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Influence of TiO2 content on the mechanical and tribological properties of Cr2O3-based coating
Materials and Design 88 (2015) 906–914
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Influence of TiO2 content on the mechanical and tribological properties of Cr2O3-based coating Nan-nan Li a,b, Guo-lu Li a,⁎, Hai-dou Wang b, Jia-jie Kang c, Tian-shun Dong a, Hai-jun Wang b a b c
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China School of Engineering and Technology, China University of Geosciences, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 April 2015 Received in revised form 14 September 2015 Accepted 15 September 2015 Available online 18 September 2015 Keywords: Cr2O3–TiO2 coating Mechanical property Surface free energy Friction coefficient
a b s t r a c t This paper systematically investigated the influence of TiO2 content on the mechanical and tribological properties of a Cr2O3–TiO2 composite coating deposited by plasma spraying technology, and comparatively analyzed their microstructures and surface free energies. The results show that the Cr2O3–TiO2 composite coating exhibited a typical stratification and precipitated a new phase of (Cr0.88Ti0.12)2O3 in comparison with a Cr2O3 coating. The porosity of the coating increased firstly and then decreased as the TiO2 content in the Cr2O3–TiO2 composite coating increased. Furthermore, the TiO2 content added into the Cr2O3–TiO2 composite coating had an obvious influence on micro-hardness. It was also found that the friction coefficient of the coating decreased as the surface free energy decreased. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Plasma sprayed Cr2O3 ceramic coating has been widely used in various fields for improving resistance to corrosion and wear of part surfaces. [1–2]. With the further development of modern industrial technology, some special parts are required for applications in relatively severe environments [3–5]. Therefore, in order to assure that all properties of coatings meet severe service conditions, it is very necessary to further develop Cr2O3 ceramic coatings with more superior performance. As a common thermal spraying material, TiO2 with moderate hardness, good toughness, and excellent resistance to wear and corrosion, can be used to improve the properties of pure Cr2O3 ceramic coating [6–11]. Therefore, a composite coating with a proper proportion of TiO2 and Cr2O3 not only can exhibit a relatively high hardness which can match that of a pure Cr2O3 coating, but also can ensure considerable toughness. Furthermore, the TiO2 addition into the Cr2O3 coating can also reduce the cracking tendency of the coating in the process of plasma spraying. To date, there were numerous studies on the performances of the Cr2O3-based plasma sprayed coating. Kim et al. [12] studied the optimum plasma spraying process of the Cr2O3–3 wt.% TiO2 coating, and the results showed that this composite coating exhibited the most superior performance when performed with a spraying power of 47 kW, a spraying distance of 90 mm, and a powder feed rate of 90 g/min. After measurement, it can be found that the hardness and density were ⁎ Corresponding author. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matdes.2015.09.085 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
higher under the above spray parameters. Zhang et al. [13] implemented epoxy sealing treatment to the Cr2O3–8 wt.% TiO2 coating and then carried out a salt spray corrosion test. Before the sealing treatment, the corrosion results showed that an etching solution entered into the space between the ceramic coating and undercoating along pores and micro-cracks during the experiment. Eventually, the corrosion belt was formed and the bonding strength between the substrate and the coating was reduced. After carrying out the sealing treatment, the amount of open pores and cracks was reduced and the corrosion rate also decreased significantly. Cetinel et al. [14] performed the frictionwear test to a Cr2O3-based coating in an acidic environment. They found that the load magnitude wasn't the dominant factor for the friction coefficient, and the wear loss of the coating was greatly affected by the friction time. In addition, Singh [15], Cho [16], Mishra [17] and other scholars also conducted more thorough researches to the Cr2O3-based coating, and further revealed the failure mechanisms of the coatings under different experimental conditions. However, previous studies of mechanical properties on the Cr2O3based ceramic coating were numerously focused on a single chemical composition (namely Cr2O3 and TiO2 at a specific percentage), and without attracting attention on the changes of mechanical properties caused by changing the second phase content. Moreover, for any material, the exchange of substance and energy will be achieved by the surface coating. Thus, it would be interesting to investigate the physical properties and energy state of the surface coating. Hence, in the present work, Cr2O3–TiO2 ceramic coatings with different TiO2 contents were produced using plasma spraying technology. The purpose of this research is to comprehensively investigate the effects of TiO2 contents
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solution by an ultrasonic apparatus for 20 min to remove the surface stains. Then sandblasting was carried out on the cleaned substrate to remove the rust. To enhance the bonding strength between the coating and the substrate, NiCr alloy powder with a size of 45 μm was deposited on the surface-treated substrate as an undercoating. The substrate was preheated to 300 °C before the deposition to avoid the phenomenon of cracking and delamination. During the spraying, the main gas was argon, and nitrogen was the secondary gas and carrier gas. The SEM images of the NiCr alloy powder and Cr2O3–TiO2 powder are shown in Fig. 1. The Cr2O3–TiO2 powders (dark green) with five different contents of TiO2 (white) were mixed by adopting a mechanical hybrid method. All powders used the same spraying parameters for comparison in the process of coating preparation. The weight ratio of Cr2O3 to TiO2 and the detailed spraying processing parameters are summarized in Table 1. 2.2. Characterization of the coatings Prior to the microscopic observation and the performance test of the coating, specimens were ground by SiC grinding papers from 400 to 2000 grade and then polished. After that, alcohol solution was used to clean the samples. The surface and cross section morphologies were observed by a Nova NanoSEM 450/650 type environmental scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The crystalline phases were analyzed by a D8 X-ray diffractometer with Cu Kα (1.54056 Å) and transmission electron microscopy (TEM). At the same magnification condition (×2000), the porosity of each coating section was measured by image processing software ImageJ2x, and more than ten micrographs were analyzed to obtain statistically averaged values. The micro-hardness of a polished cross section was determined by an HVS-1000 micro-hardness meter with 300 g load for 15 s loading duration time. More than ten indentations were performed on each sample to obtain the mean value. In this article, an OCA-30 automatic contact angle meter was introduced to measure the contact angles of the polished coating. Distilled water, ethylene glycol and diiodomethane were selected and used as the probe liquids. Surface free energies of coatings were calculated by Owens–Wendt equation (Eqs. (1)–(2)) [18] and the detailed free energy parameters of the three probe liquids are listed in Table 2. Friction coefficients of the coatings under the dry condition and the oil medium were tested by the CETR UMT-3 wear tester. The experimental parameters under the dry condition were set as: frequency of 10 Hz, time of 20 min, and load of 20 N. Experimental parameters in oil medium were set as: frequency of 10 Hz, time of 20 min, and load of 60 N. In both experimental conditions the wear scar length was 3 mm and the friction pair was ZrO2 ceramic balls with a diameter of 4 mm.
