Abstract
Alumina coatings doped with different precious metals were prepared by cathode plasma electrolytic deposition. Results show that the porosity of precious metal-doped alumina coatings (especially Al2O3-Ru) decreases, and the high-temperature cyclic oxidation resistance and spallation resistance are enhanced. The Al2O3-Ru composite coating shows better effect: its average oxidation rate K and average amount of oxide spallation G are minimum. Meanwhile, Nernst equation was used to explain the simultaneous deposition of precious metal and alumina, and the whole process and mechanism of deposition were analyzed.
Al2O3 is the most widely applied coating material, which is used in ceramic devices, pharmaceuticals, adsorbents, and carriers. Numerous techniques have been developed to produce Al2O3 coatings, including chemical vapor deposi-tio
Plasma electrolysis techniques, such as plasma micro-arc oxidation (PEO) and cathode plasma electroplating deposition (CPED), can be used for the surface treatment of material
Wang et a
Aiming to decrease the material cost, precious metals were adopted in this research for coating preparation. Al2O3-Ru and Al2O3-Pd coatings were deposited on the nickel-based K418 superalloy by CPED from aqueous solutions. The deposition mechanism was studied, and the properties of prepared composite coatings were characterized.
The schematic diagram of CPED device used for preparing Al2O3 coatings is shown in

Fig.1 Schematic diagram of CPED device
The morphologies of Al2O3 coatings doped with different precious metal particles were observed by scanning electron microscope (SEM, JMS-6480A) equipped with an energy dispersive X-ray spectroscope (EDS). The porosity of the coatings was evaluated on the basis of the area ratio of pores observed in SEM images.
In order to investigate the kinetics of oxidation resistance and spallation resistance of the coatings, the high-temperature cyclic oxidation experiments were conducted at 1273 K for 100 h. The experiment method was described in Ref.[

Fig.2 Surface morphologies of different Al2O3 coatings prepared by CPED at 130 V (a–c) and 150 V (d–f): (a, d) Al2O3-Pd coating, (b, e) Al2O3-Ru coating, and (c, f) Al2O3-Pt coating

Fig.3 SEM-BSE surface images of different Al2O3 coatings prepared by CPED at 130 V (a–c) and 150 V (d–f): (a, d) Al2O3-Pd coating, (b, e) Al2O3-Ru coating, and (c, f) Al2O3-Pt coating; EDS analysis results of point A in Fig.3a (g), point B in Fig.3b (h), and point C in Fig.3c (i)
Parameter | Al(NO3)3-Pd | Al(NO3)3-Ru | Al(NO3)3-Pt |
---|---|---|---|
pH value | 2.13 | 2.53 | 2.51 |
A | 0.5 | 0.5 | 0.5 |
P | 0 | 0 | 0.0006 |
P | 0.0006 | 0 | 0 |
(RuNO | 0 | 0.0006 | 0 |
Firstly, the hydrolysis reactions are performed, wherein the ionization reaction proceeds, as follows:
(1) |
(2) |
(3) |
(4) |
The formation reactions of Al2O3-Pt, Al2O3-Ru, and Al2O3-Pd coatings during CPED are complex. The following electrode reactions may occur on the cathode and anode surfaces:
(5) |
(6) |
(7) |
(8) |
(9) |
Electrode potential | Value/V |
---|---|
Θ | 0.401 |
Θ | 0.00 |
Θ(P | 0.95 |
Θ(P | 1.43 |
Θ[(RuNO | 0.46 |
0.45 | |
0.21 |
(10) |
where is the standard electrode potential of the electrode reaction; R is the gas constant; T is the Kelvin temperature; z is the charge number; F is the Faraday constant affecting the activity of electrode reaction substance; η is the overpotential of substance precipitated on electrode; (
According to
Electrode potential | Al(NO3)3-Pd | Al(NO3)3-Ru | Al(NO3)3-Pt |
---|---|---|---|
1.53 | 1.53 | 1.53 | |
-0.36 | 0.36 | 0.36 | |
(P | 0.86 | - | - |
[(RuNO | - | 0.37 | - |
(P | - | - | 1.38 |
Negative anode precipitation potential difference | 1.89 | 1.89 | 1.89 |

