Tribological properties of heat-treated electroless Ni-P coatings on AZ91 alloy

Influence of heat treatment regime on adhesion and wear resistance of Ni-P electroless coating on AZ91 magnesium alloy is investigated in this work. The pretreated substrate was plated using a bath containing nickel sulphate, sodium hypophosphite and sodium acetate as main constituents. The coated samples were heat treated at 400-450 °C for 1-8 h. Adhesion of coating was estimated from the scratch test with an initial load of 8.80 N. Wear resistance was studied using the pin-on-disc method. It was found that there is no significant dependence of the coating wear resistance on heat treatment regime, as the formation of Al-Ni intermetallic sub-layers that reduce coating adhesion is limited to regions where Al₁₇Mg₁₂ phase is present in the substrate. Moreover, the coating shows good sliding properties due to the formation of oxide glazes in the wear track.     

Microstructure and tribological properties of TiCu2Al intermetallic compound coating

TiCu₂Al ternary intermetallic compound coating has been in situ synthesized successfully on pure Ti substrate by laser cladding. Tribological properties of the prepared TiCu₂Al intermetallic compound coating were systematically evaluated. It was found that the friction coefficient and wear rate was closely related to the normal load and sliding speed, i.e., the friction coefficient of the prepared TiCu₂Al intermetallic compound coating decreased with increasing normal load and sliding speed. The wear rate of the TiCu₂Al intermetallic compound coating decreased rapidly with increasing sliding speed, while the wear rate first increased and then decreased at normal load from 5 to 15 N.     

Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes

Microarc oxidation coatings on AM60B magnesium alloy were prepared in silicate and phosphate electrolytes. Structure, composition, mechanical property, tribological, and corrosion resistant characteristics of the coatings was studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and microhardness analyses, and by ball-on-disc friction and potentiodynamic corrosion testing. It is found that the coating produced from the silicate electrolyte is compact and uniform and is mainly composed of MgO and forsterite Mg₂SiO₄ phases, while the one formed in phosphate electrolyte is relatively porous and is mainly composed of MgO phase. The thick coating produced from a silicate electrolyte possesses a high hardness and provides a low wear rate (3.55 × 10⁻⁵ mm³/Nm) but a high friction coefficient against Si₃N₄ ball. A relatively low hardness and friction coefficient while a high wear rate (8.65 × 10⁻⁵ mm³/Nm) is recorded during the testing of the thick coating produced from a phosphate electrolyte. Both of these types of coatings provide effective protection for the corrosion resistance compared with the uncoated magnesium alloy. The coating prepared from the silicate electrolyte demonstrates better corrosion behavior due to the compacter microstructure.     

On the synthesis and optical absorption studies of nano-size magnesium oxide powder

Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 °C for 24 h to form magnesium oxalate dihydrate [α-MgC₂O₄·2H₂O] and its decomposition at 500, 600, 800, and 1000 °C for 2 h to yield MgO powder (average crystallite size ∼6.5-73.5 nm). The sol-gel products at various stages of synthesis are characterized for their thermal behaviour, phase, microstructure, optical absorption, and presence of hydroxyl and other groups like OCO, CO, C-C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter increasing with decrease in crystallite size (t_av); typical value being ∼4.222(2) Å for t_av∼6.5 nm as against the bulk value of 4.211 Å. Infrared absorption has shown MgO to be highly reactive with water. Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap (7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron emission and reducing flickering effects. The possible application of the intermediate sol-gel products, viz., α-MgC₂O₄·2H₂O and anhydrous magnesium oxalate (MgC₂O₄) in understanding the plants and ESR dosimetry, respectively, has also been suggested.     

Coating formation by plasma electrolytic oxidation on ZC71/SiC/12p-T6 magnesium metal matrix composite

Plasma electrolytic oxidation (PEO) of a ZC71/SiC/12p-T6 magnesium metal matrix composite (MMC) is investigated in relation to coating growth and corrosion behaviour. PEO treatment was undertaken at 350 mA cm⁻² (rms) and 50 Hz with a square waveform in stirred 0.05 M Na₂SiO₃.5H₂O/0.1 M KOH electrolyte. The findings revealed thick, dense oxide coatings, with an average hardness of 3.4 GPa, formed at an average rate of ∼1 μm min⁻¹ for treatment times up to 100 min and ∼0.2 μm min⁻¹ for later times. The coatings are composed mainly of MgO and Mg₂SiO₄, with an increased silicon content in the outer regions, constituting <10% of the coating thickness. SiC particles are incorporated into the coating, with formation of a silicon-rich layer at the particle/coating interface due to exposure to high temperatures during coating formation. The distribution of the particles in the coating indicated growth of new oxide at the metal/coating interface. The corrosion rate of the MMC in 3.5% NaCl is reduced by approximately two orders of magnitude by the PEO treatment.     