Fig. 1. Micromorphology of spraying powders (SEM-SE). (a) NiCr alloy particles and (b) Cr2O3–TiO2 particles.
on the mechanical and tribological properties of the Cr2O3-based coating. 2. Materials and methods 2.1. Preparation of coating Cr2O3–TiO2 ceramic coatings (CT) with different contents of TiO2 (mass percentage) were fabricated by a highly effective supersonic plasma spray system (HEPJet) invented by the National Key Laboratory for Remanufacturing, China. The substrate used for depositing an undercoating and a ceramic coating was non-tempered ASTM1045 stainless steel with a dimension of 60 × 50 × 4 mm, and a Rockwell hardness of HRC47. Prior to spraying, the substrates were cleaned in acetone
qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi γL ð1 þ cosθÞ ¼ 2 γ DL γ DS þ γPL γ PS
ð1Þ
γS ¼ γ DS þ γPS
ð2Þ
P where θ is the contact angle,γL, γD L , and γL are the surface tension of the liquid, and its dispersive and polar components, respectively, and γS, γD S,
Table 1 Composition of Cr2O3–TiO2 and the spraying parameters. Sample
Ar/(L min−1)
N2/ (L min−1)
I/A
U/V
Spray distance/mm
Powder feed rate (g min−1)
Cr2O3 (CT0) Cr2O3–8 wt.% TiO2 (CT8) Cr2O3–16 wt.% TiO2 (CT16) Cr2O3–24 wt.% TiO2 (CT24) Cr2O3–32 wt.% TiO2 (CT32)
100
22
385
165
90
35
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Table 2 Surface tension components of the three liquids for contact angle measurements. Test liquids
Distilled water Ethylene glycol Diiodomethane
Temperature/°C
25 25 25
Surface free energy parameters/mJ m−2 γL
γD L
γPL
72.8 48.0 50.8
21.8 29.0 50.8
51.0 19.0 0
and γPS are the surface free energy of the specimen, and its dispersive and polar components, respectively. 3. Results and discussion 3.1. Surface and cross section morphologies Fig. 2 is the SEM image of the coating surface (five kinds of coatings have a similar surface morphology) and cross section (CT16 coating). As can be seen from Fig. 2(a), the coating surface shows a complex structure. There are also many “hills” with irregular shapes on the surface. It can also be found that there are many flat particles when the molten particles cooled and deposited on the substrate from the high magnification image (Fig. 2(b)). The boundary lines of flat particles have a clear image, and there are some micro-holes distributed along those lines. This is caused by the high heat and kinetic energy of particles in the spray process. Firstly, the high temperature in the chest of the spray gun made the particles melt. Then the molten particles moved into the nozzle, where they would be subject to mechanical
compression and self-magnetic compression came from the nozzle and the plasma flame, respectively. The molten particles obtained higher heat and kinetic energy in this process. When the molten particles bombarded the substrate, the cooling-down phenomenon took place rapidly before the molten particles completely fused with each other causing the volume of the particle to contract. The micro-pores and obvious edges then appeared. Fig. 2(c) shows the cross-sectional morphology of the CT16 coating. It can be found that the cross section consists of two kinds of areas―dark and light, which formed the whole cross section alternately. Viewing the cross section as a whole, the layered structure is very clear. Through the magnification of the cross section (Fig. 2(d)), we can find that the dark and light parts have the same porosity, and the distribution of the pores in the coating is relatively uniform. Compared to four kinds of coating containing TiO2, the CT0 coating did not show an obvious layered structure. The XRD patterns of all coatings are shown in Fig. 3. It can be seen that the Cr2O3 coating only contains an Eskolaite phase, as well as the composite coatings of Cr2O3 and TiO2 which are composed of three phases, namely the Eskolaite, Rutile and (Cr0.88Ti0.12)2O3 phases. Among them, the Eskolaite phase which presents a hexagonal structure or an amorphous structure plays a supportive role in the coating, while the Rutile (the Mohs hardness is 6.0–6.5) as the soft–tough phase can enhance the toughness of the coating and reduce the tendency of cracking. The dark area of the coating cross section containing TiO2 was analyzed with Fig. 4 (point A). The results showed that it can be as a binding phase containing more Ti elements, as well as a hard phase with more Cr elements presented in the light area of Fig. 4 (point B). Under high temperature, the three elements of Cr, Ti and O reacted with each other to generate the (Cr0.88 Ti0.12)2O3 phase. The new phase led to a lattice mismatch with the Cr2O3 phase in the light
Fig. 2. Morphology of a coating surface and cross section. (a) The morphology of a coating surface (SEM-SE), (b) the high magnification morphology of a coating surface (SEM-SE), (c) the morphology of a CT16 coating cross section (SEM-BE), and (d) the high magnification morphology of a CT16 cross section (SEM-BE).
