•Ag-doped BiCuSeO is synthesized by high energy ball milling & SPS by replacing Cu.•PVA results in stabilized pellets along with improved electrical conductivity.•Ag-doping enhances the Seebeck coefficient due to improved electrical conductivity.•The phonon scattering centers result 22% reduction in total thermal conductivity.•The high ZT of 0.64 was attained at 923 K for BiCu1−xAgxSeO, 16% higher BiCuSeO.BiCuSeO oxyselenide-based thermoelectric materials are attracting much attention due to their ultra-low intrinsic thermal conductivity (ĸ) and moderate Seebeck coefficient (S). However, the low conversion efficiency limits their application in energy conversion systems. This paper investigated the effect of monovalent Ag-doping in BiCuSeO at Cu site in the presence of polyvinyl alcohol (PVA). The results show that Ag is effectively doped at Cu site. The heavy atom substitution in BiCu1−xAgxSeO at Cu site can enhance S and reduce its total thermal conductivity (k) by phonon scattering. Further, the incorporation of heavy atom (Ag) with different ionic radii results in the improved ZT of 0.64 for BiCu1−xAgxSeO, which is 16% more than that for pure BiCuSeO (ZT = 0.55) at 923 K. It is believed that the Cu site should be an efficient dopant site to improve the efficiency of oxyselenide-based thermoelectric materials.The heavy element (Ag) doping at Cu site was explored to investigate its effect on the thermoelectric properties of BiCuSeO. It is found that Cu site is also an effective doping sites to interrupt the thermoelectric conductivity compared to the traditional Bi site and results in an enhanced ZT value compared to the un-doped BiCuSeO.

•Failure analysis of turbomachinery high nickel alloy steel seal ring was performed.•New and used seal ring materials were characterized and compared.•Used ring material has undergone recovery and recrystallization.•Secondary phase precipitation occurred along the grain boundaries.•Precipitation, grain refinement and microvoids ultimately lead to seal ring failure.The purpose of this study is to identify the failure cause of the steel seal ring used in nuclear steam turbines. New high nickel steel alloy seal ring was compared with the failed seal ring. Metallographic analysis, scanning electron microscopy, nanoindentation and in-situ tensile testing were used to analyze the reasons of the seal ring failure at both macroscopic and microscopic scales. The main reason of the seal ring failure is a combination of stress and elevated temperature during turbine operation. Complex work environment caused recrystallization and recovery, resulting in grain refinement and secondary phases precipitation. Many secondary phase precipitates appeared at grain boundaries during use, causing formation of microvoids and cracks, ultimately leading to ring failure.Download high-res image (389KB)Download full-size image

Testing high temperature brittle film adhesion is necessary for understanding interfacial failure at elevated temperatures. However, current brittle film adhesion measurement methods are limited to room temperature. Experimental techniques to characterize high temperature brittle film adhesion are lacking, and temperature effects on brittle film adhesion remain poorly understood. Here, a simple, yet reliable method is developed to measure the adhesion of TiN films on Si substrates with native SiO2 oxide layer from 300 °C to 500 °C, based on circular blisters induced by annealing. The circular blister size was proven to remain the same after cooling down to room temperature, based on in situ observations. Experimental results show that film adhesion energy gradually increases and then drops with annealing temperature. Thermally activated dislocation glide promotes easier nucleation of dislocations in Si substrate near the interface. This in turn increases dislocation shielding effects on the interfacial crack tip during its dynamic propagation, resulting in the initially increased adhesion with temperature. Plastic deformation of TiN film is not considered because the combination of the small grain size of less than 10 nm and the amorphous/nanocrystalline structure limits dislocation emission and grain sliding. Local phase film transformation from amorphous to nanocrystalline at the TiN/SiO2 interface was demonstrated by high resolution transmission electron microscopy, causing adhesion reduction due to interfacial embrittlement and contact mismatch at 500 °C. In addition, the drop in adhesion induces circular blisters' transition from axisymmetric to non-axisymmetric.A simple method to measure TiN film adhesion energy from 300 °C to 500 °C is presented, based on circular blisters induced by annealing.Download high-res image (184KB)Download full-size image

Residual stress in thin films and coatings strongly affects their properties and behavior in service. Comprehensive understanding and precise measurements of residual stress are prerequisites for preparing high quality films and coatings. Residual stresses in TiN films with different thickness were measured by X-ray diffraction (XRD) employing the cos2α sin2ψ method with certain optimization. Grazing incidence parallel beam optics was combined with side-inclination geometry using in-house designed sample stage to ensure results accuracy. To validate this method, TiN films with thickness ranging from 1 to 3 µm were deposited on (100) Si single crystal substrates at 300 °C by RF magnetron sputtering. High compressive −2 GPa residual stress was present in the 0.9 µm thick film and decreased with film thickness. Tensile stress of less than 0.3 GPa was present in 2 µm TiN film. Compressive-to-tensile residual stress transition was observed with the film thickness increase. Microstructure change with growth, annihilation of grain boudaries, atomic peening and recovery mechanisms are responsible for the reported stress sign transition.

