A study on microstructure and properties of calcium phosphate coatings processed using ion beam assisted deposition on heated substrates

In this study a set of thin Hydroxyapatite (HA) [Ca₁₀(PO₄)₆(OH)₂] coatings was deposited on heated silicon and titanium substrates using Ion Beam Assisted Deposition (IBAD). The effects of substrate temperature and processing parameters on the microstructural properties and composition of the coatings are being studied. Analytical techniques include transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) with an energy dispersive X-ray spectroscopy (EDS), as well as scanning electron microscopy (SEM) with EDX, X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). The current results indicate that as substrate temperature increases the Ca/P ratio of the coatings both on titanium and silicon substrates increases. The crystallinity of the coatings and the number of calcium phosphate compounds within the coating including HA also increases. STEM-EDS revealed an atomically diffused intermediate layer at the interface between the coating and substrate. XRD results along with TEM selected area diffraction (SAD) revealed that the coatings are composed of HA, other calcium phosphate, and calcium oxide compounds.     

Porosity of Electroless Nickel Coatings Investigated Using Different Porosity Tests and Their Application

The porosity of electroless nickel coatings on mild steel substrates having different substrate surface roughness and coating thickness was investigated using NSS (Neutral Salt Spray), ferroxyl, and SO₂ tests. In addition the influence of substrate surface roughness and coating thickness upon the properties of electroless nickel coatings was studied.

It was found that all porosity tests showed the same tendency for the porosity of electroless nickel coatings to decrease with decreasing substrate surface roughness and increasing coating thickness. The use of the SO² and ferroxyl test have the advantage over the NSS test as a routine quality control procedure in that they are both quicker to carry out and easier to interpret. A relationship between porosity, substrate surface roughness and coating thickness was established which might be used as an industrial quality control tool.     

Effect of temperature on deposition of LaPO4 coatings produced by ultrasonic-based coating process on steel substrates

High-adhesion LaPO₄ coatings were fabricated on steel substrates at temperatures of 150-400 °C after a 10 min treatment using an ultrasonic-based coating process. The principle underlying this process is the collision of ultrasonically accelerated hard balls with the substrate surface that is covered by loosely adhered LaPO₄ particles. The repeated substrate-to-ball collisions flatten the precoated LaPO₄ particles, bond them together and cold weld them to the substrate. The coating thickness, roughness and structure were found to depend on the substrate temperature. The LaPO₄ coatings produced at temperatures ranging from 150 to 250 °C exhibited a granular and porous structure. The treatment at temperatures higher than 300 °C enabled the production of rather dense coatings.     

Investigation of particle flattening behaviour and bonding mechanisms of APS sprayed coatings on magnesium alloys

Magnesium alloys are promising alternatives to other lightweight materials due to their high specific strength and stiffness. However, the use of magnesium alloys is limited by their poor wear behaviour and low corrosion resistance for many industrial applications. The thermal spray technology offers a wide range of possibilities to improve the surface properties of Mg-based components. In this study, three different coating materials, namely Al, NiAl5 and Al₂O₃, were applied on AZ91 and AE42 substrates using the atmospheric plasma spray technology. The investigation was focused on the bonding strength of the coatings and the related bonding mechanisms. For a better understanding of the bonding mechanisms, the flattening behaviour of the spray particles was investigated in correlation with the substrate pre-heating temperature. It was found that NiAl5-particles could well melt the substrate at the surface and deformed it locally; Al-particles did the same but to a lower extent. The dominating bonding mechanism for NiAl5-coatings could be attributed to a metallurgical bonding. For Al-coatings, this mechanism played a more important role once the substrate pre-heating temperature was increased. Al₂O₃ particles in contrast, were less able to deform the substrate in spite of their higher thermal load and the mechanical anchoring remained the main bonding mechanism. The thermo physical properties of the Mg substrate showed also to have an influence on the adhesion of the coatings.     

Influence of Impact Conditions on Feedstock Deposition Behavior of Cold-Sprayed Fe-Based Metallic Glass

Cold spray is a promising method by which to deposit dense Fe-based metallic glass coatings on conventional metal substrates. Relatively low process temperatures offer the potential to prevent the crystallization of amorphous feedstock powders while still providing adequate particle softening for bonding and coating formation. In this study, Fe₄₈Mo₁₄Cr₁₅Y₂C₁₅B₆ powder was sprayed onto a mild steel substrate, using a variety of process conditions, to investigate the feasibility of forming well-bonded amorphous Fe-based coatings. Particle splat adhesion was examined relative to impact conditions, and the limiting values of temperature and velocity associated with successful softening and adhesion were empirically established. Variability of particle sizes, impact temperatures, and impact velocities resulted in splat morphologies ranging from well-adhered deformed particles to substrate craters formed by rebounded particles and a variety of particle/substrate interface conditions. Transmission electron microscopy studies revealed the presence of a thin oxide layer between well-adhered particles and the substrate, suggesting that bonding is feasible even with an increased oxygen content at the interface. Results indicate that the proper optimization of cold spray process parameters supports the formation of Fe-based metallic glass coatings that successfully retain their amorphous structure, as well as the superior corrosion and wear-resistant properties of the feedstock powder.     

