Preparation And Properties Of ZrO₂ Doping Mo₂NiB₂ Based Cermets Prepared By Liquid Phase Sintering

Mo₂NiB₂-based cermet are expected to replace WC-Co cemented carbides in the fields of wear-resistant and corrosion-resistant coatings,molds and tools due to their good wear resistance,excellent high-temperature oxidation resistance and corrosion resistance,thus reducing the consumption of strategic metal resources.However,the grain size of Mo₂NiB₂-based cermet is prone to grain growth during high-temperature sintering,and mechanical properties such as hardness and strength are still unsatisfactory,which greatly limits their practical application.Therefore,it is urgent to improve and balance the strength and toughness of Mo₂NiB₂-based cermet.In this paper,Mo₂NiB₂-based cermets with different contents of ZrO₂ were prepared by different preparation methods,and the effects of ZrO₂doping on the microstructure,mechanical properties and corrosion resistance of Mo₂NiB₂-based cermet were studied.The mechanism of ZrO₂ doping on the microstructure and properties of Mo₂NiB₂-based cermet was analyzed.Firstly,Mo₂NiB₂-based cermet doped with different ZrO₂ content were prepared by dry ball milling and vacuum hot-pressed sintering.The microstructure of the prepared cermet was observed.It was found that the microstructure distribution of Mo₂NiB₂ hard phase and Ni-based bonding phase was more uniform with the doping of ZrO₂,and the grain size was smaller than other samples.When ZrO₂ doping content is 6.0 wt%,the maximum bending strength and fracture toughness of the material were 993.9 MPa and 16.6 MPa·m^1/2,respectively.The mechanical property results show that the Mo₂NiB₂ cermet is a typical brittle fracture.Phase transformation toughening is the main reason for the improvement of the strength and toughness of Mo₂NiB₂ cermet.In addition,the electrochemical corrosion test and corrosion weight loss test analysis show that the quantitative doping ZrO₂ can improve the corrosion resistance of Mo₂NiB₂ matrix ceramics,the corrosion morphology of the sample surface has no obvious corrosion pits,the formation of metal oxide passivation film plays an important role in improving the corrosion resistance of materials.Secondly,Mo₂NiB₂-based cermet doped with different ZrO₂ contents were prepared by dry ball milling and vacuum pressureless sintering.The Mo₂NiB₂-based cermet still consisted of two phases,the hard phase Mo₂NiB₂ and the Ni-based bonding phase,the distribution of the two phases was more uniform when the ZrO₂ doping amount was 1.0 wt%and 1.5 wt%and the maximum hardness,flexural strength,compressive strength and fracture toughness of the cermets were 828.8 HV0.3,1397.3 MPa,1811 MPa and 24.1 MPa·m^1/2,respectively.The crystallographic relationship between the two phases of Mo₂NiB₂ hard phase and Ni-based bonded phase with semi-coherent lattice made the material have high strength.In addition,the electrochemical corrosion results show that the Mo₂NiB₂-based cermet with ZrO₂ content of1.0 wt%have excellent corrosion resistance.Finally,the doped Mo₂NiB₂-based cermet with different ZrO₂ contents were prepared by high-energy ball milling and vacuum pressureless sintering.After high-energy ball milling,no new phases was formed,and the powder size was further refined to form more lamellar structures.The microstructure of Mo₂NiB₂-based cermet generated by pressureless sintering still consists of Mo₂NiB₂ phase and Ni.The hardness,fracture toughness and compressive strength of Mo₂NiB₂-based cermet with ZrO₂ doping content of 1.0 wt%were 1082.3 HV0.3,22.1 MPa·m^1/2 and 2053 MPa,respectively.At the same time,the sample has excellent corrosion resistance,and there are few pitting pits and dissolution of bond phase in the material tissue.This is mainly attributed to the passivation film of metal oxides such as Mo O3and Ni2O3 that can prevent the direct contact between the corrosion solution Na Cl and the material tissue,so as to avoid the short time corrosion of Mo₂NiB₂-based cermet.