Comparative examination of the microstructure and high temperature oxidation performance of NiCrBSi flame sprayed and pack cementation coatings

Coatings formed from NiCrBSi powder were deposited by thermal spray and pack cementation processes on low carbon steel. The microstructure and morphology of the coatings were studied by scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). Flame sprayed coatings exhibited high porosity and were mechanically bonded to the substrate while pack cementation coatings were more compact and chemically bonded to the substrate. The microhardness and the high temperature oxidation resistance of the coated samples were evaluated by a Vickers microhardness tester and by thermogravimetric measurements (TG), respectively. Pack cementation coatings showed higher hardness and were more protective to high temperature environments than the flame sprayed coatings.     

Adhesion properties of a silicon-containing calcium phosphate coating deposited by RF magnetron sputtering on a heated substrate

Silicon-containing hydroxyapatite coatings 400–700 nm in thickness are prepared by means of radio-frequency (RF) magnetron sputtering on a heated (to 200°C) titanium substrate chemically etched and treated with a pulsed electron beam. The morphology and phase composition of the coating are studied. The morphology and roughness of the composite “calcium-phosphate coating-titanium substrate” differ depending on the treatment procedure of the substrate before deposition. The scratch test method is used to assess the adhesion strength of the coatings formed at different values of bias potential applied to the substrate. It is observed that the adhesion strength of the coating changes with decreasing crystallite size.     

Investigation of Ceramic-Like Coatings Formed on Aluminum Composites by Microarc Oxidation

Ceramic-like coatings with a thickness of up to 40 μm are formed on aluminum composites without additives and with copper additives (1 and 4.5%) in a silicate-alkaline electrolyte by microarc oxidation. The composites are prepared by powder metallurgy (cold pressing and sintering in forevacuum). An increase in the copper concentration in the composites to 4.5% leads to the retardation of anode voltage growth on the initial stage of oxidation corresponding to the formation of a barrier layer. The coatings are studied by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction. The morphology of their surface corresponds to the morphology of the surface of coatings on compact aluminum alloys. According to X-ray photoelectron spectroscopy, a thin 1-μm layer forms on the surface. It consists predominantly of electrolyte components. X-ray diffraction analysis shows that the coatings mainly consist of γ-Al₂O₃ oxide as well as the η-Al₂O₃ phase, the peaks of which are broadened. This broadening is characteristic of the amorphous component and may be due to the presence of nanocrystalline regions in the coating structure. In the coatings on the composite Al + 4.5% Cu, mullite Al₂SiO₅ and copper oxide CuO are also found. The excess aluminum content may be associated with residual unoxidized aluminum inclusions in the structure of the coatings.     

Effect of pulsed plasma and high-current electron beam treatments on the structure and properties of nickel-based coatings

Using plasma-detonation technology (PDT), nickel-based powder (PGAN-33, PG-10N-01 and PG-19N-01) coatings 80–300 μm thick were deposited on a low-carbon St-3 steel substrate. Coatings were then subjected to additional treatment either by a high-current electron beam (HCEB) or by a high-velocity pulsed plasma jet until melting. The structure was analyzed using x-ray diffraction and conversion Mössbauer spectroscopy in the transmission and scattering modes. The morphology and elemental composition were studied using scanning electron microscopy and microanalysis. The micro-and nanohardness, volumetric wear, and corrosion resistance of the grown coatings were measured. It was found that, as the HCEB and PDT energy density increase, the phase composition changes, phase redistribution occurs, and new phases are formed. The last process is caused by mass transfer from the erosion plasmatron electrode and from a gas plasma jet, and interdiffusion of coating and substrate elements. After the exposure to concentrated energy fluxes, the coating surface roughness decreased. It is shown that physicochemical and mechanical characteristics of modified coatings improve in the case of optimum treatment conditions.     

