The mechanical properties are similar to those of other amorphous deposits. They have high strength, limited ductility and a high modulus of elasticity.
The ultimate tensile strength of most coatings exceeds 700 MPa (100kpsi). That is equal to many hardened steels and allows the coating to withstand a considerable amount of abuse without damage.
The ductility also varies with composition and is about 1 to 2.0% (as elongation). Whilst that is less than that of most engineering materials, it is adequate for most coating applications. Thin films of the deposit can be bent completely around themselves without fracture, and the coating has been used successfully for springs and bellows.
Electroless nickel should not be applied to articles which subsequently will be bent or drawn. Severe deformation will crack the deposit, reducing corrosion resistance and abrasion resistance. With lower phosphorus nickel deposits, or with deposits containing metallic or sulfur impurities, ductility is greatly reduced and may even approach zero.
Hardening type heat treatments reduces both the strength and the ductility of these deposits.
- Exposure to temperatures above 220oC (420oF) may cause an 80 to 90% reduction in strength and can destroy ductility, especially in lower phosphorus coatings.
- The ductility of high phosphorus coatings is not significantly reduced until heated to above 260oC (500oF).
- The modulus of elasticity of non-heat treated coatings containing 10 to 11% phosphorus is about 200 GPa (28x10psi) and is very similar to that of steel.
- The modulus of elasticity of deposits containing 7 to 8% phosphorus is only about 120 GPa (18x10psi) and is more similar to that of copper alloys.
- Heat treating the coatings at temperatures above 220oC (400oF) causes their modulus of elasticity to increase significantly.
Deposit appearance varies considerably depending on bath formulation and substrate topography. Baths can be formulated to produce deposits that vary from matte to extremely bright. Since electroless plating solutions have virtually no levelling capabilities, these coatings mirror the finish of the surface to which they are applied.
As a result, even a very bright deposit may appear dramatically less bright on a casting or blasted surface if compared to a similar deposit on a polished surface. If corrosion resistance, good deposit elongation, low stress of high thicknesses and minimum pitting are the primary requirements, matte or semi-bright deposits may be the best choice.
The adhesion of electroless nickel coatings to most metals is excellent. The initial replacement reaction, which occurs with catalytic metals, together with the associated ability of the baths to remove sub microscopic soils, allows the deposit to establish metallic as well as mechanical bonds with the substrate.
- The bond strength of MacDermid NiKlad, Elnic and Vand-aloy coatings to properly cleaned steel has been found to be 400 MPa (60 kpsi) or more.
- When electroless nickel plating aluminum and aluminum alloys the adhesion is less (than steel), but usually exceeds 300 MPa (40 kpsi).
- With non-catalytic or passive metals, such as stainless steel, an initial replacement reaction does not occur and adhesion is reduced. With proper pre-treatment and activation, however, the bond strength of the coating is normally at least 140 MPa (20 kpsi).
- The adhesion to copper alloys is usually between 300 and 350 MPa (40 and 50 kpsi).
When electroless nickel plating aluminum it is common practice to bake parts after plating for 1 to 4 hours at 130 to 200oC (270o to 400oF) to increase the adhesion of the coating. These treatments relieve hydrogen from the part and the deposit and provide a very minor amount of codiffusion between the coating and substrate.