Angelos Koutsomichalis | Hellenic Air Force Academy (original) (raw)

Papers by Angelos Koutsomichalis

Tribology and Materials

In this work, the tribological behaviour of thermally sprayed titanium and chromium ceramic-based... more In this work, the tribological behaviour of thermally sprayed titanium and chromium ceramic-based coatings was investigated under abrasive conditions. Their structure was studied by a scanning electron microscope (SEM), while their hardness was evaluated using a microhardness tester. To study the strength of these ceramic coatings under abrasion conditions, reciprocating sliding experiments were carried out in a high-precision (10 mN resolution, 1000 Hz data acquisition) pin-on-disk apparatus and by using a Ø6 mm corundum ball to generate high contact pressures (1.5 GPa). To thoroughly investigate the friction evolution of the tribo-system, three-dimensional mapping of the tangential friction forces (triboscopy) was performed. Following the abrasion experiments, the wear of these coatings was measured using confocal microscopy. The obtained friction and wear results were compared to state-of-the-art materials and coatings that are currently being used in various industrial applicati...

International Journal for Quality Research, 2007

The present paper is focused on the application of quality principles in Greek Secondary Educatio... more The present paper is focused on the application of quality principles in Greek Secondary Education and comprises of two interrelated parts. In the first part of this work definitions, a theoretical framework and multimodels of quality in education are introduced and discussed, in brief. Next, the initiatives applied for improving the quality of education are emphasized. Since 1996 and for more than a decade, a series of ESF interventions have been applied in Greece with measurable impact on improving the quality of Greek Secondary Education. The most important policy measures focused at Secondary Education together with their results are presented in the second part of the paper.

Scripta Metallurgica et Materialia, 1993

MATEC Web of Conferences, 2021

Cr2O3 and TiO2 powders were deposited by atmospheric plasma spray (APS) on steel substrates. Micr... more 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 impact cycles) and static loading tests (Vickers tests standards with 600N and 1500N) measuring the adhesion properties of the coatings. In low cycles (1x103) Titania coatings exhibited better strength, while at intermediate (4.5x105) and high (1x106) number of impact cycles, both Chromia and Titania coatings exhibited quite similar strength characteristics. At low impact force the thickness of the coatings plays critical role with better performance obtained by Chromia coatings. During static loading both coatings exhibited similar characteristics at the crater diameter but with larger crater depth for Titania. Chromia coatings exhibited hi...

1 ABSTRACT Abradable seal coatings are engineered to minimise the clearance between blade tips an... more 1 ABSTRACT Abradable seal coatings are engineered to minimise the clearance between blade tips and casing to enhance gas-turbine performance. The gas path sealing has become an important method for this purpose. Abradable seals act as sacrificial layers between the blades and the casing, and are soft enough to avoid significant wear to blade tips, thus allowing much smaller clearances. The thermal sprayed abradable seal coating has been used because of its simple manufacturing processes, easy repair of the components, easy adjustment of its properties and good sealing effectiveness. In the present paper, we report on the surface microstructure and the tribological behavior of the widely used, abradable coating Al-Si polyester. Coatings were produced by using fully-automated atmospheric thermal spraying (APS) equipment. Microscopic examination revealed rounded polyester particles embedded in the Al-Si phase without forming any interfacial product. Porosity was estimated to be about 3 %, while the coating-substrate interface was coherent. For the evaluation of the coatings tribological performance, sliding friction tests were performed on a pinon-disc apparatus with SiC as counterbody. Three normal loads (5 N, 7 N and 10 N) were used, in order to examine the influence of the contact stress on tribosystem. Abrasion was identified as the dominant wear mechanism of the coating, whilst secondary microcracking was activated when applying more severe sliding conditions. During testing, the friction coefficient remained constant, ranging from 0.21 to 0.27, for all the loads applied, and the wear volume was increasing proportionally with the applied load.

