No coating is equally suited for high temperature applications. And usually, there is a significant difference between failure in a test and failure in application.
Thermal Spray for Very High Temperatures
High temperature parts are typically coated using thermal spray. Very high temperature parts are typically coated using either plasma spray or HVOF (high velocity oxy-fuel) to produce coatings such as ceramic or Tungsten carbide which can operate continuously at temperatures up to around 540°C. Guidance on material selection for engineering applications is provided by the Engineering Council who state that service conditions should be used to specify coatings at the design and specification stage.
Hard Anodising on Aluminium
Hard anodising produces a very hard surface made of dense aluminium oxide. It is fixed to the surface of the part. The hard anodising process has an upper temperature limit. Parts that are to be used at sustained temperatures above 100 degrees C will start to degrade the oxide layer. Temperatures approaching 200 degrees C will cause the anodising to soften. This type of coating is generally best suited for use on components that are exposed to moderate temperatures. Parts to be used in engine bays or near heat exchangers are generally not suitable for hard anodising.
Electroless Nickel for Mid-Range Heat
High-phosphorus electroless nickel coatings have a continuous hardness at temperatures up to about 180°C at which the deposit starts to crystallize. Typical applications for such components are in hydraulic parts, such as pumps and valve bodies, which are usually in service at temperatures between 100°C and 150°C.
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Zinc Nickel and Its Limits
Zinc nickel plating does provide very good corrosion resistance but it is not suitable for high temperature applications. The zinc-nickel alloy begins to break down at temperatures around 120°C and this will result in the coating losing its corrosion protection. Zinc nickel plating is a good option for fasteners and other components that are used in ambient or slightly warm temperatures. It is not suitable for use in engine bays or in exhaust-adjacent locations.
Match the Coating to the Temperature
The peak service temperature that a part will experience must be determined in order to select an appropriate coating. A coating rated for 200°C operation that is exposed to temperatures of 220°C for prolonged periods will fail prematurely. Selecting a coating that is rated to operate at a temperature some what higher than the parts working temperature will provide the necessary safety margin and extend the life of the part.