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Influence of Al2O3 addition in NaAlO2 electrolyte on microstructure and high-temperature properties of plasma electrolytic oxidation ceramic coatings on Ti2AlNb alloy
Surface & Coatings Technology 370 (2019) 187–195
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Influence of Al2O3 addition in NaAlO2 electrolyte on microstructure and high-temperature properties of plasma electrolytic oxidation ceramic coatings on Ti2AlNb alloy
T
⁎
Zhao-Ying Dinga, Yuan-Hong Wanga, Jia-Hu Ouyanga,b, , Zhan-Guo Liua,b, Ya-Ming Wanga,b, Yu-Jin Wanga,b a
Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150001, China School of Materials Science and Engineering, Harbin Institute of Technology, 92 Westdazhi Street, Harbin 150001, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma electrolytic oxidation Ti2AlNb alloy Al2O3 addition Oxidation resistance Tribological properties
Plasma electrolytic oxidation (PEO) has become an effective surface modification method to form ceramic coatings on orthorhombic Ti2AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al2O3 additive were introduced into a NaAlO2 basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti2AlNb alloy. The influence of Al2O3 addition in NaAlO2 electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of Xray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-ondisc friction and wear tests. The introduction of Al2O3 into the basic electrolyte promotes the formation of Al2TiO5 phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al2O3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the least mass gain of only 1.33 mg cm−2. The micro-hardness of the PEO coatings prepared with Al2O3 additive is improved due to the presence of Al2TiO5 phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti2AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al2O3 addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.
1. Introduction Orthorhombic Ti2AlNb alloy, as a damage-tolerant, good hightemperature performance and lightweight material, has attracted great attention by jet engine manufacturers and related research labs in the worldwide since 1990s [1–3]. Due to its excellent properties of high specific strength, high fracture toughness and high creep resistance, OTi2AlNb has become a promising candidate as high-temperature structural material for applications in aircraft, automobile industries, thermal control coatings, etc., where heavy nickel-base superalloys are the only alternative today [1–4]. However, practical applications of Ti2AlNb alloy are restricted by its poor oxidation- and wear-resistance at elevated temperatures [4–13]. Surface engineering is the most
promising approach to facilitate the applications for hot-section components in aircraft and automobile industries [4]. Protective coatings are usually fabricated on the surface of Ti2AlNb with the aim of improving its high-temperature properties at operating temperatures of 650-800 °C [4–16]. Raluca Pflumm et al. [4] reviewed different coating methods to protect TiAl-based alloy from the oxidation, including their general advantages and drawbacks, among which, overlay coating on the substrates have a potential for an efficient oxidation protection of TiAl alloys. However, overlay coatings prepared by methods like PVD, thermal spraying, laser cladding, etc. are in relatively high costs and high complexity [4,12,17–19]. Coating methods in high productivity, economic efficiency and ecological friendliness are urgent to be developed. On the other hand, most researches on Ti2AlNb protection
⁎ Corresponding author at: Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150001, China. E-mail addresses: [email protected] (J.-H. Ouyang), [email protected] (Z.-G. Liu), [email protected] (Y.-M. Wang), [email protected] (Y.-J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.04.075 Received 17 December 2018; Received in revised form 12 April 2019; Accepted 24 April 2019 Available online 01 May 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 370 (2019) 187–195
Z.-Y. Ding, et al.
only focus solely on either oxidation- or wear-resistant properties. Comprehensive researches on high temperature properties of the overlaying coating are very limited. Plasma electrolytic oxidation (PEO) is an in situ growth process for fabricating ceramic coatings on metals and alloys with designed chemical composition and microstructure. The process is with high productivity, economic efficiency and ecological friendliness. As an effective surface modification method, PEO technique has been investigated extensively to form ceramic coating on titanium alloys with high bond strength, improved corrosion resistance as well as wear resistance [20,21,22]. However, to date, very limited attention has been given to the research of high-temperature properties of PEO coatings. Methods to improve the properties of PEO ceramic coatings at elevated temperatures are urgent to be discovered. PEO technique is a multi-factor controlled process, by which microstructure and properties of ceramic coatings can be tailored by electrical parameters and electrolyte compositions [23–30]. Thus, being a key role on the formation of desired PEO coatings, electrolyte composition with specific additives inside is a very important controllable factor during the PEO preparation process. It has proved that the addition of powders with different nature into electrolyte influences on both properties of obtained coatings and the rate of their formation [27–29]. With a high content of Ti in Ti2AlNb alloy, large amount of titanium oxide is formed on the surface during the PEO process. However, TiO2 cannot offer enough protection to the alloy substrate due to loose coating structure and poor oxidation resistance. Excellent oxidation resistance of ceramic coatings on Ti2AlNb substrate is mainly attributed to the formation of Al-containing oxide layer [23]. Therefore, with the aim of improving the content of aluminum oxide in the as-prepared ceramic coatings on Ti2AlNb substrate, Al2O3 powder was selected as an additive into basic electrolyte of NaAlO2 in this work. Influence of different Al2O3 contents in the NaAlO2 electrolyte on microstructure formation and high-temperature oxidation resistance were discussed to further evaluate the adhesive strength, micro-hardness and tribological properties of PEO coatings.
Table 1 Technological parameters of plasma electrolytic oxidation process. Technological parameters
Data
Voltage (V) Oxidation time (min) Frequency (Hz) Cycle duty (%) Temperature (°C) pH
550 20 600 8 30–50 12–14
coatings were characterized before and after oxidation or wear tests by a scanning electron microscope (SEM, FEI Quanta 200F, The Netherlands). Specimens were mounted into organic resins for microscopy. Isothermal oxidation tests were performed at 800 °C up to 150 h in air with a muffle furnace. Before that, specimens were cleaned with alcohol and acetone, and then were dried in oven at 60 °C for 12 h to record the mass change. Specimens were placed in alumina crucibles and were taken out of furnace at 5 h, 10 h, 20 h, 40 h, 60 h, 80 h, 100 h, 120 h and 150 h for mass measurements to obtain weight gains as a function of time. A direct pull-off tensile method was used to test the adhesive strength of PEO coatings on an Instron-1195 electronic tensile testing machine. Before testing, specimens were bonded to untreated steel samples on both sides with epoxy resin. Continual loading at a rate of 1.0 mm·min−1 was applied until the specimen was broken. Five measurements were performed to get an average value under identical test condition. Micro-hardness of PEO coatings was measured by a Vickers hardness tester with a load of 50 g and a dwell time of 15 s. Each sample was tested at least 5 times and the average was taken to ensure the accuracy. Tribological properties of PEO coatings in both room temperature and 600 °C were investigated using a ball-on-disk high-temperature wear tester (HT–1000, Lanzhou, China) under dry sliding condition. The specimens served as the disk and sintered Si3N4 ball with a diameter of 5.6 mm served as the counterpart. A sliding radius of 3 mm, a normal load of 2 N, a rotational speed of 400 rpm and a wear duration of 10 min were applied in wear tests. Wear depths and widths of the tested specimens were measured by a surface profilometer (JB-4C, Shanghai Optical Instrument Co., China). The worn surfaces of the tested specimens were characterized by SEM. The corresponding friction coefficients were recorded, and the specific wear rates of PEO coatings were calculated.
