Surface topography of CVD-W coatings on graphite substrate with a PVD-Si intermediate layer after thermal fatigue testing

• Surface topography of CVD-W coatings before and after thermal fatigue testing was characterized and investigated.
• Electron beams are only absorbed by a thin layer on coating surface owing to excellent thermal conductivity.
• No visible failure was found because the surface with fine grains can suppress the crack propagation.

This work deals with the effects of thermal fatigue testing on the surface topography of W coatings on graphite substrate with Si intermediate layers, where W coatings were deposited by chemical vapor deposition (CVD-W coatings) and Si intermediate layers were prepared by physical vapor deposition (PVD-Si layers). The surface topography of samples both as-deposited and after thermal fatigue testing were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Comparative studies show that CVD-W coatings have so excellent thermal conductivity that electronic beams are only absorbed by a thin layer on the coating surface. The absorbed thermal loads give rise to the melting of the pyramid-like W grain tips and the formation of shallow melting pools. Meanwhile, the size of the recrystallized grains on the surface is much smaller than those of the initial grains after cooling. Since the fine recrystallized grains can undoubtedly suppress the crack propagation, no visible failure was found in sample after thermal fatigue testing.