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area, which made lattice distortion to occur in the cooling process. In addition, due to the different thermal expansion coefficient between the Cr2O3 phase and the (Cr0.88 Ti0.12)2O3 phase, there are some defects existing in the light area. 3.2. TEM analysis Fig. 5 shows the transmission electron microscope (TEM) images of the Cr2O3–TiO2 coating. It can be seen that the crystal sizes of Cr2O3 were in the range of 200–300 nm (point A shown in Fig. 5(a)). The shape of the grain is similar to a regular hexagon, and there is a clear boundary between the grains. TiO2 grains with a sharpened shape can be found in the Cr2O3 matrix and its size is similar to the Cr2O3 grains (point B shown in Fig. 5(b)). Fig. 5(c) is the high resolution transmission electron microscope (HRTEM) image of TiO2. Lots of defects (such as stacking faults) can be observed in the TiO2 matrix. In addition, some tiny crystal structures are distributed among the Cr2O3 grains diffusely (point C shown in Fig. 5(d)). It is about 50 nm in diameter and smaller than the grain size of Cr2O3 and TiO2. EDS analysis was carried out on the region (Fig. 6), and the results showed that only three elements of Cr, Ti and O could be seen, and that the ratio of Cr and Ti was close to 8:1. Therefore, the tiny phase can be determined as (Cr0.88Ti0.12)2O3. Through additional pictures it can be seen that there are many defects in the interface between (Cr0.88Ti0.12)2O3 and Cr2O3. 3.3. Porosity and micro-hardness Fig. 7 shows the result of the cross-sectional porosity of five kinds of coatings at 2000 times magnification. With the increase of TiO2 content the coating porosity shows a trend of decreases after the first increase. Also, at the TiO2 content of 16% the porosity had a peak value of 8.87%. Compared to pure Cr2O3 coating (porosity of 2.93%), the internal pores of the coating increased significantly. But when the TiO2 content exceeds 16% (24%, 32%), the porosity has emerged with a clear downward trend, and the value of porosity is very close to that of the pure Cr2O3 coating. The cross section morphologies of five kinds of coatings are shown in Fig. 8. It can be seen that the CT0 coating has a denser cross section than other coatings, and the void area is very small. Moreover, the pores are mostly independently existing and there are rare large continuous holes in the cross section. There are also many larger holes with zonal distribution existing in the cross section of the other four coatings, which mostly existed in the light area. Measuring the cross section of coatings containing TiO2, it can be also found that with the increase of TiO2 content, the pores in the light area show a trend of falling
Fig. 3. XRD patterns of the five coatings.
909
after the first increase, and have a maximum value at the point of 16% TiO2. Fig. 9 shows the variation tendency of coating micro-hardness. The pure Cr2O3 coating has a highest hardness of 1697 HV0.3, and the hardness of the other coatings shows a tendency of increase and then decline with the increase of TiO2 content. When the TiO2 content is 16%, the coating has the highest hardness among the four kinds of coatings containing TiO2. Contrasting the change tendency in porosity and micro-hardness (for the four kinds of coating containing TiO2), it can be concluded that the change tendency of coating micro-hardness has a similar tendency with the porosity. Although the CT16 coating has the highest porosity value, its micro-hardness is also the highest. Analyzing the reasons, it may be related to the new phase of (Cr0.88Ti0.12)2O3. This oxide phase containing Cr and Ti may be produced through the titanium atoms that replaced part of the chrome atoms in the original Cr2O3 crystal lattice. Although the overall shape of the Cr2O3 crystal lattice did not change a lot, the differences in the atomic radius of Ti and Cr, which caused internal stress field changes in the phase transformations, will make the lattice distortion. Through calculating the mole ratio of Cr and Ti of the spraying powder and (Cr0.88Ti0.12)2O3 (ignore the mass error caused by powder preparation), it can be discovered that the closest mole ratio of powder to (Cr0.88Ti0.12)2O3 (the ratio is 7.33) is CT16 (the ratio is 5.53), followed by CT24 (3.33), CT8 (12.11) and CT32 (2.24), and the variation is very consistent with the micro-hardness and porosity of the coating containing TiO2. It can be inferred from the results that the TiO2 as a soft–tough phase in the coatings can reduce the hardness, and react with Cr2O3 to form the (Cr0.88Ti0.12)2O3 phase. The new (Cr0.88Ti0.12)2O3 phase has a
Fig. 4. EDS of points A and B in Fig. 2(d). (a) EDS of point A and (b) EDS of point B.
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Fig. 5. The TEM image of a Cr2O3–TiO2 coating. (a) Cr2O3 crystal, (b) TiO2 crystal, (c) the faults in TiO2 crystal, and (d) (Cr0.88Ti0.12)2O3 crystal.
small crystal size and acts as a support phase (grain refinement strengthening) to improve the stress resistance of a coating in geometry. The above analysis also explains why the phenomenon of coatings containing TiO2 have a consistent trend in porosity and micro-hardness.
Table 3 shows the measurement results of the contact angle and the surface free energy. It can be concluded that from the contact angle data of distilled water, the remaining coatings exhibited a hydrophilic
characteristic except for the CT24 coating. The variation trend of the distilled water contact angle of five coatings increased first and then decreased with the increase of TiO2 content. In addition, the coatings' surface free energy calculation results (Fig. 10) show that the pure Cr2O3 coating has the highest surface free energy of 75.06 mJ m−2. The surface free energy of coatings containing TiO2 appears overall to decline, showing a rising tendency after the first drop. The CT16 coating has the lowest value of 56.34 mJ m− 2. Therefore, adding TiO2 to the spraying powder weakened the hydrophilic properties of the Cr2O3 coating to some extent and reduced the surface free energy [19,20].
