Microstructure, Mechanical And Tribological Properties Of Mo₂FeB₂ Based Cermets

Mo₂FeB₂ based cermets, which possess excellent mechanical properties, wear resistance, corrosion resistance and high temperature resistance, are suitable for the application of mechanical parts under high temperature and corrosion conditions. In this work, Mo₂FeB₂ based cermets with various alloying elements such as Mn, Cr, Ni, Mo, were prepared using in-situ reaction liquid sintering. The influences of alloying elements on microstructure, mechanical properties and tribological properties of Mo₂FeB₂ based cermets have been investigated. The toughening mechanism of the cermets has been studied. The relationships between wear resistance, microstructure and mechanical properties were evaluated. The influences o f alloying elements on transfer and glazing oxide films of the cermets under elevated temperatures have also been discussed in detail. The major results are as follows:1. With Mn content increases from 0wt.% to 10 wt.%, the hard particles are distributed more uniformly in the binder phase. And the phenomenon of hard particle refinement is clearer. The mechanical properties of the cermets are improved due to particle refinement and solid solution strengthening. As a result, the cermet with superior comprehensive properties such as high fracture toughness and transverse rupture strength is achieved with 5wt.% Mn content. The hard particles change from rectangular particles to square particles due to Cr dissolution in the hard phase. This has a positive effect on relative density and transverse rupture strength of the cermet. With adding Ni into the cermet, the transverse rupture strength of the cermet decreases because of the inhomogeneous distribution of the hard particles. The agglomeration of hard particles appears with the increase of Mo content. The hardness of the cermet increases owing to the formation of brittle carbides, while the fracture toughness and transverse rupture strength of the cermet decrease.2. The hard phase particles debonding and the pl astic deformation of the binder phase are the main characters on the fracture surfaces of Mo₂FeB₂ based cermets. It is clear that the fracture mechanism is primarily based on the inter-granular fracture. Moreover, the trans-granular fracture could occur at local scales only under high stress concentration. The crack propagation path shows the toughening mechanism of the cermets mainly depends on solid solution strengthening, crack deflexion and particles pullout.3. The wear resistance of the cermets is clo sely related to their fracture toughness and hardness at room temperature. The wear mechanisms of Mo₂FeB₂ based cermets under dry sliding are primarily determined by abrasive wear, accompanying adhesive wear and oxidation wear. When alloying elements are a dded, the tribo-oxides may act as the solid lubricant and hence decrease the friction coefficient and wear rate.4. Mo₂FeB₂ based cermets exhibit an outstanding wear resistance over the entire temperature range from room temperature to 800 oC. Mo₂FeB₂ based cermets with various Mn contents show excellent wear resistance at 600 oC. As the oxidation resistance of the cermets is improved with Cr addition, Mo₂FeB₂ based cermets have low wear rate at 800 oC. When the temperature is higher than 400 oC, the oxidation will be accelerated, while the oxidation wear is the main wear mechanism of Mo₂FeB₂ based cermets. Consequently, the formation of transfer and glazing oxide films decrease distinctly the friction coefficient and wear rate at elevated temperatures.