Michail Vardavoulias

• PhD
• Managing Director at PyroGenesis SA

The hydrophilic character and the protection against pathogen proliferation are the most pivotal characteristics of leathers intended for medical purposes. To achieve these goals, dispersions of TiO2 particles incorporating three different formulations of biomimetically synthesized silica xerogels were tested. Emphasis has been given to the role of...

Four leather substrates from different animals were treated by dispersions containing hydrophilic composite silica-hyperbranched poly(ethylene imine) xerogels. Antimicrobial activity was introduced by incorporating silver nanoparticles and/or benzalkonium chloride. The gel precursor solutions were also infused before gelation to titanium oxide powd...

Four leather substrates from different animals were treated by dispersions containing hydrophilic composite silica-hyperbranched poly(ethylene imine) xerogels. Antimicrobial activity was introduced by incorporating silver nanoparticles and /or benzalkonium chloride. The gel precursor solutions were also infused before gelation to titanium oxide pow...

Two different silica conformations (xerogels and nanoparticles), both formed by the mediation of dendritic poly (ethylene imine), were tested at low pHs for problematic uranyl cation sorption. The effect of crucial factors, i.e., temperature, electrostatic forces, adsorbent composition, accessibility of the pollutant to the dendritic cavities, and...

Two different silica conformations (Xerogels and Nanoparticles) both formed by the mediation of dendritic poly (ethylene imine) were tested at low pHs on the problematic uranyl cation sorption. The effect of crucial factors i.e., temperature, electrostatic forces, adsorbent composition, accessibility of the pollutant to the dendritic cavities and M...

The capability of radially polymerized bio-dendrimers and hyperbranched polymers for medical applications is well established. Perhaps the most important implementations are those that involve interactions with the regenerative mechanisms of cells. In general, they are non-toxic or exhibit very low toxicity. Thus, they allow unhindered and, in many...

The capability of radially polymerized bio-dendrimers and hyperbranched polymers for medical applications is well established. Among them, perhaps the most important are those that involve interactions with the regenerative mechanisms of cells. Dendritic polymers due to their distinctive architecture may play a multitude of roles such as protein bi...

Hybrid organic-inorganic (dendritic polymer-silica) xerogels containing silver nanoparticles (Ag Nps) were developed as antibacterial leather coatings. The preparation method is environmentally friendly and is based on two biomimetic reactions. Silica gelation and spontaneous Ag Nps formation were both mediated by hyperbranched poly (ethylene imine...

Hybrid organic-inorganic (dendritic polymer-silica) xerogels containing silver nanoparticles (Ag-Nps) were developed as antibacterial leather coatings. The preparation method is environmentally friendly, based on two biomimetic reactions. Silica gelation and spontaneous (Ag-Np) formation were both mediated by hyperbranched poly (ethylene imine) (PE...

The use of bone implants and prostheses has contributed to a revolution in modern medicine; however, in the beginning, not much was asked from the implant and prosthetic materials per se. Therefore, the next game-changer in orthopedic research will come from new material designs which for instance can aid in prevention of implant-associated bone in...

An overview of the chemical compounds forming the rare smectic T phases is presented with references to the historical context. Thermodynamics (transition temperatures, enthalpies) along with the factors (stereochemical constraints, electrostatic interactions, aliphatic chain stacking, intermolecular forces) contributing to the adoption of tetragon...

Radially polymerized dendritic compounds are nowadays an established polymer category next to their linear, branched, and cross-linked counterparts. Their uncommon tree-like architecture is characterized by adjustable internal cavities and external groups. They are therefore exceptional absorbents and this attainment of high concentrations in their...

An overview of the chemical compounds forming the rare smectic T phases is cited with references to the historical context. Thermodynamics (transition temperatures, enthalpies) along with the factors (stereochemical constraints, electrostatic interactions, aliphatic chain stacking, intermolecular forces) contributing to the adoption of tetragonal s...

Radially polymerized dendritic compounds are nowadays an established polymer category next to their linear, branched and cross-linked counterparts. Their uncommon tree-like architecture is characterized by adjustable internal cavities and external groups. They are therefore exceptional absorbents and this attainment of high concentrations into thei...

Abrasive wear behaviour of WC-Co and Fe/Cu-based nanostructured coatings has been investigated under dry and slurry conditions in this study. The retention of nanostructured coating constituents was confirmed together with the formation of various metallic and ceramic phases. Abrasive wear rate of nanostructured WC-Co coating was one order lower th...

The objective of this paper is to manufacture free-standing solid oxide cells (SOCs) through the atmospheric plasma spray process (APS), without the aid of a metallic support nor the need for a post-process heating treatment. A five-layered cell was fabricated. Fused and crushed yttria-stabilized zirconia (YSZ) powder in the 5–22 μm particle size r...

