| 爆炸喷涂WC-Co涂层的研究 | |
| Alternative Title | Studies on Detonation Gun Sprayed WC-Co Coatings |
| 王铁钢 | |
| Subtype | 博士 |
| Thesis Advisor | 孙超 |
| 2009-07-28 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料加工工程 |
| Keyword | Wc-co涂层 爆炸喷涂 力学性能 摩擦学性能 残余应力 自润滑涂层 |
| Abstract | 爆炸喷涂作为一种先进的热喷涂技术，常用于制备高质量的耐磨涂层。由于其喷涂速度快，粉末与喷涂束流作用时间短，从而大大降低了粉末材料的氧化和分解，能够很好地保持粉末和所喷制涂层成分的一致性。爆炸喷涂WC-Co涂层由于具有较高的硬度、结合强度及优异的耐磨性能，广泛应用于众多工业领域。涂层中硬质WC颗粒起着主要的耐磨抗磨作用，粘接剂Co提供韧性和支撑，由于其熔点较低，在高温下熔化，增强了涂层与基体及涂层自身的结合强度，而且能够提高涂层的冲击韧性，从而提高涂层的耐磨性能。 爆炸喷涂涂层的力学和摩擦学性能主要受涂层组织结构影响，而涂层组织中通常存在各种孔隙、微裂纹、层片边界及未熔颗粒等缺陷，并强烈依赖于喷涂工艺参数和粉末材料。本文采用正交试验设计方法，以300μm厚WC-Co涂层为例，分别研究了氧燃率、喷涂距离对爆炸喷涂涂层物理和力学性能的影响，并据此确定WC-Co涂层最佳喷涂参数为：氧燃率1.06，喷涂距离110mm。优化后的涂层结构均匀致密、硬度高、弹性模量大、耐磨性能好，经截面与表面力学性能、磨损性能对比，涂层表现出近似各向同性特征。经研究发现涂层特征参数与力学和摩擦学性能有如下关系：涂层孔隙率越低，表面越光滑，显微硬度越高，弹性模量越大，涂层-基体结合强度越高，涂层耐磨性能越好。 针对爆炸喷涂过程中喷涂颗粒发生散射现象，通过分析散射原因，本文成功设计了一套有效的分离装置。由于喷涂流束外围的散射颗粒，最先与周围空气进行热交换而迅速凝固，分离装置的工作原理是利用分离衬板将这些散射的凝固颗粒拦截，只允许与喷枪内径同轴同尺寸区域的熔化颗粒通过，形成涂层。以WC-Co涂层为例，使用分离装置后，涂层的表面粗糙度Sa、Ra，孔隙率和磨损率分别降低了77%、41%、40%和20%；而显微硬度HV、HKcs⊥、HKcs∥，弹性模量Ecs⊥、Ecs∥，和结合强度σmax分别增加了12%、13%、13%、32%、36%和16%。但它存在一个不可避免的缺点，即相对较低的粉末利用率，因为喷涂过程中少量的散射颗粒被拦截不能形成涂层。因此，适于应用在有高性能要求，且可接受增加成本的场合。 爆炸喷涂过程中产生的残余应力，常引起涂层试样的弯曲，严重时可导致涂层的脱落或破裂。另外，各种力学性能如硬度、弹性模量和结合强度等均受残余应力强烈影响。本文基于材料力学的基础知识及一些基本的弹性假设，推导了厚涂层残余应力计算公式，结合精确的曲率半径测量结果，计算了不同厚度爆炸喷涂WC-Co涂层的残余应力，并研究其对涂层力学性能的影响。得到一些重要结论：在固定的喷涂条件下，随涂层厚度增加，涂层内残余应力逐渐由较高的拉应力转变为压应力，当涂层厚度约为365μm时，试样近似处于无应力状态；涂层内存在残余压应力可提高弹性模量、显微硬度和结合强度，而残余拉应力在一定程度上削弱了涂层性能，在一定范围内，压应力越大，涂层性能越好。 最后利用爆炸喷涂技术分别研制了单一结构和多层复合的自润滑涂层，分析涂层的物相组成、元素分布及价态，孔隙率和表面粗糙度，研究喷涂粉末中MoS2-Ni不同含量对单一结构自润滑涂层微观结构、力学性能和摩擦学性能的影响，确定了最佳含量。尽管在高温喷涂时，少量MoS2发生了氧化分解，但未影响涂层的减摩效果。测试了多层复合自润滑涂层在不同磨损阶段的摩擦学性能，分析涂层减摩机制，并与优化的单一结构自润滑涂层进行对比研究。 |
| Other Abstract | Detonation gun (D-gun) spraying is often used to fabricate high quality wear resistant coatings as an advanced thermal spraying technology. The high spraying velocity and short acting time between the feedstock powder and spraying flow greatly reduce oxidation and decomposition of powder materials, and well keep powder compositon consistency with that of the resulting coating. D-gun sprayed WC-Co coatings are widely used in many industrial fields due to the high hardness and adhesion as well as excellent wear resistance. The hard WC particles in coating play a major role in wear resistance and antiwear, and binder Co provide toughness and support. Cobalt will be melted during high-temperature spraying due to the lower melting point, which enhances the adhesion and cohesion, thus impact toughness and wear resistance of the coating are improved. Mechanical and tribological properties of D-gun sprayed coatings are mainly determined by their microstructures, whereas coating defects usually comprise of various pores, microcracks, boundaries of splat and unmelted particles, etc. which strongly depend on spraying parameters and powder materials. Based on the orthogonal experiment design method and took 300μm thick WC-Co coating as an example, the effects of oxygen-fuel ratio and spraying distance on physical and mechanical properties of D-gun sprayed coatings were respectively investigated, and accordingly oxygen-fuel ratio of 1.06 and spraying distance of 110mm were identified as the optimal spraying parameters. The optimized WC-Co coatings exhibited the uniform dense microstructures, high hardness and elastic modulus, excellent wear resistance and near-isotropic behaviors, which has been verified by the comparisons of mechanical and wear properties between cross section and surface. The following relations were obtained: lower the porosity, smoother the coating, higher the microhardness and elastic modulus, better the adhesion, and better the wear resistance. During D-gun spraying process, the spraying particles showed an undesirable dispersion, when they were spurted out from the nozzle of barrel at high velocity