Plasma Spray Deposition Research Articles

A Review of Coating Preparing Techniques of Bioactive Glass

Bioactive glass, because of the good bioactivity and biocompatibility, is considered to be the most ideal material for bone repair and replacement. However, the mechanical properties of the bioactive glass are great different from that of human bone. Coating bioactive glass on the surface of Ti or Ti-alloys that possess excellent mechanical properties can offset the disadvantage. In this paper, the research progress in the coating preparing techniques such as enameling, sol-gel, electrophoretic deposition, plasma spraying and pulsed laser deposition is summarized, the advantages and existent problems of the coating preparation techniques are discussed, and the development foreground is forecast.

Influence of plasma spraying parameters on properties of thermal barrier coatings

The aim of the present study is to investigate the influence of plasma spay deposition parameters on various properties of two standard thermal barrier coating (TBC) systems deposited on the steel substrate. Two TBC systems were investigated: NiCr–Al–Co–Y2O3/ZrO2–Y2O3 and NiCr–Al–Co–Y2O3/ZrO2–CeO2–Y2O3. Powders were deposited by atmospheric plasma spraying (APS) with two variable parameters. The first variable was substrate temperature (21–23 and 160–180°C), and the second variable was transverse speed of the spraying gun (250 and 500 mm s−1), i.e. coating thickness per one pass of the spraying gun. The quality assessments of TBC systems were made on the base of their microstructure, microhardness, tensile bond strength and thermal cyclic behaviour (thermal insulation properties and thermal cycling fatigue). Analysis of the results showed great influence of plasma spray deposition parameters on the quality of obtained TBCs.

Preparation and Characterisation of Cattle Bone Based Hydroxyapatite Implant Coatings Using Electrostatic Spray Deposition

The biocompatibility of orthopaedic or dental metallic implants can be significantly enhanced by the coatings of bioceramic materials. Currently, various deposition techniques such as plasma spraying, magnetron sputtering and pulsed laser deposition can be used for the metallic implant coating. Nevertheless, these methods are addressed as a high temperature processing and manufacturing cost. Electrostatic spray deposition (ESD) has gained interest due to its simple and economical process involving a room temperature processing. Hence, the objectives of this study have focused on preparation and characterisation of cattle bone based hydroxyapatite (cHA) stainless steel implant coatings using ESD. Process parameters including electrode-substrate distance, applied electrical voltage, powder feed rate and current were optimised. The chemical composition of cattle bone powder was investigated both before and after coating by X-ray diffractometry (XRD). The XRD pattern of cattle bone powder demonstrated that all significant peaks matched with HA both before and after coatings. The morphological characteristic of the coatings was examined by scanning electron microscope (SEM). The surface morphology of the coatings showed a homogeneous and dense layer. The average thickness was about 40 µm. The in vitro bioactivity of cHA coatings was investigated by an acellular simulated body fluid soaking experiment. The accelerated apatite precipitation process occurred on the cHA-coated stainless steel surfaces compared to the uncoated stainless steel surfaces. Thus, the use of ESD is a promising technique in producing cHA coatings for biomedical applications.

Multifunctional Composite Coating as a Wear-Resistant Layer for the Bearing in Total Hip Joint Replacement

In this study, we developed Ti-TiN composite coatings with fine lamellar structures for use as an enhanced wear-resistant layer between the bearing components of the polymer-lined acetabular cup and the metal femoral head of total hip joint replacements (THRs). A plasma spraying deposition method was used to apply the composite coatings, and the thickness of TiN layer in the composite could be controlled by varying the flow rate of N(2) atmospheric gas. The surface properties, such as roughness and hardness, were analyzed, and the friction coefficient (μ) and wear rate (k) were measured using a bovine serum wear test. A biocompatibility test was performed to evaluate the toxicity of the composite coatings. Our experimental results reveal that the friction and wear resistance of composite coatings is superior to that of the metallic implant materials, and they have a higher level of fracture toughness as compared with other ceramic coatings because of a good balance between the hardness of the TiN and the toughness of the Ti. Furthermore, these coatings possessed excellent biocompatibility. The experimental results also demonstrate that the improved wear properties can be attributed to a certain level of unavoidable porosity that is due to the rapid solidification of liquid droplets during the plasma spraying process. The pores in the coating surface play an important role as a lubricant (bovine serum) reservoir, reducing the actual contact area and friction losses.