2. Experimental procedures Ti2AlNb alloy with the chemical composition of (at.%) 53.54Ti, 21.78Al and 24.49Nb was used as the substrate. The substrates were machined into the dimension of 18 mm × 15 mm × 3 mm for isothermal oxidation tests and 15 mm × 15 mm × 3 mm for wear tests. Prior to PEO, all the specimens of Ti2AlNb substrate were ground with SiC abrasive papers, ultrasonically degreased in acetone, cleaned with deionized water and then dried at room temperature (RT). Ceramic coatings were prepared by home-made plasma electrolytic oxidation equipment with the pretreated Ti2AlNb alloy samples as the anode and the stainless steel samples as the cathode. NaAlO2 solution of 25 g·L−1 was selected as the basic electrolyte, which was demonstrated beneficial to the improvement of high temperature oxidation resistance in previous study [23]. The micron-sized Al2O3 powder is selected as the additive, and its particle size distribution ranges from 1 to 5 μm. Different contents of Al2O3 powder (2 g·L−1, 4 g·L−1 and 6 g·L−1) were introduced into basic electrolyte and the as-prepared coatings were marked as A-2A, A-4A and A-6A, respectively. Coating prepared in basic electrolyte of NaAlO2, labeled as BA, was also prepared as a comparison. Technological parameters of plasma electrolytic oxidation process were given in Table 1. After the PEO process, the coated specimens were cleaned in distilled water, and then dried at room temperature. Thicknesses of the as-prepared coatings were directly measured by a Minitest-600B eddy current coating thickness meter. To ensure the accuracy, each specimen was measured three times and an average was taken. Phase composition of PEO coatings before and after oxidation tests were characterized by X-ray diffraction with Cu-Kα radiation (XRD, Philips X'Pert, The Netherlands). Morphologies of ceramic
3. Results and discussion 3.1. Microstructure of PEO coatings Thickness and surface roughness of the as-prepared PEO coating are listed in Table 2. With increasing the Al2O3 content, the thickness of PEO coating prepared in NaAlO2-Al2O3 electrolyte increases slightly, while the surface roughness of coating increases after the introduction of 2 g·L−1 Al2O3 and then gradually decreases with further increasing Table 2 Thickness and surface roughness of PEO coatings prepared in NaAlO2-Al2O3 electrolyte. Specimens
BA A-2A A-4A A-6A
188
Na2AlO2 (g·L−1) 25 25 25 25
Al2O3 (g·L−1) 0 2 4 6
Thickness (μm)
19 18 21 24
± ± ± ±
2 2 2 2
Surface roughness (μm) 1.82 2.23 1.82 1.63
± ± ± ±
0.11 0.06 0.06 0.07
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The microstructure analysis of ceramic coatings prepared by NaAlO2-Al2O3 electrolyte shows that the suspended Al2O3 powder directly enters the micro-pores through the transient high temperature and pressure of micro arc discharge, and participates in the coating growth, resulting in the change in phase composition and microstructure of the coating. 3.2. Isothermal oxidation resistance of PEO coatings Fig. 3(a) illustrates the oxidation kinetic curves of PEO coatings plotting the weight gain per unit area vs. time in the isothermal oxidation tests at 800 °C. The bared Ti2AlNb substrate is also displayed as a reference. As shown in Fig. 3(a), the oxidation resistance of ceramic coatings is improved by the introduction of Al2O3 powder. The oxidation weight gains of all the PEO coatings prepared in electrolyte with Al2O3 addition are less than the BA coating without Al2O3 powder as an additive. With the increase of Al2O3 content, the oxidation weight gains of ceramic coating first decrease and then increase. When the Al2O3 addition is 4 g·L−1, the ceramic coating exhibits the least oxidation weight gain, with only ΔM=1.33 mg·cm−2. The improved high-temperature oxidation resistance is related to the high content of Al2TiO5 in the coating, which helps to lower the activity of oxygen in the coating. Fig. 3(b) plots the square weight gain per unit area vs. time of all specimens. The change of slope in different periods of exposure time is typical of all specimens, and indicates deviations from “ideal” parabolic oxidation behavior. Therefore, in attempt to analyze the kinetic law of oxidation mathematically, a pervasive equation for kinetics of alloy oxidation is selected to fit the ΔM − t curves:
Fig. 1. XRD patterns of PEO coatings prepared in different NaAlO2-Al2O3 electrolytes.
the Al2O3 content. Phase constituents of the as-prepared PEO coatings are illustrated in Fig. 1. As can be seen, the PEO coatings prepared in the basic NaAlO2 electrolyte is mainly composed of a large amount of R-TiO2, γ-Al2O3, αAl2O3 and a small amount of A-TiO2 phase. In addition to these phases, Al2TiO5 is formed due to the addition of Al2O3 in the electrolyte, which cannot be observed in the patterns of BA coating. With the addition of Al2O3 into electrolyte, a slight increase in the amount of γ-Al2O3 is observed when comparing the intensity of diffraction peaks at the 2θ of 46° and 47° in Fig. 1. Therefore, it can be deduced that the Al2O3 additive combines with TiO2 by the discharge of the substrate to form Al2TiO5 phase in the PEO process. It is noteworthy that the content of Al2TiO5 phase is relatively high in the ceramic coating of A-4A. The content of Al2TiO5 phase in the coatings is related to the concentration of dielectric Al2O3 particles suspended on the coating surface. When Al2O3 particles are added into the NaAlO2 electrolyte, the suspended Al2O3 near the interface between electrolyte and the coating can quickly enter the discharge micro-pores and react with the internal molten TiO2 to form Al2TiO5 phase. When the amount of Al2O3 exceeds a certain threshold, the dielectric particles at the interface will affect the discharge strength of the coating surface and thereby lead to the decrease in the number of particles entering the micro-pores, thus resulting in a slight decrease in the relative content of Al2TiO5 phase. Fig. 2 shows the surface and cross-sectional morphologies of the asprepared PEO coatings. From the top view of surface morphologies, it can be seen that the coatings prepared in the NaAlO2-Al2O3 electrolyte exhibit the similar porous structure as the ceramic coating prepared in the basic NaAlO2 electrolyte. The surface is covered with a large number of crater-like molten oxides as well as quite a few randomly distributed micro arc discharge holes in different sizes. The big holes in crater-like molten oxides are in the size of several microns, mostly < 10 μm; while the micro arc discharge holes are in the size of submicrons. The surface morphology of coating changes little with increasing the Al2O3 content. From the cross-sectional morphologies shown in Fig. 2, the coating structure becomes more compact after introducing Al2O3 compared to Fig. 2e, and there are a large number of small discharge holes in the interior of coating close to the matrix. The thicknesses of the coatings remain almost unchanged with the increase of Al2O3 content, but the number of residual discharge pores inside the coating increases gradually. It is due to the suspension of dielectric Al2O3 particles in the electrolyte, which affects the discharge strength of the substrate and the cooling rate of the molten oxidized products. As a result, the Al2O3 addition in NaAlO2 electrolyte can exert an influence on microstructure of the coating, and increase the number of micropores at the coating/substrate interface.
(1)
∆M = k1 ∙t + k2 ∙ t −2
where ΔM is the weight gain per unit area (mg·cm ), k1 is the linear oxidation rate constant (mg∙cm−2∙h−1), k2 is parabolic oxidation rate constant (mg2∙cm−4∙h−1) and t is the oxidation time (h). The calculated values of k1 and k2 for bare Ti2AlNb alloy and different PEO ceramic coatings are listed in Table 3. According to the oxidation rate constants given in Table 3, the high temperature oxidation of both Ti2AlNb alloy and specimens with PEO coatings are dominated by diffusion-controlled process. As can be seen, all the specimens with PEO ceramic coating exhibit greatly reduced parabolic oxidation rate constant k2 values. This is because the diffusion of oxygen to the alloy matrix is effectively suppressed due to the presence of surface coating on the alloy, thereby protecting the alloy matrix from oxidation and weight gain. The parabolic oxidation rate constant k2 values are further reduced when Al2O3 powders are introduced into the basic electrolyte, and exhibit a slight increase when the content of Al2O3 reaches 6 g·L−1 in the electrolyte. Specimen with A-4A PEO coating shows the smallest values of parabolic oxidation rate constant k2 (1.45 × 10−2 mg2∙cm−4∙h−1), which is less than half of the BA coating without Al2O3 additive and even less than one tenth of bare Ti2AlNb alloy, representing the best high-temperature oxidation resistance. Fig. 4 shows the surface and cross-sectional morphologies of PEO coatings after isothermal oxidation resistance tests at 800 °C for 150 h. From the surface morphology, it is observed that the surface of ceramic coatings remains porous after oxidation. From the cross-sectional morphology, ceramic coatings are still intact with substrates after oxidation. Dense oxidized layers (OL) between the coating and alloy substrate are formed, among which A-4A and A-6A coatings exhibit relatively thinner oxidized layers (Fig. 4g and h). Cross-sectional morphologies of PEO coatings at different oxidation time were also investigated. Fig. 5 shows the cross-sectional morphologies of A-6A ceramic coating after isothermal oxidation tests at 800 °C for different oxidation duration. A layer of oxide film is formed between coating and matrix during the oxidation, and the oxide layer is thickened as time prolongs. An obvious layered structure can be observed when the coating is oxidized at 800 °C for 150 h (Fig. 5d). In the initial oxidation (t = 20 h), a very thin oxidized layer with a thickness of < 189
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Fig. 2. Surface and cross-sectional morphologies of ceramic coatings prepared in NaAlO2-Al2O3 electrolyte: (a) surface of BA; (b) surface of A-2A; (c) surface of A-4A; (d) surface of A-6A; (e) cross-section of BA; (f) cross-sections of A-2A; (g) cross-sections of A-4A; (h) cross-section of A-6A.
