Synthesis and Tribological Characterization of Ti3SiC2/ZnO Composites

Dense Ti3SiC2/ZnO composites were sintered at different temperatures by spark plasma sintering (SPS). The effects of sintering temperature on composition and mechanical properties of Ti3SiC2/ZnO composites were studied. The tribological behaviors of Ti3SiC2/ZnO composites/Inconel 718 alloy tribo-pairs at elevated temperature from 25 °C to 800 °C were discussed. The experimental results showed that the initial decomposition temperature of the Ti3SiC2/ZnO composite was 1150 °C, and Ti3SiC2 decomposed into TiC. When the decomposition temperature was higher than 1150 °C, the compositions of the Ti3SiC2/ZnO composites were Ti3SiC2, ZnO, and TiC. It was found that Ti3SiC2/ZnO composites had better self-lubricating performance than Ti3SiC2 at elevated temperature from 600 °C to 800 °C, which was owing to material transfers of tribo-pairs and sheared oxides generated by tribo-oxidation reactions.


Introduction
Ti 3 SiC 2 is a ternary compound of the M n AX n + 1 (MAX) phase which combines the merits of both metals and ceramics, such as low density, high elastic modulus, favorable electrical conductivity, excellent high-temperature oxidation resistance, and good mechanical properties [1][2][3][4]. More significantly, it has potential application in self-lubricating under certain conditions [5,6].
With the development and innovation of industrial machinery, lubrication under extreme conditions has become one of the most interesting research areas [7][8][9][10]. Some traditional solid lubricants, such as graphite and MoS 2 , are not able to work effectively at elevated temperature in the fields of aerospace, aircraft, and others. Therefore, scholars have tried to prepare advanced materials for continuous lubrication in a wide-temperature range [11][12][13].
In previous studies, Ti 3 SiC 2 /Pb composites were successfully prepared, and it was found that the addition of Pb could not only improve mechanical properties, but also significantly decreased the friction coefficient and wear rate at 600-800 • C [14,15]. H.J. Wang et al. used SPS to synthesize Ti 3 SiC 2 /Al 2 O 3 composites and they found that the mechanical properties and friction-wear properties of Ti 3 SiC 2 could also be improved by adding Al 2 O 3 [12,16,17]. Islak et al. [18] prepared Ti 3 SiC 2 and Ti 3 SiC 2 /SiC composites by a mixture of Ti, Si, TiC, and Al powder and studied their mechanical properties and tribological performances, respectively. It was shown that Ti 3 SiC 2 /SiC composites had higher hardness, fracture toughness, and wear resistance than Ti 3 SiC 2 [19,20]. These studies showed that the mechanical properties and tribological performance of Ti 3 SiC 2 were relatively poor at elevated temperature. Fortunately, the friction coefficient and wear rate could be reduced with the incorporation of second phases.
Ternary layered carbide, Ti 3 SiC 2 , has good comprehensive properties. In this study, solid lubricant ZnO was used as the second phase to prepare Ti 3 SiC 2 /ZnO composites. It was expected that the addition of ZnO would improve the tribological performance of Ti 3 SiC 2 and especially promote its lubrication at elevated temperatures (600-800 • C).

Samples Preparation
The composites were prepared from powder mixture of Ti 3 SiC 2 (purity: >98%, average particle size: 2 µm, Jinhezhi Materials Ltd., Beijing, China) and ZnO (purity: >99.5%, average particle size: 1 µm, Xilong Chemical Ltd., Shantou, China). The volume fraction of ZnO in the Ti 3 SiC 2 /ZnO composite was 15 vol.%. The mixture with designed composition was firstly mixed for 6 h by ball-milling then filled into graphite dies (inner diameter: Φ25 mm) and sintered by SPS (manufacturer: Shanghai Chenhua Electric Furnace Co., Ltd., Shanghai, China, instrument model: SPS-20T10) under vacuum. The heating rate and pressure were 50 • C/min and 35 MPa, respectively. The holding time was 5 min at sintered temperatures (1100 • C, 1150 • C, and 1200 • C). Then, the furnace temperature was reduced to 1000 • C with a rate of 50 • C/min, and held at 1000 • C for 15 min. Subsequently, the sample was cooled to room temperature with the furnace. For comparison, Ti 3 SiC 2 was prepared at 1200 • C (purity: >99.5%, average particle size: 1 µm).