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.     

Oxidation behavior of arc-sprayed FeMnCrAl/Cr3C2-Ni9Al coatings deposited on low-carbon steel substrates

FeMnCr/Cr₃C₂ and FeMnCrAl/Cr₃C₂ coatings, using Ni9Al arc-sprayed coating as an interlayer on low-carbon steel substrates, were deposited by high velocity arc spraying (HVAS) on the cored wires. The high temperature oxidation behavior of the arc-sprayed FeMnCrAl/Cr₃C₂-Ni9Al and FeMnCr/Cr₃C₂ coatings on the low-carbon steel substrates was studied during isothermal exposures to air at 800 °C. The surface and interface morphologies of the coatings after isothermal oxidation after 100 h were observed and characterized by optical microscopy, field emission scanning electron microscope, energy dispersion spectrum, and X-ray diffraction. The results showed that the oxidation weight gains of the coatings were significantly lower than that of the low-carbon steel substrate. Moreover, the FeMnCrAl/Cr₃C₂-Ni9Al coating registered the lowest oxidation rate. This favorable oxidation resistance is due to the Al and Cr contents of the aforementioned coating that inhibits the generation of Fe and Mn oxides. This is attributed to the interdiffusion between the substrates and the Ni9Al arc-sprayed coating, which can convert the mechanical bonding between substrates and coatings into a metallurgical one, thereby inhibiting the oxidation of interface between the low-carbon steel and the coating.     

Influence of deposition temperature on mechanical properties of plasma-sprayed hydroxyapatite coating on titanium alloy with ZrO<Subscript>2</Subscript> intermediate layer

Hydroxyapatite coatings were plasma sprayed on the Ti6A14V substrate with and without an intermediate ZrO₂ layer; meanwhile the temperatures of substrates were varied at 90, 140, and 200 °C. The coatings were subjected to the standard adhesion test per ASTM C633-79. The purpose of the investigation was to study the effects of those processing variables on the bonding strength and failure behavior of the system. It is found that the bonding strengths of HA/ZrO₂ and HA coatings generally decrease with increasing substrate temperature, except for the HA/ZrO₂ coating deposited at 200 °C. The rationale of the results is attributed to the residual stress reported in the literature. Introducing ZrO₂ bond coat is found to significantly promote the bonding strength of HA coating. The possible strengthening mechanism is the rougher surface of ZrO₂ bond coat and the higher toughness of ZrO₂, which provide the mechanical strengthening effects. The slightly denser HA in 200 °C deposited HA coating cannot explain the high bonding strength of the HA/ZrO₂ coating, nor the mechanical strengthening effect of ZrO₂ intermediate layer should apply. It is believed that a stronger diffusion bonding is formed at the interface of HA and ZrO₂, which increases the bonding between them chemically. The bonding strengths of HA/ZrO₂ and HA coatings are correlated with the area fraction of adhesive failure of the coatings. The correlation explains the findings in this study.     

The influence of composition and processing parameters on the mechanical properties and erosion response of Ni + TiB2 coatings

Sputtered Ni + TiB₂ coatings have been shown to protect Inconel* 718 and Ti-6A1-4V substrates from solid particle erosion. However, before new erosion-resistant coatings can be efficiently designed, it is essential that the role of mechanical properties in determining erosion resistance be fully understood. In this investigation, nanoindentation techniques were used to quantify the effects of substrate preparation, coating composition, and sputtering process parameters on the elastic moduli and indentation hardness of thin coatings deposited on Ti-6A1-4V and Inconel 718 substrates. The influence of these parameters on coating adhesion was determined using a conventional scratch test. Elastic moduli, indentation hardnesses, and coating adhesion were correlated with erosion behavior. The erosion resistance of those coatings that exhibited microscopic ductility is dependent on the nodule diameter and coating properties such as hardness, elastic modulus, and fracture toughness. Inconel 718 is a trademark of The International Nickel Co.     

Effect of Vacuum Annealing on the Characteristics of Plasma Sprayed Al2O3-13wt.%TiO2 Coatings

Adhesion strength is one of the critical properties for plasma-sprayed coating. In this study, the plasma-sprayed Al₂O₃-13wt.%TiO₂/NiCrAl coatings were annealed at 300-900?°C for 6?h in vacuum. The tensile bond strength and porosity of the coatings were investigated. The microstructure and the fracture were studied using optical microscopy, scanning electron spectroscopy, and x-ray diffraction. It was found that the tensile bond strength of coatings increased with the increase of annealing temperature until 500?°C, reaching the maximum value of 41.2?MPa, and then decreased as the annealing temperature continues to increase. All coatings presented a brittle fracture and the fracture occurred inside the ceramic coatings except for the coating annealed at 500?°C, which had a brittle-ductile mixed fracture and the fracture occurred at the interface of bond coating and the substrate.     

