Indentation size effect on the Fe2B/substrate interface

This study evaluated the indentation size effect on the Fe₂B/substrate interface using the Berkovich nanoindentation technique. First, the Fe₂B layers were obtained at the surface of AISI 1018 borided steels by the powder-pack boriding method. The treatment was conducted at temperatures of 1193, 1243 and 1273 K for 4, 6 and 8 h at each temperature. The boriding of AISI 1018 steel resulted in the formation of saw-toothed Fe₂B surface layers. The formation of a jagged boride coating interface can be attributed to the enhanced growth at the tips of the coating fingers, due to locally high stress fields and lattice distortions. Thus, the mechanical properties achieved at the tips of the boride layer are of great importance in the behavior of borided steel.Applied loads in the range of 10 to 500 mN were employed to characterize the hardness in the tips of the Fe₂B/substrate interface for the different conditions of the boriding process. The results showed that the measured hardness depended critically on the applied load, which indicated the influence of the indentation size effect (ISE). The load-dependence of the hardness was analyzed with the classical power-law approach and the elastic recovery model. The true hardness in the tips of the Fe₂B/substrate interface was obtained and compared with the boriding parameters. Finally, the nanoindentation technique was used to estimate the state of residual stresses in this critical zone of the Fe₂B/substrate interface.     

Interfacial indentation and shear tests to determine the adhesion of thermal spray coatings

Adhesion is one of the most important parameters which influences the development of thermal spray coatings. Therefore, the level of adhesion should be known for a given application. Apart from the standardized Tensile Adhesive Test (TAT), more than 80 methods are reported to measure the coating adhesion. Most of them are energy consuming in terms of time, cost and equipment. Moreover, they do not fulfil the necessary requirements of accuracy, confidence and representation of the real delamination process observed in service. To address this problem, the interfacial indentation test is used here to initiate and propagate a crack at the interface between the substrate and the coating. Studying the extension of the crack, an interfacial toughness is defined and deduced analytically from the experimental results. The new shear test, developed in the frame of the EU-CRAFT-project “Shear Test for Thermally Sprayed Coatings”, is also employed to assess the coating adhesion. Both tests are compared to the standardized TAT for various spraying systems, materials, substrate roughness and coating thickness. Advantages and disadvantages of the three tests are discussed. Correlations between the tests results obtained for different coating-substrate combinations are presented and general trends are described.     

Formation and kinetics of FeB/Fe2B layers and diffusion zone at the surface of AISI 316 borided steels

The kinetics of the FeB/Fe₂B layers and diffusion zone at the surface of AISI 316 steels exposed to the powder-pack boriding process were studied in this work. FeB/Fe₂B layers and diffusion zone measurements were taken at different temperatures and exposure times to validate diffusion-controlled growth during the boriding process. In order to obtain the boron diffusion coefficients at the FeB/Fe₂B layers and diffusion zone, a mathematical model based on the mass balance at the growing interfaces was proposed. The activation energy values estimated for the FeB and Fe₂B layers were 204 and 198 kJ mol^− 1 respectively. In addition, the activation energy value obtained for the diffusion zone was 116 kJ mol^− 1. The diffusion model was extended to estimate the FeB/Fe₂B layer thicknesses, and the depth of the diffusion zone at the temperature of 1243 K with 3 and 5 h of exposure, based on the experimental parameters ascribed to the boriding process. Finally, the effects of the FeB/Fe₂B growth and diffusion zone, on the weight gain of borided steels and on the instantaneous velocity of the interfaces were incorporated in the model.     

The interfacial indentation test to determine adhesion and residual stresses in NiCr VPS coatings

The interfacial indentation test allows determining the interface toughness of a coating obtained by thermal spraying. During this test, a Vickers indentation is carried out on a cross-section of the sample. A crack is initiated and propagated along the interface. An analytical model allows defining an interface toughness representing the coating adhesion. The objective of this study is to compare this test with other tests (tensile adhesive test, shear test) and to specify its applicability. The residual stresses are also estimated by two different methods. Their influence on adherence is discussed in a third part. Those experiments were conducted on NiCr 80-20 VPS coatings with different thicknesses and roughnesses. In particular, it is shown that the interfacial indentation test is the most universal one and that compressive residual stresses improve coating adhesion.     

