Friction properties of high temperature boride coating under dry air and water vapor ambiences

Friction properties of high temperature boride coating under dry air and water vapor ambiences

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Friction properties of high temperature boride coating under dry air and water vapor ambiences Chuanbing Huanga,b,n, Bingcong Zhanga, Hao Lana, Lingzhong Dua, Weigang Zhanga,b b

a State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Yangzhou Solar Energy Materials Research and Development Center, Institute of Process Engineering, Chinese Academy of Sciences, Yangzhou 225009, China

Received 12 March 2014; received in revised form 8 April 2014; accepted 15 April 2014

Abstract NiCoCrAlY/ZrB2–B4C composite coatings were prepared by atmospheric plasma spraying using a clad powder and their microstructure, phase composition and mechanical properties were characterized. The friction and wear behavior of the coatings were evaluated from room temperature to 800 1C in dry air and water vapor atmospheres using a ball-on-disk tribometer. The results show that the NiCoCrAlY/ZrB2–B4C composite coating exhibited low porosity (6.77 0.8%), high Rockwell hardness (HR45y82 7 5) and high cohesive strength (18 73 MPa), as well as good friction and excellent anti-wear ability up to 800 1C. The friction coefficients and wear rates of the coating under water vapor ambience were lower than that under dry air. The main reason is the synergistic effect of layer structured lubricating H3BO3 and molten B2O3 films, which were produced by the reaction of the boride with O2 and H2O in the water vapor atmosphere. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Films; C. Friction; D. Borides; E. Wear parts

1. Introduction ZrB2 and B4C related materials have a wide range of applications in civil, aerospace and military fields because of their excellent mechanical and thermal properties. Generally speaking, these materials have no self-lubricating properties in the common dry ambience. However, recent studies found that boride oxide can be generated at elevated temperatures and high moisture atmosphere to form self-lubricating film [1–6]. These kinds of films not only have good stability at high temperature, but also can slowdown further oxidation rates of borides [7].So they can play important roles of environmental adaptive lubrication and anti-friction properties. Therefore, as promising coating materials that can be widely used in the field of friction and wear reduction, boride coatings have been fabricated by vapor deposition or hot-pressing method worldwide in recent n Corresponding author at: State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. Tel./fax: þ86 10 82627093. E-mail address: [email protected] (C. Huang).

years [8–10]. But since these methods tend to be time-consuming and costly, which is not suitable for complex shapes and large size structures, thus the further applications of borides coatings are restricted. Plasma spray is suitable to prepare boride-containing hard coatings, such as ZrB2, TiB2 and B4C [11,12] because of the high-temperature and high-velocity environment during the process. However, dense ZrB2 or B4C coatings are often difficult to obtain from atmospheric plasma spray technology, because of severe oxidation of borides occuring at high temperatures [13]. Also, it cannot fabricate a dense and thick ZrB2 or B4C coating using vacuum plasma spray due to its low spray pressure and flame energy density, which lead to undesirable melting of ZrB2 or B4C particles [14,15]. Tului et al. [16,17] used controlled atmosphere plasma spray technology to prepare ZrB2–SiC ultra-high temperature ceramic coating, but the tribological properties of the coating were not examined. In recent studies, B4C coatings were successfully obtained by high-pressure plasma spraying, vacuum plasma spraying [18–21] and electromagnetic acceleration plasma spraying

http://dx.doi.org/10.1016/j.ceramint.2014.04.091 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: C. Huang, et al., Friction properties of high temperature boride coating under dry air and water vapor ambiences, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.091

