Sliding wear behavior of the supersonic plasma sprayed WC-Co coating in oil containing sand

To improve the wear resistance of the cylinder liner of the engines operating in desert area, the tungsten carbide-based cermet coating (WC-12wt.%Co) was produced using the supersonic plasma spraying technique. The microstructure and phase composition of the as-sprayed coating were evaluated. The effect of sand size in lubricant on the friction and wear behavior of the coating under the lubrication of oil containing sand was investigated on a ball-on-disk tribometer. Characterization of the coating shows that the coating possesses dense microstructure with low level of porosity. The decarburization of WC is limited for the high particle velocity of the supersonic plasma spraying and only a few of W₂C phases are observed in the coating. With the higher hardness and the good adhesion with the substrate the sprayed coating exhibits a better wear resistance compared with the gray casting iron used in the current cylinder liner. Micro-structural examination shows that the wear of coating is dominated by micro-cutting, WC particles pull-out and sub-surface indentation-induced sub-surface cracking, while the main wear mechanism of the gray casting iron is micro-cutting and plastic deformation.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.     

Negative impacts of diamond magnetism on the microstructure of co-deposited composites and the effective solution

In a paper, we reported that incorporation of diamond into composite coatings could cause microstructural deteriorations (e.g., roughening the coating surface, coarsening the matrix grain and reducing the mechanical retention of diamond grains in the matrix), and suggested that all the impacts were caused by diamond magnetism resulting from metallic inclusions trapped in it. To confirm this, further microstructural observations were conducted on composites containing diamond particles that experienced different treatments before being planted, i.e., magnetization (strengthening diamond magnetism by a strong magnet) and demagnetization (weakening diamond magnetism by an alternating magnetic field), as well as on composites fabricated in the presence of an external alternating magnetic field in the vicinity of the cathode. It is shown that advance demagnetization treatment reduces the impacts while advance diamond magnetization treatment does the opposite. Moreover, the impacts could be more effectively reduced by superimposition of the external alternating magnetic field which may exert effects not only on demagnetization of the diamond grains, but also on cations' deposition process since the external magnetic field is much stronger than that produced by diamond.     

Influence of parameters of the HVOF thermal spray process on the properties of multicomponent white cast iron coatings

High velocity oxi-fuel (HVOF) thermal spray process has been used in order to deposit a new alloy known as multicomponent white cast iron. The coatings were characterized in terms of macrostructure, phase composition, porosity and hardness. Coating characteristics and properties were found to be dependent on the particles size range, spray distance, gases flow rate and oxygen to propane ratio. For set of parameters utilized in this job a narrow particle size range between 20 and 45 μm with a spray distance of 200 mm and oxygen to propane ratio of 4.6 are the preferred coating parameters. Coating porosity of 0.9% and hardness of 766 HV were obtained under these conditions.     

Influence of ZnO content and annealing temperature on the dielectric properties of ZnO/Al2O3 composite coatings

ZnO/Al₂O₃ coatings were prepared by atmospheric plasma spraying (APS) using ZnO powders and Al₂O₃ powders as starting materials. The dielectric properties of these coatings were discussed. Both the real part of permittivity and the energy loss increase greatly with increasing ZnO content over the frequency range 8.2-12.4 GHz, which can be ascribed to orientation polarization and relaxation polarization due to a higher ZnO content. The frequency-dependent maximum of the loss tangent is found to obey Debye theory. In addition, annealing temperature which leads to the change of ZnO content also plays an important role in the dielectric performance.     

Matrix microstructure deteriorations resulting from diamond incorporation in electroplated metal-diamond composites

Because of their desirable hardness and wear resistance, diamond composite coatings have been widely studied. This article first reports our observation results about the impact of diamond incorporation on the matrix microstructure, and then gives explanations. It is shown that the incorporation significantly worsens the microstructure, like coarsening matrix grain, inducing gaps between matrix and diamond, and promoting emergence of nodules, valleys and over-plating cases on matrix surface. These deterioration phenomena are aggravated by an increase in ‘troublesome’ diamond grain number, current density, or deposit thickness. All the facts are found to result from Lorentz forces generated by the interaction of electric current (moving cations) and magnetic fields produced by magnetic diamond grains that contain metallic inclusions. The interaction not only forces a magnetic diamond grain to stand on one of its corners, also forces a nearby cation that moves from anode to cathode (i.e., from the diamond top to the bottom) to rotate counterclockwise around the grain with an increasing radius (due to a repelling Lorentz force) before it passes the diamond middle, and then to rotate clockwise with a decreasing radius (due to an attracting Lorentz force) after passing the middle.     

