Zn- or Cu-Containing CaP-Based Coatings Formed by Micro-arc Oxidation on Titanium and Ti-40Nb Alloy: Part I-Microstructure, Composition and Properties

Abstract

Zn- and Cu-containing CaP‑based coatings, obtained by micro-arc oxidation process, were deposited on substrates made of pure titanium (Ti) and novel Ti-40Nb alloy. The microstructure, phase, and elemental composition, as well as physicochemical and mechanical properties, were examined for unmodified CaP and Zn- or Cu-containing CaP coatings, in relation to the applied voltage that was varied in the range from 200 to 350 V. The unmodified CaP coatings on both types of substrates had mainly an amorphous microstructure with a minimal content of the CaHPO₄ phase for all applied voltages. The CaP coatings modified with Zn or Cu had a range from amorphous to nano- and microcrystalline structure that contained micro-sized CaHPO₄ and Ca(H₂PO₄)₂·H₂O phases, as well as nano‑sized β‑Ca₂P₂O₇, CaHPO₄, TiO₂, and Nb₂O₅ phases. The crystallinity of the formed coatings increased in the following order: CaP/TiNb < Zn-CaP/TiNb < Cu-CaP/TiNb < CaP/Ti < Zn-CaP/Ti < Cu-CaP/Ti. The increase in the applied voltage led to a linear increase in thickness, roughness, and porosity of all types of coatings, unlike adhesive strength that was inversely proportional to an increase in the applied voltage. The increase in the applied voltage did not affect the Zn or Cu concentration (~0.4 at%), but led to an increase in the Ca/P atomic ratio from 0.3 to 0.7.

Keywords: Ti-40 wt% Nb alloy; adhesion strength; calcium phosphate coating; microstructure; micro‑arc oxidation; morphology; pure titanium.

Figure 1

A schematic illustration of the “Micro-Arc 3.0” experimental setup.

Figure 2

Plots of the current density vs the MAO time for the deposition of the coatings under different applied voltages: (a) CaP coating on Ti; (b) Zn-CaP coatings on Ti-40Nb; (c) Zn-CaP coating on Ti; (d) Zn-CaP coating on Ti-40Nb.

Figure 3

SEM images of the surface and cross-section of the coatings deposited on Ti-40Nb alloy at voltages of 200 V (a–f) and 300 V (g–l): (a,d,g,j) CaP coating; (b,e,h,k) Zn-CaP coating; (c,f,i,l) Cu-CaP coating.

Figure 4

Plots of the thickness (a,d), surface roughness (b,e), and porosity (c,f) of all the types of the coatings on both Ti (a–c) and Ti-40Nb (d–f) substrates against the MAO applied voltage.

Figure 5

Plots of the adhesion strength of all the types of coatings on both Ti (a–c) and Ti-40Nb (d–f) substrates against the applied voltage (a,d), the coating thickness (b,e) and surface porosity (c,f).

Figure 6

XRD patterns of all the types of coatings deposited at applied voltages of 200 V (a,b), 350 V (c) and 300 V (d) on Ti (a,c) and Ti-40Nb (b,d) substrates.

Figure 7

Typical SEM images and EDX grey-level maps of the Ca, P, Zn and Cu concentrations (marked with white color) in the Zn-CaP (a,b) and Cu-CaP (c,d) coatings deposited on Ti at applied voltages of 200 V (a,c) and 300 V (b,d).

Figure 8

Typical FT-IR spectra of the MAO coatings deposited at different voltages on both Ti (a) and Ti-40Nb (b) substrates.

Figure 9

BF TEM (a,d,g) and DF TEM (c,f,i) images and SAED patterns (b,e,h) of the particles of the CaP (a–c), Cu-CaP (d–f) and Zn-CaP (e–g) coatings deposited on Ti at 300 V. The SAED patterns were observed within the regions of interest that are highlighted in BF TEM images.