MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 382

Gas inlet

Gas exhaust Target

Lase r

Substrate

Figure 11.18. Detailed scheme of the experimental setup for the production of bioactive
coatings.

to transfer the stoichiometry of very complex materials to the coating, and a fi ne
control of the fi lm properties [Cotell, 1993 ; Serra, 1995 ; Arias, 1998 ; Gonz á lez,
2003 ].
This technique is based on melting and vaporization of a starting material
source with the required composition for obtaining the fi nal coating. A high-
power laser is used to ablate the target material, forming a plasma plume which is
directed towards the substrate to be coated. The ablation process is carried out in
vacuum or in a gas environment favouring the chemical reaction of the highly
excited ablated particles with other gaseous precursors [Chrisey, 1994; Miller,
1998]. Several processing conditions, such as the substrate temperature, gas pres-
sure and laser parameter infl uence the growth rate and the physico-chemical
properties of the coatings which can be fi nely tuned.
Figure 11.18 depicts a typical experimental system for pulsed laser deposi-
tion. The target and the substrate are situated inside a high vacuum chamber
equipped with a turbomolecular pump to ensure low base pressures. The chamber
consists of a heatable substrate holder and a target rotation system to avoid the
crater formation and, therefore, improve the homogeneity of the coating. Several
measuring devices ensure the control of the processing pressure, gas fl ow and
substrate temperature. The beam of an excimer or Nd:YAG laser is focussed onto
the target through a transparent window, and its energy density and pulse repeti-
tion rate are also controlled.
The physical mechanisms involved in the different stages of the PLD process,
which are quite complex [Chrisey, 1994; Miller, 1998], are the following:

1. interaction of the laser with the target, where the photons are absorbed by
the target material leading to the evaporation of the material;
2. expansion of the ablated products, where an ablation plume is generated,
consisting of a hot plasma at high pressure which expands in presence of
ambient gases;