导　　师： 曾德长

学科专业： 080503

授予学位： 博士

作　　者： ;

机构地区： 华南理工大学

摘　　要： 作为Cr3C2和WC的一种潜在替代者，TiB/_2陶瓷不仅具有优异的力学性能，同时还具有良好抗高温氧化和耐腐蚀性能，使其成为制备金属基陶瓷复合涂层中的最佳增强候选材料。现有的研究结果表明，M-TiBB/_2涂层如Fe/(Cr/)-TiB/_2涂层等具有比常规粗晶Cr3C2-NiCr和TiC-NiCr涂层更优的耐磨粒磨损性能，但TiB/_2的本征缺陷如高脆性、低疲劳极限和难烧结等限制了该类涂层性能的进一步提高。纳米化技术尤其是涂层纳米化可望很好地解决这一问题，它能够在很大程度上提高涂层的断裂韧性、疲劳抗性、强度和膜基结合力等，进而提高涂层的耐磨耐蚀及其它性能，成为热喷涂领域的重要发展前沿和研究热点。本课题以高能球磨法制备的纳米Ni60-TiB/_2B复合粉作为热喷涂用结构喂料，利用超音速火焰/(HVOF/)喷涂技术在中碳钢上制备出了高质量的纳米复合涂层，并较系统地研究了其微观组织及其相关性能。与相同成分的常规微米复合涂层相比，纳米复合涂层的力学性能和耐磨耐蚀性能都得到了较大幅度的提高，具有很好的综合性能和使用价值，为高性能纳米涂层的制备和应用提供了新的途径和科学依据。 研究结果表明，高能球磨技术可成功制备出适宜直接用于热喷涂的纳米Ni60-TiBB/_2结构喂料。高能球磨20 h后，纳米TiB/_2增强相粒子均匀弥散地分布于Ni60粉内，其平均晶粒尺寸减少至38 nm并达到平衡状态。该球磨粉体的团聚颗粒外形呈近球状，大部分颗粒粒径在5μm以上，具有良好的流动性，稍加过筛后可直接用于HVOF喷涂。以高能球磨的纳米级粉体为原料，采用超音速火焰喷涂技术在中碳钢基体上成功制备出了高性能的纳米Ni60-TiBB/_2复合涂层。该纳米复合涂层的显微组织均匀致密，平均晶粒尺寸约为45.7 nm。与同成分的常规微米复合涂层相比，纳米复合涂层具有更高的硬度和断裂韧性，其平均显微硬度和断裂韧性分别达到1102 kgf. mm和-2 3.5 MPa.m1//2左右。Ni60-TiBB/_2复合涂层在600和800近似遵循抛物线规律，表明其循化氧化过程主要由扩散机制控制。但在相同的循环氧化条件下纳米 涂层具有更优的抗循环氧化性能，这主要是由于涂层中晶粒纳米化能够增加扩散通道而提高原子扩散速度，有利于更快地形成完整致密的保护性SiO℃℃高温下都具有较好的抗循环氧化性能。与同成分微米复合涂层的氧化增重曲线相似，纳米复合涂层的氧化增重曲线也，复合2-Cr2O3膜；同时晶粒纳米化在一定程度上降低了涂层与氧化膜之间的内应力，高了涂层与氧化膜间的结合力。 在室温和20~60 N载荷的无润滑干滑动磨损条件下，纳米Ni60-TiBB/_2复合涂层具有比同成分常规微米Ni60-TiB/_2B复合涂层更低的滑动摩擦系数和磨损体积损失，表明纳米复合涂层的抗滑动磨损性能更优异，这主要归因于纳米复合涂层中TiB/_2增强相均匀弥散分布以及喷涂喂料中纳米特征的良好继承。纳米复合涂层在磨损过程中都呈现出粘结磨损和磨粒磨损的特征。 纳米和常规微米复合涂层的热腐蚀行为表明，复合涂层在Na2SO4-60/%V2O5（摩尔比，下同）混合盐中的抗热腐蚀性能明显优于其在Na2SO4-30/%K2SO4混合盐的抗热腐蚀性能。这主要是由于Ni60-TiBB/_2复合涂层在热腐蚀过程中形成的SiO2膜溶于碱性环境，而在酸性条件下则基本不溶。在Ni60-TiB/_2B复合涂层的热腐蚀实验中，Na2SO4-60/%V2O5混合盐为酸性盐膜，腐蚀过程中能形成完整致密的保护性Si- Cr-O膜，而Na2SO4-60/% K2SO4混合盐则为碱性盐膜，初期形成的SiO2发生碱性溶解，最终表层形成一层疏松多孔的非保护性Ni-Ti-Cr-Si氧化膜。 As a potential attractive alternative to Cr3C2 and WC, titanium diboride /(TiB/_2/) withexcellent mechanical properties, wear resistance and corrosion resistance properties, whichmakes it become a promising reinforced material for metal matrix composite coatings. Inrecent work, TiBB/_2-based coatings such as Fe/(Cr/)-TiB/_2 have been deposited and exhibitedbetter abrasive resistance than that of TiC-the conventional C-NiCr and Cr3C2-NiCrHowever,coatings.the high brittleness and low fatigue resistance for TiB/_2 have become the mainrestrictions for the conventional TiB/_2BB -based coatings. Nanotechnology especiallynanostructured coating can solve this problem perfectly by improving the performanceproperties such as hardness, toughness, membrane-based adhesion, fatigue resistance andwear-corrosion resistance, which is the frontier and research hotspot in thermal spraying.In this thesis, nanostructured Ni60-TiBB/_2 composite coating was prepared by High VelocityOxygen Fuel /(HVOF/) spraying using ball milled nanocrystalline powders. The microstructureand performances of the milled powders and HVOF coatings were also studied.Nanostructured Ni60-TiB/_2 B composite coating exhibited better mechanical properties andwear-corrosion resistance than that of its conventional counterpart, then the comprehensiveproperties of the nanostructured coating was greatly improved as well. This work is expectedto play an important role in the preparation and application of high performancenanostructured