S. Dosta

Cold spray as an emerging technology for biocompatible and antibacterial coatings: state of art

The use of coatings in biomaterials has been fundamental on the applicability of many medical devices and has helped improve mechanical properties such as wear and fatigue and biological properties such as biocompatibility and bioactivity of implant prosthesis, thus, in essence, ameliorating human quality life. The aim of the present paper is to give a review on cold spray (CS) coating systems that are emerging in orthopedics industry (internal fixation systems and prosthesis) as well as those for antibacterial purposes (in body and touch external surfaces). These studies are very new, the oldest dating from the half of last decade and most deal with the improvement of biocompatibility and bioactivity of hard tissue replacement; therefore, research on biocoatings is in constant development with the aim to produce implant surfaces that provide a balance between cell adhesion and low cytotoxicity, mechanical properties, and functionalization. CS offers many advantages over conventional high-temperature processes and seems to be able to become competitive in front of the low-temperature techniques. It is mainly cost effective, appropriate for oxygen-sensitive materials, and environmentally green. It basically involves the use of feedstock material in powder form, which is supersonically sprayed onto the appropriate substrate but without any melting as it occurs in conventional thermal spray processes. Biocompatible metallic materials and polymers have been successfully deposited by this method because it is based on the plasticity of the coating material; pure ceramic deposits, for example of hydroxyapatite, are still a challenge.

Single Impact Bonding of Cold Sprayed Ti-6Al-4V Powders on Different Substrates

The objective of this study is to investigate particle deformation and bonding over a wide range of impact conditions, cold spraying of spherical Ti-6Al-4V powders was carried out with nitrogen as process gas onto titanium grade II, Ti-6Al-4V, and aluminum and magnesium alloy substrates. The bond strength was assessed by applying a modified cavitation test. The corresponding impact morphologies and fracture morphologies, as obtained after cavitation testing, were studied by scanning electron microscopy. The investigations demonstrate that bonding and adhesion of spherical Ti-6Al-4V particles depend strongly on substrate properties. Higher bond strength is achieved for material combinations that show lower differences in properties (density, hardness, thermal and mechanical properties), e.g., in the case of Ti-6Al-4V on Ti and Ti-6Al-4V substrates. This is attributed to more pronounced formation of adiabatic shear instabilities for more compatible material combinations.

Optimisation of HVOF thermal spray coatings for their implementation as MSWI superheater protectors

Abstract The aim of this paper is to the study high velocity oxygen fuel thermal spray coatings as superheater tubes protectors against degradation problems in municipal solid waste incinerators. Ni based and Fe based high velocity oxygen fuel coatings have been sprayed through optimised spray parameters with the help of online monitoring technology. All these coatings have been tested under laboratory conditions simulating the boiler erosion and corrosion mechanisms. The influence of the spray parameters on the porosity and oxidation has been studied in order to optimise the coatings structure. Interconnected porosity and spray oxidation must be avoided to attain the best coating resistance under corrosion and erosion mechanisms identified on the municipal solid waste incinerators. Tribological and electrochemical coating properties have been determined under standardised tests. Different erosion tests have been carried out in order to determine coatings resistance. X-ray diffraction studies show the main formed phases under corrosion mechanisms for the different studied alloys. The Ni based Inconel 625 coatings have been reported as a good superheater tubes protector for its industrial application.

Enhancing the bioactivity of polymeric implants by means of cold gas spray coatings

Nanostructured anatase coatings were built-up on biocompatible polyetheretherketone (PEEK) by means of cold gas spray (CGS). Titanium layer was previously desposited, which acted as bond coat between PEEK and metal oxide. Semicrystalline polymer was not degraded during the spraying process and starting composition of titanium dioxide was not affected. TiO₂ was homogeneously obtained onto CGS Ti layer and completely covered the piece. Primary human osteoblasts were seeded onto biomaterials and in vitro cell experiments provided evidence to confirm that nanostructured anatase coatings deposited by cold gas spray improve the performance of PEEK implants.

Photocatalytic activity of nanostructured anatase coatings obtained by cold gas spray

This article describes a photocatalytic nanostructured anatase coating deposited by cold gas spray (CGS) supported on titanium sub-oxide (TiO2−x) coatings obtained by atmospheric plasma spray (APS) onto stainless steel cylinders. The photocatalytic coating was homogeneous and preserved the composition and nanostructure of the starting powder. The inner titanium sub-oxide coating favored the deposition of anatase particles in the solid state. Agglomerated nano-TiO2 particles fragmented when impacting onto the hard surface of the APS TiO2−x bond coat. The rough surface provided by APS provided an ideal scenario for entrapping the nanostructured particles, which may be adhered onto the bond coat due to chemical bonding; a possible bonding mechanism is described. Photocatalytic experiments showed that CGS nano-TiO2 coating was active for photodegrading phenol and formic acid under aqueous conditions. The results were similar to the performance obtained by competitor technologies and materials such as dip-coating P25® photocatalysts. Disparity in the final performance of the photoactive materials may have been caused by differences in grain size and the crystalline composition of titanium dioxide.

