Influence of nickel silicides presence on hardness,elastic modulus and fracture toughness of gas-borided layer produced on Nisil-alloy

The two-stage gas boriding in N₂?H₂?BCl₃ atmosphere was applied to producing a two-zoned borided layer on Nisil-alloy. The process was carried out at 910 °C for 2 h. The microstructure consisted of two zones differing in their phase composition. The outer layer contained only a mixture of nickel borides (Ni₂B, Ni₃B) only. The inner zone contained additionally nickel silicides (Ni₂Si, Ni₃Si) occurring together with nickel borides. The aim of this study was to determine the presence of nickel silicides on the mechanical properties of the borided layer produced on Ni-based alloy. The hardness and elastic modulus were measured using the nanoindenter with a Berkovich diamond tip under a load of 50 mN. The average values of indentation hardness (H_I) and indentation elastic modulus (E_I) obtained in the outer zone were respectively (16.32±1.03) GPa and (232±16.15) GPa. The presence of nickel silicides in the inner zone reduced the indentation hardness (6.8?12.54 GPa) and elastic modulus (111.79?153.99 GPa). The fracture toughness of the boride layers was investigated using a Vickers microindentation under a load of 0.981 N. It was confirmed that the presence of nickel silicides caused an increase in brittleness (by about 40%) of the gas-borided layer.     

Finite element and dimensional analysis algorithm for the prediction of mechanical properties of bulk materials and thin films

In this work, the applicability of a new algorithm for the estimation of mechanical properties from instrumented indentation data was studied for thin films. The applicability was analyzed with the aid of both three-dimensional finite element simulations and experimental indentation tests. The numerical approach allowed studying the effect of the substrate on the estimation of mechanical properties of the film, which was conducted based on the ratio h_max/l between maximum indentation depth and film thickness. For the experimental analysis, indentation tests were conducted on AISI H13 tool steel specimens, plasma nitrated and coated with TiN thin films. Results have indicated that, for the conditions analyzed in this work, the elastic deformation of the substrate limited the extraction of mechanical properties of the film/substrate system. This limitation occurred even at low h_max/l ratios and especially for the estimation of the values of yield strength and strain hardening exponent. At indentation depths lower than 4% of the film thickness, the proposed algorithm estimated the mechanical properties of the film with accuracy. Particularly for hardness, precise values were estimated at h_max/l lower than 0.1, i.e. 10% of film thickness.     

Nanoindentation characteristics of Zr69.5Al7.5 − xGaxCu12Ni11 glasses and their nanocomposites

This work deals with the indentation behavior of Zr_69.5Al_7.5 − xGa_xCu₁₂Ni₁₁ (x = 0, 1.5, 7.5 at.%) alloys. A comparison between their nanohardness and reduced elastic modulus values of the as-synthesized glassy phase with their nanocomposites has been made. The indentation characteristics of a novel Ga substituted glass composition corresponding to x = 7.5 have shown significant improvement in regard to hardness and elastic modulus. The evidence of pile up has been observed in case of as-synthesized glassy ribbons. The load (P) versus depth (h) curves for as-synthesized melt-spun ribbons displayed the presence of displacement burst, which are known as pop-ins. The amount of energy per unit volume required for the shear band formation in glassy state has been estimated based on the pop-ins observed in P-h curve. This seems to decrease with Ga addition. Based on transmission electron microscopic observations of indented glassy specimen, the possibility of nanocrystallization has been ruled out.     

Effects of mechanical properties, residual stress and indenter tip geometry on instrumented indentation data in thin films

