【作者】 陈响明；

【导师】 易丹青；

【作者基本信息】 中南大学 ， 材料学， 2012， 博士

【摘要】 涂层刀具结合了基体高强度、高韧性和涂层高硬度、高耐磨性的优点,适用于高速度、高精度的自动化加工。涂层刀具在高速切削领域有巨大的发展潜力和广阔的应用前景,使用涂层刀具可以获得明显的经济效益。本文以TiN-TiCN-Al2O3-TiN系列多层复合涂层硬质合金刀具为研究对象,对涂层的沉积热力学、流体力学以及沉积动力学,多层复合涂层的微观结构,元素在多层复合涂层中的扩散行为,残余应力分布,界面结合力以及抗氧化行为进行了详细深入的研究,得到以下几方面的结论：1)以硬质合金基体材料表面TiN-TiCN-Al2O3-TiN多层复合涂层为研究对象,计算了各子涂层的吉布斯自由能与温度的关系,计算了料盘内流体力学特征参数值如扩散长度、Peclet系数、Thiele长度和Thiele模量、雷诺数、Damkohler数以及Knudsen数,探讨了MT-TiCN及Al2O3涂层的沉积动力学；研究发现,在860℃-890℃温度范围内MT-TiCN沉积过程受表面反应动力学控制,其活化能为254kJ/mol, A12O3涂层的沉积过程主要受水蒸汽的生成反应所控制。热力学、流体力学及动力学研究为涂层制备的工艺参数设计提供了指导。2)设计和制备了iN/MT-TiCN/Al2O3/TiN多层复合涂层,利用扫描电镜、透射电镜、能谱仪以及二次离子质谱仪详细研究了多层复合涂层中各子涂层的微观结构；研究结果表明,TiN过渡层在WC晶粒上主要以外延生长为主,而在Co晶粒上主要以形核生长为主；设计与制备了C、N含量呈梯度分布的MT-TiCN涂层,不同生长阶段MT-TiCN涂层的微观结构明显不同；在MT-TiCN和Al2O3涂层间设计与制备了TiCO涂层,Al2O3在TiCO层上重新形核,Al2O3涂层呈柱状多晶体结构生长。3)探讨了基体和涂层组成元素在基体/多层复合涂层界面处的扩散行为。基体中的Co在涂层中主要沿晶界扩散,而W在涂层中的扩散由沿晶界扩散和体扩散两种方式组成,其在涂层中的扩散距离更大；探讨了Ti、Al元素在TiCO/Al2O3界面的扩散行为；研究发现,Ti、Al原子在TiCO/Al2O3界面处的扩散距离均很短或不扩散。4)计算了TiN/MT-TiCN/Al2O3/TiN多层复合涂层中的残余应力分布、涂层的断裂能、涂层产生裂纹的临界厚度、临界开裂应力和临界温度；利用X射线应力仪测试了样品中的残余应力；研究结果表明,基体和涂层间细小的热膨胀率差别会在涂层中形成较大的热应力；提高基体的韧性以及提高涂层的断裂能是提高涂层开裂临界厚度、临界开裂应力、临界开裂温度的主要途径；探讨了涂层开裂时残余应力的释放,研究发现,喷砂和喷丸可有效增加涂层中的裂纹密度、释放涂层中残余应力。5)利用划痕试验研究了多层复合涂层的结合强度,并对划痕形貌进行了深入分析；结果表明所设计的多层复合涂层与硬质合金基体之间均具有良好的结合强度。探讨了基体成份及涂层组合方式对涂层/基体界面结合强度的影响；研究发现,基体中WC的粒度、Co含量、立方碳化物相的加入以及表面是否形成富钴层均会对结合强度产生明显影响；多层复合涂层厚度的增加以及Al2O3涂层的加入使界面结合强度明显下降；用TiC代替TiCN形成TiN/TiC/Al2O3/TiN多层复合涂层并与不同的基体组合后,其在压痕试验中可承受的载荷要增加。多层复合涂层中存在多种破损形式,破损不仅发生在涂层与基体之间,也发生在各子涂层之间。6)研究了四类典型多层涂层试样在600℃-950℃和30min-120min条件下的抗氧化性能；与硬质合金材料的氧化相比,涂层试样在氧化过程中的活化能较低,更易受扩散动力学控制,涂层试样的抗氧化性能明显好于硬质合金基体试样；没有Al2O3的多层复合涂层由外及内均匀氧化,含有Al2O3子涂层样品的氧化过程主要是通过表面的热裂纹进行；致密完整的Al2O3子涂层能有效的保护多层复合涂层和硬质合金基体,并使其抗氧化性能远高于无Al2O3涂层的试样。更多还原