Cr2O3 and TiO2 powders were deposited by atmospheric plasma spray (APS) on steel substrates. Microstructural analysis of the coatings showed typical lamellar structure with good coating quality. Fatigue strength was studied by using cyclic testing (measuring with an inhouse-built apparatus the strength of the coated systems under a wide range of im...

As the field of nanoscience is rapidly evolving, interest in novel, upgraded nanomaterials with combinatory features is also inevitably increasing. Hybrid composites, offer simple, budget-conscious and environmental-friendly solutions that can cater multiple needs at the same time and be applicable in many nanotechnology-related and interdisciplina...

Special Issue Information Dear Colleagues, We would like to invite you to contribute your technological insight and research to our Special Issue on Innovative Thermal Spray Applications in Energy, Environment and Medical Sectors. Thermal spraying is a well-established technology with a wide range of applications of functional interest. These app...

As the field of nanoscience is rapidly evolving, interest for novel, upgraded nanomaterials with combinatory features is also inevitably increasing. Hybrid composites, offer simple, budget-conscious and environmental-friendly solutions that can cater multiple needs at the same time and be applicable in many nanotechnology-related and interdisciplin...

The technique of thermal spraying has been proposed since several years ago, as an alternative to Cr electrodeposition, a process characterized by the need of post-deposition handling of a large amount of toxic slurry wastes. Chromium oxide and chromium carbide coatings, as well as Cr electrodeposits find applications, mainly, on the wear protectio...

The objective of this paper is to study and compare the structure and properties of thermal spray coatings prepared with conventional and nanostructured hydroxyapatite powder, in order to assess their suitability for future use in medical implants. Four coatings were prepared via Atmospheric Plasma Spray process (APS). Two kinds of feedstock materi...

The present study focuses on porous titanium (commercially pure Ti) coatings for future use on medical implants, manufactured by specific thermal spray method. Its objective is to investigate the possibility of Ti porosity to offer room for medication storage in addition to bone growth promotion. Three specimens for each different type of coating w...

The aim of the present work is to demonstrate processing of polycrystalline free-standing Si wafers and thin Si layers on various substrates using thermal spray of relevant Si powders, including recycled ones. Silicon powders were fabricated by milling from two different Si feedstocks: (i) Si kerf produced during sawing of Si wafers from the ingots...

Thin Si layers deposited on various substrates by thermal spray have been analysed by several analytical methods: SEM, Raman, resistivity and in some cases minority carriers life-time measurements. The high-purity silicon particles were prepared by milling and sieving techniques and characterized for their morphology and size distribution using the...

Stellite 6 powder (CoCrSiW) was deposited using flame spray (FS) and high velocity oxygen fuel (HVOF) techniques on steel substrates. The microstructure of stellite coatings produced with these deposition techniques displayed a rather similar morphology consisting of cobalt-rich dendrites surrounded by hard (Cr3C7) carbide particles. The stellite (...

This paper provides a comprehensive assessment of the tensile and sliding wear behaviour of WC-10Co4Cr agglomerated and sintered powder deposited on aluminum by Hyper Velocity Oxy-Fuel (HVOF) process. Microstructural analysis (SEM) identified grains of tungsten carbide (WC) in the metal matrix of the cobalt-chromium (Co-Cr). A transformation of the...

This paper studies the effect of a cermet HVOF WC-Co-Cr coating on the mechanical behaviour of steel substrate. The HVOF WC-Co-Cr coating was found to reduce the tensile strength of the steel. The failure of the HVOF WC-Co-Cr coated steel specimens initiated by cracking on the coating surface, transverse propagation of the cracks towards the coatin...

This paper provides a comprehensive assessment of the tensile and sliding wear behaviour of WC– 10Co4Cr agglomerated and sintered powder deposited on aluminum by HVOF process. The microstructure of the HVOF sprayed WC–CoCr coatings were characterized by scanning electron microscopy and optical microscopy. Microstructural analysis identified grains...

Industrial Thermal Spray applications from PYROGENESIS sa

Abstract: The present study is focused on the evaluation of the in-service tribological performance of Cr2O3 coatings deposited by Flame Spraying, a method that allows in-situ deposition and surface enhancement of metallic components of large dimensions. The relevant experimental results concern ball-on-disc and three-body abrasion testing and are...

Abstract: The paper presents the applications of thermal spray (TS) coatings, when a high tribological performance (wear resistance, friction, lubrication) is necessary. For many decades Cr plating was applied when high wear resistance was requested. However this technique is characterized by the hazardous emission of Cr6+ ions, and the need of man...

