Abstract: A series of shock recovery experiments up to ~50 GPa were performed on reactions to form carbon
nitrides. Nitrogen-rich starting materials, included a C-N-O amorphous precursor, dicyandiamide, melamine,
and a mixture of carbon tetrahalide and sodium dicyanoamide, were used and the recovered samples were
investigated by X-ray diffraction technique, elemental analysis, transmission electron microscopy and so on.
Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock
pressure of 37 GPa, and production of amorphous C-N materials with a highest N/C ration of 1.26 from the
reaction between carbon tetrahalide and sodium dicyanoamide. We extended to the system C3N4-Si3N4 based
on the recent results on synthesis of spinel-type nitrides. Shock wave chemical reactions provide a route for
synthesizing novel materials including not only high-pressure phases but also metastable, unique substances.
Abstract: The cubic γ-Si3N4 phase was synthesized by the shock technique from the hexagonal
β-Si3N4 phase. The thermal stability of the γ-Si3N4 was investigated during heating in vacuum up to
1773 K. An exothermal heat effect was found at 1690 K and structural investigations revealed
complete transformation of γ-Si3N4 to β-Si3N4. Corresponding heat effect value was estimated as
51.3±7.7 KJ/mol. The high-pressure-high-temperature treatment (P=13 GPa, T=1300-2300 K) was
applied to γ-Si3N4 to make bulk polycrystalline non-porous samples. It was found that temperatures
below 1623 K do not change content of the cubic γ-Si3N4 while temperatures above 2273 K decrease
it substantially. Mechanical properties of these bulk samples were measured by acoustic wave and
nanoindentation techniques. The maximum values belong to cubic γ-Si3N4: hardness 39-44 GPa,
Young’s modulus 475 Gpa and bulk modulus 263 GPa. Equilibrium γ-β line position in P-T phase
diagram was estimated by using data obtained in this work. The equilibrium pressure at T=300 K was
estimated as P300=7.0±2.0 GPa.
Abstract: Widely known technologies of explosive (X) welding and explosive (X) powder
compaction are based on applications of porous composite solid or liquid explosives. Recent results
on dynamics of X-welding and X-powder compaction are presented and discussed in this paper in
the conceptual context of an orderly oscillating detonation wave (DW), a synergetic phenomena
observed in detonation of all classes of composite energetic materials, that was discovered in
LEDAP in last eight years. Regular instabilities that are induced by oscillating DW, are transmitted
through the interface of the impacted materials, causing the local instability and fluctuations in both
processes, formation of the interfacial waves (X-welding mechanism) and in an initial phase of
powder compaction. Application of high resolution optical probes (spatial resolution 250 *m,
temporal resolution 1 ns, 96 independent channels) allowed the simultaneous registration of the
oscillating DW in the X-charge and transmission of oscillations, trough the flyer plate, up to the
welding zone. Similar measurements have been made in experiments with X-compaction of
tungsten powder providing the continuous registration of shock wave velocity inside the compacted
powder, its geometrical shape, their instabilities and irregularities.
Abstract: Some of the activities of the Laboratory of Manufacturing Technology of the NTUA in
manufacturing engineering are reported, focusing onto some recent trends and developments in
advanced manufacturing of advanced materials, in the important engineering topics nowadays from
industrial, research and academic point of view: nanotechnology/nanostructured materials,
synthesis and net-shape fabrication of superconductors, biomedical engineering and solar energy
Abstract: Five types of tungsten carbide based powders with different chemical compositions (WC-12Co,
WC-17Co, WC-10Ni, WC-10Co-4Cr and WC- 20Cr-7Ni) were deposited onto ST37 mild steel
substrate using high velocity oxy fuel (HVOF) spray technique. The feedstock powders and sprayed
coatings were studied by using X-ray diffraction (XRD), and differential thermal analyzing (DTA).
