HiPIMS vs DCMS technology to produce tungsten coatings for fusion applications

Carbon-based materials have been used in thermonuclear fusion devices for a long period. The major drawback of a plasma-facing wall with carbon armour, such as graphite or Carbon Fiber Composite (CFC), is their chemical erosion and dust formation under hydrogen bombardment. Moreover the re-deposition of carbon layers containing tritium represents a serious safety concern. The use of W coatings deposited on carbon based materials can allow to overcome these problems. The employment of Tungsten as an armour material is convenient because of its high melting temperature, good thermal conductivity, low tritium retention, low sputtering rate. W films were deposited by various methods on carbon substrates: Vacuum Plasma Spray (VPS), Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) techniques. However, because of the extreme working conditions (neutron bombardement, thermal cyclic fatigue, carbon diffusion, plasma erosion) at present none of the proposed deposition techniques is completely successful. Among the different technologies, HiPIMS seems to be the most promising because it permits to deposit high quality and ultra-dense coatings with highly controlled microstructure and residual stress. This work gives a comparison between the properties of several W coatings obtained by the conventional Direct Current Magnetron Sputtering (DCMS) and High Power Impulse Magnetron Sputtering (HiPIMS). The used deposition systems have been custom designed and built. In particular, the used HiPIMS generator is the Huettinger TruPlasma Highpulse one. In front of the 4 inches HiPIMS cathode a single rotating tree was mounted providing different target substrate distance (TSD) position between 50 and 200 mm. The substrate can be heated up to 800°C and biased up to 1200V. The W coatings have been grown on graphite, metal sheets and Si substrates changing progressively the sputtering parameters to optimize the process to obtain the desired thermodynamically stable body-centered-cubic form of metallic tungsten (alfa-W). The tungsten coating final phase depends on substrate bias and temperature, deposition pressure, residual gas species. Multilayered films were produced using Mo as a glue layer because tungsten and carbon can form metallic brittle carbide phases. Furthermore Mo compensates possible thermal expansion mismatch. The crystalline structure and orientation of the obtained film is analyzed by X-ray diffraction. The crystallographic parameters are evaluated using MAUD (Material Analysis Using Diffraction). Their morphology and thickness were determined by electron scanning microscopy. Adhesion properties were evaluated by scratch tests and thermal shock tests. Hardness was estimated by Vickers indentation.