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

JUNE 26 MONDAY AFTERNOON
ETCHC-MoA-INV.3 THERMAL BEHAVIOUR OF W-SI-N HARD COATINGS IN
PROTECTIVE AND OXIDATION ENVIROMENTS. A. Cavaleiro, ICEMS -Faculdade de
Ciências e Tecnologia da Universidade de Coimbra, Dep. Eng. Mecânica, Rua Luís Reis Santos,
3030-788 Coimbra, Portugal
Extensive discussion has been raised concerning the structural arrangement in TM-Si-N
(TM=transition metal) nanocomposite coatings, particularly on the way how Si is present in the coatings.
Furthermore, abundant results on the hardness and Young´s modulus of these films is available
in the literature, particularly for TM = Ti and Zr. As a function of the phase arrangement, different
interpretations and discussions on the hardening mechanisms involved in the mechanical behaviour
of the coatings, have been raised such as, grain size, residual stress, lattice distortion. Many authors
defend that the hardness of TM-Si-N films depends only on the purity, dimensions and distribution
of the Si-N phase in relation to the TM nitride grains.
W-Si-N is a particular case in the world of TM-Si-N systems (TM=Ti, Zr), due to the different
chemical affinity among the elements. In fact, the affinity of N for W is much lower that the one of
Si, inversely to the case of e.g. Ti-Si-N where similar affinities of Ti and Si for N are observed. Such
a fact determines the arrangement of the phases during the deposition and has a huge influence on
the structural stability of the coatings when annealed at increasing temperatures. The main consequences
are: (1) much higher N 2 partial pressures are needed for depositing films with W-nitride
phases and (2) after thermal annealing no W-nitride is detected in the films. However, the coatings of
this system can also show hardness values as high as 45GPa even if the main phase (W-based) his
typically a metallic bonding type based material..
In this talk, the current knowledge on the thermal annealing of W-Si-N sputtered films in both protective
and oxidant atmospheres is reviewed. Firstly, sputter deposited single W films are presented
as a particular case of metallic element films and their thermal annealing analyzed. The transition for
the W-Si-N films permitted to conclude that the addition of Si promotes a loss of the crystallinity degree
until amorphous structures are reached. Generally, amorphous coatings (20-30GPa) are softer
than crystalline ones (25-45GPa). Afterwards, the thermal stability of W-Si-N coatings is considered
either in protective or oxidant atmospheres. Special attention is paid for amorphous films. In protective
atmosphere, it is shown that after crystallization the hardness can be higher than that of asdeposited
crystalline films with similar structure. The hardest films have a single W-metallic phase
mixed with amorphous Si-N phase.
The Si content is determinant in the oxidation resistance: the higher the Si content the lower the oxidation
rate is. As an example, the weight gain reached by a coating without Si at 650ºC is similar to
the one shown by a high Si content film (>30at.%) at 1000ºC. The oxidation resistance is attributed
to either the formation of a continuous protective Si-rich layer or to the synergetic effect of a very
low oxide grain phase and a very high compactness of the oxide scale.
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