MATEC Web of Conferences

Three Ti-6Al-4V bolts from mid-life jet aircraft failed during service operation. Each of the fai... more Three Ti-6Al-4V bolts from mid-life jet aircraft failed during service operation. Each of the failed bolts were installed on a landing gear component. Metallurgical failure analysis indicated that the fracture mechanism is low temperature creep, possibly resulting from a sustained tension load over a long time. Visual inspection revealed ductile fracture morphology with no macroscopic deformation or corrosion. For all bolts fracture occurred close to the root at the first pitches of the thread. Scanning Electron Microscope (SEM) analysis revealed dimple morphologies with micro-void coalescence. Energy Dispersive X-Ray (EDX) analysis did not reveal any material deficiency that would have precipitated failure.

IOP Conference Series: Materials Science and Engineering, 2017

This paper provides a comprehensive assessment of the tensile and sliding wear behaviour of WC-10... more 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 WC in the W 2 C phase was observed and decomposition of WC in the metal matrix. The HVOF WC-Co-Cr coating was found to decrease tensile strength of the aluminum substrate. Transverse cracks were observed to initiate on the coating surface, increasing rapidly with the increase in tensile strain and stopped on the coating-substrate interface causing decohesion. Tribological properties were examined using the pin-on-disk method under various loads. The friction coefficient rose abruptly at the start-up phase and stabilized at almost the same sliding distance independently of the applied load. Both the friction coefficient and the wear volume were found to increase with increasing applied load. Study of the wear mechanisms revealed surface micro-cracking and fragmentation of flattened coating layers with subsequent gradual pull out of the carbide particles.

Materials Science Forum, 1992

ABSTRACT Fractal geometry has been introduced to describe the roughness of machined surfaces for ... more ABSTRACT Fractal geometry has been introduced to describe the roughness of machined surfaces for providing evaluation by one or two parameters. The present con-tribution is aimed at modelling the interactions of cutting conditions with fractal dimension of surfaces obtained by turning using analysis of variance (ANOVA) and response surface methodology. Also, single interactions were studied through single regression analysis. Four metallic materials were used namely, Ck60 plain carbon steel, 304 stainless steel, 6061 aluminum alloy and a brass. Longitudinal turning was performed with constant depth of cut, and feed and cutting speed varied over a wide range of values applied in roughing, semi-finishing and finishing. In relation to the fractal dimension D, the skew-ness of the profile distribution R sk was considered in view of bearing properties of the surfaces. The predictive statistical models developed on D for all four materials considered with respect to feed and cutting speed showed fair to good correlation. Single regression models of D against feed exhibited very good correlation. Further, the findings of the present study were compared to results existing in literature and partial relevance was proved.

Surface and Coatings Technology, 1988

... 1. Introduction Argon, carbon dioxide, ruby and yttrium aluminium garnet lasers are currently... more ... 1. Introduction Argon, carbon dioxide, ruby and yttrium aluminium garnet lasers are currently used in research investigations and industrial ... Sircar [3] deposited eutectic compositions of goldgermanium and silvergermanium on n GaAs single crystals and annealed them in ...

Surface and Coatings Technology, 1998

Abstract SiN x IBAD coatings were applied to low-alloy steel (0.4 μm thick) and to high-alloy ste... more Abstract SiN x IBAD coatings were applied to low-alloy steel (0.4 μm thick) and to high-alloy steels AISI 310 and 321 (0.8 μm) at 90 keV energy nitrogen ions. The samples were subjected to a high temperature corrosion test for 2000 h; low-alloy steel in air and highalloy steels in sulphidizing-oxidizing atmosphere at temperatures 500–700°C. In both gaseous environments the application of SiN x increased the corrosion resistance of the tested steels. The protective function of SiN x coating consists of its barrier effect and in some cases it supports the formation of a compact protective SiO 2 layer.

Surface and Coatings Technology, 1990

Abstract Ni-Cr coatings were deposited on the surface of Inconel 625 spcecimens by the plasma spr... more Abstract Ni-Cr coatings were deposited on the surface of Inconel 625 spcecimens by the plasma spraying technique. During the plasma spraying procedure, elements from the substrate were observed to diffuse to the deposited Ni-Cr coating. The coated and uncoated alloy specimens were immersed in acidic NaCl solutions and their corrosion behaviour was examined. The corrosion of the coated specimen was found to be higher than that of the uncoated specimen and to change drastically after immersion in 4 M H3PO4 solution.

