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.     

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.     

Composition and corrosion resistance of coatings on the basis of nitrides of titanium and chromium

Ti-Cr-N coatings were deposited on a low-carbon steel St3 substrate by overlapping of Ti and Cr plasma flows in residual nitrogen atmosphere using ion bombardment. Auger electron spectroscopy and X-ray diffraction were used to analyze the element and phase composition of the deposited coatings. It was established that, under constant deposition conditions (the arc current, gas pressure, bias voltage), coatings possess the fcc structure of a (Ti,Cr)N solid solution with uniform distribution of elements along the depth. The growth of the substrate bias voltage leads to an increase of titanium concentration in the coating due to changes in the interaction processes (condensation and sputtering) of the deposited materials with the substrate. Corrosion tests of the coatings and steel St3 were performed in acid (1 M H₂SO₄) and salt (3% NaCl) media. It was found that the corrosion processes progress less intensely in salt solution than in acid medium.     

Interface adhesion properties of functional coatings on titanium alloy formed by microarc oxidation method

Three functional coatings (namely Al-C, Si-P-Al and P-F-Al coating) were fabricated by microarc oxidation method on Ti6Al4V alloy in different aqueous solutions. The microstructure, phase and chemical composition of coatings were investigated using scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy. The interface adhesion failure mode of the coating is revealed by shear, tensile and thermal shock methods. The coatings exhibit high adhesion strength by the quantitative shearing test, registering as 110, 70, and 40 MPa for Al-C, Si-P-Al and P-F-Al coating, respectively. The tensile test of the coated samples shows that microarc oxidation treatment does not significantly deteriorate mechanical properties of substrate titanium alloy. The observations of the coating failure after subjected to the identical tensile elongation of 3.0% are well in agreement with those results of the shear test. The thermal cycle test indicates that all the coatings have good anti-thermal shocking properties.     

Influence of Cobalt on the Adhesion Strength of Polycrystalline Diamond Coatings on WC–Co Hard Alloys

The influence of cobalt on the phase composition and adhesion strength of polycrystalline diamond coatings has been studied using scanning electron microscopy, Raman spectroscopy, and X-ray microanalysis. The coatings have been deposited on WC–Co hard alloy substrates in glow discharge plasma. It has been found that the catalytic amorphization of carbon only takes place during the direct synthesis of the diamond coating, when the cobalt vapor pressure over the substrate is high and the cobalt-related degradation of the synthesized diamond is absent.     

Microstructural inhomogeneity in plasma-sprayed hydroxyapatite coatings and effect of post-heat treatment

The microstructural inhomogeneity in the plasma-sprayed hydroxyapatite (HA) coatings was characterized by using electron probe microanalyser (EPMA). A simple and artful method was developed to detect the interface characteristics. All the samples for observation were ground and polished along the direction parallel to the coating surfaces. The BSE images directly and clearly showed the inhomogeneity in the as-sprayed coatings with the amorphous regions being bright gray and crystalline regions being dark gray. X-ray diffractometer (XRD) patterns indicated that after immersion in deionized water for 20 days, bone-like apatite and α-Ca₂P₂O₇ precipitated on the polished surfaces of the as-sprayed HA coatings. The post-heat treatment could eliminate the microstructural inhomogeneity in the coatings. Only β-Ca₂P₂O₇ precipitated on the surfaces of the heat-treated HA coatings. The immersed samples were re-polished till tiny substrate was bared to investigate the effect of immersion on interface. It was shown that the immersion decreased the cohesive strength of the as-sprayed coatings. There were more and broader cracks in the splats that came into contact with the substrate and amorphous phase increased toward the coating-substrate interface. Post-heat treatment was proved to reduce the peeling off of coating during re-polishing operation. It was proposed that the distributions of amorphous phase and cracks in as-sprayed coatings are detrimental to coating properties and should be modified through improving the plasma spraying processing.     