Influence of substrate bias on the composition of SiC thin films fabricated by PECVD and underlying mechanism

The influence of the substrate bias on the composition of SiC thin films synthesized by plasma-enhanced chemical vapor deposition was studied. Our results indicate that the ratio of Si to C in the thin films is almost stoichiometric at a bias of − 300 V, whereas excessive carbon is observed in the films if the bias is lower or higher. Very little oxygen can be detected in the film produced without biasing. The effects of the bias on the composition of the thin films can be attributed to the interaction between the positive ions in the plasma and the surface atoms. The underlying mechanism is also discussed.     

Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co-28Mo-17Cr-3Si (similar to Tribaloy-800) coating, a Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006) coating and a Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co-28Mo-17Cr-3Si coating and of the Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni-32Mo-16Cr-3Si-2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.     

Effects of saccharin and cobalt concentration in electrolytic solution on microhardness of nanocrystalline Ni-Co alloys

Cobalt content, surface morphology and microhardness of nanocrystalline Ni-Co deposits prepared by pulse plating technique at constant electrodeposition conditions with varied concentration of saccharin and cobalt sulfate in the electrolyte were investigated. It is found that appropriate amount of both additions could lead to finer structure and higher hardness of the deposit and further increase of the concentration could result in decline of the hardness, which is regarded as the result of inverse Hall-Petch relation. The maximum hardness of the Ni-Co alloy deposits is not higher than that of their pure Ni counterparts, indicating that the refinement hardening effect (Hall-Petch relation) is dominant in nanocrystalline Ni-Co alloy deposits. By adding Co ions to the electrolyte, the amount of organic refiner saccharin (responsible for introduction of sulfur and carbon impurities) needed to produce nanocrystalline deposits could be remarkably reduced.     

Tribological and anti-corrosion properties of Ni-W-CeO2 coatings against molten glass

Composite plating was used to prepare Ni-W infused rare earth oxide CeO₂ composite coatings. The high temperature friction behavior and corrosion resistance of the coatings against molten glass were investigated by using a high temperature tribometer. A microhardness tester and an environmental scanning electron microscope equipped energy dispersive spectroscopy were employed to investigate the microhardness and the surface morphology of the composite coatings respectively. The results show that the brittle fracture, high temperature tribological properties and the corrosion resistance of Ni-W infused CeO₂ coatings are superior to those of a standard Ni-W coating. CeO₂ particles decrease the friction coefficient from nearly 0.5 to about 0.25 during the composite coatings sliding against the molten glass at about 973 K, and proper quantities of CeO₂ decrease the variation of the friction coefficient value. Furthermore, CeO₂ can improve the corrosion resistance of the Ni-W coatings at high temperature effectively.     

Novel application of nanocrystalline nickel electrodeposit: Making good diamond tools easily, efficiently and economically

Electrodeposition has been identified as a feasible and economical technique for nanomaterials application. This article details an improved approach to producing better diamond tools at lower cost and with higher productivity. Pulse-electroformed nanocrystalline nickel was used as the new matrix. The pulse parameters were determined after examination of the microstructure, grain size, hardness and tensile strength of the deposits obtained at different average current densities (J_m) with constant pulse-on time and pulse-off time. It is shown that, with J_m ranging from 1 Adm^− 2 to 14 Adm^− 2, the grain size decreases sharply from 180 nm to about 10 nm while the hardness and tensile strength significantly increase at first and then reach their peaks respectively, although the strength fails to stay long. Current density J_m that produced the highest hardness and strength of deposit (with grain size of 20 nm) was chosen for new diamond tools that exhibited 20.2% longer service life than their usual Ni-Co counterparts. Therefore, nanocrystalline electrodeposits are expected to be an upgrading substitute for conventional polycrystalline matrix.     

Development and characterization of single wire-arc-plasma sprayed coatings of nickel on carbon blocks and alumina tube substrates

A single wire-arc-plasma spray torch has been used to develop metal coatings on carbon and alumina substrates under argon atmosphere for various applications. Nickel coatings of around 1 mm thickness have been deposited on selected area (60 mm × 200 mm on each side) of large size carbon blocks by intermittent arc spraying and cooling to reduce thermal stresses and possibility of coating de-lamination from the base substrate. The same process is also used for depositing about 3 mm thick nickel metal coatings (8 mm dia. × 40 mm long) on alumina tubes for developing electrical feed throughs. The nickel coated alumina tubes were tested for the vacuum compatibility of the coated material with the base tube. The coated assemblies could withstand vacuum of the order of 1 × 10^− 6 Torr and the leak rate was found to be less than 1 × 10^− 9 Std. cc/s for Helium gas, indicating excellent bonding of the coated metal with alumina ceramics and no connected open porosity in the coatings. X-ray diffraction studies were conducted for identifying the phases and the optical microscope with image analysis technique was used for studying the microstructure and porosity in the coatings.     