Fig.4 Schematic diagrams of CPED preparation process of Al2O3-M coatings

Fig.5 Current density-voltage relationship in CPED process with Al(NO3)3·9H2O system with different precious metals
The first stage is a linear stage, where the current density is increased linearly with the increase in voltage. During CPED process, H2 is produced at the cathode surface, which promotes the hydrolysis of A
The second stage is the formation of closed gas sheaths, during which the current density decreases and becomes constant. The conductivity of the solution near the cathode surface is almost zero. Thus, the rate of the cathodic reaction becomes very slow.
The third stage is plasma discharge, where the plasma begins to discharge and the current density begins to increase. At this stage, Al(OH)3 is converted into Al2O3 by the huge energy of plasma, and the precious metal particles also begin to grow at the cathode to form the composite coating. With the gradual thickening of composite coating, the electrical conductivity is decreased continuously, whereas the critical breakdown field strength of the coating is increased gradually. Meanwhile, the energy density of the arc on the cathode surface gradually decreases, and the current density begins to decrease and finally becomes stable. Eventually, the arcing is basically stopped, and the composite coating is completely deposited. When the composite coating begins to form on the cathode surface, the dielectric material changes from the original continuous gas sheath to a two-layer structure consisting of a continuous gas sheath and an Al2O3-Pd (or Al2O3-Ru) composite coating. According to the Maxwell-Wagner model, the electric field strength of the gas sheath (Egas) and the electric field strength of the coating (Ecoating) are not equal to the average electric field strength (E
(11) |
(12) |
where αgas is the electrical conductivity of the gas sheath, αcoating is the electrical conductivity of the coating, dgas is the thickness of the gas layer, and dcoating is the thickness of the coating layer.
According to Eq.(
Under the same electrolyte concentration and voltage, the plasma energy density is basically the same, so the plasma temperature is basically the same. The melting points of Pt, Pd, and Ru are 1768, 1554, and 2250 °C, respectively. Under the plasma, Ru melts less and it is slightly bombarded from the coating, so Ru is more diffusely distributed in the Al2O3 coating. Therefore, the critical field strength of the Al2O3-Ru composite coating is lower than that of Al2O3-Pd composite coating, and its coating porosity is lower. The coating field and plasma energy density reduce when the coatings are prepared at low voltage. More precious metal particles can be deposited in the coating, and the porosity of coatings is larger. However, due to the decrease in plasma energy, the precious metal particles show excessive agglomeration in the coating, as shown in Fig.
(13) |
(14) |

Fig.6 Oxidation kinetics curves (a) and spallation kinetics curves (b) of different Al2O3 coatings prepared by CPED at 150 V
where m1 is the total mass of the sample and the crucible after tests for 100 h; m2 is the total mass of the sample and the crucible after tests for 50 h; m3 is the sample mass after tests for 100 h; m is the mass of calcined crucible; S is the area of the sample; K is average oxidation rate; G is average amount of oxide spallation.
According to Eq.(
Coating | m1/g | m2/g | m3/g | m/g | K/g·c | G/g·c |
---|---|---|---|---|---|---|
Al2O3-Pt | 7.695 02 | 7.694 95 | 4.341 62 | 3.353 00 | 0.002 413 | 0.689 7 |
Al2O3-Ru | 8.273 01 | 8.272 96 | 4.305 06 | 3.967 58 | 0.001 724 | 0.637 9 |
Al2O3-Pd | 8.165 67 | 8.165 61 | 4.281 44 | 3.883 73 | 0.002 069 | 0.862 1 |
Al2O3 | 8.424 10 | 8.423 95 | 4.485 50 | 3.937 84 | 0.005 172 | 1.310 3 |
It is apparent that the Al2O3 coatings containing dispersed precious metal particles have better resistance against the high-temperature oxidation and spallation, compared with Al2O3 coating. Comparing the high-temperature oxidation resistance of Al2O3-Pt, Al2O3-Pd, and Al2O3-Ru coatings, the Al2O3-Ru composite coating shows optimal properties, and its values of K and G are minimum. Because the Ru particles are more distributed in the Al2O3 coating, the porosity of the resultant composite coating is lower. Therefore, its resistance against the high-temperature oxidation is better. Three reasons support the abovementioned results, as follows.
(1) Because precious metal particles have high melting points and thermodynamic stability, they do not react physically or chemically with Al2O3 at high temperatures.
(2) There is a blocking effect caused by the composite coating. Ref.[
(3) Precious metal particles provide a toughening effect. The cracking and peeling of the coating at high temperatures are caused by the crack propagation, but precious metal particles can absorb the energy of crack propagation by plastic deformation, preventing the crack expansion and achieving the toughening effect by sealing cracks and reducing defects in the coatin

