Model based laser-ultrasound determination of hardness gradients of gas-carburized steel

Gas carburizing is a common industrial process utilized for case hardening of low carbon steels. However, there is a lack of non-destructive evaluation systems for the measurement of hardness-depth profiles. We propose a novel measurement method for the determination of hardness-depth profiles of two-step gas carburized steel specimens. The method is based on the measurement of broadband laser excited Rayleigh waves. Rayleigh waves were generated by a pulsed Nd: YAG laser in the thermoelastic regime and measured with a heterodyne Mach-Zehnder interferometer in the near-field. From two measurements with different source to receiver distances the dispersion diagrams were calculated by means of the phase spectral analysis method. In order to simulate the observed dispersive behavior of the Rayleigh waves, first the two-step gas carburizing process was simulated using solutions of the diffusion equation. The resulting continuous hardness profile was then discretized into up to 100 layers. Thereafter the Rayleigh wave dispersion diagram was calculated from the discretized stack of layers using a delta-matrix formulation of the Thomson-Haskell transfer matrix method. In order to obtain best fitting hardness profiles, the simulated dispersion diagrams were fitted to measurements with a curve fitting algorithm. Comparison of the Rayleigh wave inversion method with destructively obtained Vickers hardness profiles shows good quantitative agreement.     

High temperature tribology of heavily boron doped diamond films against steel

Despite the fact that Fe, Co, and Ni catalyze the phase transition of diamond into graphite, the question of the applicability of diamond as a functional coating of a metal-cutting tool is still open. For this reason, our work contains investigation of wear and friction of heavily boron doped diamond films against steel at elevated temperature, as well as influence of boron concentration on diamond film oxidation resistance. The obtained data indicated that minimum CoF value is achieved in the temperature range within 570–670 °C and strongly depends on boron concentration in coating (CoF decreases with increasing of boron concentration). Wear rate has the same dependence as the CoF, whereas oxidation resistance decreases with increasing of boron concentration. Besides, the presented results are first obtained for boron doped diamond films synthesized under high B/C ratio conditions (of up to 333 ppb).     

Synthesis of amorphous carbon films by plasma-based ion implantation with simultaneous application of DC and pulse bias

Carbon films were synthesized on a Si wafer by simultaneous application of pulse bias and DC bias by a plasma-based ion implantation system using an electron cyclotron resonance (ECR) plasma source with a mirror field. The relationship between the pulse biasing voltage and the properties of carbon films was investigated. The hardness and tribological properties of the carbon film improved as the pulse bias voltage was decreased from −10 kV to −2 kV. Diamond-like carbon (DLC) films with a low friction coefficient were formed by simultaneous application of a low pulse bias voltage, such as −2 kV, and a DC bias. During the friction test of the DLC film, excellent tribological properties were observed under a high conducted load, such as 20 N, which shows that not only the friction coefficient but also the durability during the friction test was improved. The improvement of the tribological property was attributed to the formation of a mixed layer at the interface between the DLC film and the Si substrate.     

Experimental analysis of bending fatigue strength of plain and notched case-hardened gear steels

Fatigue tests were performed on two commercial case-hardened steels using plain and notched specimens and results were compared with data available in the literature for similar steels. The purpose of the paper was to compare performances of different materials under the same testing conditions. Moreover, the results obtained may form the basis to apply ISO Standard methods mentioned in the Annex A, which estimate the fatigue life of gears starting from experimental data generated from specimens. Even though ISO Standard suggests primarily the use of experimental results generated from testing gears, specimens’ testing is faster and cheaper to evaluate different materials during the design process. From this point of view, specimens’ based approaches seem promising. Plane bending as well as axial fatigue tests were carried out, along with static tensile tests. Fatigue design coefficients relevant to the notch support effect, surface finish, mean stress effect in bending and load type (axial or bending) were derived from the experimental test results and, whenever possible, were compared with ISO Standard recommendations. Microstructures, hardness profiles, residual stresses and fracture surfaces were analyzed as well. To evaluate the accuracy of specimens-based approaches mentioned in the Annex of ISO Standard, additional work is needed in order to compare theoretical estimations with experimental results obtained from gears.     

热喷涂技术制备铝涂层及其在3.5%NaCl溶液中耐腐蚀性的研究现状

铝是一种应用十分广泛的耐腐蚀材料,热喷涂技术作为表面工程领域的重要技术之一,在钢铁材料表面喷涂铝涂层,能够对钢铁材料起到很好的耐腐蚀保护作用,延长钢铁的使用寿命,减少对钢材的维护与保养。目前通常采用火焰喷涂技术、电弧喷涂技术和冷喷涂技术制备铝涂层,对此三种热喷涂技术制备铝涂层的涂层特点和耐腐蚀性能进行综述。系统归纳了这三种热喷涂技术的热源温度、粒子飞行速度和喷涂距离对形成涂层特点的影响机制,以及铝涂层在3.5%NaCl溶液中的耐腐蚀机理,揭示出铝涂层内部孔隙是影响其耐腐蚀性能的最主要因素,孔隙含量可由孔隙率表示,并指出随着孔隙率的增大,其耐腐蚀性能降低。但是并未详细指出涂层内部孔隙的含量和形状大小对涂层耐腐蚀性能的影响,因此通过进一步优化热喷涂技术制备铝涂层的工艺,研究不同孔隙含量的铝涂层和不同形状大小孔隙的铝涂层在实际服役工况下的具体耐腐蚀程度,对今后热喷涂铝涂层的实际应用具有重要的科学意义,是今后的重点研究方向之一。     

