High Velocity Suspension Flame Spraying Research Articles

Copper-enriched hydroxyapatite coatings obtained by high-velocity suspension flame spraying. Effect of various gas parameters on biocompatibility

Hydroxyapatite (HAp)-coated bone implants are frequently used for orthopaedic or dental implants since they offer high biocompatibility and osteoconductivity. Yet, problems such as infections, e.g. periprosthetic joint infections, occur when implanting foreign material into the body. In this study, HAp coatings were produced via high-velocity suspension flame spraying (HVSFS). This method allows for the production of thin coatings. We investigated the effects of different gas parameters on the coating properties and on the biocompatibility, which was tested on the human osteosarcoma cell line MG63. Furthermore, Copper (Cu) was added to achieve antibacterial properties which were evaluated against standard microorganisms using the airborne assay. Three gas parameter groups (low, medium, and high) with different Cu additions (0 wt.%, 1 wt.% and 1.5 wt.%) were evaluated. Our findings show that porosity as well as hardness can be controlled through gas parameters. Furthermore, we showed that it is possible to add Cu through external injection. The Cu content in the coating as well as the release varies with different gas parameters. Both antibacterial efficacy as well as biocompatibility are affected by the Cu content. We could significantly reduce the amount of colony-forming units (CFU) in all coatings for E. coli, CFU for S. aureus was reduced by adding 1.5 wt.% of Cu to the coating. The biocompatibility testing showed a cytotoxicity threshold at a Cu-release of 14.3 mg/L in 120 hours. Based on our findings, we suggest medium gas parameters for HVSFS and the addition of 1 wt.% Cu to the coating. With these parameters, a reasonable antibacterial effect can be achieved while maintaining sufficient biocompatibility.Graphical

Suspension-Sprayed Calcium Phosphate Coatings with Antibacterial Properties.

Prosthesis loosening due to lack of osteointegration between an implant and surrounding bone tissue is one of the most common causes of implant failure. Further, bacterial contamination and biofilm formation onto implants represent a serious complication after surgery. The enhancement of osteointegration can be achieved by using bioconductive materials that promote biological responses in the body, stimulating bone growth and thus bonding to tissue. Through the incorporation of antibacterial substances in bioconductive, biodegradable calcium phosphate (CaP) coatings, faster osteointegration and bactericidal properties can be achieved. In this study, Cu-doped CaP supraparticles are spray-dried and suspension-sprayed CaP ceramic coatings with antibacterial properties are prepared using high-velocity suspension flame spraying (HVSFS). The objective was to increase the coatings' porosity and investigate which Cu-doped supraparticles have the strongest antibacterial properties when introduced into the coating layers. Biocompatibility was tested on human Osteosarcoma cells MG63. A porosity of at least 13% was achieved and the supraparticles could be implemented, enhancing it up to 16%. The results showed that the addition of Cu-doped supraparticles did not significantly reduce the number of viable cells compared to the Cu-free sample, demonstrating good biocompatibility. The antimicrobial activity was assessed against the bacterial strains Escherichia coli and Staphylococcus aureus, with Safe Airborne Antibacterial testing showing a significant reduction in both Gram-positive and Gram-negative strains on the Cu-doped coatings.

Microstructure and Corrosion Behavior of Zinc/Hydroxyapatite Multi-Layer Coating Prepared by Combining Cold Spraying and High-Velocity Suspension Flame Spraying.

