【作者】 吕和平；

【导师】 孙宝德； 王俊；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2010， 博士

【摘要】 热镀锌是目前钢铁材料防腐的主要手段之一,但是熔融的锌或锌铝合金对镀锌部件具有强烈的腐蚀性,如热镀锌锅中的沉没辊、稳定辊等镀锌部件,经常受到熔液的腐蚀而过早失效。镀锌部件除了受到熔液的浸蚀之外,还会受到熔液中的锌渣等硬质颗粒的磨蚀,因此其工况条件非常恶劣。热喷涂陶瓷涂层因具有良好的耐磨、耐蚀性能而常被用于热镀锌部件的防护,但是热喷涂陶瓷涂层的本征脆性常常导致其剥落失效,从而限制了其在热镀锌部件防护中的应用。针对这一问题,本研究通过分析目前国内外耐熔锌或锌铝合金腐蚀防护技术的研究现状,同时结合熔锌或熔锌铝合金对材料的腐蚀机理,开发出了一种新型不含金属粘结相的三元硼化物基陶瓷涂层,并且涂层中包含一定量的非晶、纳米晶组织,有望利用纳米晶提高材料强韧性的特点来解决普通陶瓷涂层的脆性问题,同时解决金属陶瓷涂层不耐蚀的问题。因此,本研究具有很高的应用价值和理论意义。本文研究技术路线如下:首先采用CALPHAD （Calculation of Phase Diagram）技术,对所研究的Co-Mo-B三元系进行相平衡热力学计算,以此指导初始粉体的成分配比。初始粉体成分配比确定后,通过湿法球磨、喷雾干燥、高温烧结以及筛分过程,成功地制备出了具有高松装密度和良好流动性、呈球形或近球形结构、平均粒度分布约为32μm的三元硼化物基陶瓷粉体。采用超音速火焰（HVOF）喷涂技术在316L不锈钢基体上制备出了三元硼化物基陶瓷涂层,利用扫描电镜（SEM）、透射电镜（TEM）、X射线衍射（XRD）、差示扫描量热仪（DSC）、示差热分析（DTA）,热膨胀仪,显微压痕和压汞法等分析检测手段,对涂层的组织结构特征、力学和物理性能进行了研究,并对涂层中非晶、纳米晶的形成进行了热力学分析。最后研究了涂层在熔融锌铝（Al-43.5Zn%-1.5Si）液中的耐腐蚀性能及其耐腐蚀机理。主要研究内容及结论如下:初始粉体的成分配比约为:Co:22.92wt.%,MoB:59.08wt.%,CrB:18wt.%。Borides-Co复合浆料具有剪切稀化的假塑性流体特征,其粘度随着剪切速率的增大而减小;当分散剂、粘结剂含量分别为固相含量的0.5wt.%和2.4wt.%,固相含量为浆料总质量的45wt.%时,复合浆料保持低的粘度和良好的分散稳定性,适合后续的喷雾造粒。喷雾干燥的最佳温度范围为300-325℃,在此温度区间能够得到最大的干粉收集率为73%,并且大部分颗粒形态是球形,流动性较好;通过热分析确定团聚粉体最佳的烧结温度约在1290℃左右,经检验在此温度下得到的烧结粉体中包含CoMo2B2和CoMoB两相,以及二元硼化物MoB、CrB,没有发现氧化物等其它物相,并且粉体内部呈多孔结构、颗粒之间不发生粘连现象。HVOF喷涂三元硼化物基陶瓷涂层组织结构致密,孔隙分布均匀,没有发现裂纹或大量未熔颗粒。涂层孔隙率约为6.38%,平均孔隙直径约为0.16μm,涂层的孔隙大小分布呈双态分布。经XRD检测分析,涂层中的物相主要有四种晶态相CoMoB、CoMo2B2、MoB、CrB和非晶相组成,各相的百分含量分别为CoMo2B2:45.8wt.%,CoMoB:19.7wt.%,MoB:2.3wt.%,CrB:16.4wt.%,非晶:15.8wt.%。纳米晶的尺寸小于10nm。涂层中非晶的形成是由于喷涂液滴的快速冷却及合适的粉末成分;由于后续熔融液滴的堆积对已形成的涂层产生退火效应,纳米晶以均匀形核与非均匀形核的方式分别在非晶内部和非晶与硼化物的界面形成。三元硼化物基陶瓷涂层显示出了较高的显微硬度、弹性模量和断裂韧性,这得益于涂层中非晶、纳米晶以及高硬度硼化物的存在。涂层的显微硬度、弹性模量和断裂韧性都表现出各向异性特征,Weibull分布显示:涂层表面和截面的显微硬度呈双态分布。涂层的结合强度随涂层厚度的增加而逐渐减小。三元硼化物基陶瓷涂层、粘结层和基体的热膨胀系数分别约为8.9×10^-6/K、14.3×10^-6/K、17.2×10^-6/K。三元硼化物基陶瓷涂层在熔融锌铝液中的浸蚀实验表明,涂层具有极好的耐腐蚀性能,在熔液中的寿命接近600h,远高于WC-Co（WC）、低碳WC-Co（LW）涂层。熔液和未进行热处理陶瓷涂层（TB）的润湿性要优于熔液和热处理陶瓷涂层（HTTB）的,这是由于TB涂层中包含非晶造成的。在熔液中浸蚀50h后,TB涂层表面局部有大约20μm左右厚的浸蚀区域,而HTTB涂层表面没有被浸蚀迹象;浸蚀600h后,TB涂层的浸蚀区面积基本没有变化,HTTB涂层仍然没有发现被浸蚀现象。这是因为TB涂层中包含非晶组织,虽然一般认为非晶是以金属键进行结合的,但其在热力学上处于一种亚稳态,其原子的活性要远高于晶态合金。所以在润湿的过程中,非晶合金中的原子容易浸蚀、扩散到熔融液中;涂层在600℃左右的熔融Zn-Al液中浸蚀一定时间后,涂层中的非晶逐渐转变为晶态组织,熔液对涂层的浸蚀也就基本停止。TB涂层浸蚀600h后,失效形式表现为涂层内部出现了许多平行于陶瓷层/粘结层界面的横向裂纹,而粘结层内部以及粘结层/基体界面没有裂纹的出现,这一部分原因是由于旋转浸蚀的过程中试样和锌铝渣硬质颗粒的碰撞造成的机械损伤,另外一个重要原因是由于热膨胀系数不匹配所造成的热应力损伤。更多还原

【Abstract】 Hot-dip galvanizing is widely used as one of the most important methods for resisting corrosion in metal products. However, the immersed bath hardware （e.g. the sink and stabilizer rolls） is subject to corrosive attack by the molten bath material （Zn or Zn-Al alloy） and often lead to premature failure. In addition to the corrosive effects of the molten bath material, the bath hardware is subject to erosive and abrasive attack from the hard intermetallic particles formed within the bath, so the working condition is very agressive. Thermal sprayed coatings are often used to protect the immersed bath hardware because of its excellent abrasive and erosion resistance. However, the intrinsic brittleness of the ceramic coating often lead to spallation failure, which limits its application range. In this paper, a novel ternary borides based ceramic coating containing amorphous and nanocrystalline phase was developed and does not contain metal binding phase by analyzing the