| 其他摘要 | Detonation gun (D-gun) spraying is often used to fabricate high quality wear resistant coatings as an advanced thermal spraying technology. The high spraying velocity and short acting time between the feedstock powder and spraying flow greatly reduce oxidation and decomposition of powder materials, and well keep powder compositon consistency with that of the resulting coating. D-gun sprayed WC-Co coatings are widely used in many industrial fields due to the high hardness and adhesion as well as excellent wear resistance. The hard WC particles in coating play a major role in wear resistance and antiwear, and binder Co provide toughness and support. Cobalt will be melted during high-temperature spraying due to the lower melting point, which enhances the adhesion and cohesion, thus impact toughness and wear resistance of the coating are improved.
Mechanical and tribological properties of D-gun sprayed coatings are mainly determined by their microstructures, whereas coating defects usually comprise of various pores, microcracks, boundaries of splat and unmelted particles, etc. which strongly depend on spraying parameters and powder materials. Based on the orthogonal experiment design method and took 300μm thick WC-Co coating as an example, the effects of oxygen-fuel ratio and spraying distance on physical and mechanical properties of D-gun sprayed coatings were respectively investigated, and accordingly oxygen-fuel ratio of 1.06 and spraying distance of 110mm were identified as the optimal spraying parameters. The optimized WC-Co coatings exhibited the uniform dense microstructures, high hardness and elastic modulus, excellent wear resistance and near-isotropic behaviors, which has been verified by the comparisons of mechanical and wear properties between cross section and surface. The following relations were obtained: lower the porosity, smoother the coating, higher the microhardness and elastic modulus, better the adhesion, and better the wear resistance.
During D-gun spraying process, the spraying particles showed an undesirable dispersion, when they were spurted out from the nozzle of barrel at high velocity and pressure. Through analyzing the dispersion causes, an effective separation device was successfully designed and constructed. Because the dispersed particles in periphery of spraying flame first exchange heat with ambient atmosphere and rapidly solidify, the working principle of separation device is intercepting these dispersed and solidified particles by separation liner plate, and only allowing the melted particles in the designated region which is coaxial and same size as the inner diameter of barrel to pass and form coating. After using the separation device, the surface roughness Sa, Ra, the porosity, and wear rate of the D-gun sprayed WC–Co coatings respectively decreased by 77%, 41%, 40%, and 20%, and the microhardness HV, HKcs⊥, HKcs∥, the elastic modulus Ecs⊥, Ecs∥, and the adhesive strength σmax increased by 12%, 13%, 13%, 32%, 36%, and 16%, respectively. At the same time, there is an inevitable disadvantage for using the separation device, i.e., the relatively lower utility rate of the feedstock powder, because small amount of dispersed particles will be intercepted and can not form coating. Therefore, the separation device is suitable to be applied in occasions of high-performance requirements where increased costs are acceptable.
The residual stresses generated during D-Gun spraying process act as pre-existing stresses and can give rise to deformation of coated specimens and spallation or cracking of the coating. In addition, various mechanical properties, such as hardness, elastic modulus and adhesion strength, etc. are strongly influenced by the nature and magnitude of the residual stress. Based on fundamental knowledge of material mechanics and some basic elastic assumptions, a new calculation formula was developed to estimate the residual stresses of thick coatings. Combining with accurate curvature radius results, the residual stresses of D-Gun sprayed WC-Co coatings with different thickness were calculated. Furthermore, the effects of residual stress on mechanical properties of the coatings were also investigated, and some valuable conclusions were drawn. Under the fixed spraying conditions, the residual stress of D-Gun sprayed WC-Co coating varied gradually from a high tensile stress to a compressive stress with increasing the coating thickness. Meanwhile, the coating was in an approximately stress-free state when its thickness reached around 365 μm. Compressive residual stress could significantly improve the coating elastic modulus, microhardness and adhesion, whereas tensile residual stress impaired the coating properties to some extent. In a certain range, the larger compressive residual stress was, the better the coating properties.
Finally, the single structured and multilayer composite self-lubricating coatings were prepared by D-gun spraying technology. The phase constituents of the coatings, element distribution and valence state, porosity and surface roughness were analyzed in detail. The effects of MoS2-Ni content in the feedstock powder on the microstructures, mechanical and tribological properties of the resulting single structured coatings were investigated systematically, and to determine the optimum content. Although a little of MoS2 were oxidized and decomposed during the high-temperature spraying process, the friction reducing effects of the coatings were not significantly affected. To analyze the friction reducing mechanism of multilayer composite self-lubricating coating whose tribological properties in different stages of wear were tested and compared with the optimized single structured self-lubricating coating. |
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