【作者】 钱峰；

【导师】 周霞；

【作者基本信息】 大连理工大学 ， 工程力学， 2010， 硕士

【摘要】 镁合金由于具有生物可降解性从而可作为生物体植入材料,因而在生物医学领域具有非常重要的应用意义,但镁合金存在着耐磨和耐蚀性较差的缺点,在镁合金表面进行刷镀则能很好的改善镁合金的这一缺点。本文主要研究了在镁合金表面电刷镀纳米羟基磷灰石颗粒(Hydroxyapatite particles, HAP)与纳米碳管(Carbon nanotubes, CNTs)混杂增强镍钴合金纳米复合镀层的微细观结构和力学行为。在Ni-Co/(HAP+CNTs)复合镀层中,Ni-Co合金作为金属基体,而纳米HAP及CNTs用做混杂增强相。该复合镀层表面光亮致密、与镁合金结合牢固、镀层综合力学性能优良,与人体组织具有良好的生物相容性,因此在生物工程中是一种具有潜在应用前景的新型生物复合镀层。本研究根据镁合金特点确定了其优化的刷镀工艺,制备出不同含量和混杂比的纳米颗粒与纤维增强的复合镀层,并运用金相显微镜(OM)、扫描与透射电子显微镜(SEM)、拉伸试验等手段分析了复合镀层的表面与截面形貌、微观结构以及膜基体系的拉伸力学性能等。结果表明：复合镀层中纳米增强相分布比较均匀,颗粒及纤维增强的复合镀层/基体体系拉伸性能优于纯镍钴镀层/基体体系,随着混杂增强相中颗粒含量的增多,镀层拉伸性能随之进一步提高；但随着纤维含量的增加,镀层结合力变差,镀层的性能在一定程度上随之下降；在实验范围内,当镀液中颗粒浓度为及纤维的混杂浓度分别为5g/L和0.1g/L时,镀层具有较高的强度和硬度等优良的综合性能。同热喷涂、气相沉积等涂层沉积过程相比,虽然电刷镀过程中的温度变化较小,但由于镁合金的热膨胀系数较大,所以对镀层热应力及其影响因素的分析是非常必要的,它直接关系到镀层的表面质量。本文利用有限元软件ABAQUS对镀层热残余应力进行了模拟分析。为了进一步表征热残余应力对镀层性能的影响,本文还对其进行了压痕过程的有限元模拟分析。结果表明,随着刷镀过程中温度的增高,镀层的最大热残余应力变大,这一点已通过理论计算与有限元模拟进行了双重检验。在压痕模拟过程中,对于相同的压深,初始温度越高,所需的压力越小,镀层的硬度也相应的越低。为了进行对比,文中对带有粗糙度的膜基体系进行了热分析及压痕模拟,结果表明,带有粗糙度的膜基体系的结合力更大,涂层的硬度也相对更高。此外,对热残余应力状态下的垂直裂纹镀层在拉伸作用下的开裂过程及裂纹尖端处的应力进行了模拟计算,计算结果有助于优化镀层的结构设计。对于复合镀层本身的拉伸力学性能,基于镀层微观结构的高分辨率扫描电子显微镜(SEM)及透射电子显微镜图像(TEM),采用软件coreltrace进行图形矢量化,建立了基于实际微观结构的复合镀层有限元模型,根据代表体元法对其施加周期性边界条件,对其准静态拉伸力学性能进行了模拟计算,并研究了镀层内颗粒与纤维的体积分数、混杂比、粒度分布等对镀层拉伸性能的影响。结果表明,纤维含量越多,镀层的抗拉性能越好,镀层内盒维数(分维数)越大,颗粒与纤维分布越均匀,镀层的抗拉性能也越好,拉伸模拟结果与拉伸实验结果相吻合。复合镀层力学性能大为改善的主要原因可归结为位错强化、晶粒细化、基体和混杂增强相的热胀失配及复合承载效应共同作用的结果。更多还原

【Abstract】 Because magnesium and its alloys can be used as biodegradable material for implants, the corresponding research has very important significance in the biomedical field. However, magnesium alloy has poor wear and corrosion resistance and brush-plating on its surface can overcome the shortcoming. In this paper, the microstructure and mechanical behavior of nanocomposite coating reinforced with HAP (Hydroxyapatite) and CNTs (Carbon nanotubes) on the surface of AZ91D Mg alloy have been studied. The Ni-Co was used as the metal matrix, and nano-HAP and CNTs were chosen as the hybrid reinforcements. The composite coating is bright and compact, and it has strong interface binding strength and excellent biocompatibility. So the coating is a new type of potential biomaterial in bone tissue engineering.According to the characteristics of magnesium alloys, its optimal brush plating process was determined and composite coatings with different content and ratio of nano-sized HA particles and CNTs were prepared. The surface and cross section morphology, microstructure and tensile performance of the coatings were analyzed by the means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tensile testing. The results show the hybrid nano-reinforcements is well distributed in the coating, the tensile properties of particle and fiber reinforced composite coatings are better than that of pure nickel and cobalt coating. The tensile properties of the coating are increased with the increase of particle content, while the tensile properties and interface bonding strengths are decreased with the increase of fiber content. In the experimental range, the coating has excellent comprehensive properties such as higher strength and hardness when the concentration of HAP is 5g/L and the concentration of CNTs is 0.1 g/L.Comparing with thermal spraying and vapor deposition etc., there are small temperature changes in the brush plating process. However, there often existed large thermal stress due to bigger thermal expansion coefficient changes. Using ABAQUS finite element software, the thermal residual stresses of the coating during the cooling process are calculated and the simulation of indentation is also conducted to describe the influence of the thermal residual stresses. The results show that the maximum thermal residual stress is increased with the increase of temperature, which has been verified by theoretical calculation and FEM. In the simulation of indentation, for the same impression depth, the higher the initial temperature is, the smaller the pressure is, and so the lower the hardness of the coating is. In order to compare with the above results, the thermal stress analysis and indentation simulation of rough film-substrate systems are also carried out.The results show that the binding strength is greater and the coating hardness is relatively higher for rough film-substrate systems.Cracking process and its stress distribution of vertical crack under thermal residual stresses have been simulated.Using periodic boundary conditions, the finite element model based on graphical vectorization of SEM and TEM images by software coretrace is built, and the tensile properties of coating are simulated. Effects of volume fraction, hybrid ratio and size distribution on the coating are calculated. The results show that when more CNTs are added and the box dimension is bigger, the more uniform the distribution of the particles and fibers are, thus the better the tensile properties of coatings are. The simulation results are consistent with that of the experiment. The mechanical property improvements of the hybrid nanocomposites can be attributed to CTE mismatch between the matrix and the reinforcements, Orowan strengthening, grain refinement, and the load bearing effects.更多还原