A common question is how electroless nickel plating behaves at the extreme temperatures associated with atmospheric re-entry. There is no single study that can predict the performance of every electroless nickel coating under these conditions. However, the known behaviour of nickel-phosphorus (Ni-P) coatings as temperature increases provides useful guidance.
Temperature alone does not determine coating performance. The type and duration of heat exposure are also important. Performance can be affected by direct heat, frictional heating, heat transferred from an adjacent component, or heat conducted through the substrate.
Up to approximately 200°C
Up to around 200°C, electroless nickel would generally be expected to perform well across a wide range of applications, assuming the coating and substrate have been correctly specified.
At these temperatures, significant changes to the coating structure are not generally expected to a concern. The coating can therefore continue to provide its intended combination of wear resistance, hardness and corrosion protection.
Actual suitability will, however, depend on the specific application and exposure conditions.
200–300°C
Between approximately 200°C and 300°C, changes within the nickel-phosphorus coating begin to become relevant.
As temperature increases, phosphorus can begin to precipitate from the nickel-phosphorus structure, while the nickel structure can become more ordered as crystallisation takes place. These changes can alter the coating’s properties from its original plated condition.
The coating may remain physically intact and continue to function, but its performance can begin to change with increasing temperature.
300–400°C
Between approximately 300°C and 400°C, suitability becomes increasingly dependent on the requirements of the application.
Crystallisation would be expected to occur, and the coating can become very hard. At the same time, stresses within the coating may increase, potentially affecting adhesion between the coating and substrate.
Corrosion resistance may also decrease as the coating structure changes.
As a result, a coating that performs well at lower temperatures may not provide the same performance after exposure in the 300–400°C range. Whether this is acceptable depends on the application.
Above 400°C
Above approximately 400°C, further changes to the electroless nickel coating can occur. Embrittlement and other temperature-related effects may also become concerns for some substrates, while oxidisation and diffusion can occur within the coating.
This does not mean that electroless nickel will automatically fail above 400°C. Rather, the complete coating/substrate system and the specific operating conditions need to be considered, and alternative coating systems may need to be evaluated.
What about atmospheric re-entry?
Atmospheric re-entry presents a very different environment from a typical high-temperature industrial application.
Pure nickel has a melting point of approximately 1,455°C, while a high-phosphorus nickel-phosphorus alloy containing around 11–12% phosphorus melts at approximately 880°C. As phosphorus content decreases, the melting point rises towards that of pure nickel.
If a high-phosphorus electroless nickel coating were directly exposed to the atmosphere during re-entry and its temperature exceeded its melting point, the coating would become molten. In the high-speed airflow during re-entry, the molten coating would likely be removed from the surface by the intense aerodynamic forces.
However, not every component of a re-entry vehicle will necessarily reach the same temperature. Where a heat protection system is present, temperatures on protected components can be significantly lower. In these circumstances, the electroless nickel coating may remain partially, or potentially fully, intact depending on the temperature reached, exposure time and nature of the exposure.
Temperature is only part of the equation
When considering electroless nickel for a high-temperature application, the maximum temperature should not be considered in isolation. Important factors include:
A coating exposed briefly to a high temperature may behave differently from one held continuously at the same temperature. Similarly, frictional heating can produce different conditions from heat transferred from a neighbouring component.
A useful guide, rather than a fixed limit
The temperature ranges above should be treated as a general guide rather than absolute operating limits.
Ultimately, the suitability of electroless nickel depends not only on the maximum temperature, but also on how that temperature is generated, how long the coating is exposed to it, the substrate involved and what the coating is required to achieve.
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