Wear resistance of reactive plasma sprayed and laser remelted TiB2-TiC0.3N0.7 based composite coatings against medium carbon steel

Wear resistance of reactive plasma sprayed TiB₂-TiC_0.3N_0.7 based composite coatings and the as-sprayed coating with laser surface treatment was investigated using plate-on-plate tests. Wear tests were performed at different normal loads and sliding speeds under dry sliding conditions in air. The surface morphologies of counterparts against as-sprayed and laser remelted coatings were investigated. The microstructure and chemical composition of wear debris and coatings were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The results show that the wear resistance of the laser remelted coating is improved significantly due to their increased microhardness and reduced flaws. The primary wear mechanism of the remelted coating is oxidation wear and its minor wear mechanisms are grain abrasion and fatigue failure during the course of wear test. In contrast, the primary wear mechanism of the as-sprayed coating is grain abrasion at the low sliding speed (370 rpm) and fatigue failure at the high sliding speed (549 rpm). The oxidation wear mechanism is a minor contributor for the as-sprayed coating.     

Adhesion and stress of magnesium oxide thin films: Effect of thickness, oxidation temperature and duration

Nanoscale magnesium oxide thin films have been deposited on glass substrate by thermal oxidation (in air) of vacuum evaporated magnesium films. X-ray diffraction (XRD) showed orientation along (2 0 0) and (2 2 0) directions. The mechanical properties of the MgO thin films were found to be the function of thickness (300, 450 and 600 nm), oxidation temperature (573, 623 and 673 K) and oxidation duration (90 and 180 min). As oxidation temperature and oxidation duration increases, adhesion and intrinsic stress were found to increase. Intrinsic stress decreased whereas adhesion increased due to increase in thin film thickness. The value of intrinsic stress was in range 28.902-73.212 (×10⁷ N/m²) and that of adhesion was 12.1-27.4 (×10⁴ N/m²) for the thin film of thickness 300 nm.     

Dry sliding tribological behavior of AZ31 magnesium alloy irradiated by high-intensity pulsed ion beam

The dry sliding tribological behavior of AZ31 magnesium alloy irradiated by high-intensity pulsed ion beam (HIPIB) at energy density of 3.4 J/cm² with 10 shots is investigated by dry sliding wear tests in order to explore the effect of HIPIB irradiation on tribological property of magnesium alloy. Surface morphologies, composition and structure of the irradiated AZ31 magnesium alloys are examined by electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The results indicated that HIPIB irradiation led to the increase in surface microhardness and the reduction in friction coefficient and wear rate. Wear rate for both the original and the irradiated samples increased with increasing sliding load from 0.1 to 0.5 N. The transition from severe metallic wear to mild oxidative wear induced by HIPIB irradiation was observed by a combined analysis in surface morphology and chemical composition of wear tracks, mechanically mixed materials and wear debris, which is mainly attributed to the significant increase in microhardness resulting from grain refinement on the irradiated surface. In addition, defects induced by HIPIB irradiation promoted the diffusion of oxygen during sliding wear and therefore led to the formation of compact mixed materials and protective films on the wear tracks surface, which also contributes to the transition in wear mechanism of AZ31 magnesium alloy induced by HIPIB irradiation.     

Yttrium ion implantation on the surface properties of magnesium

Owing to their excellent physical and mechanical properties, magnesium and its alloys are receiving more attention. However, their application has been limited to the high reactivity and the poor corrosion resistance. The aim of the study was to investigate the beneficial effects of ion-implanted yttrium using a MEVVA ion implanter on the surface properties of pure magnesium. Isothermal oxidation tests in pure O₂ at 673 and 773 K up to 90 min indicated that the oxidation resistance of magnesium had been significantly improved. Surface morphology of the oxide scale was analyzed using scanning electron microscope (SEM). Auger electron spectroscopy (AES) and X-ray diffraction (XRD) analyses indicated that the implanted layer was mainly composed of MgO and Y₂O₃, and the implanted layer with a duplex structure could decrease the inward diffusion of oxygen and reduce the outward diffusion of Mg²⁺, which led to improving the oxidation resistance of magnesium. Potentiodynamic polarization curves were used to evaluate the corrosion resistance of the implanted magnesium. The results show yttrium implantation could enhance the corrosion resistance of implanted magnesium compared with that of pure magnesium.     

