【作者】 朱庆军；

【导师】 邹增大；

【作者基本信息】 山东大学 ， 材料加工工程， 2008， 博士

【摘要】 随着现代工业的发展，对机械产品零件的表面性能要求越来越高，不少工件要求在高速、高压、重载及腐蚀介质工况下可靠持续的工作。在普通金属材料表面制备复合材料涂层，可以改善材料的表面性能，延长机电产品的使用寿命、减少环境污染。非晶-纳米晶复合材料由于具有较高的硬度、强度及良好的耐腐蚀性，在材料表面改性领域具有潜在的应用价值。本文用激光作为热源，熔覆预涂覆在45钢表面的合金粉末，利用激光快速加热及冷却的特性，制备出Fe-Ni-Si-B-V系列非晶一纳米晶复合涂层。并对熔覆层的微观组织、非晶相、纳米晶相的形成机制、微合金化对合金系的影响及熔覆层的磨损性能等进行了系统分析，研究了影响熔覆层组织及其性能的因素及规律。预涂覆合金粉末的组分及合金中的元素存在状态是影响激光熔覆制备Fe基非晶-纳米晶复合涂层中非晶相与晶化相形成的重要因素。在Fe-Ni-Si-B-V合金系中，B以硼铁的状态加入合金系比以B粉状态加入更有利于抑制熔覆层中晶化相的产生、降低熔池的液态温度，而且工艺性好、价格低廉，能够与母材获得结合良好的熔覆层，这归咎于硼铁的低共晶熔点以及硼铁中的硼相对单质硼更稳定的特性。Si作为合金系的组元之一，适量增加Si的含量，可增加熔覆层中非晶的形成倾向，同时还能防止合金系中其他组成元素氧化烧损，因此有利于提高熔覆层中非晶相、纳米晶相的含量；但过量的加入容易使合金系偏离其深共晶成分，不利于熔覆层中非晶相、纳米晶相的形成。在本试验条件下，根据各种加入元素量的比较结果，其中Fe-Ni-Si-B-V合金有利于非晶相、纳米晶相形成的最佳成分为Fe₃₆Ni₃₂Si₁₆B₁₄V₂。利用激光熔覆制备的Fe₃₆Ni₃₂Si₁₆B₁₄V₂非晶-纳米晶复合涂层没有裂纹、孔洞等宏观缺陷。利用X射线衍射、扫描电镜、透射电镜等分析结果表明，由于微区成分和冷却条件的差异，熔覆层的构成由表及里依次为细小的树枝晶区、粒状晶及非晶-纳米晶区和外延生长的树枝晶区。其中晶化相主要为Fe₂B和γ-Fe，Ni。熔覆层物相组成呈现明显的分层结构，而且熔覆层中的非晶-纳米晶相含量沿熔覆层的厚度方向变化。熔覆层的微观结构呈现为高度弥散的细小晶化相、纳米晶相、非晶相等复合组织，非晶相-纳米晶相主要存在于距熔覆层表面0．4～0．5mm区域。熔覆层中非晶相与晶化相相间分布，局部呈现纳米晶相镶嵌于非晶态基体上或存在于晶化相内部。熔合区晶粒以外延生长方式向熔覆层内部生长，基体与熔覆层呈现良好的冶金结合。Fe₃₆Ni₃₂Si₁₆B₁₄V₂非晶-纳米晶复合涂层的显微硬度从熔覆层表层至熔覆层中部存在着一个显微硬度缓慢上升的区域，熔覆层中部的显微硬度最高，从熔覆层中部至母材间显微硬度持续下降，呈现梯度分布。在同样载荷、同样磨损时间的情况下，Fe₃₆Ni₃₂Si₁₆B₁₄V₂非晶-纳米晶复合涂层的磨损体积只有母材磨损体积的1／6-1／10。熔覆层的摩擦系数与母材相比平均低0．1～0．2，摩擦过程也变得比较平缓。在激光熔覆的条件下，C、Nb、Ce等不同合金元素对Fe-Ni-Si-B-V合金微合金化作用不同，对晶化相、非晶-纳米晶相形成条件影响不同；其相对含量不同，对熔覆层的性能也有不同的影响。研究表明，C的加入有利于抑制熔覆层中晶化相的生长，促进非晶-纳米晶相的生成。与Fe₃₆Ni₃₂Si₁₆B₁₄V₂熔覆层相比，Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁熔覆层中部由复杂的纳米晶、非晶组成，其中纳米晶相为组成的主体，非晶相分布于纳米晶相之间，非晶相构成非常复杂，微观成分不均匀。Fe₂B等晶粒也变得细小、分布更加均匀。这是由于C的加入改变了液态熔池的Fe-C短程有序团簇的数量与分布，进而影响到其他元素与Fe的结合能力。磨损试验研究指出非晶-纳米晶含量的增加可有效提高材料的耐磨性，其机理为改变了磨损机制，在相同条件下Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁熔覆层的磨损体积仅有母材的1／15～1／22，主要磨损机制为磨粒磨损和粘着磨损。强碳化物元素Nb不利于Fe-Ni-Si-B-V合金中非晶相的形成，Nb的加入促进了NbC及（Nb,V）C形成的倾向，易于生成NbC-VC复合晶粒，为高温熔体提供了异质形核核心，因而降低了Fe₃₆Ni₃₂Si₁₆B₁₄V₂合金的玻璃形成能力，使得熔覆层中的非晶-纳米晶相含量下降。熔覆层的显微硬度下降，同时由于硬质点碳化物的增加，熔覆层整体显微硬度变化幅度加大。Ce作为微合金化元素，可以改变Fe-Ni-Si-B-V合金熔体的结晶体系，Ce的加入量小于1．0at．％时，抑制Fe₂B的结晶、促进γ-Fe，Ni相的产生；Ce含量为2．0at．％时，两种晶化相的结晶均受到抑制。Ce的添加形成了更多的非晶相，降低了起到强化作用的纳米晶相、Fe₂B等硬质相的含量，与Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁熔覆层相比，Fe₃₆Ni₃₀Si₁₆B₁₄V₂Ce₂熔覆层组织均匀、非晶相相含量更高，熔覆层的显微硬度下降、熔覆层变得更加均匀。由于Fe基非晶合金是高硬度、低断裂韧度的脆性材料，使Fe₃₆Ni₃₀Si₁₆B₁₄V₂Ce₂熔覆层的抗磨损性能变差。其磨损机制以以剥层磨损为主，磨损面光滑。研究表明，微量元素C、Nb、Ce中，C对Fe₃₆Ni₃₂Si₁₆B₁₄V₂熔覆层耐磨性的提高影响最大。更多还原

【Abstract】 Surface properties of machine parts need to be promoted increasingly because of the rapid development of modern industry. Most of them must work reliably and continuously under a high speed, high pressure, over loading or corrosion circumstance. Composite coatings developed on common metal benefit for improving surface properties of materials, prolonging the working life of machine parts and decreasing environment pollution. Amorphous-nanocrystalline composite materials are promising in the area of surface modification because of its high microhardness, strength and excellent