【作者】 李鹏；

【导师】 马勤； 贾建刚；

【作者基本信息】 兰州理工大学 ， 材料学， 2011， 硕士

【摘要】 0Cr18Ni9奥氏体不锈钢在非常苛刻的氧化条件下,开始阶段氧化增重缓慢,但是在某一个临界点其氧化增重现象明显,导致氧化皮的大量剥落。因此0Cr18Ni9不锈钢很难应用在对环境有严格要求的场合。Fe₃Si作为金属硅化物的一种,具有优越的高温抗氧化性能,其原因是保护性氧化膜SiO₂的生成。但是,Fe₃Si的室温脆性限制了其作为结构材料方面的应用。因此,将Fe₃Si用于材料的表面改性涂层材料则可以避开Fe₃Si的脆性,从而拓展Fe₃Si的应用领域。通过在0Cr18Ni9不锈钢表面渗硅并形成Fe₃Si金属间化合物渗层不仅提高不锈钢表面强度、硬度、耐磨性的同时,同时提高0Cr18Ni9不锈钢的高温抗氧化性。基于上述思想,本研究通过熔融盐非电解法在0Cr18Ni9不锈钢表面制备了不同含硅量的Fe₃Si型金属硅化物渗层。采用X射线衍射仪（XRD）分析了渗硅层的物相组成,用扫描电子显微镜（SEM）和附带能量色散谱仪（EDS）研究了渗层截面的形貌和成分,通过调整熔盐体系、渗硅温度和渗硅时间等工艺参数,研究了渗硅层对0Cr18Ni9不锈钢的综合力学性能特别是高温抗氧化性能,得到如下几条具有意义的结论:（1）利用NaCl:KCl:NaF=2:2:1中性熔融盐作为载体, Na₂SiF₆和Si粉做为渗硅剂采用不同的渗硅工艺,可以在0Cr18Ni9不锈钢表面获得不同质量和厚度的金属硅化物渗层。XRD分析表明:渗硅体系1和2在800℃下保温10h后的渗层表面物相均以Fe₃Si相为主。SEM对硅化物渗层断面分析表明:渗硅体系1和2在800℃下渗硅处理后渗层和基体结合紧密,并且在不同体系中保温不同时间后在基体和渗层交界处仍然存在由于Si,Fe原子扩散系数不一样而导致的柯肯达尔空隙带。渗层断面EDS分析表明:在渗硅处理过程当中发生了Fe,Cr,Ni元素向基体外的扩散和Si元素的向内扩散,并且在渗层和基体组织交界处发生了Si元素含量的突变。（2）对经渗硅处理5h和10h后的0Cr18Ni9不锈钢轴向拉伸力学性能分析表明,渗硅试样的比例极限大于不经过渗硅处理的0Cr18Ni9不锈钢,渗硅处理5h和10h后对0Cr18Ni9不锈钢的抗拉强度影响不大,硅化物渗层除了在试样表面呈台阶状断裂外,同时还沿柯肯达尔孔隙带断开,导致渗层的横截面积减小,是导致试样在弹性形变过程中出现不同弹性模量的原因。（3）研究了渗硅温度,渗硅时间,和渗硅体系对硅化物渗层生长速度和质量的影响,试验表明:渗硅温度越高硅化物渗层生长速度越快;当硅化物渗层达到一定厚度的时候,保温时间的延长并不能够很有效的增加硅化物渗层的厚度;在相同的保温时间和渗硅温度下,体系2所获得的渗层厚度明显大于体系1,但是相对于体系1而言体系2制备的渗层却存在比较多的缺陷。（4）高温氧化试验表明:Fe₃Si基硅化物渗层表现出了较0Cr18Ni9不锈钢优越的高温抗氧化性能。Fe₃Si基硅化物渗层在800℃和900℃条件下的高温循环氧化动力学曲线均表现为二次抛物线型。由于高温下生成了连续的SiO₂保护膜,使得900℃下的抗氧化性能更优。试验分析表明,试样在800℃下氧化层由Fe₂O₃和SiO₂组成,而在900℃下氧化层则是由Fe₂O₃、SiO₂和Cr₂O₃组成的混合氧化物。渗硅试样在800℃循环氧化过程中,硅化物渗层局部和0Cr18Ni9不锈钢基体之间结合比较紧密;而在900℃下循环氧化过程中,渗层与基体之间通过Si、Cr元素相互扩散使二者结合强度得到了增强。渗硅试样在900℃下形成的氧化膜比800℃下形成氧化膜更为致密,800℃下渗层表面氧化100h后的物相以Fe₂O₃为主,另外还有少量的SiO₂;900℃下SiO₂在氧化层表面的衍射峰大大加强。更多还原