Friction and wear performance of some thermoplastic polymers and polymer composites against unsaturated polyester

Wear experiments have been carried out with a range of unfilled and filled engineering thermoplastic polymers sliding against a 15% glass fibre reinforced unsaturated polyester polymer under 20, 40 and 60 N loads and 0.5 m/s sliding speed. Pin materials used in this experimental investigation are polyamide 66 (PA 66), poly-ether-ether-ketone (PEEK) and aliphatic polyketone (APK), glass fibre reinforced polyamide 46 (PA 46 + 30% GFR), glass fibre reinforced polytetrafluoroethylene (PTFE + 17% GFR), glass fibre reinforced poly-ether-ether-ketone (PEEK + 20% GFR), glass fibre reinforced poly-phylene-sulfide (PPS + 30% GFR), polytetrafluoroethylene filled polyamide 66 (PA 66 + 10% PTFE) and bronze filled pofytetrafluoroethylene (PTFE + 25% bronze) engineering polymers. The disc material is a 15% glass fibre reinforced unsaturated polyester thermoset polymer produced by Bulk Moulding Compound (BMC). Sliding wear tests were carried out on a pin-on-disc apparatus under 0.5 m/s sliding speed and load values of 20, 40 and 60 N. The results showed that the highest specific wear rate is for PPS + 30% GFR with a value of 1 × 10⁻¹¹ m²/N and the lowest wear rate is for PTFE + 17% GFR with a value of 9.41 × 10⁻¹⁵ m²/N. For the materials and test conditions of this investigation, apart from polyamide 66 and PA 46 + 30% GFR polymers, the coefficient of friction and specific wear rates are not significantly affected by the change in load value. For polyamide 66 and PA 46 + 30% GFR polymers the coefficient of friction and specific wear rates vary linearly with the variation in load values.     

Sol-gel synthesis of highly luminescent magnesium oxide nanocrystallites

An attempt has been made to prepare MgO nano-crystallites through decomposition of sol-gel derived magnesium oxalate dihydrate in air, oxygen and nitrogen ambient at 500, 600, 800, and 1000 °C for 2 h each and study them with regard to their phase, average crystallite size, morphology, and photoluminescence (PL) behaviour. They are shown to possess f.c.c. (NaCl-type) structure with lattice parameter a∼4.211 Å, average crystallite size in the range of 3.0-73.5 nm, 〈1 1 1〉 preferred orientation at decomposition temperature of 500 °C (in nitrogen and oxygen ambient), and a distorted rod-like morphology. The PL peaks observed around 395 and 440 nm have been assigned to ²T_1u→²A_1g and ³B_1u→¹A_g transitions associated with the relaxation of excited states of F⁺- and F₂²⁺-centres, respectively. Further, the emission band intensity is found to depend on decomposition temperature and gas ambient, crystallite size, and their morphology. However, in nitrogen ambient above 800 °C, several other PL peaks observed at 491.8, 501.8, 503.5, 509.3, 561.5, and 563.0 nm arise due to aggregates of F centres and/or extra energy levels created in the energy band gap by nitrogen incorporation. A mechanism for nitrogen trapping in MgO has been suggested. Further, emission intensity depends on both colour centres and surface states.     