【Abstract】 Combining high strength and ductility of matrix with high hardness and abradability of coating, coated cutting tools are widely used for high rate and accuracy automatic process. Due to their great potential and broad prospect in high-speed cutting field, coated cutting tools have great economic benefits. In this paper, TiN-TiCN-Al2O3-TiN multi-layer coated carbide tools were studied. Deposition thermodynamics, fluid mechanics, deposition dynamics of coating, the microstructure of multi-layer coating, diffusion of elements in multi-layer coating, residual stress distribution, interface cohesion and oxidation resistant were investigated in details. The conclusions were as followings:1)TiN-TiCN-Al2O3-TiN multi-layer composite coated carbide tools were investigated, relation between Gibbs free energy and temperature of sub-coating as well as fluid mechanics characteristic parameters of tray, such as:diffusion length, Peclet, Thiele length, Thiele modulus, Reynolds, Damkohler and Knudsen were calculated, deposition thermodynamics of MT-TiCN and Al2O3coatings was discussed. The results showed that the deposition of MT-TiCN coating was controlled by surface reaction kinetics in860℃-890℃, activation energy was calculated to be254KJ/mol. The deposition of Al2O3coating was mainly controlled by formation of water-vapor. A guide for design of processing parameters was supplied by calculating thermodynamics, fluid mechanics and dynamics.2) TiN/MT-TiCN/Al2O3/TiN multilayer composite coating were designed and prepared, and the microstructure of every sub-coating in multilayer system were studied in detail by SEM, TEM, EDS and SIMS. The results indicated that the growth pattern of TiN transition layer in WC grain was primarily epitaxial growth, whereas that in Co grain was primarily nucleation growth. MT-TiCN coating which the content of C and N was gradient distributed were designed and prepared, and the microstructure of MT-TiCN coating in different growth stage was obviously different. TiCO coating was designed and prepared inside MT-TiCN coating and Al2O3coating. Al2O3was nucleated renewedly, and the growth pattern of Al2O3coating was cylindrical polycrystalline growth.3) The diffusion behavior of elements in matrix/multilayer interface was discussed. The results showed that Co in matrix diffused along grain boundary inside the coating, whereas the diffusion pattern of W was grain boundary diffusion and volume diffusion, and the diffusion distance of W inside the coating was larger than other elements. The diffusion behavior of Ti and Al in TiCO/Al2O3interface indicated that the diffusion distances of Ti and Al in TiCO/Al2O3interface were both short, and they even did not diffuse.4) The residual stress distribution, fracture energy, critical thickness of crack in the coating, critical crack stress and critical temperature of TiN/MT-TiCN/Al2O3/TiN multilayer composite coating were calculated. The residual stress was measured by XRD analysis. The results demonstrated that small difference of thermal expansion rate between matrix and coating can form relatively large thermal stress. It is the primary methods that the improvement of critical thickness, critical crack stress and critical temperature of crack in the coating can be improved by enhancing the ductility of the matrix and fracture energy of the coating. The relaxing of residual stress was studied when the crack of coating happened. The results pointed out that sand blasting and shot peening can effectively increase the crack density, and release the residual stress of the coating.5) The bond strength of mutil-layer coating was investigated by scratch test, and the morphology of sample scratched was also analyzed. The results indicated that good combination between mutil-layer composite coating and cemented carbide matrix was formed. The influence of component of matrix and combination modes of coating on the bond strength was discussed. The bond strength was affected obviously by particle size of WC, contents of Co, addition of cubic carbide and the formation of cobalt-rich layer. The bond strength decreased clearly with the increasing of coating thickness and the formation of Al2O3coating. The strength increased in indentation test caused by the substitution of TiC in TiN/TiC/Al2O3/TiN multilayer with different matrix. There are various kinds of fracture form exist in multi-layer composite coating, and fracture was not only formed between coating and matrix but also exist in each sub-coating.6) The oxidation resistance of four kinds of multilayer coating sample was studied under the condition of600℃-950℃and30min-120min. Compared with the oxidation of cemented carbide, the activation energy of coating sample is lower and was controlled by diffusion dynamics more easily. Hence, coating sample exhibiting much better oxidation resistance compared with cemented carbide matrix. Both internal and external of multilayer coating without Al2O3were oxidized, and furthermore, the oxidation of Al2O3sub-coating was processed along with the heating crack in surface. Dense and integrated Al2O3sub-coating can protect the multilayer coating and matrix effectively, which also improved the oxidation resistance obviously.更多还原