【作者】 杜宝帅；

【导师】 邹增大；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 磨损作为工程构件的三大主要失效形式（疲劳、磨损、腐蚀）之一,在工程应用中造成巨大的经济损失。在普通金属材料表面制备耐磨熔覆层,改善材料表面的物理、化学性质增强构件的抗磨损能力,成为了提高产品使用性能,发展维修与再制造技术,延长机械产品使用寿命的重要途径。本文利用激光（CO₂、Nd:YAG）作为热源,结合原位自生技术在低碳钢基体上熔覆制备了TiB₂、TiC、TiB₂+TiC增强Fe基耐磨熔覆层,并对熔覆层的微观组织、物相构成、增强相的生长机制、熔覆层磨损性能进行了系统研究,分析了影响熔覆层组织及性能的因素和规律。预涂合金粉末的组分和工艺性能是激光熔覆制备原位自生陶瓷相增强Fe基熔覆层的关键因素。利用FeTi30+FeB16预置粉末,采用CO₂激光熔覆制备了TiB₂增强Fe基熔覆层,所得Fe-Ti-B复合熔覆层中TiB₂呈条、块状均匀分布于基体之中。当熔覆粉末中B,Ti原子比例在1.8:1-2:1之间时可以获得以TiB₂+α-Fe为物相组成的熔覆层,该熔覆层具有较好的抗裂性。采用FeTi30+石墨作为预置粉末,CO₂激光熔覆可以制备原位合成TiC/Fe熔覆层,试验表明在综合考虑石墨和Ti元素在熔覆过程中的烧损量的情况下,预置粉末中Ti,C原子比为1:1.3的配比可以促进TiC增强相的生成,提高其在熔覆层中的含量并避免脆性相Fe₂Ti及高碳马氏体的产生。所得熔覆层中TiC以花瓣状和枝晶状存在于Fe基基体之中。采用Nd:YAG固体激光,以Fe+Ti+B₄C为预置粉末,制备了TiB₂+TiC联合增强Fe基熔覆层。当预置合金粉末中Ti和B₄C含量按照反应3Ti+B₄C=2TiB₂+TiC配制,采用较高功率密度时,易因熔覆过程中Ti元素的烧损导致熔覆层中产生Fe₃（B,C）脆性相,而且所得熔覆层中增强相的含量较低。优化试验表明,采用Fe45-Ti41.12-B₄C13.88（wt.%）作为预置粉末,较低功率密度时,所得熔覆层物相组成为TiB₂,TiC和α-Fe,避免了脆性相的产生,同时增强相TiB₂和TiC的含量较高。熔覆层致密、无缺陷,且同基材呈良好冶金结合,TiB₂和TiC增强相均匀分布于熔覆层之中,TiB₂呈条、块状,TiC粒子为尺寸较小的等轴状和花瓣状。TiB₂和TiC可以单独形核、生长,达到双相粒子复合强化的效果。而且增强相生长浓度环境的改变以及增强相之间的竞争生长,使得其在熔覆层基体中的分布更加分散。随着熔覆层稀释率的减小,增强相的含量和尺寸变大;随着熔覆层稀释率的增加,增强相的含量和尺寸减小。对熔覆层合金体系进行热力学分析表明Fe-Ti-B,Fe-Ti-C以及Fe-Ti-B-C体系中TiB₂、TiC在300K-2000K区间为稳定存在物相,预置粉末中增强相生成元素的相对原子比例对熔覆层的物相组成具有重要影响。预置合金粉末在激光作用下,首先经历加热过程形成低熔共晶,之后通过元素在熔体中的扩散、化合反应生成细小增强体（TiB₂、TiC）,在激光持续加热和反应放热的耦合作用下,生成的增强体还可溶解于熔体之中,在随后的冷却过程中增强相通过形核-长大方式生长。原位合成的TiB₂和TiC增强相均表现出小平面相特征,激光熔覆快冷过程并未带来增强相生长界面从光滑向粗糙的转变。TiB2增强相的（0001）,{10（?）0}界面能较低,具有较慢的生长速度,增强相粒子的形态表现为以（0001）为基面,{10（?）0}为侧面的棱柱形貌。TiC在形成过程中由于晶核在界面前沿熔体扩散驱动力以及成分过冷的作用下易发生界面失稳,枝晶主干沿着[001]方向发展使增强相生长为花瓣和枝晶状,枝晶端部的显露面为密排{111}平面。此外,在冷凝过程中还会因共晶反应形成细小的棒状和分枝状TiC。（TiB₂+TiC）/Fe熔覆层中TiB₂和TiC可以独立形核长大,其晶体生长惯习性并未改变,但在熔覆层内局部区域发现了TiB₂依附TiC长大的现象。通过原位反应生成的增强相同基体界面结合良好、洁净、无附着物及非晶相。室温干滑动磨损试验表明,原位合成TiB₂/Fe、TiC/Fe和（TiB₂+TiC）/Fe熔覆层具有良好的抗磨损性能,在同样磨损条件下,熔覆层的摩擦系数平均比母材低0.1-0.15。熔覆层内大量增强相（TiB₂、TiC、TiB₂+TiC）的存在使得磨轮在摩擦过程中对材料的粘着和犁削作用明显减弱,而且增强相在磨损过程中还起到承受载荷以及对熔覆层基体钉扎强化作用,因此使得熔覆层抗磨损性能得到显著提高。原位合成TiB₂/Fe熔覆层磨损体积为低碳钢Q235的1/17-1/19,其磨损机制主要为显微切削和硬质相剥落;原位合成TiC/Fe熔覆层的磨损体积为Q235基材的1/15-1/16,其磨损机制为显微切削和区域选择性粘着磨损;原位合成（TiB₂+TiC）/Fe熔覆层的磨损机制为显微切削、划擦,由于TiB₂和TiC的互补效应,增强相的分布均匀而且平均自由间距减小,从而使得熔覆层表现出最优的抗磨损能力,其磨损体积约为Q235金属的1/21。更多还原

【Abstract】 As one of the three most commonly encountered failure modes （fatigue, wear, corrosion） of engineering components, wear causes great economic loss in commercial application. The fabrication of wear resistant coatings on metallic materials through modifying the physical and chemical properties of surface becomes an important way to improve the quality of products, develop the technology of maintenance and reproduction, and extend the service time of mechanical products. In the present study, laser （CO₂, Nd: YAG） was employed with combination of in