原位自生颗粒增强镍基激光熔覆涂层研究

【作者】 王文丽；

【导师】 晁明举； 梁二军；

【作者基本信息】 郑州大学 ， 光学， 2007， 硕士

【摘要】 激光熔覆陶瓷颗粒增强金属基复合涂层是一项先进的表面技术，它可改善材料表面性能，如耐磨性、耐蚀性、抗氧化、抗热震能力等。在该技术中，通常是将陶瓷颗粒增强相直接加入到涂敷材料中，但由于外加陶瓷相与合金基体的热物性参数差异较大，相容性差，使得增强相／基体界面结合强度较低，成为涂层中产生微裂纹等缺陷的潜在源，影响涂层强度。而激光熔覆原位自生陶瓷增强复合涂层的方法是在激光照射下，通过元素之间或元素与化合物之间的原位反应，在涂层内原位生成一种或几种高强度、高弹性模量的陶瓷增强相，从而达到强化基体的效果。由于这种增强体是原位形核、长大的热力学稳定相，其表面无污染，因而避免了与基体相容性不良的问题，且界面结合强度高。本文采用激光熔覆技术制备了几种原位自生颗粒增强的镍基复合涂层，对其制备、组织和耐磨性等进行了系统研究，结果和主要结论如下：一、激光熔覆原位自生TaC颗粒增强镍基复合涂层研究（1）在A3钢表面激光熔覆Ni60+（Ta₂O₅+C）混合粉末，首次成功制备出形貌良好、性能改善、原位自生TaC颗粒增强的镍基复合涂层；最佳制备工艺为：（Ta₂O₅+C）含量20wt％，离焦量47mm，激光功率1.6kW，扫描速度2mm／s；（2）Ni60+20wt.％（Ta₂O₅+C）熔覆层组织为：原位生成的TaC颗粒和Cr的碳化物枝状增强相均匀分布在γ（Ni）固溶体和晶间共晶体双相基体中；（3）原位自生TaC颗粒增强镍基复合涂层平均硬度HV_0.31100，与纯Ni60熔覆层(平均硬度HV_0.3800)相比，提高37.5％。摩擦试验表明，其耐磨性是纯Ni60涂层的5倍。原位TaC颗粒增强相的生成及其均匀分布是其显微硬度和耐磨性得以大大提高的关键因素。二、WC增强镍基合金激光熔覆层研究（1）在A3钢表面上激光熔覆Ni60+（WO₃+C）合金粉末，可获得形貌完好、组织致密、耐磨性优良的镍基复合涂层。合适的工艺参数为：（WO₃+C）含量15％、离焦量50mm，激光功率1.6kW，扫描速度2mm／s；（2）Ni60+15wt.％（WO₃+C）熔覆层组织组织结构主要为WC颗粒相和枝状相弥散分布在γ（Ni）基体中。由于WO₃与C反应最初在高温下生成W₂C，继续反应才生成WC。又激光熔覆过程是一个急热急冷的过程，所以很难控制WO₃的含量和保证WC的生成，在熔覆层中原位生成的WC含量较少；（3）Ni60+15wt.％（WO₃+C）熔覆层平均硬度达HV_0.31100，与纯Ni60熔覆层相比，耐磨性提高1.7倍。这是由于加入15％（WO₃+C）的复合涂层的细晶强化、固溶强化及马氏体相变强化的共同作用。三、原位合成硼化物颗粒增强镍基合金激光熔覆层研究（1）在A3钢表面激光熔覆Ni60+（MoO₃+B₂O₃）混合粉末，可以得到无裂纹的光滑的激光熔覆层。合适的工艺参数为：（MoO₃+B₂O₃）含量15％、离焦量50mm，激光功率1.6kW，扫描速度2mm／s；（2）Ni60+15wt.％（MoO₃+B₂O₃）激光熔覆层中上部组织为硼、碳化合物的网状共晶体分布在γ（Ni）基体中；（3）与纯Ni60熔覆层相比，加入适当比例（MoO₃+B₂O₃）的镍基复合熔覆层，虽然硬度稍有降低，但其耐磨性明显提高。这是由于加入（MoO₃+B₂O₃）后的涂层中粗大块状相消失，韧性相增加，以及涂层中共晶组织细化的结果。更多还原