Modified tensile adhesion test for evaluation of interfacial toughness of HVOF sprayed coatings

Tensile adhesion test is widely used to evaluate the adhesion strength of coatings sprayed by High Velocity Oxy-Fuel (HVOF) technique. But there are two issues to be improved. Firstly, when the coatings have high adhesion strength, failure occurs in an adhesive layer, and secondary, the edge of a substrate is heavily deformed and rounded due to the high impact energy of sprayed particles. This deformation causes large scatter of adhesion test results. In this paper, a new technique to evaluate the interfacial fracture toughness has been proposed by introducing pre-crack at the interface of a conventional tensile adhesion test specimen. The asymptotic analytical formula was derived for interfacial toughness evaluation. Numerical analysis was also carried out for comparison. The difference between the numerical and the theoretical data was less than 5%. The developed procedure was applied for the SUS316 L steel coatings and the significant effects of the surface roughness and preheating temperature on adhesion strength were reconfirmed quantitatively.     

Determination of Vickers indentation fracture toughness of boronised alloyed ductile iron

In the present study, copper (Cu), nickel (Ni), and molybdenum (Mo)-alloyed ductile iron was pack boronised at 800–850°C for 3–6?h and subsequently the microhardness and the microstructures of boride layers under different process parameters were investigated in detail. Further, Vickers indentation fracture toughness tests were executed on borided surfaces under 200?g load. The fracture toughness of borided layers was estimated separately by a series of equations and the half-length of corner cracks and the half diagonals of Vickers indents were used as variables. Generally, the values of the obtained fracture toughness were found to be higher than those of previous studies. It was found that thinner boride layers were formed at lower boronising temperatures. The highest toughness value and the thickest boride layer were obtained in the sample boronised at 850°C for 6?h.     

A numerical fracture analysis of a stationary semi-circular interface crack during interfacial indentation test

The aim of this work is to investigate the fracture behavior of a stationary crack lying along the interface in a coated system during interfacial indentation test. One traditional 3D numerical model and one cohesive element numerical model have been made to study the Vickers indentation test process. A crack-block approach is applied to generate 3D meshes containing the crack front along the interface. The stress intensity factors and the energy release rates (G) on the crack front are computed. The fracture mode mixity is also presented. It is found that the near surface propagation of the crack can be prevented during loading due to compressive stresses. It may occur during unloading due to residual stresses resulting from plastic strain.     

Nd2Fe14B／Fe3B基纳米复合磁体的热磁行为

研究了快淬方法制备的Nd4Fe80B16和Nd4Fe76Co3Hf0.5Ga0.5二种合金的磁性和热行为。发现添加Hf和Ga能有效地降低Nd2Fe14B/Fe3B复合磁体的晶化速度。矫顽力（iHc)随后时效处理的不同仅有轻微的变化,这表明iHc对晶粒尺度和交换耦合是磁不敏感参量。实验所观察的Mr和Hc 随晶粒的增大变化不大,这与在临界晶粒尺寸（dc）随近所做的数字计算理论不一致。本文对在各向同性纳米复合材料中的剩磁和矫顽力与晶粒尺寸的关系进行了分析。     

The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB)

In this study, we introduce a new electrochemical boriding method that results in the formation of a single-phase Fe₂B layer on low carbon steel substrates. Although FeB phase is much harder and more common than Fe₂B in all types of boriding operations, it has very poor fracture toughness; hence, it can fracture or delaminate easily from the surface under high normal or tangential loading. We call the new method “phase homogenization in electrochemical boriding” (PHEB), in which carbon steel samples undergo electrochemical boriding for about 15 min at 950 °C in a molten electrolyte consisting of 90% borax and 10% sodium carbonate, then after the electrical power to the electrodes is stopped, the samples are left in the bath for an additional 45 min without any polarization. The typical current density during the electrochemical boriding is about 200 mA/cm². The total original thickness of the resultant boride layer after 15 min boriding was about 60 μm (consisting of 20 μm FeB layer and 40 μm Fe₂B layer); however, during the additional phase homogenization period of 45 min, the thickness of the boride layer increased to 75 μm and consisted of only Fe₂B phase, as confirmed by glancing-angle x-ray diffraction and scanning electron microscopy in backscattering mode. The microscopic characterization of the boride layers revealed a dense, homogeneous, thick boride layer with microhardness of about 16 GPa. The fracture behavior and adhesion of the boride layer were evaluated by the Daimler-Benz Rockwell C test and found to be excellent, i.e., consistent with an HF1 rating.     