Microstructure and corrosion properties of thick WC composite coating formed by plasma cladding

The thick Ni-coated WC coatings, in a matrix of Nickel-based alloys, were prepared on AISI 1045 steel using plasma cladding equipment. A pre-placed layer of uniform mixture, with different weight fractions of Ni-coated WC powder and Nickel-based alloy powder, on the steel substrate was melted at the high temperature of the plasma jet. The coating composition, microstructure and microhardness were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and microhardness testing. The experimental results show that the metallurgical bond was formed between the coating and substrate. The XRD results show that the coatings contain γ-Ni, carbides (such as M₂₃C₆ and M₇C₃) and boride (such as Fe₂B, Fe₃B phases). SEM shows that all the coatings are crack-free with lower porosity (<1%). It is found that the microhardness and the electrochemical behavior of the coatings are depended on the content of Ni-coated WC powder. The corrosion mechanism for the coatings may be due to the microgalvance corrosion between the phases in the cladding coatings.     

Surface characteristics of Zinc–TiO2 coatings prepared via micro-arc oxidation

Titanium (Ti) and its alloys are widely used as metallic biomaterials for fabrication of dental and orthopedic implants due to their favorable biocompatibility and corrosion resistance in a body environment. However, the thin oxide layer (TiO₂) on Ti substrate formed naturally in air or in many aqueous environments is bioinert and surrounded by fibrous tissues without producing any chemical or biological bond to bone when implanted. In the present work, Zinc-incorporated porous TiO₂ coatings (Zn–TiO₂) were prepared on Ti substrate by micro-arc oxidation (MAO) technique in the zinc gluconate-containing electrolyte. The surface morphology, cross-sectional morphology, composition, and phase of the coatings were analyzed using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometry, respectively. Surface topography and roughness of the coatings were investigated by atomic force microscopy operated in tapping mode. The results showed that Zn was successfully incorporated into the porous TiO₂ coatings, which did not alter apparently its surface topography and phase composition. In conclusion, the formation of porous Zn–TiO₂ coatings endow Ti with potential bioactivity and antibacterial activity, and we believe that the porous Zn–TiO₂ coatings on Ti by MAO technique might be promising candidates for orthopedic and dental implants     

Influence of substrate hardness transition on built-up of nanostructured WC-12Co by cold spraying

The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to deposit nanostructured WC-Co coatings. In the present study, the deposition behavior of nanostructured WC-12Co coating was examined through the surface morphology and cross-sectional structure of the deposited single WC-12Co particle impacting on the substrates with different hardness using a nanostructured WC-12Co powder. Substrates included stainless steel, nickel-based self-fluxing alloy coatings with different hardness. It was observed from the top surface and cross-section of individual WC-12Co particles that the penetration leading to particle deposition depends on substrate hardness. When the substrate surface is covered by WC-12Co particles, the hardness of the newly formed substrate, i.e. the coating, increases greatly. The significant increase of the surface hardness leads to the rebounding off of impacting particles and erosion of the deposited particles, which prohibits effective built-up of coating. However, it was found that with spray jet fixed, a deposit with a thickness up to over 700 μm can be built-up. A model involving in substrate hardness transition during deposition is proposed to explain such phenomenon, which can be employed to optimize the conditions to build up a uniform nanostructured WC-12Co coating.     

Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy

Oxide coatings were produced on AM60B magnesium alloy substrate making use of microarc oxidation (MAO) technique. The effect of KF addition in the Na₂SiO₃-KOH electrolytic solution on the microarc oxidation process and the structure, composition, and properties of the oxide coatings was investigated. It was found that the addition of KF into the Na₂SiO₃-KOH electrolytic solution caused increase in the electrolyte conductivity and decrease in the work voltage and final voltage in the MAO process. Subsequently, the pore diameter and surface roughness of the microarc oxidation coating were decreased by the addition of KF, while the coating compactness was increased. At the same time, the phase compositions of the coatings also varied after the addition of KF in the electrolytic solution, owing to the participation of KF in the reaction and its incorporation into the oxide coating. Moreover, the coating formed in the electrolytic solution with KF had a higher surface hardness and better wear-resistance than that formed in the solution without KF, which was attributed to the changes in the spark discharge characteristics and the compositions and structures of the oxide coatings after the addition of KF.     

