金属基陶瓷涂层弹性模量和界面断裂韧度

对双面涂层三点弯曲试验方法进行了改进(两面涂层不必等厚度),基于该理论对一种铁基陶瓷涂层进行了试验研究,并与单面涂层的三点弯曲试验结果进行了比较.结果表明,随涂层厚度的增大,弹性模量增大.分析认为,一方面涂层越厚,测量精度越高;另一方面涂层厚度增大,孔隙率降低,使得弹性模量增大.研究发现涂层弹性模量与喷涂方向有关,呈现了各向异性的特性.文中还介绍了测量涂层/基体界面断裂韧度的研究进展,着重介绍了测量界面临界应变能量释放率的四点弯曲试验、改进的拉伸试验方法以及采用威布尔应力评价界面强度的局部法.     

Miniaturized bend tests on partially stabilized EB-PVD ZrO2 thermal barrier coatings

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests.Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined.In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.     

Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate

Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5-40 μm). Young's modulus E_f of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300-600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo E_f = 40 GPa and for 32 wt.% of Mo E_f = 23 GPa.     

Characterization of Microplasma Sprayed Hydroxyapatite Coating

Microplasma sprayed (MIPS) HAP coatings on SS316L substrates were characterized by x-ray diffraction, Fourier transformed infrared spectroscopy, optical microscopy, scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), atomic force microscopy and image analysis. The coating showed a high degree of crystallinity ~92%, a high porosity level of 20 vol.% and a moderate bonding strength of about 13 MPa. The displacement controlled three-point bend tests and associated results of optical microscopy indicated that crack deflection, crack branching, and also local crack bridging occurred during crack propagation in the coating. The nano-hardness (H) and Young’s modulus (E) of the MIPS-HAP coatings as measured by nanoindentation technique were about 6 and 92 GPa, respectively. The fracture toughness (K _ic) of the coating was ~0.6 MPa·m^0.5. From the nano-scratch experiments, the critical normal load at which localized microcracking led to delamination was measured to be ~400 mN.     

Study of Cr2O3 coatings Part II: Adhesion to a cast-iron substrate

The interfacial indentation technique for determining toughness is applied to plasma-sprayed Cr₂O₃ coatings. In this investigation, another adhesion test, i.e. the four-point bend test coupled with acoustic emission (AE), is performed. AE is monitored during the test from the initiation of load application until fracture in order to detect the damaging of the coating and to identify the different crack growth proc-esses. The residual stresses of coatings are determined by a step-by-step hole drilling method. Correlation of the residual stresses and the two determined parameters to failure (crack length in the case of indenta-tion test and failure displacement in the case of bending test) is discussed.     

金属基陶瓷涂层/钢基体界面裂纹的应力强度因子

将三点弯曲断裂力学试验与有限元分析(FEA)结合起来计算LX88A涂层与Q345钢界面裂纹的复应力强度因子.结果表明,对于三点弯曲试样,当发生界面断裂的临界载荷较小时,涂层试样的界面裂纹尖端附近存在K控制区,但K因子随临界载荷的增大,K因子控制区消失,发生失效的现象.对于三点弯曲试样,当临界载荷超过一定值时,线弹性断裂力学已经不能描述界面裂纹尖端场.因此,在后续研究中有必要使用弹塑性断裂力学和概率断裂力学对此类界面裂纹进行分析.     

Mechanical properties of thermal barrier coatings after isothermal oxidation.: Depth sensing indentation analysis

Thermal barrier coatings (TBC) are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. Isothermal oxidation degrades the performance of these coatings, so this work analyses the mechanical properties (Young's modulus, E, and hardness, H) of TBC and its evolution after thermal exposure in air. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBC were isothermally oxidized in air at 950 °C and 1050 °C for 72, 144 and 336 h. Depth sensing indentation tests were carried out on the ceramic coating to evaluate E and H in the as-sprayed materials and after isothermal oxidation. An approach based on multiple tests at different loads was used to determine size independent apparent E an H. These mechanical properties, measured perpendicular to the surface, clearly decreased after isothermal oxidation as a consequence of microcracking within the ceramic coating.     

