Effect of arc spraying power on the microstructure and mechanical properties of Zn-Al coating deposited onto carbon fiber reinforced epoxy composites

This paper investigates the effect of arc spraying power on the microstructure and mechanical properties of Zn-Al coatings deposited on carbon fiber reinforced epoxy composites (CFRE composites). The bond strength between the Zn-Al coatings and the substrates was tested on a RGD-5 tensile testing machine. The microstructures and phase composition of the as-sprayed coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results showed that both the melting extent of Zn-Al particles and the bond strength of the coatings were evidently improved by increasing the spraying power. Moreover, the content of crystalline Zn-Al coatings was slightly changed. Observation of fracture surfaces showed that the Zn-Al coatings could bond well with the carbon fiber bundles using 40 kW spraying power.     

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.     

Structure and adhesion of MoSi2/Ni interfaces: Evaluation of MoSi2 as an alternative bond coat alloy

We use density functional theory to evaluate the stability of molybdenum disilicide coatings on a nickel substrate, as a possible bond coat alloy for high temperature coating applications. We consider the MoSi₂(0 0 1)/Ni(1 1 1), MoSi₂(1 0 0)/Ni(1 1 1), and MoSi₂(1 1 0)/Ni(1 1 1) interfaces and predict quite strong (3.5-3.8 J/m²) adhesion of this metal-silicide ceramic to nickel. The origin of this strong adhesion is elucidated by examining the geometric and electronic structure of the interfaces. We predict that Mo and Si atoms at the interface primarily occupy Ni 3-fold hollow sites, the typical adsorption site on Ni(1 1 1). Projected local densities of states and electron density difference plots reveal a mixture of localized, covalent Si-Ni bonds and more delocalized metallic Mo-Ni bonding, as the origin of the strong interfacial bonding. As emphasized in our earlier work, creation of strong covalent bonds at interfaces results in very strong adhesion. Such strong adhesion makes MoSi₂ a potential candidate for use in thermal barrier applications, in conjunction with a yttria-stabilized zirconia topcoat.     

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.     

Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO₄ into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO₄ in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO₄. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO₄ demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa.     

Diffusion research between Ni3Al coating and titanium alloy produced by plasma spraying process

A Ni₃Al coating was prepared by plasma spraying technique on the surface of titanium alloy. Ni-Al mixed powders, coatings and reaction products were investigated by scanning electron microscope, EDS, DSC and XRD. A tight bonding between the coating and the substrate was formed. The X-ray diffraction analysis of the patterns showed that the coating not only had Ni₃Al phase, but also had NiO and Al₂O₃ phase microcontent. Comparing Ni coated Al to Ni₃Al at 900 °C, the diffusion was stronger and the diffusion layer was thicker. A minute pore structure was formed at 1200 °C in the front edge of solid-state reaction layer. So Ni₃Al restrained the solid-state reaction of the coating with the substrate, and as a whole weakened the entry of oxygen atoms into the substrate and quenched the out-diffusion of titanium.     

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.     

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3-8 kW) and vortex which produces the main plasma with high power level (10-40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.     

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.     

Nanostructured TiN coating prepared by reactive plasma spraying in atmosphere

In the present study, the nanostructured TiN coatings are obtained by means of reactive plasma spraying in the air. The XRD analysis shows the coating was mainly composed of TiN (max. 86.3%) and a small quantity of Ti₃O. Their microstructures were observed by SEM and TEM. The TEM pictures show the TiN coatings were composed of the nanoscaled grains (range from 70 to 90 nm), and the Scherrer equation analysis on the average grain size corresponded with that result. The effects on the average grain size also were studied by changing the spraying power and the spraying distance.     

Plasma sprayed Al2O3/FeCrAl composite coatings for electromagnetic wave absorption application

Al₂O₃/FeCrAl composite coatings were fabricated by atmosphere plasma spraying technique. Microstructure and dielectric properties in the frequency range from 8.2 to 12.4 GHz were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles with litter pores and microcracks in the composite coatings. The relaxation polarization and interfacial polarization in the coatings would contribute to enhance ?′ with rising FeCrAl content, and the associated loss could be considered as a dominating factor enhancing ?″. By calculating the microwave-absorption as a single-layer absorber, for the composite coatings with 41 wt.% FeCrAl content, the reflection loss values exceeding −10 dB are achieved in the frequency range of 9.1-10.6 GHz when the coating thickness is 1.3 mm.     