The study aims to enhance the corrosion resistance and bioactivity of Mg alloy substrates through the development of a zinc/hydroxyapatite multi-layer (Zn/HA-ML) coating. The Zn/HA-ML coating was prepared by depositing a cold-sprayed (CS) Zn underlayer and a high-velocity suspension flame sprayed (HVSFS) Zn/HA multi-layer and was compared with the CS Zn coating and the Zn/HA dual-layer (Zn/HA-DL) coating. Phase, microstructure, and bonding strength were examined, respectively, by X-ray diffraction, scanning electron microscopy, and tensile bonding testing. Corrosion behavior and bioactivity were investigated using potentiodynamic polarization, electrochemical impedance spectroscopy, and immersion testing. Results show that the HVSFS Zn/HA composite layers were mainly composed of Zn, HA, and ZnO and were well bonded to the substrate. The HVSFS HA upper layer on the CS Zn underlayer in the Zn/HA-DL coating exhibited microcracks due to their mismatched thermal expansion coefficient (CTE). The Zn/HA-ML coating exhibited good bonding within different layers and showed a higher bonding strength of 27.3 ± 2.3 MPa than the Zn/HA-DL coating of 20.4 ± 2.7 MPa. The CS Zn coating, Zn/HA-DL coating, and Zn/HA-ML coating decreased the corrosion current density of the Mg alloy substrate by around two-fourfold from 3.12 ± 0.75 mA/cm2 to 1.41 ± 0.82mA/cm2, 1.06 ± 0.31 mA/cm2, and 0.88 ± 0.27 mA/cm2, respectively. The Zn/HA-ML coating showed a sixfold decrease in the corrosion current density and more improvements in the corrosion resistance by twofold after an immersion time of 14 days, which was mainly attributed to newly formed apatite and corrosion by-products of Zn particles. The Zn/HA-ML coating effectively combined the advantages of the corrosion resistance of CS Zn underlayer and the bioactivity of HVSFS Zn/HA multi-layers, which proposed a low-temperature strategy for improving corrosion resistance and bioactivity for implant metals.

Deposition of Fluoresceine-Doped HAp Coatings via High-Velocity Suspension Flame Spraying

In current medicine, joint revision surgery plays an important role in the treatment of degenerative joint diseases. Infections of the artificial joints are an iatrogenic, accompanying symptom after joint replacement procedures. A new approach is to functionalize the bioactive coatings of the implants by infiltrating them with anti-inflammatory drugs. This work aims at a one-step approach in manufacturing drug-doped, porous hydroxyapatite coatings by high-velocity suspension flame spraying (HVSFS). Thermal exposure of the temperature-sensitive drugs is critical. Therefore, a new process setup, combining one axial and one radial suspension line, is used for coating deposition. The lower dwell time of the suspension in the flame through radial injection helps in controlling the temperature exposition of the contained drug to the flame. Additionally, the influence of the powder morphology, used as a carrier for the drug in the suspension, is investigated by carrying out spray experiments with suspensions from three different granule types. To analyze the temperature exposition within the coating process, fluoresceine was used as a model drug, as the decomposition of the organic, fluorescing molecules can be easily controlled by fluorescence intensity measurements. It could be shown that the deposition of temperature-sensitive organic molecules is possible without degrading the molecular structure by a modified HVSFS process. This knowledge offers new possibilities in the cost-effective one-step manufacturing of functionalized, anti-inflammatory bioceramic coatings on orthopaedic implants.

High velocity suspension flame spraying of AlN/Al2O3 composite coatings

In applications such as electronics and optoelectronics, where high heat exchange performance is required, the thermal conductivity of aluminium oxide is not always sufficient. To enhance the thermal conductivity, direct spraying of AlN using a supersonic suspension flame spray (HVSFS) process was investigated. Aqueous suspensions containing AlN powders, or mixtures of AlN and Al2O3 were used as a feedstock. Different combinations of axial and radial injections into the HVOF torch have been investigated and results are compared. The coating morphology and phase composition were analysed using optical microscope, SEM, WDS and X-Ray diffraction. Mechanical properties like microhardness and surface roughness have been measured. The aim is to create coatings with a high AlN content and thus higher thermal conductivity. Having these properties, this type of coating could be of excellent use in numerous electronic applications.The results show the highest AlN content in the coatings with axially injected pure AlN suspension. Moreover, the thermal conductivity values clearly correlate with the AlN content in the coating.

Evaluation of Ti/Al alloy coated with biogenic hydroxyapatite as an implant device in dogs\u2019 femur bones