Fig. 6. EDS analysis of region C in Fig. 5.
Fig. 7. Porosity change trend of five coatings.
3.4. Surface free energy and friction coefficient
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Fig. 8. The section morphology of five coatings. (a) CT0 coating, (b) CT8 coating, (c) CT16 coating, (d) CT24 coating, and (e) CT32 coating.
The results of the friction coefficient under dry friction and oil lubrication conditions are shown in Fig. 11. It can be found that the dry friction coefficient has a dramatic fluctuation (Fig. 11(a)), and the results are distributed in the range of 0.56–0.91. The curve of friction coefficient changes with time is not very stable (Fig. 11(b)), while the friction coefficient curve of oil lubrication changes with time has a stable state. Its amplitude is in the range of 0.15–0.16. By comparing the curves (Fig. 11(a)), both in dry friction and oil lubrication conditions, the tendency of friction coefficients is very similar to the capital letter “N”.
The minimum values both present at the point of 16% and the minimum friction coefficient are 0.56 and 0.15, respectively. Since the coating and friction pair were in a run-in period in the initial stage of dry friction, the higher protrusions of the coating surface were gradually being removed. Thus it formed an unstable resistance in the move process, which caused a great mutant in the friction coefficient [10,21]. With the increase of the test time, the surface protrusions disappeared gradually and the friction coefficient became stable. Under oil lubrication conditions, some oil will be stored in the coating surface
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Fig. 9. Micro-hardness of five coatings.
Table 3 Results of contact angle and surface free energy. Sample
CT0 CT8 CT16 CT24 CT32
Contact angle/°
Surface free energy parameters/mJ m−2
Distilled water
Ethylene glycol
Diiodomethane
γS
γdS
γpS
53.57 70.80 97.97 80.67 56.70
35.17 55.53 62.50 51.70 46.57
39.80 38.37 50.88 43.27 42.53
81.11 68.00 56.34 66.02 74.74
75.06 65.85 56.08 65.63 68.58
6.05 2.16 0.26 0.39 6.16
between the two projections to form a “micro-oil tank”. When the friction pair moves to the hollow between two projections, the lubricating oil cannot be discharged, and it will cause an upward force in highpressure to reduce the oscillation of the friction pair in the move process (Fig. 12) [22]. In addition, the lubricant film formed on the coating surface can reduce the direct contact between the friction pair and the coating, which effectively reduced the resistance of the friction pair during the movement [23,24]. Fig. 13 shows the surface morphology of coatings under two lubrication conditions. We can find that the morphology of the coating under oil lubrication conditions is relatively flat (Fig. 13(a)), and the morphology of the coating without lubricating oil presents more pits (Fig. 13(b)).
Fig. 11. The friction coefficients of five coatings. (a) The tendencies of friction coefficient, (b) dry friction coefficient, and (c) oil friction coefficient.
Fig. 10. Variation trend of coating surface free energy.
For these reasons, the friction coefficient under the oil lubrication conditions is significantly smaller than the coefficient under dry conditions, and its run-in period is obviously shorter than the dry friction. Comparing the coating surface free energy curve (Fig. 10) with the friction coefficient curve (Fig. 11(a)), we can conclude that the friction coefficient of the four kinds of coatings containing TiO2 declined with the reduction of the surface free energy of the coatings. For a wearresistant coating, the surface free energy not only affects the bonding strength between the coating and the substrate, but also has an effect on the bonding resistance between the coating and the friction pair [25,26]. If the surface free energy of the coating is high, the bonding phenomenon between the friction pair and the coating will take place easier in the sliding process and form the resistance force. Thus the friction coefficient under the same load conditions will increase [27–29]. Conversely, it will form a small cohesive force when the coating is at a lower surface free energy state, which leads to a relatively easy
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4. Conclusion
Fig. 12. The stress diagram of a friction pair in the oil lubrication process.
movement between the coating and the friction pair. It is characterized by a low friction coefficient. However, it does not seem to meet the above rules for the CT0 coating. This may be due to the transformation of coatings that changed with the addition of TiO2 and further theoretical and experimental studies are needed.
Microstructural characteristics, mechanical properties and surface free energy of different weight percentages of Cr2O3–TiO2 coatings were investigated in this work. It was found that, the coating with TiO2 added has an alternating distribution with two colors. The dark areas are mainly the soft–tough phase of TiO2, which can reduce the tendency of cracking. The light areas are mainly the Cr2O3 phase and the (Cr0.88Ti0.12)2O3 phase, which play a supportive role in the internal coating. The generation of (Cr0.88Ti0.12)2O3 increased the number of internal voids. The amounts of the (Cr0.88Ti0.12)2O3 can also reach a maximum value when the atomic ratio of Cr and Ti is at an appropriate value. Even though the porosity is higher, the generation of the (Cr0.88Ti0.12)2O3 phase allows the coating to have a higher microhardness (e.g. CT16 coating). For the coating system containing TiO2, when the surface free energy is low, the adhesion between the coating and the friction pair is small. The coating then has a low friction coefficient. Acknowledgements The paper was financially supported by NSFC (51275151), the 973 Project (2011CB013405), and the Distinguished Young Scholars of NSFC (51125023). References
Fig. 13. The morphologies of coatings under different lubrication conditions. (a) The morphology of a coating under oil lubrication. (b) The morphology of a coating without lubrication oil.