Diode-end-pumped passively Q-switched Nd:Y0.8Lu0.2VO4 laser with Cr:YAG crystal

A diode end-pumped passively Q-switched Nd: Y_0.8Lu_0.2VO₄ laser with a Cr⁴⁺: YAG crystal is first demonstrated in this paper. The maximum continuous wave (CW) output power of 5.59 W is obtained at the incident pump power of 13.07 W with the output transmission T = 20%, resulting in an optical-to-optical efficiency of 42.7%. For Q-switching operation, the measured pulse duration of 8.5 ns, the pulse energy of 45.24 μJ and the peak power of 5.32 kW are respectively obtained for the output transmission of 50% when the Cr⁴⁺: YAG crystal is used with an initial transmission (T₀) of 60%.     

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.     

Effect of deposition temperature on boron-doped carbon coatings deposited from a BCl3-C3H6-H2 mixture using low pressure chemical vapor deposition

A mixture of propylene, hydrogen and boron trichloride was used to fabricate boron-doped carbon coatings by using low pressure chemical vapor deposition (LPCVD) technique. Effect of deposition temperature on deposition rate, morphologies, compositions and bonding states of boron-doped carbon coatings was investigated. Below 1273 K, the deposition rate is controlled by reaction dynamics. The deposition rate increases with increasing deposition temperature. The activation energy is 208.74 kJ/mol. Above 1273 K, the deposition rate decreases due to smaller critical radius r_c and higher nuclei formation rate J with increasing temperature. Scanning electron microscopy shows that the structure changes from glass-like to nano-laminates with increasing deposition temperature. The boron concentration decreases with increasing deposition temperature, corresponding with increasing carbon concentration. The five types of bonding states are B-C, B-sub-C, BC₂O, BCO₂ and B-O. B-sub-C and BC₂O are the main bonding states. The reactions are dominant at all temperatures, in which the B-sub-C and PyC are formed.     

Improvement in the properties of plasma-sprayed metallic, alloy and ceramic coatings using dry-ice blasting

Dry-ice blasting, as an environmental-friendly method, was introduced into atmospheric plasma spraying for improving properties of metallic, alloy and ceramic coatings. The deposited coatings were then compared with coatings plasma-sprayed using conventional air cooling in terms of microstructure, temperature, oxidation, porosity, residual stress and adhesion. It was found that a denser steel or CoNiCrAlY alloy coating with a lower content of oxide can be achieved with the application of dry-ice blasting during the plasma spraying. In addition, the adhesive strength of Al₂O₃ coating deposited with dry-ice blasting exceeded 60 MPa, which was nearly increased by 30% compared with that of the coating deposited with conventional air cooling. The improvement in properties of plasma-sprayed metallic, alloy and ceramic coatings caused by dry-ice blasting was attributed to the decrease of annulus-ringed disk like splats, the better cooling efficiency of dry-ice pellets and even the mechanical effect of dry-ice impact.     

Intrinsic mechanical properties of ultra-thin amorphous carbon layers

In this work, we extracted the film's hardness (H_F) of ultra-thin diamond-like carbon layers by simultaneously taking into account the tip blunting and the substrate effect. As compared to previous approaches, which did not consider tip blunting, this resulted in marked differences (30-100%) for the H_F value of the thinner carbon coatings. We find that the nature of the substrate influences this intrinsic film parameter and hence the growth mechanisms. Moreover, the H_F values generally increase with film thickness. The 10 nm and 50 nm thick hydrogenated amorphous carbon (a-C:H) films deposited onto Si have H_F values of, respectively, ∼26 GPa and ∼31 GPa whereas the 10 nm and 50 nm thick tetrahedral amorphous carbon (t-aC) films deposited onto Si have H_F values of, respectively, ∼29 GPa and ∼38 GPa. Both the a-C:H and t-aC materials also show higher density and refractive index values for the thicker coatings, as measured, respectively by X-ray reflectometry and optical profilometry analysis. However, the Raman analysis of the a-C:H samples show bonding characteristics which are independent of the film thickness. This indicates that in these ultra-thin hydrogenated carbon films, the arrangement of sp² clusters does not relate directly to the hardness of the film.     