In this work, an axisymmetric two-dimensional finite element model was developed to simulate instrumented indentation testing of thin ceramic films deposited onto hard steel substrates. The level of film residual stress (σ_r), the film elastic modulus (E) and the film work hardening exponent (n) were varied to analyze their effects on indentation data. These numerical results were used to analyze experimental data that were obtained with titanium nitride coated specimens, in which the substrate bias applied during deposition was modified to obtain films with different levels of σ_r. Good qualitative correlation was obtained when numerical and experimental results were compared, as long as all film properties are considered in the analyses, and not only σ_r. The numerical analyses were also used to further understand the effect of σ_r on the mechanical properties calculated based on instrumented indentation data. In this case, the hardness values obtained based on real or calculated contact areas are similar only when sink-in occurs, i.e. with high n or high ratio Y/E, where Y is the yield strength of the film. In an additional analysis, four ratios (R/h_max) between indenter tip radius and maximum penetration depth were simulated to analyze the combined effects of R and σ_r on the indentation load-displacement curves. In this case, σ_r did not significantly affect the load curve exponent, which was affected only by the indenter tip radius. On the other hand, the proportional curvature coefficient was significantly affected by σ_r and n.     

Fractal analysis of crack paths in Al2O3-TiC-4%Co composites

1 Introduction Al2O3-TiC composite (denoted by AT) is an important material for structural components due to the high strength, hardness, as well as chemical stability and wear resistance. However, the low fracture toughness still cannot match the comman…     

Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Si-based ceramics (e.g., SiC and Si₃N₄) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H₂O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al₂O₃·2SiO₂) interlayer and (1 ? x)BaO·xSrO·Al₂O₃·2SiO₂, 0 ?? x ?? 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H₂O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.     

Microindentation fracture behavior of surface modified alumina ceramic using glass infiltration

This study investigates the effect of a low expansion glass (Mg₃Al₂Si₆O₁₈) treatment on the surface fracture toughness of sintered alumina. The surface fracture toughness was determined by direct indentation method (Vickers indentations), carried out at different loads ranging from 9.8 to 196 N. The crack lengths on the surface at each load were found to be decreased (8–12%) by glass treatment and the corresponding crack resistance values increased by about 17–20%. Both sintered and glass treated specimens showed rising trend in crack resistance values as the indentation load was increased. There was also a significant increase in the Weibull modulus value of crack resistance. Improved properties of glass treated sample were attributed to the formation of a relatively larger process zone surrounding the crack, crack arrest behavior due to the compressive stresses and the crack bridging phenomena. The compressive stresses were generated from the thermo-elastic properties mismatch: (a) between the glass and the ceramic in the glass infiltrated zone, and (b) the glass–ceramic composite layer and the ceramic substrate.     

Investigation of some physical properties of Gd added Bi-2223 superconductors

X-ray powder diffraction (XRD), scanning electron microscopy (SEM), dc electrical resistivity, critical current density and static microindentation measurements are performed to investigate some physical properties of Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd_xO_y superconducting samples with x = 0.0, 0.1, 0.3 and 0.5. We observe from the transport measurements that, for the Gd added sample, the critical transition temperature (T_c) and the critical current density (J_c) are decreased in comparison with that of undoped sample. In addition, surface morphology and grain connectivity of the samples are degraded and the high-T_c phase of the samples decreases with increasing Gd addition. The indentation load versus diagonal length of the samples under different indentation loads in the range of 0.245-2.940 N are measured. The microindentation measurements showed that, for the Gd added sample, the load dependent (apparent) microhardness value (H_v) is lower in comparison with that of the pure sample (x = 0). The values of H_v are found to be load dependent. In addition, we extract the load independent (true) microhardness using the Kick's law, proportional specimen resistance (PSR), modified proportional specimen resistance (MPSR) and the Hays-Kendall (HK) approach and compare the true hardness with the apparent hardness. The possible reasons for the observed degradation in microstructure, superconducting and mechanical properties due to Gd addition are discussed.     

Gradients in elastic modulus for improved contact-damage resistance. part ii: the silicon nitride–silicon carbide system