The results were shown during HVOF thermal spraying, WC-M powders become partially melted
before being sprayed on the surface of the substrate with supersonic speed. In these types of
coatings, the crystallographic structures are normally non equilibrium, because the cooling rates of
the deposited splats are very high due to the cold substrate acting as a thermal sink. These partially
melted powders are then rapidly solidified to an amorphous phase. XRD analysis showed that the
amorphous phase was existed in all of the as sprayed coatings. The amorphous phase in WC-12Co,
WC-17Co and WC-10Ni coatings was transformed to crystalline phases by heat treatment at high
temperature. Heat treatment of these coatings at high temperature also resulted in partially
dissolution of WC particles and formation of new crystalline phases. In cobalt base coatings, the
new phases were eta carbide phases like Co6W6C and Co3W3C but in WC-10Ni coating a NiW
intermetallic phase was formed. Heat treatment of WC-10Co-4Cr and WC-20Cr-7Ni coatings did
not change the amorphous phases in these coatings. Differential thermal analysis of cobalt
containing coatings revealed an exothermic reaction at approximately 880°C. This exothermic
reaction may be related to the transformation of the amorphous phase to eta phases. On the contrary,
DTA analysis of feedstock powders of these coatings showed an endothermic reaction at
approximately 1000°C. DTA analyses of nickel containing cermets also showed similar results.
Differential thermal analysis of chromium containing cermets did not show any noticeable
exothermic or endothermic reactions.
Abstract: In this work, WC-17Co powder was thermally sprayed onto mild steel using HVOF
spray technique. The coated specimen was heat treated at 1100°C in a vacuum chamber and was
then studied by using transmission electron microscopy (TEM).
Post heat treatment resulted in recrystallization of the amorphous phase, formed during thermal
spraying, into low carbon eta phase like Co6W6C. TEM results of the heat treated specimens
showed that these new nucleated eta phases had very clear crystallographic structure without any
crystalline defects. Heat treatment could also transform high carbon carbides like WC and W2C in
the as sprayed samples to high carbon eta phases like Co3W3C. High density of dislocations and
staking faults noticed in TEM of these phases might be an indication of possible shear mechanism
in formation of these carbides.
Abstract: This paper proposes a seamless microforming technique from 100 micrometer order
bulging all the way down to micrometer-order embossments by using hydro spark forming method.
Sub-millimeter order bulging with 300 and 100 m diameter aluminum thin foils are demonstrated
to be easily available without accurate positioning as in the conventional methods. The present
technique is successfully applicable also to surface embossments of micrometer order, for a coin
surface, an IC chip and hologram surfaces.
Abstract: The method for prescribing the site of the first buckling lobe in the axial impact of the
tubular structure is proposed. The inertia force induced by the solid mass attached to the tube during
the impact is exploited to trigger the first buckling lobe. When the tube with the solid mass
undergoes a large acceleration, the inertia force of the solid mass is expected to bend on the tube
wall. In the experiment, the rectangular solid mass was attached to the aluminum alloy square tube.
The tube fixed to the drop-hammer was impacted against the stationary rigid plate at the velocity of
5 or 7.7 m/s. For the case of the tube without the solid mass, the site of the first buckling lobe varied
and the slight wavy plastic deformation remained further than the buckling lobes. On the other hand,
when the tube with the solid mass was impacted, the onset of the first buckling lobe was observed at
the portion where the solid mass was attached and the wavy deformation stated above was
suppressed. The corresponding computation was also conducted using the dynamic explicit finite
element method. The result showed a good agreement with the experimental one.
Abstract: The attenuation effect of barrier materials, which covers an explosive completely, on blast
waves was studied. The density of the barrier materials was examined to make the barrier materials
light and low in volume. Water gel, small spheres of foam polystyrene, and mixtures of these two
materials were used as the barrier materials, and the density of the mixture was varied from 0.12
g•cm-3 to 1.0 g•cm-3 by changing the mixed volume ratio. Natural silica sand was also tested for
comparison. A spherical PMMA container was filled with the barrier materials and a spherical
pentolite (100 g) was ignited at the center of container. The blast pressure around the container was
measured. The mixture of the density of approximately 0.55 g•cm-3 maximized the attenuation of the
blast wave for the same volume. The attenuation effect depends not only on the weight of the barrier
materials but also on the porosity. A mixture of a density of approximately 0.13 g•cm-3 maximized the
attenuation of the blast wave for the same weight. Using porous materials, relatively light barrier
materials can attenuate the blast wave effectively, if the volume is not restricted. The attenuation
effect of sand was greater than that of water gel and a mixture for the same volume.