Fig. 3. Oxidation kinetic curves of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C up to 150 h: (a) Mass gain per unit area vs. time plot; (b) square mass gain per unit area vs. time plot of all specimens.
36° and 54°, the BA coating exhibits the largest amount of R-TiO2 after oxidation, while the A-4A coating exhibits the least. It is worth noting that the content of Al2TiO5 shows an obvious decrease for the PEO coatings prepared in electrolyte with Al2O3 additive. This is due to the instability of Al2TiO5 at high temperature. It decomposes to R-TiO2 and α-Al2O3 during the isothermal oxidation tests which follows chemical reaction (2):
Table 3 Values of k1 and k2 for bare Ti2AlNb alloy and different PEO ceramic coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C. Specimens
k1×10−2 (mg∙cm−2∙h−1)
k2×10−2 (mg2∙cm−4∙h−1)
Ti2AlNb BA A-2A A-4A A-6A
0.59 0.86 0.74 0.76 1.00
17.08 4.56 3.21 1.45 2.47
Al2TiO5 → R‐TiO2 + α‐Al2O3
(2)
This reaction is accompanied by a certain volume expansion (11% molar volume), which helps to increase the density of the coating and further inhibits oxygen diffusion. It is rather remarkable that the Al2TiO5 remains the most for the coating of A-4A with the formed RTiO2 phase being the least. This result agrees with the minimum oxidation weight gain and minimum oxidation rate constant of A-4A coating mentioned above. Based on the analysis of isothermal oxidation test at 800 °C up to 150 h, it can be concluded that the high-temperature oxidation resistance of PEO coatings is significantly improved by the introduction of Al2O3 addition into the electrolyte. On one hand, the relatively dense
1 μm is formed at the interface between coating and substrate; with oxidation time extending (t = 60 h), the oxide layer increases to 4 μm; when the oxidation time reaches 100 h, the oxidation layer is thickened to 6 μm; at the end of the isothermal oxidation test (t = 150 h), the oxidation layer is around 8 μm. XRD patterns of the PEO coatings after oxidation at 800 °C for 150 h are illustrated in Fig. 6. For all the specimens after oxidation, the content of R-TiO2 increases a lot with a small amount of A-TiO2 formed at the same time. From the diffraction peaks of R-TiO2 at the 2θ of 27°, 190
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Fig. 4. Surface and cross-section morphologies of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C for 150 h: (a) surface of BA; (b) surface of A-2A; (c) surface of A-4A; (d) surface of A-6A; (e) cross-section of BA; (f) cross-section of A-2A; (g) cross-section of A-4A; (h) cross-section of A6A.
diffusion of oxygen in the coating. When oxygen in air diffuses into the coating through the porous structure, a dense oxide layer is subsequently formed in the interlayer of the PEO coatings adjacent the substrate, to protect the substrate from further oxidation. By improving the internal structure density of the coating and increasing the content of aluminum oxide in the coating, the oxidation weight gain of the Ti2AlNb alloy can be effectively reduced and the high temperature
structure of PEO coating after introducing Al2O3 hinders the diffusion of oxygen to some extent. On the other hand, the presence of Al2TiO5 phase, formed by the reaction between Al2O3 and TiO2 in the PEO process, reduces the oxidation activity inside coating. The high-temperature oxidation mechanism of the PEO ceramic coating is dominated by diffusion control. The presence of the PEO coating first prevents the oxygen in air from directly contacting the alloy matrix and hinders the
(a)
(b)
PEO
OL
PEO
(c)
OL
(d)
PEO
OL
PEO
OL
Fig. 5. Cross-sectional morphologies of PEO coatings prepared in NaAlO2 electrolyte with 6 g·L−1 Al2O3 addition after isothermal oxidation tests at 800 °C for different time: (a) t = 20 h; (b) t = 60 h; (c) t = 100 h; (d) t = 150 h. 191
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alloy when it is in contact at the initial stage of the 600 °C wear test. With the occurrence of wear, the friction pair is affected by the adhesive friction and the furrow force. Temperature in the wear surface rises rapidly, while a complete protective film is not yet formed on the surface, and the friction coefficient gradually increases. For another reason, the rapid drop of friction coefficient may be attributed to the lubrication effect of titanium oxides formed on frictional surfaces during the running-in period [10,31]. However, it is known that Al2O3 and TiO2 have a contrary effect to friction coefficient [10]. The significant increase in Al2O3 content leads to a high friction coefficient in the wear test at elevated temperatures. The samples with PEO coatings also show an increased friction coefficient than that at room temperature. The reason behind can be explained by the high surface roughness of the PEO coatings and the influence of the increasing amount of Al2O3 formed upon heating as well. Both PEO coatings prepared in the basic NaAlO2 and the NaAlO2Al2O3 electrolytes exhibit better tribological properties than Ti2AlNb alloy, but with large variations at high temperatures. The reason for the fluctuation in high temperature friction coefficient is the formation of a large amount of hard oxide wear-debris due to brittle fracture on the coating surface during tribo-stressing sliding at high temperature. The friction coefficient of A-4A coating almost coincides with that of BA coating at 600 °C. Table 5 lists the wear volume and the calculated volumetric wear rate ω based on Eq. (3)
Fig. 6. XRD patterns of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation test at 800 °C for 150 h.
oxidation resistance is thereby improved. The volume expansion caused by the decomposition reaction of Al2TiO5 phase will also, in turn, increase the density of the coating, thereby increasing the anti-oxidation performance of the coating. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the best high-temperature oxidation resistance due to the high content of Al2TiO5 phase and the dense inner structure.
