【作者】 常鹰；

【导师】 李溪滨；

【作者基本信息】 中南大学 ， 材料学， 2006， 博士

【摘要】 纳米材料在力学、热学、摩擦学等方面具有优良的性能，纳米粉体是纳米材料得以应用的基础，而纳米涂层是已获得应用的纳米材料之一。国内外对纳米粉体的制备研究进行了二十多年，但获得分散性优异、球形度良好且产率较高的纳米粉体仍然是其技术难点之一。另外，近年国内外均开展了热喷涂纳米涂层的制备及性能的研究，尽管这方面的研究报道不多，但已显示了良好的应用前景。本论文围绕这两个重要课题，采用乳浊液和均匀沉淀结合法制备球形纳米氧化锆粉体，在此基础上，采用大气等离子喷涂制备了纳米氧化锆涂层。鉴于目前国内外对纳米氧化锆涂层的研究绝大多数采用纳米涂层和传统的微米涂层横向比较的形式，作者在纳米氧化锆涂层的制备及性能表征等方面在纳米涂层间进行纵向比较，对于纳米氧化锆涂层作为新一代的热障涂层而言，着重研究等离子喷涂工艺参数对纳米涂层物相组成、显微结构和热冲击性能的影响；并专门就该涂层的热学性能、摩擦学性能以及力学性能进行了较为具体的研究，本论文的研究内容如下：（1）首次采用价格低廉的二甲苯为油相，span—80为表面活性剂，以及水相含量较高的乳浊液系统，将草酸二甲酯、Zr（NO₃）₄和Y（NO₃）₃溶入水相中，在水浴过程中使草酸二甲酯分解生成沉淀剂，使沉淀物直接在乳液包围的水相中均匀析出形成球形颗粒，从而采用乳浊液和均匀沉淀相结合的方法制备出了具备良好分散性和较窄粒度分布的球形纳米ZrO₂（Y₂O₃）粉末，并对该方法获得球形纳米氧化锆粉末的机理进行了分析。同时，为了获得理想的粉末，在充分考虑乳浊液的影响因素和均匀沉淀影响因素等的同时，在探索性实验的基础上，选择纳米氧化锆前驱体粉末的比表面BET作为正交实验考察指标，采用正交实验对制粉工艺进行了优化，其最佳工艺条件为：水相在乳浊液中的体积百分数为20、反应温度是50℃、草酸二甲酯与Zr（NO₃）₄摩尔比为2：1、活性剂在油相中的体积百分数为7、反应时间为3h。（2）计算了该纳米氧化锆粉末中各相的体积分数，即四方相为0.78，立方相为0.22，并利用晶面间距公式与Bragg方程，借助XRD衍射谱求出各相的晶格常数，包括立方相的点阵常数a_c，四方相的点阵常数a_c和c_t及四方相的晶轴比c_t／a_t值，这些值分别为：a_c=0.514357nm；a_t=0.51067nm，c_t=0.51799nm，晶轴比c_t／a_t=1.01433。在此基础上，计算了制备粉末的密度为6.0810g／cm²，并与公式d=6／ρS_BET（该公式前提是假设颗粒呈球形）中的理论物质密度相对比，从而从理论上说明了采用上述方法制备的纳米氧化锆粉末具有良好的球形度。（3）采用喷雾干燥技术制备了适合等离子喷涂的纳米氧化锆团聚粉末，考察了该粉末的性能。在进行探索性实验的基础上，确定了等离子喷涂的影响因素及水平，根据其影响因素及水平设计了正交表，按正交表的参数利用大气等离子喷涂技术分别制备了纳米结构氧化锆涂层，以涂层在1100℃水淬的抗热震寿命作为正交实验考察指标的基础上对喷涂工艺进行了优化，其优化的喷涂工艺参数应选择如下：功率为45KW；喷涂距离为120mm；送粉率为25g／min；氩气流量为40slpm；电流为580A；并较为系统地研究了喷涂工艺参数对涂层的显微结构和抗热震性能的影响。（4）计算了按优化喷涂工艺参数制备的纳米氧化锆涂层中各相的体积分数及各相的晶格常数，即四方相为0.91，立方相为0.09；另外，涂层四方相的晶格常数a_ct=0.51079nm，c_ct=0.51762nm，晶轴比c_ct／a_ct=1.01337；涂层内立方相的晶格常数a_ct=0.51460nm；将以上数值与纳米氧化锆粉末的数值进行对比，从而对等离子喷涂形成纳米氧化锆涂层过程中相转变机理进行了探索；并根据各相的晶格常数计算了该纳米氧化锆粉末及涂层中Y₂O₃的理论含量，通过与其实际含量相对比，确定了纳米氧化锆粉末及其涂层的物相组成。（5）在按优化的喷涂工艺参数成功获得纳米氧化锆涂层的基础上，对该涂层进行了如真空热处理、封孔等后处理，采用对比分析的方式考察了涂层及其处理后的抗热震性能和高温氧化性能，实验结果表明：对纳米氧化锆涂层进行封孔和真空热处理均对其抗热震寿命有不利的影响，封孔对纳米氧化锆涂层的抗氧化性能帮助不大，而真空热处理不利于涂层的抗高温氧化；另外，分析了涂层的相结构及表面形貌，并结合热冲击试验结果阐述了涂层的抗热震失效机理，即涂层在热震加热过程中，孔隙和早期存在的微裂纹附近的纳米颗粒发生长大，当大部分纳米颗粒或全部的纳米颗粒长大后，纳米氧化锆涂层相应的转变成准微米氧化锆涂层，随后其热震失效方式按传统涂层的失效方式进行；同时结合等温氧化实验结果对纳米氧化锆涂层的氧化机理进行了分析，即纳米氧化锆涂层的氧化机理遵从Wagner氧化理论。对纳米氧化锆涂层的热扩散系数进行了测量，分析了涂层及处理后涂层的热扩散系数的差异，同时，对热扩散机理进行了分析。（6）采用最佳喷涂工艺参数分别对两种喷雾造粒粉末进行等离子喷涂，制备了两种纳米氧化锆涂层S1涂层和B1涂层，对比研究了两种涂层的显微硬度、抗裂纹扩展力以及结合强度。结果表明：两种涂层的显微硬度测试值服从weibull分布，无论是表面还是截面的显微硬度值，S1涂层均优于B1涂层，两种涂层截面的显微硬度值均优于其对应表面的显微硬度值；另外，S1涂层的抗裂纹扩展力与结合强度均优于B1涂层。（7）对S1涂层和B1涂层摩擦学性能研究表明：两种氧化锆涂层的摩擦系数均随载荷增大而减小。在较低载荷（100N）条件下，S1涂层与不锈钢的摩擦系数低于B1涂层与不锈钢的摩擦系数；而在较高（400N）载荷下，两种氧化锆涂层的摩擦系数开始趋于一致。在摩擦过程中，S1涂层具有比B1涂层更好的抗磨损性能。在较低的载荷下，两种涂层与不锈钢摩擦副的摩擦磨损机制是不同的，S1涂层磨损的磨损属于粘着磨损，B1涂层的磨损属于磨粒磨损；在较高载荷下，两种涂层的磨损机制趋于一致，均为粘着磨损。更多还原

【Abstract】 Nanostructured materials possess excellent properties in mechanics,calorifics and tribological. Nano-powders are the base of application ofnanostructured material, and nanostructured coating is one of the appliednanostructured materials. The study on the preparation of nano-powdershas been engaged all over the world for two decades, but there is still oneof technology difficulties in gaining nano-powders with excellentdispersion, spherical shape and high productivity. On the other hand, thestudy in preparation and properties of nanostructured coating by thermalspraying is on the way. Though the research in this field is