Cobalt-containing layers on titanium

Article abstract of DOI:10.1134/S0020168507060167

Surface oxide layers up to 60 μm in thickness, containing up to 40 wt % cobalt, 5-10 wt % tungsten, and 8-10 wt % phosphorus, have been produced on titanium by plasma electrolytic oxidation. The layers contained crystalline CoO, Co₂O₃

Full text of DOI:10.1134/S0020168507060167

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 6, pp. 642–644. © Pleiades Publishing, Inc., 2007.
Original Russian Text © V.S. Rudnev, M.S. Vasil’eva, M.V. Bondarenko, V.G. Kuryavyi, N.B. Kondrikov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 6,
pp. 729–731.
Cobalt-Containing Layers on Titanium
a
b
b
V. S. Rudnev , M. S. Vasil’eva , M. V. Bondarenko ,
a
b
V. G. Kuryavyi , and N. B. Kondrikov
a
Institute of Chemistry, Far East Division, Russian Academy of Sciences,
pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia
b
Institute of Chemistry and Applied Ecology, Far East State University,
ul. Sukhanova 8, Vladivostok, 690600 Russia
e-mail: rudnevvs@ich.dvo.ru
Received May 10, 2006
Abstract—Surface oxide layers up to 60 µm in thickness, containing up to 40 wt % cobalt, 5–10 wt % tungsten,
and 8–10 wt % phosphorus, have been produced on titanium by plasma electrolytic oxidation. The layers con-
tained crystalline CoO, Co O , Co O , CoWO , Na TiO , and Na Co (PO ) , depending on the oxidation and
2
3
3
4
3
2
3
3
9
4 7
annealing conditions.
DOI: 10.1134/S0020168507060167
INTRODUCTION
added 2–30 g/l Co(CH COO) · 4H O. The electrolytes
3 2 2
were prepared from commercially available reagent-
To produce surface layers containing di-, tri-, and
polyvalent metal compounds on valve metals and alloys
by plasma electrolytic oxidation (PEO) (anodizing of
grade chemicals and distilled water.
To determine the elemental composition and exam-
metals in electrolytes at high potentials, resulting in ine the microstructure of the surface layer, we used a
spark and arc discharges), aqueous alkaline electrolytes JEOL JXA-8100 electron probe x-ray microanalyzer.
have been recently proposed which enable precipitation The phase composition of the coatings was determined
of appropriate metal hydroxides [1, 2]. In particular, by x-ray diffraction with a D8 ADVANCE diffractome-
Ti/TiO /Mn O –Mn O layered structures were pro- ter (Germany) in a Bragg–Brentano geometry (CuKα
2
2
3
3
4
duced on titanium using aqueous electrolytes contain- radiation, sample rotation). In phase analysis, we used
ing Na B O and Mn(CH COO) [2]. Manganese EVA software and ICDD PDF-2 data.
2
4
7
3
2
oxides were formed around discharge channels as a
result of the thermolysis of the electrolytic deposit con-
taining manganese(II) hydroxides. The composites thus
prepared were shown to catalyze CO oxidation at tem-
peratures from 200 to 350°ë [3, 4].
The samples were air-annealed at a preset tempera-
ture for 2 h in a muffle furnace.
RESULTS AND DISCUSSION
The cobalt oxides CoO and Co O are known to be
The films grown on titanium in the PBW electrolyte
3
4
among the best oxide catalysts for CO oxidation [5, 6]. with cobalt acetate additions were uniform in appear-
In this paper, we describe the preparation of cobalt-con- ance. With increasing cobalt acetate concentration, the
taining surface structures on titanium by PEO using the films change color from light gray (in pure PBW) to
approach in question.
dark blue. The thickness of the layers varies from sev-
eral microns to 60 µm, depending on the cobalt(II) ace-
tate concentration and processing time.
EXPERIMENTAL
The elemental and phase compositions of the PEO
structures produced on titanium are listed in the table.
Increasing the cobalt acetate concentration in the PBW
electrolyte to ꢀ22 g/l increases the cobalt content in the
top layer of the coatings and reduces the titanium con-
tent. Note that the variation of the voltage between the
electrodes with processing time, V(τ), changes consid-
erably in this range of cobalt(II) acetate concentrations.
At a concentration of 2 g/l and PEO time of 5 min, the
Oxide layers were produced on VT1-0 titanium
plates 2.3 × 2.3 × 0.1 cm in dimensions, at a constant
2
current density j = 0.08 A/cm . The anodizing time was
5
to 20 min. After PEO processing, the samples were
washed with distilled water and then dried in air at
room temperature. The sample preparation procedure,
electrochemical cell, and current source were similar to
those described elsewhere [2–4].
The starting alkaline electrolyte contained 25 g/l voltage between the electrodes at the end of the gal-
Na PO · 12H O, 13 g/l Na B O · 10ç é, and 2 g/l vanostatic PEO process is V ꢀ 180 V. With increasing
3
4
2
2
4
7
2
f
Na WO · 2H O (PBW) [7, 8]. To this electrolyte was cobalt acetate concentration, V decreases, down to 30–
2
4
2
f
6
42

