【作者】 路彩虹；

【导师】 井晓天； 葛利玲；

【作者基本信息】 西安理工大学 ， 材料科学与工程， 2008， 硕士

【摘要】 纳米晶体材料晶粒细小,界面密度高,表现出独特的力学、物理和化学性能,但是目前很难制备理想的三维块体纳米晶;而材料的失稳(比如磨损,腐蚀,疲劳等)多发生在材料表面,若在材料的表面制备出一定厚度的表面纳米层,可利用纳米晶的优异特性来提高材料的表面的性能。金属表面自身纳米化处理可以在金属材料的表面获得纳米晶组织,且纳米结构层与基体之间没有明显的界面,在使用过程中不会因为外界条件的变化而发生剥层和分离,能够提高材料整体的服役行为。气体渗氮是工业生产中提高材料表面性能的常用技术之一。表面晶粒细化可有效地加速化学热处理过程,纳米材料拥有丰富的晶界、位错等缺陷,这些缺陷为原子扩散提供了快速扩散的通道,可大大降低渗氮的温度,缩短渗氮的时间,提高元素渗入的浓度和深度。本文采用超音速微粒轰击(SFPB)技术对40Cr和20Cr钢表面进行轰击处理,在材料表层获得纳米晶组织。利用X衍射、光学电镜、透射电镜、显微硬度测量仪和摩擦磨损试验机等测试技术对样品的微观结构和力学性能进行了测试分析,对表面纳米化的形成机理及其对性能的影响进行了初步的探讨。并对表面纳米化处理后的试样分别在300℃,350℃,400℃,450℃下进行了气体渗氮实验,利用金相法,硬度法和XRD法对渗氮后的试样进行了表征,讨论了实现低温气体渗氮的原理。主要研究结果如下:1)SFPB处理以后,40Cr钢和20Cr钢试样表面均已形成等轴,随机取向的纳米结构层,最表面晶粒尺寸达到10nm左右。40Cr钢SFPB表面纳米化最佳工艺为0.4MPa气压下,轰击时间为60s～480s;20Cr钢最佳工艺轰击气压0.2MPa,轰击480s～3600s,纳米层厚度为30μm左右;随着距表面距离的增加,晶粒尺寸增大,塑性变形量减小;2)SFPB处理后表面显微硬度明显增大,为原始试样的二倍以上;摩擦磨损性能也有所提高,表现为摩擦系数减小,磨损量减少;3)SFPB处理的试样,在300℃低温,渗氮时间9h即可实现快速渗氮,表面形成化合物主要以γ′相为主,并含有少量的ε相。明显降低气体渗氮的温度,并缩短了渗氮的周期,降低成本。4)渗氮温度降低,时间缩短的主要因素为SFPB表面纳米化处理,晶粒细化为氮原子的扩散提供了大量的通道;晶界上存在大量的非平衡缺陷,提供晶界储存能,相对降低氮化物形成的自由能;试样表面粗糙度增加,与氮原子的接触面积增多,也有利于渗氮的进行。更多还原

【Abstract】 Nanocrystalline materials have attracted considerable interest because of their novel mechanical, physical and chemical properties originating from a large volume fraction of grain boundaries or interfaces with respect to the conventional coarse-grained polycrystalline materials. Most failures of materials occur on surface, including fatigue fracture, corrosion and wear etc., which are very sensitive to microstructure and properties of the surface. It is expected to achieve surface modification by the generation of a nanostructured surface layer, referring as surface nanocrystallization(SNC), and there is not obvious interface between the layer of nanostructures and the matrix, thus the nanostructures layer don’t separate and appear break-crust because of the change of external condition, so that the properties and behavior of materials are significantly improved.Gas nitriding is widely used in industrial production to improve the surface properties of materials. The surface nanocrystalization can accelerate the process of chemical heat treatment. Nanocrystal materials have abundant interface and dislocation, these defects supply ideal entryway for the diffuseness of chemical elements, and can markedly lower the temperature and time of chemical treatment and enhance infiltration concentration and deepness.A nanostructured surface layer was fabricated on a 40Cr and 20Cr steel by using supersonic fine particles bombarding (SFPB) technique. The microstructure and mechanics property of the surface layer of sample was characterized by means of X-ray diffraction, transmission electron microscope ,optical microscope, microhardness testing machine and friction and wear testing machine. The development mechanism and mechanics property of the surface layer was essentially probed. The nanocrystalized samples are nitridized respectively at the temperature of 300℃, 350℃,400℃,450℃and characterized by multiform test methods, such as optical microscope, microhardness and XRD. Then we bring forward the principium of low temperature nitriding. Experimented evidence show that:1) After SFPB, equiaxed nanocrystallines with random crystallographic orientations were obtained in the surface layer of 20Cr and 40Cr steel, the average grain size of the nanocrystallines approximates to10nm. Under the pressure of 0.4MPa, the prime duration of 40Cr steel ranges from 60s to 480s; Under the pressure of 0.2MPa, the prime duration of 20Cr steel ranges from 480s to 3600s. The depth of the nanocrystal layer is about 30μm.With the increase of the distance from the top surface, the crystal dimension increases and the plastic deformation decreases.2) After SFPB, the surface microhardness is obviously increased, is about twice of the original sample before SDFPB; the abrasion-resistance of the surface layer is significantly enhanced compared with that of original sample, which can be primarily attributed to the grain refinement. The friction coefficient decreases and the weight of wear reduces.3) The result shows that the nitriding temperature of the nanocrystalized samples could be as low as 300℃, which was much lower than the traditional nitriding temperature. The critical nitrogen potential to formεphase from SFPB was obviously reduced with respect to that of the coarse-grained form. And the depth of the nitriding layer was increased obviously.4) The principium of the low temperature nitriding treatment is: the surface naocrystallization supplies plenty of entryway for the diffuseness of nitrogen elements; abundant interface and defects supply interface deposited energy for lowing the formation free energy of nitride; the coarseness of the sample is increased after SFPB, which increase the contact area for nitrogen atom. These three factors benefit for nitriding treatment.更多还原