Reactive plasma cladding of TiC/Fe cermet coating using asphalt as a carbonaceous precursor

A new process of preparing Ti-Fe-C composites powder for reactive plasma cladding, precursor carbonization-composition process, was developed. TiC/Fe cermet coatings were synthesized by reactive plasma cladding of the composite powder. XRD and SEM were employed to analyze the phase composition and microstructure of the composite powder and coating. The hardness and wear resistance of the coating was tested. Results show that: The compound powder prepared by precursor carbonization-composition process has very tight structure, which can avert the question of raw powder breaking-up in cladding process. The TiC/Fe cermet coating by reactive plasma cladding consists of alternate, laminated layers as following: the layers in which the round nanoscale TiC particles are dispersed within the α-Fe matrix and the layers of TiC accumulation. The TiC/Fe cermet coating by reactive plasma cladding shows superior hardness and wear resistance: The surface hardness of the TiC/Fe cermet coating is 68 ± 6 (HR30 N). In the same fretting conditions, the wear resistance of Ni60 coating is twelve times than that of the TiC/Fe cermet coating.     

Comparison of diamond film adhesion on molybdenum substrates with different surface morphologies

To evaluate the effect of substrate morphology on the adhesion of diamond film, two types of substrate morphology of molybdenum (Mo) were compared. The two morphology types were formed by polishing a Mo substrate with SiC abrasive paper along one direction (anisotropic morphology) and by polishing the Mo substrate with diamond powder in a random direction (isotropic morphology).Ultrasonic cavitation tests were conducted to evaluate the adhesion of the diamond films on these Mo substrates. In the case of low surface roughness, there was very little difference between the effects of SiC abrasive paper polishing and diamond powder polishing. In the case of high surface roughness, the adhesion of the diamond film on the SiC paper polished Mo substrate was larger than that of the diamond film on the diamond powder polished Mo substrate. Detachment of the diamond film from the SiC paper polished Mo substrate progressed along the polishing direction; while detachment of the diamond film from the diamond powder polished Mo substrate progressed in a random direction. It was thought that the detachment of the diamond film from a Mo substrate having an anisotropic polishing trace was suppressed because the anisotropic grooves restricted the formation of connections between the points of detachment at right angles to the groove direction. Therefore, the anisotropic surface morphology of the Mo substrate is effective for improving the adhesion of diamond film.     

Tribological coatings with a chromium-based composite structure obtained by reactive deposition in vacuum

The peculiarities inherent to the chemical and phase compositions, microstructure, and tribological behavior of Cr-O, Cr-C-O, Cr-C-H and Cr-C-N-H coatings, obtained by plasma-assisted physical vapor deposition in a chemically active gas atmospheres (activated reactive deposition and magnetron sputtering methods), have been studied. It has been demonstrated that all coatings have a micro- and/or nanocomposite structure comprising nanoscale chromium-based domains, as well as chromium oxide, carbide and nitride phases with different stoichiometries. The tribological properties of the coatings investigated in a broad temperature range under unlubricated friction contact conditions, as well as their correlation with microstructures and reactive-deposition technology parameters, are discussed.     

Lateral layer-by-layer nanostructuring of thermal barrier coatings of zirconium dioxide during plasma spraying

Results of experiments on development of the method of the lateral layer-by-layer nanostructuring of coatings prepared by plasma spraying in order to improve their working characteristics under thermal cycling loads are presented. Coating deposition is performed through a mask with holes while maintaining dynamic vacuum in the chamber. Analysis of the morphology of the zirconium-oxide coatings using a scanning electron microscope shows that the structure of the coating obtained in the “shaded” regions almost completely consists of submicron particles and nanoparticles. An estimate explaining the formation of regions in the coating consisting of nanoparticles due to heterogeneous condensation of the vapor phase of the sprayed material in the Prandtl-Meyer expansion formed in the supersonic plasma flow around the barrier is given.     