【Abstract】 Laser cladding of ceramics particulate reinforced metal matrix composites （MMCs） on various traditional substrates is an advanced superficial modification technology, which can change the material surface performances, such as resistance to wear, erosion resistance, anti-oxidation and so on. In this technology, the reinforcements are usually formed by directly adding ceramics particulates into the coating materials. However, it is well known that there is great difference in the thermo-mechanical properties between the additional reinforcement ceramics particles and the matrix. The bad compatibility results in lower bond strength on the reinforcements/matrix interface that become the latent source of the microcrack and other flaws and eventually affects the coating’s strength. However, in the in-situ synthesis of composite coatings by laser cladding, one or more kinds of ceramics reinforcements with high strength and high elasticitic coefficient are formed by reacting between added elements or compounds under the laser illumination, which consequently strengthens bonding to the substrate. Because this kind of enhancement body is the thermodynamics steady phase that nucleates and grows from the matrix. The surface is pollution-free. The reinforcements are more compatible with the matrix and the interface bond strength is high. Therefore, in this thesis several kinds of in-situ synthesized particulates reinforced Ni-based composite coatings were produced by laser cladding and their microstructure and resistance to wear and so on were systematically investigated.The main results and conclusions are as the following:1. Investigation on in-situ synthesis of TaC particulate reinforced Ni-based composite coatings by laser cladding:（1） Good finish and in situ synthesized TaC particulate reinforced Ni-based composite coatings can be successfully achieved on A3 steel by laser cladding by employing a proper amount of Ni60+（ Ta₂O₅+C）-doping.. The optimum conditions are 20wt.% content of （Ta₂O₅+C）-doping, 1.6 kW laser power, 2 mm/s scanning velocity and 47mm defocusing length. （2） The microstructure of the coating is mainly composed of in-situ synthesized TaC white particulate phases and the dendrite phases consisting of mainly chromium carbide dispersed in the gray dendriteγ（Ni） solid solution and intercrystalline eutectic biphase matrix.（3） The composite coating gives very high average hardness of Hv_0.31100, which is 37.5 percent higher than the average hardness of Hv_0.3800 of pure Ni60 coatings, and excellent wear resistance which is five times as high as that of pure Ni60 coatings. In-situ Synthesis and uniform distribution of TaC particulates are the key factors of the coatings excellence hardness and wear resistance.2. Investigation of WC particulate reinforced Ni-based composite coatings by laser cladding:（1） An excellent Ni-based composite coating by laser cladding can be successfully achieved by laser cladding on A3 steel by employing a proper amount of Ni60+（WO₃+C）-doping. The optimum conditions are 15wt.% content of （wO₃+C）-doping, 1.6kW laser power, 2mm/s scanning velocity and 50mm defocusing amount.（2） The microstructure of the coating is composed of mainly WC particulate phases and dendrite phases dispersed in the gray dendriteγ（Ni） matrix. At first, WO₃ reacts with C to form W₂C, and then W₂C reacts with C to form WC. Since the laser cladding process is a rapid heating and cooling process, the content of WO3 is not easily controlled to form WC so that the content of WC is very small in the laser cladding.（3） Because of the combined action of grain strengthening, solidsolution strengthening and martensitic phase transformation strengthening, the composite coating under 15% content of （WO₃+C）-doping gives a very high average hardness of HV_0.31100 and excellent wear resistance which is 1.7 times as high as that of pure Ni60 coatings.3. Investigation on in-situ synthesis of boride particulate reinforced Ni-based composite coatings by laser cladding:（1） An excellent coating can be achieved by cladding proper content of Ni60+（MoO₃+B₂O₃）-doping into Ni60 under 1.6kW laser power, 2mm/s scanning velocity, 15% content of （MoO₃+B₂O₃）-doping and 50mm defocusing length.（2） The microstructure of the coating is composed of mainly reticulation eutectic of borides and carbides dispersed inγ（Ni） matrix.（3） The wear resistance is considerably improved at little expense of a decrease in hardness. It is due to the disappearance of larger brittle phases and increase of tough phases with proper content of （MoO₃+B₂O₃ ）-doping.更多还原