The Influence of the Substrate on the Deposition of Cold-Sprayed Titanium: An Experimental and Numerical Study

The deposition of cold-sprayed titanium on various substrates is studied in this work. A rather coarse powder of titanium (−70 + 45 μm) was sprayed under uniform spraying conditions using a cold spray system onto five different substrates: two aluminum-based alloys (AISI 1050-H16 and AISI 2017-T4), copper, stainless steel AISI 304L, and Ti-6Al-4V. All the spraying experiments were carried out using alternatively nitrogen (N₂) or helium (He) as the process gas. Thick coatings were formed on the various substrates, with the exception of the AISI 2017 substrate. When N₂ was used as the process gas, only a few particles remained adhering to the AISI 2017. The thick pre-existing superficial oxide layer on AISI 2017, which was detected by Electron MicroProbe Analysis (EPMA), appeared to prevent adhesion of cold-sprayed titanium particles. The interaction of the sprayed particles with the various substrates was also studied by means of numerical simulations to better understand the adhesion mechanisms. The microstructure and the characteristics of the coatings were investigated. Deposition efficiency and coating density were found both to be strongly improved by spraying helium as the process gas.     

Manufacture and properties of cold spray deposited large thickness Cu coating material for sputtering target

In this study, the manufacture of a large thickness Cu coating layer as sputtering target material via a cold spray coating process was undertaken. The microstructure and properties of the Cu layer as the sputtering target material (before and after the annealing heat treatment) were evaluated, compared, and analyzed. To evaluate the purity, density, grain size and uniformity, microstructure, and properties of the Cu-coated layer as a sputtering target, X-ray diffraction, ICP analysis, SEM, EBSD, porosity analysis, and Vickers hardness tests were performed. The result of the observation of the layer’s purity and microstructure showed that a purity level (99.47%) similar to that of the early powder 2N5 was maintained and that the manufacture of a cold spray deposited, ∼20 mm thick Cu coating material for the sputtering target was performed successfully. As a result of the EBSD mapping, the average grain size near the interface and around the center measured 1.48 μm and 1.49 μm; the grains were small and non-uniform compared with the 1.91 μm size near the surface. Note, however, that the recrystallization and grain growth (caused by annealing) increased the grain size to 1.82 μm (near the interface), 1.83 μm (near the center), and 1.87 μm (near the surface) and improved the level of uniformity. Moreover, through post heat treatment, the overall porosity declined (0.44 % porosity/400 °C/h heat treatment), and the grain texture became uniform. The possibility of controlling the microstructure as a large thickness sputtering target by conducting an annealing heat treatment was also confirmed. Nonetheless, the differences in the porosity and hardness associated with the coating thickness changes were partially maintained. Based on the aforementioned findings, this study suggests that by using cold spray deposition, Cu coating layers with large thicknesses can be applied as a sputtering target.     

Residual Stress Analysis of Boronized AISI 1018 Steel by Synchrotron Radiation

AISI 1018 steel substrates were powder-pack, diffusion boronized at 850 °C for 4 h, followed by air quenching. Optical microscopy in conjunction with color etching was used to obtain the average penetration depth of the iron monoboride layer (9 μm) and the iron diboride layer (57 μm). X-ray diffraction by synchrotron radiation, conducted at the National Synchrotron Light Source in Brookhaven National Laboratory, confirmed the presence of iron monoboride and iron diboride in the boronized plain steel substrates. The sin² ψ technique was employed to calculate the residual stress found in the iron monoboride layer (−237 MPa) and in the substrate layer (−150 MPa) that is intertwined with the needle-like, iron diboride penetration.     

Laser surface modification of AISI 410 stainless steel with brass for enhanced thermal properties

Brass coating was applied to AISI410 steel using high power laser in a laser engineered net shaping (LENS™) system. The influence of laser treatment on interfacial microstructure and thermal performance was evaluated as a function of coating thickness. Laser deposition resulted in a diffused and metallurgically sound interface between metallurgically incompatible brass coating and AISI410 steel substrate. The thermal conductivity of AISI410 steel increased from 27 W/mK to a maximum of 37 W/mK depending on the coating thickness, almost 50% gain. The absence of sharp interface between the coating and the substrate, as a result of laser processing, resulted in a low interfacial thermal contact resistance. Thermal performance tests showed that the brass coating can enhance the heat transfer rate of stainless steel substrate. These results show that novel and efficient feature based coatings can be exploited using laser-based advanced manufacturing technologies for various industrial applications.     