Synthesis and heat treatments of NiAl-Ni3Al high-temperature sprayed coatings by in-flight combustion synthesis (CAFSY) Amalia Marinou1,2, Galina Xanthopoulou1, George Vekinis1, Angeliki Lekatou2, Michail Vardavoulias3 1 Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310, Athens, Greece 2 Department of Materials Science and Eng...

The influence of the cermet fraction in cermet/ metal composite coatings developed by High-Velocity Oxyfuel Flame (HVOF) spraying on their tribological behaviour was studied. Five series of coatings, each one containing different proportion of cermet-metal components, prepared by premixing commercially available feedstocks of NiCrFeBSiC metallic an...

Composite coatings with five different proportions of WC-Co/Cr and NiCrFeBSiC components were deposited on stainless steel by HVOF spraying. Cavitation erosion tests were performed and the material removal micro-mechanisms were identified by SEM of both the eroded areas and the specimens’ cross-sections. Waves’ propagation and deflection at the wea...

Titanium is most commonly associated with jet engines and airframes, but the most recent researches indicate its applicability for fittings for prosthetic devices and artificial heart. Titanium castings are often more economical than small fabricated parts, metal injection moulded, stamped and forged parts. The paper presents an experimental factor...

Abstract: A new, cost-efficient and on-site-applicable thermal spraying process for depositing NiAl metallic overlay or bond-coat coatings for high temperature applications by synthesizing the desired intermetallic phases in-flight during oxy-acetylene flame spraying is presented. Base-metal powders were used for spraying and, by adjusting the spra...

Alumina dispersed FeCuAl-based nanostructured cermet coatings were deposited from nanostructured powders by atmospheric plasma spraying on low carbon steel substrates. Nanostructuring was retained in the deposited coatings which exhibit up to four distinctive phases as revealed by electron microscopy. In this study, the friction behavior of the dis...

The purpose of this work is to check the potential of innovative processes for the Si wafers production toward the solar cell industry. Studies have been focused on a comparative analysis of mechanical properties of such wafers, since: (i) reduced wafer strength leads to a high breakage rate during subsequent handling and solar cell processing step...

Nanostructured WC–12Co coating with crystal sizes below 30 nm were deposited on low carbon steel substrate by high velocity oxy-fuel spraying. In this study, corrosion behavior of such coatings was investigated in artificial sea water electrolyte and compared with micron-sized (internal structure) coating of similar composition. Besides that, role...

This work introduces thermal spraying of an agglomerated high energy ball milled powder produced by a commercial, martensitic steel. The nanostructure of the powder feedstock was examined by means of Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Varying levels of HVOF process spraying parameters were employed to deposit the nanost...

Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainles...

Thin-film epi-Si solar cells on low-cost Si based substrates (Si wafer equivalent approach) are a promising alternative to bulk Si solar cells. ThinSi is a medium-scale focused research project founded by the European Commission, which aims to develop a solar cell processing chain for high throughput, cost-effective manufacturing of such thin film...

A stainless steel powder of a mixed amorphous and crystalline structure was HVOF sprayed in an effort to produce coatings with a large glass fraction. In the first part of this work, the microstructure and annealing behavior of powder and coatings are studied. The coatings consisted of a glassy part and a martensitic part, the latter with boride, b...

High velocity oxygen fuel (HVOF) thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Powder morphology varies according to production methods while new powder manufacturing techniques produce porous powders containing air voids which are not interconnected. The porous microst...

High-velocity oxy-fuel (HVOF) thermal spraying can generate dense depositions without melting the powders during spraying. Our previous study showed that most HVOF-sprayed particles are in solid state prior to impact on the substrate. The deposition of solid particles requires sufficient deformation of the particles as a result of a high impact. Th...

A partially amorphous FeCrMoWBCSi powder has been HVOF sprayed in order to produce highly amorphous coatings. The extinction or retention of crystalline phases due to the spraying process is discussed. Amorphicity in coatings is associated with a high melting degree. The latter is attained by a high particle temperature and sufficient residence tim...

High energy ball milled nanostructured Al2O3 (“N”), fused/crushed conventional Al2O3 (“C”) and sintered nanostructured Al2O3 (“S”) powders were air plasma sprayed on 304 stainless steel. The nanostructured powder was composed of nanoparticle agglomerates, whereas the conventional powder consisted of solid granules. The average crystal size of the p...

Coatings have been deposited by air plasma spraying of alumina powders in the form of conventional particles (C), nanostructured agglomerates (N) and sintered–nanostructured agglomerates (S). Sintering alleviated the stresses introduced in the nanopowder by the manufacturing process (high energy ball milling). The coating porosity is a direct conse...

