An investigation on the tribological behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys at elevated temperatures

The wear behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys was investigated under a normal load of 20 N at the wear testing temperatures of 25–250 °C and sliding speeds of 0.4 and 1 m s⁻¹. As the sliding speed increased from 0.4 to 1 m s⁻¹ at the wear temperature of 25 °C, the wear rates of AZ91 and AZ91 + 3 wt% RE alloys decreased by about 8% and 60%, respectively. With an increase in the wear temperature to 100 °C, the wear rate of AZ91 alloy was reduced by 58% at a sliding speed of 0.4 m s⁻¹, while the wear rate was sharply increased at a sliding speed of 1 m s⁻¹. At higher wear temperatures, the wear of the AZ91 alloy at both sliding speeds soared as a result of the softening of β-Mg₁₇Al₁₂ phase. However, the wear rate of AZ91 + 3 wt% RE alloy showed a minimum at the wear temperatures of 100 and 200 °C at sliding speeds of 1 and 0.4 m s⁻¹, respectively. Superior wear behavior of AZ91 + 3 wt% RE at the elevated temperatures could be attributed to its higher thermal stability and strength. Furthermore, a rise in sliding speed led to a 55% reduction in the wear rate of AZ91 + 3 wt% RE alloy at the wear temperature of 100 °C due to the formation of stable oxide layers on the wear surface.     

Effect of melt temperature on the oxidation behavior of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The oxidation behaviors of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres at temperatures between 660 °C and 760 °C have been studied. The experimental results show that with the increase of melt temperature, the oxidation rate of molten AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres increased and the oxidation kinetics changed from parabolic law to linear law. On the other hand, the amount of MgF₂ in the oxide film formed on AZ91D decreased, and the amount of MgO increased. The effect of melt temperature on the oxidation behaviors is primarily related to the relative content of MgO and MgF₂ in the film, as well as the diffusion rate and the evaporation rate of magnesium through the film.     

Amorphous InGaZnO thin film transistors with SiO2/HfO2 double-layer gate dielectric fabricated at low temperature

Amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) with double-layer gate dielectric were fabricated at low temperature and characterized. A stacked 150 nm-thick SiO₂/50 nm-thick HfO₂ dielectric layer was employed to improve the capacitance and leakage characteristics of the gate oxide. The SiO₂/HfO₂ showed a higher capacitance of 35 nF/cm² and a lower leakage current density of 4.6 nA/cm² than 200 nm-thick SiO₂. The obtained saturation mobility (μ_sat), threshold voltage (V_th), and subthreshold swing (S) of the fabricated TFTs were 18.8 cm² V^?1 s^?1, 0.88 V, and 0.48 V/decade, respectively. Furthermore, it was found that oxygen pressure during the IGZO channel layer deposition had a great influence on the performance of the TFTs.     

Laser welding of AZ31B magnesium alloy to Zn-coated steel

The characteristics of laser lap welding of AZ31B magnesium alloy to Zn-coated steel were investigated. Welding was difficult when the laser beam was irradiated onto the AZ31B alloy and the processing parameters were set to obtain a keyhole welding mode. The difference in the physical properties between the two materials resulted in unstable welding process particularly when the laser beam penetrated into the steel specimen and a keyhole was formed therein. By switching to a conduction mode, the process stability was improved and successful welding could be achieved because the liquid metal film remained unbroken and the laser beam did not penetrate into the material. A 25 mm wide joint failed in tensile shear testing at loads exceeding 6000 N. This high joint strength was attributed to the formation of a 450 nm thick layer of Fe₃Al intermetallic compound on the steel surface as a result of the interaction between Al from the AZ31B alloy and Fe. The presence of Zn-coating layer was essential to eliminate the negative effects of oxides on the joining process.     