Microstructure, bonding strength and thermal shock resistance of ceramic coatings on steels prepared by plasma electrolytic oxidation

Ceramic coatings were successfully prepared on steel by plasma electrolytic oxidation (PEO) in aluminate electrolyte and silicate electrolyte, respectively. The microstructure of the coatings including surface morphology, phase and element composition were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The bonding strength between the ceramic coating and the substrate was tested using different methods including tensile tests and shearing tests. The thermal shock resistance of the coatings was also evaluated. The results indicated that coatings obtained in both electrolytes were porous and coarse. The average diameters of the pores were below 10 μm. PEO coatings obtained in aluminate electrolyte were composed of Fe₃O₄ and FeAl₂O₄, while those obtained in silicate electrolyte were in a noncrystal state. PEO coatings obtained in aluminate electrolyte showed similar change trend of tensile strength and shearing strength with increasing treating time, namely, a relatively high values with middle time treating and low value with short and long time treating. The best coating was the samples treated with 30 min, whose tensile strength was 20.6 MPa and shearing strength was 16 MPa. The tensile strength and shearing strength of coatings obtained in silicate electrolyte were not strongly influenced by the treating time, the values of which were range in 14 ± 2 MPa and 11 ± 2 MPa, respectively. Coatings obtained in both electrolytes showed the best thermal shock resistance with middle time treating. Coatings obtained in silicate electrolyte show a little better thermal shock resistance than those obtained in aluminate electrolyte.     

Effect of vacuum heat treatment on tensile strength and fracture performance of cold-sprayed Cu-4Cr-2Nb coatings

The previous study [1] indicated that dense thick Cu-4Cr-2Nb coatings could be formed by cold spraying, and the post-spray heat treatment could significantly influence the microstructure and microhardness of the as-sprayed Cu-4Cr-2Nb coatings. In this study, the tensile strength and fracture performance of the Cu-4Cr-2Nb coatings after annealing were investigated. The vacuum heat treatment was conducted under 10⁻² Pa at 850 °C for 4 h. Results showed that the heat treatment had a great contribution to the healing-up of the incompleteness of the interfaces between the deposited particles. In addition, the coating microhardness decreased from 156.8 ± 4.6 Hv_0.2 for the as-sprayed coatings to 101.7 ± 4.5 Hv_0.2 for the annealed ones. The mean tensile strength of the annealed coatings was approximately 294.1 ± 36.1 MPa compared to that of 45.0 ± 10.5 MPa for the as-sprayed ones, which results from the partially metallurgically bonded zones between the deposited particles inducing by the heat treatment process.     

Influence of treating frequency on microstructure and properties of Al2O3 coating on 304 stainless steel by cathodic plasma electrolytic deposition

Alumina ceramic coatings were fabricated on 304 stainless steel by cathodic plasma electrolytic deposition (CPED). Influence of treating frequency of the power supply on the microstructure and properties of the coatings were studied. The results indicated that coatings obtained at various frequencies on 304 stainless steels were all composed of α-Al₂O₃ and γ-Al₂O₃, and α-Al₂O₃ was the dominant phase. The contents of α-Al₂O₃ decreased gradually in a very small rate with increasing the frequency and γ-Al₂O₃ gradually increased. The surface of alumina ceramic coating was porous. With increasing the frequency, the coating surface gradually became less rough and more compact, resulting in low surface roughness. The bonding strength of Al₂O₃ coating was higher than 22 MPa and was not strongly affected by treating frequency. With increasing the frequency, the alumina coated steels showed better and gradually increasing corrosion resistance than the uncoated one in 3.5% NaCl solution. The coating steel with desirable corrosion resistance was obtained at 800 Hz whose corrosion current potential and corrosion density were −0.237 V and 7.367 × 10⁻⁸ A/cm², respectively.     

Characterization and corrosion behavior of hydroxyapatite/zirconia composite coating on NiTi fabricated by electrochemical deposition

NiTi alloy is used as biomaterial due to its unique properties, but the high content of Ni (about 50 at.%) in biomedical NiTi is concerned. Hydroxyapatite and hydroxyapatite/zirconia composite coatings were directly electrodeposited on NiTi alloy surface. The coated samples were characterized using X-ray diffraction, scanning electron microscopy, infrared spectroscopy, bonding strength test, polarization and EIS. Results showed that when ZrO₂ was added into the electrolyte, morphology of HAP was changed from thin flake-flower-like crystals to needle-flower-like crystals, and coating was denser. Besides, HAP crystal grains in the coating were preferentially arranged in the [0 0 1] direction. Addition of ZrO₂ could improve the bonding strength between the coating and the substrate. Corrosion resistance of NiTi in the simulated body fluid at 37 °C was significantly improved by almost 60 times by electrodeposition of the hydroxyapatite/zirconia composite coating.