ω=
V N×L
(3)
3.3. Mechanical and tribological properties of PEO coatings Besides the high temperature properties, mechanical and tribological properties are also important for practical applications of PEO coatings prepared on Ti2AlNb alloy. Specimens of A-4A, which exhibit the best high-temperature oxidation performance, are selected in this part to characterize the mechanical and tribological properties. Table 4 listed the adhesive strength and micro-hardness of the A-4A coating with BA coating on Ti2AlNb alloy as references. It can be seen that BA coating prepared in the basic NaAlO2 electrolyte without any additive shows an average adhesive strength of 31 MPa. However, the adhesive strength of the A-4A coating is slightly weakened, being 24 MPa, which is attributed to the formation of various micro-defects inside the coatings. It can be seen from Table 4 that the micro-harness of the A-4A coatings (388 HV0.05) is significantly improved compared to the Ti2AlNb (313 HV0.05) and BA coating (327 HV0.05). The improved micro-hardness of the A-4A coating is ascribed to the existence of hard Al2TiO5 phase. Fig. 7 shows the friction coefficients of the bare Ti2AlNb alloy and the PEO coatings as a function of wear duration at room temperature and 600 °C. At room temperature, the bare Ti2AlNb alloy exhibits the highest friction coefficient, which keeps at 0.67. The A-4A coating has a friction coefficient of 0.62 at room temperature, which is comparable to the BA coating. In the wear test at 600 °C, the friction coefficient of bare Ti2AlNb alloy shows a rapid drop in the beginning of the wear test and increases gradually, with an average value of 0.89. The friction coefficient changing from high to low during the running-in period can be ascribed to the formation of a rough oxide film on the surface of the Table 4 Adhesive strength and micro-hardness of PEO coatings prepared in NaAlO2Al2O3 electrolytes on Ti2AlNb alloy. Specimens
Average adhesive strength (MPa)
HV0.05
Ti2AlNb BA A-4A
– 31 24
313 327 388
−1
−1
where ω is the wear rate (mm ·N m ), V is the wear volume (mm3) and L is the sliding distance (m). The wear of each specimen is more severe at 600 °C than that obtained at room temperature, with the wear volume and wear rate increased by one order of magnitude. According to the above analysis, the friction and wear behavior of all specimens change after the test temperature is increased. The specimens with a PEO coating exhibit a relatively low wear rate at room temperature, demonstrating that the PEO coatings play a wear-resistant role. However, specimens prepared with Al2O3 addition show a slightly high wear rate as compared with bare Ti2AlNb alloy at 600 °C, which may be related to crack propagation in the coating. Fig. 8 gives the surface morphologies of BA and A-4A coatings and Ti2AlNb alloy after wear tests at room temperature. The wear scar of Ti2AlNb alloy is wide. A plough zone can be found on the surface, which suggests that abrasive wear being the main wear mechanism. The wear scar of the BA and A-4A coatings are much shallow and the worn surfaces are relatively smooth. No visible furrows can be observed. The BA coating has only slight wear. The porous structure of ceramic coatings still remains on the wear scar. The improved wear resistance is owing to high hardness and the filling of wear debris into the discharge holes in the surface of the PEO coating, by which a smooth zone is thus formed on the surface. The wear volume of the A-4A coating is slightly large but still comparable to that of the BA coating at room temperature. Fig. 9 shows the surface morphologies after wear tests at 600 °C. Obviously, the wear behavior is deteriorated at high temperature. With a clear trace of serious plowing and plastic smearing, Ti2AlNb alloy is mainly dominated by both abrasive wear and adhesive wear. For the specimens with PEO coatings, the substrates are also exposed in the middle of the wear scar, and the coating cannot provide complete protection for the Ti2AlNb substrate any more. The wear scars of the PEO coatings are featured with wide and rough wear tracks. The wear of BA coating at high temperature is mainly abrasive wear. During the wear test, the coating is gradually stripped under the dual effect of friction shear force and wear load. Flaked friction products are formed on worn surfaces from the debris by repeated grinding and extrusion. The exposed substrate in the middle of wear scar forms plough zone after contacting with the grinding ball. The wear mechanism of A-4A 3
192
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Fig. 7. Friction coefficients of Ti2AlNb alloy and different PEO coatings room temperature as a function of wear duration at (a) room temperature and (b) 600 °C. Table 5 Friction coefficient, wear volume and wear rate of Ti2AlNb alloy and different PEO coatings at room temperature and 600 °C. Specimens
RT Ti2AlNb BA A-4A
0.67 0.61 0.62
Wear rate (mm3·N−1 m−1)
Wear volume (mm3)
Friction coefficient 600 °C
RT
0.89 0.71 0.71
(a)
600 °C −2
1.45 × 10 5.41 × 10−3 6.45 × 10−3
0.13 0.11 0.15
(b)
600 °C −5
9.64 × 10 3.59 × 10−5 4.28 × 10−5
8.63 × 10−4 7.40 × 10−4 1.03 × 10−3
(c)
500µm
(d)
RT
500µm
(e)
500µm
(f)
50µm
50µm
50µm
Fig. 8. Surface morphologies of different specimens after wear tests at room temperature: (a) (d) Ti2AlNb; (b) (e) BA coating; (c) (f) A-4A coating.
with the coupled Si3N4 ball in the initial stage of wear (1). At this time, the normal load concentrates on the top of a few high micro-protrusions with a very small contact area. Then, the micro-protrusions are cut off by transverse shearing force, thus leaving wear debris on the worn surface. In the further wear process (2), the lower micro-protrusions on the coating surface are exposed to the grinding balls, and the tribostressing area is increased. The debris accumulated on the worn surface becomes finer after repeated rolling and grinding. In the steady period of wearing (3), the porous structure on the coating surface is filled with fine wear debris, and a compact film is thus formed on the surface of the wear scar to act as a solid lubricant, which can prevent the coating from directly contact with the grinding balls. The wear mechanism of the
coating is a combination of abrasive wear with adhesive wear. The cover layer on the surface is formed by an accumulation of grinding. The improved wear resistance at high temperature could be also ascribed to the micro-pores in PEO coatings, where the large amount of fine wear debris fills into. By this means, the coatings are protected from serious delamination at 600 °C. The hard phase of Al2TiO5 also plays a role in reducing friction between the friction pairs, with which the coating can effectively support normal load during the wear process. Fig. 10 illustrates schematically the wear mechanism of PEO ceramic coatings at both tested temperatures. At room temperature, as shown in Fig. 10(a), micro-protrusion in the coating is first in contact 193
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(a)
(b)
(c)
500µm
(d)
500µm
500µm
(e)
(f)
50µm
50µm
50µm
Fig. 9. Surface morphologies of different specimens after wear tests at 600 °C: (a) (d) Ti2AlNb; (b) (e) BA coating; (c) (f) A-4A coating.
(1)
(2)
(3)
Si3N4 ball
(a) PEO coating
Ti2AlNb substrate
(4)
Si3N4 ball
Si3N4 ball
PEO coating
PEO coating
Ti2AlNb substrate
(5)
Ti2AlNb substrate
(6)
Si3N4 ball Si3N4 ball
(b) PEO coating
Ti2AlNb substrate
PEO
Si3N4 ball PEO
Ti2AlNb substrate
Ti2AlNb substrate
Fig. 10. Schematic illustrations of wear mechanism of PEO ceramic coatings: (a) at room temperature; (b) at high temperature.
part of the substrate alloy is exposed directly to the grinding ball with prolonging the wear duration. Brittle fracture occurs at the edge of the wear scar and a large amount of wear debris is formed after the cracks being subjected to the transverse shearing force. The wear surface is distributed with discrete large pieces of wear debris, some of which come from fragmented coatings and some of which are from the matrix.
coating at room temperature is mainly mechanical polishing and brittle micro-fracture. When the coating contains a large amount of hard phase, namely Al2TiO5 in this work, the coating can effectively support the normal load during the wear process, and the generated debris play a role in reducing the friction between the friction pairs, thereby improving the wear resistance of ceramic coatings. The wear of the PEO coating shows deterioration at high temperatures, with increased values of not only the friction coefficient but also the wear volume. The wear mechanism of the coating is more complicated with prolonging the wear duration, including fatigue wear and partial delamination in addition to mechanical polishing and brittle micro-fracture. As can be seen in Fig. 10(b), part of the coating fails during the wear process due to the thermal expansion mismatch, while
4. Conclusions (1) The addition of Al2O3 powder into the NaAlO2 basic electrolyte promotes the formation of Al2TiO5 phase in PEO coatings, and the main phases in the coating are Al2TiO5, R-TiO2, A-TiO2, γ-Al2O3 and α-Al2O3, which leads to the increase in coating microhardness.
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(2) Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings prepared with Al2O3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the least mass gain of only 1.33 mg cm−2. (3) The adhesive strength of the coating prepared electrolyte with 4 g L−1 Al2O3 additive is slightly weakened compared to the coating prepared in the basic electrolyte, which is due to the formation of defects inside the ceramic coating. The micro-hardness of PEO coatings prepared with 4 g L−1Al2O3 additive in the electrolyte is improved due to the presence of Al2TiO5 phase. (4) The wear resistance of Ti2AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coating prepared with Al2O3 additive is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.