Wear resistance of laser cladding and plasma spray welding layer on stainless steel surface

The effect of coatings, which are formed with laser cladding and plasma spray welding on 1Cr18Ni9Ti base metal, on wear resistance is studied, A 5-kW transverse flowing CO2 laser is used for cladding Co base alloy powder pre-placed on the substrate. Comparing with the plasma spray coatings, the spoiled rate of products with laser clad layers was lower and the rate of finished products was higher. Their microstructure is extremely fine. They have close texture and small size grain. Their dilution resulting from the compositions of the base metal and thermal effect on base metal are less. The hardness, toughness,and strength of the laser cladding layers are higher. Wear tests show that the laser layers have higher properties of anti-friction, anti-scour and high-temperature sliding strike. The wear resistance of laser clad layers are about one time higher than that of plasma spray welding layer.     

Synthesis of TiN thin films by a new combined laser/sol-gel processing technique

In this work a novel method for synthesising TiN coatings is reported. A high-power diode laser at different powers and traverse speeds was applied to a mild steel substrate, coated with a slurry of titania sol-gel, urea and graphite. The reaction chemical thermodynamics was investigated to estimate the compositions, temperature range, and the required reaction enthalpy for producing TiN. A one-dimensional heat transfer model was used to optimise the processing parameters. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscope, and field emission gun scanning electron microscope which reveals deposition of very thin layer about 0.3 μm of pure TiN and the presence of sub-micron crystalline structure of TiN forming a metal matrix composite inter-layer with the substrate below the film which suggest a good metallurgically bonding with the substrate. Chemical composition was determined by energy dispersive X-ray analysis. The phases were identified by X-ray diffraction which confirms the synthesis of TiN film for all the samples. Results of nano-hardness measurements revealed a hardness value of the order of 22-27 GPa.     

Microstructure and properties of TiC-Fe36Ni cermet coatings by reactive plasma spraying using sucrose as carbonaceous precursor

This study is aimed to introduce an innovative precursor pyrolysis process to prepare Ti-Fe-Ni-C compound powder and to discuss and evaluate the relationship between microstructure and properties of TiC-Fe36Ni cermet coatings in-situ synthesized by reactive plasma spraying (RPS) of these compound powders. The main characteristic of the pyrolysis process is that sucrose (C12H22O11) is used as a source of carbon as well as a binder to bind reactive constituent particles. The compound powder with high bonding strength can avoid the problem that reactive constituent particles are separated during spraying. The TiC-Fe36Ni cermet coatings present typical splat-like morphology of thermally sprayed coatings and consist of two different areas: one is a composite reinforcement area where spherical fine TiC particles (100-500 nm) homogeneously distribute within the Fe36Ni matrix; the other is an area of TiC accumulation. The surface hardness of the coatings reaches about 90 ± 2 (HR15N). The maximum and average microhardness values of the coatings are 1930 HV_0.2 (Vicker Hardness) and 1640 HV_0.2, respectively. The average bonding strength of the coatings is about 62.3 MPa. The wear resistance property of the coatings is much more than that of Ni60 alloys coatings.     

Correlation between Al2O3 particles and interface of Al-Al2O3 coatings by cold spray

Al-Al₂O₃ composite coatings with different Al₂O₃ particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al₂O₃) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al₂O₃ particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al₂O₃ particles. In addition, when the large hard particles of fused Al₂O₃ were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages.     