重载条件下钢基体表面涂层裂纹及分层失效

用有限元法分析了Hertz接触应力下钢基体表面涂层裂纹和分层失效状况,采用无预制裂纹状况下的扩展有限元(XFEM)技术和内聚力(Cohesive)模型研究了涂层的裂纹扩展和分层失效过程。分析表明:涂层的裂纹萌生于涂层表面,并向内部扩展;涂层弹性模量越大越易产生裂纹,且裂纹扩展越深。涂层的分层失效主要是由涂层-基体界面切应力造成;由于畸变应力的存在,较薄涂层(如物理气相沉积涂层),涂层越厚越易产生分层;较厚涂层(如等离子喷涂涂层),涂层越薄越易产生分层。在另一方面,涂层的裂纹和分层会相互影响,分层会使涂层更易产生裂纹,使裂纹扩展越深,影响裂纹扩展速度;涂层裂纹会使涂层更易分层,使分层区域在远离接触区域方向上不断扩展。对比分析表明,研究结果与前人的理论及实验结果吻合较好,为今后进行涂层失效数值模拟提供了依据。     

Microstructure and tribological properties of CrN and CrSiCN coatings

Three CrN based coatings were deposited on 17-4PH precipitation hardening stainless steel substrate using plasma enhanced magnetron sputtering (PEMS) technique. The three coatings evaluated in this study assumed the nominal compositions of Cr_0.68N_0.32 (sample CrN), Cr_0.55Si_0.013C_0.14N_0.3 (sample CrSiCN-1), and Cr_0.43Si_0.034C_0.25N_0.29 (Sample CrSiCN-2)_. The microstructure, mechanical properties and wear and erosion resistance of the coatings were evaluated to examine the effect of Si and C additions to CrN. The results indicated that with the incorporation of Si and C, the microstructure transformed from hexagonal Cr₂N (for CrN coating) to B1 structure containing crystalline Si₃N₄ (for CrSiCN-2). The initial addition of Si (1.3 at.%) and C resulted in increase of hardness (H), Young's modulus (E) and the ratio of H³/E². With further increase in Si (3.4 at.%) and C, the hardness and Young's modulus decreased. The coefficient of friction was observed to decrease with the addition of Si and C, irrespective of microstructure changes. The combination of reduced coefficient of friction and microstructure modifications has resulted in improved wear resistance for sample CrSiCN-2 (with a wear rate ∼ 60% lower than CrN). The erosion resistance test results showed brittle erosion characteristics for samples CrN and CrSiCN-1 where erosion rate increased with erodent impingement angle and reached the highest rate at 75° and 90°, respectively. CrSiCN-2 coating, while exhibiting higher erosion rate at low impingement angle, demonstrated reduced erosion rate at higher angle due to the ductile nature of the coating under erosion test condition.     

Oxidation and interfacial fracture behaviour of NiCrAlY/Al2O3 coatings on an orthorhombic-Ti2AlNb alloy

Al₂O₃ diffusion barriers of various thicknesses have been fabricated by filtered arc ion plating between the NiCrAlY coating and the O-Ti₂AlNb alloy. Isothermal oxidation tests and three-point bend tests have been conducted to investigate the influence of the Al₂O₃ diffusion barriers on the oxidation and interfacial fracture behaviour of the coatings. The results indicate that the Al₂O₃ diffusion barrier defers interdiffusion and gives oxidation resistance of the NiCrAlY coatings. The thickness of the Al₂O₃ interlayer not only influences the oxidation behaviour but also affects the interfacial fracture properties. Additionally, thermal exposure affects the critical load in three-point bend tests.     

Effect of bilayer period on properties of Cr/CrN multilayer coatings produced by laser ablation

Multilayers of alternate Cr/CrN coatings deposited on AISI 301 steel by a pulsed Nd:YAG laser have been tested by microindentation to measure both hardness and Young's modulus. Critical load L_C has been determined by scratch test. The total thickness of multilayers was 1 µm, and the bilayer period Λ varied from between 60 nm and 1000 nm. TEM and EDS techniques were used for investigating and analyzing the microstructure of the multilayers. The results of these measurements show that the mechanical properties of Cr/CrN multilayers depend on the bilayer period. The maximum improvement in performance over a single layer of CrN has been found to be Λ at 250 nm, particularly in terms of critical load and Young's modulus.     