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Influence of Al2O3 addition in NaAlO2 electrolyte on microstructure and high-temperature properties of plasma electrolytic oxidation ceramic coatings on Ti2AlNb alloy
T
⁎
Zhao-Ying Dinga, Yuan-Hong Wanga, Jia-Hu Ouyanga,b, , Zhan-Guo Liua,b, Ya-Ming Wanga,b, Yu-Jin Wanga,b a
Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150001, China School of Materials Science and Engineering, Harbin Institute of Technology, 92 Westdazhi Street, Harbin 150001, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasma electrolytic oxidation Ti2AlNb alloy Al2O3 addition Oxidation resistance Tribological properties
Plasma electrolytic oxidation (PEO) has become an effective surface modification method to form ceramic coatings on orthorhombic Ti2AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al2O3 additive were introduced into a NaAlO2 basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti2AlNb alloy. The influence of Al2O3 addition in NaAlO2 electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of Xray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-ondisc friction and wear tests. The introduction of Al2O3 into the basic electrolyte promotes the formation of Al2TiO5 phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al2O3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the least mass gain of only 1.33 mg cm−2. The micro-hardness of the PEO coatings prepared with Al2O3 additive is improved due to the presence of Al2TiO5 phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti2AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al2O3 addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.
1. Introduction Orthorhombic Ti2AlNb alloy, as a damage-tolerant, good hightemperature performance and lightweight material, has attracted great attention by jet engine manufacturers and related research labs in the worldwide since 1990s [1–3]. Due to its excellent properties of high specific strength, high fracture toughness and high creep resistance, OTi2AlNb has become a promising candidate as high-temperature structural material for applications in aircraft, automobile industries, thermal control coatings, etc., where heavy nickel-base superalloys are the only alternative today [1–4]. However, practical applications of Ti2AlNb alloy are restricted by its poor oxidation- and wear-resistance at elevated temperatures [4–13]. Surface engineering is the most
promising approach to facilitate the applications for hot-section components in aircraft and automobile industries [4]. Protective coatings are usually fabricated on the surface of Ti2AlNb with the aim of improving its high-temperature properties at operating temperatures of 650-800 °C [4–16]. Raluca Pflumm et al. [4] reviewed different coating methods to protect TiAl-based alloy from the oxidation, including their general advantages and drawbacks, among which, overlay coating on the substrates have a potential for an efficient oxidation protection of TiAl alloys. However, overlay coatings prepared by methods like PVD, thermal spraying, laser cladding, etc. are in relatively high costs and high complexity [4,12,17–19]. Coating methods in high productivity, economic efficiency and ecological friendliness are urgent to be developed. On the other hand, most researches on Ti2AlNb protection
⁎ Corresponding author at: Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150001, China. E-mail addresses: [email protected] (J.-H. Ouyang), [email protected] (Z.-G. Liu), [email protected] (Y.-M. Wang), [email protected] (Y.-J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.04.075 Received 17 December 2018; Received in revised form 12 April 2019; Accepted 24 April 2019 Available online 01 May 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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only focus solely on either oxidation- or wear-resistant properties. Comprehensive researches on high temperature properties of the overlaying coating are very limited. Plasma electrolytic oxidation (PEO) is an in situ growth process for fabricating ceramic coatings on metals and alloys with designed chemical composition and microstructure. The process is with high productivity, economic efficiency and ecological friendliness. As an effective surface modification method, PEO technique has been investigated extensively to form ceramic coating on titanium alloys with high bond strength, improved corrosion resistance as well as wear resistance [20,21,22]. However, to date, very limited attention has been given to the research of high-temperature properties of PEO coatings. Methods to improve the properties of PEO ceramic coatings at elevated temperatures are urgent to be discovered. PEO technique is a multi-factor controlled process, by which microstructure and properties of ceramic coatings can be tailored by electrical parameters and electrolyte compositions [23–30]. Thus, being a key role on the formation of desired PEO coatings, electrolyte composition with specific additives inside is a very important controllable factor during the PEO preparation process. It has proved that the addition of powders with different nature into electrolyte influences on both properties of obtained coatings and the rate of their formation [27–29]. With a high content of Ti in Ti2AlNb alloy, large amount of titanium oxide is formed on the surface during the PEO process. However, TiO2 cannot offer enough protection to the alloy substrate due to loose coating structure and poor oxidation resistance. Excellent oxidation resistance of ceramic coatings on Ti2AlNb substrate is mainly attributed to the formation of Al-containing oxide layer [23]. Therefore, with the aim of improving the content of aluminum oxide in the as-prepared ceramic coatings on Ti2AlNb substrate, Al2O3 powder was selected as an additive into basic electrolyte of NaAlO2 in this work. Influence of different Al2O3 contents in the NaAlO2 electrolyte on microstructure formation and high-temperature oxidation resistance were discussed to further evaluate the adhesive strength, micro-hardness and tribological properties of PEO coatings.
Table 1 Technological parameters of plasma electrolytic oxidation process. Technological parameters
Data
Voltage (V) Oxidation time (min) Frequency (Hz) Cycle duty (%) Temperature (°C) pH
550 20 600 8 30–50 12–14
coatings were characterized before and after oxidation or wear tests by a scanning electron microscope (SEM, FEI Quanta 200F, The Netherlands). Specimens were mounted into organic resins for microscopy. Isothermal oxidation tests were performed at 800 °C up to 150 h in air with a muffle furnace. Before that, specimens were cleaned with alcohol and acetone, and then were dried in oven at 60 °C for 12 h to record the mass change. Specimens were placed in alumina crucibles and were taken out of furnace at 5 h, 10 h, 20 h, 40 h, 60 h, 80 h, 100 h, 120 h and 150 h for mass measurements to obtain weight gains as a function of time. A direct pull-off tensile method was used to test the adhesive strength of PEO coatings on an Instron-1195 electronic tensile testing machine. Before testing, specimens were bonded to untreated steel samples on both sides with epoxy resin. Continual loading at a rate of 1.0 mm·min−1 was applied until the specimen was broken. Five measurements were performed to get an average value under identical test condition. Micro-hardness of PEO coatings was measured by a Vickers hardness tester with a load of 50 g and a dwell time of 15 s. Each sample was tested at least 5 times and the average was taken to ensure the accuracy. Tribological properties of PEO coatings in both room temperature and 600 °C were investigated using a ball-on-disk high-temperature wear tester (HT–1000, Lanzhou, China) under dry sliding condition. The specimens served as the disk and sintered Si3N4 ball with a diameter of 5.6 mm served as the counterpart. A sliding radius of 3 mm, a normal load of 2 N, a rotational speed of 400 rpm and a wear duration of 10 min were applied in wear tests. Wear depths and widths of the tested specimens were measured by a surface profilometer (JB-4C, Shanghai Optical Instrument Co., China). The worn surfaces of the tested specimens were characterized by SEM. The corresponding friction coefficients were recorded, and the specific wear rates of PEO coatings were calculated.
