The high velocity oxy-fuel sprayed(HVOF)WC-Co coatings can improve the wear resistance of materials employed under 550?C and achieve the remanufacture of surface damaged machine parts.WC-Co coatings fabricated by using the powder with ceramic particle micro and nano size exhibited higher hardness,facture toughness and wear resistance.Nevertheless,the melting,decomposing and decarburization of nano WC as well as the formation of detrimental phases during HVOF spraying usually deteriorate the wear performance of the coating,which is recognized the issues for fabricating high performance WC-Co coating.Therefore,it is significant to study the phase and microstructure evolution of the cermet particles composing two kinds of hard phases with miro and nano size during HVOF processes and its influence on coating microstructure and properties for developing the new kinds of WC based cermet suitable to fabricate well wear resistance coatings.In this thesis,the six kinds of WC-Cr3C2-CoNiLa cermet powders with different content of micro and nao Cr3C2,Ni and La2O3 were designed and fabricated by agglomerated and sintered method as well as were used to deposit coating by HVOF in different spray parameters.By examination the surface morphologies and cross section microstructures of individual splats attached on substrate,and the cross sectional surface microstructures of the coatings,the variation of hard phases and binder phase during particles deposition in different conditions as well as their effect on the interfaces bonding within coatings were studied.The microhardness,elastic modulus and fracture of the coatings were evaluated by micro-indentation method using relevant standard test procedures.The adhesive strength of the coating was estimated by using the tensile test.The wear performance of the coating was characterized by two body abrasive wear test using pin-on-disc abrasive tester.The wear mechanism and the effect of micro and nano Cr3C2 on inhabiting the decarburization of WC and on the strengthening of the coatings were explored.The experiment results showed that most of the micro-nano WC-Cr3C2-CoNiLa powder particles exhibited spherical morphology which consisted of different size and content of Cr3C2 and WC,different content of Co,Ni and La2O3 in addition to some pores.Both of WC and Cr3C2 presented an angular outline and embedded in binder phase.The particle size of six kinds of powders were 8.68 to 58.9μm,9.86-51.8μm、8.68-58.9μm,11.2-58.9μm,9.86-58.9μm,9.86-58.9μm,respectively.It is showed that,quite different from the original particles,most of the splats on mild steel substrate surfaces presented a cake shape with the steam like outline and emboss smooth morphology,and with some radial ejectors on its periphery.The cross sectional morphologies showed that the splats size in the direction parallel to substrate surface was much large than that in particle impact direction and average diameter of original particles,and most of the splats substrate interfaces was well contacted and compact.It is demonstrated that the powder composition and spray parameters possessed key role to the composition,microstructure and phase constitution of deposited coatings.The coatings exhibited some typical lamellar structures consisted of splats which were composed of hard phase and binder phase and unbond areas.The hard phases within coatings were mainly WC and a few of(WCr)2C,Cr3C2 and a very little of Co3W3C.Different from that in original powders,most of WC phase presented an irregular particle shape with smooth outline.The Cr3C2 phase presented an irregular blocks or plate morphology.The sizes both of WC and Cr3C2 particles were reduced somewhat and the interfaces between two kind of hard phases and binder phase enhanced obviously.The binder phase in the coatings presented some band or stripe morphologies being different from that in original powders.The experimental results showed that the mechanical properties of the coatings were significantly influenced by spray parameters and powder compositions.The microhardness,elastic modulus,fracture toughness,adhesive strength and abrasive wear weight loss of the coating deposited using 78%WC-10%Cr3C2-12%(80%Co20Ni)1.5%La powder and in the oxygen flow of 8 M3/h,propane flow of 1500 L/h and spray distance of 170 mm,were 14.68±0.39 GPa,154.49±25.46 GPa,2.7±0.03MPa?m1/2,50.85±0.47 MPa and 5.28±0.18 mg,respectively.The mechanical performance of the coatings deposited using six kinds of original powders and in the same spray parameters was different.The coating deposited by using68WC5Cr3C2(w)15Cr3C2(n)12Co1.5La powder exhibited the highest microhardness(14.86±1.28 GPa)and lowest wear weight loss(5.08±0.42mg).The coating deposited by using 68WC5Cr3C2(w)15Cr3C2(n)12(Co80Ni20)1.5La powder presented the highest elastic modulus(173.48±84.11 GPa),fracture toughness(3.13±0.38 MPa?m1/2)and adhesive strength(62.59±2.63 MPa),respectively.Nevertheless,in identical spray parameters the coatings deposited using other five kinds of powders exhibit lower wear weight loss than the coating deposited using commercially WC-12Co powder except of the coating deposited using the78WC10%Cr3C2(n)12%(80Co20Ni)2.0La powder which exhibit comparable wear weight loss to that of commercially WC-12Co powder,By examination of the worn surface,it is revealed that the wear of the coatings happed mainly in the form of ploughing,plastic deformation and peeling off of carbide particles and splats. |