Research On Synthesis, Structure And Properties Of [Zr(Cr)]₂Al₄C₅ Carbides Ceramic

Zr-Al-C ceramics are a class of layered ternary ceramics with widely prospect in applications in the aerospace industry, which of physical-chemical stability during the high temperature due to their high melting point, high thermal conductivity and good oxidation resistance. This thesis is to substitute Zr in Zr₂Al₄C₅ ceramics with part of Cr to synthesis [Zr(Cr)]₂Al₄C₅ ceramics on purpose of improving oxidation resistance of Zr-Al-C ceramics. [Zr(Cr)]₂Al₄C₅ ceramics were fabricated by hot-pressing sintering method with ZrC, Al, Cr₃C₂, C powders as raw materials. The phase composition and microstructure of materials were analyzed and the mechanical properties of the materials at room temperature were also tested; at the same time, static oxidation experiment was carried out. The static oxidation mechanism of the materials and the effect of the Cr content on structure and mechanical properties of materials were accordingly analyzed.A system of [Zr(Cr)]₂Al₄C₅ ceramics was designed in order to study the effect of different Cr content, and [Zr(Cr)]₂Al₄C₅ ceramics were fabricated by hot-pressing sintering method. The sintering parameters as follows: the sintering temperature is 1900℃, the sintering pressure is 30 MPa, and insulation for 1 hour.The main phase of the material is [Zr(Cr)]₂Al₄C₅ with a small amount of Zr₃Al₄C₆,ZrC,Cr₂AlC,Cr₉Al₁₇,Cr₃C₂ and Cr7C3 phase besides the main phase. The add-in of Cr can reduce the size of Zr₂Al₄C₅ grains, however, the enrichment of Cr can lead to the emergency of the second phase if the content is too much. With the increasing of the Cr content, the flexural strength of [Zr(Cr)]₂Al₄C₅ increases at first, then decreases, but the fracture toughness have been being improved all the way.The static oxidation resistance of the materials was evaluated and found out that the static oxidation dynamics curves and curves of oxidized layer thickness along with the oxidized time of [Zr(Cr)]₂Al₄C₅ at the oxidation temperature of 1000℃to 1200℃were similar to Zr₂Al₄C₅, and both of which followed k·tn parabolic law. And [Zr(Cr)]₂Al₄C₅ ceramics provided lower mass change and thickness of oxidized layer, which indicated that, compared with Zr₂Al₄C₅, oxidation resistance of [Zr(Cr)]₂Al₄C₅ has been improved significantly. [Zr_0.9Cr_0.1]₂Al₄C₅ showed better static oxidation resistance compared to other two [Zr(Cr)]₂Al₄C₅ ceramics in the same state of static oxidation. Acorrding to the analysis of the oxidized layer of [Zr(Cr)]₂Al₄C₅, at the initial stage of oxidation, the oxidized products are ZrO2 and Al2O3; and as oxidation proceeds, the composition of the oxidized layer turns into ZrO₂ and (Al_0.9Cr_0.1)₂O₃. The oxidized layer can be roughly divided into three layers: the rich Cr oxidized layer on the surface, the porous Al₂O₃/ZrO₂ layer beneath this layer and the transition layer between oxidized layer and the matrix. The improved behavior was mainly attributed to the the outmost rich Cr oxidized layer, which acted as an effective barrier against the inward diffusion of oxygen.