Effect of negative pulsed high-voltage-bias on diamond-like carbon thin film preparation using capacitively coupled radio-frequency plasma chemical vapor deposition

Mechanical properties of diamond-like carbon films by an effective addition of negative pulsed high-voltage-bias have been investigated in capacitively coupled CH₄/Ar radio-frequency plasma. The results revealed that hardness and elastic modulus of the deposited film, estimated from nano-indentation load-displacement curve for about 1 μm-thick films, increase with the increase of the absolute value of high-voltage V_NPHV for V_NPHV < 5 kV and then saturate. Elastic recovery R got the highest value (> 90%) at V_NPHV = 5 kV.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

Effect of ion implantation layer on adhesion of DLC film by plasma-based ion implantation and deposition

High adhesive diamond-like carbon (DLC) film on SUS304 was obtained using carbon ion implantation between DLC film and substrate material by plasma-based ion implantation and deposition (PBIID). Implantation of mixed silicon and carbon ions to the substrate resulted in much higher adhesion strength than that of the epoxy resin. Effect of ion implantation on adhesion of DLC film was studied by cross sectional STEM observation and EDS element analysis. Enhancement in adhesive strength by ion implantation of mixed carbon and silicon was ascribed to the formation of the multilayer interface consisting of mixed carbon and silicon ion implanted layer and the amorphous layer of carbon and silicon.     

A comparison between the corrosion resistances of some HVOF-sprayed metal alloy coatings

This study compares the corrosion resistance of one Co-based alloy coating, namely Co-28Mo-17Cr-3Si (similar to Tribaloy-800), four Ni-based alloy coatings, namely Ni-17Cr-4Fe-4Si-3.5B-1C (Diamalloy-2001), Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006), Ni-22Cr-9Mo-4Nb-5Fe (similar to Inconel-625), Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700), and a (WC-12Co)-33Ni-9Cr-3.5Fe-2Si-2B-0.5C cermet-Ni alloy blend coating. They were produced by liquid-fuelled HVOF spraying onto AISI1040 steel plates. Electrolytic hard chrome (EHC) plating was characterised as a reference material, to verify whether some HVOF coatings are suitable as an EHC replacement. The microstructure of the coatings was examined by SEM and XRD. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution; the corrodkote test (ASTM B380-97R02) was also performed, to rank coatings qualitatively.The lowest corrosion current densities (I_corr) were recorded for EHC and Tribaloy-700. The latter coating contained few secondary phases and little porosity; the damage was mainly due to corrosion activation along lamellae boundaries. Diamalloy-2001 exhibited the highest I_corr and was significantly damaged after the polarization test, as its multi-phase microstructure had triggered severe galvanic corrosion. During free corrosion in 0.1 M HCl, Tribaloy-700 and Diamalloy-4006 retained rather stable polarization resistance (R_p), whereas the R_p of EHC decreased significantly. Tribaloy-700 survived 40 h of corrodkote test with no apparent damage and EHC underwent limited pitting corrosion. All other coatings had visible corrosion. The Inconel-625 coating failed to protect the substrate after 20 h of testing, due to inadequate processing conditions.     

The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

Amorphous-like nanocrystalline γ-Al2O3 films prepared by MOCVD

Alumina (Al₂O₃) films were prepared by metalorganic chemical vapor deposition using aluminum tri-acetylacetonate as a precursor. The effects of deposition conditions on film phase, microstructure, and deposition rate were investigated. γ-Al₂O₃ films were obtained at substrate temperatures ranging between 1173 and 1373 K and total chamber pressures ranging between 400 and 1000 Pa, whereas α-Al₂O₃ films incorporating a small amount of the γ phase were obtained at 1373 K and 800 Pa. Al₂O₃ films prepared at 1173 K showed a halo in X-ray diffraction patterns, consistent with amorphous structures. However, TEM observations suggested that these films consisted of a nanocrystalline γ-Al₂O₃ phase containing trace amounts of carbon.     

Electrochemical deposition of octacalcium phosphate micro-fiber/chitosan composite coatings on titanium substrates

Calcium phosphate/chitosan composites have been extensively studied as bone substitutes, tissue engineering scaffolds, or bone cements. In the present study, we have prepared octacalcium phosphate (OCP) micro-fiber/chitosan composite coatings through an electrochemical deposition method. OCP coatings with microporous structure, which are woven by micro-fibers with 20-30 μm in length and 0.1-1 μm in width, have a good ability to incorporate chitosan. This novel OCP micro-fiber/chitosan composite coating could have broad applications in the biomedical engineering.