【作者】 聂璞林；

【导师】 蔡珣； 沈耀；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2009， 博士

【摘要】 涂层与基体的结合性能是膜基结构系统的重要力学性能之一,其表征评价方法多种多样,但目前还没有一种方法可适用于所有的膜基系统。对于致密性较差、且存在较多缺陷的热喷涂层而言,可采用能量法评价体系,即通过界面裂纹扩展能量释放率与应力相角表征热喷涂层与基体的结合性能。相对于载荷法得到的膜基脱离所需临界载荷值,建立在断裂力学基础上的能量法评价结果具有更清晰的物理意义,且更适合表征复合应力状态下的界面结合性能。本文在国家自然科学基金(No. 50601018)和上海市纳米专项基金(No. 0359nm005)资助下,对MoB-CoCr热喷涂层与2Cr13钢基体结合性能进行了系统地研究,对能量法评价体系进行了有益尝试和探讨,主要研究结果如下:采用能量法评价体系时,必须首先知晓涂层的弹性模量,才能采用力学模型计算出界面断裂韧性,而对涂层弹性模量的测量需根据涂层组织结构和性能特点合理地选择测量方法。本文采用纳米压痕法、弯曲法、屈曲法对2Cr13钢和Al块体材料、(Ti,Al)N和Ti薄膜以及MoB-CoCr热喷涂层的弹性模量进行了系统的研究。研究中我们发现,对于宏观上组织结构和性能均匀的块状样品,纳米压痕法和弯曲法均可取得满意的结果。然纳米压痕法测试结果反映的是测试微区的力学性能,对微区组织结构相当敏感,分散性大,且在大压入载荷/深度条件下还受基底材料的影响。对于(Ti,Al)N薄膜/316L不锈钢基体样品,只有当压入深度<10 %膜厚时,基底材料的影响才能忽略,所测得弹性模量值才是可信的,否则为膜层与基底材料共同作用的综合弹性模量。弯曲法测试范围大,测试结果反映的是试样整体的力学性能,数据重复性好。因此,对致密度较差、且存在大量缺陷的MoB-CoCr热喷涂层,弯曲法更适合测量其弹性模量。由于孔隙的存在会降低涂层样品的整体刚度,在局部致密处采用纳米压痕法所测得的弹性模量通常高于反映涂层整体性能的弯曲法所测的弹性模量值。至于屈曲法,则在厚度非常薄的薄膜测量中更具优势。本文利用屈曲法对沉积在107硫化橡胶表面厚度仅100 nm Ti薄膜的弹性模量进行测量,得到弹性模量值为127 GPa。针对MoB-CoCr涂层与2Cr13钢结合性能测量,本文在三点弯曲和四点弯曲试验基础上,设计两套研究方案。三点弯曲试验使用单面涂层试样,并利用涂层弯曲变形产生的拉应力促使裂纹从涂层表面萌生并扩展到膜基界面上。四点弯曲试验使用sandwich结构试样,外层基体中有一预制缺口,且缺口底部靠近膜基界面。四点弯曲试验通过缺口处集中的应力(正应力)作用于界面,促使界面裂纹萌生扩展。利用有限元模型对三点弯曲和四点弯曲试验过程模拟,计算出系统能量的变化及裂纹尖端应力场,结合断裂理论计算出能量释放率与应力相角,完成对涂层与基体结合性能的评价。本文的弹塑性模型分析模型将金属基体塑性变形所耗散的能量考虑到能量释放率的计算模型中,相对于过去传统的四点弯曲法弹性分析模型,更适合于热喷涂层/金属基体系统。两种研究方案对MoB-CoCr与2Cr13钢结合性能评价结果相近似:三点弯曲试验得到能量释放率为73 J/m~2、应力相角为36.8 o;四点弯曲试验得到能量释放率为76 J/m~2、应力相角为37.1 o。由于涂层与基体材料的物性参数不同,在制备过程中涂层内总存在有残余应力。涂层中残余应力会影响裂纹扩展行为及界面断裂韧性,进而影响能量法结合性能评价。本文通过cohesive单元有限元模型模拟了裂纹在涂层中存在残余应力的条件下沿界面扩展过程,发现残余应力会影响裂纹萌生及扩展所需外力、裂纹长度、挠曲涂层的位移及系统消耗的能量。在利用这些参量评价界面结合性能时,评价的结果必然受到残余应力的影响。本文根据弹性力学,解出涂层中存在残余应力条件下圆形界面裂纹扩展能量释放率的解析表达式。该表达式将加载区作为有一定半径的圆形区域,避免了以往将加载区近似为点所带来的较大计算误差,较点载荷近似模型应用范围更广。此外,该表达式将残余应力引入到分析模型中,更适合分析涂层中有残余应力的情况。利用该表达式和有限元模型,对不同残余应力条件下的能量释放率进行数值分析,结果显示残余应力对能量释放率的影响呈非线性特征,其影响程度受加载区及界面裂纹几何尺寸的影响。更多还原