2. Experimental procedures Ti2AlNb alloy with the chemical composition of (at.%) 53.54Ti, 21.78Al and 24.49Nb was used as the substrate. The substrates were machined into the dimension of 18 mm × 15 mm × 3 mm for isothermal oxidation tests and 15 mm × 15 mm × 3 mm for wear tests. Prior to PEO, all the specimens of Ti2AlNb substrate were ground with SiC abrasive papers, ultrasonically degreased in acetone, cleaned with deionized water and then dried at room temperature (RT). Ceramic coatings were prepared by home-made plasma electrolytic oxidation equipment with the pretreated Ti2AlNb alloy samples as the anode and the stainless steel samples as the cathode. NaAlO2 solution of 25 g·L−1 was selected as the basic electrolyte, which was demonstrated beneficial to the improvement of high temperature oxidation resistance in previous study [23]. The micron-sized Al2O3 powder is selected as the additive, and its particle size distribution ranges from 1 to 5 μm. Different contents of Al2O3 powder (2 g·L−1, 4 g·L−1 and 6 g·L−1) were introduced into basic electrolyte and the as-prepared coatings were marked as A-2A, A-4A and A-6A, respectively. Coating prepared in basic electrolyte of NaAlO2, labeled as BA, was also prepared as a comparison. Technological parameters of plasma electrolytic oxidation process were given in Table 1. After the PEO process, the coated specimens were cleaned in distilled water, and then dried at room temperature. Thicknesses of the as-prepared coatings were directly measured by a Minitest-600B eddy current coating thickness meter. To ensure the accuracy, each specimen was measured three times and an average was taken. Phase composition of PEO coatings before and after oxidation tests were characterized by X-ray diffraction with Cu-Kα radiation (XRD, Philips X'Pert, The Netherlands). Morphologies of ceramic
3. Results and discussion 3.1. Microstructure of PEO coatings Thickness and surface roughness of the as-prepared PEO coating are listed in Table 2. With increasing the Al2O3 content, the thickness of PEO coating prepared in NaAlO2-Al2O3 electrolyte increases slightly, while the surface roughness of coating increases after the introduction of 2 g·L−1 Al2O3 and then gradually decreases with further increasing Table 2 Thickness and surface roughness of PEO coatings prepared in NaAlO2-Al2O3 electrolyte. Specimens
BA A-2A A-4A A-6A
188
Na2AlO2 (g·L−1) 25 25 25 25
Al2O3 (g·L−1) 0 2 4 6
Thickness (μm)
19 18 21 24
± ± ± ±
2 2 2 2
Surface roughness (μm) 1.82 2.23 1.82 1.63
± ± ± ±
0.11 0.06 0.06 0.07
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The microstructure analysis of ceramic coatings prepared by NaAlO2-Al2O3 electrolyte shows that the suspended Al2O3 powder directly enters the micro-pores through the transient high temperature and pressure of micro arc discharge, and participates in the coating growth, resulting in the change in phase composition and microstructure of the coating. 3.2. Isothermal oxidation resistance of PEO coatings Fig. 3(a) illustrates the oxidation kinetic curves of PEO coatings plotting the weight gain per unit area vs. time in the isothermal oxidation tests at 800 °C. The bared Ti2AlNb substrate is also displayed as a reference. As shown in Fig. 3(a), the oxidation resistance of ceramic coatings is improved by the introduction of Al2O3 powder. The oxidation weight gains of all the PEO coatings prepared in electrolyte with Al2O3 addition are less than the BA coating without Al2O3 powder as an additive. With the increase of Al2O3 content, the oxidation weight gains of ceramic coating first decrease and then increase. When the Al2O3 addition is 4 g·L−1, the ceramic coating exhibits the least oxidation weight gain, with only ΔM=1.33 mg·cm−2. The improved high-temperature oxidation resistance is related to the high content of Al2TiO5 in the coating, which helps to lower the activity of oxygen in the coating. Fig. 3(b) plots the square weight gain per unit area vs. time of all specimens. The change of slope in different periods of exposure time is typical of all specimens, and indicates deviations from “ideal” parabolic oxidation behavior. Therefore, in attempt to analyze the kinetic law of oxidation mathematically, a pervasive equation for kinetics of alloy oxidation is selected to fit the ΔM − t curves:
Fig. 1. XRD patterns of PEO coatings prepared in different NaAlO2-Al2O3 electrolytes.
the Al2O3 content. Phase constituents of the as-prepared PEO coatings are illustrated in Fig. 1. As can be seen, the PEO coatings prepared in the basic NaAlO2 electrolyte is mainly composed of a large amount of R-TiO2, γ-Al2O3, αAl2O3 and a small amount of A-TiO2 phase. In addition to these phases, Al2TiO5 is formed due to the addition of Al2O3 in the electrolyte, which cannot be observed in the patterns of BA coating. With the addition of Al2O3 into electrolyte, a slight increase in the amount of γ-Al2O3 is observed when comparing the intensity of diffraction peaks at the 2θ of 46° and 47° in Fig. 1. Therefore, it can be deduced that the Al2O3 additive combines with TiO2 by the discharge of the substrate to form Al2TiO5 phase in the PEO process. It is noteworthy that the content of Al2TiO5 phase is relatively high in the ceramic coating of A-4A. The content of Al2TiO5 phase in the coatings is related to the concentration of dielectric Al2O3 particles suspended on the coating surface. When Al2O3 particles are added into the NaAlO2 electrolyte, the suspended Al2O3 near the interface between electrolyte and the coating can quickly enter the discharge micro-pores and react with the internal molten TiO2 to form Al2TiO5 phase. When the amount of Al2O3 exceeds a certain threshold, the dielectric particles at the interface will affect the discharge strength of the coating surface and thereby lead to the decrease in the number of particles entering the micro-pores, thus resulting in a slight decrease in the relative content of Al2TiO5 phase. Fig. 2 shows the surface and cross-sectional morphologies of the asprepared PEO coatings. From the top view of surface morphologies, it can be seen that the coatings prepared in the NaAlO2-Al2O3 electrolyte exhibit the similar porous structure as the ceramic coating prepared in the basic NaAlO2 electrolyte. The surface is covered with a large number of crater-like molten oxides as well as quite a few randomly distributed micro arc discharge holes in different sizes. The big holes in crater-like molten oxides are in the size of several microns, mostly < 10 μm; while the micro arc discharge holes are in the size of submicrons. The surface morphology of coating changes little with increasing the Al2O3 content. From the cross-sectional morphologies shown in Fig. 2, the coating structure becomes more compact after introducing Al2O3 compared to Fig. 2e, and there are a large number of small discharge holes in the interior of coating close to the matrix. The thicknesses of the coatings remain almost unchanged with the increase of Al2O3 content, but the number of residual discharge pores inside the coating increases gradually. It is due to the suspension of dielectric Al2O3 particles in the electrolyte, which affects the discharge strength of the substrate and the cooling rate of the molten oxidized products. As a result, the Al2O3 addition in NaAlO2 electrolyte can exert an influence on microstructure of the coating, and increase the number of micropores at the coating/substrate interface.
(1)
∆M = k1 ∙t + k2 ∙ t −2
where ΔM is the weight gain per unit area (mg·cm ), k1 is the linear oxidation rate constant (mg∙cm−2∙h−1), k2 is parabolic oxidation rate constant (mg2∙cm−4∙h−1) and t is the oxidation time (h). The calculated values of k1 and k2 for bare Ti2AlNb alloy and different PEO ceramic coatings are listed in Table 3. According to the oxidation rate constants given in Table 3, the high temperature oxidation of both Ti2AlNb alloy and specimens with PEO coatings are dominated by diffusion-controlled process. As can be seen, all the specimens with PEO ceramic coating exhibit greatly reduced parabolic oxidation rate constant k2 values. This is because the diffusion of oxygen to the alloy matrix is effectively suppressed due to the presence of surface coating on the alloy, thereby protecting the alloy matrix from oxidation and weight gain. The parabolic oxidation rate constant k2 values are further reduced when Al2O3 powders are introduced into the basic electrolyte, and exhibit a slight increase when the content of Al2O3 reaches 6 g·L−1 in the electrolyte. Specimen with A-4A PEO coating shows the smallest values of parabolic oxidation rate constant k2 (1.45 × 10−2 mg2∙cm−4∙h−1), which is less than half of the BA coating without Al2O3 additive and even less than one tenth of bare Ti2AlNb alloy, representing the best high-temperature oxidation resistance. Fig. 4 shows the surface and cross-sectional morphologies of PEO coatings after isothermal oxidation resistance tests at 800 °C for 150 h. From the surface morphology, it is observed that the surface of ceramic coatings remains porous after oxidation. From the cross-sectional morphology, ceramic coatings are still intact with substrates after oxidation. Dense oxidized layers (OL) between the coating and alloy substrate are formed, among which A-4A and A-6A coatings exhibit relatively thinner oxidized layers (Fig. 4g and h). Cross-sectional morphologies of PEO coatings at different oxidation time were also investigated. Fig. 5 shows the cross-sectional morphologies of A-6A ceramic coating after isothermal oxidation tests at 800 °C for different oxidation duration. A layer of oxide film is formed between coating and matrix during the oxidation, and the oxide layer is thickened as time prolongs. An obvious layered structure can be observed when the coating is oxidized at 800 °C for 150 h (Fig. 5d). In the initial oxidation (t = 20 h), a very thin oxidized layer with a thickness of < 189
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Fig. 2. Surface and cross-sectional morphologies of ceramic coatings prepared in NaAlO2-Al2O3 electrolyte: (a) surface of BA; (b) surface of A-2A; (c) surface of A-4A; (d) surface of A-6A; (e) cross-section of BA; (f) cross-sections of A-2A; (g) cross-sections of A-4A; (h) cross-section of A-6A.