A Study on Microstructure and Dielectric Performances of Alumina/Copper Composites by Plasma Spray Coating

In this study, surfaces of copper plates were coated with a thick alumina layer by the plasma spray coating to fabricate a composite with a dielectric performance that made them suitable as substrates in electronic devices with high thermal dissipation. The performance of alumina dielectric layer fabricated by the plasma spray coating and traditional screen-printing process was compared, respectively. Effects of the spraying parameters and size of alumina particles on the microstructure, thickness, and the surface roughness of the coated layer were explored. In addition, the thermal resistance perpendicular to the interface of copper and alumina and the breakdown voltage across the alumina layer of the composite were also investigated. Experimental results indicated that alumina particles with 5-22 μm in diameter tended to form a thicker layer with a poorer surface roughness than that of the particles with 22-45 μm in diameter. The thermal resistance increased with the surface roughness of the alumina layer, and the breakdown voltage was affected by the ambient moisture, the microstructure and the thickness of the layer. The optimal parameters for plasma spray coating were an alumina powder of particles size between 22 and 45 μm, a plasma power of 40 kW, a spraying velocity of 750 m/s, an argon flow rate of 45 L/min, a spraying distance of 140 mm, and a spraying angle of 90°. It can be concluded that an alumina layer thickness of 20 μm provided a low surface roughness, low thermal resistance, and highly reliable breakdown voltage (38 V/μm).     

Bond Strength of Multicomponent White Cast Iron Coatings Applied by HVOF Thermal Spray Process

Multicomponent white cast iron is a new alloy that belongs to system Fe-C-Cr-W-Mo-V, and because of its excellent wear resistance it is used in the manufacture of hot rolling mills rolls. To date, this alloy has been processed by casting, powder metallurgy, and spray forming. The high-velocity oxyfuel process is now also considered for the manufacture of components with this alloy. The effects of substrate, preheating temperature, and coating thickness on bond strength of coatings have been determined. Substrates of AISI 1020 steel and of cast iron with preheating of 150 °C and at room temperature were used to apply coatings with 200 and 400 μm nominal thickness. The bond strength of coatings was measured with the pull-off test method and the failure mode by scanning electron microscopic analysis. Coatings with thickness of 200 μm and applied on substrates of AISI 1020 steel with preheating presented bond strength of 87 ± 4 MPa.     

On the limit drawing ratio of magnetron sputtered aluminium–scandium foils within micro deep drawing

Two different foils out of the alloy aluminium-scandium with a thickness of about 15 μm were produced by the d.c. magnetron-sputtering process applying different substrate temperatures, i.e. S37 at the substrate temperature of 37°C and S160 at the substrate temperature of 160°C. They show different forming properties, e.g. flow stress. In this work these two different foils were used as blank material in micro deep drawing with a punch diameter of 0.75 mm to investigate the formability of these foils. A limit drawing ratio of 1.6 was achieved for both foils. Using the strip drawing test the friction coefficients between the foils and the tools were acquired experimentally, i.e. μ = 0.12 on the smooth side and μ = 0.16 on the rough side for the foil of S37 and μ = 0.15 on the smooth side and μ = 0.17 on the rough side for the foil of S160.     

Formation of composite coatings based on titanium carbide via electrospark alloying

This work concerns studying the coatings prepared via electrospark alloying. To deposit the coatings, we used STIM-2/30 electrodes derived from a combination of self-propagating high-temperature synthesis (SHS) and extrusion. It was found that the composite coating is formed from titanium carbides and a solid solution of nickel in iron and contains both large (4–5 μm) and small carbides (less than 1 μm); in addition, large grains of titanium carbide are formed in the central portion of the coating, and the grain size decreases to 100 nm while approaching the transition zone. Large grains of titanium carbide in the coating consist of dispersed carbides with sizes less than 1 μm. It is determined that the composition of the substrate has an effect on the size of the transition and diffusion zones. It ranges from 17 μm for steel 9KhSA to 26 μm for steel 20; that is, the higher the degree of alloying, the less the depth of the modified layer.     