corrosion resistant property. In this paper, alloy powders are pre-coated on 45 steel and Fe-Ni-Si-B-V amorphous-nanocrystalline composite coatings are developed by laser cladding. Microstructures of the coatings, growth mechanism of the amorphous phase and nanocrystalline phase, influence of microalloying on the alloy system and wear properties of the coatings are investigated in detail. At the same time, factors and rules influencing the coatings are studied.Composition and the element existing status of the pre-coated alloy powders are the key role of preparing Fe-based amorphous-nanocrystalline coatings by laser cladding. As far as Fe-Ni-Si-B-V alloy is concerned, it benefit for forming the amorphous phase and depressing the crystalline phase when B is added into the alloy in the status of boron iron instead of B powder. Further more, forming temperature of the molten pool can be decreased and the coatings have good metallic bonding with the substrate. At the same time, it is cheaper and has excellent processing properties. Si severs as not only one component of the alloy, but also the element preventing other component from oxidation. The content of the amorphous phase and nanocrystalline phase in the coatings can be improved by increasing the content of Si properly. However, the alloy deviates from eutectic composition and the amorphous phase and nanocrystalline phase is depressed if Si is over-added. The suitable component of Fe-Ni-Si-B-V alloy in favor of forming amorphous phase and nanocrystalline phase is Fe₃₆Ni₃₂Si₁₆B₁₄V₂ by laser cladding.Fe₃₆Ni₃₂Si₁₆B₁₄V₂ amorphous-nanocrystalline composite coating fabricated by laser cladding has no cracks and holes. The coatings are studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Because of the composition difference in micro-zone and crystalline conditions, the coating is build up by fine dendrites zone, cellular grains and amorphous-nanocrystalline zone and epitaxial growth dendrites zone from the surface to the bottom. The main crystalline phases are identified as Fe₂B and y- Fe, Ni. The coatings show a layer-structure obviously. At the same time, content of the amorphous-nanocrystalline phase in it vary along the depth of the coatings. The amorphous-nanocrystalline phase concentrates in the zone beneath 0.4⁰.5mm away from the surface. The amorphous phase and crystalline phase exist side by side in the coating. Moreover, nanocrystalline phase embedded in the amorphous substrate or in the crystalline phase locally. In the bond zone, the grains develop into the coatings in the form of epitaxial growth and the coatings have a good metallic bonding with the substrate. Microhardness of the Fe₃₆Ni₃₂Si₁₆B₁₄V₂ amorphous-nanocrystalline composite coatings increases slightly from the surface to the middle of the coatings. The hardest area is in the middle of the coatings. The microhardness of the coatings decreases from the middle zone to the base metal and