Microstructure and properties of TiB2-containing coatings prepared by arc spraying

Low cost arc spraying and cored wires were used to deposit composite coatings consisting of TiB₂ and TiB₂/Al₂O₃ hard particles in a Ni(Cr) and stainless steel 304L matrix. Four coatings were prepared namely Ni(Cr)-TiB₂, Ni(Cr)-TiB₂/Al₂O₃, 304L-TiB₂ and 304L-TiB₂/Al₂O₃. The microstructural characteristics of powders and coatings were observed by scanning electron microscopy (SEM). Phase compositions of powders were analyzed by X-ray diffraction (XRD). Although all the analyzed coatings exhibited similar lamella structure, remarkable differences not only in the morphology of hard phase and matrix but also in the size and distribution of hard phases were observed from one coating to another. Tribological behavior of the coatings was analyzed in room temperature dry sliding wear tests (block-on-ring configuration), under 75 N at low velocity (0.5 m/s). The coatings showed far high wear resistance than low carbon steel substrate under same conditions examined. Wear loss of 304L-TiB₂ and Ni(Cr)-TiB₂ coatings were lower nearly 15 times than that of steel substrate. TiB₂ hard phases in coatings bonded well with metal matrix contributed to high wear resistance.     

Pulsed laser deposition of Mg–Al layered double hydroxide with Ag nanoparticles

Powdered layered double hydroxides (LDHs)—also known as hydrotalcite-like (HT)—compounds have been widely studied due to their applications as catalysts, anionic exchangers or host materials for inorganic or organic molecules. Assembling thin films of nano-sized LDHs onto flat solid substrates is an expanding area of research, with promising applications as sensors, corrosion-resistant coatings, components in optical and magnetic devices. The exploitation of LDHs as vehicles to carry dispersed metal nanoparticles onto a substrate is a new approach to obtain composite thin films with prospects for biomedical and optical applications. We report the deposition of thin films of Ag nanoparticles embedded in a Mg–Al layered double hydroxide matrix by pulsed laser deposition (PLD). The Ag-LDH powder was prepared by co-precipitation at supersaturation and pH = 10 using aqueous solutions of Mg and Al nitrates, Na hydroxide and carbonate, and AgNO₃, having atomic ratios of Mg/Al = 3 and Ag/Al = 0.55. The target to be used in laser ablation experiments was a dry pressed pellet obtained from the prepared Ag-LDH powder. Three different wavelengths of a Nd:YAG laser (266, 532 and 1064 nm) working at a repetition rate of 10 Hz were used. X-Ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and secondary ions mass spectrometry (SIMS) were used to investigate the structure, surface morphology and composition of the deposited films.     

Microstructure and mechanical properties of alumina coatings prepared by double glow plasma technique

Low-temperature growth (600 °C) of α-Al₂O₃ coatings on the stainless steel substrate by double glow plasma technique was achieved. The compositions and microstructures of the coatings prepared at different oxygen flow rates were characterized, respectively, by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrometry. A phenomenological mechanism for the formation of the Al₂O₃ ceramic coatings during the oxidation process was proposed on the basis of the experimental results. It was obvious that the oxygen flow rates had a great effect on the surface structure of the prepared Al₂O₃ coatings. The dense and smooth Al₂O₃ coatings were prepared at the oxygen flow rate of 15 sccm. In addition, the correlations between the mechanical properties of Al₂O₃ coating and oxygen flow rates were also discussed. The coating prepared at 15 sccm oxygen flow rate exhibited the best mechanical properties with a maximum hardness of 31 GPa and elastic modulus of 321 GPa. The corresponding critical load of scratch adherence for this sample was 47 N.     

AlN/Al dual protective coatings on NdFeB by DC magnetron sputtering

AlN/Al dual protective coatings were prepared on NdFeB by DC magnetron sputtering in a home-made industrial apparatus. Comparing with Al coating, AlN/Al coatings have a denser structure of an outmost AlN amorphous layer following an inner Al columnar crystal layer. The coatings and NdFeB substrate combine well, and moreover, there is occurrence of metallurgy bonding in the interface layer. Both Al and AlN/Al coatings have a good protective ability to NdFeB. Especially, the corrosion resistance of AlN/Al coated NdFeB is improved largely. AlN/Al and Al protective coatings not only do not deteriorate the magnetic properties of NdFeB, but contribute to their slight increase.