Mechanical properties and tribological characteristics of B4C-SiC ceramic composite in artificial seawater

ABSTRACT To improve the mechanical properties and tribological characteristics of boron carbide (B4C) ceramic, silicon carbide (SiC) as an incorporation phase was added into boron carbide matrix. The results showed that, the incorporation of SiC phase led to denser microstructure and lower porosity. When SiC content was 20 wt.%, the bending strength and fracture toughness of the ceramic composite were 447.6MPa and 7.21 MPa·m1/2, respectively. Under seawater lubrication, the friction coefficient of B4C-20%SiC was lowered to 0.052, and the wear rate of ceramic composite was lower than the magnitude of 10−6mm3/Nm. The excellent tribological performance was attributed to lubricating, cooling of seawater, and tribochemical reaction.


Introduction
It is well-known that the moving components (such as sealing element, bearing, valve seat, pump body and plunger) operating are usually subjected to the combined actions of friction and wear during service. Especially, the seals in seawater are usually susceptible to the combined action of wear and corrosion. It is an effective way to improve the stability and reliability of the moving component to adopt the materials with anti-wear, anti-corrosion, and self-healing properties [1][2][3][4]. Ceramic materials possess a series of interesting properties, such as high hardness, high strength, and good chemical stability and wear resistance. At present, more and more attention has been paid to the application of ceramic materials in seawater environment sealing [5][6][7].
Boron carbide (B 4 C) ceramics possess excellent mechanical properties, chemical stability in acid-base environment and unique nuclear shielding properties [8,9]. However, as engineering friction materials, boron carbide ceramic material has low fracture and poor tribological performance, which limits its further application [10,11]. Usually, the strength, toughness and tribological properties of boron carbide ceramics have been improved by adding a second phase. Ojalvo [12] added Ti-Al powders into B 4 C ceramic matrix in order to improve the toughness and wear resistance of B 4 C ceramics. And, the results showed that, the toughness of ceramic composite was only 3.5 MPa·m 1/2 , though the friction coefficients of B 4 C-based composite sliding against diamond-coated SiC reached to a range of 0.04-0.05. Meanwhile, Li [13,14] tried to add hBN into B 4 C ceramic matrix and fabricated B 4 C-hBN ceramics by hot-press sintering. And, the experimental results showed that, the dry friction coefficient of B 4 C-hBN was reduced to 0.321 by adding hBN, but the incorporation of hBN significantly deteriorates the toughness of B 4 C ceramics (from 5.62 MPa·m 1/2 for B 4 C to 3.85 MPa·m 1/2 for B 4 C-hBN). Moreover, Sedlák [15] introduced GPLs into B 4 C ceramic matrix and found that, the incorporation of GPLs could slightly improve its tribological properties, but the hardness significantly was reduced to 18.21 GPa. As described above, it is necessary to further study how to improve the mechanical and tribological properties of boron carbide ceramics simultaneously.
In recent years, some scholars have found that, the mechanical properties of B4C ceramics can be improved by introducing SiC phase into the ceramic matrix [16][17][18]. Chawon [19] fabricated the B 4 C-preceramic polymer derived SiC composite, and found that the mechanical properties of the composite were very closed to those of single-phase ceramics. Meanwhile, Sung [20] fabricated B 4 C-SiC ceramic composites by hot pressing sintering, and found that the fracture toughness of composite was 3.59 MPa·m 1/2 at a sintering temperature of 2000°C and pressure of 40 MPa. Similarly, Du [21] prepared the B 4 C-SiC ceramic composites with high density by hot-press sintering at a sintering temperature of 1950°C and pressure of 30MPa, and the study results showed that the hardness and fracture toughness of composite could reach to 33.2GPa and 5.64MPa·m 1/2 , respectively. Obviously, adding SiC can improve the mechanical properties of boron carbide ceramics, but the research conclusions are not uniform so far. In particular, studies on the tribological properties of the ceramic composite are rare.
Based on the mentioned above, this paper systematically studied the mechanical properties and tribological performance of B 4 C-SiC ceramic composites in seawater environment. The special plastic polyetheretherketone (PEEK) which is resistant to seawater corrosion was selected as the mating material [22]. The friction and wear properties were revealed by the wear test of pin-on-disc, and the microstructure, wear surface and tribological products were analyzed by various testing and analysis methods. Then, the wear mechanism was further discussed.