Mechanical Property
Densities of the composites were determined by using Archimedes principle. Microhardness was measured on a Micro-hardness Tester (Hengyi Technology Co., Ltd., Shanghai, China, instrument model: MH-5-VM) applying a load of 500 g with a dwell time of 10 s. Flexural strength and compression strength were measured using a universal material tester (SANS-CMT5205, MTS China, Shenzhen, China). The three-point bending test was conducted to obtain the flexural strength and the samples were cut to 3 mm high × 4 mm wide × 20 mm long. The cross-head speed and span were 0.05 mm/min and 16 mm, respectively. The sizes of the samples for the compression strength test were Φ 5 mm × 12.5 mm and the crosshead speed was 0.2 mm/min. All samples of mechanical properties were prepared by wire cutting.

Friction and Wear Test
The friction and wear tests were conducted on a high temperature tribometer with a pin-on-disk configuration (CSM Instruments SA, Peseux, Switzerland). Friction tests under each condition were repeated three times. The pin was a Ti 3 SiC 2 and Ti 3 SiC 2 /ZnO composite with a size of Φ 6 mm × 12 mm by wire cutting. The disk was Inconel 78 alloy with a size of Φ 32 mm × 8 mm by wire cutting. The surface roughness (Ra) of the disk was 0.06 µm after grinding and polishing. The test was conducted at room temperature with relative humidity in the range of 20-50%. The sliding speed was 0.5 m/s and the normal load was 5 N. The sliding distance was 200 m. The friction coefficient was be recorded by a computer during the test. The wear volume was calculated by measuring the volume loss of the Ti 3 SiC 2 and Ti 3 SiC 2 /ZnO pin and the cross-sectional area of the wear track on Inconel 718 disk using optical microscopy and 3D surface profilometry. The wear rates were calculated from the wear volume divided by sliding distance and load. In addition, scanning electron microscope (SEM, JEOL Company, Tokyo, Japan) was used to observe the wear surface morphology, X-ray diffraction (XRD, Rigaku Corporation, Tokyo, Japan) was used to analyze the phase of the wear surface, and X-ray photoelectron spectroscopy (XPS, Physical Electronics Corporation, Chanhassen, MN, USA) was used to analyze the oxidation on the wear surface. Figure 1 shows the XRD patterns of Ti 3 SiC 2 /ZnO composites at different sintering temperatures. The XRD pattern at 1100 • C shows that the main components of sintered samples were Ti 3 SiC 2 and ZnO. The results indicate that no chemical reaction occurred at 1100 • C between Ti 3 SiC 2 and ZnO, and Ti 3 SiC 2 did not decompose. At 1150 • C and 1200 • C, diffraction peaks of Ti 3 SiC 2 , TiC and ZnO were found in the XRD pattern, which indicated that TiC was generated by partial decomposition of Ti 3 SiC 2 . This result was consistent with the previous research results [15]. Figure 2 shows the micrographs of Ti 3 SiC 2 /ZnO composites at 1100 • C, 1150 • C, and 1200 • C. As seen from the figure, at 1100 • C, the grain of Ti 3 SiC 2 /ZnO composites is coarse and irregular in shape, and many pores are distributed among them. With the increase of temperature, the grain size was refined, and its pores were obviously reduced. Figure 1 shows the XRD patterns of Ti3SiC2/ZnO composites at different sintering temperatures. The XRD pattern at 1100 °C shows that the main components of sintered samples were Ti3SiC2 and ZnO. The results indicate that no chemical reaction occurred at 1100 °C between Ti3SiC2 and ZnO, and Ti3SiC2 did not decompose. At 1150 °C and 1200 °C, diffraction peaks of Ti3SiC2, TiC and ZnO were found in the XRD pattern, which indicated that TiC was generated by partial decomposition of Ti3SiC2. This result was consistent with the previous research results [15]. Figure 2 shows the micrographs of Ti3SiC2/ZnO composites at 1100 °C, 1150 °C, and 1200 °C. As seen from the figure, at 1100 °C, the grain of Ti3SiC2/ZnO composites is coarse and irregular in shape, and many pores are distributed among them. With the increase of temperature, the grain size was refined, and its pores were obviously reduced.     