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methods [22,23]. These methods can reduce the oxidation of boron carbide, and the increased energy density of plasma jet can also improve melting state of the powder particles. However, the workpiece was restricted by the size of the vacuum chamber. Meanwhile, due to the complexity of the process, harsh conditions like high pressure, vacuum and expensive equipment along with the high cost restrict application of those technologies. Thus, it is urgent to improve the powder preparation process to minimize the oxidation of ZrB2 and B4C during plasma spraying. In addition, for the huge difference in thermal expansion coefficient between the coating and substrate, the thermal shock resistance of plasma sprayed B4C coating can only resist up to 550 1C [18]. Meanwhile, due to the high porosity of plasma sprayed B4C coating, the bonding strength and thermal conductivity of B4C coating are usually low [24]. Adding alloying elements in the B4C coating can enhance coating's toughness and improve bonding strength between the coating and substrate, and by this method a dense coating can be obtained [25]. In this study, NiCoCrAlY/ZrB2–B4C composite powder was prepared by centrifugal spray granulation, high-pressure hydrogen reduction and solid alloy methods. Then atmospheric plasma spray technology was used to deposit self-lubricating wear-resistant NiCoCrAlY/ZrB2–B4C coating to overcome the adverse factors, such as the intrinsic brittleness of the boride materials, processing difficulties and severe oxidation during the spraying process, in order to expand its tribological application in high temperature and/or high humidity.

2. Experimental The ZrB2, B4C powders (r 5 μm), water and dispersant were blended together to make agglomerated reconstitution core powders by spray drying. Then the core powders were coated by NiCoCrAlY layer using pressurized hydrogen reduction and solid state alloying technologies, which were introduced in our previous studies [26–28]. The nominal composition, flowability and apparent density of the feedstock powder are shown in Table 1. Table 1 Nominal composition and some properties of feedstock powder. NiCoCrAlY (wt%)

ZrB2 (wt%)

B4C (wt%)

Apparent density (g/cm3)

Flowability (s/50g)

Shape

60

24

16

1.35

55

spherical

NiCoCrAlY/ZrB2–B4C coatings are deposited on stainless steel substrates by APS-2000K plasma spraying system using the spray parameters as shown in Table 2. Before coating, the substrate was blasted clean with coarse SiC particles and then plasma-sprayed for about 0.1 mm thick NiCrAl bond coating. The NiCoCrAlY/ZrB2–B4C coatings were then applied at a thickness of no less than 0.4 mm. The flowability and apparent density of NiCoCrAlY/ZrB2– B4C powder were tested by the standard Hall flowmeter. Microstructural characterization of the feedstock powder and as-sprayed coating were performed using a scanning electron microscope (SEM, FEI Quanta 200 FEG), equipped with an energy dispersive X-ray analysis system (EDX). The constituents of the powder and coating were characterized by X-ray diffraction (XRD) with a Philips X’Pert Pro diffractometer (PANalytical, Holland) using filtered CuKα radiation (λ ¼ 0.1541 nm) at 40 kV, 40 mA. The coating's porosity was measured by image analysis with a Olympus PMG3 microscope and the Adobe Photoshop software according to GB/T 3365-82 standard method. To evaluate the coating’s adherence strength, a universal testing machine (model WDW2100E) was used at a cross head speed of 1 mm/min (GB/T 8642-2002), and the sample size is Ø25 mm  5 mm. Four samples were tested to get the average results of the coating’s adherence strength. The hardness was tested in Rockwell superficial hardness tester according to GB/ T 8640-1988. The friction and wear tests are carried out using a modified HT-1000 ball-on-disk high-temperature tribometer as shown in Fig. 1 [29]. The disk was made of NiCoCrAlY/ ZrB2–B4C coatings (Ø25 mm  4.5 mm), while the counterpart ball was made of Si3N4 ceramic (Ø6 mm). The friction and wear tests were carried out with a load of 9.8 N and sliding speed of 0.188 m/s, for duration of 30 min. The water vapor was produced by a vapor generator, in which the temperature was about 120 1C and therefore the water vapor pressure was about 0.2 MPa. The water vapor was introduced to the chamber of tribometer by a steel pipe (Ø4 mm). The friction coefficient was calculated from the measured friction torque and the applied normal load. The wear volumes of coatings were measured using the Rank Taylor Hobson Talysurf 5P-120 system. The wear rates of specimens were calculated as w ¼ V/ FS, where w is the wear volume in mm3, F is the normal load in Newtons, and S is the total sliding distance in meters. Repeated tests were performed for each frictional pair and the averaged results of the three repeated tests are reported in this study. The surface morphologies of the coatings before and after tribological tests were observed by the SEM with EDX.

