Biomedical Engineering Reference
In-Depth Information
obtained by covering the ceramic substrate with a first layer of high-
melting glass (ground coat), and one or more layers of a low-melting
and less-reactive bioactive glass (cover glass).
Another method is based on glass-matrix composite coatings, obtained
by adding a second phase into the bioactive glass, aiming to accommo-
date the mismatch in the substrate and the coating thermal expansion
coefficients, acting as well as an alumina diffusion barrier. For example,
aluminum oxide can diffuse from the substrate into the softened glass
during the coating preparation.
8.4 PLASMA SPRAYING
Thermal spraying is a well-established technology commonly used to
produce coatings for a wide variety of applications, including coating
hip replacement prostheses with synthetic hydroxyapatite [11]. Thermal
spray processes can be grouped into three major categories: plasma-arc
spray, flame spray, and electric wire-arc spray. These three categories
use different energy sources to heat a coating material (generally metals,
ceramics, polymers, or their mixtures in powder form) to a molten
or softened state. The heated particles are then accelerated toward a
substrate. When the particles hit the substrate, a bond forms between
the molten particles and the substrate.
The development of bioactive glass coatings and composites on metal
substrates (Ti and its alloys) was made easier by the plasma spray
technique [11, 12]. The process takes place by injecting a gas flow
(argon, nitrogen, hydrogen, or helium) into a chamber where a high-
temperature plasma flame is produced by means of an electric arc.
The gas temperature increases up to 10 000-30 000K. Powders of an
appropriate grain size (generally around 80
m) are injected into the
chamber, rapidly heated, and accelerated through a nozzle toward the
substrate. The hot material impacts on the substrate surface and rapidly
cools, forming a coating. The speed of the particles may range between
100 and 350m/s, and therefore the flight time is of the order of only
10
−
3
s. Upon impact, the molten particles yield their thermal and kinetic
energy to the substrate; they undergo deformation to a lenticular shape
and solidify in less than 10
−
6
s. By multiple scanning of the substrate,
the particles deposit on top of one another, making up a coating of the
desired thickness, usually around 100-150
μ
m.
Some disadvantages of this technique are its high cost, structural or
morphological changes of thermodynamically unstable coating materi-
als, and the presence of residual macro- or microporosity. However,
μ
