Preparationand Performance Of New Thick Tungsten Composite Coating

The key problem of nuclear fusion energy application is materials. Tungsten is one of the most promising PFM with the advantages of high melting point, high sputtering threshold, high thermal conductivity and low storage with tritium. Tungsten is a rare resource, and its high melting point, hard, brittle and other physical characteristics make it difficult to be deformed, so tungsten is not suitable to be used as structural material. However, it can be used as outer wall material to connect to the heat sink structural material with good thermal conductivity. Nevertheless, the physical properties of tungsten and those of the heat sink material are hugely different, especially the thermal expansion coefficient, which might cause high thermal stresses and lead to failures during the process of preparation or service, so the design and preparation of thick tungsten coating with great performance is important and urgent.In this paper, air powered spray technology and metal organic chemical vapor deposition technology had been used successively to prepare tungsten coating to achieve an effective connection between the tungsten and the heat sink material (CuCrZr alloy). In order to compromise the defects and shortcomings in these two coating technologies, these two methods were combined to prepare the thick tungsten coating. The structure and properties of the coating were also studied. The main contents and conclusions are as follows:(1) W-Ni-Fe coatings with different components were prepared on CuCrZr alloys by air power spray technique and the thickness had been raised notably. With the increasing of the percent of tungsten powder, coating thickness and hardness both increased rapidly and porosity increased, while thermal shock resistance declined. Failures of coated were begging from the tungsten concentration area at the edge of the coating. In terms of 75+93-gradient W coating, the thickness was close to 2mm, porosity2.44% and hardness 481.4Hv.(2) The author has made several improvements to the metal organic chemical vapor deposition equipment which was independently developed by my group. In this paper, we prepared a pure nanostructure tungsten coating on CuCrZr alloy by this equipment using tungsten hexacarbonyl as gaseous precursor. The author optimized the process parameters and found that when the deposition temperature was 550℃, the deposition distance was 25mm, the carrier gas was hydrogen and the flow rate of carrier gas was 80ml/min, the coating prepared had a smooth surface, a uniform and dense microstructure without any cracks and holes. The coating was body-centered cubic crystal structure and had only a-W phase, with a (110) preferred orientation, and the thickness of the coating was 4.5μm, hardness was 3.8GPa, modulus of elasticity was 105.3GPa, the deposition rate was 26.2 x 10-3 g·cm-2·h-1.(3) Bonding mechanism of cold gas dynamic spray method (CGDSM) coating and formation mechanism of metal-organic chemical vapor deposition (MOCVD) coating were both studied. The author found that CGDSM-W-Ni-Fe coating had a lamellar structure, and the bond of the coating and substrate was mainly mechanical bond, between tungsten and niekel-iron existed diffusion layer, part of the particle collision area existed local metallurgical bond; MOCVD-W coating conformed to the cohesion center theory and forming process existed the process of adsorption, chemical reactions, condensation forming at the center, nucleation and growth, growth and combination of the island, and so on.(4) And then combined with MOCVD technique, the surface flatness and pits of tungsten coating had been improved, the hardness increased to 643.7Hv, bond strength and thermal shock resistance of the coating did not decline. The combined strength of the coating was greater than 35MPa, and it can undergo 325 times of thermal shock cycle at 500℃,55 times of thermal shock cycles at 800℃.CGDSM technology combined with MOCVD technology can be used to preparation a new thick tungsten composite coating with low porosity, high hardness, high bond strength and good thermal shock resistance.