Table 1 shows some mechanical properties of the Ti3SiC2 and Ti3SiC2/ZnO composites at different sintering temperatures. When the sintering temperature was 1100 °C, the temperature was so low that it caused the formation of coarse grains and many holes in the composites with poor mechanical properties. Therefore, these results were not listed in the table. Compared with Ti3SiC2, the relative density, microhardness, flexural strength, and compression strength of the Ti3SiC2/ZnO composites were decreased. In addition, with the increase of sintering temperature, the mechanical properties of the Ti3SiC2/ZnO composites had certain changes: the flexural strength decreased by about 15.4%, compression strength increased by about 23.5%, and Vickers hardness increased by only 3.7%. It can be concluded that Ti3SiC2/ZnO composites have relatively good mechanical properties with sintering at 1200 °C, and their microhardness, bending strength, and compression strength were 4.67 ± 0.93 GPa, 115 ± 5 MPa, and 557 ± 3 MPa, respectively. Figure 3 shows the compression stress-strain curve of the Ti3SiC2/ZnO composites sintered at a temperature of 1200 °C and the bending fracture morphology of the composites at different sintering temperatures. As seen from Figure 3a, the stress-strain curve of the Ti3SiC2/ZnO composites was close to a straight line with a certain slope before the fracture. Consequently, the composites only displayed elastic deformation before the fracture, which was consistent with the characteristic of brittle fracture. In addition, the fracture mechanism of Ti3SiC2 was also brittle fracture [15]. Moreover, the addition of ZnO had no obvious effect on the fracture mode. Figure 3b-d shows the bending fracture morphology of materials at different sintering temperatures. From the perspective of macroscopic morphology, the fracture mode of the Ti3SiC2/ZnO composites at room temperature was brittle fracture.  Table 1 shows some mechanical properties of the Ti 3 SiC 2 and Ti 3 SiC 2 /ZnO composites at different sintering temperatures. When the sintering temperature was 1100 • C, the temperature was so low that it caused the formation of coarse grains and many holes in the composites with poor mechanical properties. Therefore, these results were not listed in the table. Compared with Ti 3 SiC 2 , the relative density, microhardness, flexural strength, and compression strength of the Ti 3 SiC 2 /ZnO composites were decreased. In addition, with the increase of sintering temperature, the mechanical properties of the Ti 3 SiC 2 /ZnO composites had certain changes: the flexural strength decreased by about 15.4%, compression strength increased by about 23.5%, and Vickers hardness increased by only 3.7%. It can be concluded that Ti 3 SiC 2 /ZnO composites have relatively good mechanical properties with sintering at 1200 • C, and their microhardness, bending strength, and compression strength were 4.67 ± 0.93 GPa, 115 ± 5 MPa, and 557 ± 3 MPa, respectively. Figure 3 shows the compression stress-strain curve of the Ti 3 SiC 2 /ZnO composites sintered at a temperature of 1200 • C and the bending fracture morphology of the composites at different sintering temperatures. As seen from Figure 3a, the stress-strain curve of the Ti 3 SiC 2 /ZnO composites was close to a straight line with a certain slope before the fracture. Consequently, the composites only displayed elastic deformation before the fracture, which was consistent with the characteristic of brittle fracture. In addition, the fracture mechanism of Ti 3 SiC 2 was also brittle fracture [15]. Moreover, the addition of ZnO had no obvious effect on the fracture mode. Figure 3b-d shows the bending fracture morphology of materials at different sintering temperatures. From the perspective of macroscopic morphology, the fracture mode of the Ti 3 SiC 2 /ZnO composites at room temperature was brittle fracture.