铀表面Al/Ti复合镀层热应力的有限元分析

对铀表面Al/Ti复合镀层的热应力进行了热弹塑性有限元分析,表明Ti镀层内为压应力,Al镀层内为拉应力,并达到铝的屈服强度,靠近试样侧边,存在边缘效应引起的应力分布不均匀性,离试样侧边2倍镀层厚度处,不均匀性逐渐消失,试样侧边U-Al界面剪切应力大于中部区域.对沉积温度、镀层厚度及镀层力学性能对镀层热应力和塑性应变的研究表明,随着沉积温度升高,镀层内热应力和塑性应变明显增大,减薄Al镀层和增厚Ti镀层可降低镀层内热应力和塑性应变,Al镀层屈服强度及Ti镀层弹性模量对镀层热应力和塑性应变有重要影响.     

Interfacial indentation test of FeB/Fe2B coatings

The present study uses interfacial indentation testing to estimate the adhesion of the FeB/Fe₂B coating formed on the surface of borided AISI 316 steel. This technique creates and propagates a crack along the FeB/Fe₂B interface and defines the apparent fracture toughness, which can then be related to the adhesion and mechanical support of the aforementioned interface. The boriding process was performed on the surface of AISI 316 steel by means of the powder-pack method at temperatures of 1123, 1173, 1223 and 1273 K with 2, 4, 6, 8 and 10 h for each temperature. The Young's modulus for each surface layer was obtained by Knoop microindentation at a constant indentation load. Vickers microindentation fracture technique was used to generate microcracks at the FeB/Fe₂B interface with varying indentation loads. The applied load, Young's modulus, hardness, and lateral crack lengths generated from the corners of the indentations, along with the depth of the FeB layer, were used to determine the apparent fracture toughness and adhesion of the FeB/Fe₂B interface. The apparent fracture toughness of the FeB/Fe₂B interface varied between 3.56 and 4.45 MPa . Finally, the intensity of residual stress at the FeB/Fe₂B interface was estimated as a function of the FeB layer thickness.     

CMAS对热障涂层界面裂纹和残余应力的影响

随着航空发动机涡轮叶片工作温度的提升,使得一种主要由CaO,MgO,Al₂O₃和SiO₂组成的玻璃态物质(CMAS)对热障涂层的危害越来越严重,从而对热障涂层的性能和耐久性有了更高的要求。本文以电子束物理气相沉积热障涂层为研究对象,利用有限元方法研究了CMAS的渗入对界面裂纹扩展及CMAS对陶瓷层(TC)内部残余应力的影响规律。采用波长固定、振幅变化的正弦曲线表示不同粗糙度的涂层界面,同时考虑了CMAS的弹性模量变化的影响及不同界面形貌与CMAS之间的相互作用。结果表明：CMAS弹性模量的增加对界面裂纹具有抑制作用,并且TGO幅值和厚度越小,抑制作用越明显。CMAS弹性模量对TC层最大残余应力S₂₂的影响存在临界点,在临界点之前,CMAS弹性模量的变化对TC层最大残余应力的影响较大,随着CMAS弹性模量的增加,TC层最大残余应力大幅度减小;在临界点之后,TC层最大残余应力基本不受CMAS弹性模量变化的影响。这些结果对电子束物理气相沉积喷涂的热障涂层失效机理的研究具有重要意义,可以为热障涂层界面的优化提供指导。     

Transverse fracture in thin-film coatings under spherical indentation

The competition between transverse surface and sub-surface cracks in a thin, hard coating bonded to polycarbonate substrate due to spherical indentation is investigated in real-time as a function of coating thickness and indenter radius. Fine grain (Y-TZP) and medium grain (alumina) ceramics and pre-abraded amorphous glass are used for the coating. As the coating thickness is reduced, the familiar star-shape sub-surface damage is suppressed, resulting in the top-surface ring crack as the dominant fracture mode. In the intermediate thickness range, the sub-surface damage occurs as a set of off-axis cracks. LEFM in conjunction of a large-strain FEM contact code is used to predict the onset of transverse fracture in the coating. Guided by the test results, the damage on both coating surfaces is assumed as a cylindrical surface crack. In consistency with their polycrystalline nature, the coatings are assumed to contain a distribution of cracks, with the least fracture load among all possible crack lengths taken as the critical load. The numerical predictions compare well with the tests results, and they help identify the applicability range of the simpler point loading case as well as a fracture analysis that is based on a critical stress criterion in terms of the system parameters.     

