摘要
Zr-Al-C陶瓷是一类在航天航空领域具有广阔应用前景的三元层状陶瓷,具有高熔点、高热导率及良好的抗氧化性能等特性,在超高温环境中能够保持物理和化学稳定性。本文以部分Cr元素替代Zr_2Al_4C_5陶瓷中的Zr形成[Zr(Cr)]_2Al_4C_5陶瓷,以提高Zr-Al-C碳化物陶瓷的抗氧化性。采用ZrC、Al、Cr_3C_2、炭黑粉为原料,利用热压烧结法制备了[Zr(Cr)]_2Al_4C_5陶瓷,并研究了材料的相组成及显微组织结构,进行了室温力学性能测试和静态氧化实验,并讨论了材料的静态氧化机理及Cr含量对材料组织性能影响。
本文为了讨论Cr含量对材料性能的影响,设计了[Zr(Cr)]_2Al_4C_5陶瓷材料体系,采用热压烧结法成功制备了[Zr(Cr)]_2Al_4C_5陶瓷,其制备工艺参数为:烧结温度1900℃、烧结压力30 MPa、保温1h。
烧结产物中的主相为[Zr(Cr)]_2Al_4C_5,除此之外还含有少量的Zr_3Al_4C_6、ZrC、Cr_2AlC、Cr_9Al_(17)、Cr_3C_2和Cr_7C_3相。Cr元素的加入能一定程度上减小Zr_2Al_4C_5的晶粒尺寸,但当含量过高时,Cr元素部分富集而形成第二相。而随着Cr含量的增加,[Zr(Cr)]_2Al_4C_5材料的弯曲强度先升高后降低,而断裂韧性则一直提高。
考察了材料的静态抗氧化性能,发现[Zr(Cr)]_2Al_4C_5材料在1000℃-1200℃下的静态氧化动力学曲线及氧化层厚度随时间变化曲线与Zr_2Al_4C_5材料类似,均遵循k·tn形式的抛物线规律,而[Zr(Cr)]_2Al_4C_5材料具有较小的单位表面增重和氧化层厚度,说明其抗氧化性能与Zr_2Al_4C_5材料相比有了相当明显地改善。在氧化条件相同的情况下,[Zr_(0.9)Cr_(0.1)]_2Al_4C_5材料的抗氧化性最好。材料在氧化初始阶段,生成的氧化产物为ZrO_2和Al_2O_3,随着氧化过程的进行,氧化层的成分转变为ZrO_2和(Al_(0.9)Cr_(0.1))_2O_3。氧化层可大致分为三层:表面的富Cr氧化层、中间较为疏松的ZrO_2/Al_2O_3层及氧化层与基体之间的氧化过渡层。在氧化最外层形成的富Cr氧化层,能起到有效的“屏障”作用,阻止氧的侵入,是其具有较好的抗氧化性能的关键。
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 Zr_2Al_4C_5 ceramics with part of Cr to synthesis [Zr(Cr)]_2Al_4C_5 ceramics on purpose of improving oxidation resistance of Zr-Al-C ceramics. [Zr(Cr)]_2Al_4C_5 ceramics were fabricated by hot-pressing sintering method with ZrC, Al, Cr_3C_2, 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)]_2Al_4C_5 ceramics was designed in order to study the effect of different Cr content, and [Zr(Cr)]_2Al_4C_5 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)]_2Al_4C_5 with a small amount of Zr_3Al_4C_6、ZrC、Cr_2AlC、Cr_9Al_(17)、Cr_3C_2 and Cr7C3 phase besides the main phase. The add-in of Cr can reduce the size of Zr_2Al_4C_5 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)]_2Al_4C_5 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)]_2Al_4C_5 at the oxidation temperature of 1000℃to 1200℃were similar to Zr_2Al_4C_5, and both of which followed k·tn parabolic law. And [Zr(Cr)]_2Al_4C_5 ceramics provided lower mass change and thickness of oxidized layer, which indicated that, compared with Zr_2Al_4C_5, oxidation resistance of [Zr(Cr)]_2Al_4C_5 has been improved significantly. [Zr_(0.9)Cr_(0.1)]_2Al_4C_5 showed better static oxidation resistance compared to other two [Zr(Cr)]_2Al_4C_5 ceramics in the same state of static oxidation. Acorrding to the analysis of the oxidized layer of [Zr(Cr)]_2Al_4C_5, 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 ZrO_2 and (Al_(0.9)Cr_(0.1))_2O_3. The oxidized layer can be roughly divided into three layers: the rich Cr oxidized layer on the surface, the porous Al_2O_3/ZrO_2 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.
引文
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