Study On Phase Composition,Microstructure And Growth Mechanism Of CVD Tungsten Carbide Coating

In this paper,tungsten carbide coatings are prepared on the substrate of copper by chemical vapor deposition(CVD),and dimethyl ether(DME)is used as the carbon source,tungsten hexafluoride(WF₆)as the tungsten source,and hydrogen(H₂)as the reducing gas at atmospheric pressure.Shapes of substrates include disc and hexagonal prism.The phase composition,microstructure and microhardness of the coatings are characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and nanoindenter(DSI).The thermodynamics and growth kinetics of the coatings were studied,and the coating growth model was established.Details of the study are as follows:(1)The partial pressure of dimethyl ether(P_DME)has a significant effect on the phase composition,microstructure and growth rate of tungsten carbide coatings.With the increase of P_DME,the C content of the substrate surface reaction increased,and the excess C blocked the surface active sites,resulting in the decrease of the number of free active sites and the decrease of the coating growth rate.The phase and microstructure also changed.When the deposition temperature was 600?,the coating phase was W₃C and W when 0<P_DME<0.038×10⁵Pa,and had a lamellar structure.When 0.038×10⁵Pa<P_DME<0.076×10⁵Pa,the coating phases were W₂C and W with a lamellar structure.The coating phase were W₂C and W when 0.076×10⁵Pa<P_DME<0.163×10⁵Pa,and the lamellar structure coexists with the fibrous tissue.In addition,coating growth was inhibited at 0.163×10⁵Pa<P_DME;(2)When the coating phase is W₂C and W,the mechanical properties are better.P_DMEaffects the crystallographic orientation of W₂C and W in the coating and thus affects the macro performance of the coating.W₂C and W in the tungsten carbide coating have a semi-coherent relationship with the coating delaminating When 0.038×10⁵Pa<P_DME<0.076×10⁵Pa.Howere fine W grains are distributed in the W₂C grains and there are also the structure that W₂C grains of 3-5 nm are inlaid in the amorphous when 0.076×10⁵Pa<P_DME<0.163×10⁵Pa,thereby improving the plasticity of the coating and reducing the internal stress,and the coating has no cracking and delamination.(3)With the continuous growth of the coating,in the low P_DME,the relative content of W₃C is gradually decreased and the relative content of W is increased;in the high P_DME,the relative content of W₂C is gradually increased and the relative content of W is decreased.The reason is the consumption of C when C is depleted and the accumulation of C when C is rich.Moreover,when the P_DME continues to increase to a certain value,the preferred orientation of W changes from(100)to(110) during coating growth.(4)Combining with the above studies and based on the growth kinetics model of tungsten by chemical vapor deposition,taking into account the adsorption of DME,the relationship between the deposition rate of tunshten carbide coating prepared by atmospheric chemical vapor deposition and PDME is as follows:(?)(5)When P_DME is 0.032×10⁵Pa and 0.044×10⁵Pa,the coating phases are W₃C+W and W₂C+W,respectively.The W content and deposition rate of the coating on top surface of substrate are lower than that of the side surface.When P_DME is 0.090×10⁵Pa,the coating phase is W₂C+W,and the W content and deposition rate of the coating on top surface are higher than that of the side.This phenomenon confirms the influence of P_DME on the growth rate of tungsten carbide coating in the above research.