【Abstract】 In very harsh Oxidation conditions,the weight gain of a0Cr18Ni9 austenitic stainless steel, is very slow ,But at some point the growth rate become largely ,lead to Large Oxide loss. For this reason 0Cr18Ni9 stainless steel is hard to use in Such occasions that have Strict requirements.Fe₃Si as a metal silicide has excellent oxidation resistance,the formation of SiO₂ play an imporant role in the oxidation resistance , but the room temperature brittleness limite silicide Fe₃Si as a structural material. But Fe₃Si used as a coating in the field of surface modification of materials which can avoid the brittleness of Fe₃Si and to expand the applications of Fe₃Si. Diffusion Coatings on 0Cr18Ni9 austenitic stainless steel in one hand can significantly increase strength, hardness, wear resistance on the surface of the 0Cr18Ni9 austenitic stainless steel and in the other hand can also increase High temperature oxidation resistance.Based on the above thoughts,Fe₃Si type silicide layer containing Cr and Ni alloying elements with different Si concent deposited on 0Cr18Ni9 austenitic stainless steel stainless steel were formed in molten salts. phase composition of silicon layer Analysis by x-ray diffraction By scanning electron microscopy （SEM） and energy dispersive spectrometer attached （EDS） of the diffusion layer cross-section of the morphology and composition.By means of Adjustment Molten Salt System, Temperature, and time.To improve the structure and performance of silicon coating and thus increase Mechanical properties and high temperature oxidation resistance of 0Cr18Ni9 austenitic stainless steel. Some useful conclusions are drawed :（1） using molten mixture of Na₂SiF₆:Si=8:2 as siliconizing agent and the molten halogenide of alkali metals of NaCl:KCl:NaF=2:2:1 as siliconizing agent carrier,can get different quality silicide coating. XRD analysis shows that: the surface of the sample after siliconized for 10h at 800℃in Silicon infiltration system 1 and 2 is Fe₃Si multivariate transition metal silicide.SEM image of cross-section of the silicide layer indicate that : combition betwween silicide coating and substrate. Is very tight but Kekendae Porosity zone exist betwween silicide coating and substrate dispite different Experimental parameters ,EDS analysis on cross–section showed that:in proceed of the experment Fe Cr Ni element spread from substrate to Silicide coating In contrast with the Si element,in junction between substrate and Silicide coating Occurred Sudden change of Si element concent.（2） Analysis showed that the axial tensile mechanical properties Proportional limit that siliconized for 10h and 5h larger than 0Cr18Ni9 austenitic stainless steel, siliconized for 10h and 5h have little effect on the tensile strength of stainless steel, silicide coating in addition to step-like fracture surface of the sample was outside, but also with a break along the Kekendaer pores, leading to the cross section of coating area decreases,and the reason for different elastic modulus of the sample.（3） The study concern influence of siliconized Temperature, siliconized time and siliconized System on Growth rate and quality of the Silicide coating. Analysis shows that: Growth rate Increase With siliconized Temperature up , When the silicide coating reaches a certain thickness, holding time is not very effective to increase the thickness of coating, In the same holding time and temperature , the infiltration system 2 obtained significantly greater than the thickness of system 1, but there is more penetrating layer defects infiltration system 2 than infiltration system 1.（4） Aspects of high temperatur oxidation Fe₃Si Silicide coating have good High-temperature oxidation resistance than bare 0Cr18Ni9 austenitic stainless steel . Cyclic oxidation kinetics of Fe₃Si Silicide coating at 800℃、900℃Performance Parabolic law In the high temperature oxidation process. As quickly form a continuous SiO₂ protective film,making the 900℃for better oxidation resistance. Experimental analysis shows that the composition of the Oxide layer is mixed with Fe₂O₃, SiO₂ and Cr₂O₃. In the high temperature oxidation process,at 800℃Silicide combine well with the 0Cr18Ni9 Stainless steel substrate In the high temperature oxidation process at 900℃Si ,Cr atom diffusion Strengthening the combine of the coating with the substrate. Oxide film on Silicide coating at 900℃is dense than at 800℃, Fe₂O₃ phase and small amount of SiO₂ was found in the Oxide layer ,the Peaks of SiO₂ at 900℃enhance greatly .更多还原