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Influence of TiO2 content on the mechanical and tribological properties of Cr2O3-based coating Nan-nan Li a,b, Guo-lu Li a,⁎, Hai-dou Wang b, Jia-jie Kang c, Tian-shun Dong a, Hai-jun Wang b a b c
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China National Key Lab for Remanufacturing, Academy of Armored Forces Engineering, Beijing 100072, China School of Engineering and Technology, China University of Geosciences, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 April 2015 Received in revised form 14 September 2015 Accepted 15 September 2015 Available online 18 September 2015 Keywords: Cr2O3–TiO2 coating Mechanical property Surface free energy Friction coefficient
a b s t r a c t This paper systematically investigated the influence of TiO2 content on the mechanical and tribological properties of a Cr2O3–TiO2 composite coating deposited by plasma spraying technology, and comparatively analyzed their microstructures and surface free energies. The results show that the Cr2O3–TiO2 composite coating exhibited a typical stratification and precipitated a new phase of (Cr0.88Ti0.12)2O3 in comparison with a Cr2O3 coating. The porosity of the coating increased firstly and then decreased as the TiO2 content in the Cr2O3–TiO2 composite coating increased. Furthermore, the TiO2 content added into the Cr2O3–TiO2 composite coating had an obvious influence on micro-hardness. It was also found that the friction coefficient of the coating decreased as the surface free energy decreased. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Plasma sprayed Cr2O3 ceramic coating has been widely used in various fields for improving resistance to corrosion and wear of part surfaces. [1–2]. With the further development of modern industrial technology, some special parts are required for applications in relatively severe environments [3–5]. Therefore, in order to assure that all properties of coatings meet severe service conditions, it is very necessary to further develop Cr2O3 ceramic coatings with more superior performance. As a common thermal spraying material, TiO2 with moderate hardness, good toughness, and excellent resistance to wear and corrosion, can be used to improve the properties of pure Cr2O3 ceramic coating [6–11]. Therefore, a composite coating with a proper proportion of TiO2 and Cr2O3 not only can exhibit a relatively high hardness which can match that of a pure Cr2O3 coating, but also can ensure considerable toughness. Furthermore, the TiO2 addition into the Cr2O3 coating can also reduce the cracking tendency of the coating in the process of plasma spraying. To date, there were numerous studies on the performances of the Cr2O3-based plasma sprayed coating. Kim et al. [12] studied the optimum plasma spraying process of the Cr2O3–3 wt.% TiO2 coating, and the results showed that this composite coating exhibited the most superior performance when performed with a spraying power of 47 kW, a spraying distance of 90 mm, and a powder feed rate of 90 g/min. After measurement, it can be found that the hardness and density were ⁎ Corresponding author. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matdes.2015.09.085 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
higher under the above spray parameters. Zhang et al. [13] implemented epoxy sealing treatment to the Cr2O3–8 wt.% TiO2 coating and then carried out a salt spray corrosion test. Before the sealing treatment, the corrosion results showed that an etching solution entered into the space between the ceramic coating and undercoating along pores and micro-cracks during the experiment. Eventually, the corrosion belt was formed and the bonding strength between the substrate and the coating was reduced. After carrying out the sealing treatment, the amount of open pores and cracks was reduced and the corrosion rate also decreased significantly. Cetinel et al. [14] performed the frictionwear test to a Cr2O3-based coating in an acidic environment. They found that the load magnitude wasn't the dominant factor for the friction coefficient, and the wear loss of the coating was greatly affected by the friction time. In addition, Singh [15], Cho [16], Mishra [17] and other scholars also conducted more thorough researches to the Cr2O3-based coating, and further revealed the failure mechanisms of the coatings under different experimental conditions. However, previous studies of mechanical properties on the Cr2O3based ceramic coating were numerously focused on a single chemical composition (namely Cr2O3 and TiO2 at a specific percentage), and without attracting attention on the changes of mechanical properties caused by changing the second phase content. Moreover, for any material, the exchange of substance and energy will be achieved by the surface coating. Thus, it would be interesting to investigate the physical properties and energy state of the surface coating. Hence, in the present work, Cr2O3–TiO2 ceramic coatings with different TiO2 contents were produced using plasma spraying technology. The purpose of this research is to comprehensively investigate the effects of TiO2 contents
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solution by an ultrasonic apparatus for 20 min to remove the surface stains. Then sandblasting was carried out on the cleaned substrate to remove the rust. To enhance the bonding strength between the coating and the substrate, NiCr alloy powder with a size of 45 μm was deposited on the surface-treated substrate as an undercoating. The substrate was preheated to 300 °C before the deposition to avoid the phenomenon of cracking and delamination. During the spraying, the main gas was argon, and nitrogen was the secondary gas and carrier gas. The SEM images of the NiCr alloy powder and Cr2O3–TiO2 powder are shown in Fig. 1. The Cr2O3–TiO2 powders (dark green) with five different contents of TiO2 (white) were mixed by adopting a mechanical hybrid method. All powders used the same spraying parameters for comparison in the process of coating preparation. The weight ratio of Cr2O3 to TiO2 and the detailed spraying processing parameters are summarized in Table 1. 2.2. Characterization of the coatings Prior to the microscopic observation and the performance test of the coating, specimens were ground by SiC grinding papers from 400 to 2000 grade and then polished. After that, alcohol solution was used to clean the samples. The surface and cross section morphologies were observed by a Nova NanoSEM 450/650 type environmental scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The crystalline phases were analyzed by a D8 X-ray diffractometer with Cu Kα (1.54056 Å) and transmission electron microscopy (TEM). At the same magnification condition (×2000), the porosity of each coating section was measured by image processing software ImageJ2x, and more than ten micrographs were analyzed to obtain statistically averaged values. The micro-hardness of a polished cross section was determined by an HVS-1000 micro-hardness meter with 300 g load for 15 s loading duration time. More than ten indentations were performed on each sample to obtain the mean value. In this article, an OCA-30 automatic contact angle meter was introduced to measure the contact angles of the polished coating. Distilled water, ethylene glycol and diiodomethane were selected and used as the probe liquids. Surface free energies of coatings were calculated by Owens–Wendt equation (Eqs. (1)–(2)) [18] and the detailed free energy parameters of the three probe liquids are listed in Table 2. Friction coefficients of the coatings under the dry condition and the oil medium were tested by the CETR UMT-3 wear tester. The experimental parameters under the dry condition were set as: frequency of 10 Hz, time of 20 min, and load of 20 N. Experimental parameters in oil medium were set as: frequency of 10 Hz, time of 20 min, and load of 60 N. In both experimental conditions the wear scar length was 3 mm and the friction pair was ZrO2 ceramic balls with a diameter of 4 mm.