shows a graded distribution. The wear volume losses of the coatings are one sixth to one tenth of the substrate under the same load and wearing time. Compared to the substrate, the friction coefficients of the coatings are lowered 0.1-0.2 than the substrate and the course of the wear are more gentle.When amorphous-nanocrystalline composite coatings are fabricated by laser cladding, different microalloying elements, such as C, Nb and Ce, have different influence on the forming conditions of amorphous-nanocrystalline phase and crystalline phase. Furthermore, the properties of the coatings vary with their different content.It is shown that the addition of C favorites of the forming of amorphous-nanocrystalline phase and depressing the generation of crystalline phase. Compared to the Fe₃₆Ni₃₂Si₁₆B₁₄V₂ coatings, the middle zone of the Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁ coatings is composed of complex nanocrystalline phase and amorphous phase. The majority one is nanocrystalline phase. Structurally, amorphous phase distributes among the nanocrystalline phase and is complexity and non-homogeneous. The grains of Fe₂B turn to more small and distribute more homogeneously. It is shown that the wear property of the materials is promoted by developing more amorphous-nanocrystalline phase, which alters the wear mechanism of the materials. The wear volume loose of the Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁ coatings are only one fifteenth to one twenty second of the substrate under the same load and wearing time. The main wear mechanisms are abrasive wear and cohesive wear.Nb element is unfavour of the developing of amorphous phase. Because of the generation of NbC-VC composite grains, when Nb is added to the Fe₃₆Ni₃₂Si₁₆B₁₄V₂ alloy, the high-temperature molten pool has cores for heterogeneity nucleation and the GFA （Glass Forming Ability） of the Fe₃₆Ni₃₂Si₁₆B₁₄V₂ alloy is depressed. This causes the decrease of amorphous-nanocrystalline phase and increase of the variation amplitude of microhardness.When Ce is added to the alloy, the crystalline system of Fe₃₆Ni₃₂Si₁₆B₁₄V₂ alloy is changed. The crystallization of Fe₂B phase is depressed and the crystallization ofγ-Fe, Ni phase is promoted when the addition of Ce is less than 1.0at. %. When the addition of Ce reaches 2.0at. %, both of the crystalline phases are depressed and the amorphous phase is promoted. Compared to the Fe₃₆Ni₃₁Si₁₆B₁₄V₂C₁ coatings, microstructure of Fe₃₆Ni₃₀Si₁₆B₁₄V₂Ce₂ coatings are more homogeneous and have much more amorphous phase. However, the microhardness of the coatings is lower. The wear properties of Fe₃₆Ni₃₀Si₁₆B₁₄V₂Ce₂ coatings lower because of the Fe-based amorphous alloys are hard and brittle materials. The main wear mechanism is peeling and the worn scars are smooth.It is shown that C has the most important influence on the wear property of Fe₃₆Ni₃₂Si₁₆B₁₄V₂ coatings among the microalloying elements.更多还原