Preparation And Property Study Of NiCr-Cr₃C₂ Gradient Coating Sprayed By HVOF On Copper Alloy Substrate

In this paper,the gradient coating with different metallic bond layer,whose microhardness and coefficient of thermal expansion were gradual transited from CrZrCu substrate to metal-ceramic layer,were prepared by using HVOF technique equipment?WokaStar 640?.The highpracticability gradient coating was designed to improve the surface properties of copper mold plate by adjusting the types of metallic bond layer,Cr₃C₂ ceramic content in metal-ceramic layer,spray parameters and annealing process.The phase structure,adhesion strength,microhardness and wear resistance of the designed gradient coating were systematicallyanalyzed and compared by x-ray diffractometer?XRD?,scanning electron microscope?SEM?,tensile testing machine,microhardness tester,ball on disk friction wear tester and step profiler.The results showed that:The main phase of HVOF sprayed NiCr-Cr₃C₂ metal-ceramic layer were NiCr and Cr₃C₂ as well as a little Cr₇C₃ and Cr₂O₃,the decreasing of Oxygen flux had a great effect on reducing the decomposition of Cr₃C₂ and oxidation of NiCr phase.There was very few decomposition of Cr₃C₂ carbide and little oxidation of NiCr phase in metal-ceramic layer when the gradient coating was annealed under 800?,which indicated that the gradient coating could ensure a good oxidation resistance at high temperatures under 800?.The structure of HVOF sprayed NiCr metallic bond layer and NiCr-Cr₃C₂metal-ceramic layer were classical plate,the porosityexisted in metal-ceramic layer was decreased slightly with the increasing of Cr₃C₂ content and annealing temperature.The metal-ceramic layer microhardness of designed gradient coating GC1 and GC2 were 370 HV and 600 HV,which were decreased to 300 HV and 550 HV after annealing treatment respectively.The microhardness decreasing ratio of Cr ZrCu substrate was 8.22%,21.86% and 50.62% after annealed at 600?,700?and 800? respectively.The adhesion strength of designed gradient coating GC1-Ni-NiCr,GC1-Ni and GC1-NiCr were 24 MPa,69 MPa and 15 MPa respectively,after annealing treatment the adhesion strength of gradient coating GC1-NiCr had a great improvement?more than 70 MPa?.The adhesion strength of designed gradient coating was increased with the decreasing of oxygen flux and increasing of Cr₃C₂ content in metal-ceramic layer.The adhesion of gradient coating GC2-Ni and GC2-NiCr were 73 MPa and 40 MPa respectively,and the adhesion strength of GC2-Ni and GC2-NiCr were all above 70 MPa.Under the condition of 20 N wear loading,the wear resistance of gradient coating with NiCr metallic bond layer was slightly better than gradient coating with Ni metallic bond layer,however,the gradient coating with Ni Cr and Ni double-deck metallic layer possessed the worst wear resistance and the annealing treatment had a great improvement on the wear resistance of designed gradient coating.While the wear loading was 5 N wear loading,the wear resistance of designed gradient coating had no relationship to types of metallic bond layer,but it was inversely proportional to the microhardness of metal-ceramic layer.The wear mechanism of designed gradient coating was mainly adhesive wear and oxidation wear,as well as slightly abrasive wear.