research status of corrosion protection technique against molten Zn or Zn-Al, and combining with the corrosion mechanism of bath hardware against molten Zn or Zn-Al. Research results indicates that the ceramic coating contain amorphous phase and nano-crystal. The brittleness of ceramic coating is expected to be resolved because that nano-crystal can improve the toughness of materials and poor corrosion resistance of cermet coating also is expected to be effectively overcome. So the work has important theoretical and practical values.The technical routine of this research is as follows: firstly, we calculate the phase equilibrium thermodynamics of Co-Mo-B three system using CALPHAD （Calculation of Phase Diagram） technique, in order to guide the design for the composition of original powder. Following, the ternary borides based ceramic powder was prepared by the process of wet ball-milling, spray drying, high temperature sintering and calssifying. The sintered powder has good spherical shape and small size distribution so as to have good fluidity and high loose packing density, the mean particle size is about 32μm. Ternary borides based ceramic coating was prepared on the surface of 316L stainless steel by high velocity oxy-fuel （HVOF） thermal spray. The microstructures, physical and mechanical properties of the ceramic coating and powder were characterized using X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, differential thermal analysis （DTA）, differential scanning calorimerty （DSC）, thermal dilatometer, micro-indentation and mercury intrusion porosimeter （MIP）. The formation of amorphous phase and nanocrystalline phase were analyzed by thermodynamics principle. Finally, the corrosion resistance and mechanism of the ternary borides based coating against molten 55%Al-43.5%Zn-Si alloy were studied. The main work and conclusions in this study are described as the following:The composition of the original powder is: Co: 22.92wt.%,MoB: 54.32wt.%,CrB: 18wt.%. The borides-Co slurry exhibits characteristic of pseudoplastic fluid within basic scope. As the shear rate increases, the viscosity decreases. The optimum content of binder system was 0.5wt.% referred to solid, the best content of A15 as dispersant was 2.4wt.%, and the solid concentration is about 45wt.%. The optimized slurry shows good dispersity and stability and fit for spray pelletization.The suitable temperature range for the spray pelletization is 300-325℃. In this scope, the maximum collection rate of dry agglomerations is 73% and the agglomerations have good spherical shape and good fluidity. The optimum sintering temperature for the agglomerations is 1290℃,the sintered powder is composed of CoMo2B2, CoMoB, MoB and CrB,but no oxide and others phase were detected. The morphology of the sintered powder is characterized by a porous internal structure of particles and no conglutination phenomena were observed.The HVOF sprayed ternary borides basesd coating is a very dense coating with homogeneously dispersed micropores, while cracking or large amounts of unmelted particles are not observed. The surface connected porosity and mean pore diameter of the coating are 6.38% and 0.16μm, respectively. The surface connected porosity in the coating presents a typical bimodal pore size distribution. XRD results indicate that the coating consisted of CoMo2B2（45.8wt.