Study On High-temperature Mechanical,Friction And Wear Behavior Of Ternary Transition-metal Borides

As an important member of ultra-high temperature ceramics,binary transition-metal boride ceramics have broad application prospects in the field of hypersonic vehicles due to their high melting point and high hardness,but their intrinsic brittleness and poor oxidation resistance are the main obstacles that limit its application.The high toughness and high damage tolerance of MAX phase ceramics suggest that one can achieve the purpose of toughening by inserting Al atoms into the binary borides to form a ternary transition-metal boride MAB phase ceramic.In order to better understand these materials,two typical MAB phase materials MoAlB and Fe₂AlB₂ were chosen as subjects of research,and their basic mechanical properties,high temperature mechanical behavior and friction and wear properties were studied.The change of the phase composition and density of MoAlB with the main process parameters such as sintering temperature,holding time and raw material ratio was studied by using powders of MoB and Al as starting materials.The MoAlB bulk with higher purity can be prepared under the experimental conditions of the ratio of raw materials of MoB and Al to 1:1.3 and the holding time is more than 2h.The relative density of MoAlB bulks prepared by holding at 1200?and 30 MPa for 6 h can reach.More than 90%.Among all the process parameters,the holding time has the most obvious effect on the density of the synthesized product.The lattice constant measured by XRD is in good agreement with the theoretical value.The backscattered image combined with the energy spectrum analysis revealed that the MoAlB bulk contained Al₂O₃ and Mo-Al intermetallic compounds not found in the XRD pattern.The process parameters have a great influence on the mechanical properties of the prepared MoAlB.The hardness of MoAlB synthesized at 1 h was higher than that of MoAlB synthesized under other conditions,and both were higher than Fe₂AlB₂.However,the process parameters have little effect on the fracture toughness of MoAlB,which is higher than Fe₂AlB₂,but its fracture work is lower than Fe₂AlB₂.The bending strength of MoAlB is between 350 and 400 MPa,and the compressive strength is between 1500 and 2200 MPa.The flexural strength and compressive strength of MoAlB prepared after 6 h of heat preservation were lower than those of MoAlB prepared under other experimental conditions.For all MoAlB prepared in this paper,the main factor affecting the bending strength is the grain size rather than the porosity,and the compressive strength is controlled by the grain size and the porosity.The high temperature bending strength of Fe₂AlB₂ has no obvious attenuation in the temperature range of 0-1000?,and the failure mode is brittle fracture.According to the load displacement curve,the brittle-ductile transition temperature?BDTT?of Fe₂AlB₂ is close to but greater than 1000?in the bent state.The compressive strength of Fe2AlB2 decreases with increasing temperature and decreases to 245±7MPa at 1000?.According to the stress-strain curve and the damaged sample,it can be confirmed that the BDTT in the compressed state of Fe₂AlB₂ falls within the range of 800-900?.The higher shear stress under compression than the three-point bending load results in a lower BDTT.Using GCr15 as the friction disc,the friction wear behavior of Fe₂AlB₂ and MoAlB was studied by the pin-disk friction and wear tester.The stable friction coefficient of Fe₂AlB₂ is about 0.3,and the wear rate is between 0.4×10-⁵³.0×10^-5mm³/?N·m?.The stable friction coefficient of MoAlB is very close to that of Fe₂AlB₂,and the wear rate is between 0.4×10^-55 and 5.0×10^-55 mm³/?N·m?.The sliding rate controls the friction coefficient by changing the contact time of the adhesion point.The sliding rate increases and the friction coefficient decreases.The effect of the load on the friction coefficient is not obvious.The wear rate increases with increasing load and slip rate,and the main wear mode is adhesive wear.During the friction process,along with the formation and falling off of the oxide film,fatigue wear occurs under high speed and high load.