Table 2 Thermal spray parameters. System

Voltage (V)

Current (A)

Ar pressure (MPa)

Ar flow rate (L/min)

APS-2000 K H2 pressure (MPa) 0.65

70 H2 flow rate (L/min) 2

500 Carrier gas pressure, N2 (MPa) 0.3

0.6 Powder feed rate (g/min) 35

35 Spray distance (mm) 130

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Fig. 1. Schematic diagram of the high-temperature friction and wear tester with water vapor introducing generator.

3. Results and discussions 3.1. Characterization of NiCoCrAlY/ZrB2–B4C powder and coating Fig. 2(a) is the SEM morphology of NiCoCrAlY/ZrB2–B4C powder. It can be seen that the particle distribution is relatively uniform between 50 and 100 μm. NiCoCrAlY/ZrB2–B4C powder is spherical, and the core of ZrB2–B4C is uniformly dense coated with a layer of 3 μm NiCoCrAlY alloy. The NiCoCrAlY alloy, which helps prevent oxidation, ablation and rebound of ZrB2–B4C during plasma spraying also functions as a high-temperature bonding phase, to provide the essential mechanical and oxidation resistance properties. The core is a hard wear-resistant phase that can significantly enhance the strength and abrasion performance of the composite coating. And it is also the source of the B2O3 and H3BO3 hightemperature solid lubrication phases, which can reduce the friction coefficients and wear rates of the coating and counter parts. ZrB2 and B4C powders were granulated and clad by NiCoCrAlY alloy, density of each NiCoCrAlY/ZrB2–B4C particle is substantially the same, thus the density difference is small. After assembly, the thermo-physical parameters of each NiCoCrAlY/ZrB2–B4C particle are also substantially the same. Since the NiCoCrAlY alloy and the core particle are chemically bonded, as long as the NiCoCrAlY alloy melts during the spraying process, a coating with good structure can be obtained. Having a good resistance to oxidation at high temperatures, the clad alloy offers an effective protection for the internal ZrB2–B4C components. Therefore, this can significantly reduce the oxidation, ablation and rebound loss of the borides during the spraying process. Further more, the components of the powder prepared in this way are uniform, and the components cannot be easily separated during the storage, transportation, and the spraying process. The flow rate of NiCoCrAlY/ZrB2–B4C powder is about 55 s/50 g, and the apparent density is 1.35 g/cm3. These meet the needs of plasma spraying.

Fig. 2. (a) SEM morphologies of NiCoCrAlY/ZrB2–B4C powder; (b) XRD analyses of NiCoCrAlY/ZrB2–B4C powder and coating.

Fig. 2(b) is the XRD phase analysis of NiCoCrAlY/ZrB2– B4C powder and coating. It can be seen that the powder diffraction peaks are sharp, and the major constitutional phases

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of NiCoCrAlY/ZrB2–B4C coating are nearly the same as the feedstock powder. This indicated that no significant oxidation and decomposition of materials occurred during the plasma spraying and resolidification processes. Compared with the XRD of the powder, the wider width of diffraction peaks in the coating is mainly attributed to the presence of a certain amount of nano-scale grain broadening in the coating. Also, the coating XRD pattern shows much lower intensity because of a significant amount of amorphous material in the coating. Fig. 3 is the typical microstructure of the plasma sprayed NiCoCrAlY/ZrB2–B4C coating. From Fig. 3, the substrate, NiCrAl bonding layer and the composite coating can be clearly seen from bottom to top. It is evident that the coating and the substrate had a good mechanical bonding and the coating showed a typical flat lamellar structure. The average value of coating’s adherence strength is about 18 MPa, and the fracture occurred mainly within the NiCoCrAlY/ZrB2–B4C coating. This indicated the bond strength between substrate and coating was more than 18 MPa. Flat particles are linked closely together, and the coating without coarse pore showed a low

porosity of about 7%. The large pores are mainly distributed in the bonding interface of layered particles, and the small pores are mainly distributed within the layered particles. From Fig. 3(c) and (d), it can be seen that the NiCoCrAlY phase was in gray regions, zirconium boride phase was in white regions, and boron carbide and Al2O3 phase were distributed in black regions. The Al2O3 phase particle was larger than boron carbide phase, because Al2O3 phase was resulted from relatively minimal oxidation of plastic NiCoCrAlY phase during the plasma spraying process, while brittle boron carbide got impacted and crushed to fine particles. The boride is located inside the layered particles, showed as fragmentation particulates. From the sectional microstructure of the coating, it can be clearly seen that most boride particles were irregular triangles. The results showed that most borides were unmelted during the spraying process. Therefore, the coating structure is formed by a two-phase liquid–solid particle collision with the substrate and the surface of the coating. Because borides have relatively high mass fraction in the coating, the mean hardness value of the coating is evaluated to