The preferred growth orientation of the sputtered lead selenide (PbSe) thin films on Si(100) substrates was thermodynamically simulated and calculated on the basis of the density functional theory. The results showed that the total free energy variation during the grain growth is dominated by the interface and strain energy minimization under certain conditions, indicating that the preferred growth orientation and related optical properties of the PbSe thin films can be effectively modified by these two energy variations. Thermodynamically, the PbSe[200] and PbSe[220] preferred orientations are obtained when the interface and strain energy minimization dominate the total free energy variation, respectively. A texture map related to the interface and strain energy revolution was obtained, which can be used to predict the structure and optical properties of the sputtered PbSe thin films, and its applicability was confirmed by the real X-ray diffraction and Fourier transform infrared spectroscopy experimental results of four midfrequency sputtered PbSe thin films with designed thickness and microstrain deposited on Si(100) substrates.Keywords: energy contribution; lead selenide; preferred orientation; thermodynamic modeling; thin film

•Pit-like corrosion attack occurred only on the Cr-containing steels.•Cr-containing steel was more likely to attract SO42− and Cl− ions in the crevice.•Cr and Cu have synergistic effects on increasing the crevice corrosion.The effects of Cr and Cu on the corrosion behavior of low-alloy steel have been studied by immersion tests and electrochemical measurements combined with scanning electron microscopy and analysis of polarization curves. The low-alloy steels exhibit higher uniform corrosion resistance than do carbon steel. Crevice corrosion morphology study indicated that the pit-like corrosion attack occurred only on the Cr-containing steels. The crevice corrosion tests indicated that Cr and Cu have synergistic effects on increasing the crevice corrosion of the steel.

Morphology, structure, residual stress, and surface energy of magnetron-sputtered titanium nitride (TiN) thin films, deposited at 300 °C with a thickness in the 0.5-1.7 μm range, were characterized. Film microstructure, the origin of residual stress, and its effect on the surface energy were analyzed. The grain size increased with the film thickness. X-ray diffraction showed (200) to (111) preferred orientation transitions with the increasing film thickness. The stress in the TiN films changed from compressive −0.3 GPa to tensile with the film thickness reaching 0.3 GPa. Larger grain size, initial porosity, and sub-grains generation are the reasons for significant changes in the residual stress. Surface energy was investigated by contact angle of water and glycerol droplets, which both show a significant change with the different stress state and crystal preferred orientation. The TiN films form a contact angle larger than 100° with water as a test liquid, demonstrating their hydrophobicity. While the residual stress changes from compressive to tensile, the contact angle reaches 118°, and the corresponding surface energy changes from 38.8 to 24.2 mJ/m2. One can expect to achieve a certain desired surface state of TiN films for potential applications.

•Oxygen doped PbSe films were grown on Si (100) using magnetron sputtering.•The band gap of doped PbSe films decreased from 0.278 to 0.21 eV.•The band gap increased almost linearly with the lattice constant.•The change rate between the light and dark resistance increased to 64.76%.Oxygen doped PbSe thin films with different thickness were grown on the Si (100) substrates by magnetron sputtering, and characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and physical properties measurement system. As the film thickness increased, the intensity of the (200) PbSe prominent diffraction peak increased, while the (220) peak almost vanished, indicating the primary growth direction. The change rate between the light and dark resistance increased with the film thickness, and the maximum of 64.76% was obtained. According to the density functional theory calculations and the experimental results, the band gap of the PbSe thin films decreased from 0.278 eV to 0.21 eV when doped with oxygen. Doping with oxygen during the deposition process is a viable way to prepare PbSe thin films with a tunable band gap. The band gap increased almost linearly with the lattice constant, confirmed by the calculated and experimental results.

Nickel films were deposited by radio frequency magnetron sputtering on top of polycarbonate substrates. Surface energy of the substrate was measured by means of the contact angle technique. Effects of sputtering parameters on the critical load between the film and the substrate were determined by the universal mechanical testing system. Optimized fabrication parameters and their influence on the critical load between sputtered nickel films and polymer substrate were studied by means of the orthogonal experimental design. Increasing radio frequency power and time improved film critical load. The radio frequency power had a more pronounced effect on critical load than the sputter power. The plasma pretreatment with Ar gas modified the surface, leading to an increased surface energy, improving the chemical bonds between nickel and carbon atoms, and thereby enhanced the critical load. The adhesion mechanism is also discussed in this paper.

The tribological properties of magnetron sputtered titanium nitride coating on 316L steel, sliding against Si3N4 ceramic ball under dry friction and synthetic perspiration lubrication, were investigated. The morphology of the worn surface and the elemental composition of the wear debris were examined by scanning electron microscopy and energy dispersive spectroscopy. TiN coatings and 316L stainless steel had better tribological properties under synthetic perspiration lubrication than under dry friction. Among the three tested materials (316L, 1.6 and 2.4 μm TiN coatings), 2.4 μm TiN coating exhibits the best wear resistance. The difference in wear damage of the three materials is essentially due to the wear mechanisms. For the TiN coating, the damage is attributed to abrasive wear under synthetic perspiration lubrication and the complex interactive mechanisms, including abrasive and adhesive wear, along with plastic deformation, under dry friction.

