Properties of Electroless Nickel
The properties of electroless nickel including corrosion resistance are influenced by the type of alloy. This can be a low phosphorous nickel coating, a medium phosphorous nickel coating, a high nickel phosphorous coating or a nickel boron coating.
The table below outlines the various properties and nickel alloy type. Click on the property for further explanation.
| Property / Level of alloy | Higha | Mida | Mid-Lowa | Lowa | Mid-Low Bb | Low Bb |
|---|---|---|---|---|---|---|
| % Phosphorous | 10 - 13 | 7 - 9 | 4 - 6 | 1 - 3 | - | - |
| % Boron | - | - | - | - | 3 - 5 | 0.2 - 1 |
| Deposit density Range (g/cm3) | 7.6 - 7.9 | 8.0 - 8.2 | 8.3 - 8.5 | 8.6 - 8.8 | 8.25 | 8.8 |
| Plating rate (µ/Hr) | 7.5 - 15 | 7.5 - 15 | 18 - 30 | 11 - 19 | - | - |
| Hardnessc,f | 400 - 525 | 500 - 600 | 625 - 750 | 725 - 800 | 650 - 750 | 600 - 700 |
| Rockwell C (Rc) Hardness | 41 - 46 | 45 - 51 | 53 - 59 | 57 - 61 | 54 - 59 | 51 - 56 |
| Hardness after heat treatc | 850-950 | 850-1000 | 850-1100 | 900-1100 | 1100-1200 | 500-600 |
| Taber wear indexe,f | 22 - 24 | 16 - 20 | 10 - 14 | 7 - 12 | 3 - 10 | 7 - 9 |
| Coefficient of thermal expansiong | 8 - 10 | 10 - 15 | 11 - 14 | 12 - 15 | - | - |
| Electrical resistivityh | 75 - 110 | 40 - 70 | 15 - 45 | 10 - 30 | 40 - 90 | 10 - 20 |
| Thermal conductivityi | 0.010 | 0.012 | 0.016 | 0.015 | - | |
| Tensile strength (MPa) | 650-900 | 800-1000 | 350-600 | 200-400 | - | - |
| Elongation | 1 - 1.25 | 0.5 - 1 | 0.5 -1 | 0.5 - 1.5 | 0.2 | - |
| Modulus of elasticity (GPa) | 55 - 70 | 50 - 65 | 45 - 65 | 55 - 65 | 120 | - |
| Melting range oC | 880-900 | 880-980 | 1100-1300 | 1250-1360 | 1040-1080 | 1350-1390 |
| Coercivity (Oe) | 0 | 1 - 8 | 10 -15 | 15 - 80 | - | - |
| Magnetic propertiesf | Non | Slightly | Magnetic | Magnetic | - | Weakly |
| Internal stressf | Neutral to comp | Slightly tensile | Slightly comp | Slightly tensile | Tensile | |
| % Phosphorous | 10 - 13 | 7 - 9 | 4 - 6 | 1 - 3 | - | - |
| % Boron | - | - | - | - | 3 - 5 | 0.2 - 1 |
a) nickel-phosphorous b) nickel-boron c) HK100 e) mg/1000 CYCLES - CS-10 WHEEL, 100 g LOAD f) as plated g)mm/m/°C h)resistivity uOHM-CM i)CAL/CM/SEC/°C
ENGINEERING PROPERTIES
Deposit Uniformity
A primary benefit of electroless nickel is its uniform coating thickness. With electroplated coatings, thickness can vary significantly depending on component configuration and its proximity to the anodes. These variations can affect the ultimate performance of the coating and cause additional finishing to be required after plating.
With electroless nickel phosphorous and nickel boron coatings the thickness is the same on any section of the part exposed to fresh plating solution. Grooves, slots, blind holes, and even the inside of tubing will have the same amount of coating as the outside part.
Deposit thickness
Plating thickness can easily be controlled to suit the application. Coatings as thin as 2.5μm (0.1 mil) are commonly applied to nickel boron coatings or low phosphorous nickel deposits for electronic components. For highly corrosion resistant coatings, nickel phosphorous deposits as thick as 75 to 125μm (3 to 5 mils) are typical.
Coatings thicker than 250 μm (10 mils) are used for salvage and repair of worn or mis-machined parts.
Deposit Structure
Hypophosphite reduced electroless nickel is one of the very few metallic glasses used as an engineering material. Depending on the bath formulation, deposits may contain from 1% (low phosphorous nickel) to 13% (high nickel phosphorus).
Although electroless nickel boron plating to AMS 2433 is also an option, phosphorus is the most common alloy. Bath formulations can have a dramatic effect on deposit structure through changes in plating rate, deposit content and deposit stress levels.
The structure of these coatings depends upon their composition.
Deposits:
- Containing more than 8.5% phosphorus (high nickel phosphorus) have no crystalline structure or separate phases and are normally amorphous to x-rays
- From 5-8.5% phosphorus contain different phases of nickel and are partly crystalline
- Below 5% in phosphorus content (low phosphorous nickel) and crystalline are typically laminar in structure
PHYSICAL PROPERTIES
Melting Point
Electroless nickel is a eutectic alloy with a wide melting range. Unlike a pure compound, it does not have a true melting point. The melting point range for electroless nickel coatings varies depending on the phosphorus content of the deposit.
All these coatings begin to melt at approximately 880oC (1620oF), which is the eutectic temperature for nickel phosphate (Ni3P). The temperature at which the coating is completely liquid, however, increases with decreasing phosphorus content:
- From about 880oC (1620oF) at 11% – the eutectic point
- To approximately 1450oC (2640oF) for pure nickel
Thus, the melting range becomes wider as the phosphorus content is reduced. Practically, this means that all commercial coatings contain large quantities of liquid material at temperatures above 880oC (1620oF). For example, at 900oC (1650oF) coatings containing 5.8 and 10.5% phosphorus are 46, 74 and 100% melted respectively.
Density
The density of electroless nickel coatings is inversely proportional to their phosphorus content. It varies from about 8.5 g/cm3 for very low phosphorus nickel deposits, to 7.75 g/cm3 for high nickel phosphorous deposits containing about 10.5% (phosphorus).
Electrical Resistivity
The electrical properties of these coatings also vary with composition. For high nickel phosphorus deposits, electrical resistivity is generally about 90 μΩ-cm. Accordingly, these coatings are significantly less conductive than conventional conductors such as copper. For low phosphorus nickel deposits, electrical resistivity is about 20 μΩ-cm. Because of the relatively thin layers used, however, for most applications the resistance of these coatings are not significant.
Heat Treatments
Heat treatments precipitate phosphorus from the alloy and can increase the conductivity of electroless nickel by 2 to 4 times. The formulation of the plating solution can also affect conductivity. Tests with baths complexed with sodium acetate and with succinic acid showed electrical resistivities of 61 and 804μΩ /cm, respectively. A nickel boron coating yields the lowest resistivity of any commercial electroless nickel types. Phosphorus content also has a strong effect on the thermal expansion of the deposit.
MECHANICAL PROPERTIES
The mechanical properties are similar to those of other amorphous deposits. They have high strength, limited ductility and a high modulus of elasticity.
Tensile Strength
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.
Ductility
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
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
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.
Bond Strength
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.
