Study On High Temperature Resistance And Interface First-principles Calculation Of Rare Earth Modified WC-Co Coatings

Nowadays,WC/Co ceramic coatings prepared by high velocity oxy fuel?HVOF?spraying possess high hardness,high bonding strength and low porosity.Due to its excellent wear and corrosion resistance,it is widely used in high-temperature gas turbines,hot roller shafts and other engineering components as protective or repair coatings.The complex conditions such as high temperature and high speed requiregreater demands on service life and reliability of ceramic coatings.The introduction of rare earth elements?RE?to improve the microstructure of the coating and strengthen its high temperature performance,thereby expanding the application of the coating is of great significance in engineering practices.In this paper,WC-12Co coatings with different content of CeO₂ were prepared by high velocity oxy fuel?HVOF?spraying on Q235 steel.The microstructure,phase constitution,porosity and hardness of the coatings were employed to investigate the effects of rare earth on the WC-12Co coating properties.The results showed that the addition of rare earth can inhibit WC grain growth and decarburization,and increase the uniformity of dispersion of WC particles in Co phase.The coating with 1.0wt.%CeO₂ additive presented best of all,whose microhardness increased by 10%,porosity decrease by 59.3%,and the critical load of failure was increased from 95N to 147N.Therefore,considering the one with 1.0wt.%CeO₂ as the modified coating.,The static high temperature cyclic oxidation test and the pin-disc high temperature wear test were carried out to discuss effects of RE on high temperature resistance of coatings.And the conclusions were obtained as follows:?1?The oxidation products of WC-12Co coatings were mainly composed of WO₃ and CoWO₄ in the temperature range of 450°C⁷50°C.The oxidation kinetics of the coating followed the parabolic law within temperature range of 450°C⁶50°C,while the oxidation curve followed straight line.The coating cannot be used as a wear resistant protective coating above 650°C.?2?The addition of rare earth effectively reduced the oxide rate of WC-12Co coating,and promoted the formation of a dense oxide film.Compared with the coatings without CeO₂ additive,the modified coatings showed better resistance to high temperature oxidation.?3?High temperature friction and wear tests for the two coatings at 450°C,550°C,and 650°C showed that the wear mechanism at 450°C was abrasive wear caused by Co phase ploughing.Above 550°C,the wear mechanism showed a combination of abrasive wear,oxidation and adhesive wear.?4?Unmodified coatings rapidly lost its ability to resist wear above 550°C,while the modified coatings still maintained good carrying capacity at 650°C.The main reason is that RE can slowed the oxidation process and effectively reduced the loss of strong component elements of coatings.On the other hand,it can promote the formation of continuous and dense oxide film based on CoWO₄,then reduced friction and increased wear-resistance.In addition,the first-principles method was employed to investigate the strengthening and toughening mechanism of RE on improving the interfacial properties of coatings.The interface properties of the ideal WC/Fe interface,WC/Co interface and doping of rare earth Ce atoms are calculated.The interfacial atom,electronic structures and interfacial bonding/elastic energy were systematicly analysized.The calculated results supported that when the interface was C-terminated,the interface strength was derived from the strong polar covalent C-Fe bonds.When the interface was W-terminated,the adhesion was provided by weak ionic bonds and metallic bonds formed between W-Fe or W-Co.Moreover,the rare-earth Ce atoms can dissolve into the interface by replacing the metal atom,which affected the charge transfer of the interfacial atoms,improving the interface bonding strength and toughness.When Ce atom displaced W atom of the WC/Fe interface,the maximum interfacial binding energy increased from 8.78 J/m² to 12.44 J/m².When Ce atom displaced Co atom of the WC/Co interface,the maximum interfacial binding energy increased from 9.52 J/m² increased to 11.39 J/m².