The main target of the present research was a full assessment of the toxicity effects and biocompatibility of a Ti/Al-alloy device coated with biogenic hydroxyapatite (bHA) when implanted in dogs in comparison with those of an uncoated Ti/Al-alloy device. The coating of the alloy was carried out using controlled high-velocity suspension flame spray (HVSFS) technique. Both coated and uncoated devices were implanted in dogs’ femur bones for different time periods (45 days and 90 days). Bone-formation ability and healing were followed up, and blood analysis was performed, at Time zero (immediately post surgery), and then at 3 days, 45 days, and 90 days post surgery. Bone mineral density checks, radiological scans of the femur bone, and histological analysis were also conducted. The in-vivo study results proved that implantation of a device made from bHA-coated Ti/Al alloy in dogs’ femur bones is completely safe. This is due to the high osteoconductivity of the coated alloy, which enables the formation of new bone and a full connection between new and original bone material. At 90 days post surgery, the coated alloy had been completely digested within the original bone; thus, it appeared as a part of the femur bone and not as a foreign body. Both the scanning electron microscopy with energy-dispersive X-ray and histology analysis findings affirmed the results. Furthermore, the blood tests indicated no toxicity effects during the 90 days of implantation.

Microstructure and Corrosion Properties of Biodegradable Mg/MgO Composite Coating on Mg Alloy Prepared by High Velocity Suspension Flame Spraying

Microstructure and Corrosion Properties of Biodegradable Mg/MgO Composite Coating on Mg Alloy Prepared by High Velocity Suspension Flame Spraying

Different Coating Methods of Titanium Dioxide on Metal Substrates for Orthopedic and Dental Applications: A Review

The outstanding physico-chemical characteristics to assist bone regeneration and cell development, titanium dioxide (TiO2) based materials have showed significant promise for applications in implants. Due to its excellent performance in a wide variety of applications, chemical stability, and inexpensive cost, this metal oxide has received the more attention. Coating techniques for creating surfaces made of this substance have been thoroughly investigated. The aim of this review article is to look at the current status of TiO2 technology for orthopedic and dental implants. Over the years, researchers have investigated several TiO2 coating deposition techniques on metal implants, with the goal of improving adhesion strength and long-term dependability. This review examines a variety of TiO2 deposition techniques on metal substrates in depth. Anodization, sol-gel method, plasma spray coating, cold spray coating, high velocity oxy-fuel spray, high velocity suspension flame spraying, pulsed laser deposition (PLD), ion beam deposition (IBD), magnetron sputtering deposition, electrophoretic deposition (EPD), electrochemical deposition and biomimetic deposition are among the methods examined.

Investigation of the sedimentation behaviour of micron-sized particles in aqueous suspension for high velocity suspension flame spraying

Suspension for thermal spraying has to exhibit long-term storage stability and safe handling for becoming established in industry. Especially micron-sized particles are affected by sedimentation and have to be dispersed before use. A possible way to prevent the settlement is to increase suspension viscosity. This work investigates the effect of a rheological additive to ensure storage stability of a 30 wt% α-alumina suspension. The additive changes the flow curve from newtonian to shear thinning and a frequency sweep assesses the sedimentation behaviour. The suspension was sprayed with a High Velocity Suspension Flame Spraying (HVSFS) setup resulting in a finely structured coating. The microstructure, hardness and phase compositions were analysed. The coating shows low porosity, high hardness and a high amount of γ-alumina.

High Velocity Suspension Flame Spraying (HVSFS) of Metal Suspensions.

Thermal spraying of metal materials is one of the key applications of this technology in industry for over a hundred years. The variety of metal-based feedstocks (powders and wires) used for thermal spray is incredibly large and utilization covers abrasion and corrosion protection, as well as tribological and electrical applications. Spraying metals using suspension- or precursor-based thermal spray methods is a relatively new and unusual approach. This publication deals with three metal types, a NiCr 80/20, copper (Cu), and silver (Ag), sprayed as fine-grained powders dispersed in aqueous solvent. Suspensions were sprayed by means of high-velocity suspension spraying (HVSFS) employing a modified TopGun system. The aim was to prepare thin and dense metal coatings (10–70 µm) and to evaluate the process limits regarding the oxygen content of the coatings. In case of Cu and Ag, possible applications demand high purity with low oxidation of the coating to achieve for instance a high electrical conductivity or catalytic activity. For NiCr however, it was found that coatings with a fine dispersion of oxides can be usable for applications where a tunable resistivity is in demand. The paper describes the suspension preparation and presents results of spray experiments performed on metal substrates. Results are evaluated with respect to the phase composition and the achieved coating morphology. It turns out that the oxidation content and spray efficiency is strongly controlled by the oxygen fuel ratio and spray distance.