Structure and properties of a hard alloy deposited on a copper substrate by means of a pulsed plasma spray technology

A pulsed plasma spray generator based on a new principle is developed for depositing ceramicmetal, ceramic, and metal coatings on solid substrates. Calculations of the plasma-generator parameters are presented. A hard alloy (W-Co) was deposited on a copper substrate to demonstrate the use of the plasma generator. A Rutherford backscattering (RBS) technique, x-ray diffraction microanalysis of phase composition, diffraction transmission electron microscopy (TEM), and hardness and adhesion measurements were used to examine the hard-alloy coating. It is shown that the coating consists of W-Co crystals with hexagonal and cubic lattices and contains ∼25-nm cobalt α-and β-phase crystallites, with W₃Co₃C particles revealed at the crystallite edges.     

Study of hafnium-oxide coatings by means of nuclear backscattering spectrometry

Single-layer coatings made of hafnium oxide and double-layer structures with an intermediate nickel layer, the total thickness of which is 70 μm, are obtained via plasma spraying with a supersonic jet in a rarefied atmosphere. A nozzle extension capable of implementing a Prandtl–Meyer expansion fan is used to generate nanostructured coatings. The coatings are investigated via the nuclear backscattering of spectrometry 7.6 MeV protons, scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The studies of the surface and transverse microsection of the coatings indicate that they comprise not only deformed particles of the sprayed powder with sizes of greater than 20 μm but also layers and conglomerates of nanoparticles with sizes of 30–60 nm. Depth profiling of the elemental composition performed by means of the nuclear backscattering spectrometry of protons demonstrates that transition layers exist at the interface between the substrate and coating layers characterizing the average size of the coating microparticles. A comparison of thicknesses defined by two methods allows estimation of the overall porosity of the hafnium-oxide layer. In accordance with X-ray diffraction data, the cubic and monoclinic phases of hafnium oxide with a high fraction of the amorphous component are formed in the coating.     

The mechanism of PEO process on Al–Si alloys with the bulk primary silicon

This study focuses on mechanism of ceramic coating on Al–Si alloys with bulk primary Si using plasma electrolytic oxidation (PEO) technology. Al–Si alloys with 27–32%Si in weight were used as substrates. The morphologies, composition and microstructure of PEO coatings were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray system (EDX). Results showed that the PEO process had four different stages. The effect of bulk Si is greatly on the morphology and composition of coatings at first three stages. Anodic oxide films formed on Al and Si phases, respectively. When the voltage exceeded 40 V, glow appeared and concentrated on the localized zone of interface of Al and Si phase. Al–Si–O compounds formed and covered on the dendrite Si phase surface, and the coating on bulk Si, which was silicon oxide, was rougher than that on other phase. If the treatment time was long enough, the coatings with uniform surface morphologies and elements distribution will be obtained but the microstructure of inner layer is looser due to the bulk Si.     

The physical basics of structure formation in electroexplosive coatings

It is shown that, under an electroexplosive spraying of coatings of various systems, the dynamic rotation of the particles deposited is the basis for forming a vortex hierarchically organized structure in both the coating and upper layers of the substrate, including their interface. This causes dispersion of all the structural elements of the cover, their mutual penetration into each other, and the absence of a sharp interface between the coating and the substrate. The impact of multiple shocks on the deposited coating by a highenergy plasma jet from the discharge of a plasma accelerator creates the forged structure in the material, which is characterized by high strength, wear resistance, and relaxation capability.     

Synthesis and microstructure observation of titanium carbonitride nanostructured coatings using reactive plasma spraying in atmosphere

In the present study, nanostructured titanium carbonitride (TiCN) coatings were successfully deposited by reactive plasma spraying (RPS) technology using a self-designed gas tunnel mounted on a normal plasma spray torch. The phase composition and microstructure of the TiCN coatings were characterised by XRD, SEM and TEM. The results indicated that the main phase of the coatings was FCC TiC_0.2N_0.8 with a small amount of Ti₃O. The coating that was deposited using 35 kW displayed better microstructure and properties. The coating exhibited a typical nanostructure including 90 nm diamertrical equiaxed grains and 400 nm long columnar grains by TEM images. The SEM observation further revealed that the equiaxed grains in parallel direction to the substrate surface in TEM images were actually the columnar grains perpendicular to the substrate surface. The formation mechanism of the nanostructured coatings was also discussed. The measured microhardness value of the coating was approximately 1659 Hv_100 g, and the calculated crack extension force was about 34.9 J/m².     

