Titania coating formation on hydrostatically extruded pure titanium by micro-arc oxidation method

doi:10.1016/j.jmst.2021.09.019

Abstract

Abstract:

In this work, the microstructure of titania coating fabricated on the surface of hydrostatically extruded titanium grade 4 with the use of the micro-arc oxidation method was studied. The surface topography and microstructure investigations performed with atomic force microscopy and scanning and transmission electron microscopy revealed that, by using an Na₂HPO₄ electrolyte, a well-adherent porous coating is produced on the top surface and side walls of the extruded rod. The distribution of chemical elements was analyzed by using energy dispersive X-ray spectroscopy. The chemical elements dissolved in the electrolyte (Na, P and O) incorporated into the coating. Sodium locates preferentially in the outer part of the coating, while phosphorus and oxygen are distributed throughout the whole coating. The most relevant finding shows that a grain refinement caused by a hydrostatic extrusion provoked an increase in density of high-angle grain boundaries (HAGB), which in turn secured the formation of a continuous amorphous layer close to the substrate. The presence of this layer compensates for the effect of anisotropic substrate, producing a comparable and homogenous microstructure with a large number of micropores.

Key words: Micro-arc oxidation, Titanium of commercial purity, Hydrostatic extrusion, Surface topography, Microstructure

Ł. Maj, D. Wojtas, A. Jarzębska, M. Bieda, K. Trembecka-Wójciga, R. Chulist, W. Kozioł, A. Góral, A. Trelka, K. Janus, J. Kawałko, M. Kulczyk, F. Muhaffel, H. Çimenoğlu, K. Sztwiertnia. Titania coating formation on hydrostatically extruded pure titanium by micro-arc oxidation method[J]. J. Mater. Sci. Technol., 2022, 111: 224-235.

Figures/Tables 15

Table 1. Chemical composition of titanium grade 4 according to the standard.

Standard	Maximum concentration (wt.%)
Standard	N	C	H	Fe	O	Ti
UNS R 50,700	0.050	0.080	0.015	0.500	0.400	balance

Table 2. Subsequent steps of treatment of substrate material.

Parameter	Initial extrusion	Thermal treatment and turning	1st pass of HE	2nd pass of HE	3rd pass of HE	RS
Reduction in diameter [mm]	50 → 30	Annealing at 700 °C for 2 h and turning 30 → 20	20 → 12	12 → 8	8 → 6	6 → 5
Cumulative true strain ε	0.5	-	1	1.4	1.7	1.9

Fig. 1. Scheme of MAO coating on the top of titanium grade 4 after hydrostatic extrusion and SEM/SE images presenting the base and walls of coated cylinder.

Fig. 2. SEM/EBSD orientation maps of substrate material of titanium grade 4 after 3-pass hydrostatic extrusion and RS in (a) TCS and (b) LCS with marked HAGBs (black lines) as well as (c) calculations of HAGB, LAGB density and average grain size. Color-coding based on inverse pole figure with respect to the ED according to the shown standard triangle. Black areas correspond to the points with the CI lower than 0.1 and were excluded from analysis.

Fig. 3. TEM/BF microstructure images of substrate material of titanium grade 4 after 3-pass of hydrostatic extrusion and RS in (a) TCS and (b) LCS.

Fig. 4. .(0002),($10\bar{1}0$) and ($11\bar{2}0$) pole figures of titanium grade 4 subjected to 3-pass hydrostatic extrusion and RS calculated based on EBSD data (A1-A2 denotes the TCS plane).

Fig. 5. XRD pattern of the MAO-produced coatings: the top spectrum was acquired from side wall of the coated rod, while the bottom one was obtained - from its top surface.

Fig. 6. SEM/SE images of the surface topography of the MAO coating produced on (a) top surface and (b) side wall of titanium grade 4 after 3-pass hydrostatic extrusion and RS.

Fig. 7. AFM maps of the surface topography of the MAO coating produced on (a) top surface and (b) side wall of titanium grade 4 after 3-pass hydrostatic extrusion and RS.

Fig. 8. SEM/BSE microstructure images of the MAO coating produced on titanium grade 4 after 3-pass hydrostatic extrusion and RS presented in the (a, c) transversal and (b, d) longitudinal section to ED.

Fig. 9. TEM/BF microstructure images of MAO coating produced on the top surface of titanium grade 4 subjected to 3-pass hydrostatic extrusion and RS: (a) general overview, (b) interface and (c) coating area.

Fig. 10. TEM/BF microstructure images of MAO coating produced on the sidewall of titanium grade 4 subjected to 3-pass hydrostatic extrusion and RS: (a) general overview, (b) interface and (c) coating area.

Fig. 11. TEM/BF microstructure image of MAO coating produced on titanium grade 4 subjected to the 3-pass hydrostatic extrusion and RS (a) and SAED diffraction patterns from (b) interface area, (c) ultrafine grain and (d) amorphous matrix.

Fig. 12. SEM/BSE microstructure image of the MAO coating produced on titanium grade 4 after the 3-pass hydrostatic extrusion and RS and the EDS maps of distribution of chemical elements.

Fig. 13. STEM-HAADF microstructure image (a) and EDS map of phosphorus (b) incorporated into the MAO coating.

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