Fig.7 High-temperature oxidation resistance (a) and spallation resistance (b) of Al2O3-Pd coatings prepared at different voltages after cyclic oxidation test at 1273 K for 100 h

Fig.8 High-temperature oxidation resistance (a) and spallation resistance (b) of Al2O3-Ru coatings prepared at different voltages after cyclic oxidation test at 1273 K for 100 h
1) The high-temperature oxidation resistance and spallation resistance of the Al2O3-Pt, Al2O3-Pd, and Al2O3-Ru composite coatings are improved by the dispersion distribution of precious metals.
2) The Al2O3-Ru composite coating shows optimal performance with the minimum average oxidation rate and the minimum average amount of oxide spallation.
References
Ogita Y I, Ohsone S, Kudoh T et al. Thin Solid Films[J], 2008, 516(5): 836 [Baidu Scholar]
Wang J, Binner J, Pang Y et al. Thin Solid Films[J], 2008, [Baidu Scholar]
516(18): 5996 [Baidu Scholar]
Musil J, Blažek J, Zeman P et al. Applied Surface Science[J], 2010, 257(3): 1058 [Baidu Scholar]
Wei X L, Xia Y, Liu X M et al. Electrochimica Acta[J], 2014, 136: 250 [Baidu Scholar]
Gupta P, Tenhundfeld G, Daigle E O et al. Surface & Coatings Technology[J], 2007, 201(21): 8746 [Baidu Scholar]
Yerokhin A L, Nie X, Leyland A et al. Surface & Coatings Technology[J], 1999, 122(2–3): 73 [Baidu Scholar]
Yang X, Ding X F, Hao G J et al. Plasma Chemistry & Plasma Processing[J], 2016, 37: 177 [Baidu Scholar]
Liu C X, Zhang J, Zhang S G et al. Surface & Coatings Technology[J], 2017, 325: 708 [Baidu Scholar]
Yerokhin A, Pilkington A, Matthews A. Journal of Materials Processing Technology[J], 2010, 210(1): 54 [Baidu Scholar]
Liu C X, Zhao Q, Wang L X et al. RSC Advances[J], 2017, 7(63): 39824 [Baidu Scholar]
Zhou S, He Y D, Wang D R et al. Transactions of Materials & Heat Treatment[J], 2013, 34(12): 171 [Baidu Scholar]
Wang Y, Jiang Z, Liu X et al. Applied Surface Science[J], 2009, 255(21): 8836 [Baidu Scholar]
Bahadori E, Javadpour S, Shariat M H et al. Surface & Coatings Technology[J], 2013, 228(9): S611 [Baidu Scholar]
Zhang Y P, Meng Y, Shen Y H et al. Applied Surface Science[J], 2017, 419(15): 357 [Baidu Scholar]
Wang P, He Y D, Zhang J. Materials Chemistry & Physics[J], 2016, 184: 1 [Baidu Scholar]
Liu C X, Zhang J, He Y D et al. Materials Research Express[J], 2017, 4(3): 036306 [Baidu Scholar]
Wang P, He Y D, Deng S J et al. International Journal of Minerals, Metallurgy and Materials[J], 2016, 23(1): 92 [Baidu Scholar]
Deng S J, Wang P, He Y D et al. International Journal of Minerals, Metallurgy and Materials[J], 2016, 23(6): 704 [Baidu Scholar]
Bunget I, Popescu M. Physics of Solid Dielectrics[M]. Amsterdam: Elsevier, 1984 [Baidu Scholar]