Design Of Strengthening-toughening And Investigation On High-temperature Mechanical And Tribological Properties For Transition Metal Diborides

Transition-metal diborides（TMB₂）possess high thermal conductivity,low electronic resistance,superior chemical inertness,a high melting point,and facile synthesis conditions,establishing them as exceptional superhard/hard materials ideal for intricate environments.However,their elevated hardness often coincides with increased brittleness（low toughness）,hindering effective crack prevention and leading to sudden fractures during service,compromising reliability and longevity.Augmenting material toughness effectively mitigates brittle fracture.Yet,in ceramics,balancing hardness and toughness presents a challenging paradox.Consequently,achieving concurrent elevation of both properties in TMB₂ is imperative.This paper proposes a chemical-tuned solid solutions method from the standpoint of component regulation and crystal structure design.It aims to guide the development of strengthened and toughened TMB₂,offering a feasible scheme to optimize its high-temperature mechanical strength and tribological behavior.The specific research contents and conclusions are detailed as follows:1.We systematically computed elastic constants for 9 binary TMB₂ and 81 dual-transition-metal diboride solid solutions（dual-TM diborides）.Extending the valence electron concentration（VEC）mechanical properties descriptor from cubic TMN and TMC systems to hexagonal TMB₂,we discovered that increasing VEC triggers a brittle-ductile transformation in TMB₂.However,elastic constants among different TMB₂ with the same VEC exhibit scattered distribution,limiting VEC’s accuracy in describing TMB₂’s mechanical properties.In light of this,the relative electronegativity（REN）between solvent and solute metal atoms in TMB₂ emerges as crucial in delineating mechanical property variations.TMB₂’s charge distribution pattern consistently shifts with increasing REN for the same VEC,effectively characterizing elastic constant ranges among different TMB₂ with identical VEC.The amalgamation of VEC and REN substantially enhances the accuracy of the VEC descriptor.This composite VEC-REN descriptor,rooted in fundamental physics and chemistry principles,serves as a potent tool for elucidating TMB₂’s mechanical properties.2.In order to understand the influence of strain modes on the structural evolutions and mechanical properties of TMB₂,and further optimize the structural design of TMB₂and improve its mechanical properties,we systematically calculated the mechanical responses of Ta B₂ under complex strain modes.The theoretical calculation results show that the dynamic instability of Ta B₂ does not occur when the（001）lattice plane undergoes Berkovich indentation strain mode,and the indentation strength reaches 46-49 GPa.However,Ta B₂ exhibit strain-induced dynamic instability in advance when the（110）and（100）lattice planes suffer the same strain mode,which limits the stress responses of Ta B₂ on these two crystal planes and leads to a significant reduction in indentation strength（<30 GPa）.Subsequently,（001）preferred orientation Ta B₂ film with high crystal quality was synthesized,and the nanoindentation hardness reached45.9 GPa.These results highlight a robust structural engineering approach that holds promise for extending a range of intrinsic superhard TMB₂ by identifying and constructing stronger while avoiding weaker structural configurations,opening up an effective way to selectively optimize the mechanical properties of TMB₂.3.We further explore the possibility of obtaining superhard yet high-toughness TMB₂ by adjusting the electronegativity of the solute metal atoms and controlling the crystal orientation based on the aforementioned two works.The Ta₃Zr B₈ solid solution was obtained by introducing low-electronegative Zr to partially replace the high-electronegative Ta in Ta B₂.Nanoindentation test results show that the Ta B₂ hardness（45.9±1.0 GPa）is improved by forming Ta₃Zr B₈ solid solution（49.5±2.2 GPa）.Meanwhile,Ta₃Zr B₈ solid solution also has enhanced toughness,specifically,there is no crack initiation and propagation induced by indentation.Theoretical calculation reveals that Zr atoms with low electronegativity exhibit weak charge-absorbing ability in Ta₃Zr B₈ with Berkovich indentation strain,which delays the charge deplete rate in Ta₃Zr B₈.Consequently,the B-B and Ta-B main load-bearing bonds in Ta₃Zr B₈ are strengthened,and thus improving the overall stress and strain responses,leading to the strengthening and toughening of Ta₃Zr B₈.This work demonstrates the reasonable matching of electronegativity between solute and solvent metal atoms is important in simultaneously improving the strength and toughness,and thus provides a new idea for designing and optimizing the mechanical properties of TMB₂.4.We conducted an investigation into the impact of Zr（Ta）B₂ solid solution formation on the high-temperature mechanical properties and tribological behaviors of Ta B₂ and Zr B₂,employing a combined approach of calculation and experimentation.In the Zr-Ta-B system,the solid solution shows a significant 100%increase in ultimate strain amplitude compared to Zr B₂,leading to enhanced high-temperature shear strength.Additionally,Zr’s lower electronegativity minimizes electron loss in the solid solution’s robust ionic/covalent bonds during strain,strengthening these load-bearing bonds and consequently elevating the solid solution’s high-temperature shear strength above that of Ta B₂.Further examination of four films at 500°C indicates that the solid solution displays the lowest high-temperature wear rate in contrast to Zr B₂ and Ta B₂.This outcome stems from the combined effects of heightened solution strengthening at high temperatures and improved oxidation resistance within the solid solution,which mitigate ploughing and oxidation-related wear during high-temperature friction processes.Simultaneously,the solid solution exhibits considerably reduced friction coefficients during high-temperature friction tests.This reduction is primarily due to the solid solution’s tendency to form Ta₂O₅,possessing a higher ion potential than Zr O₂,owing to the relatively stronger adsorption energy between Ta atoms and O₂ at elevated temperatures.Consequently,the resulting lubricating Ta₂O₅ and B₂O₃ spread uniformly along the solid solution’s wear tracks during the 500°C friction test,reducing coefficients of friction to the range of 0.2-0.3.These findings inspire optimism for the ongoing exploration and development of lubricated,wear-resistant TMB₂ materials tailored specifically for high-temperature environments.