Manufacturing and Macroscopic Properties of Cold Sprayed Cu-In Coating Material for Sputtering Target

This study attempted to manufacture a Cu-In coating layer via the cold spray process and to investigate the applicability of the layer as a sputtering target material. In addition, changes made to the microstructure and properties of the layer due to annealing heat treatment were evaluated, compared, and analyzed. To examine the microstructural and property changes made to the Cu-In coating layer and Cu coating layer (comparison material), ICP, XRD, SEM, and other tests were conducted; purity, density, hardness, porosity, and bond-strength were measured. The results showed that coating layers with thickness of 20 mm (Cu) and 810 μm (Cu-In) could be manufactured via cold spraying under optimal process conditions. With the Cu-In coating layer, the pure Cu and intermetallic compounds of Cu₇In₃ and CuIn₄ were found to exist inside the layer regardless of annealing heat treatment. The preannealing inconsistent microstructure of the layer, whose phases were difficult to distinguish was found to have transformed into one with clearer phase distinction and fine, consistent grains following thermal treatment via a progress of recovery, recrystallization, and grain growth. The porosity and hardness values of the coating layers were 1.4% and 133.9 HV, respectively, for Cu and 3.54% and 476.6 HV, respectively, for Cu-In. The values of the Cu-In layer were higher than those of the Cu layer in terms of porosity and hardness, which declined drastically after annealing. With the porosity of the Cu-In coating layer in particular, the higher value found during the preannealing stage dropped to 0.36% after heat treatment of 773 K/1 h as the level on a par with pure Cu (0.44%), thus indicating the improved quality of the Cu-In layer. Moreover, the results of the bond-strength measurement performed on the Cu-In coating layer and annealing treated materials revealed the strength to be relatively high for heat treated coating layers. Based on the findings of this study and on the comparison and discussion of the properties that are typically required of the target material, the Cu-In coating layer manufactured via cold spray process and annealing heat treatment can be said to be applicable as sputtering target in the future.     

Corrosion behaviour of magnetron sputtered α- and β-Ta coatings on AISI 4340 steel as a function of coating thickness

The corrosion behaviour of magnetron sputtered α- and β-Ta coated AISI 4340 steels was studied with potentiodynamic polarization and electrochemical impedance spectroscopy. The coating porosity was observed to decrease with increasing coating thickness. For coatings less than 10 μm thick (α- or β-Ta), porosity was significant and open pores resulted in severe localized corrosion of the steel substrate, coating delamination, and overall coating failure. Additionally, the β-Ta coatings were more susceptible than the α-phase to delamination. As for the 50 and 100 μm thick α-Ta coatings, the electrochemical impedance behaviour was comparable to that of Ta foil, demonstrating the coating viability and corrosion resistance.     

Influence of the nitriding and TiAlN/TiN coating thickness on the sliding wear behavior of duplex treated AISI H13 steel

AISI H13 die steel substrates were low pressure gas nitrided to different thicknesses and hardness values. Nitrided and non nitrided samples were subsequently coated with bi-layer TiAlN/TiN to two different thicknesses. The hardness was measured across the coating thickness and observed to be higher when a thinner coating was deposited over nitrided substrates. The hardness behavior across relatively thin (3 μm) coatings was not affected by the nitrided surface hardness or thickness of the nitride layer in the range of values examined here (80-150 μm). On the other hand, the hardness behavior of thicker coatings (8um) was affected by the nitrided layer, as the thicker coatings were soft due to their columnar structure. The specific wear rate of the duplex coatings was affected by the coating thickness and hardness distribution across the coating system.     

Hydrogen permeation behavior of nickel electroplated AISI 4340 steel

The role of the electroplated nickel layer on hydrogen permeation through AISI 4340 steel was investigated by a electrochemical hydrogen permeation test. The permeation test, composed of three steps, was conducted to measure the hydrogen diffusivity and surface hydrogen concentration. A constant current of 20 mAcm^-2 and a constant potential of -100 mV vs. Ag/AgCl electrode were applied to the hydrogen entry and exit cells, respectively. The thickness of the electroplated nickel layer on AISI 4340 steel increased in a linear fashion with an increase in electroplating time. The nickel coated layer contributed to a decrease in the hydrogen permeability of nickel coated AISI 4340 steel specimens. This is due to the fact that the surface hydrogen concentration and hydrogen diffusivity in nickel coated layer were lower than those of AISI 4340 steel substrate. Especially, low hydrogen diffusivity decreased significantly with hydrogen permeability. The critical effective hydrogen diffusivity for barrier of nickel electroplated AISI 4340 steel specimens was higher than the hydrogen diffusivity of AISI 4340 steel specimen. It is proposed then that the thin nickel layer on AISI 4340 steel acts as a barrier for hydrogen permeation through AISI 4340 steel.     