situ technology to fabricate TiB₂, TiC, TiB₂+TiC reinforced Fe-based wear resistant coatings on low carbon steel. Systematic analysis was carried out to study the microstructure and phase constituent of coating, formation mechanism of reinforcement and wear properties of coating. Besides, factors that have influence on microstructure and properties of coating were also investigated.It is found that constituent of preplaced powder and the related effect on processing are key factors of synthesizing in situ ceramic phase reinforced Fe-based coating by laser cladding. Using FeTi30+FeB16 as precursor, TiB₂ reinforced Fe-based coatings were produced by CO₂ laser cladding. TiB₂ particles with blocky and strip shape are distributed uniformly in the clad layer. Phase constituent of TiB₂+α-Fe is obtained when the atomic ratio of Ti to B in the preplaced powder is between 1:1.8 to 1:2. These coatings also show better cracking resistance. In situ TiC reinforced Fe-based coatings were fabricated by CO₂ laser cladding using FeTi30+graphite as precursor. By considering the burning amount of graphite and Ti during laser cladding, the selection of atomic ratio of Ti:C=1:1.3 can facilitate the formation of TiC, improve its content and avoid the presence of brittle phases such as Fe₂Ti and high-carbon martensite. TiC with flower-like and dendritic morphology is dispersed in the Fe-based matrix.In situ synthesized TiB₂+TiC reinforced Fe-based coating was fabricated by Nd:YAG laser cladding using preplaced powder of Fe+Ti+B₄C Brittle Fe₃（B,C） phase presented in the coating and the content of reinforcements was relatively low when the amount of Ti and B4C in the preplaced powder was selected based on the reaction of 3Ti+B₄C=2TiB₂+TiC and relatively higher power density was used. By using Fe45-Ti41.12-B₄C13.88（wt.%） as precursor and smaller power density, phases presented in the coating evolved into TiB₂, TiC andα-Fe, and the content of reinforcements also increased. Dese and defect-free coating with metallurgical joint to the substrate was obtained. Reinforcements dispersed uniformly in the matrix, and TiB₂ shows the morphology of block and strip while TiC takes on the shape of equiaxed and flower. This is because of the competing growth and deviation of concentration environment for TiB₂ and TiC. Moreover, due to the influence of dilution ratio, the content and size of reinforcements increase with the increasing of scan speed while the increasing of laser power results in the decreasing of content and size of reinforcements.It is shown by thermodynamic analysis that in Fe-Ti-B, Fe-Ti-C and Fe-Ti-B-C system TiB₂ and TiC possess lower Gibbs free energy from 300K to 2000K, and thus they are stable