HVOF sprayed WC-Co nanostructured coatings deposited on low carbon steel and alumina substrates were investigated in artificial sea water. The objective is to get a better insight into anodic/cathodic behaviour of such nanostructured materials in comparison to common micro-structured ones. Impedance spectroscopic data were recorded and fitted with...

Nanostructured and conventional Al2O3 powders have been plasma sprayed to produce coatings. The parameters for retaining a fraction of the nanostructure were investigated. Dissimilarities were observed between the two types of coating, regarding properties and phase proportions, which are related with the different percentages of semimolten particl...

Copper, titanium and CoNiCrAlY coatings were obtained using cold spray, a rela- tively new thermal spray process. Negligible porosity and oxygen content, absence of cracks and no defects were observed in the case of copper. Twins were observed in some of the copper coatings. The microhardness varied between 95 HV0.3 and 150 HV0.3 for the different...

Nanostructured and conventional Al2O3 powders have been plasma sprayed, under the same parameters, to produce coatings on 304 stainless steel. The nanopowder was fabricated by mechanical attrition techniques and composed of agglomerates of nanoparticles, whereas the conventional powder consisted of solid granules. In order to maintain the beneficia...

The aim of this study was to investigate the microstructure (including surface roughness, microstructure and mi-crohardness) of cold sprayed copper coatings on 2017 Al alloy, before and after annealing at 350 o C for 1h. It seemed that keeping constant the powder flow rate and increasing the gas pressure from 2.0 to 2.5 MPa, the sur-face roughness...

Nanostructured alumina (Al2O3) and nanostructured cermet coatings containing alumina dispersed in a FeCu or FeCuAl matrix, were deposited by atmospheric plasma spraying (APS) from nanostructured powders. These coatings were characterized by SEM, EDAX, TEM, XRD and nanoindentation. Friction and wear behaviour were investigated by sliding and abrasio...

Nanostructured and conventional Al2O3 powders have been plasma sprayed to produce coatings. The parameters for retaining a fraction of the nanostructure were investigated. Dissimilarities were observed between the two types of coating, regarding properties and phase proportions, which are related with the different percentages of semimolten particl...

Compositionally graded self-standing thick films (0.4mm) have been fabricated using the airflow deposition method. Films were made of five layers with different composition Ba(1−x)SrxTiO3 (BST, x=0, 0.1, 0.2, 0.3 and 0.4). The layers of different thicknesses, ranging from 80 to 30μm, presented similar Vickers microhardness of about 0.25GPa. The ave...

Nanostructured WC-Co based coatings were investigated. The main focus was given to the effect of Al and Cr addition on their tribological behaviour. These coatings were successfully deposited from engineered nanosized powders using high velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS). Porosity level was < 3%. Crystal sizes of around...

The present study has investigated the microstructural, tribological and mechanical properties of three coatings (Cr2O3, NiAl and Cr2O3–bond coating NiAl), obtained by atmospheric plasma spraying on medium carbon steel. More precisely, the effect of the bond coating (NiAl) on the properties of ceramic coating is investigated. The results showed tha...

The purpose of the present work was the investigation and characterization of the quality of air in a thermal spray industry, in Greece. The activities that take place in the specific facility, as well as in most other similar industries, include thermal spraying and several mechanical and metallurgical tasks that generate airborne particles, such...

Plasma atomizing is the unique process for producing high purity biomedical titanium powders with a good sphericity. The paper presents some theoretical aspects obtained by establishing and analyzing the equation of the thermal balance at the plasma atomizing process. The equation emphasizes the numbers of the total particles obtained in the proces...

The corrosion and corrosion-wear behaviour of a thermal sprayed nanostructured FeCu/WC-Co coating were investigated in a Hank's solution and compared to stainless steel AISI 304 and nanostructured WC-Co coatings. Electrochemical noise and potentiodynamic polarization measurements were conducted along with an ex situ scanning electron microscopy to...

The purpose of the present study is to investigate the influence of critical HVOF spray parameters on mechanical properties of WC–12Co coatings obtained from nanophased powder used as feedstock material. A Taguchi design of experiments approach was used for the selection and optimization of process parameters and coating properties. An L9 table was...

Greece is a country with limited tradition in the heavy manufacturing industry. Only a few original equipment manufacturers (OEMs) are installed in the country, and therefore, the need for protective or functional coatings on newly manufactured components is limited. On the other hand, a high volume of mechanical equipment is in operation in indust...

In this work, the high temperature oxidation mechanism of a high chromium (28 wt-%) cast stainless steel containing 3%Ni, and with an austenitic-ferritic microstructure, has been studied and compared to that of a ferritic stainless steel with similar chromium and carbon contents but without any nickel additions. Two different oxidation temperatures...