36,491 research outputs found
Carbon and titanium diboride (TiB2) multilayer coatings.
Titanium Diboride, (TiB2) is a metal-based refractory ceramic material that has
been investigated in industrial applications ranging from, cutting tools to wear
parts and for use in the aerospace industry. The unique properties which make
this material so fascinating are, its high hardness, high melting point and its
corrosion resistance. TiB2 is prevented from wider mainstream use because of its
inherent brittle nature. With a view to overcome this in coating form and with
the aim of providing in addition inherent lubricity, in this study 50 layer
TiB2/C multilayer stacks have been fabricated, with varying volume fractions of
ceramic, whereby the interfaces of the layers limit crack propagation in the
TiB2 ceramic. TiB2 has been multilayered with carbon, to make use of the unique
and hybrid nature of the bonding in carbon coatings. DC magnetron sputtering
with substrate bias was the preferred route for the fabrication of these
coatings. AISI tool steel has been used as the substrate material. By varying
the amount of TiB2 ceramic from 50% to 95%, the Hardness of the coating is seen
to increase from 5 GPa to 17GPa. The Hardness is observed to decrease as a
function of increasing carbon content, agreeing with other studies that the
carbon layers are not load-bearing. The graphitic nature of the sp2 bond,
however, acts as a lubricant layer
Geodesic Structure of Lifshitz Black Holes in 2+1 Dimensions
We present a study of the geodesic equations of a black hole space-time which
is a solution of the three-dimensional NMG theory and is asymptotically
Lifshitz with and as found in [Ayon-Beato E., Garbarz A., Giribet
G. and Hassaine M., Phys. Rev. {\bf D} 80, 104029 (2009)]. By means of the
corresponding effective potentials for massive particles and photons we find
the allowed motions by the energy levels. Exact solutions for radial and
non-radial geodesics are given in terms of the Weierstrass elliptic ,
, and functions.Comment: 10 pages, 6 figures, accepted for publication in Eur. Phys. J.
The golden ratio in Schwarzschild-Kottler black holes
In this paper we show that the golden ratio is present in the
Schwarzschild-Kottler metric. For null geodesics with maximal radial
acceleration, the turning points of the orbits are in the golden ratio . This is a general result which is independent of the value and
sign of the cosmological constant
On the normalization of Killing vectors and energy conservation in two-dimensional gravity
We explicitly show that, in the context of a recently proposed 2D dilaton
gravity theory, energy conservation requires the ``natural'' Killing vector to
have, asymptotically, an unusual normalization. The Hawking temperature
is then calculated according to this prescription.Comment: 7 pages, Latex, no figure
Testing a dissipative kinetic k-essence model
In this work, we present a study of a purely kinetic k-essence model,
characterized basically by a parameter in presence of a bulk
dissipative term, whose relationship between viscous pressure and energy
density of the background follows a polytropic type law , where , in principle, is a parameter without
restrictions. Analytical solutions for the energy density of the k-essence
field are found in two specific cases: and
, and then we show that these solutions posses the
same functional form than the non-viscous counterpart. Finally, both approach
are contrasted with observational data from type Ia supernova, and the most
recent Hubble parameter measurements, and therefore, the best values for the
parameters of the theory are founds.Comment: 9 pages, 5 figures, accepted in EPJ
- …