Osteoblast interaction with laser cladded HA and SiO2-HA coatings on Ti–6Al–4V

In order to improve the bioactivity and biocompatibility of titanium endosseous implants, the morphology and composition of the surfaces were modified. Polished Ti–6Al–4V substrates were coated by a laser cladding process with different precursors: 100 wt.% HA and 25 wt.% SiO₂-HA. X-ray diffraction of the laser processed samples showed the presence of CaTiO₃, Ca₃(PO₄)₂, and Ca₂SiO₄ phases within the coatings. From in vitro studies, it was observed that compared to the unmodified substrate all laser cladded samples presented improved cellular interactions and bioactivity. The samples processed with 25 wt.% SiO₂-HA precursor showed a significantly higher HA precipitation after immersion in simulated body fluid than 100 wt.% HA precursor and titanium substrates. The in vitro biocompatibility of the laser cladded coatings and titanium substrate was investigated by culturing of mouse MC3T3-E1 pre-osteoblast cell line and analyzing the cell viability, cell proliferation, and cell morphology. A significantly higher cell attachment and proliferation rate were observed for both laser cladded 100 wt.% HA and 25 wt.% SiO₂-HA samples. Compared to 100 wt.% HA sample, 25 wt.% SiO₂-HA samples presented a slightly improved cellular interaction due to the addition of SiO₂. The staining of the actin filaments showed that the laser cladded samples induced a normal cytoskeleton and well-developed focal adhesion contacts. Scanning electron microscopic image of the cell cultured samples revealed better cell attachment and spreading for 25 wt.% SiO₂-HA and 100 wt.% HA coatings than titanium substrate. These results suggest that the laser cladding process improves the bioactivity and biocompatibility of titanium. The observed biological improvements are mainly due to the coating induced changes in surface chemistry and surface morphology.     

Free-standing,roll-able,and transparent silicone polymer film prepared by using nanoparticles as cross-linking agents

A free-standing, roll-able, and transparent silicone-based polymer film with a tensile modulus of ca. 7.8 MPa and strain at the break point of 0.76% was successfully prepared by reaction between a reactive silicone oligomer with methyl- and methoxy-side groups and hydrophilic SiO₂ nanoparticles. First, SiO₂ nanoparticles were grafted with silicone chains by a controlled wet chemical sol–gel-type reaction with the reactive oligomers. The solvent of the resulting solution was evaporated to form a viscous suspension, casted into a film, and finally heat-treated at 100 °C and 150 °C. A hydrolysis and condensation reaction among silicone-grafted SiO₂ nanoparticles and free silicone oligomers in the final heat treatment resulted to produce free-standing, roll-able, and transparent silicone-based polymer film. The fact that the silicone film cannot be synthesized without the presence of SiO₂ nanoparticles suggests that these nanoparticles act as cross-linking agents of silicone components providing the improved mechanical properties to the composite film. The rate-controlled mixing and heating of the SiO₂ aqueous/alcohol suspension and the silicone oligomer/alcohol solution was found to be the key step in the synthesis of the free-standing transparent film. While rapid addition/mixing resulted in a fragile and opaque film, a transparent material was achieved when those solutions were slowly mixed. The effect of the synthesis process on the macroscopic and microscopic properties of the prepared films is discussed along with their formation mechanism.     

Anodic oxide film growth on thin magnetron sputter-deposited titanium layer

A ~ 100 nm thick sputter-deposited titanium layer on electropolished aluminium is used for the investigation of anodic film growth in 1 M H₃PO₄. It is found that the thin titanium layer could not provide sufficient current efficiency for titanium anodic film growth when anodized to the voltage over 80 V due to the occurrence of oxygen evolution. The ruptures of titanium anodic film and the sputtering layer are induced by the development of oxygen bubbles. The penetrated phosphoric acid electrolyte in the ruptured regions of sputtering titanium layer contacts with the aluminium substrate, which is leading to the anodic oxide growth of aluminium. The thickness of the titanium anodic film increases from 10 to 100 V, however, it is reduced from 100 to 150 V due to the thinning of titanium layer. Over 80 V, the sputtering layer at some regions where it is completely ruptured, the anodic film growth of titanium could not be created. A thicker aluminium anodic film is formed on such regions due to the direct connection with the electrolyte.     