•Diamond-like carbon thin films were deposited by RF hollow cathode method.•The deposition rate of 45 nm/min was achieved.•A higher plasma density results in a higher deposition rate.Diamond-like carbon (DLC) thin films were deposited on p-type Si (100) substrates by RF hollow cathode method under different RF power and pressure, using ethane as the precursor gas. The deposition rate of 45 nm/min was achieved, almost 4 times higher than by conventional radio frequency plasma enhanced chemical vapor deposition. The mechanism of fast DLC films deposition is attributed to high plasma density in RF hollow cathode method, discussed in this paper. Scanning electron microscopy and Raman spectroscopy were used to investigate the microstructure of DLC films. The film hardness and Young's modulus were measured by nanoindentation.

•Grain size increased and subgrains were detected with increasing film thickness.•Stress changed from compressive to tensile with increasing film thickness.•Film residual stress and fracture toughness are strongly correlated.Morphology, structure, residual stress, hardness, and fracture toughness of magnetron sputtered titanium nitride (TiN) thin films, deposited at 300 °C with a thickness in the 0.3- to 2-μm range, were characterized. Film microstructure, the origin of residual stress, and its effect on the fracture toughness and hardness were analyzed. The grain size increased with the film thickness, with 1- to 2-μm-thick films having high pore density. For the 2-μm film, subgrains appeared at grain boundaries. X-ray diffraction showed (200) to (111) preferred orientation transition. The stress in the TiN films changed from highly compressive (− 1.1 GPa) to tensile with the film thickness, reaching 0.68 GPa. Larger grain size, initial porosity, and subgrain generation are reasons for significant changes in the residual stress. Average hardness measured by nanoindentation is 23.2 ± 0.6 GPa. The hardness of the films in compression is higher than in tension. Hardness variation with the film thickness is mainly due to the grain size and microstructure effects. The fracture toughness decreases with the film thickness, depending on the stress state and value. Compressive stress can significantly improve TiN film fracture toughness, while tensile stress seriously degrades it.

Multilayer Cr(1 − x)AlxN films with a total thickness of 2 μm were deposited on high-speed steel by medium frequency magnetron sputtering from Cr and Al–Cr (70 at.% Al) targets. The samples were annealed in air at 400 °C, 600 °C, 800 °C and 1000 °C for 1 hour. Films were characterized by cross-sectional scanning electron microscopy and X-ray diffraction analysis. The grain size of the as-deposited multilayer films is about 10 nm, increasing with the annealing temperature up to 100 nm. Interfacial reactions have clearly changed at elevated annealing temperatures. As-deposited films' hardness measured by nanoindentation is 22.6 GPa, which increases to 26.7 GPa when the annealing temperature goes up to 400 and 600 °C, but hardness decreases to 21.2 GPa with further annealing temperature increase from 600 to 1000 °C. The multilayer film adhesion was measured by means of the scratch test combined with acoustic emission for detecting the fracture load. The critical normal load decreased from 49.7 N for the as-deposited films to 21.2 N for the films annealed at 1000 °C.

Ti/Ti-doped diamond-like carbon (DLC) and Ti/AlTiN/Ti-DLC composite coatings were deposited by magnetron sputtering on W18Cr4V high speed steel substrates. The effect of the AlTiN support layer on the properties of these composite coatings was investigated through microstructure and mechanical properties characterization, including hardness, elastic modulus, coefficient of friction and wear properties measured by scanning electron microscopy, Raman spectroscopy, scratch and ball-on-disk friction tests. Ti and AlTiN interlayers have a columnar structure with 50–80 nm grains. The hardness and elastic modulus of Ti/Ti-DLC and Ti/AlTiN/Ti-DLC coatings is 25.9 ± 0.4, 222.2 ± 6.3 GPa and 19.3 ± 1, 205.6 ± 6.7 GPa, respectively. Adhesion of Ti-DLC, Ti/AlTiN/Ti-DLC and AlTiN/Ti-DLC coatings expressed as the critical lateral force is 26.5 N, 38.2 N, and 47.8 N, respectively. Substrate coefficient of friction without coatings is 0.44, and it is 0.1 for Ti/Ti-DLC and Ti/AlTiN/Ti-DLC coatings. Wear resistance of Ti/AlTiN/Ti-DLC composite coatings is much higher than Ti/Ti-DLC coatings based on the wear track width of 169.8 and 73.2 μm, respectively, for the same experimental conditions.

Single and multi-layer Cr/Cr2O3 coatings were deposited by reactive magnetron sputtering with the total thickness of 7 μm on steel substrates. X-ray diffraction analysis showed that single and multi-layer Cr/Cr2O3 coatings have different preferred crystal orientations. Columnar microstructure was detected by transmission electron microscopy both in metal chromium and ceramic chromium oxide layers. Grain size increased with the coating thickness. The value of single and multi-layer coating's fracture toughness is between 4 and 6 MPa·m1/2 measured with the Berkovich tip indentation, and it is between 2.8 and 3.9 MPa·m1/2 when measured with the Vickers indenter. The adhesion is about 192.1 and 246.7 J/m2 for single and multi-layer coatings, respectively.