COBALT-CONTAINING LAYERS ON TITANIUM
643
Effects of cobalt acetate concentration in the PBW electrolyte, processing time, and annealing temperature on the elemental
and phase compositions of surface layers on titanium
Elemental composition, wt %
C, g/l τ, min t, °C
Phase composition
Co
Ti
O
W
P
1
1
2
3
0
6
4
0
5
5
5
2
–
–
X-ray amorphous
15.4
19.4
42.6
–
28.2
21.9
5.4
–
37.4
35.0
29.4
–
5.6
7.0
5.5
–
8.5
8.4
5.0
–
TiO (anatase)
2
–
CoO
–
X-ray amorphous
–
CoO
CoO
Co O
44.4
–
4.1
–
29.6
–
8.3
–
8.0
–
5
300
5
00
00
–
3
4
8
TiO (rutile), CoWO , Co O , Na Co (PO )
4 7
2
4
3
4
3
9
1
1
2
0
5
0
CoO, TiO (rutile), Na TiO
3
43.0
33.9
38.7
3.9
7.8
2.8
27.1
28.2
30.7
8.8
12.0
7.4
9.0
7.7
2
2
–
CoO, Na TiO , TiO (rutile)
2 3 2
–
CoO, Na TiO , TiO (rutile), Co O
3
10.2
2
3
2
2
5
0 V at a concentration of 22–24 g/l, and remains x-ray amorphous samples. In studies of PEO layers
unchanged at lower concentrations. These results seem produced on titanium or aluminum in aqueous electro-
to be evidence of a change in the mechanism of layer lytes containing Mn(II) polyphosphate complexes,
growth, presumably, associated with changes in the Tyrina et al. [10] found that the elemental composition
state of the electrolyte. To clarify this issue, further of the layers and sample weight remained unchanged at
research is needed.
heat-treatment temperatures of up to 800°C. The x-ray
amorphous layers grown on titanium in aqueous H PO
The above changes in elemental composition are
accompanied by microstructural changes (Fig. 1). At
cobalt acetate concentrations no higher than 12 g/l, we
obtain homogeneous, porous coatings (in Fig. 1, pores
appear as black areas). Such surface microstructures
are typical of PEO layers produced in phosphate elec-
trolytes (see, e.g., [9]). At Co(ëç ëéé) concentra-
tions from 14 to 16 g/l, the coating surface appears
melted, and the visible pores become fewer but larger.
In the concentration range 22–24 g/l, the surface
becomes rougher and less homogeneous, similar to
what was reported for manganese oxide layers pro-
duced on titanium by PEO in alkaline tetraborate elec-
trolytes containing manganese(II) acetate [2].
3
4
solutions with additions of fine hydroxyapatite powder
contained crystallites about 50 nm in size [11]. Anneal-
ing at 800°ë in air or vacuum led to the formation of
crystallites up to 1–6 µm in size, and the x-ray diffrac-
tion patterns of the layers showed reflections from crys-
talline CaTi (PO ) (predominant phase), TiO , and
4
4
2
3
2
TiP O .
2
7
Thus, the present results demonstrate that the
approach proposed in [1, 2] offers the possibility of pro-
ducing cobalt-containing layers on titanium by PEO
processing. Varying the cobalt(II) acetate concentration
in the PBW electrolyte (based on Na PO , Na B O ,
3
4
2
4
7
and Na WO ), one can prepare PEO layers on titanium
2
4
As seen from the table, the layers grown over a
period of 5 min in the PBW electrolyte containing no
more than 22 g/l cobalt(II) acetate are x-ray amorphous
with a controlled cobalt content of up to 30–40 wt %.
The tungsten and phosphorus contents of the layers are
5
–10 and 8–10 wt %, respectively. The growth mecha-
or contain crystalline TiO . The layers produced at
2
nism and surface microstructure of the forming PEO
cobalt(II) acetate concentrations above 22 g/l contain layers depend on the cobalt(II) acetate concentration in
CoO. Increasing the processing time, as well as subse- the PBW electrolyte. The surface layers grown in this
quent air annealing, increases the number of crystalline study are mixed-phase. The content and number of
phases detectable by x-ray diffraction. In particular, we crystalline phases in the layers are influenced by the
observe crystallization of Co O and a sodium cobalt cobalt(II) acetate concentration in the PBW electrolyte,
2
3
orthophosphate. According to Tyrina et al. [10] and PEO processing time, and subsequent air annealing.
Sagymbaev et al. [11], the phases crystallizing during The layers produced in 5 min at cobalt acetate concen-
heat treatment are, most likely, already present in the trations in the PBW electrolyte above 24 g/l contain
INORGANIC MATERIALS Vol. 43 No. 6 2007

6
44
RUDNEV et al.
This work was supported by the Russian Foundation
for Basic Research (grant no. 06-03-32184) and the
Presidium of the Far East Division of the Russian
Academy of Sciences (grant nos. 06-I-P8-012 and 06-
1
-OKhNM-137).
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Fig. 1. Surface microstructures of coatings produced by
PEO for 5 min in the PBW electrolyte containing (a) 10,
(b) 14, and (c) 24 g/l of cobalt(II) acetate.
crystalline CoO. Varying the process conditions and
subsequent air annealing of the samples lead to crystal-
lization of Co O , Co O , CoWO , Na TiO , and
2
3
3
4
3
2
3
1
0. Tyrina, L.M., Rudnev, V.S., Bulanova, S.B., et al., Ther-
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ACKNOWLEDGMENTS
11. Sagymbaev, E.E., Bushnev, L.S., Kolobov, Yu.R., and
Karlov, A.V., Structure and Phase Composition of
Microarc Oxidation Coatings on Titanium, Izv. Vyssh.
Uchebn. Zaved., Fiz., 2000, no. 6, pp. 72–76.
We are grateful to T.A. Kaidalova for the x-ray dif-
fraction characterization of the surface structures.
INORGANIC MATERIALS Vol. 43 No. 6 2007