Materials Letters, 1993

fuming oxide was plasma sprayed on the surface of Al+% Cu ahoy substrate. The mo~holo~ of the coa... more fuming oxide was plasma sprayed on the surface of Al+% Cu ahoy substrate. The mo~holo~ of the coating was studied thoroughly using optical and scanning electron microscopy. The abrasive wear behaviour of the alumina coating was examined using the disk on disk method (Taber abraser). The wear rate of the coating was higher at the beginning of the wear tests and depended on the applied load. A linear relationship was observed between the wear rate and the initial surface roughness of the

Materials and Manufacturing Processes, 1995

ABSTRACT Copper-nickel-indium coatings were deposited on the surface of titanium alloy specimens ... more ABSTRACT Copper-nickel-indium coatings were deposited on the surface of titanium alloy specimens by plasma spraying. The structure and properties of the coatings were studied thoroughly using optical and scanning electron microscopy. The mechanical behavior of the Cu-Ni-In coated titanium substrates was examined. The coating was found to decrease the tensile strength of the titanium alloy substrate. The elastic modulus and the ultimate tensile strength of the coated system were found to decrease with increasing coating thickness. The residual stresses of the coated system were also calculated.

Journal of Materials Science, 1994

Magnesium alloy (AZ31 B) was irradiated in air using a pulsed KrF excimer laser. The surface of t... more Magnesium alloy (AZ31 B) was irradiated in air using a pulsed KrF excimer laser. The surface of the irradiated magnesium exhibited a wavy topography and its surface roughness was found to depend on the laser power density. The corrosion behaviour of the laser-treated magnesium was studied and it was found to be slightly better than that of the untreated magnesium. 90 .__

Journal of Laser Applications, 1996

An AlCuMg alloy was irradiated using a KrF pulsed excimer laser. A microstructural study showed t... more An AlCuMg alloy was irradiated using a KrF pulsed excimer laser. A microstructural study showed the presence of a laser treated zone having a uniform depth of approximately 20 Î¼m. The surface layer of the laser treated aluminum alloy exhibited a wavy topography and its surface roughness was found to depend on the number of laser pulses per step. A X-ray diffraction analysis revealed the presence of aluminum oxides and nitrides on the surface of the laser irradiated specimens. Corrosion measurements showed that the laser treated aluminum alloy exhibited a higher corrosion resistance. The corrosion behavior of the irradiated specimens is related to the energy density of the incident laser pulses.

This paper addresses the question of how the corrosion resistance of EDMachined tool steel (AISI ... more This paper addresses the question of how the corrosion resistance of EDMachined tool steel (AISI D2) surfaces is related to topographic formations and characteristics, as machined, and during and after the application of a laboratory accelerated corrosion test. Correlation of machining conditions with the evolution and the final magnitudes of corrosion damage was finally obtained.

ABSTRACT In the present paper we report on the effect of shot peening on the surface integrity an... more ABSTRACT In the present paper we report on the effect of shot peening on the surface integrity and the tribological behaviour of tool steels (a premium AISI H13 and a modified AISI P20 grades). Shot peening of steel discs was performed by using two different shot sizes (S110 and S230) combined with two different blast pressures (40 psi and 80 psi). For the evaluation of surface integrity parameters (surface roughness, topography and microhardness) typical standardized methods were used. For the evaluation of friction and wear characteristics sliding friction tests were performed on a state-of-the-art pin-on-disc apparatus. Main findings are summarized as follows: (i) A surface morphology characterized by randomly overlapping craters with dimensions increasing with the increase of shot size and/or pressure was identified in all cases. (ii) A thin work-hardened zone was developed on all specimens; the extent of this zone and the position of peak microharness value depends upon the peening parameters. (iii) It was verified that, in general, shot peening exerts a beneficial effect on tribological behaviour reducing wear and friction coefficient.