Phase composition and tribological properties of Ti-Al coatings produced on pure Ti by laser cladding

Ti-Al coatings with ∼14.7, 18.1, 25.2 and 29.7 at.% Al contents were fabricated on pure Ti substrate by laser cladding. The laser cladding Ti-Al coatings were analyzed with X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). It was found that with the increase of Al content, the diffraction peaks shifted gradually to higher 2θ values. The laser cladding Ti-Al coatings with 14.7 and 18.1 at.% Al were composed of α-Ti and α₂-Ti₃Al phases, while those with 25.2 and 29.7 at.% Al were composed of α₂-Ti₃Al phase. With the increase of Al content, the cross-sectional hardness increased, while the fracture toughness decreased. For the laser cladding Ti-Al coatings, when the Al content was ≤18.1 at.%, the wear mechanism was adhesive wear and abrasive wear; while when the Al content ≥25.2 at.%, the wear mechanism was adhesive wear, abrasive wear and microfracture. With the increase of Al content, the wear rate of laser cladding Ti-Al coatings decreased under 1 N normal load, while the wear rate firstly decreased and then increased under a normal load of 3 N. Due to its optimized combination of high hardness and high fracture toughness, the laser cladding Ti-Al coating with 18.1 at.% Al showed the best anti-wear properties at higher normal load.     

The microstructure, mechanical and friction properties of protective diamond like carbon films on magnesium alloy

Protective hard coatings deposited on magnesium alloys are believed to be effective for overcoming their poor wear properties. In this work, diamond-like carbon (DLC) films as hard protective films were deposited on AZ91 magnesium alloy by arc ion plating under negative pulse bias voltages ranging from 0 to −200 V. The microstructure, composition and mechanical properties of the DLC films were analyzed by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation. The tribological behavior of uncoated and coated AZ91 magnesium alloy was investigated using a ball-on-disk tribotester. The results show that the negative pulse bias voltage used for film deposition has a significant effect on the sp³ carbon content and mechanical properties of the deposited DLC films. A maximum sp³ content of 33.3% was obtained at −100 V, resulting in a high hardness of 28.6 GPa and elastic modulus of 300.0 GPa. The DLC films showed very good adhesion to the AZ91 magnesium alloy with no observable cracks and delamination even during friction testing. Compared with the uncoated AZ91 magnesium alloy, the magnesium alloy coated with DLC films exhibits a low friction coefficient and a narrow, shallow wear track. The wear resistance and surface hardness of AZ91 magnesium alloy can be significantly improved by coating a layer of DLC protective film due to its high hardness and low friction coefficient.     

Corrosion and wear properties of PEO coatings formed on AM60B alloy in NaAlO2 electrolytes

Coatings with a thickness of 22-32 μm were formed on an AM60B magnesium alloy by plasma electrolytic oxidation (PEO) in electrolytes containing 12.0-24.0 g/l NaAlO₂ and other additives. SEM analyses of the coated samples showed that the coatings were compact with relatively low porosity. X-ray diffraction revealed that the coatings consisted of mainly MgAl₂O₄ and MgO phases. The relative amount of MgAl₂O₄ in the coating increased with increasing NaAlO₂ concentration. The relatively compact and thick coatings provide good corrosion protection for magnesium, as indicated by the results of potentiodynamic polarization tests. In addition, the PEO treatment also significantly improved the wear resistance of the alloy. Pin-on-disk wear tests showed that the PEO treatment reduced the wear volume loss by a factor of 10.     

The effect of heat treatment on the performance of the Ni/(Zr-Sm oxide) catalysts for carbon dioxide methanation

The active catalysts for methane formation from the gas mixture of CO₂ + 4H₂ with almost 100% methane selectivity were prepared by reduction of the oxide mixture of NiO and ZrO₂ prepared by calcination of aqueous ZrO₂ sol with Sm(NO₃)₃ and Ni(NO₃)₂. The 50 at%Ni-50 at%(Zr-Sm oxide) catalyst consisting of 50 at%Ni-50 at%(Zr + Sm) with Zr/Sm = 5 calcined at 650 or 800 °C showed the highest activity for methanation. The active catalysts were Ni supported on tetragonal ZrO₂, and the activity for methanation increased by an increase in inclusion of Sm³⁺ ions substituting Zr⁴⁺ ions in the tetragonal ZrO₂ lattice as a result of an increase in calcination temperature. However, the increase in calcination temperature decreased BET surface area, metal dispersion and hydrogen uptake due to grain growth. Thus, the optimum calcination temperature existed.     