Microstructural evolution of Fe3B/Nd2Fe14B nanocomposite magnets microalloyed with Cu and Nb

The microalloying effect of Cu and Nb on the microstructure and magnetic properties of an Fe₃B/Nd₂Fe₁₄B nanocomposite permanent magnet has been studied by transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). Additions of Cu are effective in refining the nanocomposite microstructure and the temperature range of the heat treatment to optimize the hard magnetic properties is significantly extended compared with that of the ternary alloy. Combined addition of Cu and Nb is further effective in reducing the grain size. Optimum magnetic properties obtained by annealing a melt-spun Nd_4.5Fe_75.8B_18.5Cu_0.2Nb₁ amorphous ribbon at 660°C for 6 min are B_r=1.25 T, H_cJ=273 kA/m and (BH)_max=125 kJ/m³. The soft magnetic Fe₂₃B₆ phase coexists with the Fe₃B and Nd₂Fe₁₄B phases in the optimum microstructure of the Cu and Nb containing quinternary alloy. Three-dimensional atom probe (3DAP) results show that the finer microstructure is due to the formation of a high number density of Cu clusters prior to the crystallization reaction, which promote the nucleation of the Fe₃B phase. The Nb atoms appear to induce the formation of the Fe₂₃B₆ phase when the remaining amorphous phase is crystallized.     

Evaluation of microhardness,fracture toughness and residual stress in a thermal barrier coating system: A modified Vickers indentation technique

The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67 MPa m^1/2, and the corresponding residual stress near the indented region varies from − 36.8 to − 243 MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81 MPa m^1/2, and residual stress varies from − 5 to − 30 MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

The adhesion strength and indentation toughness of plasma-sprayed yttria stabilized zirconia coatings

In this work, yttria stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spray (APS) technique under various powder feed rates and spray distances. The microstructure and phase analysis were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Tensile adhesion test (TAT) and interfacial indentation test (IIT) methods were used to evaluate adhesion of coatings. The effect of spray parameters on the coatings adhesion as well as its toughness was investigated. The obtained results revealed that with increasing powder feed rate and spray distance, adhesion of the coatings reaches to a maximum and then reduces. Interfacial toughness values change in the same manner. Adhesion/toughness behaviors of the YSZ coatings were related to microstructural characteristics including the volume fraction and size of pores and unmelted particles.     

Evaluation of interfacial strength by an instrumented indentation method and its application to an actual TBC vane

The thermal fatigue behaviour of an air plasma sprayed thermal barrier coating was investigated. And also the interfacial strengths of thermal barrier coated specimens subjected to thermal fatigue, as well as a retired TBC vane were also evaluated by means of an instrumented indentation machine. The results indicated that, (1) the TGO grew at the interface during thermal fatigue cycle as a function of the exposure time at elevated temperature; (2) the microcracks were initiated in the top coating and at the...     

Strength and residual stresses of functionally graded Al2O3/ZrO2 discs prepared by electrophoretic deposition

Biaxial strength testing of functionally graded Al₂O₃/ZrO₂ discs revealed that the strength of such discs, prepared by electrophoretic deposition, was almost doubled from 288 MPa for pure Al₂O₃ to 513 MPa for the graded discs; this was due to the compressive surface residual thermal stresses in the Al₂O₃ surface layer caused by the graded compositional profile. The surface compressive stress measured by means of X-ray diffraction was compared with the analytically calculated stress distribution in the graded component.     