limited, itshows excellent foreground in its application. This dissertation encirclesthe two points mentioned above. The ZrO₂ nano-powders with excellentdispersing, spherical shape and narrow particle size distribution havebeen prepared via the coupling route of w/o emulsion with methyl oxalatehomogenous precipitation. At the same time, nanostructured zirconiacoating has been prepared by atmosphere plasma spraying. Consideringthe landscape orientation comparison study of nanostructured zirconiacoating and microstructured zirconia coating in the majority of theprevailing studies, the author longitudinally compared the preparationmethods and properties of nanostructured zirconia coating. The effects ofplasma spraying technologic parameters on phase composition,microstructure and thermal shock property of nanostructured zirconiacoating were examined. In addition, mechanics properties,thermophysical properties and tribological properties of nanostructuredzirconia coating were also studied.（1）The emulsion system containing high water phase was used forthe first time, in which system cheap xylol was used as the oil phase,span-80 as the surfactant, and aqueous solution containing Zr（NO₃）₄,Y（NO₃）₃ and methyl oxalate as the water phase. During the process ofheating water, methyl oxalate hydrolysis occurs to come into beingprecipitator, and precipitate become spherical shape grains in the waterdroplets of emulsion. Through this method, the ZrO₂ nano-powders containing 3.5% Y₂O₃ with excellent dispersion, spherical shape andnarrow particle size distribution have been prepared via the couplingroute of w/o emulsion with methyl oxalate homogenous precipitation.The principle of the coupling route of emulsion with homogenousprecipitation to attain spherical shape particle was analyzed. At the sametime, in order to attain ideal powder, considering the effect factors ofemulsion and homogenous precipitation, the author selected BET of ZrO₂precursor powders as review target of orthogonal test, and usedorthogonal test to optimize preparation process. The optimum technicalconditions were obtained as follows: The volume percentage of waterphase in emulsion is 20; the mol ratio of methyl oxalate and Zr （NO₃）₄ is2:1; the reaction temperature is 50℃; the reaction time is 3h; and G is 7.（2）The phase volume fractions of nano-powders were calculated.Tetragonal phase is 0.78 and cubic phase 0.22. Forthermore, the phasecrystal lattice constants were calculated, that is a_c is 0.514357nm, a_t is0.51067nm, c_t is 0.51799nm and c_t/a_t is 1.01433. At the same time, inorder to explain in theory the prepared powders possessing excellentspherical shape, the theoretical density was calculated.（3）Nanoscale zirconia powders, which are suitable for plasmaspraying, were reconstituted by spraying and drying before the plasmaspraying. The properties of the powder were reviewed. Basing onexploring experiments, the effect factors and levels of plasma sprayingwere confirmed and tetragonal table was planed. According to thetechnical parameters of tetragonal table, nanostructured zirconia coatingswere deposited by atmosphere plasma spraying. The author selectedthermal shock life of ZrO₂ coating as review target of orthogonal test, andused orthogonal test to optimize technical parameters. The optimumtechnical conditions were obtained as follows: Power is 45kw, sprayingdistance is 120mm, the ratio of powder feeding is 25g/min, the flowvelocity of argon is 40slpm and current is 580A. In addition, the effect ofspraying technical parameters on microstructure and thermal shockproperty of ZrO₂ coatings was systematically studied.