Thermoelasticity problem for a multilayer coating-substrate assembly irradiated by a laser beam

An analytical solution of a two-dimensional thermoelasticity problem is determined for a thin coating attached to a substrate. The coating is treated as a stratified multilayer structure composed of layers with varying thermophysical and optical characteristics. The developed mathematical procedure of solution is based one expansion of Laplace and Hankel transforms of temperature, displacement and strain distributions in series of small parameter which is equal to the ratio of a coating thickness to a laser beam radius. Expressions for determination of the two-dimensional nonstationary temperature, strain and stress distributions in a multilayer assembly are derived in a closed analytical form which can be used for development of an algorithm of laser photothermal method for detection of failures in the multilayer coating-substrate systems.     

b-direction anomalous thermal expansion behavior in SmCrO3

The ceramic composites of Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) have been prepared by solid state reaction. X-ray diffraction patterns demonstrate that Bi-doped polycrystalline samples match with standard pattern of SmCrO₃. After Bi doping, the lattice constants in three directions increase slightly. The thermal expansion properties were studied by X-ray powder diffraction over the temperature range from 100 to 400 K. The results showed that the compound SmCrO₃ exhibits positive thermal expansion behavior in the a and c directions in the whole measurement temperature range. The anomaly is that in the b direction the lattice exhibits positive thermal expansion behavior at low temperatures, while above 200 K it exhibits near zero thermal expansion behavior. The thermal expansion behaviors of all Bi-doped SmCrO₃ are similar to that of SmCrO₃, but unexpectedly, negative thermal expansion behaviors were observed in the b direction above 200 K in the highly Bi doped samples. The dielectric measurement shows that there are two thermal activation processes in all Sm_1-xBi_xCrO₃ (x = 0, 0.1, 0.2, 0.3) respectively, with the low-temperature activation corresponding to the grain itself, while the high-temperature activation comes from the grain boundary. After low-temperature thermal activation, with increasing temperature, the enhanced vibration of the corner oxygen atoms of the CrO₆ octahedron is mainly perpendicular to the b axis, which leads to the near zero thermal expansion behavior in the b direction. Moreover, after high-temperature thermal activation, as the temperature increases, the enhanced interaction between Bi³⁺ and the free oxygen atom (space oxygen atom) at the grain boundary will further lead to the negative thermal expansion behavior in the b-direction of the high concentration doped samples.     

Comparative investigation of physical,X-ray and neutron radiation shielding properties for B2O3-MnO2-CdO borate glasses

B₂O₃-MnO₂-CdO ternary oxide glasses with amorphous properties were synthesised using the melt-quenching method. Structural and physical property analyses showed that the amount of non-bridging oxygen and the system stiffness increases, and the oxide network filling is more tightly packed as cadmium oxide (CdO) gradually replaces manganese Oxide (MnO₂) in the glass. The radiation-shielding performance of the B₂O₃-MnO₂-CdO glasses was evaluated using the shielding parameters calculated by the MCNPX simulation and the Phy-X program. Glasses with 40% and 50% CdO loading exhibited an average specific lead equivalent (PbE) of 0.241 and 0.294 mmPb/mm in the medical X-ray diagnostic area at 0.03–0.08 MeV; thus, they could fully meet the specific PbE requirements for application as "protection devices against diagnostic medical X-radiation". Furthermore, their photon attenuation capability is superior to that of various commercial shielding glasses in fast-neutron nuclear applications at 1°–10³ MeV. In addition, compared to the borate glass systems studied in the literature, B₂O₃-MnO₂-CdO glasses have fast neutron removal cross-sections of 0.125 cm⁻¹ at a smaller density of 3.9043–4.8135 g/cm³, making them potentially excellent fast neutron absorbers.     

Tunable antiferroelectric ceramic polarization via regulating incommensurate phase and domain features

Due to the high demand for dielectric materials with high energy density, the energy storage performance of antiferroelectric ceramic capacitors has always gained much attention. Polarization intensity is a key factor that is closely related to the energy storage density. However, thus far, there has been a lack of research studies or successful methods to effectively modulate polarization intensity. The behavior of the polarization process is complex and contains domain nucleation, growth, and flip-flapping. Based on this finding, the introduction of Nb⁵⁺ at the B-site was designed to influence the three stages of antiferroelectric polarization by regulating the balance between the ferroelectric and antiferroelectric phases, and eventually realized regulation of the saturation polarization intensity in the (Pb_1-1.5xLa_x)(Zr_0.5Sn_0.43Ti_0.07)O₃ antiferroelectric ceramics. The saturation polarization intensity has increased from 25.56 to 42 μC/cm² with Nb⁵⁺ content increases from 0 to 4 mol% and the hysteresis was kept low, Pb_0.94La_0.04(Zr_0.65Sn_0.35)_0.975Nb_0.02O₃ is the optimal component with a high releasable energy density of 8.26 J/cm³ and an energy storage efficiency of 90.31%. This work provides an in-depth explanation of the microscopic mechanism of antiferroelectric ceramic polarization and presents a novel approach for the composition design of high-energy storage density antiferroelectric ceramics.