keronite thermal barrier, thermal cycling, stability

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Thermal Protection

THERMAL PROPERTIES

Thermal insulation
The thermal conductivity of Keronite is two orders of magnitude lower than that of aluminium or magnesium. For Keronite on aluminium, the thermal conductivity is approximately 1.6 W/m/K (around two orders of magnitude lower than that of the aluminium substrate itself). In the case of magnesium it is even lower at 0.8 W/m/K. A Keronite surface conversion layer can therefore provide a degree of thermal protection to the surfaces of aluminium or magnesium parts subjected to high heat fluxes.

Thermal stability
Keronite on aluminium us proven to be thermally stable up to 1000 °C. It can even withstand exposure to temperatures as high as 2000 °C for short periods. Keronite on magnesium will tolerate short exposures to temperatures around 1000 °C.

Thermal cycling
Because they are created by a conversion process, Keronite layers have excellent adhesion to the substrate alloy. In addition to this interfacial toughness, they layers possess a low modulus which renders them more strain tolerant: for a given strain, they will experience lower stresses than a stiffer coating. This makes them well suited to applications where components are exposed to extremes of temperature or repeated thermal cycling.

Independent tests have indicated that Keronite on aluminium can improve the resistance to shock considerably. A 60 µm layer of Keronite on 6082 aluminium alloy was subjected to alternate immersion in boiling water and liquid nitrogen. The sample remained immersed in each bath for 30 seconds and the operation was repeated 50 times. There was no sign of delamination or cracking of the Keronite layer and the ceramic layer remained well bonded to the aluminium substrate.

In the case of both diesel and gasoline engines, a layer of Keronite ceramic on the piston crown can provide a thermal barrier, allowing higher operating temperatures. This is critical in terms of engine performance and since aluminium begins to lose its mechanical properties at temperatures of 250-300 °C, every degree is important. Early tests demonstrated that just 40 µm of Keronite can support an additional thermal gradient of as much as 30 °C.

A further benefit in the case of gasoline engines is the fact that the piston crown is protected against the powerful impact of spontaneous detonation and the engine can therefore be run closer to the detonation region of its operating range. The engine will either achieve higher specific power or there will be significant improvements in fuel economy and the corresponding reduction in emissions, depending upon the way in which the engine is tuned: for Formula 1 racing, or for the family saloon.