Materials
B 4 C powder (Shanghai Yao Tian Nano Material Co., Ltd., China, average particle size: 0.8 μm, the purity: 99.9%) and SiC powder (Weifang Hua Rong Ceramic Material Co. Ltd., China, average particle size: 0.45 μm, the purity: 98.5%) were utilized as raw materials to prepare B 4 C-SiC ceramic composites. Monolithic B 4 C ceramic was also prepared as references for comparison with the mechanical and tribological properties of the composites. Additional Al 2 O 3 powder (Aijia NewMaterial Science & Technology Ltd., China, average particle size: 1.17 μm, purity: 99.9%) and Y 2 O 3 powder (Aijia NewMaterial Science & Technology Ltd., China, average particle size 0.37 μm, purity 99.9%) were added in all samples as sintering additives. The composition and number of the B 4 C-SiC ceramics are shown in Table 1.
The mixed powders were milled in ethanol for 12 h by using ZrO 2 balls at a speed of 90 r/min, and later dried. The weight ratio of ZrO 2 ball to mixed powder was 2:1. SEM images and XRD results of starting powders are shown in Figure 1. After slurry was dried, the  powders were hot-press sintered in flowing N 2 at a temperature of 1900°C and pressure of 40MPa for dwell time of 30 min using an inductive hot-pressing vacuum furnace (ZT-40-21Y, Shanghai Chenhua Electric Furnace Co., Ltd., China), and a disc with a size of Φ45 × 6mm was prepared. The sintering process is schematically shown in Figure 2. The test piece with a size of 5 mm×5 mm×20 mm was cut from disc for testing bending strength and Vickers hardness; the test piece in a size of 2.5 mm×5 mm×20 mm with a 2.5 mm notch was also cut from disc for fracture toughness; and the pin sample in a size of 5 mm×5 mm×10 mm was fabricated from disc for wear test.

Test procedure
The ceramic samples were deeply etched in a solution of NaOH for 2 min, and the microstructure of ceramic composite was observed by scanning electron microscope (SEM). And, the phase composition of composite was analyzed by X-ray diffract meter (XRD). The density and porosity of the composite was measured by using the Archimedes method. Three-point-bending method was used to measure the bending strength over a 16 mm span at a crosshead speed of 0.5 mm/min. The Vickers hardness was measured on polished surface with a load of 98 N for 10 s. Single edge notched bending (SENB) test was performed to reveal the fracture toughness of ceramic composite at a crosshead speed of 0.05 mm/min. Wear testing was carried out on MMW-1 type pin-on -disc wear test rig (which was produced by Jinan Hengxu Testing Machine Group Co., Ltd., China). In this test rig, an upper pin contacts a stationary disc in artificial seawater as shown in Figure 3, and the artificial seawater was prepared according to the standard of ASTM 1141-98. The pin specimen (BS0-BS20 in Table  1) with a filleted square end was used to form flat contacts, so the contact surface area is about 20 mm 2 ; the disc, as the friction pairs, was machined from PEEK, in a size of 44 mm in diameter and 5 mm in thickness. The chemical structure and main performance parameters of PEEK have been given in the other paper [23]. The PEEK disc was finished by grinding to achieve a surface roughness (Ra) of about 0.05 μm, and the surface roughness (Ra) of the B 4 C-SiC composite ceramic pins were around 0.1 μm.
Both pin and disc were ultrasonically cleaned in fresh alcohol. The disc was fixed, and the ceramic pin was rotated at a speed of 500 r/min (0.836 m/s) and a normal load of 20 N. Total sliding distance was 1500 m. The initial running-in period was not accounted for the calculation of friction coefficient (f) and wear rate (w). The friction coefficient is directly determined by the tester. The wear rate is defined by w = Δm/(ρPL), where Δm represents the mass wear volume assessed by weight loss using a microbalance (accuracy = 0.1 mg), P is the normal load, L is the sliding distance, and ρ is the density. Friction coefficients and wear rates were obtained from the average of the values taken from three runs. The morphological analysis and chemical characterization of the wear surfaces were made by SEM/EDS, XPS and Raman spectrum.

Phase composition and microstructure
The XRD diffraction patterns of B 4 C-SiC composite ceramics are presented in Figure 4. As shown, the ceramic composites were mainly composed of B 4 C and SiC, and the composites have a carbon-rich phase. Meanwhile, the microstructures of B 4 C-SiC composites are shown in Figure 5. It can be clearly observed that the grain is significantly refined with the increase of SiC content, and the denser microstructure is also observed.