Microstructure and corrosion resistance behavior of ceramic coatings on biomedical NiTi alloy prepared by micro-arc oxidation

In this paper, ceramic coatings were prepared on biomedical NiTi alloys by micro-arc oxidation (MAO) in constant voltage mode. The current density-time response was recorded during the MAO process. The microstructure, element distribution and phase composition of the coatings prepared at different MAO treatment times were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), thin-film X-ray diffraction (TF-XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion behavior of the coatings in 0.9% NaCl solution was evaluated by the potentiodynamic polarization test. It is found that the coatings become more compact with increasing the MAO treatment time, and the growth rate of coating decreases. The results of TF-XRD, EDS and XPS indicate that the coatings are composed of a large amount of γ-Al₂O₃ and a little α-Al₂O₃, TiO₂ and Ni₂O₃. The Ni content of the coatings is about 3 at.%, which is greatly lower than that of NiTi substrate. The bonding strength of coating-substrate is higher than 40 MPa for all the samples in this study. The corrosion resistance of the coatings is about two orders of magnitude higher than that of the uncoated NiTi alloy.     

Study of physicochemical and biological properties of calcium phosphate coatings prepared by RF magnetron sputtering of silicon-substituted hydroxyapatite

Coatings based on pure silicon and silicon-substituted hydroxyapatite were grown by RF magnetron sputtering. The coating surface morphology, phase and elemental composition were studied by scanning electron microscopy, energy-dispersive X-ray analysis, and infrared spectroscopy. It was found that coatings are X-ray-amorphous, their elemental composition being controlled by the sputtered target composition. The distribution of elements over the coating surface is homogeneous. Medical and biological properties of coatings were studied in vivo and in vitro. Osteogenic properties of coatings were studied. Coatings grown by sputtering of a stoichiometric hydroxyapatite target are biocompatible without osteoinductive activity. The introduction of silicate ions into the hydroxyapatite structure that forms an electrode target significantly enhances the in vivo effect of CaP magnetron coatings on the osteogenic activity and stromal bone-marrow stem cells.     

激光熔覆Al+SiC涂层对镁合金表面耐磨性能的改性

通过脉冲激光器(Nd-YAG)在AZ91D镁合金基底上熔覆Al+SiC粉体。采用扫描电子显微镜、能量色散谱(EDS)和X-射线衍射测定分析熔覆层的显微组织、化学成分和物相组成。研究表明:Al+SiC涂层主要由SiC,β-Mg_(17)Al_(12)及Mg和Al相组成,激光熔覆层与镁合金基底表现出良好的冶金结合。所有样品都具有树枝状结构,且随着SiC质量分数的增大,树枝状和胞状结构的间隔变得更大。熔覆涂层的表面硬度高于基底,并且随着熔覆层中的SiC质量分数的增加而增大,SiC质量分数为40%的熔覆层具有最大的显微硬度,达到180 HV,然而质量分数为10%的熔覆层硬度为136 HV。销盘滑动磨损试验表明,复合涂层中的SiC颗粒和原位合成的Mg_(17)Al_(12)相显著提高了AZ91D镁合金的耐磨损性,其中,SiC质量分数从10%增加到30%过程中磨损体积损失逐渐减少,SiC质量分数在20%~30%时熔覆层具有最好的耐磨性。     

In situ formation of low friction ceramic coatings on carbon steel by plasma electrolytic oxidation in two types of electrolytes

In situ formation of ceramic coatings on Q235 carbon steel was achieved by plasma electrolytic oxidation (PEO) in carbonate electrolyte and silicate electrolyte, respectively. The surface and cross-section morphology, phase and elemental composition of PEO coatings were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). The bond strength of the coating was determined using a direct pull-off test. The hardness as well as tribological properties of the ceramic coating was primarily studied. The results indicated that the coating obtained in carbonate electrolyte was Fe₃O₄, while the coating achieved from silicate electrolyte was proved to be amorphous. Both kinds of coatings showed coarse and porous surface. The Fe₃O₄ coatings obtained in carbonate electrolyte showed a high bonding strength to the substrate up to 20 ± 2 MPa and the value was 15 ± 2 MPa for the amorphous coatings obtained in carbonate electrolyte. The micro hardness of the amorphous coating and the Fe₃O₄ coating was 1001 Hv and 1413 Hv, respectively, which was more than two and three times as that of the Q235 alloy substrate (415 Hv). The friction coefficient exhibited by amorphous coating and Fe₃O₄ coating was 0.13 and 0.11, respectively, both lower than the uncoated Q235 substrate which ranged from 0.17 to 0.35.     

Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium （TA2） substrates in a Na2SiO3NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope （SEM） and x-ray diffraction （XRD） are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile （TiO2）. With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.     

Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte

In the present work, four processes were carried out to produce ceramic coatings on aluminum substrate in two kinds of electrolytes (silicate and phosphate solution systems) using plasma electrolytic oxidation (PEO) technology. The voltage-time responses were recorded during different PEO processes. SEM/EDX and XRD were adopted to investigate the microstructure, elements distribution and phase composition of the coatings prepared in the two electrolyte systems. It is found that coatings produced in the silicate electrolyte have a more homogeneous morphology than those produced in the phosphate system. EDX analysis shows that silicon element tends to present primarily in the outer region of the coatings while phosphorus distributes uniformly throughout the coating thickness. According to the conventional anodic oxidation mechanism, a model is set up to explain the different characteristics of ceramic coatings fabricated in different electrolytes which is helpful to understand the growth mechanism of PEO coatings.     

Physical and technological principles of designing layer-gradient multicomponent surfaces by combining the methods of ion-diffusion saturation and magnetron- and vacuum-arc deposition

Using a technological system proposed by the authors, a combined process is developed for formation of stratified-gradient surface layers and multicomponent coatings. It is implemented under the conditions of a combined serial-parallel operation of a hot-cathode gas plasma generator and a duomagnetron with two targets and two electric-arc evaporators. The extended functional potential is ensured by using advanced multi-element and multi-phase cathode targets made of borides, carbides, silicides, and sulfides of metals produced by the SHS-process followed by their immediate compaction. The variations in composition, structure, and physicomechanical properties in the cross-section of the stratified-gradient surface layers and coating is provided by a predetermined alternating replacement of the sputtered cathode targets of the plasma sources, the plasma flow intensity ratios, and variation in the particle energy incident on the substrate, which is determined by the accelerating voltage on the substrate.     

Characterization and oxidation behavior of NiCoCrAlY coating fabricated by electrophoretic deposition and vacuum heat treatment

Electrophoretic deposition (EPD) was showed to be a feasible and convenient method to fabricate NiCoCrAlY coatings on nickel based supperalloys. The microstructure and composition of the NiCoCrAlY coatings after vacuum heat treatment were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). Isothermal-oxidation test was performed at 1100 °C in static air for 100 h. The results show that the major phases in electrophoretic deposited and vacuum heat treated NiCoCrAlY coating are γ-Ni and γ′-Ni₃Al phases, also there is an extremely small quantity of Al₂O₃ in the coating. Composition fluctuations occur in the coating and a certain amount of titanium diffuse from the superalloy substrate to the top of the coating during vacuum heat treatment. The oxidation test results exhibit that the oxidation kinetics of this coating has two typical stages. The protective oxide layer is mainly formed in the initial linear growth stage and then the oxide layer hinders further oxidation of the coating in the subsequent parabolic growth stage. The coating can effectively protect the superalloy substrate from oxidation. A certain amount of rutile TiO₂ is formed in the coating during oxidation and it is adverse to the oxidation resistance of the coating.     

Study on the laser treatment of electroless Ni-P-SiC composite coatings

Effect of the laser treatment on electroless Ni-P-SiC composite coatings was investigated. The microscopic structure, surface morphology, ingredient, and performance of the Ni-P-SiC composite coatings were synthetically analyzed by the use of X-ray diffraction apparatus, scanning electron microscope, energy distribution spectrometer, micro-hardness tester, wear tester and so on. It was found that the composite coatings did make crystalloblastic transformation after laser heating. Structural analysis confirmed that some new types of phase Ni2Si or Ni3Si compound would emerge in the Ni-P-SiC coatings after laser treatment. The micro-hardness measurement results showed that when the laser power was 450 W with scanning speed of 0.5 m/min, the hardness of the coating was superior to the coating obtained by the conventional furnace heating, and wear resistance of the composite coating after laser treating could also improve.