Coating of carbon fiber‐reinforced polyetheretherketone implants with titanium to improve bone apposition

Carbon fiber-reinforced polyetheretherketone (CF/PEEK) is a thermoplastic composite biomaterial exhibiting properties suitable for load-bearing orthopedic implants. However, the hydrophobic surface of CF/PEEK implants induces the deposition of a peri-implant fibrous tissue capsule preventing bone apposition. However, if bone apposition was improved, the use of CF/PEEK in orthopedics could be increased as it has many advantages compared with metallic implants. In this study, CF/PEEK screws were coated with titanium (Ti) using two different techniques, namely vacuum plasma spraying (VPS) and physical vapor deposition (PVD) with uncoated screws as controls. These coatings were characterized and implanted in a loaded sheep tibia model. In the characterization of the screw surfaces using microscopy techniques, the uncoated screws were seen to have an irregular surface. The PVD coating appeared smooth and consistent, whereas the VPS coating appeared to be a rough coating with some inhomogeneities, which did not cover the entire surface area. Nevertheless, in the ex vivo analysis the VPS-coated screws had a screw removal torque which was statistically greater than uncoated and PVD-coated screws (p ≤ 0.002 for both comparisons). Additionally, the VPS-coated screws had a statistically higher bone contact area than the uncoated screws (p = 0.006), whereas no statistical difference was detected between VPS and PVD coating types (p = 0.11). Thereby illustrating that Ti coating of CF/PEEK screws significantly improve bone apposition and removal torque compared with uncoated CF/PEEK screws.

Influence of plasma and cold spray deposited Ti Layers on high-cycle fatigue properties of Ti6Al4V substrates

This paper presents a summary of the research on the influence of plasma and cold spray deposition of bio-grade Ti powder on the high-cycle fatigue properties of Ti6Al4V substrates.Four sets of flat specimens (as-received, grit-blasted, plasma and cold sprayed) were prepared and subjected to cantilever–beam cyclic bend loading with a constant deflection of the free end.It was found that the grit-blasting procedure significantly increased the fatigue lives of the specimens compared to the as-received set (1.81× increase). The deposition of the coatings onto grit-blasted specimens led to deterioration of fatigue properties. The average fatigue lives of the plasma sprayed and cold sprayed samples reached 1.16× and 0.91× of the as-received specimens, respectively.In order to understand the positive effect of the grit-blasting procedure and the adverse effect of the coatings deposition on fatigue lives, chemical analyses, fractographic analysis, microstructural investigations as well as layers elastic moduli estimation were carried out. It was found that the cold spray deposition retained the composition of the powder feedstock while a complete transformation of Ti into nitrides and oxides was detected in the plasma deposited layers. Owing to the different coating build-up principles and their respective different porosity levels, the moduli of the coatings were found to vary (7.2GPa for plasma sprayed and 36.7GPa for cold sprayed layers). The fatigue crack initiation sites and propagation directions in the plasma and cold sprayed specimens were found to differ substantially. Based on the obtained results, two explanations of the fatigue loading results are suggested in the paper.

Model of a low-pressure radio-frequency inductive discharge in Ar/O2 used for plasma spray deposition

A global (volume-averaged) model of a low-pressure radio-frequency (RF) inductive discharge used for nanostructured zinc oxide thin film deposition, the so-called spray-plasma device, is proposed. The plasma reactor is fed with an admixture of argon and oxygen and the pressure is typically several tens of mTorr. In the first step of the modelling, the injector and the substrate holder are not taken into account, and therefore zinc-containing species are not considered. The global model is based on the numerical integration of the particle balance equations and the electronic power balance equation. The model is first run until the steady state is reached to determine the equilibrium discharge parameters that are the species densities and the electron temperature. A parametric study is carried out varying the gas pressure, the RF power and the O2 fraction in the reactor. A parameter of great importance for the deposition process is the flux of the reactive species on the substrate holder and the model allows a fast exploration of this parameter. For continuous plasmas, the ratio of the reactive species flux to the total positive ion flux can be controlled varying the three basic parameters cited before (pressure, power and dilution). In the last part of the paper, we also investigate pulsed plasmas and the effect of the duty cycle variations on the neutral/ion flux ratio.

Enhancement of amorphous phase formation in alumina–YSZ coatings deposited by suspension plasma spray process

In the present work on pseudo-eutectic alumina–yttria stabilized zirconia (YSZ) composite, the influence of spray parameters on amorphous phase formation during suspension plasma spray deposition is investigated. Several variables were evaluated as the most probable key factors influencing the amorphous phase formation. These variables include powder feed size, in-flight particle characteristics (temperature and velocity), spraying robot travel speed, preheating the substrate, number of deposition passes and the presence of bond coat. It was found that larger particle size, higher robot speed and substrate preheating lead to larger amounts of amorphous phase. Moreover, it is shown that particle velocity and temperature need to be reasonably low to get greater amorphous content while the presence of a bond coat has no significant influence on the amorphous phase formation. In contrast, increasing the number of deposition passes is detrimental to this phase. This work also discusses some correlations observed between the amorphous phase content and the in-flight particle characteristics and coating grain sizes.