Atomic layer deposition of an HfO2 thin film using Hf(O-iPr)4

A thin film of hafnium dioxide (HfO₂) was formed on the surface of Si(100) by atomic layer deposition (ALD) using Hf(O-iPr)₄ (Hf(OCH(CH₃)₂)₄, hafnium tetrakis-iso-propoxide) as an Hf source and O₂ as an oxidant. The temperature window of the process was 250–350 °C, which is about 100 °C lower than that of a process using Hf(O-tBu)₄ as a source. This result was in accordance with the decomposition characteristics of the Hf precursor, as investigated by the temperature-programmed decomposition of the compound in an ultra-high vacuum and by thermogravimetric analysis in air. The thickness of a film deposited under the above conditions increased in proportion to the ALD cycles, indicating that the film-growth rate per cycle remained nearly constant during the process. The deposited film consisted of a monoclinic crystal phase included in an amorphous matrix, which was confirmed by X-ray diffraction. The film showed an equivalent-oxide thickness (EOT) of 2.1 nm and a leakage current density of 8.9 × 10^? 6 A/cm² at ? 1 V. The leakage current was three orders of magnitude lower than that of SiO₂ with the same EOT.     

Studies on optical,chemical, and electrical properties of rapid SiO2 atomic layer deposition using tris(tert-butoxy)silanol and trimethyl-aluminum

Rapid SiO₂ atomic layer deposition (ALD) was used to deposit amorphous, transparent, and conformal SiO₂ films using tris(tert-butoxy)silanol (TBS) and trimethyl-aluminum (TMA) as silicon oxide source and catalytic agent, respectively. The growth rate of the SiO₂ films drastically increased to a maximum value (2.3 nm/cycle) at 200 °C and slightly decreased to 1.6 nm/cycle at 275 °C. The SiO₂ thin films have C–H species and hydrogen content (～8 at%) at 150 °C because the cross-linking rates of SiO₂ polymerization may reduce below 200 °C. There were no significant changes in the ratio of O/Si (～2.1) according to the growth temperatures. On the other hand, the film density slightly increased from 2.0 to 2.2 although the growth rate slightly decreased after 200 °C. The breakdown strength of SiO₂ also increases from 6.20 ± 0.82 to 7.42 ± 0.81 MV/cm. These values suggest that high cross-linking rate and film density may enhance the electrical property of rapid SiO₂ ALD films at higher growth temperature.     

Properties and thermal stability of solution processed ultrathin,high-k bismuth titanate (Bi2Ti2O7) films

Ultrathin bismuth titanate films (Bi₂Ti₂O₇, 5–25 nm) are deposited onto SiO₂/Si substrates by aqueous chemical solution deposition and their evolution during annealing is studied. The films crystallize into a preferentially oriented, pure pyrochlore phase between 500 and 700 °C, depending on the film thickness and the total thermal budget. Crystallization causes a strong increase of surface roughness compared to amorphous films. An increase of the interfacial layer thickness is observed after anneal at 600 °C, together with intermixing of bismuth with the substrate as shown by TEM-EDX. The band gap was determined to be ～3 eV from photoconductivity measurements and high dielectric constants between 30 and 130 were determined from capacitance voltage measurements, depending on the processing conditions.     

Interface microstructure and mechanical properties of zinc–aluminum thermal diffusion coating on AZ31 magnesium alloy

The zinc–aluminum (Zn–Al) alloy coating with excellent wear and corrosion resistance was fabricated on the surface of magnesium substrate (AZ31) using thermal diffusion technique. The microstructure, phase constitution and chemical composition were investigated. The experimental observation exhibited that the interfacial microstructures were composed of network eutectic structures and lamellar eutectoid structures at heating temperature of 350 °C for holding time of 30 min under 0.1 MPa in a vacuum of 10⁻³ Pa. X-ray diffraction (XRD) pattern analysis identified that α-Mg, Mg₇Zn₃ and MgZn phases were formed in the diffusion layer. The interdiffusion of Mg and Al atoms were restricted by Mg–Zn intermetallic compounds (IMCs). The value of microhardness at the diffusion layer increased due to the formation of Mg–Zn eutectic phases. This technique is beneficial to improving poor wear and corrosion resistance of magnesium alloy.     