In an effort to create elastic-modulus (E) graded materials for contact-damage resistance—free of substantial amounts of glass—silicon nitride (Si₃N₄)-silicon carbide (SiC) graded materials were processed. The structure of these graded materials is such that Si₃N₄ (E=300 GPa) is at the contact surface and SiC (E=400 GPa) is in the interior, with a stepwise gradient in composition existing between the two over a depth of 1.6 mm. A pressureless, liquid-phase co-sintering method, in conjunction with a powder-layering technique, was used to achieve this structure. The liquid phase used was yttrium aluminum garnet (YAG). Under spherical indentation, cone-cracks did not form in the graded material, but some inelastic shear deformation was observed. Cone cracks formed in both the monolithic Si₃N₄ and the monolithic SiC end member materials under identical indentation conditions. Finite element analysis (FEA) of the stresses associated with indentation revealed that the maximum principal tensile stresses outside the Hertzian contact circle, which drive the classical cone-cracks, are reduced by approximately 12% in the graded material relative to the monolithic silicon nitride case. This reduction is significantly lower than what was calculated for the Si₃N₄-glass case (Part I), owing to the shallower, linear E-gradient over a 1.6 mm depth in Si₃N₄-SiC, as compared with the power-law, steeper E-gradient over 0.4 mm depth in the Si₃N₄-glass. It appears that, in addition to the E-gradient, the inelastic deformation contributes to the suppression of cone cracks in the Si₃N₄-SiC graded material. It is suggested that compressive residual stresses may be present in the Si₃N₄-SiC graded material, which are also likely to aid in the suppression of cone-cracks.     

Finite element analysis for conical indentation unloading of elastoplastic materials with strain hardening

Finite element analysis is conducted for various elastoplastic solids with linear strain-hardening that are contacted by cone indenters having three different inclined face angles [β=10.0°, 19.7° (Vickers equivalent angle), and 30.0°]. The indentation load P vs. penetration depth h relationship during unloading is intensively examined. Although the P–h unloading relationship for a cone indentation is apparently non-quadratic [P=α(h−h_r)^m; m<2.0], it is emphasized from the geometrical similarity of cone indentation that the unloading process conforms to a quadratic relation [P=k₂(h−h_r)²; h_r is the residual depth of impression after a complete unload] in its essential physical process. The unloading parameter k₂ is, then, directly related to the elastic modulus E^′ of the material on the basis of the concept of “effective face angle β_eff” of indenter. The indentation unloading process is well described by the loading process of a conical indenter with the effective face angle of β_eff(=β−β_r) pressed into a flat elastic half-space, in which the inclined face angles of the indenter used and of the residual hardness impression formed are defined by β and β_r, respectively. The non-quadratic term included in unloading P–h relation results from the locally distorted convex surface profile of the residual impression.     

Thermal kinetic analysis of a complex process from a solid-state reaction by deconvolution procedure from a new calculation method and related thermodynamic functions of Mn0.90Co0.05Mg0.05HPO4·3H2O

Three individual peaks of thermal solid-state reaction processes of the synthesized Mn_0.90Co_0.05Mg_0.05HPO₄·3H₂O were observed corresponding to dehydration I, dehydration II and polycondensation processes. An alternative method for the calculation of the extent of conversion was proposed from the peak area of the individual DTG peak after applying the best fitting deconvolution function (Frazer-Suzuki function). An iterative integral isoconversional equation was used to compute the values of the apparent activation energy E_α and they were found to be 65.87, 78.16 and 119.32 kJ/mol for three peaks, respectively. Each individual peak was guaranteed to be a single-step kinetic system with its unique kinetic parameters. The reaction mechanism functions were selected by the comparison between experimental and model plots. The results show that the first, second and final individual peaks were two-dimensional diffusion of spherical symmetry (D₂), three-dimensional diffusion of spherical symmetry (D₃) and contracting cylinder (cylindrical symmetry, R₂) mechanisms. Pre-exponential factor values of 3.91×10⁶, 1.35×10⁷ and 2.15×10⁷ s^?1 were calculated from the E_α values and reaction mechanisms. The corresponded standard thermodynamic functions of the transition-state (activated) complexes were determined and found to agree well with the experimental data.     

Modified phase transition and electrical properties of [Li0.05(Na0.535K0.48)0.95]-(Nb0.94Sb0.06)O3 lead-free piezoelectric ceramic by sintering temperature

Li/Sb-doped (Na,K)NbO3 with a nominal composition of [Li0.05(Na0.535K0.48)0.95](Nb0.94Sb0.06)O3 ceramic was synthesized by normal sintering. The phase structure, microstructure, and electrical properties were investigated with a special emphasis on the influence of the sintering temperature. A polymorphic phase transition (PPT) from orthorhombic to tetragonal symmetry was observed when the sintering temperature was raised from 1040 to 1050 ℃, whereby the piezoelectric coefficient d33 and the electromechanic...     

