TY - BOOK

T1 - Surface Damage of Rotating Anodes and the Impact on the X-ray Emission

AU - Siller, Maximilian

N1 - embargoed until 01-08-2026

PY - 2021

Y1 - 2021

N2 - In medical X-ray tubes used for diagnostic imaging, X-rays are generated in the focal spot of the anode by the deceleration of impinging electrons. These electrons will deposit most of their carried energy as heat in the focal spot which can lead to extremely high temperatures. High performance rotating anodes in modern X-ray tubes reach temperatures of more than 1500 °C with additional microsecond-long temperature rises of another 1000 °C – 1500 °C.Due to their excellent thermophysical properties and suitability for generating X-rays, tungsten-rhenium alloys have been used in the focal spot of rotating anodes for many years. The initiated thermal shock and thermal fatigue in the focal spot due to the repeated thermal loading leads to the degradation of the anode surface trough the formation of cracks and partial melting.These surface modifications change the characteristics of the emitted X-rays and jeopardize the operational stability of the X-ray tube. Consequently, the X-ray output of current high performance X-ray tubes is partly limited to the thermal loadability of the focal spot.The main goal of this work was to investigate the impact of the rotating anode's typical thermal cycling on the surface degradation of tungsten-rhenium alloys composed of different microstructures. Further, other adaptations like surface modification and small chemical variations were investigated. In addition, the impact of the initiated surface damage in relation to the changing X-ray output was revealed.Microstructural and fractographic investigations by optical light-, laser- and electron- microscopy formed the broad foundation of this task. Finite element modeling enabled to predict temperatures, thermal stresses and strains of the different testing methods and allowed for comparison of the thermal situations of rotating anodes in operation. The material tests included testing by electron beam exposure in a conventional electron beam welding machine and tests of stationary anodes. For the stationary anode tests a novel test system was designed and put into operation. This enabled testing materials comparable to rotating anodes with a high reproducibility and throughput.Tungsten-rhenium alloys with columnar grain microstructure and synthetic surface modifications showed significantly improved thermal fatigue behavior. The positive effect was attributed to the early formation of oriented and stable crack networks which prevented further surface deformation and erosion due to relaxation of the surface.For materials with an orientated microstructure, recrystallization and the related vanishing of the initial microstructure led to a loss of these beneficial effects. However, when cracks where initiated before full recrystallization, the detrimental effects of recrystallization were reduced. The mechanisms of recrystallization were determined to be dependent on the depth of thermal cycling which was different between the used testing techniques. Additionally, the tested samples showed dynamic recrystallization at the surface initiated by the cyclic plastic deformation during repeated thermal loading.The surface morphology in the focal spot was correlated with the measured X-ray output and showed a directly proportional geometric relation when the measured focal spot intensity distribution was taken into account. The formation of local melting was found to be especially detrimental when aiming for high X-ray output. This further emphasizes the crucial factor of crack propagation below the surface which was found to be the main contributor towards local overheating and melting. It was also possible to point out the importance of the alignment of synthetic and orientated surface modifications towards the detector.This work manages to link the thermal loading, the material response and the final reduction of X-ray output of modern high-performance rotating anodes to each other. It shows the driving factors of the initiated surface damage and proposes ways how the current application limits can be overcome.

AB - In medical X-ray tubes used for diagnostic imaging, X-rays are generated in the focal spot of the anode by the deceleration of impinging electrons. These electrons will deposit most of their carried energy as heat in the focal spot which can lead to extremely high temperatures. High performance rotating anodes in modern X-ray tubes reach temperatures of more than 1500 °C with additional microsecond-long temperature rises of another 1000 °C – 1500 °C.Due to their excellent thermophysical properties and suitability for generating X-rays, tungsten-rhenium alloys have been used in the focal spot of rotating anodes for many years. The initiated thermal shock and thermal fatigue in the focal spot due to the repeated thermal loading leads to the degradation of the anode surface trough the formation of cracks and partial melting.These surface modifications change the characteristics of the emitted X-rays and jeopardize the operational stability of the X-ray tube. Consequently, the X-ray output of current high performance X-ray tubes is partly limited to the thermal loadability of the focal spot.The main goal of this work was to investigate the impact of the rotating anode's typical thermal cycling on the surface degradation of tungsten-rhenium alloys composed of different microstructures. Further, other adaptations like surface modification and small chemical variations were investigated. In addition, the impact of the initiated surface damage in relation to the changing X-ray output was revealed.Microstructural and fractographic investigations by optical light-, laser- and electron- microscopy formed the broad foundation of this task. Finite element modeling enabled to predict temperatures, thermal stresses and strains of the different testing methods and allowed for comparison of the thermal situations of rotating anodes in operation. The material tests included testing by electron beam exposure in a conventional electron beam welding machine and tests of stationary anodes. For the stationary anode tests a novel test system was designed and put into operation. This enabled testing materials comparable to rotating anodes with a high reproducibility and throughput.Tungsten-rhenium alloys with columnar grain microstructure and synthetic surface modifications showed significantly improved thermal fatigue behavior. The positive effect was attributed to the early formation of oriented and stable crack networks which prevented further surface deformation and erosion due to relaxation of the surface.For materials with an orientated microstructure, recrystallization and the related vanishing of the initial microstructure led to a loss of these beneficial effects. However, when cracks where initiated before full recrystallization, the detrimental effects of recrystallization were reduced. The mechanisms of recrystallization were determined to be dependent on the depth of thermal cycling which was different between the used testing techniques. Additionally, the tested samples showed dynamic recrystallization at the surface initiated by the cyclic plastic deformation during repeated thermal loading.The surface morphology in the focal spot was correlated with the measured X-ray output and showed a directly proportional geometric relation when the measured focal spot intensity distribution was taken into account. The formation of local melting was found to be especially detrimental when aiming for high X-ray output. This further emphasizes the crucial factor of crack propagation below the surface which was found to be the main contributor towards local overheating and melting. It was also possible to point out the importance of the alignment of synthetic and orientated surface modifications towards the detector.This work manages to link the thermal loading, the material response and the final reduction of X-ray output of modern high-performance rotating anodes to each other. It shows the driving factors of the initiated surface damage and proposes ways how the current application limits can be overcome.

KW - X-ray tube

KW - rotating anode

KW - tungsten rhenium

KW - surface roughening

KW - R�ntgenr�hre

KW - Drehanode

KW - Wolfram-Rhenium

KW - Oberfl�chenaufrauung

M3 - Doctoral Thesis

ER -