Titanium–SiO2 nanocomposites and their scaffolds for dental applications

There have been a number of attempts to modify the properties of titanium implants to improve osseointegration. These modifications include alterations of the chemistry and roughness of the surface of the implant. In this work, Ti–10 wt.% SiO₂ nanocomposites and their scaffolds were synthesized using a combination of mechanical alloying and a “space-holder” sintering process. The phase and microstructure analysis was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and the properties were measured using hardness and corrosion testing equipment. An amorphous structure was obtained at 20 h of milling. The crystallization of the amorphous phase upon annealing led to the formation of a nanostructured Ti–10 wt.% SiO₂ composite with a grain size of approximately 40 nm. The Vickers hardness of the Ti–10 wt.% SiO₂ nanocomposites reached 670 HV_0.2. The in vitro cytocompatibility of these materials was evaluated and compared with conventional microcrystalline titanium, where normal human osteoblast (NHOst) cells from Cambrex (CC-2538) were cultured. The morphology of the cell cultures obtained on the bulk Ti–10 wt.% SiO₂ nanocomposite was similar to those obtained on the microcrystalline titanium. However, on the porous scaffold, the cells adhered to the insert that penetrated the porous structure with their entire surface, whereas on the polished surface, more spherical cells were observed with a smaller surface of adhesion. Porous Ti–10 wt.% SiO₂ scaffolds have been developed in order to promote bone ingrowth and to induce prosthesis stabilization.     

Effect of substrate on microstructure and mechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Lead-free ferroelectric (K, Na)NbO₃ (KNN) thin films (～200 nm thickness) were prepared using a modified sol–gel method by mixing K and Na acetates with the Nb–tartarate complex, deposited by spin-coating method on Pt/Al₂O₃ and Pt/SiO₂/Si substrates and sintered at 650 °C. Pure perovskite phase of K_0.65Na_0.35NbO₃ in film on silicon were revealed, while film on alumina contained also small amount of secondary pyrochlore Na₂Nb₈O₂₁ phase. Homogenous microstructure of film on Si substrate was smoother with the lower roughness (～7.4 nm) and contained spherical (～50 nm) particles. The mechanical properties of films were characterized by nanoindentation. The modulus and hardness of KNN films were calculated from their composite values of film/substrate systems using discontinuous and modified Bhattacharya model, respectively. The KNN film modulus was higher on alumina substrate (91 GPa) in comparison with silicon substrate (71 GPa) and values of film hardness were the same (4.5 GPa) on both substrates.     

A universal electrolyte for the plasma electrolytic oxidation of aluminum and magnesium alloys

The plasma electrolytic oxidation (PEO) of aluminum and magnesium alloys is carried out in electrolytes which contain the same reactants, but in fundamentally different concentrations. In this research the possibility of the PEO of aluminum and magnesium alloys in a universal electrolyte is studied. The two most commonly encountered alloys, namely, aluminum alloy 5052 and magnesium alloy AZ91D, are chosen. The oxide layers obtained are studied using SEM, EDX, XRD, and a microhardness tester. The corrosion properties are determined using a potentiostat. The effect of variation of the silicate concentration in the electrolyte on the growth kinetics of the coating and its qualitative characteristics is discussed. It is shown that at Na₂SiO₃·5H₂O concentrations in the electrolyte ranging from 3.2 to 32 g l^− 1, the rate of growth of the oxide layer increases from 15 to 55 μm h^− 1 with significant variation of the phase composition of the coating. The greatest hardness of an oxide ceramic layer was obtained in the outer sublayer on the magnesium alloys (874.7 HV₁₀) and in the inner sublayer on the aluminum alloys (1123 HV₁₀). The most favorable combination of physical and chemical properties for both alloys is obtained in an electrolyte containing 12.72 g l^− 1 Na₂SiO₃·5H₂O.     