Determining the fracture resistance of B4C-NanoSiB6 nanocomposite by Vickers indentation method and exploring its mechanical properties

In this research, the effects of NanoSilicon hexaboride (NanoSiB₆) additive, as a sintering aid, on mechanical properties and the sinterability of Boron Carbide (B₄C) have been investigated. In addition, the results of several equations used for determining the Indentation fracture resistance (K_IFR) by Vickers indentation test have been evaluated. For this purpose, 0, 2, 4, 6 and 8 wt% NanoSiB₆ additive have been added to B₄C in order to improve its sinterability at temperatures 2050, 2150 and 2250 °C. The findings indicate that by adding NanoSiB₆, up to 4 wt%, to B₄C, its properties such as relative density, Young's modulus, microhardness and K_IFR improve as the sintering temperature rises; however, these properties diminish with the further increase of the mentioned sintering aid. Also, a comparison between K_IFR values shows the closeness of the results obtained by different equations and the satisfactory accuracy of the equation for determining K_IFR by crack area method compared to the results of other equations; with a difference of < 20% between the two results.     

Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips – Berkovich and spherical – are employed to examine the SMAs’ indentation responses as a function of the representative strain, ε_R. In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, ε_R is systematically varied and its effect on the depth recovery ratio, η_d, is examined. Results indicate that RTA has higher η_d than RTM, but the difference decreases with increasing ε_R such that η_d values for both the alloys would be similar in the fully plastic regime. The experimental trends in η_d vs. ε_R for both the alloys could be described well with a η_d ∝ (ε_R)^?1 type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum ε_R, below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found.     

纳米压入结合有限元模拟确定金属材料的塑性性能

材料具有相同的弹性模量 E以及代表性应力与代表性应变(σr,εr)时,可以获得相同的纳米压痕加载曲线,而与材料的应变强化指数 n无关。基于此,利用纳米压入结合有限元数值模拟建立一种确定金属材料塑性性能参数的改进方法。首先,不考虑金属材料的加工硬化,通过不断调整代表性应力的假设值,当模拟与实验载荷?位移曲线的加载阶段相吻合时,确定其代表性应力。其次,对金属材料假设不同的应变强化指数,采用相同的方法确定其代表性应变。最后,通过调整应变强化指数的假设值,使模拟曲线与实验曲线的卸载阶段相吻合来确定金属材料的真实应变强化指数,继而利用幂强化本构方程确定金属材料的初始屈服极限。将该方法应用于AISI 304不锈钢、铁及铝合金三种金属,其有效性得到验证。     

Crystal plasticity finite element simulations of pyramidal indentation in copper single crystals

Vickers and Berkovich indentation experiments in single crystals are examined via three-dimensional finite element simulations. Annealed and strain-hardened copper are used as model materials to assay the physical consistency of the continuum crystal plasticity theory used in the simulations. Agreement between experiments and simulations is predicated in terms of hardness, instrumented indentation applied load (P)–penetration depth (h_s) curves, and material pileup and sinking-in development at the contact boundary. It is shown that while hardness is essentially insensitive to loading direction, some degree of anisotropy is present in the P–h_s curves. The influence of crystal slip anisotropy and of prior straining on the dislocation density patterns underneath sharp indenters is examined. A similarity is found between contact responses obtained with the J₂ associated-flow theory (describing elasto-plastic deformation in isotropic polycrystalline metals) and those from present crystal plasticity simulations. The implications for mechanical property extractions are discussed.     