Formation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Diffusion Barrier Between Cr-Contained Stainless Steel and Cold-Sprayed Ni Coatings at High Temperature

A novel approach to prepare a coating system containing an in situ grown Cr₂O₃ diffusion barrier between a nickel top layer and 310SS was reported. Cold spraying was employed to deposit Ni(O) interlayer and top nickel coating on the Cr-contained stainless steel substrate. Ni(O) feedstock was prepared by mechanical alloying of pure nickel powders in ambient atmosphere, acting as an oxygen provider. The post-spray annealing was adopted to grow in situ Cr₂O₃ layer between the substrate and nickel coating. The results revealed that the diffusible oxygen can be introduced into nickel powders by mechanical alloying. The oxygen content increases to 3.25 wt.% with the increase of the ball milling duration to 8 h, while Ni(O) powders maintain a single phase of Ni. By annealing the sample in Ar atmosphere at 900 °C, a continuous Cr₂O₃ layer of 1-2 μm thick at the interface between 310SS and cold-sprayed Ni coating is formed. The diffusion barrier effect evaluation by thermal exposure at 750 °C shows that the Cr₂O₃ oxide layer effectively suppresses the outward diffusion of Fe and Cr in the substrate effectively.     

TEM Analysis on Micro-Arc Oxide Coating on the Surface of Magnesium Alloy

By micro-arc oxidation (MAO), the oxide coatings were prepared on the surface of magnesium alloys in a composite electrolytic solution. The microstructures of the coating layer and the interface between coating and substrate were analyzed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The oxide coating consists of two layers (the outer and the inner layer). Although both layers are composed of microcrystalline MgO and amorphous phase, the inner layer is more compact and rich in fluorine with a thickness of about 1-2 μm. Fluorine plays an important role in the inner dense layer formation. The inner layer, like a barrier wall, blocks the thickness of the oxide coating to increase and improves corrosion resistance. The formation mechanism of the inner layer is also discussed.     

The Effect of Coating Thickness on Corrosion Resistance of Hydroxyapatite Coated Ti6Al4V and 316L SS Implants

Hydroxyapatite (HAP) has been coated onto Ti6Al4V and 316L SS substrates by sol-gel method. The coating thicknesses for the analysis were about 40 and 72 μm. Adhesion and corrosion tests have been conducted on uncoated and HAP-coated substrates. The coatings were characterized by XRD, SEM, and adhesion analysis. The corrosion resistance was examined in vitro by potentiodynamic polarization technique in Ringer’s solution at room temperature. Electrochemical analysis indicated that the highest corrosion susceptibility was found on 72-μm-coated 316L SS, and the 40-μm HAP-coated Ti6Al4V showed the highest corrosion resistance. It was observed that the coating thickness was an effective parameter on both adhesion and corrosion resistance. It was shown that adhesion and corrosion resistance decreased with increasing coating thickness on both substrates.     

Steam oxidation resistance and thermal stability of chromium aluminide/chromium hybrid coating on alloy steels formed at low temperatures

This study investigated the feasibility of forming a hybrid coating with a structure consisting of a top Cr-aluminide layer and an inner Cr layer on alloy steels by using a two step process: electro-Cr plating and then pack aluminising at low temperatures. The oxidation resistance of the coating so formed was tested in ultrasupercritical steam of 650 °C and 30 MPa. The factors affecting the oxidation kinetics of the coating were studied by comparing its oxidation behaviour with that of the pack Fe-aluminide coating tested in the same ultrasupercritical steam. The thermal stability of the coating at 650 °C was investigated by a series of isothermal annealing experiments in argon atmosphere. It was demonstrated that the outer Cr-aluminide layer of the coating can improve the steam oxidation resistance of the steel substrate. The inner Cr layer can function as an effective barrier preventing the outward diffusion of Fe from the steel substrate; it can also act as a buffer zone, substantially reducing the rate of the inward Al diffusion process.     

