Low-temperature densification of high-entropy(Ti,Zr,Nb,Ta,Mo)C-Co composites with high hardness and high toughness

In order to prepare high toughness(Ti,Zr,Nb,Ta,Mo)C ceramics at low temperatures while maintaining high hardness,a liquid-phase sintering process combined with Co-based liquid-phase extrusion strategy was adopted in this study.The densification temperature can be lowered to 1350℃,which is much lower than the solid-state sintering temperature(~2000℃)generally employed for high-entropy carbide ceramics.When sintered at 1550℃and 30 MPa applied pressure,part of the Co-based liquid-phase was squeezed out of the graphite mold,such that only~3.21 vol%of Co remained in the high-entropy ceramic.Compared to the Co-free solid-state sintered(Ti,Zr,Nb,Ta,Mo)C ceramics,prepared at 2000℃and 35 MPa,the hardness was slightly decreased from 25.06±0.32 to 24.11±0.75 GPa,but the toughness was increased from 2.25±0.22 to 4.07±0.13 MPa·m^(1/2).This work provides a new strategy for low-temperature densification of high-entropy carbides with both high hardness and high toughness.

Fabrication and modelling of Si_(3)N_(4)ceramics with radial grain alignment generated through centripetal sinter-forging

Silicon nitride(Si_(3)N_(4))based ceramics are one of the most attractive advanced engineering materials,which have been widely used under high-speed rotational operation or for mechanical contacts across a curved surface.In the present study,rotationally symmetric texturing of Si_(3)N_(4),with radial grain align-ment,was obtained by centripetal sinter-forging(CSF)of a partially sintered sample.The average values of the included angles between the c-axis of the local Si_(3)N_(4)grain and radial direction were approxi-mately 16.4°and 11.0°,on the section plane perpendicular to the pressing direction,and parallel to both the pressing and radial directions,respectively.The compressive strain in the pressing direction forced the ceramic body to flow towards the central axis,resulting in compressive strain in the tangential di-rection and tensile strain in the radial direction.A fundamental physical model was created to simulate the grain rotation during the 3-dimentional strain reorientation,which revealed the rod-like grain would preferentially rotate toward the center of the sample under the CSF process.In addition,due to the fric-tion between the sample surface and the pressing punch,the increased shear strain could enhance the Si_(3)N_(4)grain alignment.Consequently,ceramics with rod-like grains perpendicular to the curved side sur-face could be anticipated by applying the centripetal forming concept in a controlled manner.

Textured and toughened high-entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C-SiCw ceramics

High-entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C ceramics,with different contents(0,5,10,and 20 vol.%)of Si C whiskers(SiCw),were fabricated by spark plasma sintering using raw powders synthesized via carbothermal reduction.The application of a uniaxial compaction force led to texture development of the SiCw within the(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C matrix.Fracture toughness increased with the increase in SiCw content,while Vickers hardness remains almost unchanged.The toughness of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C-20 vol.%SiCw ceramics reached 4.3±0.3 MPa m^(1/2),which was approximately 43%higher than that of the monolithic(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C ceramic(3.0±0.2 MPa m1/2).The main toughening mechanisms were attributed to crack deflection,whisker debonding,and whisker pullout.