Study on Vacuum Plasma Spray Boron Carbide Coating

Boron carbide (B4C) is a kind of non-oxide ceramic with strong covalent bond. It has the features of high melting point, low density, high hardness, elastic modulus, etc. It can be used as anti-abrasion material and anti-nuclear radiation material. The B4C coating deposited by CVD method is dense and uniform, and the hardness is extremely high. However, the CVD method has disadvantages such as high substrate temperature and thin coating. The use of gaseous B203 reacts with high temperature (2000D carbon substrate) and can also form a B4C coating, but the purity of the coating is difficult to control. A sintering method is to apply the powdered boride and carbide to the substrate with an organic binder. On the surface, the coating is formed at a high temperature (2400*C), but the B4C coating prepared by this method tends to have uneven thickness and inconsistent surface morphology.

1 Contact Person: Zheng Xuebin (Shanghai Institute of Ceramics, Chinese Academy of Sciences, 12000 Shanghai Dingxi Road, Shanghai 200050) The plasma spraying method has the characteristics of high jet temperature, controllable coating thickness, high bonding strength, and easy operation. Used as b4c coating preparation 14. This article uses vacuum plasma spraying method to deposit B4c coating on the surface of stainless steel, and the structure, composition, hardness and bonding strength of the coating were observed and measured.

2. Experimental method A vacuum (VPS) plasma spraying device (SulzerMetco AG) was used to deposit a B4C coating on a stainless steel substrate.

For comparison, a B4C coating was prepared using an atmospheric (APS) plasma spray method. In VPS and APS spraying, argon and hydrogen were used as plasma spraying gas.

The cross-sectional morphology of the B4C coating was observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) was used to examine the phase composition of the coating. The bond strength of the coating to the substrate was measured by the ASTM C-633 method. The Vickers hardness (Hv0.2) of the coating was measured with a HX-1000 microhardness tester.

3. Results and Discussion The vacuum plasma spraying method can be used to prepare B4C coatings with a thickness of 200*500nm. It was determined that the bond strength of the coating can reach 40 MPa and the hardness (Hv0.2) is 14.5 GPa. However, in atmospheric spraying, it is difficult to deposit a B4C coating with a thickness exceeding 3 nm, and the coating is loose and has more powdery particles. Can be scraped off with a blade.

The XRD pattern of B4C original powder and coating is shown in . As can be seen from the figure, the main ingredient of the powdered raw material is the B4C phase, and there is also a very small amount of free graphite phase. Compared with the XRD pattern of the original powder, the carbon peak disappeared in the atmospheric plasma sprayed B4C coating, and a distinct B203 peak appeared. This indicates that the powder reacted with oxygen in the air during spraying.

After the carbon reaction generates C02 escape. Oxidation of the generated B203 in atmospheric plasma spraying can lead to loose coating structures and reduced strength. (c) shows that no obvious B203 phase appeared in the vacuum plasma sprayed B4C coating, indicating that B4C oxidation can be effectively avoided in vacuum spray coating. When vacuum plasma spraying technology is used for spraying, the vacuum chamber is a certain pressure of inert gas Ar gas. It has been reported that the physical and chemical properties of boron carbide powders in the atmosphere and Ar atmosphere have obvious differences 51. Thermogravimetric analysis results show that boron carbide in the atmospheric atmosphere 55 (TC began to increase weight, while in the Ar atmosphere Next, until 1400X: No significant weight gain.Argon atmosphere can well prevent the oxidation of boron carbide powder at high temperatures.

The deposition efficiency of the vacuum plasma sprayed B4C coating at different spray powers is shown. It can be seen from the figure that the deposition efficiency of the coating exceeds 50%. With the increasing of the spraying power, the deposition efficiency also increases, and the deposition efficiency at the power of 45KW is 72%. B4C melting point 篼 2450*C The higher spraying power helps to improve the melting state of B4C powder, which facilitates its deposition on the substrate.

A photomicrograph of the cross-section of a vacuum plasma sprayed B4C coating prepared using the original powder having an average particle size of 20 pm and 12 pm, respectively, is shown. As can be seen from the figure, the bond between the coating and the substrate is good. At the same time, it can be seen that the coating prepared with a powder having a powder particle size of 12 nm is denser. The smaller particle size powder melts better in the plasma flame, resulting in a more dense coating.

(b) Atmospheric Plasma Spraying BW Coating (c) Vacuum Plasma Spraying BX Coating X Boron Carbide Powder Raw Materials and Coatings Different XRD Patterns for Spray Power Pneumatic Plasma Spraying BW Spreading Deposition Efficiency (b) Coatings Prepared at 12pm BX Powders Layers of different particle size powder prepared by vacuum plasma spray BX coating cross-section morphology 4, the conclusion of the use of vacuum plasma spraying method, to avoid the oxidation phenomenon in atmospheric spraying, can be successfully prepared thickness 200-500pm B4C coating. The higher power and finer powder material can improve the melting of B4C during the spraying process, contributing to the deposition of the coating. The bond strength and hardness (Hv0.2) of the prepared B4C coating are 40 MPa and 145 GPa, respectively. 5. Acknowledgements: This article was funded by the National High Technology Research and Development (863) Program (2003AA305950).

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