The role of operations after the deposition on the performance of SiOx films in optoelectronics devices

In this study, we have investigated phase separation, silicon Nano crystal (Si-NC) formation and optoelectronics properties of Si oxide (SiO_x, 0.7 < x < 1.3) films in high-vacuum annealing and ion bombardment conditions. The SiO_x films were deposited by physical vapor deposition (PVD). The internal structure properties of these films were the main factor in applications of optoelectronic. Possible changes in the structure, composition and electro physical properties were investigated by FTIR and TEM spectroscopy. The measurements show that SiO_x film is the dominant phase in the ultra-thin layer. Also, high-temperature annealing ion bombardment results in further increase of the phase separation of the whole layer.     

Role of environmental and annealing conditions on the passivation-free in-Ga–Zn–O TFT

We examined the characteristics of passivation-free amorphous In–Ga–Zn–O thin film transistor (a-IGZO TFT) devices under different thermal annealing atmospheres. With annealing at higher temperature, the device performed better at the above-threshold operation region, which indicated the film quality was improved with the decrease of defects in the a-IGZO active region. The mobility, threshold voltage and subthreshold swing of a-IGZO TFT annealed at 450 °C was 7.53 cm²/V s, 0.71 V and 0.18 V/decade, respectively. It was also observed that the a-IGZO was conductive after thermal annealing in the vacuum, due to the ease of oxygen out-diffusion from the a-IGZO back channel. The oxygen deficiency resultantly appeared, and provided leaky paths causing electrical unreliability when TFT was turned off. In contrast, the annealing atmosphere full of O₂ or N₂ would suppress the oxygen diffusion out of the a-IGZO back channel. The worst V_th degradation of a-IGZO TFT after positive gate bias stress and negative gate bias stress (NGBS) was about 2 V and ? 2 V, respectively. However, the V_th shift in the NGBS testing could be suppressed to ? 0.5 V in vacuum chamber. Material analysis methods including X-ray photoelectron spectroscopy and scanning electron microscopy were used to investigate the change of a-IGZO film after different thermal annealing treatments. The variation of O 1s spectra with different annealing atmospheres showed the consistence with our proposed models.     

CdS nanoparticles incorporated onion-like mesoporous silica films: Ageing-induced large stokes shifted intense PL emission

CdS nanoparticles (NPs) were generated in onion-like ordered mesoporous SiO₂ films through a modified sol–gel process using P123 as a structure directing agent. Initially Cd²⁺ doped (12 equivalent mol% with respect to the SiO₂) mesoporous SiO₂ films were prepared on glass substrate. These films after heat-treatment at 350 °C in air yielded transparent mesoporous SiO₂ films having hexagonally ordered onion-like pore channels embedded with uniformly dispersed CdO NPs. The generated CdO NPs were transformed into CdS NPs after exposing the films in H₂S gas at 200 °C for 2 h. The as-prepared CdS NPs incorporated mesoporous SiO₂ films (transparent and bright yellow in color) showed a band-edge emission at 485 nm and a weak surface defect related emission at 530 nm. During ageing of the films in ambient condition the band-edge emission gradually weakened with time and almost disappeared after about 15 days with concomitant increase of defect related strong surface state emission band near 615 nm. This transformation was related to the decay of initially formed well crystalline CdS to relatively smaller and weakly crystalline CdS NPs with surface defects due to gradual oxidation of surface sulfide. At this condition the embedded CdS NPs show large Stokes shifted (∼180 nm) intense broad emission which could be useful for luminescent solar concentrators. The detailed process was monitored by UV–Visible, FTIR and Raman spectroscopy, XPS, XRD and TEM studies. The evolution of photoluminescence (PL) and life times of CdS/SiO₂ films were monitored with respect to the ageing time.     

