Mechanical and Tribological Properties of Ca/P-Doped Titanium Dioxide Layer Produced by Plasma Electrolytic Oxidation: Effects of Applied Voltage and Heat Treatment

Plasma electrolytic oxidation (PEO) is a technique that produces a hard oxide layer on the titanium surface where its properties can be tailored by changing the process parameters or by a posterior heat treatment (HT). In this work, a TiO₂ layer with different crystallinity was produced by PEO with different applied voltages (250 to 400 V) and post-HT at 600°C. Our aim was to evaluate the influence of the PEO voltage and HT on the mechanical and tribological properties of anodized Ti. There is an increase in pore size, oxide thickness, and Ca/P ratio for the oxide layer with the applied voltage during the PEO process. X-ray diffraction (XRD) results indicated an increase in the crystalline rutile phase in the oxide layer with voltage and HT. Nanoindentation shows an increase in the oxide hardness and elastic modulus with increased voltage and HT, leading to an improvement in the wear resistance.     

Microstructural evolution and micromechanical properties of thermally sprayed hydroxyapatite coating

ABSTRACT

The effects of preheating and annealing processes on the micromechanical features of thermally sprayed hydroxyapatite (HA) coatings were investigated. The results indicated that subsequent heat treatment at 700°C for 60?min promotes the development of a crystalline HA coating. The EDS line scan showed that the oxygen content was homogeneous along the thickness direction from the coating surface to the titanium–HA interface, whereas the calcium and phosphorus concentration gradually decreased at 7?μm from the interface. From the roughness profiles, the coatings on preheated substrates gave lower roughness compared to the coatings at room temperature. According to the nanoindentation results, the sample preheated at 300°C after annealing at 700°C exhibited an elastic modulus of 108.1?±?6?GPa and hardness of 5.97?±?0.3?GPa, which were almost 3% lower and 171% higher than the bare substrate, respectively.     

Unsteady pressure measurements in a high-speed centrifugal compressor

This paper presents the unsteady data acquisition system used to measure the pressure field in high speed compressors. Details and electronic sketches are given for the conditioners developed in-house that have been used to amplify and to filter the pressure signal with the aim of acquiring data up to 150 kHz. A discussion of the experimental results carried out in a centrifugal compressor is proposed. Through different processing of the pressure signals and a comparison with URANS simulations, the excitation of the pressure transducers by the pressure waves generated by shock waves that occur between the impeller and the diffuser is highlighted. The levels of pressure fluctuations measured when entering into surge are also presented and reveal very repetitive behaviour of the flow instabilities.     

Incipient Boiling on Exponentially Heated Surfaces

Calculations have been performed on the asymptotic angular neutron flux, critical thickness and extrapolation distance in the axial direction for a cylindrical system of finite length. Neutrons are assumed to be monoenergetic. Applying the finite Laplace transformation in the axial direction and buckling approximation in the radial, we find a solution of the transport equation that satisfies the boundary conditions of no incoming neutron and symmetry. This method is an extension of Kobayashi's Laplace transform method for slab problem, and may easily be applied to other geometries. For a rectangular prism system, numerical results based on our method are compared with results from P₃-approximation.     

Measurement of the coolant flow rate using correlation of temperature fluctuations

The transit time of the coolant, and thus its velocity, has been measured using the temperature fluctuation at the outlet of a reactor core. An impulse response function estimation is introduced, which substitutes the widely used cross-correlation measurement technique. It is shown in theory and practice that the time delay estimation is improved when using the impulse response function instead of the cross-correlation function in parameter estimation. Extremely low velocities (down to 2 cm/sec) have been measured in a natural circulation regime in a research reactor.     