Fig. 1. Micromorphology of spraying powders (SEM-SE). (a) NiCr alloy particles and (b) Cr2O3–TiO2 particles.
on the mechanical and tribological properties of the Cr2O3-based coating. 2. Materials and methods 2.1. Preparation of coating Cr2O3–TiO2 ceramic coatings (CT) with different contents of TiO2 (mass percentage) were fabricated by a highly effective supersonic plasma spray system (HEPJet) invented by the National Key Laboratory for Remanufacturing, China. The substrate used for depositing an undercoating and a ceramic coating was non-tempered ASTM1045 stainless steel with a dimension of 60 × 50 × 4 mm, and a Rockwell hardness of HRC47. Prior to spraying, the substrates were cleaned in acetone
qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi γL ð1 þ cosθÞ ¼ 2 γ DL γ DS þ γPL γ PS
ð1Þ
γS ¼ γ DS þ γPS
ð2Þ
P where θ is the contact angle,γL, γD L , and γL are the surface tension of the liquid, and its dispersive and polar components, respectively, and γS, γD S,
Table 1 Composition of Cr2O3–TiO2 and the spraying parameters. Sample
Ar/(L min−1)
N2/ (L min−1)
I/A
U/V
Spray distance/mm
Powder feed rate (g min−1)
Cr2O3 (CT0) Cr2O3–8 wt.% TiO2 (CT8) Cr2O3–16 wt.% TiO2 (CT16) Cr2O3–24 wt.% TiO2 (CT24) Cr2O3–32 wt.% TiO2 (CT32)
100
22
385
165
90
35
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Table 2 Surface tension components of the three liquids for contact angle measurements. Test liquids
Distilled water Ethylene glycol Diiodomethane
Temperature/°C
25 25 25
Surface free energy parameters/mJ m−2 γL
γD L
γPL
72.8 48.0 50.8
21.8 29.0 50.8
51.0 19.0 0
and γPS are the surface free energy of the specimen, and its dispersive and polar components, respectively. 3. Results and discussion 3.1. Surface and cross section morphologies Fig. 2 is the SEM image of the coating surface (five kinds of coatings have a similar surface morphology) and cross section (CT16 coating). As can be seen from Fig. 2(a), the coating surface shows a complex structure. There are also many “hills” with irregular shapes on the surface. It can also be found that there are many flat particles when the molten particles cooled and deposited on the substrate from the high magnification image (Fig. 2(b)). The boundary lines of flat particles have a clear image, and there are some micro-holes distributed along those lines. This is caused by the high heat and kinetic energy of particles in the spray process. Firstly, the high temperature in the chest of the spray gun made the particles melt. Then the molten particles moved into the nozzle, where they would be subject to mechanical
compression and self-magnetic compression came from the nozzle and the plasma flame, respectively. The molten particles obtained higher heat and kinetic energy in this process. When the molten particles bombarded the substrate, the cooling-down phenomenon took place rapidly before the molten particles completely fused with each other causing the volume of the particle to contract. The micro-pores and obvious edges then appeared. Fig. 2(c) shows the cross-sectional morphology of the CT16 coating. It can be found that the cross section consists of two kinds of areas―dark and light, which formed the whole cross section alternately. Viewing the cross section as a whole, the layered structure is very clear. Through the magnification of the cross section (Fig. 2(d)), we can find that the dark and light parts have the same porosity, and the distribution of the pores in the coating is relatively uniform. Compared to four kinds of coating containing TiO2, the CT0 coating did not show an obvious layered structure. The XRD patterns of all coatings are shown in Fig. 3. It can be seen that the Cr2O3 coating only contains an Eskolaite phase, as well as the composite coatings of Cr2O3 and TiO2 which are composed of three phases, namely the Eskolaite, Rutile and (Cr0.88Ti0.12)2O3 phases. Among them, the Eskolaite phase which presents a hexagonal structure or an amorphous structure plays a supportive role in the coating, while the Rutile (the Mohs hardness is 6.0–6.5) as the soft–tough phase can enhance the toughness of the coating and reduce the tendency of cracking. The dark area of the coating cross section containing TiO2 was analyzed with Fig. 4 (point A). The results showed that it can be as a binding phase containing more Ti elements, as well as a hard phase with more Cr elements presented in the light area of Fig. 4 (point B). Under high temperature, the three elements of Cr, Ti and O reacted with each other to generate the (Cr0.88 Ti0.12)2O3 phase. The new phase led to a lattice mismatch with the Cr2O3 phase in the light
Fig. 2. Morphology of a coating surface and cross section. (a) The morphology of a coating surface (SEM-SE), (b) the high magnification morphology of a coating surface (SEM-SE), (c) the morphology of a CT16 coating cross section (SEM-BE), and (d) the high magnification morphology of a CT16 cross section (SEM-BE).