Comparison of the Mechanical and Electrochemical Properties of WC-25Co Coatings Obtained by High Velocity Oxy-Fuel and Cold Gas Spraying

AbstractCold gas spray (CGS) coatings were previously produced by spraying WC-25Co cermet powders onto Al7075-T6 and low-carbon steel substrates. Unlike conventional flame spray techniques (e.g., high-velocity oxy-fuel; HVOF), no melting of the powder occurs; the particles are deformed and bond together after being sprayed by a supersonic jet of compressed gas, thereby building up several layers and forming a coating. WC-Co cermets are used in wear-resistant parts, because of their combination of mechanical, physical, and chemical properties. XRD tests were previously run on the initial powder and the coatings to determine possible phase changes during spraying. The bonding strength of the coatings was measured by adhesion tests. Here, WC-25Co coatings were also deposited on the same substrates by HVOF spraying. The wear resistance and fracture toughness of the coatings obtained previously by CGS and the HVOF coatings obtained here were studied. Their corrosion resistance was determined by electrochemical measurements. It was possible to achieve thick, dense, and hard CGS coatings on Al7075-T6 and low-carbon steel substrates, with better or the same mechanical and electrochemical properties as those of the HVOF coatings; making the former a highly competitive method for producing WC-25Co coatings.

Improved bonding strength of bioactive cermet Cold Gas Spray coatings

The fabrication of cermet biocompatible coatings by means Cold Gas Spray (CGS) provides prosthesis with outstanding mechanical properties and the required composition for enhancing the bioactivity of prosthetic materials. In this study, hydroxyapatite/Titanium coatings were deposited by means of CGS technology onto titanium alloy substrates with the aim of building-up well-bonded homogeneous coatings. Powders were blended in different percentages and sprayed; as long as the amount of hydroxyapatite in the feedstock increased, the quality of the coating was reduced. Besides, the relation between the particle size distribution of ceramic and metallic particles is of significant consideration. Plastic deformation of titanium particles at the impact eased the anchoring of hard hydroxyapatite particles present at the top surface of the coating, which assures the looked-for interaction with the cells. Coatings were immersed in Hanks solution for 1, 4 and 7 days; bonding strength value was above 60 MPa even after 7 days, which enhances common results of HAp coatings obtained by conventional thermal spray technologies.

Nanostructured Cermet Coatings with Enhanced Properties Produced by HVOF Thermal Spray

There is an increasing interest in the last years for materials with nanometric grain size because of the enhanced properties that could be achieved when reducing to the nanometric scale. Three coatings using conventional, nanostructured and bimodal (mixture of conventional and nano) WC-Co powders were obtained through High Velocity Oxygen-Fuel (HVOF) technique. The powders were sprayed under different spraying conditions in order to improve trybological properties for nanostructured powders. The relationship between spraying conditions and decomposition has been studied using in-flight measurements and XRD techniques. The nanostructured coating showed more decomposition than the other ones during spraying process, but this decomposition was reduced through the new conditions. Nanostructured coating showed more hardness using producer conditions, but the bimodal coating showed better abrasive and friction wear resistance. Nanostructured and bimodal coatings also provided an enhanced corrosion protection to the substrate when compared with the conventional one. An important improvement in the abrasive wear resistance for the nanostructured coating was obtained with the new conditions, due to the reduction in the decomposition.

Hierarchical structures of anodised cold gas sprayed titanium coatings

ABSTRACT Cold gas spray (CGS) titanium coatings have been produced to obtain porous and rough coatings with enhanced mechanical performance. The coatings from optimal spraying conditions reached tensile strength values up to 40 MPa, shear strength up to 39 MPa and a loss mass of 37 mg/100 cycles in abrasive testing, values in accordance with the ASTM standards to be applied for orthopaedic joint prostheses. An innovative hierarchical structure (micro-nano) consisted of a TiO2 nanotubes top layer obtained by anodisation onto a CGS Ti coating. The present paper focuses on the characterisation of both surfaces, as-sprayed CGS Ti layer and double Ti-TiO2 layer, in terms of mechanical properties, surface topography and wettability (contact angle). There were not significant changes in micro-roughness, Ra∼40 µm and Ra∼30 µm, but a significant decrease in contact angle, from ≈26° up to 0°, was observed between these two structures. This behaviour indicates that the combination of the CGS + anodising results in promising high roughness superhydrophilic surfaces, ideal for biomedical applications.

Osteoblastic cell response on high-rough titanium coatings by cold spray

AbstractHighly rough and porous commercially pure titanium coatings have been directly produced for first time by the cold spray technology, which is a promising technology in front of the vacuum plasma spray for oxygen sensitive materials. The wettability properties as well as the biocompatibility evaluation have been compared to a simply sand blasted Ti6Al4V alloy substrate. Surface topographies were analysed using confocal microscopy. Next, osteoblast morphology (Phalloidin staining), proliferation (MTS assay), and differentiation (alkaline phosphatase activity) were examined along 1, 7 and 14 days of cell culture on the different surfaces. Finally, mineralization by alizarin red staining was quantified at 28 days of cell culture. The contact angle values showed an increased hydrophilic behaviour on the as-sprayed surface with a good correlation to the biological response. A higher cell viability, proliferation and differentiation were obtained for highly rough commercial pure titanium coatings in comparison with sand blasted substrates. Cell morphology was similar in all coatings tested; at 14 days both samples showed extended filopodia. A higher amount of calcium-rich deposits was detected on highly rough surfaces. In summary, in-vitro results showed an increase of biological properties when surface roughness increases.