Fig. 3. Cross-sections of as-sprayed microstructure of the NiCoCrAlY/ZrB2–B4C. coating: (a) secondary electron image; (b) and (c) back-scattered electron image; (d) EDX analysis of different regions in (c). Please cite this article as: C. Huang, et al., Friction properties of high temperature boride coating under dry air and water vapor ambiences, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.091

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Fig. 3. (continued)

be about HR45y of 82. The high Rockwell hardness of the coating is significant to improve the coating’s wear resistance. 3.2. Sliding friction and wear behavior of NiCoCrAlY/ZrB2–B4C coating Fig. 4(a,b and c) presents the friction coefficients of NiCoCrAlY/ZrB2–B4C coating sliding against Si3N4 balls from room temperature to 800 1C under air and water vapor ambience. It can be seen that the NiCoCrAlY/ZrB2–B4C coating exhibits an average friction coefficient of 0.68–0.82 under air ambience, and the friction coefficient curves show large fluctuations. However, the NiCoCrAlY/ZrB2–B4C coating exhibits an average friction coefficient of 0.32–0.42 under

water vapor ambience, and the friction coefficient curves also become relatively stable, which may be due to the formation of higher oxide content under water vapor ambience than that under air ambience. In other words, the contact interface demonstrated obvious friction chemical reaction during high speed sliding, and borides can react with water vapor to form B2O3. The B2O3 can further react with water vapor to form lamellar structured H3BO3. At elevated temperatures, the reactions can be listed as follows: (1) (2) (3) (4)

2ZrB2 þ 5O2-2ZrO2 þ 2B2O3 2B4C þ 7O2-4B2O3 þ 2CO B2O3 þ 3H2O-2H3BO3 H3BO3-HBO2-H2B4O7-B2O3

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Borides (B2O3) continue to generate, consume and regenerate, as well as the boric acid, and they are consumed during the sliding friction process. The lubrication film goes through forming, destroying, falling and reforming cycle, to continue providing surface lubrication, decreasing the friction coefficients of coating at elevated temperatures. Because H3BO3 has a layered atomic structure, its bonding characteristics are very similar to common MoS2 and graphite solid lubricants. The same layer atoms in H3BO3 are joined together by a strong bond, but the atoms between the layers are connected by weak van der Waals forces. Due to the weak resistance to shear between layers, the layers can easily slide under friction to show lubricating properties, which is the main reason of the decrease of the friction coefficient [9,30]. So the synergistic effects of lamella-slip of H3BO3 and the molten B2O3 films significantly decreased the friction coefficients of NiCoCrAlY/ ZrB2–B4C coating. Furthermore, it can be seen that the friction coefficient of NiCoCrAlY/ZrB2–B4C coating is about 0.32 under water vapor ambience at room temperature, and it is much lower than that under dry air ambience. This may be attributed to the formation of water film on the sliding surface, which can significantly reduce the friction in ball-on-disk sliding wear test. Fig. 4(d) shows the wear rates of NiCoCrAlY/ZrB2–B4C coating from room temperature to 800 1C under air and water vapor ambience. It can be seen that at room temperature, the

wear rates of NiCoCrAlY/ZrB2–B4C coating under both conditions are close to 3.8  10  4 mm3/N m. However, it can be seen that the wear rates of NiCoCrAlY/ZrB2–B4C coating under water vapor ambience are much lower than that under air ambience. The wear rates of coating increase and decrease somewhat respectively with the increasing temperature from room temperature to 800 1C. The increased wear rate of coating may be due to slight oxidation wear and adhesive wear, which are induced by the high flash temperature at the contact area and the plastic deformation.