Delamination Strength of WC-Co Thermal-Sprayed Coating Under Combined Stresses by Torsion-Tension Pin-Test Method

In this report, the delamination strength of WC-Co thermal-sprayed coatings under combined torsion and tension is evaluated using a newly developed method, which is called the torsion-tension pin-test. First, the effects of both the pin diameter and the coating thickness on the apparent delamination strength were investigated experimentally. Second, the stress distributions around the interface edge between the pin and the coating were numerically obtained by using the finite element analysis program “MARC.” It was confirmed that the fractured plane of the torsion pin coincides with the interfacial plane between the coating and the pin. The apparent delamination strength obtained experimentally decreased linearly with increasing pin diameter and increased with increasing coating thickness t, but it was stable at t of 400 μm or more. The shear delamination strength decreased with increasing tensile stress. Similar stress distributions were observed at the interface when delaminations occurred for rather thick coatings, independent of the pin diameter. The critical combination of the strength of shear stress fields (Ks) with that of tensile stress fields (Ka), i.e., the delamination criteria of the coating under combined shear and tensile loadings, was obtained for a WC-12Co thermal-sprayed coating. These combinations were found to be independent of pin diameter and coating thickness.     

Tensile Test Specimens with a Circumferential Precrack for Evaluation of Interfacial Toughness of Thermal-Sprayed Coatings

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K _Int at the tip of the precrack, which can be expressed as , where σ₀ is the apparent average stress, a the crack length, R the specimen radius, and F _I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F _I for various values of a/R. In the experiments, the flat surface of a pin was grit-blasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m^1/2. As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.     

喷涂层对十字接头焊趾裂纹应力强度因子的影响

利用有限元分析软件ANSYS分别建立了非承载十字接头焊趾裂纹的二维和三维断裂模型,并计算了焊趾裂纹尖端的应力强度因子.分析了等离子喷涂层弹性模量对裂纹尖端应力强度因子的影响,以及裂纹尺寸对裂纹尖端应力强度因子的影响.计算结果表明,等离子喷涂层使焊趾裂纹尖端应力强度因子明显降低.对于确定的裂纹尺寸、形状,裂纹尖端的应力强度因子随涂层弹性模量的增加而降低.当Ec/Ep=0.6,二维模型中a/t=0.125时,喷涂后裂纹尖端应力强度因子降低了62.5%;三维模型中a/t=0.125,a/c=0.2时,喷涂后裂纹尖端的应力强度因子降低了42.6%.     

Evaluation of interfacial mechanical properties under shear loading in EB-PVD TBCs by the pushout method

A new simple pushout technique for evaluation of interfacial shear mechanical properties in thermal barrier coatings has been developed. The technique is similar to the pushout test of fiber-reinforced ceramics, except for the specimen shape and support method. The technique has been applied to evaluation of interfacial delamination toughness, Γ_i, of the electron beam physical vapor deposition (EB-PVD) ZrO₂ thermal barrier coating (TBC) system. The change of Γ_i in the EB-PVD system with thermal exposure is measured and discussed in terms of microstructural change and delamination crack path. The measured delamination toughness varied from Γ_i = 10 to 115 J/m². The delamination path and TGO growth were found to be closely related. The delamination toughness significantly decreases due to the formation and growth of a spinel phase in the TGO layer. The relation between delamination toughness and delamination behavior is discussed.     

Effect of a laser pre-quenched steel substrate surface on the crack driving force in a coating–steel substrate system

A mechanical model of a coating/laser pre-quenched steel substrate specimen with a crack oriented perpendicular to the interface between the coating and the hardened layer is developed to quantify the effects of the residual stress and hardness gradient on the crack driving force in terms of the J-integral. It is assumed that the crack tip is in the middle of the hardened layer of the pre-quenched steel substrate. Using a composite double cantilever beam model, analytical solutions can be derived, and these can be used to quantify the effects of the residual stress and the hardness gradient resulting from the pre-quenched steel substrate surface on the crack driving force. A numerical example is presented to investigate how the residual compressive stress, the coefficient linking microhardness and yield strength and the Young’s modulus ratio of the hardened layer to the coating influence the crack driving force for a given crack length.