Thin Degradable Coatings for Optimization of Osteointegration Associated with Simultaneous Infection Prophylaxis.

One of the most common causes of implant failure is aseptic prosthesis loosening. Another frequent complication after prosthesis implant is the microbial colonization of the prosthesis surface, which often leads to a replacement of the prosthesis. One approach to reduce these complications is the application of bioactive substances to implant surfaces. Both an antibiotic prophylaxis and a faster osteointegration can be obtained by incorporation of bactericidal active metals in degradable calcium phosphate (CaP) coatings. In this study, thin degradable calcium phosphate ceramic coatings doped with silver (Ag), copper (Cu), and bismuth (Bi) on a titanium substrate were prepared with the aid of the high-velocity suspension flame spraying (HVSFS) coating process. To characterize the samples surface roughness, brightfield microscopy of the coatings, X-ray diffraction (XRD)-analysis for definition of the phase composition of the layers, Raman spectroscopy for determination of the phase composition of the contained metals, element-mapping for Cu-content verification, release kinetics for detection of metal ions and ceramic components of the coatings were carried out. The aim of this study was to evaluate in vitro biocompatibility and antimicrobial activity of the coatings. For biocompatibility testing, growth experiments were performed using the cell culture line MG-63. Cell viability was investigated by Giemsa staining and live/dead assay. The WST-1 kit was used to quantify cell proliferation and vitality in vitro and the lactate dehydrogenase (LDH) kit to quantify cytotoxicity. The formation of hydroxyapatite crystals in simulated body fluid was investigated to predict bioactivity in vivo. The Safe Airborne Antibacterial Assay with Staphylococcus aureus (S. aureus) was used for antimicrobial testing. The results showed good biocompatibility of all the metal doped CaP coatings, furthermore Cu and Ag doped layers showed significant antibacterial effects against S. aureus.

A Review on Suspension Thermal Spray Patented Technology Evolution

Since the introduction of suspension thermal spray (STS) in 1997, the rapid evolution of this technology has resulted in a critical number of inventions justifying a comprehensive review. Today STS is one of the most important research and development topics in thermal spray. Due to the efforts of different industries, universities, and governmental institutions, STS has reached a certain maturity for implementation in different applications. STS includes any thermal spray process that is based on injection of a suspension of solid particles in a liquid carrier into the gas jet, such as suspension plasma spray, suspension high-velocity oxy-fuel, and high-velocity suspension flame spray. This review is on the state of the art of the STS technology and includes an overview of the current patent situation including the number of patents published in English language, patenting institutions and invention domains. Apparatus and methods, feedstock, and new material systems and applications were identified as the three main domains of focus for the STS inventions. The presented patents show a general perspective of the current situation and present the technology advancements in each domain. It also shows the potential of implementing this technology in new applications based on the needs of thermal spray market through further technology development.

Microstructure and Corrosion Behavior of Thermal-Sprayed Hydroxyapatite/Magnesium Composite Coating on the Surface of AZ91D Magnesium Alloy

To slow down the corrosion rate and increase the bioactivity of magnesium alloys, biodegradable hydroxyapatite/magnesium (HA/Mg) composite coating was deposited onto AZ91D alloy substrate by high-velocity suspension flame spray technique using nano-sized HA and micron-sized Mg powders. Microstructures and phase constitutions of HA/Mg composite coating were analyzed by scanning electron microscopy and x-ray diffraction. Corrosion resistance of composite coating was also evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy in Hanks’ balanced salt solution (HBSS). The result shows that the HA/Mg composite coating mainly consisted of Mg and HA in addition to a few of MgO phases. The coating showed a rough surface morphology with homogeneous elemental distribution and a lamellar structure in cross section as well as well-bonded coating/substrate interface. Mg particles presented superior deposition during spraying due to their higher melting degree compared to HA particles. AZ91D alloy substrate was significantly protected from corrosion in HBSS by the HA/Mg composite coating with Mg dissolution on surfaces of the composite coating. This strategy can offer a new route to fabricate HA/Mg composite coatings on other metal substrates for use in biodegradable implant fields.

