Characteristics of the hydroxyapatite (HA) plasma coatings are directly related to performances of the plasma jet used in the deposition process. Previous research, using predominantly the conventional plasma gun, suggested the use of the lower plasma power, about 11-25 kW, to avoid significant chemical decomposition of HA occurring at higher powers. However, coatings obtained at this power level, have a poor microstructure, resulting in their unsatisfactory cohesion/adhesion properties. In contrast, using our PJ-100 plasma installation a good quality of HA coatings can be produced at high power (52 kW) with a plasma jet in laminar flow mode. For two HA powders, containing particles differing in mass by a factor of 20, a set of optimum deposition parameters has been defined leading to the coatings with high crystallinity (80-90%), excellent microstructure (porosity 1-2%) and high adhesion strength (60 and 40 MPa for the coating thicknesses of 120 and 350µm, respectively). For the first time, it has been shown that higher plasma power does not necessarily lead to a higher HA decomposition. The parameter determining coating crystallinity is the degree of the particle dehydroxylation during its flight from the plasma jet to the substrate.

Microstructure of the coating
as-deposited (a) and etched (b)

XRD paterns of the
powder (a) and coating (b)

Adhesion of the coating as a function of a) coating thickness
b) substrate temperature

Coated implants

Reviewer's comment on our scientific paper:

"Probably the authors have successfully deposited a very dense, adhesive HA coating with high crystallinity, they should give more evident informations, convincing us that this fantastic work happened."
(Unknown Reviewer)

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