Samir  Hamid Awad, 
    Double-Layer Coatings for Surface Improvement of Al 6061 Alloys by Anodizing and Micro
Plasma Oxidation Process MPO using New Hydroxy Apatite and Rock Additives Modified
Electrolytes,
    Journal of Engineering and Applied Sciences,
    Volume 15,Issue 16,
    2020,
    Pages 3112-3121,
    ISSN 1816-949x,
    jeasci.2020.3112.3121,
    (https://makhillpublications.co/view-article.php?doi=jeasci.2020.3112.3121)
    Abstract: In this research, duplex ceramic coatings were
deposited on Al 6061 alloy, using anodizing and Micro
Plasma process (MPO). MPO electrolyte was modified
using hydroxy apatite and 15 g L<sup>&#150;1</sup> rock additives rich
with Calcium Carbonate (CaCo<sub>3</sub>). Phase composition of
the coatings, micro structure, micro-hardness, topography,
thickness, roughness and wear resistance were evaluated
by X-Ray Diffraction (XRD), (SEM), a Vickers indenter,
AFM and Microprocessor coating thickness meter,
roughness test and Micro test, respectively. Results
showed that the samples coatings contained &#947;-alumina and
their thickness and hardness increased by the increasing
of deposition time. The research demonstrates that a
relatively hard, thick and uniform coatings with good
wear resistance, can successfully be deposited on Al alloy
using hydroxy apatite and 15 g L<sup>&#150;1</sup> CaCo<sub>3</sub> rock additives
containing electrolytes as an natural rock additives.
    Keywords: Micro arc oxidation;Aluminium alloy;Rock additive;Hardness;wear resistance