Effects of Al content on the corrosion properties of Mg–4Y–xAl alloys

The corrosion performances of Mg–4Y–xAl (x = 1, 2, 3, and 4 wt%) alloys in the 3.5% NaCl electrolyte solution are investigated by electrochemical tests, weight loss measurement and corrosion morphology observation. The results indicate that corrosion modes for the alloys are localized corrosion and the filiform type of attack. With Al concentration increasing from 1 to 4 wt%, the corrosion rate of Mg–4Y–xAl alloys decreases firstly and then increases, and WA42 alloy shows the best corrosion resistance. The addition of Al element to Mg–4Y alloys leads to the formation of Al₂Y and Al₁₁Y₃ intermetallic compounds and reduces the proportion of Mg₂₄Y₅ phase. Corrosion resistance of the Mg–4Y–xAl alloys mainly depends on the size and distribution of the second phases. Besides, the addition of excessive Al can greatly consumes the Y element in the matrix, thus leading to a less protective film on the alloys. The effect of the relative Volta potential changes between the second phases and α-Mg on corrosion resistance of Mg–4Y–xAl alloys is insignificant. The main corrosion products of the Mg–4Y–xAl alloys are Mg(OH)₂, Mg₃(OH)₅Cl·4H₂O, Mg_0.72Al_0.28(CO₃)_0.15(OH)_1.98(H₂O)_0.48, and Mg₄Al₂(OH)₁₂CO₃·3H₂O.     

纳米压痕法测量Sn-Ag-Cu无铅钎料BGA焊点的力学性能参数

对Sn-3．0Ag-0．5Cu无铅体钎料和Sn-4．0Ag-0．5Cu无铅钎料BGA(ball grid array)焊点进行了Berkovich纳米压痕法实验,通过改变不同的加载速率研究了钎料的蠕变特征．钎料压痕载荷位移曲线蠕变部分表现出了明显的加载速率相关性．基于Oliver-Pharrr法确定的体钎料和BGA焊点的Young’S模量分别为9．3和20GPa．基于压痕做功概念确定的体钎料和BGA焊点的应变速率敏感指数分别为0．1111和0．0574．钎料的力学性能有着明显的尺寸效应。     

Determination of the controlling parameters for dislocation nucleation in SrTiO3: An investigation by nanoindentation

We conduct nanoindentation to investigate dislocation nucleation in SrTiO₃ (STO) single crystals with surface orientations of (0 0 1), (0 1 1), and (1 1 1) with loading/unloading rates of 25, 250, and 2500 μN/s. Results reveal that the critical loads (P_c) at which “pop-in” event occurs depend strongly on surface orientations, but slightly related to loading rate. Based on P_c, the critical shear stress that triggers dislocation nucleation was determined by extracting the maximum resolved shear stress (τ_max) along the slip systems of STO using the Hertzian solution. The dislocation activation shear stress (τ_a) was determined by averaging τ_max. The determined τ_a is 9.0–12.0 GPa, close to the shear strength (∼G/2π) of STO, indicating that homogeneous dislocation nucleation dominates the pop-in events. The consistency of the determined τ_a demonstrates that the frameworks for nanoindentation pop-in analysis established for metals can be extended to ceramics, whereas the influence of the limited slip systems should be taken into consideration. Additionally, we estimated the activation volume and the activation energy via the statistical model proposed by Schuh et al. The small values of the determined activation volume (0.6–9.8 Å³) and the activation energy (0.13–0.70 eV) indicate that the dislocation nucleation possibly begins from a single-atom migration and local point defects may participate in the dislocation nucleation process. That is, heterogeneous nucleation may exist initially but the homogeneous dislocation nucleation dominates the pop-in events.     

Microstructure and characterization of Ag1−xLixNbO3 ceramics

Silver–lithium niobate, Ag_1−xLi_xNbO₃, is a promising candidate for lead-free piezoelectrics. Ceramic samples for x equal 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.1 were synthesized by solid state reaction. Room temperature X-ray studies indicate that all samples consist mainly of perovskite phase. Only small amounts of secondary phases, namely Ag₂Nb₄O₁₁, Ag and LiNbO₃, appear. For x≈0.06 the orthorhombic–rhombohedral morphotropic phase boundary is observed. Excellent piezoelectric properties, recently reported for x>0.1, may be the result of proximity of morphotropic phase boundary. Scanning electron microscopy and secondary ion mass spectroscopy investigations suggest that the secondary phases occur chiefly at the grain boundaries. Dielectric measurements, carried out in a wide temperature range, revealed that Ag_1−xLi_xNbO₃, for x≤0.05, undergoes the same complex sequence of phase transitions as AgNbO₃. On the other hand, for x>0.06, the temperature dependence of dielectric permittivity exhibits only two maxima, related to transitions from the ferroelectric R phase to the antiferroelectric M phase and followed by the paraelectric O phase. The dielectric results obtained confirm the existence of the morphotropic phase boundary.