Fig. 3. Oxidation kinetic curves of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C up to 150 h: (a) Mass gain per unit area vs. time plot; (b) square mass gain per unit area vs. time plot of all specimens.
36° and 54°, the BA coating exhibits the largest amount of R-TiO2 after oxidation, while the A-4A coating exhibits the least. It is worth noting that the content of Al2TiO5 shows an obvious decrease for the PEO coatings prepared in electrolyte with Al2O3 additive. This is due to the instability of Al2TiO5 at high temperature. It decomposes to R-TiO2 and α-Al2O3 during the isothermal oxidation tests which follows chemical reaction (2):
Table 3 Values of k1 and k2 for bare Ti2AlNb alloy and different PEO ceramic coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C. Specimens
k1×10−2 (mg∙cm−2∙h−1)
k2×10−2 (mg2∙cm−4∙h−1)
Ti2AlNb BA A-2A A-4A A-6A
0.59 0.86 0.74 0.76 1.00
17.08 4.56 3.21 1.45 2.47
Al2TiO5 → R‐TiO2 + α‐Al2O3
(2)
This reaction is accompanied by a certain volume expansion (11% molar volume), which helps to increase the density of the coating and further inhibits oxygen diffusion. It is rather remarkable that the Al2TiO5 remains the most for the coating of A-4A with the formed RTiO2 phase being the least. This result agrees with the minimum oxidation weight gain and minimum oxidation rate constant of A-4A coating mentioned above. Based on the analysis of isothermal oxidation test at 800 °C up to 150 h, it can be concluded that the high-temperature oxidation resistance of PEO coatings is significantly improved by the introduction of Al2O3 addition into the electrolyte. On one hand, the relatively dense
1 μm is formed at the interface between coating and substrate; with oxidation time extending (t = 60 h), the oxide layer increases to 4 μm; when the oxidation time reaches 100 h, the oxidation layer is thickened to 6 μm; at the end of the isothermal oxidation test (t = 150 h), the oxidation layer is around 8 μm. XRD patterns of the PEO coatings after oxidation at 800 °C for 150 h are illustrated in Fig. 6. For all the specimens after oxidation, the content of R-TiO2 increases a lot with a small amount of A-TiO2 formed at the same time. From the diffraction peaks of R-TiO2 at the 2θ of 27°, 190
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Fig. 4. Surface and cross-section morphologies of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation tests at 800 °C for 150 h: (a) surface of BA; (b) surface of A-2A; (c) surface of A-4A; (d) surface of A-6A; (e) cross-section of BA; (f) cross-section of A-2A; (g) cross-section of A-4A; (h) cross-section of A6A.
diffusion of oxygen in the coating. When oxygen in air diffuses into the coating through the porous structure, a dense oxide layer is subsequently formed in the interlayer of the PEO coatings adjacent the substrate, to protect the substrate from further oxidation. By improving the internal structure density of the coating and increasing the content of aluminum oxide in the coating, the oxidation weight gain of the Ti2AlNb alloy can be effectively reduced and the high temperature
structure of PEO coating after introducing Al2O3 hinders the diffusion of oxygen to some extent. On the other hand, the presence of Al2TiO5 phase, formed by the reaction between Al2O3 and TiO2 in the PEO process, reduces the oxidation activity inside coating. The high-temperature oxidation mechanism of the PEO ceramic coating is dominated by diffusion control. The presence of the PEO coating first prevents the oxygen in air from directly contacting the alloy matrix and hinders the
(a)
(b)
PEO
OL
PEO
(c)
OL
(d)
PEO
OL
PEO
OL
Fig. 5. Cross-sectional morphologies of PEO coatings prepared in NaAlO2 electrolyte with 6 g·L−1 Al2O3 addition after isothermal oxidation tests at 800 °C for different time: (a) t = 20 h; (b) t = 60 h; (c) t = 100 h; (d) t = 150 h. 191
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alloy when it is in contact at the initial stage of the 600 °C wear test. With the occurrence of wear, the friction pair is affected by the adhesive friction and the furrow force. Temperature in the wear surface rises rapidly, while a complete protective film is not yet formed on the surface, and the friction coefficient gradually increases. For another reason, the rapid drop of friction coefficient may be attributed to the lubrication effect of titanium oxides formed on frictional surfaces during the running-in period [10,31]. However, it is known that Al2O3 and TiO2 have a contrary effect to friction coefficient [10]. The significant increase in Al2O3 content leads to a high friction coefficient in the wear test at elevated temperatures. The samples with PEO coatings also show an increased friction coefficient than that at room temperature. The reason behind can be explained by the high surface roughness of the PEO coatings and the influence of the increasing amount of Al2O3 formed upon heating as well. Both PEO coatings prepared in the basic NaAlO2 and the NaAlO2Al2O3 electrolytes exhibit better tribological properties than Ti2AlNb alloy, but with large variations at high temperatures. The reason for the fluctuation in high temperature friction coefficient is the formation of a large amount of hard oxide wear-debris due to brittle fracture on the coating surface during tribo-stressing sliding at high temperature. The friction coefficient of A-4A coating almost coincides with that of BA coating at 600 °C. Table 5 lists the wear volume and the calculated volumetric wear rate ω based on Eq. (3)
Fig. 6. XRD patterns of PEO coatings prepared in NaAlO2-Al2O3 electrolyte after isothermal oxidation test at 800 °C for 150 h.
oxidation resistance is thereby improved. The volume expansion caused by the decomposition reaction of Al2TiO5 phase will also, in turn, increase the density of the coating, thereby increasing the anti-oxidation performance of the coating. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the best high-temperature oxidation resistance due to the high content of Al2TiO5 phase and the dense inner structure.
