Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 26 MONDAY MORNING
ETCHC-MoM-OR.7 TUNGSTEN OXIDE WITH DIFERENT OXYGEN CON-TENT:
SLIDING PROPERTIES. T. Polcar, N.M.G. Parreira and A. Cavaleiro a * ICEMS – Grupo de
Materiais e Engenharia de Superfícies, Faculdade de Ciências e Tecnologia da Universidade de
Coimbra – Pólo II, 3030-201 Coimbra, Portugal. * To whom all correspondence should be addressed
(albano.cavaleiro@dem.uc.pt)
Tungsten oxides were studied in the past due to their electro-optical properties (e.g. exhibit electrochromic
behaviour) and, more recently, due to their applications in gas sensor devices. These studies
were particularly focus on the stoichiometric compound WO 3 or WO 3-x (x = 0–1) prepared by different
method. However, it is difficult to prepare tungsten oxide with low oxygen content by equilibrium
process, since the solubility of oxygen in tungsten in such conditions is very low. Therefore, the
non-equilibrium processes must be used being reactive sputtering the most versatile.
The sliding properties of tungsten oxide coatings are not well known. Tungsten trioxide studied by
Lugscheider et al. showed good tribological properties. It is supposed, that sub-stoichiometric tungsten
trioxide can act as a solid lubricant due to presence of the so-called “Magneli” phases. These
phases exhibit a wide range of structures, which leads to the crystallographic shear planes with reduced
binding strength.
In this study, we prepared tungsten oxide by reactive magnetron sputtering with oxygen contents of
13 and 75 at.%. Detailed analyses of the XRD patterns of the former W–O coating showed the b.c.c.
α-W phase. However, the position of the diffraction peak moved to the lower diffraction angles
compared to α-W phase of pure tungsten. This means that oxygen is allocated at the interstitial positions
in the lattice, which leads to higher lattice parameters and induces compressive residual stress.
Concerning the W 25 O 75 coating, the diffraction peaks of XRD pattern suggest a nanocrystalline
structure. At the moment, it is difficult to state that it corresponds to the WO 3 monoclinic (the equilibrium
structure), since the ICDD patterns ofW 20 O 58 , W 5 O 14 , WO 2.90 , and others forms of WO 3 (cubic,
orthorhombic, triclinic) also have diffraction peaks in the same region as the WO 3 monoclinic
phase.
The hardness and Young’s modulus were evaluated by depth sensing indentation. W 87 O 13 and
W 25 O 75 coatings exhibited hardness of 26 GPa (similar to that of pure tungsten) and 7 GPa, respectively.
The coatings displayed a relatively low adhesion (critical load Lc