3.3. Worn morphology analysis Water vapor plays an important role on the tribological properties of NiCoCrAlY/ZrB2–B4C coatings as the tribological chemical reaction occurring under high temperatures. During the high-speed sliding wear process, high flash temperature was generated in the contact area between the coating and the Si3N4 ball, which supplies the momentum for borides to react with oxygen and water vapor. As mentioned above, the formation of layer structured H3BO3 can reduce the friction coefficient and wear rates of NiCoCrAlY/ZrB2–B4C coatings. In order to find out the wear mechanism, worn surfaces of NiCoCrAlY/ZrB2–B4C coatings after tribological tests were investigated.

Fig. 4. Coefficients of friction (a, b and c) and wear rates (d) of the NiCoCrAlY/ZrB2–B4C coatings at different test temperatures. Please cite this article as: C. Huang, et al., Friction properties of high temperature boride coating under dry air and water vapor ambiences, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.091

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Fig. 5. SEM morphologies of worn surface of the NiCoCrAlY/ZrB2–B4C coating in different temperatures under air ambience: (a) room temperature; (b) 400 1C; (c) 600 1C; (d) 800 1C; (e) area EDS analysis of (a); (f) area EDS analysis of (d).

Fig. 6. SEM morphologies of worn surface of the NiCoCrAlY/ZrB2–B4C coating in different temperatures under water vapor ambience: (a) room temperature; (b) 400 1C; (c) 600 1C; (d) 800 1C; (e) area EDS analysis of (a); (f) area EDS analysis of (d); (g) XRD of (d). Please cite this article as: C. Huang, et al., Friction properties of high temperature boride coating under dry air and water vapor ambiences, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.04.091

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Fig. 5 shows the worn surface morphology of coatings in different temperatures under air ambience. NiCoCrAlY/ZrB2– B4C coatings show fracture surface, including inter-splat fracture and severe delamination from room temperature to 800 1C. For the as-sprayed NiCoCrAlY/ZrB2–B4C coatings, pores, intersplat boundaries and other low density areas existed, as mentioned in Section 3.1. On further deformation during the tribological tests, these voids and cracks shear to the surface, yielding large wear debris that can generate secondary wear, and the severe scratches appear on the worn surface. This is a reason for the high wear rates of NiCoCrAlY/ZrB2– B4C coatings under air ambience. It can be assumed that brittle fracture and delamination are the dominant wear mechanisms of NiCoCrAlY/ZrB2–B4C coatings under air ambience. Fig. 6 shows the worn surface morphology of coatings at different temperatures under water vapor ambience. It can be seen that the worn surface of NiCoCrAlY/ZrB2–B4C coatings are relatively smooth and covered by fewer amount of debris and fracture. The reason for this is the addition of water vapor which is beneficial to the formation of lubrication B2O3 and H3BO3 films. As the testing temperature increases, continuous lubricating films and mild scratches appear on worn surface of the NiCoCrAlY/ZrB2–B4C coatings. EDX analysis shows the lubrication film contained large amounts of B and O, which suggest that the lubrication film consist of B2O3 and H3BO3. As a conclusion, the lamella-slip of H3BO3 and the molten B2O3 films are the main factors leading to low friction coefficients and wear rates of NiCoCrAlY/ZrB2–B4C coatings under water vapor ambience. 4. Conclusions In conclusion, a self-lubricating wear-resistant NiCoCrAlY/ ZrB2–B4C coating has been prepared by the atmospheric plasma spraying technology using clad powders. The coatings show low porosity, high Rockwell hardness and cohesive strength. The NiCoCrAlY/ZrB2–B4C coating also shows good tribological properties from room temperature to 800 1C under water vapor atmosphere, which is due to the synergistic effect of layer structured lubricating H3BO3 and molten B2O3 films. These advantages make NiCoCrAlY/ZrB2–B4C coating highly potential for industrial and environmental tribological applications. Acknowledgment The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant no.51001093) and the Natural Science Foundation of Jiangsu Province, China (Grant no. BK2011452). References [1] C.M. Lin, H.L. Tsai, C Yang, Effects of microstructure and properties on parameter optimization of boron carbide coatings prepared using a vacuum plasma-spraying process, Surf. Coat. Technol. 206 (2012) 2673–2681.

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