Studying On Preparation,Mechanical And Tribological Properties Of Mo-Si-B/ZrB₂ Composites

Mo-Si-B alloy and its composites have superior application prospects in traditional superalloys.It can be used for the application of critical mechanical parts,but the shortcomings of the material such as poor wear resistance and poor oxidation resistance at high temperatures limit the development of the alloy.Therefore,it is of great significance to systematically study the tribological properties of Mo-Si-B alloy under the premise of solving the problem of oxidation resistance of Mo-Si-B alloy.In this paper,four Mo-12Si-8.5B/x ZrB2composites（x=2.5,5,7.5,10 wt.%）were prepared by spark plasma sintering.The effects of ZrB2 content on the microstructure,mechanical properties and tribological properties of the composites were investigated.The main research contents of this paper are as follows:（1）Mo-12Si-8.5B alloy powder and ZrB2 powder were prepared by mechanical alloying and self-propagation high-temperature synthesis methods,respectively.The powder of Mo-12Si-8.5B/ZrB2 composite was prepared by mixing the alloy powder with ZrB2 powder via low-energy ball milling.The Mo-12Si-8.5B/ZrB2 composite was prepared by spark plasma sintering.The microstructure analysis of the composite showed that ZrB2 particles were dispersed and distributed in the Mo matrix,but ZrB2 particles had an agglomeration phenomenon.The composite is mainly composed ofα-Mo,Mo3Si,and Mo₅Si B₂ three-phase.The mechanical properties of the composite showed that the hardness and strength of the composite were improved by the increase of ZrB2 content and its ZrB2 particles have dispersion strengthening,solution strengthening and load transfer strengthening.In addition,due to the low density of ZrB2,the increase in its content leads to a gradual decrease in the density of the composite.Due to the increase of ZrB2 content,the brittleness and porosity of the composite increased,which results in the reduction of the fracture toughness of the composite.（2）The tribological properties of Mo-12Si-8.5B/ZrB2 composites at room temperature mainly depend on the content of ZrB2 and the type of counterpart.When the Al2O3 ceramic ball is the friction counterpart,the friction coefficient is independent of ZrB2 content and remains between 0.59～0.65.The wear rate decreased from 3.85×10^-5 mm³N^-1m^-1 to 1.47×10^-5 mm³N^-1m^-1 with the increase of ZrB₂ content.When the GCr15 ball is the friction counterpart,the friction coefficient remains between 0.73～0.85 and has an upward trend with the increase of ZrB₂ content.The wear rate of GCr15 counterpart decreased from 10.95×10^-7 mm³N^-1m^-1to 2.71×10^-7 mm³N^-1m^-1 with the increase of ZrB₂ content.The wear mechanism of the Al₂O₃counterpart is mainly abrasive wear and its adhesion,while the GCr15 counterpart is mainly based on adhesive wear,resulting in a higher friction coefficient of the GCr15 counterpart.The increased content of ZrB2 leads to a decrease in adhesion,resulting in more abrasive wear and an increase in the coefficient of friction.Since the strength and hardness of Al2O3 are greater than those of the GCr15 ball,the wear effect of the former on the surface of the composite is greater than that of the latter,so the wear rate of Al2O3 is higher.In addition,due to the strengthening effect of ZrB2,the wear rate of the two friction counterparts is reduced.（3）The high-temperature tribological properties of Mo-12Si-8.5B/ZrB2 composites mainly depend on the changes in temperature and ZrB2 content.At 200℃～400℃,the friction coefficient of the composite material is high,which remains between 0.65～0.85.At600℃～800℃,the friction coefficient of the composite maintains between 0.26～0.49.The coefficient of friction was stable with the increase of ZrB2 content at 200℃and increased with the increase of ZrB2 at 400°C.The friction coefficient decreases with the increase of ZrB2 content at 600°C,while the friction coefficient increases with the increase of ZrB2content at 800°C.This is due to the fact that the wear mechanism at 200℃～400℃is mainly the abrasive wear of adhesive wear and brittle fracture,resulting in a large friction coefficient.At 600℃～800℃,oxidation wear and the lubrication effect of its oxide film lead to a decrease in the friction coefficient.Due to the self-lubricating effect of the oxide film and the strengthening effect of the composite,the wear rate at 600℃decreases with the increase of ZrB2 content.At 800℃,the coefficient of friction increases due to the decrease in the lubrication effect of the oxide film due to the volatilization of Mo O3,and the increase in the content of ZrB2 leads to the increase in the strength of the oxide film and the material,resulting in a decrease in the wear rate.