Physical and mechanical properties
The density and porosity of the B 4 C-SiC composite are shown in Figure 6. As shown, it can be seen that the density increases with the increase of SiC content, and the porosity decreases from 1.79% of BS0 to 1.02% for BS20. The incorporation of SiC increases the density of the ceramics, which should be attributed to the higher density of 3.21 g/cm 3 for second phase SiC (which is higher than that of 2.49 g/cm 3 for B 4 C) and lower porosity for the composite. And, the lower porosity of 1.02% for BS20 is consistent with the dense microstructure (as shown in Figure 5(c)). Table 2 shows the Vickers hardness of the B 4 C-SiC composite, and it can be seen that, with the incorporation of SiC, the hardness of boron carbide ceramic    decreased slightly. And, the Vickers hardnesses of ceramic composites were all at a range from 38.7GPa to 32.6GPa. As is well known, the hardness of SiC is lower than that of B 4 C ceramic. In this study, on one hand, the content of SiC was 20%, which has a relatively smaller effect on the hardness of boron carbide compared with other studies [24]. On the other hand, the grain is obviously refined with the incorporation of SiC (seen from Figure 5). So, it still shows a higher hardness of 32.6GPa. Figure 7 shows the bending strength and the fracture toughness of the B 4 C-SiC ceramic composite. With the increase of SiC content, the bending strength of B 4 C-SiC ceramics increases from 393.6 MPa to 447.6 MPa, and the fracture toughness also increases from 6.44 MPa·m 1/2 to 7.21MPa·m 1/2 . As a whole, B 4 C-20%SiC ceramics presents better strength and toughness.
It is well known that the thermal coefficients of B 4 C and SiC ceramics are 4.4 × 10 −6 K −1 and 4.8 × 10 −6 K −1 , and their Young modulus are 447GPa and 440GPa, respectively. So, when SiC as incorporation phase was added into B 4 C ceramic, the residual stress appeared at the interface of the two phases. In this case, the residual stress drove the crack to deflect, and consumed a lot of energy. Therefore, the B 4 C-SiC composites presented better bending strength and toughness. Figure 8 gives the friction coefficients of BS0/PEEK, BS10/PEEK and BS20/PEEK pairs in artificial seawater as a function of sliding distance. From Figure 8, the friction coefficient of BS0/PEEK pair increases from 0.14 to 0.20 with the progress of the friction process, and maintains a continuous rising trend until a distance of 1500 m. Meanwhile, for BS10/PEEK and BS20/PEEK pairs, the friction coefficients do not increase with the increase of friction distance, maintaining a relatively stable friction state. Among them, the friction coefficient of BS10/PEEK is stable at around 0.1, and that of BS20/PEEK is stable at 0.05. Figure 9 presents the average friction coefficient and wear rates of B 4 C-SiC/PEEK pairs. From Figure 9 (a), it could be easily found that, the addition of SiC to the B 4 C matrix significantly reduces the friction coefficient from 0.192 to 0.052. From Figure 9(b), all the wear rates of the composite pin and PEEK disc were no more than the order of 10 −6 mm 3 /Nm. It is obvious that the BS20/PEEK pair has a better comprehensive friction and wear performance in artificial seawater.