Effect of geometry on void formation in commercial electroplating of thin strips to copper

Three plating trials comprised of six samples were performed to evaluate the ability to attach thin strips of varying cross section to a copper substrate via commercial nickel electroplating in a nickel sulfamate bath. Nickel plated to the top, bottom, and sides of the nickel strips, as well as to the substrate. A significant void formed beneath rectangular nickel strips for all geometries studied, including strip widths of 500 µm to 5000 µm and gap thicknesses from 100 µm to 1500 µm. This is due to the starvation of ions when two regions of growing grains impinge and entrap a volume of electrolyte, surrounding it completely by deposited nickel. Impingement often occurs just past the edge of the strip which causes the void width to be greater than the strip width and form seams at the edge of the void. The rate of plating is greatest in areas where sharp corners exist, due to higher current density. More plating reaches under the strip as the width decreases and/or gap thickness increases. Thus, the void size decreases with decreasing aspect ratio, defined as the strip width over gap thickness. The use of a cylindrical strip produces sound plating with no voids. This is because the lack of corners allows impingement to occur first beneath the center of the circular cross section, so liquid is never surrounded. The results of this study demonstrate how to minimize or avoid void formation in commercial electroplating of suspended strips which is of great importance in the installation of sensor strips in the coating layer of continuous casting molds.     

Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel

The present research involves the deposition of pulsed DC CFUBM sputtered TiN on nickel plated steel discs and electroplated monolayer cBN wheels at seven different target frequencies and ten different bias voltages separately. The coating microstructures and the interaction between TiN and nickel were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Phase detection was carried out using grazing incidence X-ray diffraction (GIXRD) technique. The cohesive and adhesive strengths of nickel layer were assessed by scratch test. After grinding of low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100), the conditions of the uncoated and coated cBN wheels were observed under Stereo Zoom Microscope and SEM.Average column size of TiN was found to decrease with increase in both target frequency and negative bias voltage. The structure of the coating gradually transformed from porous and open columnar (at 0 V bias) to very compact, dense and featureless (at − 80 V bias). EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel-titanium intermetallic phases at their interface. The cohesive strength of nickel layer was not effectively enhanced with increase in target frequency, whereas the same was significantly improved with increase in negative bias voltage. Seemingly, TiN coated wheel could not perform better than the uncoated wheel in grinding AISI 1020 steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheels. On the other hand, many fractured grits and some grit pull-out were observed in the uncoated wheel when grinding AISI 52100 steel, whereas almost no pull-out along with much less fractured grits were observed in the wheels coated at bias voltages like − 60 V and − 90 V.     

Formation of WC-iron silicide (Fe5Si3) composite clad layer on AISI 316L stainless steel by high power (CO2) laser

An attempt was made to produce WC-iron silicide cladded layer on AISI 316L stainless steel by laser processing to obtain high hardness and lesser variations in hardness distribution in the layer. Different compositions of coating materials (WC, Si and Ni) and laser processing parameters were used. A good and defect free cladded layer of WC-iron silicide was obtained for an energy density of 22.5 J/mm² and coating composition of 40WC-40Si-20Ni (wt.%). The layer exhibited average hardness of about 883 HV with lesser variations in the hardness distribution and also higher wear resistance compared to the substrate.     

Effect of the Current Density on Morphology,Porosity, and Tribological Properties of Electrodeposited Nickel on Copper

In the steel industry, nickel coating on copper has increased the lifespan of continuous ingot casting molds. The objective of this work is to estimate the porosity of nanocrystalline nickel electrodeposited onto copper. Characteristics of nickel coating such as hardness, wear resistance, porosity, morphology, and adhesion are very important for maximum performance of molds. The effective porosity in nickel coating was determined by using anodic voltammetry. The porosity of electrodeposited nickel onto copper increased from 0.16% up to 6.22% as the current density increased from 1.5 up to 8.0 A dm⁻². The morphology of the nickel electrodeposited at lower current densities was more compact. Tribological properties were studied using hardness measurements, and calotest. Results of calotest indicated a wear coefficient of 10⁻⁶ for all samples. An extremely low friction coefficient of 0.06-0.08 was obtained for the sample deposited with a current density of 1.5 A dm⁻², and a friction coefficient of 0.15-0.21 was measured for the nickel coating electrodeposited at a current density of 5 A dm⁻². Effects of the current density of the electrodeposition process on the morphology, porosity, and tribological properties were evaluated.