Laser In-situ Fabricated, Microstructures, And Properties Of Ni₃Si Based Composites

Intermetallic compounds based on Ni₃Si have been considered as the basis of high-temperature structural materials and chemical parts because the Ni₃Si displays an increasing strength with temperature increasing and shows excellent oxidation resistance and corrosion resistance in sulfur dioxide environments.In the present wok,Ni₃Si based intermetallic composites were investigated using metheds of alloying and of composite by laser cladding.The Ni₃Si based intermetallic composite coating was fabricated by laser cladding using the raw material of Ni,Si,Ti,Nb,C,Cr alloy powder at different ratio.The mierostructure of the laser clad intermetallic composite coating was characterized by XRD,OM,SEM,EPMA and DSC.The room temperature wear resistance of the coatings was evaluated under the sliding wear test condition,the high temperature isothermal oxidation resistance was tested at 1100℃in air.The wear and oxidation mechanisms were discussed.The laser clad Ni75Si25 coating which the microstructure consists of Ni₃Si/Ni₅Si₂ and Ni(Si).The microhardness of Ni75Si25 coating shows 740HV; while the substrate shows 350HV.With the addition of Cr the microstrueture of the coating is mainly composed of Ni(Si),Ni₃Si/Ni₅Si₂ and the Ni75Si25-5at.%Nb coating consist of Ni₃Si,Ni(Si),Nb₃Ni₂Si and Ni₅Si₂,Ni75Si25-5at.%Ti coating consist of Ni(Si)and Ni₃(Si,Ti).With the Cr,Ti and Nb addition,the microhardness of coatings was 700HV, 640HV,and 765HV respectively.The Ni78Si13Ti9+(Ti+C)clad layers were mainly composed of the Ni solid solution,Ni₃(Si,Ti)intermetallic compound and TiC carbide,the TiC was in-situ formed.TiC carbide was dispersed with global shape,clubbed shape,and flower-like shape.The crystal structure of TiC,the ratio of Ti/C,and the energy of laser beam play an important role in controlling the growth morphology of carbide. The volume fraction and the size of TiC carbide gradually increased from the bottom to the top of the clad layer.With the 20wt.%[Ti+C]addition,the microhardness of clad layer was 747HV.Phases of laser cladding Ni78Si13Ti9+WC were composed of Ni solid solution,Ni₃(Si,Ti)intermetallic compound and(Ti,W)C carbide.With increasing the percentage of WC,content of carbides formed in the coatings increased,the microhardness of clad layers was also enhanced.The NbC particle reinforced Ni₃Si composite coating was prepared by in situ contact reaction with preplaced powder mixtures of NioSi-Nb-C.It was found that compound material cladding coating mainly consists of NbC particles,Ni₃Si intermetallic and Ni solid solution.The NbC particle was about 2-4μm in size and dispersivly distributed in the coating with different morphologies.The microhardness of the clad layer showed a gradient increased with the increasing of[Nb+C]content.The wear mechanism of the particle reinforced Ni₃Si composite coatings were micro-cutting,slight adhesion and abrasive.During the wear process,carbide reinforcement serve as load-carring particles,carbide in-situ formed which have a good bonding with base.The friction coefficient decrease with the increasing of the carbide reinforcement.The carbide reinforced intermetallic matrix coatings exhibit excellent corrosion resistance.The coatings have an outstanding isothermal elevated temperature oxidation resistance in the high temperature addition.The oxidation kinetic curves mainly followed line-parabola rate law.In the initial course of the oxidation,the oxide scales mainly consisted of NiO,exposures up to 100h,the oxides which foemed in Ni-Si-Ti-C coatings consisted of TiO₂ and SiO₂,while the oxides which foemed in Ni-Si-Nb-C coatings consisted of SiO₂,the formation of SiO₂ under the NiO layer caused the NiO peeled.