and pressure. Through analyzing the dispersion causes, an effective separation device was successfully designed and constructed. Because the dispersed particles in periphery of spraying flame first exchange heat with ambient atmosphere and rapidly solidify, the working principle of separation device is intercepting these dispersed and solidified particles by separation liner plate, and only allowing the melted particles in the designated region which is coaxial and same size as the inner diameter of barrel to pass and form coating. After using the separation device, the surface roughness Sa, Ra, the porosity, and wear rate of the D-gun sprayed WC–Co coatings respectively decreased by 77%, 41%, 40%, and 20%, and the microhardness HV, HKcs⊥, HKcs∥, the elastic modulus Ecs⊥, Ecs∥, and the adhesive strength σmax increased by 12%, 13%, 13%, 32%, 36%, and 16%, respectively. At the same time, there is an inevitable disadvantage for using the separation device, i.e., the relatively lower utility rate of the feedstock powder, because small amount of dispersed particles will be intercepted and can not form coating. Therefore, the separation device is suitable to be applied in occasions of high-performance requirements where increased costs are acceptable. The residual stresses generated during D-Gun spraying process act as pre-existing stresses and can give rise to deformation of coated specimens and spallation or cracking of the coating. In addition, various mechanical properties, such as hardness, elastic modulus and adhesion strength, etc. are strongly influenced by the nature and magnitude of the residual stress. Based on fundamental knowledge of material mechanics and some basic elastic assumptions, a new calculation formula was developed to estimate the residual stresses of thick coatings. Combining with accurate curvature radius results, the residual stresses of D-Gun sprayed WC-Co coatings with different thickness were calculated. Furthermore, the effects of residual stress on mechanical properties of the coatings were also investigated, and some valuable conclusions were drawn. Under the fixed spraying conditions, the residual stress of D-Gun sprayed WC-Co coating varied gradually from a high tensile stress to a compressive stress with increasing the coating thickness. Meanwhile, the coating was in an approximately stress-free state when its thickness reached around 365 μm. Compressive residual stress could significantly improve the coating elastic modulus, microhardness and adhesion, whereas tensile residual stress impaired the coating properties to some extent. In a certain range, the larger compressive residual stress was, the better the coating properties. Finally, the single structured and multilayer composite self-lubricating coatings were prepared by D-gun spraying technology. The phase constituents of the coatings, element distribution and valence state, porosity and surface roughness were analyzed in detail. The effects of MoS2-Ni content in the feedstock powder on the microstructures, mechanical and tribological properties of the resulting single structured coatings were investigated systematically, and to determine the optimum content. Although a little of MoS2 were oxidized and decomposed during the high-temperature spraying process, the friction reducing effects of the coatings were not significantly affected. To analyze the friction reducing mechanism of multilayer composite self-lubricating coating whose tribological properties in different stages of wear were tested and compared with the optimized single structured self-lubricating coating. |
| Pages | 137 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17153 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 王铁钢. 爆炸喷涂WC-Co涂层的研究[D]. 金属研究所. 中国科学院金属研究所,2009. |
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