【Abstract】 Interfacial adhesion is one of the most important mechanical properties for coating/substrate system. In the past, a number of techniques were developed and applied to different coating / substrate systems. However, none of them can produce results under all conditions and each technique has intrinsic limitations. Among those techniques, energy evaluation system is based on well-known fracture theory and can be applied to study the bond strength of thermal spray coating in porous nature on the metallic substrate. Compared to those techniques that evaluate the adhesion with critical load needed to detach the coating, the energy evaluation system has advantages of clear analysis model and application in mixed-mode loading conditions. Under the support of national science foundation (No. 50601018) and shanghai Nano-foundation (No. 0359nm005), this work systemically studies the interfacial adhesion measurement of a MoB-CoCr coating/2Cr13 steel system. The main results include:The elastic modulus of the coating is required for fractural analysis of interfacial toughness by energy evaluation system. There are lots of techniques can be used to determine the modulus. Bit different results are often obtained by different techniques for one coating. In order to obtain reasonable result, the microstructure and performance of the coating should be considered in the selection of the measurement. The work studies the applications of the nanoindentation, bending and buckling techniques for bulk materials (2Cr13 steel and Al), films ((Ti,Al)N film and Ti film) and coating (MoB-CoCr thermal spray coating). For those samples with large thickness and homogenous mechanical properties such as 2Cr13 steel and Al bulk samples, the satisfied results that agree with the results of standard tensile test can be achieved by nanoindentation and bending techniques. However the results of the nanoindentation only reflect the mechanical properties of small volume and are sensitive to the microstructure in the local indentation region. If the indentation depth is relatively large compared to coating thickness, the results are also influenced by the mechanical properties of substrate materials. For (Ti,Al)N film, reliable results without influence of the substrate can be obtained when the indentation depth is smaller than one tenth of the coating thickness. In large indentation depth condition, the determined elastic modulus is the combined modulus of coating and substrate. The results of bending test reflect the integral properties of the overall sample structure, because whole sample experiences deformation in the bending test. For thermal spray coating in porous nature, bending technique is more appropriate to determine its elastic modulus if the integral mechanical properties of the sample are the objective. The modulus of the thermal spray coating determined by bending technique is smaller than that determined by nanoindentation, because the porous nature of the thermal spray coating. For the film of rather small thickness, buckling technique exhibits excellent performance. In the work, the buckling technique successfully determined the elastic modulus of Ti film, which was 100nm in thickness and covered on 107 vulcanized rubber, as 127 GPa.Two new techniques were designed for studying the interfacial adhesion of MoB-CoCr coating on 2Cr13 steel. One is based on three-point bending test, which uses single-faced coating samples. During the bending, the crack is induced by the tensile stresses in the surface of coating, followed by extending to the interface. The other is based on four-point bending test, which uses sandwich structured samples. A notch is prefabricated in one substrate covered coating, and the bottom of the notch is near the interface. During the bending, the normal stress in the interface is arisen by the stress concentration in the notch region, and results in the initiation and extension of interfacial crack. Using finite element analysis (FEA), the cracking processes in the bending tests are simulated for calculating the stress field at the crack tip and the consumed energy. With the data extracted by the simulation, the critical energy release rate and phase angle are determined for evaluating the interfacial adhesion. Compared to other reported techniques, the cracking ways and mechanical analysis models of those two techniques are different. Especially the dissipated energy by plastic deformation in steel is considered in the calculation. Therefore our designed techniques are more suitable for the thermal spray coating/metallic substrate system. Using those two techniques, a MoB-CoCr coating/2Cr13 steel substrate system was studied. The energy release rates and phase angle are respectively determined as 73 J/m~2 and 36.8 o by 3PB test, 76 J/m~2 and 37.1 o by 4PB testDue to the mismatch physical performance between the coating and substrate such as thermal expansion coefficient, the residual stresses always exist in the coating/substrate system. The presence of the residual stresses influence the cracking and interfacial toughness. The cohesive element FEA simulation shows that the residual stresses influence the load needed to induce the initiation and extension of the crack, the length and deflection of the delaminated coating, and the energy consumed in the cracking. Therefore the fracture analysis for evaluating the interfacial adhesion based on those mechanical quantities is also influenced by the residual stresses.A new close-form solution on energy release rate was obtained for analyzing the effects of the residual stress. Compared to other reported solutions, our work exhibits two features. One is the loading spot is a circular region with certain radius. Another is the extension of the crack is under the interactive action of the external force and residual stresses. Therefore our solution has wider range of application than the point loading model and is more suitable for studying the condition that the coating is in presence of residual stresses than free residual stresses model. Numerical analysis using the solution and FEA model shows that the influences of the residual stresses on the interfacial toughness present non-linear features and the degree to which the residual stresses influence the energy release rate varies with the geometry of the loading spot and interfacial crack.更多还原