不同燃料超音速火焰喷涂NiCr-Cr3C2涂层

采用超音速火焰喷涂工艺在20CrMo钢圆片上成功制备25%NiCr-Cr3C2金属陶瓷涂层,研究了不同燃料对喷涂层组织性能的影响。结果表明,以丙烷为主要燃料所得喷涂层的组织不均匀,存在典型的层状结构,孔隙率约为3.2%,显微硬度仅为836 HV,涂层断裂韧性KIC为2.08 MPa.m3/2;以煤油为主要燃料所得喷涂层的组织分布均匀,细小致密,孔隙率约为2.4%,显微硬度可达1045 HV,涂层断裂韧性KIC为2.56 MPa.m3/2。前者的组成相为Ni(Cr)固溶体、Cr3C2和微量NiCrO4;后者的组成相除Ni(Cr)固溶体和Cr3C2外,还有Cr7C3、Cr23C6和Ni(Cr)相出现。     

硬质相晶粒尺寸对MO2FeB2基金属陶瓷断裂韧性的影响

以Mo、Fe、FeB等为原材料,采用最高烧结温度（保温时间）分别为1170℃（0min）、1250℃（0min）以及1250oC（40min）的真空烧结工艺制备了硬质相晶粒尺寸不同的M02FeB：基金属陶瓷,所得烧结体的硬质相晶粒尺寸分别为1．12、1．31和1｜73wm。利用压痕法测定了M02FeB：基金属陶瓷的断裂韧性。结果表明：M02FeB：基金属陶瓷的断裂韧性随着硬质相晶粒尺寸的增加而增大,当晶粒尺寸从1．12μm增加到1．73μm时,Mo2FeB2基金属陶瓷的断裂韧性从11．4MPa·m^1/2增加到14．2MPa·m^1/2。随着硬质相晶粒尺寸增加,裂纹偏转增强,并出现裂纹桥联和晶粒拔出现象,导致裂纹扩展路径和能量消耗增大,从而提高了Mo2FeB2基金属陶瓷的断裂韧性。     

Fracture toughness of thermal spray ceramic coatings determined by the indentation technique

The indentation technique for determining material toughness is applied to spinel and yttria-stabilized zirconia plasma-sprayed coatings in this investigation. Fracture toughness of the coatings ranged from 1.9 to 3.4 MPa√m for spinel and 2.0 to 3.3 MPa√m for yttria-stabilized zirconia. These results are in good agreement with those obtained by other experimenters for bulk materials.     

Microstructure and adhesion of Cr3C2-NiCr vacuum plasma sprayed coatings

Vacuum plasma spraying (VPS) was used to spray a Cr₃C₂-NiCr coating of ∼ 150, 300 and 450 μm in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr₃C₂ phase, besides Cr₇C₃ and Cr₂O₃. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m^1/2 when the thickness increases from 150 to 450 μm. Also, it has been found that the parameter Kca_o, determined by linear regression from the Kca versus 1 / e² curve by means of the interfacial indentation model advanced by Chicot et al., has a value of ∼ 9.8 MPa m^1/2.     

Residual stresses as a factor in the selection of tungsten carbide coatings for a jet engine application

Tungsten carbide thermal spray coatings are important to the aerospace industry for the mitigation of midspan damper wear on jet engine fan and compressor blades. However, in some cases the coating can fail due to spallation and cracking, and in other situations the fatigue life of a fan or compressor blade is reduced when a coating is applied. Coating failures can result in decreased engine performance and costly maintenance time. A comprehensive experimental research program was conducted to evaluate coating crack resistance in bending, low-cycle fatigue properties of the coating and substrate, coating performance in jet engine tests, and microstructures for a wide range of coating compositions and application processes. Coating residual stress distributions also were evaluated. Eleven coatings were ranked according to their performance relative to the other coatings in each evaluation category. Results from the bend and low-cycle fatigue evaluations were compared to the experimentally evaluated residual stresses. Comparisons of rankings indicate a strong correlation between performance and the residual stresses in the coatings. Results from the program were used to select a suitable coating system for final in-service use based on two important criteria: (1) the coating must not fail while in service, and (2) the coating must not induce crack propagation into the substrate of the midspan damper.