We performed investigations to determine the lifetime of tungsten coatings on carbon under high heat flux with respect to tungsten carbide formation. Tungsten coatings with a molybdenum interlayer were deposited by combined magnetron sputtering and ion implantation. These coatings were heat treated in a vacuum furnace. Subsequently the fraction of the coating thickness which had converted into carbide, was determined by focussed ion beam preparation and electron microscopy. This thickness was found to extend from 10m to 18m for heat treatments between 2 and 20 hours at 1350°C. Such coatings were then exposed to high heat flux pulses of 16.5MW/m2 with pulse durations of 1.5s. A clear threshold behaviour was found: While after 2 hours of heat treatment no coating failure was found, the delaminated surface fraction was up to 5% after treatment for 5 hours and up to 25% after treatment for 20 hours at 1350°C.