Suspension and coating characterization of high velocity suspension flame sprayed (HVSFS) mixed titanium oxide–titanium carbide coatings

Three different suspensions of pure titanium carbide were thermally sprayed through high velocity suspension flame spraying (HVSFS) in order to deposit mixed titanium oxide–titanium carbide coatings. One suspension was prepared by using milled feedstock powder (TC1), while two suspensions were prepared using non-milled carbide powder dispersed in pure deionized water (TC2) and deionized water with additives (TC3), respectively. As reference, a pure titanium oxide HVSFS coating was investigated, which was deposited from a commercial aqueous suspension W 740 X. Each suspension was set to a solid content of 20 wt% and showed shear thinning behavior. Through the use of pure titanium carbide suspensions, it was possible to achieve a two-phase titanium oxide–titanium carbide coating with significant oxidation of the carbide phase. In the composite coatings, the retained titanium carbide particles with their higher Young's modulus act as reinforcement of the titanium oxide matrix (anatase and rutile phases) and therefore raised the microhardness up to 1014 HV0.05 compared with 527 HV0.05 of the pure titanium oxide coating, composed of anatase and rutile phases. The lowest porosity and the highest hardness values among the mixed coatings were achieved by the use of suspension TC1, milled powder with a median particle size of 2.66 μm. Each composite coating showed high surface roughness with Ra values over 15 μm. The addition of an organic additive to the suspension and the modification of its pH value by ammonia water resulted in coatings with homogeneously distributed titanium carbide particles. By comparison, the pure titanium carbide coating exhibited the lowest porosity beneath 1% and a significantly lower surface roughness than the mixed oxide–carbide coatings.

Comparative study of HA/TiO2 and HA/ZrO2 composite coatings deposited by high-velocity suspension flame spray (HVSFS)

To compare with enhancing effects of TiO2 and ZrO2 reinforcement particles in hydroxyapatite (HA) coatings, HA/TiO2 and HA/ZrO2 composite coatings have been deposited on 316 L stainless steel substrates by high-velocity suspension flame spray (HVSFS) technique with the addition contents of 10 wt% and 30 wt% for both TiO2 and ZrO2. Phase structures and compositions of all coatings were analyzed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). Surface morphologies and microstructures were characterized by scanning electron spectroscopy (SEM). Vickers hardness, Young's modulus, tensile strength, wear rate and corrosion resistance were also measured and compared for both HA/TiO2 and HA/ZrO2 composite coatings. Results show that TiO2 particles induced thermal decompositions of HA in HA/TiO2 composite coatings rather than ZrO2 in HA/ZrO2 composite coatings. Both the HA/TiO2 and HA/ZrO2 composite coatings exhibited rough surfaces with spherical/flattened splats and nanoparticle agglomerates as well as bonded coating/substrate interfaces. Although mechanical properties of all composite coatings were improved with increasing addition contents of reinforcements, HA/TiO2 composite coatings showed slightly higher mechanical properties than HA/ZrO2 composite coatings at the same addition content due to higher melting degrees of HA/TiO2 powders. Both HA/TiO2 and HA/ZrO2 composite coatings exhibited comparable and increased corrosion-resist performances in Hanks' simulation body fluids compared to the bare substrate.

Deposition of chromium oxide-chromium carbide coatings via high velocity suspension flame spraying (HVSFS)

A chromium carbide containing suspension was thermally sprayed through high velocity suspension flame spraying (HVSFS). Therefore, an aqueous suspension with 20 wt% of pure Cr3C2 and an organic stabilizing additive with a pH of 9.6 was prepared. Due to oxidation of the carbide particles in the flame the deposited lamellar coating is composed of both chromium oxide and carbide, namely Cr2O3 phase and Cr7C3 phase as identified via XRD measurements. The HVSFS coatings are analysed regarding their microstructure, chemical composition, microhardness and surface roughness. The gas parameters, specifically the oxygen-to-ethene ratio and the total gas amount, determine the flame temperature and gas velocity and therefore control the microstructure and chemical composition. Substoichiometric gas compositions lead to higher deposition efficiencies. They create a reducing atmosphere that hinders the oxidation, whereas oxygen excess favours the oxidation of the chromium carbide particles. The higher the chromium oxide content, the higher is the coating's microhardness. When the surface temperature during the deposition process gets high enough, post-oxidation of the as-sprayed surface takes place and finally results in increased surface roughness and a layered microstructure with alternating chromium carbide and chromium oxide layers.