phases. The atomic ratio of elements for forming reinforcements is of importance to the phase constituent of coatings. During laser cladding, the laser-materials interaction induces the heating effect and formation of low-melting point eutectic. Fine reinforcements （TiB₂ and TiC） can be formed by diffusion and reaction. Furthermore, the coupling effect of laser heating and exothermic reaction make the reinforcement remelt into the liquid alloy. Reinforcements are formed by nucleation and growth. In situ synthesized reinforcements of TiB₂ and TiC exhibit faceted nature, which shows that the rapid solidification process during laser cladding does not transform the solid-liquid interface from smooth to rough.The （0001） and {10（1|-）0} planes of TiB₂ have lower interface energy and thus tend to grow more slowly, leading to the final morphology of TiB₂ with （0001） asbasal plane and {10（1|-）0} planes as prismatic faces. Due to the driving force induced bydiffusion and constitutional supercooling in front of the interface of nucleus, TiC tends to lose its interface stability. The dendrite arm of TiC grows along the [001] direction, which leads to the flower-like and/or radial dendrite shape of TiC. The faces shown at the tip of TiC dendrite are close-packed {111} planes. In addition, club-shaped and branch-like TiC was found owing to the eutectic reaction during solidification. In the （TiB₂+TiC）/Fe coating, TiB₂ and TiC nucleate separately, but their growing habitus is not changed. Phenomenon of TiB₂ growing on TiC particles was observed. The interface between in situ synthesized reinforcement and matrix remains strong, clean and free from deleterious and amorphous phase.It was shown by dry sliding wear test at room temperature that in situ synthesized TiB₂/Fe, TiC/Fe and （TiB₂+TiC）/Fe possess superior wear resistance. Compared with the substrate, wear coefficient of the coating decreased by 0.1-0.15 under the same wear environment. Uniformly distributed reinforcements with high amount can effectively decrease the adhesion and abrasion during sliding with the counter-wheel, resulting in the substantial increase in wear resistance. Wear mechanism of in situ TiB₂/Fe coating is micro-ploughing and peeling of reinforcement, and its wear volume is 1/17-1/19 of that of Q235 substrate; Wear mechanism of in situ TiC/Fe coating is micro-ploughing and selective adhesion, and its wear volume is 1/15-1/16 of that of Q235 substrate; Wear mechanism of in situ （TiB₂+TiC）/Fe coating is micro-ploughing and scratching, and due to the simultaneous presence of TiB₂ and TiC, reinforcements are distributed more uniformly with smaller free distance, leading to the best wear resistance （1/21 of Q235 metal）.更多还原