（4）The phase volume fractions of nanostructured coating werecalculated. Tetragonal phase is 0.91 and cubic phase is 0.09. In addition, the phase crystal lattice constants were obtained by calculation: a_ct is0.51079nm, a_cc is 0.51460nm, c_ct is 0.51762nm and c_ct/a_ct is 1.01337.Through comparing these values of nano-powder and nanostructuredcoating, the principle of phase transformation during the process ofplasma spraying was studied. Y₂O₃ theoretic contents of nano-powder andnanostructured coating were calculated according to the phase crystallattice constants, and the phase composition of nano-powder andnanostructured coating were confirmed by comparing Y₂O₃ theoreticcontent and real content.（5）On the base of successfully gaining nanostructured zirconiacoatings via optimize spraying technical parameters, the coatings weresealed holes or heat-treated in vacuum. The thermal shock property andhigh temperature oxidation property of as-coatings were examinedthrough analytical comparison. Experimental results indicate that sealinghole or heat-treating in vacuum go against thermal shock life and hightemperature oxidation property of the coating. What’s more, the phasestructure and surface configuration of the coating were analyzed, and thethermal shock failure principle and oxidation principle of coating wereexplained basing on experiment results. At last, thermal diffusivities ofnanostructured zirconia coatings and coatings after treating weremeasured and analyzed contrastively, and thermal diffuseness principlewas analyzed.（6）Adopting the optimize spraying technical parameters, S1 coatingand B1 coating were prepared by plasma spraying with two kinds ofreconstituting and drying powders. Microhardness, adhesion strength andresist crack spread force of as-spraying coatings were studiedcontrastively, and results indicate that microhardnesses of S1 coating andB1 coating go with Weibull distribution, and microhardness, adhesionstrength and resist crack spread force of S1 coating excel those of B1coating. Cross-section microhardnesses of two coatings excel theirsurface microhardnesses.（7）The wear properties of S1 coating and B1 coating were examined.The experimental results show that the frictional coefficients of twocoatings reduce with load rising, the frictional coefficient of S1 coating against stainless steel in low load condition is lower than that of B1coating against stainless steel. But in high load condition, the frictionalcoefficients against stainless steel of two coatings tend to accord. Duringfriction process, wear properties of S1 coating excel that of B1 coating.At the same time, the wear principles of two coatings against stainlesssteel in different loads were studied.更多还原