Tribological properties of high velocity suspension flame sprayed (HVSFS) ceramic coatings

Ceramic coatings were manufactured by high-velocity suspension flame spraying (HVSFS). Finely dispersed isopropanol-based suspensions with submicron- and nanoscaled Al2O3, 3-YSZ and TiO2 powders were used as feedstock material. Microhardness, surface, microstructure, and phase composition of the final coating were characterized. The tribological properties were investigated via ball-on-disc test in NaCl solution and compared to those of high velocity oxy liquid fuel (HVOLF) sprayed WC/Co coatings. The ceramic coatings exhibited material specific hardness. The high surface roughness of Al2O3, mainly composed of γ-phase, and TiO2, composed of anatase and rutile, could be explained by process inhomogeneities. While WC/Co was subjected to continuous wear due to abrasive processes and surface disruption, ceramic surfaces underwent a layer build-up, causing separation of the interacting surfaces and thus protection from increased wear. Considering tribochemical interactions between contacting materials and liquid medium, high chemical reactivity leads to high wear rates but moderate friction, whereas low reactivity results in low wear but high friction. Regarding potential application in pumps with SiC as standard material for sliding parts, 3-YSZ might be the most promising coating candidate due to moderate tribochemical interactions and therefore stable friction coefficients on a tolerable level and a similar wear rate compared with WC/Co.

Microstructures, mechanical properties and electrochemical behaviors of nano-structured HA/Ti composite coatings deposited by high-velocity suspension flame spray (HVSFS)

In order to improve the mechanical and electrochemical properties of hydroxyapatite (HA) coating, nanostructured HA/Ti (nHA/Ti) composite coatings with different Ti contents of 30, 50 and 70 wt% were deposited on 316L stainless steel (SS) substrates by high-velocity suspension flame spray (HVSFS) and compared to HVSFS-deposited nanostructured HA (nHA) coatings. Microstructures and phase compositions of as-sprayed coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The deposition mechanism of nanostructured HA/Ti composite coatings was also examined. Microhardness and Young's modulus were measured. Electrochemical corrosion behaviors of all coatings were tested in Hanks’ simulated body fluid using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the nHA/Ti composite coatings contained HA and Ti in addition to some Ti-O compounds. Compared to nHA coatings, nHA/Ti composite coatings presented dense and typical lamella structures formed mostly by flattened splats. Microhardness and Young's modulus of nHA/Ti composite coatings increased with increasing Ti content. Potentiodynamic polarization and EIS measurements illustrated that nHA/Ti composite coatings had excellent corrosion resistance compared to nHA coatings and bare SS substrate.

Improvement in mechanical properties of nano-structured HA/TiO2 multilayer coatings deposited by high velocity suspension flame spraying (HVSFS)

In order to improve mechanical properties of HA coatings, nano-structured HA/TiO2 multilayer coatings composed of TiO2 bond layer, HA/TiO2 intermediate layer with different TiO2 contents and HA top layer were prepared by a high velocity suspension flame spraying (HVSFS) method using nano-sized HA and TiO2 powders. Phase constitution, compositions and microstructures were characterized by XRD, XPS, FTIR and SEM. Mechanical properties of multilayer coatings were investigated in comparison with the pure HA coating. Results show that multilayer coatings were mainly composed of HA and TiO2 phases, and presented well-bonded coating/substrate interfaces and porous surfaces as well as graded chemical compositions in coating thickness. Microhardness and Young's modulus of HA/TiO2 intermediate layers were increased with the TiO2 contents due to its reinforcement, and HA/TiO2 multilayer coatings revealed gradual mechanical properties from the TiO2 bond layer to HA top layer. Tensile strengths of HA/TiO2 double-layer coatings were improved due to TiO2 reinforcement in comparison with the pure HA coating. Wear properties of HA/TiO2 multilayer coatings were tested by a pin-on-disc method and were improved due to the presence of intermediate layers resulting in lower wear rates than that of the pure HA coating.

High velocity suspension flame spraying (HVSFS) of metal doped bioceramic coatings

•Bioceramic coatings doped with metals for antibacterial effect.•Combination of suspension and precursor flame spraying.•Successful deposition of submicron metallic particles in bioceramic matrix.