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area, which made lattice distortion to occur in the cooling process. In addition, due to the different thermal expansion coefficient between the Cr2O3 phase and the (Cr0.88 Ti0.12)2O3 phase, there are some defects existing in the light area. 3.2. TEM analysis Fig. 5 shows the transmission electron microscope (TEM) images of the Cr2O3–TiO2 coating. It can be seen that the crystal sizes of Cr2O3 were in the range of 200–300 nm (point A shown in Fig. 5(a)). The shape of the grain is similar to a regular hexagon, and there is a clear boundary between the grains. TiO2 grains with a sharpened shape can be found in the Cr2O3 matrix and its size is similar to the Cr2O3 grains (point B shown in Fig. 5(b)). Fig. 5(c) is the high resolution transmission electron microscope (HRTEM) image of TiO2. Lots of defects (such as stacking faults) can be observed in the TiO2 matrix. In addition, some tiny crystal structures are distributed among the Cr2O3 grains diffusely (point C shown in Fig. 5(d)). It is about 50 nm in diameter and smaller than the grain size of Cr2O3 and TiO2. EDS analysis was carried out on the region (Fig. 6), and the results showed that only three elements of Cr, Ti and O could be seen, and that the ratio of Cr and Ti was close to 8:1. Therefore, the tiny phase can be determined as (Cr0.88Ti0.12)2O3. Through additional pictures it can be seen that there are many defects in the interface between (Cr0.88Ti0.12)2O3 and Cr2O3. 3.3. Porosity and micro-hardness Fig. 7 shows the result of the cross-sectional porosity of five kinds of coatings at 2000 times magnification. With the increase of TiO2 content the coating porosity shows a trend of decreases after the first increase. Also, at the TiO2 content of 16% the porosity had a peak value of 8.87%. Compared to pure Cr2O3 coating (porosity of 2.93%), the internal pores of the coating increased significantly. But when the TiO2 content exceeds 16% (24%, 32%), the porosity has emerged with a clear downward trend, and the value of porosity is very close to that of the pure Cr2O3 coating. The cross section morphologies of five kinds of coatings are shown in Fig. 8. It can be seen that the CT0 coating has a denser cross section than other coatings, and the void area is very small. Moreover, the pores are mostly independently existing and there are rare large continuous holes in the cross section. There are also many larger holes with zonal distribution existing in the cross section of the other four coatings, which mostly existed in the light area. Measuring the cross section of coatings containing TiO2, it can be also found that with the increase of TiO2 content, the pores in the light area show a trend of falling
Fig. 3. XRD patterns of the five coatings.
909
after the first increase, and have a maximum value at the point of 16% TiO2. Fig. 9 shows the variation tendency of coating micro-hardness. The pure Cr2O3 coating has a highest hardness of 1697 HV0.3, and the hardness of the other coatings shows a tendency of increase and then decline with the increase of TiO2 content. When the TiO2 content is 16%, the coating has the highest hardness among the four kinds of coatings containing TiO2. Contrasting the change tendency in porosity and micro-hardness (for the four kinds of coating containing TiO2), it can be concluded that the change tendency of coating micro-hardness has a similar tendency with the porosity. Although the CT16 coating has the highest porosity value, its micro-hardness is also the highest. Analyzing the reasons, it may be related to the new phase of (Cr0.88Ti0.12)2O3. This oxide phase containing Cr and Ti may be produced through the titanium atoms that replaced part of the chrome atoms in the original Cr2O3 crystal lattice. Although the overall shape of the Cr2O3 crystal lattice did not change a lot, the differences in the atomic radius of Ti and Cr, which caused internal stress field changes in the phase transformations, will make the lattice distortion. Through calculating the mole ratio of Cr and Ti of the spraying powder and (Cr0.88Ti0.12)2O3 (ignore the mass error caused by powder preparation), it can be discovered that the closest mole ratio of powder to (Cr0.88Ti0.12)2O3 (the ratio is 7.33) is CT16 (the ratio is 5.53), followed by CT24 (3.33), CT8 (12.11) and CT32 (2.24), and the variation is very consistent with the micro-hardness and porosity of the coating containing TiO2. It can be inferred from the results that the TiO2 as a soft–tough phase in the coatings can reduce the hardness, and react with Cr2O3 to form the (Cr0.88Ti0.12)2O3 phase. The new (Cr0.88Ti0.12)2O3 phase has a
Fig. 4. EDS of points A and B in Fig. 2(d). (a) EDS of point A and (b) EDS of point B.
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Fig. 5. The TEM image of a Cr2O3–TiO2 coating. (a) Cr2O3 crystal, (b) TiO2 crystal, (c) the faults in TiO2 crystal, and (d) (Cr0.88Ti0.12)2O3 crystal.
small crystal size and acts as a support phase (grain refinement strengthening) to improve the stress resistance of a coating in geometry. The above analysis also explains why the phenomenon of coatings containing TiO2 have a consistent trend in porosity and micro-hardness.
Table 3 shows the measurement results of the contact angle and the surface free energy. It can be concluded that from the contact angle data of distilled water, the remaining coatings exhibited a hydrophilic
characteristic except for the CT24 coating. The variation trend of the distilled water contact angle of five coatings increased first and then decreased with the increase of TiO2 content. In addition, the coatings' surface free energy calculation results (Fig. 10) show that the pure Cr2O3 coating has the highest surface free energy of 75.06 mJ m−2. The surface free energy of coatings containing TiO2 appears overall to decline, showing a rising tendency after the first drop. The CT16 coating has the lowest value of 56.34 mJ m− 2. Therefore, adding TiO2 to the spraying powder weakened the hydrophilic properties of the Cr2O3 coating to some extent and reduced the surface free energy [19,20].
