Study On Preparation And Properties Of (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x-Based High-Entropy Carbide Ceramics

Compared to transitionally metallic monocarbide ceramics,high-entropy carbide ceramics offer the advantages of higher hardness,better wear resistance and oxidation resistance,and are a new type of structural ceramics with wide application prospects.However,the purity of high-entropy carbide powder is low,the fracture toughness is low,and the sintering temperature required is very high.Regarding the above-mentioned issues,(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C high entropy ceramics were employed as the research object of this study.Research on synthesis of high-purity(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x powder,low temperature densification and improvement of mechanical properties of ceramics were carried out,and the possibility of applying high-entropy carbides in the field of ceramic cutting tools was studied for the first time.In this study,(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x(x=0.55-1.00)powders with different carbon contents were first synthesized by carbothermal reduction,and high-entropy(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics were then prepared via spark plasma sintering.The effect of carbon content on powder purity and particle size as well as ceramic mechanical properties was investigated.Then,the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 ceramics were toughened by the addition of Si C.In addition,the toughness was further improved and the densification temperature was reduced by the addition of Ni/Co binder.Finally,(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composites with high hardness and high toughness were prepared by spark plasma sintering combined with liquid-phase extrusion strategy.The main research results obtained in this study are summarized as follows:(1)Using transitionally metallic oxides and graphite as raw meterials,high-entropy(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x(x=0.55-1.00)powders with different carbon contents were synthesized by carbothermal reduction at 1650°C.When the carbon content was at x=0.55,the Zr O₂ impurities remained within the powder due to the insufficient carbon content.When the carbon content was at x=1.00,the graphite impurities present within the powder due to the excess carbon.In the range of carbon content from x=0.70 to x=0.85,the as-synthesized high-entropy powder was composed of a single-phase,and the other samples possessed two carbide phases with face-centered cubic structure.The powder morphology was equiaxed,and the average particle size of the high-purity powder was 286-360 nm.The as-synthesized powders contained extensive carbon vacancies,and a small amount of oxygen was also present as in the form of solid solution within the crystal structure.(2)Using the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x powders synthesized with different carbon contents as raw meterials,the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics were densified at2000°C by SPS.A single-phase high-entropy carbide ceramics without oxide and graphite impurities could be obtained when x=0.70-0.85,both the hardness and toughness increase with increasing the carbon content.The grain size,hardness,and toughness of the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics were?2-4?m,?23-25 GPa,and?2-3 MPa·m^1/2,respectively.Chipping and catastrophic failure occurred after a short period of cutting time(13 min)due to the low fracture toughness of the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 ceramic cutting tool,resulting in the short cutting tool life.(3)In order to improve the toughness of high-entropy carbides ceramics,the toughened high-entropy(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Si C composites were prepared with different Si C contents(10-30 vol%).With the Si C content increased,the toughness of high-entropy carbide composites was increased.Compared to the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 ceramic,the toughness of(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Si C composite with 30 vol%Si C was increased from 2.25±0.27 MPa·m^1/2 to 3.11±0.25 MPa·m^1/2.The hardness of high-entropy(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Si C composites remained above 23 GPa.Chipping and catastrophic failure was not occurred during cutting,due to the toughness improvement of(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Si Ccompositecuttingtools.The(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Si C composite cutting tool with 10 vol%Si C exhibited the highest cutting lifetime of 43 min,and the main failure mechanisms of tool included adhesion wear,oxidation wear,abrasive wear,and micro-chipping.(4)In order to further improve the toughness and reduce the sintering temperature of(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 ceramic,(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Ni/Co composites with15 vol%Co and/or Ni binder were prepared by pressureless sintering.By adding the Co and/or Ni binder,the densification temperature was reduced to 1650°C by pressureless sintering.With the introduction of the Ni and Co binder,the toughness of the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Ni/Co composites was greatly improved,and the toughness of the composite with 15 vol%Ni measured was as high as 8.90±0.51 MPa·m^1/2,which was?3-4 times higher than that of pure(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 ceramic.However,the hardness of all samples was greatly reduced to only?16-17 GPa.Due to the significant reduction in hardness,the cutting performance of the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Ni/Co composite cutting tools were not improved significantly.Note that the insert with 15 vol%Co had the longest cutting lifetime of 39 min.The failure mechanisms of tools included adhesive wear,abrasive wear,and oxidative wear.(5)In order to prepare high-entropy carbides with high hardness and high toughness,the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composites with low Co content were prepared by SPS combined with liquid-phase extrusion strategy.The effects of sintering temperature(1350-1550°C)and sintering pressure(30 MPa and 4.3 MPa)on the phase composition,microstructure and mechanical properties of(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composites,and cutting performance of inserts were studied.The(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composites was densified at 1350°C and 30 MPa with a grain size of 0.44±0.09?m.The high hardness of 21-24 GPa and the high toughness of?4-5 MPa·m^1/2 of the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composites were achieved.Due to the the advantages of high hardness,high toughness and fine-grain,the cutting experiment results showed that the(Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8-Co composite cutting tool prepared at 1450°C and 4.3 MPa exhibited the best cutting performance,and the cutting lifetime reached 67 min.The main failure mechanisms of the tools identified were adhesive wear,abrasive wear,and oxidative wear.