Preparation and ablation properties of ZrC-SiC coating for carbon/carbon composites by solid phase infiltration

A novel ZrC-SiC coating was prepared on carbon/carbon (C/C) composites surface by solid phase infiltration and the ablation properties of the ZrC-SiC coated C/C composites under oxyacetylene flame were studied. The results show that the coating prepared on the condition of optimum process parameters exhibits dense surface and outstanding anti-ablation ability. After ablation for 20 s, the mass ablation rates of the coated C/C composites can be lowered to 2.36 × 10⁻³ g/s, 37.1% reduction compared with uncoated C/C composites. The oxide layer composed of ZrO₂ and SiO₂ acts as oxygen diffusion barrier and the evaporation of ZrO₂ and SiO₂ absorbs a great amount of heat from the flame and reduces the erosive attack on the coating.     

Preparation of a novel Ni/Co-based alloy gradient coating on surface of the crystallizer copper alloy by laser

A high wear-resistant gradient coating made of Ni/Co-based alloys on the surface of a Cu alloy substrate was synthesized using a YAG laser induced in situ reaction method. The coating consists of three layers: the first is a Ni-based alloy layer, the second and third are Co-based alloy layers. The microhardness increases gradually from 98 HV in the Cu alloy substrate to the highest level of 876 HV in the third layer. The main phase of the Co-based alloy layer is CoCr₂(Ni,O)₄, coexisting with the Fe₁₃Mo₂B₅, Cr(Co(Mo, and FeCr_0.29Ni_0.16C_0.06 phases. Wear tests indicate that the gradient coating has good bond strength and wear properties with a wear coefficient of 0.31 (0.50 for the Cu alloy substrate). Also, the wear loss of the coating is only 0.01 g after it has been abraded for 60 min, which is only one fifth of that of the Cu alloy of the crystallizer. Wear tests of the gradient coating reveal good adhesive friction and wear properties when sliding against steel under dry conditions. This novel technique may have good application to make an advanced coating on the surface of the Cu alloy crystallizer in a continuous casting process.     

Selective MOCVD of titanium oxide and zirconium oxide thin films using single molecular precursors on Si(1 0 0) substrates

Titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films have been deposited on modified Si(1 0 0) substrates selectively by metal-organic chemical vapor deposition (MOCVD) method using new single molecular precursor of [M(OⁱPr)₂(tbaoac)₂] (M=Ti, Zr; tbaoac=tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (μCP) method was adapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has -CH₃ terminal group. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) by modifying standard synthetic procedures. Selective depositions of TiO₂ and ZrO₂ were achieved in a home-built horizontal MOCVD reactor in the temperature range of 300-500 °C and deposition pressure of 1×10⁻³-3×10⁻² Torr. N₂ gas (5 sccm) was used as a carrier gas during film depositions. TiO₂ and ZrO₂ thin films were able to deposit on the hydrophilic area selectively. The difference in surface characteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO₂ or Si-OH layer on the Si(1 0 0) substrate led to the site-selectivity of oxide thin film growth.     

Magnesium oxide nanocrystals via thermal decomposition of magnesium oxalate

The present work reports the synthesis of magnesium oxide (MgO) nanocrystals via a thermal decomposition route and the study of physicochemical properties of products. The MgO nanocrystals were prepared from magnesium oxalate powders as precursor. Transmission electron microscopy (TEM) analysis demonstrated MgO nanocrystals with an average diameter of about 20−25 nm. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electronic diffraction (SAED), and Fourier transform infrared (FT-IR) spectroscopy. Optical absorption and photoluminescence emission properties of MgO nanocrystals were investigated.     

A new large – Scale synthesis of magnesium oxide nanowires: Structural and antibacterial properties

Large-scale one-dimensional magnesium oxide (MgO) nanowires with diameters of 6 nm and lengths of 10 μm have been successfully synthesized by a new facile and simple reaction. This production was performed via a microwave hydrothermal approach at low temperature growth of 180 °C for 30 min. The structure of as synthesized MgO nanowires were investigated by means of X-ray diffraction (X-ray), Fourier Transformation Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) and Energy Dispersive X-ray (EDS). The antibacterial behavior of MgO nanowires concentration in solid media against Gram negative and Gram positive for different bacteria has been tested in details. The results show that the MgO nanowires have bacteriostatic activity against Escherichia coli and Bacillus sp. The antibacterial activity increases with increasing MgO nanowires concentration. Furthermore, the presence of one-dimensional MgO nanowires has high antibacterial efficacy and damages the membrane wall of bacteria. Finally, this study offered the prospect of developing ultrafine nanoscale devices utilizing MgO nanowires and implementing their useful potential in biological control.