Experimental and simulation approach to plasma spray deposition of yttrium oxide

Plasma sprayed coatings of yttrium oxide, exhibiting good adhesion and thermal shock resistance, were developed on various substrates using thermal spray grade yttrium oxide powder developed in our laboratory. Numerical simulation of particle behaviour in the thermal plasma jet was combined with experimental method to develop the coatings. Phase structure and microstructural characterisation of the deposits were evaluated by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Deposition efficiencies and adhesion strength of the coatings deposited at different power levels could be well explained with the predictions provided by the model. The process was optimised using deposition efficiency as the response parameter.

Process development and coating characteristics of plasma spray-PVD

Plasma spray physical vapor deposition (PS-PVD) was developed with the aim of depositing uniform and relatively thin coatings with large area coverage. At high power input (~150kW) and very low pressure (~100Pa) the plasma jet properties change considerably compared to conventional plasma spraying and it is even possible to evaporate the powder feedstock material enabling advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition (PVD). Moreover, the resulting microstructures are unique and can hardly be obtained by other processes.In this paper, plasma characteristics of different gas mixtures are investigated. The measurements and calculations provide indications of the growth modes and help to explain the resulting microstructures and coating chemistries. Coatings sprayed from different ceramic powders are discussed.

Magnetic Properties of Nanostructured Co-Based Alloys Produced by Dynamic Compaction and Plasma Spray Deposition

The bulk nanostructured Co-P alloys were prepared by dynamic compaction and a plasma spray deposition techniques. The magnetic characteristics, such as the saturation magnetization, M0, the Bloch constant, B, the local magnetic anisotropy field, Ha, the ferromagnetic resonance linewidth, ΔH, the coercivity, Hc, were investigated. A comparison between the magnetic properties of the Co-P alloys prepared by dynamic compaction and a plasma spray deposition techniques was carried out.

Nanocrystalline structure of the surface layer of plasma-sprayed hydroxyapatite coatings obtained upon preliminary induction heat treatment of metal base

Biocompatible nanocrystalline hydroxyapatite (HA) coatings for intrabone titanium implants have been obtained by plasma spraying. The HA coatings have an average grain size within 10–30 nm and are characterized by improved characteristics of morphological heterogeneity that is acquired due to the induction heat treatment (IHT) of substrates prior to plasma spray deposition. Based on the data of scanning electron microscopy with computer-aided processing of images, it is established that the average grain size depends on the IHT temperature.

Ti-49.5Ni (at%)합금의 다공성 구조가 뼈 세포 흡착에 미치는 영향

Ti-Ni alloys are widely used in numerous biomedical applications (e.g., orthodontics, cardiovascular science, orthopaedics) due to their distinctive thermomechanical and mechanical properties, such as the shape memory effect, superelasticity and low elastic modulus. In order to increase the biocompatibility of Ti-Ni alloys, many surface modification techniques, such as the sol-gel technique, plasma immersion ion implantation (PIII), laser surface melting, plasma spraying, and chemical vapor deposition, have been employed. In this study, a Ti-49.5Ni (at%) alloy was electrochemically etched in 1M <TEX>$H_2SO_4$</TEX>+ X (1.5, 2.0, 2.5) wt% HF electrolytes to modify the surface morphology. The morphology, element distribution, crystal structure, roughness and energy of the surface were investigated by scanning electron microscopy (SEM), energy-dispersive Xray spectrometry (EDS), X-ray diffractometry (XRD), atomic force microscopy (AFM) and contact angle analysis. Micro-sized pores were formed on the Ti-49.5Ni (at%) alloy surface by electrochemical etching with 1M <TEX>$H_2SO_4$</TEX>+ X (1.5, 2.0, 2.5) wt% HF. The volume fractions of the pores were increased by increasing the concentration of the HF electrolytes. Depending on the HF concentration, different pore sizes, heights, surface roughness levels, and surface energy levels were obtained. To investigate the osteoblast adhesion of the electrochemically etched Ti-49.5Ni (at%) alloy, a MTT test was performed. The degree of osteoblast adhesion was increased at a high concentration of HF-treated surface structures.

Characteristics of Ceramic Coatings Made by Thin Film Low Pressure Plasma Spraying (LPPS-TF)

The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150 kW) and very low pressure (~100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO2, Al2O3 or MgAl2O4, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications.