%）, CoMoB（19.7wt.%）, MoB（2.3wt.%）, CrB（16.4wt.%） and amorphous phase（15.8wt.%）, the particle size of the nanocrystalline phase is less than 10nm.The formation of the amorphous phase in the coating was attributed to the high cooling rates of molten droplets and proper powder composition. The nanocrystalline phase could result from annealing of the pervious amorphous matrix during subsequent deposit of molten droplets, existing in the inner of amorphous phase and the interface between amorphous phase and borides by homogeneous and heterogeneous nucleation, respectively.The coating exhibit high microhardness, elastic modulus and fracture toughness due to composing of amorphous, nano-crystal and borides. Micro-mechanical test results show that anisotropy exists in microhardness, elastic moduls and fracture toughness between the cross section and the surface of the ceramic coating. The ceramic coating exhibits a bimodal distribution of microhardness values, as evidenced by their weibull plots. The adhesive strength of the coating decreases with increasing the thickness of the coating.?Coefficient of thermal expansion （CTE） for the ternary borides based coating, bond coating and substrate are 8.9×10^-6/K, 14.3×10^-6/K and 17.2×10^-6/K, respectively.The immersed test shows that the lifetime of the ceramic coating is as high as 600h in the molten Zn-Al alloy and is more higher than those of WC-Co and low-carbon WC-Co cermet coating. The equilibrium contact angle of Zn-Al alloy melt on the non heat-treated ceramic coating （TB） is smaller than that of the heat treated ceramic coating （HTTB）, which is attributed to the amorphous phase contained in the TB coating.After 50h immersion test, an erosion area with the thickness of 20μm appears at the TB coating while erosion area was not found at the HTTB coating. After 600h immersion test, the thickness of the erosion area at TB coating was not enlarged and the erosion area still does not appear at the HTTB coating. The corrosion mechanism of the TB coating is associated with the amorphous phase contained in the ceramic coating. Although it is generally accepted that the atomic bonding of amorphous alloy is primarily metallic, atoms in amorphous alloy are comparatively more active than those in crystalline alloy because the amorphous alloys are in thermodynamically metastable states. Atoms in amorphous alloy easily dissolve and diffuse in the molten alloy during wetting process. In molten Zn-Al alloy of 600℃, the amorphous phase would crystallized at a certain time, the erosion of the molten Zn-Al also stopped.After immersed for 600h, the failure modes of the TB coating exhibit as spallation and delamination at the ceramic coating. Horizontal microcracks parallel to the ceramic coating/bond coating interface are generated at the ceramic coating, which result in the failure of the ceramic coating. The microcracks in the ceramic coating are mainly caused by two reasons:1. Damage caused by collision between the ceramic coating and hard intermetallic particles formed within the bath during the rotation process;2. Thermal stress damage originated from mismatch of CTE of ceramic coating and that of substrate.更多还原