Magnetic field effects in decamethyl ferrocenium tetracyanoethanide

The specific heat C(H,T) of (DMeFc) (TCNE) in the temperature range of 2–12 K in the presence of applied magnetic fields is reported. Previous results had shown anomaly at 4.82 K corresponding to a transition at T_c from a high temperature one-dimensional (1D) magnet (above T_c) to a three-dimensional (3D) macroscopic ferromagnet (below T_c). The results show a dramatic behavior of the peak associated with the ferromagnetic transition. The peak height decreases with increasing magnetic field, i.e. magnetic fields smear the transition. For small values of applied magnetic fields the peak associated with the ferromagnetic transition is completely smeared. The results are indicative of anisotropic chains with enhanced 1D coherence and frustration in interchain coupling. The spin-wave spectrum is dominated by the 1D chains below T_c.     

Electrochemical and SEM study on Type 254 SMO stainless steel in chloride solutions

An investigation was conducted to examine the critical crevice potential (E_crev) and the critical protection potential (E_prot) for Type 254 SMO stainless steel in 4% NaCl solution by using potentiodynamic cyclic anodic polarization (PCAP) technique at temperature ranging from 30 to 90 °C. The critical crevice temperature (CCT) and the critical crevice protection temperature (T_prot) were determined by plotting the values of breakdown potential and E_prot versus solution temperature, respectively. The values of CCT and T_prot were recorded at the abrupt transition with increasing the temperature from transpassive corrosion to crevice corrosion and were found to be at 55 and 52 °C, respectively. Above CCT (70 °C) the following points were recorded. The E_crev and E_prot decreased linearly with log [Cl⁻]. The addition of bromide ions to chloride ions at a fixed halide content of 4% increased both E_crev and E_prot. The E_crev value in 4% NaCl increased linearly with increasing pH in the range 1-10. The addition of 0.5 M bicarbonate ions to 4% NaCl completely removed the crevices effect while increasing the addition of sulphate ions to 4% NaCl increased both of E_crev and E_prot. The morphology of the crevice corrosion produced on the steel surface was examined by scanning electron microscope (SEM) after PCAP treatment under different test conditions.     

Microstructure and mechanical properties of hot-pressed WC–MgO composites with Cr3C2 or VC addition

The influence of Cr₃C₂ and VC addition on the microstructure and mechanical properties of WC–MgO composites hot-pressed at 1650 °C for 90 min was comprehensively investigated. The grain growth of WC was significantly retarded and the homogeneity of MgO particulate dispersion was effectively improved with the addition of 0.5 wt.% Cr₃C₂ or 0.5 wt.% VC. The indentation size effect (ISE) on hardness was restrained and the load-independent hardness was increased by doping grain growth inhibitors. Improvements on fracture toughness of hot-pressed samples were also observed due to the refined WC grains and uniformly dispersed MgO particulates. In addition, experimental results demonstrated that Niihara's equation was preferable for estimating the indentation fracture toughness, by comparing the fracture toughness evaluated using the single-edge V-notch beam (SEVNB) method with the values estimated through the Vickers indentation technique.     

Nanomechanical properties and fracture toughness of hard ceramic layer produced by gas boriding of Inconel 600 alloy

Gas-boriding in N₂−H₂−BCl₃ atmosphere resulted in the formation of a thick layer on Inconel 600 alloy. The microstructure of layer produced at 920 °C for 2 h consisted of a mixture of chromium borides and nickel borides. The objective of investigations was to determine the influence of the chemical and phase compositions of borided layer on its mechanical properties. The nanoindentation was carried out using Berkovich diamond tip under a load of 50 mN. The gas-borided layer was characterized by high indentation hardness H_IT from 1542.6 HV to 2228.7 HV and high elastic modulus E_IT from 226.9 to 296.4 GPa. It was found that the mixture with higher percentage of chromium borides was the reason for the increase in H_IT and E_IT values. The fracture toughness (K_C) was measured using Vickers microindentation technique under a load of 0.98 N. The presence of high compressive stresses in normal direction to the top surface caused the strong anisotropy of the borided layer, in respect of fracture toughness. The high difference between the lowest (0.5763 MPa·m^1/2) and the highest (4.5794 MPa·m^1/2) fracture toughness was obtained. This situation was caused by the differences in chemical and phase compositions of tested areas, presence of porosity and residual stresses. Generally, the higher K_C values were obtained in areas with lower chromium content.