The Effect of Laser Glazing on High Temperature Oxidation Resistance of Thermal Barrier Coatings

Thermal barrier coatings (TBCs) have been widely used by aero and land based gas turbines to protect hot section parts from oxidation and thermal loads. These coatings are generally consisting of multiple layers of coating (usually two) with each layer having a specific function. TBCs are generally deposited with air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD) techniques. In this paper plasma sprayed TBCs were deposited on to 304 stainless steel substrates then ceramic surfaces were glazing with Nd-YAG laser. Metallographic examinations were applied to the samples to investigate microstructural changes in glazed ceramic layer. Both glazed and as-coated samples were subjected to oxidation tests to measure the high temperature oxidation resistance. The tests showed that, laser glazing is beneficial to oxidation resistance of TBCs. This improvement is attributed to sintering of zirconia layer which act as oxygen barrier and formed during glazing process.

Effect of microstructure of interface between MCrAlY coating and substrate on damping property

The damping property of MCrAlY coating prepared by air plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD) was studied with dynamic mechanical thermal analyzer. The damping property of interface microstructure of MCrAlY coating was discussed, and the ANSYS finite element analysis method was used to analyze the σx and σy of interface structure of MCrAlY coating. The damping energy dissipation of “horizontal” interface structure and “vertical” interface structure was researched. The results have shown that, the damping of “vertical” interface structure was slightly higher than the damping of “horizontal” interface structure in the same bending vibration loading environment. The mechanical energy dissipation produced by APS coating structure and EB-PVD coating structure may be different. The slipping of interface between coating and substrate was dominated in the interface damping of APS coating, while in the interface damping of EB-PVD coating structure the both interface slip and crack extension contributed. Reasonable design “vertical” interface structural may be able to help mechanical energy dissipation in the bending vibration environment and improve damping performance.

Direct synthesis of nanostructured V 2O 5 films using solution plasma spray approach for lithium battery applications

We demonstrate for the first time, the synthesis of vanadium pentoxide (V 2O 5) nanoparticles and nanorods in the films using a high throughput solution plasma spray deposition approach. The scalable plasma spray method enables the direct deposition of large area nanostructured films of V 2O 5 with controllable particle size and morphology. In this approach, the solution precursors (vanadium oxychloride and ammonium metavanadate) were injected externally into the plasma jet, which atomizes and pyrolyzes the precursors in-flight, resulting in the desired films on the current collectors. The microstructure analysis of the as synthesized films revealed pure nanocrystalline phase for V 2O 5 with particles in the size range of 20–50 nm. The V 2O 5 film based electrodes showed stable reversible discharge capacity in the range of 200–250 mAh g −1 when cycled in the voltage window 2–4 V. We further discuss the mechanism for controlling the particle growth and morphology, and also the optimization of reversible lithium storage capacity. The nanorods of V 2O 5 formed after the anneal treatment also show reversible storage capacity indicative of the potential use of such film based electrodes for energy storage.

Numerical study of semi-molten droplet impingement

Due to the low thermal conductivity of ceramics large temperature gradients are present through the powder particles during plasma spray deposition. As a result the particles often impinge at the substrate in a semi-molten form; which in turn substantially affects the final characteristics of the coating being formed. This study is dedicated to the novel modelling development and simulation of a semi-molten droplet impingement. The study examines the impingement process during impact, spreading and solidification of semi-molten zirconia. The simulation provides an insight to the heat transfer process during impact and solidification of a semi-molten powder particle and illustrates the freezing-induced break-up mechanism at the splat periphery.

Modelling Sequential Impact of Molten Droplets on a Solid Surface in Plasma Spray Process

Plasma spray deposition is one of the most important technologies available for producing the high-performance surfaces required by modern industry. In this process, powder of the coating material is fed into high-temperature plasma, which melts and accelerates the powder; the molten particles subsequently hit and solidify on the surface to be coated. To obtain good quality coating, the powder particle must be at least partially molten and hit the substrate with a high velocity. The flattening characteristics of the droplets impinging on a substrate are important determinants in governing the eventual quality of the plasma spray coating. Different codes have been developed in recent years to simulate the overall thermal spraying process, as well as the growth of the 3D coatings, in which entrained particles are modeled by stochastic particle models, fully coupled to the plasma flow. The present investigation was carried out to have an approach to systematize the atmospheric plasma spraying process in order to create a basis for numerically modeling the plasma dynamics, the coating formation mechanisms and to predict the particle thermo- kinetic state at impact.

Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H 2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al 2O 3 ceramic and stainless steel substrates produced at different Ar and H 2 gas flow rates. The Ar and H 2 gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H 2 gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.