Study On The Preparation And Properties Of Ternary Layered Carbide Cr₂AlC

Lately,the layered-ternary carbides or nitrides have received more attention because of their mixture of ceramic and metallic properties.The so-called 413 phases,312 phases or 211 phases belong to a large class of solids with the general formula M_n+1AX_n,where n=1-3,M is an early transition metal,A is anâ…¢A orâ…£A element,and X is C and/or N.As a member of the MAX family,the good elastic properties of Cr₂AlC predicted by calculated material scientists and the possible good high-temperature oxidation resistance by the formation of Al₂O₃ and Cr₂O₃ during oxidation,which is even better than Ti₃SiC₂,indicate that Cr₂AlC might be a kind of potential and fascinating material.Therefore,the study on synthesis and properties, especially phase formation sequence and the effect of starting powders,composition and sintering techniques on phase assembly and density of Cr₂AlC ceramics was carried out.Then the mechanical,electrical,thermal,static chemistry corrosion and oxidation resistance properties of Cr₂AlC ceramics were determined after preparation of the material by using the optimized processing parameters.Summarizing the results of XRD and DTA to form Cr₂AlC phase using Cr₃C₂,Al and Cr as starting materials for composition n(Cr₃C₂)∶n(Al)∶n(Cr)=0.5∶1.2∶0.5,Cr₅Al₈ and Cr₂Al were two main intermediate phases.The amount of Cr₂AlC phase was gradually increased with increasing in sintering temperature by reactions between Cr-Al intermetallic compounds,un-reacted Cr₃C₂ and Cr,and it became as a pure phase with disappearance of intermediate phases at 1350℃.The results of study on the fabrication of bulk Cr₂AlC indicated that with the increment of additional Al from 10 at%to 30at%in the composition,the amount of Cr₂AlC phase in the samples that were hot-pressed under Ar increased together with decrease in the amount of the second phase,Cr₃C₂,and Cr₂AlC became as only phase in the sample when additional Al to be 20 at%.On the other hand,bulk densities of the samples decreased gradually with increase in additional Al in the composition.For fixed composition n(Cr₃C₂)∶n(Al)∶n(Cr)=0.5∶1.2∶0.5,hot-pressing at 1350℃for 2h made the sample to have Cr₂AlC as a major phase together with very small amount of Cr₃C₂.Further increase in temperature,evidence of decomposition appeared at 1500℃and the bulk densities of the samples decreased little;With the extension of dwell time from 0.5h to 2h,the amount of Cr₂AlC phase increased and the amount of the second phase decreased,but the change of bulk densities is not obvious.Dense and predominantly single-phase samples of Cr₂AlC together with a trace amount of Cr₃C₂ were obtained by hot pressing using the above-mentioned optimized composition and processing parameters.The hardness,flexural strength and compressive strength of Cr₂AlC samples were 4.7～5.2 GPa,498 MPa and 627 MPa respectively,which was comparable to those of Ti₃AlC₂ and Nb₂AlC.It was found that the thermal conductivity of Cr₂AlC ceramics increased slightly with increase in temperature in the range of room temperature to 600℃.The electrical resistivity increased linearly with increase in temperature range 300K～900K,indicating that Cr₂AlC ceramics was electronic conductor.The coefficient of thermal expansion in the temperature range of room temperature to 1300℃,the thermal conductivity at 200℃,the heat capacity and the electrical conductivity at room temperature were 1.36×10^-5K^-1,13.35W/m·K,619J/kg·K and 1.6×10⁶ S·m^-1 respectively.The corrosion behavior of composites indicated that the sample immersion in HCl had the largest corrosion rate and the sample immersion in NaOH had the smallest corrosion rate.After cyclic oxidation for 30 times at 1000℃～1200℃,the surface of Cr₂AlC formed compact oxide layer consisting of two layers:the Al₂O₃ oxide layer and a narrow Cr₃C₂ underlayer without scale spallation.At 1000℃and 1100℃,the oxide layer was less than 3μm-thick, Cr₂AlC displayde excellent cyclic oxidation resistance and better than Ti₂AlN and Ti₃SiC₂.The oxide layer was primarily formed by Al ions moving outward and O ions moving inward.