Tribological characteristics
The worn surface morphologies of BS0, BS10 and BS20 pins are shown in Figure 10. On the worn surface of SN0 pin (as shown in Figure 10(a)), fracture, debris and peeling trace can be clearly observed. While, the worn surface of SN10 pin is significantly smoother than SN0 pin, and a small amounts of fracture and debris appears on the worn surface composed of smooth area and rough area (as shown in Figure 10(b)). And, the wear surface of SN20 pin is the smoothest with only a small amount of debris observed on the surface (as shown in Figure 10(c)). Figure 11 shows the EDS analysis results of the worn surface for BS20 pin against PEEK disc in seawater, and the results shows that O, Si, Ca, Mg, and Na elements are detected on the worn surface of SN20 pin. Figure  12 shows the Raman spectrum analysis results of the worn surfaces for BS20 pin and PEEK disc. The analysis  4 , Mg(OH) 2 , and CaCO 3 are formed on the worn surfaces of BS20/PEEK pair. Some relevant papers [25,26] reported that SiC and B 4 C could react with water molecule during the friction process in high humidity environment, as the following reactions.
In our previous studies [27][28], we have found Si 3 N 4 and hBN could react with water molecule during the friction process. In this study, under seawater lubrication, there are also a large amount of water molecules, which provides conditions for the occurrence of tribo-chemical reactions. Under seawater environment, when B 4 C-SiC composites slid against PEEK, B 4 C-SiC composite reacted with water molecule from seawater to form tribochemical products (e.g., B 2 O 3 , SiO 2 , H 3 BO 3 , and Si(OH) 4 ). These tribochemical reaction products protected and smoothed the wear surface of B 4 C-SiC/PEEK. This phenomenon was similar to our previous work, namely: Si 3 N 4 -hBN could react with water molecule during the friction process, and the tribochemical products (B 2 O 3 , H 3 BO 3 , and SiO 2 ) smoothed the wearing surface under liquid environment.
It is well known that, when the friction pair slides against each other in a liquid environment, the direct contact area is influenced by the bearing effect of liquid. According to friction binomial, the friction coefficient is positively correlated with the direct contact area, as the following equations.  F ¼ αA þ βW Where,F is friction force; f is friction coefficient; A is real contact area; W is normal load; α and β are the coefficients determined by the physical and mechanical properties of the friction surface, respectively. It can be seen from the above equations, the friction force and friction coefficient decrease with the decrease of the real direct contacte. In this study, when B 4 C-20%SiC composites slid against PEEK under seawater environment, the hard micro-bulges on the ceramic surface plowed the soft polymer surface (PEEK) at the beginning stage. Due to the denser microstructure and better mechanical properties, the abrasive wear was weakened in seawater. And then, with the increase of friction process, the wear surface of ceramic pin gradually became smooth due to tribochemical reaction. Correspondingly, the wear surface of PEEK disc also became smoother, as shown in Figure 13. Under the bearing effect of     seawater, the real contact area decreased, so the friction force and friction coefficient decreased (as shown in Figure 8).
When B 4 C slid against PEEK, the polymer disc surface was also plowed by the micro-bulges on the pin surface at the beginning stage, but the wear surfaces of ceramic did not become smooth with the increase of friction process. Larger spalling pits, cracks and wear particles were formed on the worn surface of BS0 pin, and apparent plastic deformation was also observed on the surface of the polymer disc, as shown in Figure  14. Lower fracture toughness and severe abrasive wear should be the fundamental reasons for higher friction coefficient, even with the lubrication, cooling and tribochemical removal of seawater.
In this study, with the addition of 20 wt.%SiC, B 4 C ceramic presented denser microstructure and better mechanical properties. When the ceramic composite slid against PEEK in seawater, the wear surface of ceramic composite gradually became smooth under the combined effects of lubrication, cooling of seawater and tribochemical reaction. When two smooth surfaces slide against each other under liquid lubrication, the sliding pair presented the lower friction and wear behaviors [29]. Figure 15 shows the schematic diagram of the friction process of BS20/PEEK pair in seawater.

Conclusions
B 4 C-SiC ceramics with 10 wt.% and 20 wt.% SiC were fabricated by hot-pressed sintering. The microstructures, density, mechanical properties, and tribological properties in seawater of the B 4 C-SiC ceramic composite have been studied in this study, and get the following conclusions.
(1) B 4 C-SiC ceramics were mainly composed of B 4 C and SiC, and no new phase formed in the ceramic composite. When the SiC content was 20 wt.%, the microstructure significantly became denser.
(2) With the addition of SiC, the density of ceramic composite obviously increased, and the porosity presented a downward trend. Meanwhile, the hardness of ceramic slightly decreased by the addition of SiC phase, while the bending strength and fracture toughness were obviously improved due to the incorporation of SiC phase.
(3) When B 4 C-SiC ceramics (with better mechanical properties) slid against PEEK in seawater, the incorporation of SiC significantly enhanced the tribological characteristics of ceramic composite. Under the combined effects of lubricating, cooling of seawater, and tribochemical reaction, the wear surfaces of ceramic composite became smooth, and the real contact area decreased. Thus, the B 4 C-20 wt.%SiC/PEEK pair have a better friction and wear performance.
(4) When B 4 C ceramic slid against PEEK in seawater, larger spalling pits, cracks and wear particles were formed on the wear surfaces, and higher friction coefficient and wear rate of sliding pair were obtained. This result should be related to the lower fracture toughness and severe abrasive wear.