Tribological Characteristics Of Plasma Sprayed MoSi₂-based Composite Coatings

MoSi₂ has been extensively researched due to its high melted point,good high-temperature oxidation resistance,rather low density and excellent electrical conductivity.It has been widely used as heating elements operating at high temperature.Its main application is oxidation resistant coatings on C/C,graphite,Mo various substrates.Furthermore,MoSi₂ possessed high elastic modulus and hardness,which is particularly promising for service in harsh abrasive environments at room and elevated temperatures.However,there are few studies on the wear resistance of MoSi₂ coating.Plasma spray technology is characterized by high temperature plasma jet,providing the deposited coatings with a compact microstructure and controllable thickness.In addition,the fabrication process is stable and controllable.Plasma spray has been widely used for the preparation of wear-resistance coatings.In this thesis,atmospheric plasma spray?APS?was applied to fabricate MoSi₂ coating and its wear resistance behaviors slid against WC-Co and 440-C stainless steel were evaluated.The addition of Ni,ZrO₂ and TiN on the microstructure and tribological of MoSi₂ have been investigated.The main results obtained were described as follows:?1?MoSi₂ and MoSi₂-Ni coatings were fabricated by atmospheric plasma spray technology.The addition of Ni could reduce the porosity and surface roughness,enhance the bond strength and toughness.However,the hardness of the coating decreased.The wear test carried out under5-50 loads with the velocity of 0.5m/s and 900 s sliding time.The wear rates of coatings exhibited growing tendency with two counterparts under load increased.The MoSi₂-Ni coating behaved better wear resistance than MoSi₂ coating when sliding against WC-Co.However,MoSi₂ coating showed better wear resistance sliding with stainless steel in comparison with MoSi₂-Ni coating.When slid WC-Co,The dominant wear mechanism of MoSi₂ and MoSi₂-Ni coatings were oxidation,adhesive wear and plastic deformation,oxidation,respectively.When slid stainless steel,the main wear mechanism of MoSi₂ composite coatings included abrasive,adhesive,oxidation wear.?2?MoSi₂-ZrO₂ coatings were fabricated by atmospheric plasma spray technology.The addition of ZrO₂ could reduce the porosity and surface roughness,enhance the bond strength toughness and hardness.The COFs and wear rates of MoSi₂-ZrO₂ coatings were both decreased when slid against WC-Co and stainless steel.Therefore,MoSi₂-ZrO₂ composite coatings show favourable wear resistance,which was probably due to improve the toughness and hardness of the coatings.The main wear mechanisms were fatigue,oxidation and adhesive.?3?MoSi₂-TiN coatings were fabricated by atmospheric plasma spray technology.The addition of TiN could reduce the porosity and surface roughness,enhance the bond strength and hardness.The COFs and wear rates of MoSi₂-TiN coatings were both decreased,resulting from increasement of the hardness of the coatings,when slid against stainless steel.The main wear mechanisms were fatigue,oxidation and adhesive.