coating. The results show that nanocrystalline Ni60-TiBB/_2 feedstock with qualified fluidity and particlesize distribution for thermal spraying is successfully synthesized using mechanical alloying.After 20 h mechanical alloying, nanocrystalline TiB/_2B particles are evenly dispersed in the ballmilled powders and their average particle crystal size approachs a constant value of 38 nmwith equilibrium. Most of the milled powders are above 5μm and are in approximatelyspherical shape with excellent fluidity, which are suitable for preparing nanostructuredcomposite coating by HVOF spraying with sized. Nanostructured Ni60-TiBB/_2 composite coating with excellent properties is successfullyfabricated via HVOF technique using high energy ball milled powders. The nanostructuredcomposite coating exhibits a compact and uniform structure with the average grain size of about 45.7 nm. The micro-hardness and fracture toughness of the nanostructured coating are1102 kgf. mm and 3.5 MPa.m respectively, which are much higher than those of theconventional Ni60-TiB/_2-2 1//2B composite coating. Both conventional and nanostructured Ni60-TiBB/_2 composite coating exhibt excellentcycle oxidation resistance at 600 and 800 . Their cycle oxidation kinetic behaviours allapproximately follow the parabolic law. This indicates that the oxidation processes arecontrolled by a diffusion mechanism. The nanostructured coating has better cycle oxidationresistance than that of the conventional coating under the same oxidation condition. Thereason for this improvement can be attributed to the information of the intact SiO℃℃2-Cr2O3protective layer, and the enhanced adhesion between oxide film and nanostructure coating.The sliding wear properties of HVOF-sprayed conventional and nanostructuredNi60-TiBB/_2 composite coating were studied comparatively at room temperature under ambientpressure and with applied load varied from 20 to 60 N. nanostructured composite coating possesses lower sliding wear coefficient and smallervolume loss at all applied load than that of the conventional composite coating, whichindicate that the nanostructured coating has better sliding wear resistance. The reason for thisimprovement can be attributed to the microstructual homogenization and the well preservednanostructure characteristic of the ball milled powders. Adhesive and abrasive wears arefound to be responsible for the wear down mechanisms of the nanostructured Ni60-TiB/_2The results show that theBcomposite coating. The results of hot corrosion behaviours indicate that the hot corrosion resistances ofcomposite coatings coated Na2SO4-60/% V2O5 are much better than that of composite coatingscoated with Na2SO4-30/% K2SO4. The reason is that the SiO2 layer formed during hotcorrosion tests can dissolve in Na2SO4-30/% K2SO4 with alkaline environment to form aporous and non-protective Ni-Ti-Cr-Si-O oxide film, but it has excellent chemical stability inNa2SO4-60/% V2O5 with acidic environment to form an intact Si-Cr-O protective film finally.

关 键 词： 高能球磨 超音速火焰喷涂 纳米 复合涂层 力学性能 高温热腐蚀性能 摩擦磨损性能

分 类 号： [TG174.442]

领　　域： [金属学及工艺] [金属学及工艺]