are very important. Coatings with tensile strengths of approximately 40 to 50 kg/mm2 can be obtained at temperatures from 50 to 70°C. Their ultimate elongation, which characterizes ductility, can be as high as 6% to 8%. Copper coatings obtained at room temperature are more brittle. Highly ductile coatings can only be obtained from solutions containing special additives. The ductility increases when the deposited coating is heated at temperatures between 300 and 500°C in an inert atmosphere.
Ni-P and Ni-B coatings are relatively hard; after deposition, their hardness depends on the amount of P and B, Ni–P coating is 350 to 600 kg/mm2 (3400 to 5900 MPa), Ni–P coating The layer is 500 to 750 kg/mm2 (4900 to 7400 MPa) B coating, 800 to 1000 kg/mm2 for Ni-P and 1000 to 1250 kg/mm2 for Ni-B after heating at about 400°C. Therefore, this coating has the same hardness as a chrome coating. The tensile strength of the Ni-P coating is in the range of 40 to 80 kg/mm2.
Nickel coatings are quite ductile if their hardness is taken into account: they have an ultimate elongation of less than 2%. This combination of hardness, wear resistance and ductility is unique.
The conductivity of electroless deposited coatings is generally lower than that of the corresponding pure metal. Thin copper coatings (0.5 to 1.0 μm) deposited at room temperature have a resistivity of 3 to 4 × 1.0–8 Ω m—twice that of pure bulk copper. These coatings have a surface resistance of 0.03 to 0.07 Ω/m. However, the resistivity of ductile copper coatings obtained at temperatures between 50 and 70°C is 2 × 10–8 Ω·m, which is close to that of pure copper.
The resistivity of Ni-P and Ni-B coatings depends on the content of non-metallic components, usually in the range of 3 to 9 × 10-7 Ω m; this is much higher than that of pure bulky nickel (0.69 × 10-7 Ω m m). Heating causes the resistivity to decrease.
The magnetic properties of coatings of ferromagnetic materials such as nickel and cobalt can vary over a very wide range. As the phosphorus content in the nickel coatings increases, their ferromagnetism decreases, and coatings with a phosphorus content of more than 8% by mass or a boron content of more than 6.5% by mass are nonmagnetic.
Co-P, Co-B, and cobalt alloys have highly different magnetic properties from coatings of other metals.
These depend on the composition of the coatings, their structure and their thickness, and they can be controlled by changing the composition, pH and temperature of the electroless plating solution. Typically, cobalt coatings exhibit high coercivity (15 to 80 kA/m); however, soft magnetic coatings (0.1 to 1.0 kA/m) can also be deposited.
The optical properties of the coating do not change much and are not much different from those of pure metals. Chemically deposited coatings are usually dull; when special additives are introduced, shiny coatings can be obtained. Since they are not used as finish decorative coatings, the properties of appearance and brightness are usually not necessary.
Silver and gold coatings are often used as mirrors, but the reflective side is usually the inner surface, adjacent to the smooth glass surface. Thin films of chemically deposited gold act as optical filters; they pass visible light but reflect infrared and radio waves.
Electroless deposited coatings are generally less porous than corresponding electroplated coatings; therefore, they better protect the base metal from corrosion. The corrosion resistance of the coating itself may vary depending on the structure and composition. Ni-P and Ni-B coatings are more corrosion resistant than nickel plating; this may be due to their fine crystalline structure.