Brazing of SiO2f/SiO2 composite modified with few-layer graphene and Invar using AgCuTi alloy

Due to the poor wettability of the AgCuTi alloy on the SiO_2f/SiO₂ composite, direct brazing of the composite with an Invar alloy could hardly achieve a reliable joint. To overcome that, the SiO_2f/SiO₂ composite was decorated with few-layer graphene (FLG) by a plasma enhanced chemical vapor deposition (PECVD) method. Sessile drop experiments indicate that the contact angle dropped from 123.8° to 50.7° after FLG was grown on the surface of the SiO_2f/SiO₂ composite. Afterwards, the effects of brazing temperature and Ti contents on the microstructure evolution and mechanical properties of joints (Invar/SiO_2f–SiO₂ modified with FLG) were investigated. The typical interface structure of the joint is SiO_2f–SiO₂/Ti₅Si₃ + TiO₂ + Cu_xTi_6 − xO(x = 2,3)/Ag(s,s) + Cu(s,s) + Cu–Ti blocks/wave-like Fe₂Ti + Ni₃Ti/Ag(s,s) + Cu(s,s) + Fe₂Ti + Ni₃Ti blocks/Invar. As the brazing temperature and Ti contents increase, the reaction layer on the SiO_2f/SiO₂ side becomes thicker and cracks gradually propagate. Meanwhile, a few dispersive Fe₂Ti + Ni₃Ti phases change into large-area wave-like compounds and more Cu–Ti compounds form with the increase of the Ti content. The microstructure evolution significantly affects the shear strength of the brazed joints. The highest shear strength is 26 MPa brazed at 860 °C for 10 min with 4.5 wt.% Ti content.     

Growth and characterization of (Pb,La)TiO3 films with and without a special buffer layer prepared by RF magnetron sputtering

The (Pb, La)TiO₃ (PLT) ferroelectric thin films with and without a special buffer layer of PbO_x have been deposited on Pt/Ti/SiO₂/Si(100) substrates by RF magnetron sputtering technique at room temperature. The microstructure and the surface morphology of the films annealed at 600 °C for 1 h have been investigated by X-ray diffraction (XRD) and atomic force microscope (AFM). The surface roughness of the PLT thin film with a special buffer layer was 4.45 nm (5 μm × 5 μm) in comparison to that of 31.6 nm (5 μm × 5 μm) of the PLT thin film without a special buffer layer. Ferroelectric properties such as polarization hysteresis loop (P–V loop) and capacitance–voltage curve (C–V curve) of the films were investigated. The remanent polarization (P_r) and the coercive field (E_c) are 21 μC/cm² and 130 kV/cm respectively, and the pyroelectric coefficient is 2.75 × 10^− 8 C/cm² K for the PLT film with a special buffer layer. The results indicate that the (Pb, La)TiO₃ ferroelectric thin films with excellent ferroelectric properties can be deposited by RF magnetron sputtering with a special buffer layer.     

Microwave synthesis of calcium bismuth niobate thin films obtained by the polymeric precursor method

The crystal structure, surface morphology and electrical properties of layered perovskite calcium bismuth niobate thin films (CaBi₂Nb₂O₉-CBN) deposited on platinum coated silicon substrates by the polymeric precursor method have been investigated. The films were crystallized in a domestic microwave and in a conventional furnace. X-ray diffraction and atomic force microscopy analysis confirms that the crystallinity and morphology of the films are affected by the different annealing routes. Ferroelectric properties of the films were determined with remanent polarization P_r and a drive voltage V_c of 4.2 μC/cm² and 1.7 V for the film annealed in the conventional furnace and 1.0 μC/cm² and 4.0 V for the film annealed in microwave furnace, respectively. A slight decay after 10⁸ polarization cycles was observed for the films annealed in the microwave furnace indicating a reduction of the domain wall mobility after interaction of the microwave energy with the bottom electrode.