ω=
V N×L
(3)
3.3. Mechanical and tribological properties of PEO coatings Besides the high temperature properties, mechanical and tribological properties are also important for practical applications of PEO coatings prepared on Ti2AlNb alloy. Specimens of A-4A, which exhibit the best high-temperature oxidation performance, are selected in this part to characterize the mechanical and tribological properties. Table 4 listed the adhesive strength and micro-hardness of the A-4A coating with BA coating on Ti2AlNb alloy as references. It can be seen that BA coating prepared in the basic NaAlO2 electrolyte without any additive shows an average adhesive strength of 31 MPa. However, the adhesive strength of the A-4A coating is slightly weakened, being 24 MPa, which is attributed to the formation of various micro-defects inside the coatings. It can be seen from Table 4 that the micro-harness of the A-4A coatings (388 HV0.05) is significantly improved compared to the Ti2AlNb (313 HV0.05) and BA coating (327 HV0.05). The improved micro-hardness of the A-4A coating is ascribed to the existence of hard Al2TiO5 phase. Fig. 7 shows the friction coefficients of the bare Ti2AlNb alloy and the PEO coatings as a function of wear duration at room temperature and 600 °C. At room temperature, the bare Ti2AlNb alloy exhibits the highest friction coefficient, which keeps at 0.67. The A-4A coating has a friction coefficient of 0.62 at room temperature, which is comparable to the BA coating. In the wear test at 600 °C, the friction coefficient of bare Ti2AlNb alloy shows a rapid drop in the beginning of the wear test and increases gradually, with an average value of 0.89. The friction coefficient changing from high to low during the running-in period can be ascribed to the formation of a rough oxide film on the surface of the Table 4 Adhesive strength and micro-hardness of PEO coatings prepared in NaAlO2Al2O3 electrolytes on Ti2AlNb alloy. Specimens
Average adhesive strength (MPa)
HV0.05
Ti2AlNb BA A-4A
– 31 24
313 327 388
−1
−1
where ω is the wear rate (mm ·N m ), V is the wear volume (mm3) and L is the sliding distance (m). The wear of each specimen is more severe at 600 °C than that obtained at room temperature, with the wear volume and wear rate increased by one order of magnitude. According to the above analysis, the friction and wear behavior of all specimens change after the test temperature is increased. The specimens with a PEO coating exhibit a relatively low wear rate at room temperature, demonstrating that the PEO coatings play a wear-resistant role. However, specimens prepared with Al2O3 addition show a slightly high wear rate as compared with bare Ti2AlNb alloy at 600 °C, which may be related to crack propagation in the coating. Fig. 8 gives the surface morphologies of BA and A-4A coatings and Ti2AlNb alloy after wear tests at room temperature. The wear scar of Ti2AlNb alloy is wide. A plough zone can be found on the surface, which suggests that abrasive wear being the main wear mechanism. The wear scar of the BA and A-4A coatings are much shallow and the worn surfaces are relatively smooth. No visible furrows can be observed. The BA coating has only slight wear. The porous structure of ceramic coatings still remains on the wear scar. The improved wear resistance is owing to high hardness and the filling of wear debris into the discharge holes in the surface of the PEO coating, by which a smooth zone is thus formed on the surface. The wear volume of the A-4A coating is slightly large but still comparable to that of the BA coating at room temperature. Fig. 9 shows the surface morphologies after wear tests at 600 °C. Obviously, the wear behavior is deteriorated at high temperature. With a clear trace of serious plowing and plastic smearing, Ti2AlNb alloy is mainly dominated by both abrasive wear and adhesive wear. For the specimens with PEO coatings, the substrates are also exposed in the middle of the wear scar, and the coating cannot provide complete protection for the Ti2AlNb substrate any more. The wear scars of the PEO coatings are featured with wide and rough wear tracks. The wear of BA coating at high temperature is mainly abrasive wear. During the wear test, the coating is gradually stripped under the dual effect of friction shear force and wear load. Flaked friction products are formed on worn surfaces from the debris by repeated grinding and extrusion. The exposed substrate in the middle of wear scar forms plough zone after contacting with the grinding ball. The wear mechanism of A-4A 3
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Fig. 7. Friction coefficients of Ti2AlNb alloy and different PEO coatings room temperature as a function of wear duration at (a) room temperature and (b) 600 °C. Table 5 Friction coefficient, wear volume and wear rate of Ti2AlNb alloy and different PEO coatings at room temperature and 600 °C. Specimens
RT Ti2AlNb BA A-4A
0.67 0.61 0.62
Wear rate (mm3·N−1 m−1)
Wear volume (mm3)
Friction coefficient 600 °C
RT
0.89 0.71 0.71
(a)
600 °C −2
1.45 × 10 5.41 × 10−3 6.45 × 10−3
0.13 0.11 0.15
(b)
600 °C −5
9.64 × 10 3.59 × 10−5 4.28 × 10−5
8.63 × 10−4 7.40 × 10−4 1.03 × 10−3
(c)
500µm
(d)
RT
500µm
(e)
500µm
(f)
50µm
50µm
50µm
Fig. 8. Surface morphologies of different specimens after wear tests at room temperature: (a) (d) Ti2AlNb; (b) (e) BA coating; (c) (f) A-4A coating.
with the coupled Si3N4 ball in the initial stage of wear (1). At this time, the normal load concentrates on the top of a few high micro-protrusions with a very small contact area. Then, the micro-protrusions are cut off by transverse shearing force, thus leaving wear debris on the worn surface. In the further wear process (2), the lower micro-protrusions on the coating surface are exposed to the grinding balls, and the tribostressing area is increased. The debris accumulated on the worn surface becomes finer after repeated rolling and grinding. In the steady period of wearing (3), the porous structure on the coating surface is filled with fine wear debris, and a compact film is thus formed on the surface of the wear scar to act as a solid lubricant, which can prevent the coating from directly contact with the grinding balls. The wear mechanism of the
coating is a combination of abrasive wear with adhesive wear. The cover layer on the surface is formed by an accumulation of grinding. The improved wear resistance at high temperature could be also ascribed to the micro-pores in PEO coatings, where the large amount of fine wear debris fills into. By this means, the coatings are protected from serious delamination at 600 °C. The hard phase of Al2TiO5 also plays a role in reducing friction between the friction pairs, with which the coating can effectively support normal load during the wear process. Fig. 10 illustrates schematically the wear mechanism of PEO ceramic coatings at both tested temperatures. At room temperature, as shown in Fig. 10(a), micro-protrusion in the coating is first in contact 193
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(a)
(b)
(c)
500µm
(d)
500µm
500µm
(e)
(f)
50µm
50µm
50µm
Fig. 9. Surface morphologies of different specimens after wear tests at 600 °C: (a) (d) Ti2AlNb; (b) (e) BA coating; (c) (f) A-4A coating.
(1)
(2)
(3)
Si3N4 ball
(a) PEO coating
Ti2AlNb substrate
(4)
Si3N4 ball
Si3N4 ball
PEO coating
PEO coating
Ti2AlNb substrate
(5)
Ti2AlNb substrate
(6)
Si3N4 ball Si3N4 ball
(b) PEO coating
Ti2AlNb substrate
PEO
Si3N4 ball PEO
Ti2AlNb substrate
Ti2AlNb substrate
Fig. 10. Schematic illustrations of wear mechanism of PEO ceramic coatings: (a) at room temperature; (b) at high temperature.
part of the substrate alloy is exposed directly to the grinding ball with prolonging the wear duration. Brittle fracture occurs at the edge of the wear scar and a large amount of wear debris is formed after the cracks being subjected to the transverse shearing force. The wear surface is distributed with discrete large pieces of wear debris, some of which come from fragmented coatings and some of which are from the matrix.
coating at room temperature is mainly mechanical polishing and brittle micro-fracture. When the coating contains a large amount of hard phase, namely Al2TiO5 in this work, the coating can effectively support the normal load during the wear process, and the generated debris play a role in reducing the friction between the friction pairs, thereby improving the wear resistance of ceramic coatings. The wear of the PEO coating shows deterioration at high temperatures, with increased values of not only the friction coefficient but also the wear volume. The wear mechanism of the coating is more complicated with prolonging the wear duration, including fatigue wear and partial delamination in addition to mechanical polishing and brittle micro-fracture. As can be seen in Fig. 10(b), part of the coating fails during the wear process due to the thermal expansion mismatch, while
4. Conclusions (1) The addition of Al2O3 powder into the NaAlO2 basic electrolyte promotes the formation of Al2TiO5 phase in PEO coatings, and the main phases in the coating are Al2TiO5, R-TiO2, A-TiO2, γ-Al2O3 and α-Al2O3, which leads to the increase in coating microhardness.
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(2) Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings prepared with Al2O3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al2O3 additive exhibits the least mass gain of only 1.33 mg cm−2. (3) The adhesive strength of the coating prepared electrolyte with 4 g L−1 Al2O3 additive is slightly weakened compared to the coating prepared in the basic electrolyte, which is due to the formation of defects inside the ceramic coating. The micro-hardness of PEO coatings prepared with 4 g L−1Al2O3 additive in the electrolyte is improved due to the presence of Al2TiO5 phase. (4) The wear resistance of Ti2AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coating prepared with Al2O3 additive is comparable to those formed in the basic electrolyte at both room temperature and 600 °C.
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