Study On Structure Characteristics And Tribological Propreties Of Trivalent Chromium Coatings

In this thesis, the structure and tribological properties of the Cr-C alloy coating andcopper doped Cr-C alloy coating were systematically evaluated. First, the influence of pulsecurrent (PC) and direct current (DC) on the coatings' structure and friction properties wereinvestigated. Then, two different kinds of Cr-C Coatings were heated at different temperature,and comparatively studied the structure and friction properties; The effects of Cu ions on thetribological properties of the Cr-C coating were also investigated. The primary conclusionsare listed as follows:1. The current form plays a significant role on the structure and tribological properties ofthe trivalent chromium coatings. The trivalent chromium coating which electrodeposited byPC has a glossy surface structure. Furthermore, the hardness and tribological properties of thecoatings which electrodeposited by PC are superior to the coatings electrodeposited by DC.2. On the one hand, because of the change of heat treatment temperature, the two kindsof Cr-C alloy Coatings presents the same phenomenon, that is microcrystalline to crystallinetransition. So the hardness of the two coatings will increase with the increase of heattreatment temperature. On the other hand, when heated to a certain temperature, the surfaceand internal of the coatings will present some new hard phase, such as Cr2O3, Cr7C3andCr23C6, these new hard phases to some extent will promote the hardness of the coatings. Innormal circumstances, the tribological properties of the metal coating are enhanced with theincrease of the hardness of the coating. But the Cr-C coating is not the same, regardless of thecoating which electrodeposited by PC or DC, the as-deposited coating has a relatively goodtribological property. These hardness almost comparable to the Cr-C alloy coating whichheated at maximum temperature in this article. Furthermore, because the existence of carbon,the carbon may exist in the coating with form of graphite and play a role of solid lubrication.Therefore, the as-deposited coating has the minimum friction coefficient compared with theCr-C alloy coating which heated at a high temperature.3. The trivalent thick chromium coating mainly existed as an amorphous phase structure.During friction test, the different friction conditions may lead different local temperature on the coating surface. Therefore, the phase structure of the coating which seated at the wearareas also changed at a different degrees and resulted a difference in wear performance.4. Due to the incorporation of the metal Cu in the Cr-C-Cu alloy coating, the Cu-C-Cualloy coating shows a different performance in its structure and properties. The study foundthat the as-deposited Cr-C-Cu alloy coating show excellent tribological performance thanas-deposited Cr-C alloy coatings. But the appearance and color of the Cr-C-Cu alloy coatingsare not as smooth and bright as the Cr-C alloy coatings, because of some mist and blanch areapresented on the surface of the coatings. However, due to the incorporation of Cu, the latticeof the coatings distorted and then increased the hardness of the coatings. Coupled with thesolid lubrication effect of the copper, so that the Cr-C-Cu alloy coating can play a role in longtime, low load and low friction frequency.