Preparation And Properties Of Cladding First Wall Tungsten Composite Coating

Plasma Facing Materials(PFMs)are fabricated into plasma-facing components such as First Wall,Divertor,and Limiter.They are required to withstand high surface heat load,good resistance to radiation damage,low neutron activation,etc,which are extremely important in thermonuclear fusion devices.The first wall material occupies the widest area in the plasma-facing component,has extremely high requirements for its preparation efficiency,in-situ repair convenience and cost-effectiveness.Tungsten has high melting point,high thermal conductivity,low vapor pressure,relatively low corrosion rate and low enthalpy retention,and has been considered to be the most promising plasma-oriented material.However,bulk tungsten has a high ductile-brittle transition temperature and is difficult to process.Therefore,it is more feasible to apply tungsten in PFMs by coating a layer of tungsten coating on the heat sink or structural material,especially the first wall of the cladding.Most of them are curved surfaces,and the steady-state thermal load to be subjected to is small(<1 MW/m~2).It is considered that a thick tungsten coating is applied to the first wall material on a low-activated steel base.However,due to the nearly four times difference in thermal expansion coefficient between tungsten and steel,the preparation of tungsten coating on steel base causes large thermal stress due to temperature change,which affects the bonding strength between the coating and the substrate,resulting in failure of material detachment and cracking.Therefore,in this study,a tungsten steel mixed transition layer was introduced between the tungsten coating and the steel base,forming a tungsten steel composite coating to improve the bonding strength and keep the structure more stable under the steady heat load.To date,the most common atmospheric plasma spray(APS)for the preparation of tungsten coatings has the following potential advantages:easy to large-area and non-planar shapes,high deposition rate,low cost and convenient in situ repair.However,atmospheric plasma spraying is not conducive to the preparation of mixed transition coatings with high requirements on oxygen content,porosity and bonding strength.Considering that the coating prepared by explosive spraying has the characteristics of high bonding strength,high density,and low introduction of oxygen impurities.This paper combines atmospheric plasma spraying and explosive spraying to prepare steel-based tungsten composite coating.Firstly,the tungsten composite coating suitable for the experimental preparation was designed by simulation calculation,and the tungsten-steel mixed powderswere designed by spray drying method.Secondly,the prepared tungsten-steel mixed powders were sprayed on the steel base by a D-Gun apparatus to form a tungsten-steel mixed transition layer.Finally,an APSdevice is used to spray tungsten powders on the tungsten-steel mixed transition layer to form a steel-based tungsten composite coating.By characterizing and analyzing the properties of the prepared coatings such as microstructure,density,oxygen content,hardness and bonding strength,the results show that:The average bond strength of the steel-based tungsten composite coating is58.7 Mpa,which is nearly 2.5 times higher than that of the steel-based APS-W coating.SEM observations show that the two interfaces at the tungsten composite coating have no voids and are closely combined.The tungsten steel in the transition layer is uniformly distributed and the overall density is close to the steel substrate.The tungsten steel transition layer has a porosity of only 1.9%and a pore size of less than70 nm.XRD and EDS results indicate that the oxidation in the tungsten steel transition layer is relatively low.At the same time,the preparation method was applied to the first-wall tungsten steel water-cooled parts of the cladding for preliminary thermal load test.The results show that the first-wall tungsten steel material remains basically stable under the steady heat load of about 1.5 MW/m~2.Therefore,the preparation method of the steel-based tungsten composite coating provided in this paper is convenient for preparing the tungsten steel composite coating material with high bonding strength,and has the potential to be applied to the first wall material of the plasma cladding component in the future thermonuclear fusion reactor.