Fig. 6. EDS analysis of region C in Fig. 5.
Fig. 7. Porosity change trend of five coatings.
3.4. Surface free energy and friction coefficient
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Fig. 8. The section morphology of five coatings. (a) CT0 coating, (b) CT8 coating, (c) CT16 coating, (d) CT24 coating, and (e) CT32 coating.
The results of the friction coefficient under dry friction and oil lubrication conditions are shown in Fig. 11. It can be found that the dry friction coefficient has a dramatic fluctuation (Fig. 11(a)), and the results are distributed in the range of 0.56–0.91. The curve of friction coefficient changes with time is not very stable (Fig. 11(b)), while the friction coefficient curve of oil lubrication changes with time has a stable state. Its amplitude is in the range of 0.15–0.16. By comparing the curves (Fig. 11(a)), both in dry friction and oil lubrication conditions, the tendency of friction coefficients is very similar to the capital letter “N”.
The minimum values both present at the point of 16% and the minimum friction coefficient are 0.56 and 0.15, respectively. Since the coating and friction pair were in a run-in period in the initial stage of dry friction, the higher protrusions of the coating surface were gradually being removed. Thus it formed an unstable resistance in the move process, which caused a great mutant in the friction coefficient [10,21]. With the increase of the test time, the surface protrusions disappeared gradually and the friction coefficient became stable. Under oil lubrication conditions, some oil will be stored in the coating surface
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Fig. 9. Micro-hardness of five coatings.
Table 3 Results of contact angle and surface free energy. Sample
CT0 CT8 CT16 CT24 CT32
Contact angle/°
Surface free energy parameters/mJ m−2
Distilled water
Ethylene glycol
Diiodomethane
γS
γdS
γpS
53.57 70.80 97.97 80.67 56.70
35.17 55.53 62.50 51.70 46.57
39.80 38.37 50.88 43.27 42.53
81.11 68.00 56.34 66.02 74.74
75.06 65.85 56.08 65.63 68.58
6.05 2.16 0.26 0.39 6.16
between the two projections to form a “micro-oil tank”. When the friction pair moves to the hollow between two projections, the lubricating oil cannot be discharged, and it will cause an upward force in highpressure to reduce the oscillation of the friction pair in the move process (Fig. 12) [22]. In addition, the lubricant film formed on the coating surface can reduce the direct contact between the friction pair and the coating, which effectively reduced the resistance of the friction pair during the movement [23,24]. Fig. 13 shows the surface morphology of coatings under two lubrication conditions. We can find that the morphology of the coating under oil lubrication conditions is relatively flat (Fig. 13(a)), and the morphology of the coating without lubricating oil presents more pits (Fig. 13(b)).
Fig. 11. The friction coefficients of five coatings. (a) The tendencies of friction coefficient, (b) dry friction coefficient, and (c) oil friction coefficient.
Fig. 10. Variation trend of coating surface free energy.
For these reasons, the friction coefficient under the oil lubrication conditions is significantly smaller than the coefficient under dry conditions, and its run-in period is obviously shorter than the dry friction. Comparing the coating surface free energy curve (Fig. 10) with the friction coefficient curve (Fig. 11(a)), we can conclude that the friction coefficient of the four kinds of coatings containing TiO2 declined with the reduction of the surface free energy of the coatings. For a wearresistant coating, the surface free energy not only affects the bonding strength between the coating and the substrate, but also has an effect on the bonding resistance between the coating and the friction pair [25,26]. If the surface free energy of the coating is high, the bonding phenomenon between the friction pair and the coating will take place easier in the sliding process and form the resistance force. Thus the friction coefficient under the same load conditions will increase [27–29]. Conversely, it will form a small cohesive force when the coating is at a lower surface free energy state, which leads to a relatively easy
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4. Conclusion
Fig. 12. The stress diagram of a friction pair in the oil lubrication process.
movement between the coating and the friction pair. It is characterized by a low friction coefficient. However, it does not seem to meet the above rules for the CT0 coating. This may be due to the transformation of coatings that changed with the addition of TiO2 and further theoretical and experimental studies are needed.
Microstructural characteristics, mechanical properties and surface free energy of different weight percentages of Cr2O3–TiO2 coatings were investigated in this work. It was found that, the coating with TiO2 added has an alternating distribution with two colors. The dark areas are mainly the soft–tough phase of TiO2, which can reduce the tendency of cracking. The light areas are mainly the Cr2O3 phase and the (Cr0.88Ti0.12)2O3 phase, which play a supportive role in the internal coating. The generation of (Cr0.88Ti0.12)2O3 increased the number of internal voids. The amounts of the (Cr0.88Ti0.12)2O3 can also reach a maximum value when the atomic ratio of Cr and Ti is at an appropriate value. Even though the porosity is higher, the generation of the (Cr0.88Ti0.12)2O3 phase allows the coating to have a higher microhardness (e.g. CT16 coating). For the coating system containing TiO2, when the surface free energy is low, the adhesion between the coating and the friction pair is small. The coating then has a low friction coefficient. Acknowledgements The paper was financially supported by NSFC (51275151), the 973 Project (2011CB013405), and the Distinguished Young Scholars of NSFC (51125023). References
Fig. 13. The morphologies of coatings under different lubrication conditions. (a) The morphology of a coating under oil lubrication. (b) The morphology of a coating without lubrication oil.
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