温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响

采用超音速火焰喷涂(HVOF)制备了WC-WB-CoCr涂层,研究了温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响。通过SEM、XRD和显微硬度仪对涂层的微观组织、相结构和力学性能进行表征。通过摩擦磨损试验机和拉曼光谱仪研究了WC-WB-CoCr涂层的高温摩擦学性能和氧化产物,采用台阶仪扫描磨痕形貌并计算WC-WB-CoCr涂层的磨损率。结果表明：WC-WB-Co-Cr涂层主要由WC和CoW₂B₂组成,涂层结构致密,与基体结合紧密;随着磨损试验温度升高,涂层的摩擦系数从0.66降低到0.57,涂层的磨损率随着温度的升高而升高,但是其磨损率增长程度随着温度的升高而降低。在高温磨损过程中,磨痕表面的氧化膜主要由WO₃和CoWO₄组成,且CoWO₄比WO₃表现出更好的耐高温磨损性能。涂层的主要磨损机制为氧化磨损、疲劳磨损和粘着磨损。     

Effect of Spray Distance on Microstructure and Tribological Performance of Suspension Plasma-Sprayed Hydroxyapatite–Titania Composite Coatings

Hydroxyapatite (HA)–titania (TiO₂) composite coatings prepared on Ti6Al4V alloy surface can combine the excellent mechanical property of the alloy substrate and the good biocompatibility of the coating material. In this paper, HA–TiO₂ composite coatings were deposited on Ti6Al4V substrates using suspension plasma spray (SPS). X-ray diffraction, scanning electron microscopy, Fourier infrared absorption spectrometry and friction tests were used to analyze the microstructure and tribological properties of the obtained coatings. The results showed that the spray distance had an important influence on coating microstructure and tribological performance. The amount of decomposition phases decreased as the spray distance increased. The increase in spray distance from 80 to 110 mm improved the crystalline HA content and decreased the wear performance of the SPS coatings. In addition, the spray distance had a big effect on the coating morphology due to different substrate temperature resulting from different spray distance. Furthermore, a significant presence of OH^? and CO₃ ^2? was observed, which was favorable for the biomedical applications.     

Effect of substrate temperature on adhesion strength of plasma-sprayed nickel coatings

We plasma-sprayed nickel coatings on stainless steel and cobalt alloy coupons heated to temperatures ranging from room temperature to 650 °C. Temperatures, velocities, and sizes of spray particles were recorded while in-flight and held constant during experiments. We measured coating adhesion strength and porosity, photographed coating microstructure, and determined thickness and composition of surface oxide layers on heated substrates. Coating adhesion strength on stainless steel coupons increased from 10–74 MPa when substrate temperatures were raised from 25–650 °C. Coating porosity was lower on high-temperature surfaces. Surface oxide layers grew thicker when substrates were heated, but oxidation alone could not account for the increase in coating adhesion strength. When a coupon was heated to 650 °C and allowed to cool before plasma-spraying, its coating adhesion strength was much less than that of a coating deposited on a surface maintained at 650 °C. Cobalt alloy coupons, which oxidize much less than stainless steel when heated, also showed improved coating adhesion when heated. Heating the substrate removes surface moisture and other volatile contaminants, delays solidification of droplets so that they can better penetrate surface cavities, and promotes diffusion between the coating and substrate. All of these mechanisms enhance coating adhesion.     

Effect of plasma surface treatment and heat treatment ambience on mechanical and corrosion protection properties of hybrid sol-gel coatings on aluminum

Hybrid sol-gel coatings derived from a base catalyzed hydrolysis of tetraethylorthosilicate and methyltriethoxysilane were deposited on aluminum substrates by a dip coating technique. Some of the coatings were deposited on substrates whose surfaces were pre-treated using atmospheric-air plasma prior to coating in order to study the effect of surface activation by plasma pre-treatment. The coated substrates were heat treated in different ambiences like air, flowing N₂ and vacuum to see the effect of heat treatment ambience on the properties of the coatings. Characterization of the coatings after heat treatment was carried out with respect to coating thickness, pencil scratch hardness, adhesion, water contact angle and their microstructure. Corrosion testing for all the coatings was carried out by electrochemical polarization measurements as well as electrochemical impedance spectroscopy in 3.5% NaCl solution for 1 h exposure time to investigate on their corrosion resistance. Coating thicknesses ranging from 1 μm-5 μm were obtained by varying the withdrawal speeds. Heat treatment in a controlled atmosphere with low oxygen content was seen to improve the hydrophobicity of coated surface, as measured by water contact angles (20^o — air; 71^o — N₂; 95^o — vacuum), thereby improving the corrosion resistance. Surface pre-treatment using open-air plasma was seen to improve the adhesion of the sol-gel coatings thus making it possible to obtain adherent and thick coatings in a single dip coating process. Both the methods of processing the coatings reduced the corrosion rate of aluminum from 1.95 mpy to 0.004 mpy in case of coatings densified in nitrogen and to 0.00068 mpy for coatings deposited on a plasma treated substrate and densified in air.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Al-Al₂O₃ composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al₂O₃ particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al₂O₃ particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al₂O₃ content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al₂O₃ bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.     

