TUNGSTEN AND TUNGSTEN ALLOY COATINGS ON CFC COMPOSITES FOR HIGH TEMPERATURE APPLICATION

摘要

Carbon-carbon composites possess many properties that make them attractive for applications in rotary anode x-ray tube products. In particular, the properties of high thermal conductivity, high thermal capacity, high strength, low density, and high thermal emissivity, suggest that these materials are well suited for x-ray target substrates. An x-ray target substrate is coated with a layer of X-ray emitting material such as Tungsten or Tungsten alloys. However, the challenge lies in the fact that the thermal mismatch between a CFC composite substrate and a focal track layer results in a high residual thermal stress in this layer which may result in its severe cracking and even delamination upon cool-down from the deposition process. Furthermore, temperatures of the focal track/substrate are in excess of 1700℃ during the x-ray tube operation. This may lead to further cracking and deterioration of the layer. While cracking due to the CTE mismatch is almost unavoidable, it is possible to reduce number of cracks and to control crack patters. Parameters such as CFC composite substrate surface texture and coating thickness/geometry were investigated in the effort to mitigate the formation of cracks within the target layer while maintaining its adhesion to the CFC substrate.