Thermal conductivity of EB-PVD ZrO2-4 mol% Y2O3 films using the laser flash method

The variation in thermal conductivity and thermal diffusivity of ZrO₂-4 mol% Y₂O₃ coatings deposited onto Inconel substrates by EB-PVD is examined as a function of coating thickness using the laser flash method. The coatings are found to consist of columnar grains with a feather-like microstructure. The thermal conductivities of the coatings are calculated using two methods: the first involves separating the coating from the substrate and measuring the thermal diffusivity directly; the second uses thermal diffusion results from coatings still attached to the substrate and is based on the response function method. The results of both methods are in excellent agreement, and show that the thermal conductivities of the coatings increase with increasing coating thickness. The results also confirm that the double layer method can be used successfully to calculate the thermal conductivities of thin film coatings.     

Corrosion Behavior of Thermally Sprayed NiCrBSi Coating on 16MnR Low-Alloy Steel in KOH Solution

NiCrBSi coatings were selected as protective material and air plasma-sprayed on 16MnR low-alloy steel substrates. Corrosion behavior of 16MnR substrates and NiCrBSi coatings in KOH solution were evaluated by polarization resistance (R _p), potentiodynamic polarization curves, electrochemical impedance spectroscopy, and immersion corrosion tests. Electrolytes were solutions with different KOH concentrations. NiCrBSi coating showed superior corrosion resistance in KOH solution compared with the 16MnR. Corrosion current density of 16MnR substrate was 1.7-13.0 times that of NiCrBSi coating in the given concentration of KOH solution. By contrast, R _p of NiCrBSi coating was 1.2-8.0 times that of the substrate, indicating that the corrosion rate of NiCrBSi coating was much lower than that of 16MnR substrate. Capacitance and total impedance value of NiCrBSi coating were much higher than those of 16MnR substrate in the same condition. This result indicates that corrosion resistance of NiCrBSi coating was better than that of 16MnR substrate, in accordance with polarization results. NiCrBSi coatings provided good protection for 16MnR substrate in KOH solution. Corrosion products were mainly Ni/Fe/Cr oxides.     

The Influence of Temperature on Frictional Behavior of Plasma-Sprayed NiAl-Cr2O3 Based Self-Adaptive Nanocomposite Coatings

Frictional behavior of nano and hybrid-structured NiAl-Cr₂O₃-Ag-CNT-WS₂ adaptive self-lubricant coatings was evaluated at a range of temperatures, from room temperature to 700 °C. For this purpose, hybrid structured (HS) and nanostructured (NS) composite powders with the same nominal compositions were prepared by spray drying and heat treatment techniques. A series of HS and NS coating samples were deposited on steel substrate by an atmospheric plasma spraying process. The tribological behavior of both coatings was studied from room temperature to 700 °C at 100° intervals using a custom designed high temperature wear test machine. Scanning electron microscopy was employed for the evaluation of the composite coatings and worn surfaces. Experimental results indicated that the hybrid coating had inferior tribological properties when compared to the nanostructured coating, showing the attractive frictional behavior on the basis of low friction and high wear resistance; the NS coating possessed a more stable friction coefficient in the temperature range of 25-700 °C against alumina counterface. Microstructural examinations revealed more uniformity in NS plasma-sprayed coatings.     

Characterisation of tungsten coatings by pulsed current plating from molten salt in air atmosphere

Pure and compact tungsten coatings with high (200)-orientation were successfully electrodeposited from Na₂WO₄–WO₃ binary oxide molten salt by direct and pulse current techniques on copper based alloy substrate in air atmosphere at 1173 K. The tungsten coatings and copper alloy substrates combined well with no obvious voids and cracks. In addition, the influence of pulse plating on the microstructure, crystal structure and Vickers microhardness of coatings, adhesive strength between coatings and substrates as well as the current efficiency was investigated. It was also observed that applied pulse current led to tungsten coatings with better mechanical properties than those obtained by direct current plating. In addition, a tungsten coating of 500 μm which had lower oxygen content and higher adhesive strength than the coating obtained by the plasma spray method was obtained.