Friction Coefficients and Wear Rates
As mentioned before, the Ti3SiC2/ZnO composites sintered at 1200 °C (named TSC-ZN) had favorable mechanical performance. The tribological and wear properties of TSC-ZN and Inconel 718 alloys were studied at 25-800 °C. In comparison, the tribological and wear properties of Ti3SiC2 sintered at 1200 °C (named TSC) were also tested. Figure 4 shows the friction coefficients of the Ti3SiC2 and Ti3SiC2/ZnO composites as a function of sliding distances and their average friction coefficients at different temperatures. From 25 °C to 400 °C, the friction coefficients of the Ti3SiC2/ZnO composites were higher than that of Ti3SiC2, and their friction coefficients fluctuated largely at 25°C. However, the friction coefficients of the Ti3SiC2/ZnO composites were obviously lower than that of Ti3SiC2 from 600 °C to 800 °C. As seen from the figure, the friction coefficients of the Ti3SiC2/ZnO composites decreased gradually with the increase of temperature at 25-

Friction Coefficients and Wear Rates
As mentioned before, the Ti 3 SiC 2 /ZnO composites sintered at 1200 • C (named TSC-ZN) had favorable mechanical performance. The tribological and wear properties of TSC-ZN and Inconel 718 alloys were studied at 25-800 • C. In comparison, the tribological and wear properties of Ti 3 SiC 2 sintered at 1200 • C (named TSC) were also tested. Figure 4 shows the friction coefficients of the Ti 3 SiC 2 and Ti 3 SiC 2 /ZnO composites as a function of sliding distances and their average friction coefficients at different temperatures. From 25 • C to 400 • C, the friction coefficients of the Ti 3 SiC 2 /ZnO composites were higher than that of Ti 3 SiC 2 , and their friction coefficients fluctuated largely at 25 • C. However, the friction coefficients of the Ti 3 SiC 2 /ZnO composites were obviously lower than that of Ti 3 SiC 2 from 600 • C to 800 • C. As seen from the figure, the friction coefficients of the Ti 3 SiC 2 /ZnO composites decreased gradually with the increase of temperature at 25-800 • C and there was a marked decreased at 600 • C. For Ti 3 SiC 2 , the friction coefficients were relatively stable at 25-600 • C, and they showed a significant decrease at 800 • C. In addition, the average friction coefficients of the Ti 3 SiC 2 /ZnO composites were higher than those of Ti 3 SiC 2 at 25-400 • C, but they were lower at 600-800 • C. It could be concluded that Ti 3 SiC 2 showed excellent self-lubricating properties at 800 • C, while the Ti 3 SiC 2 /ZnO composites showed excellent self-lubricating properties at 600 • C.
Materials 2021, 14, x FOR PEER REVIEW 6 of 11 800 °C and there was a marked decreased at 600 °C. For Ti3SiC2, the friction coefficients were relatively stable at 25-600 °C, and they showed a significant decrease at 800 °C. In addition, the average friction coefficients of the Ti3SiC2/ZnO composites were higher than those of Ti3SiC2 at 25-400 °C, but they were lower at 600-800 °C. It could be concluded that Ti3SiC2 showed excellent self-lubricating properties at 800 °C, while the Ti3SiC2/ZnO composites showed excellent self-lubricating properties at 600 °C.  Figure 5a shows the variation in wear rates as a function of the pin of the Ti3SiC2/Inconel 718 alloy and Ti3SiC2/ZnO-Inconel 718 alloy tribo-pairs at different temperatures. For the Ti3SiC2/ZnO-Inconel 718 alloy tribo-pair, no obvious change was found in the wear rates of the pin (Ti3SiC2/ZnO) at 25-400 °C and its wear rates decreased significantly at above 400 °C and reduced to about 7.32 × 10 −5 mm 3 /N m at 800 °C. For the Ti3SiC2/Inconel 718 alloy tribo-pairs, the wear rates of the pin (Ti3SiC2) decreased gradually with the increase of temperature (25-800 °C) and reduced to about 5.34 × 10 −6 mm 3 /N m at 800 °C. Figure 5b shows the variation in wear rates as a function of the disks of the Ti3SiC2/Inconel  Figure 5a shows the variation in wear rates as a function of the pin of the Ti 3 SiC 2 /Inconel 718 alloy and Ti 3 SiC 2 /ZnO-Inconel 718 alloy tribo-pairs at different temperatures. For the Ti 3 SiC 2 /ZnO-Inconel 718 alloy tribo-pair, no obvious change was found in the wear rates of the pin (Ti 3 SiC 2 /ZnO) at 25-400 • C and its wear rates decreased significantly at above 400 • C and reduced to about 7.32 × 10 −5 mm 3 /N m at 800 • C. For the Ti 3 SiC 2 /Inconel 718 alloy tribo-pairs, the wear rates of the pin (Ti 3 SiC 2 ) decreased gradually with the increase of temperature (25-800 • C) and reduced to about 5.34 × 10 −6 mm 3 /N m at 800 • C. Figure 5b shows the variation in wear rates as a function of the disks of the Ti 3 SiC 2 /Inconel 718 alloy and Ti 3 SiC 2 /ZnO-Inconel 718 tribo-pairs at different temperatures. For the Ti 3 SiC 2 /ZnO-Inconel 718 alloy tribo-pair, the disk (Inconel 718 alloy) showed occured at 25 • C; however, it turned into negative wear above 200 • C. Additionally, with the increase of temperature, the negative wear rates increased gradually and reached about −2.27 × 10 −4 mm 3 /N m at 800 • C. In addition, the wear rates of the disk (Inconel 718) of the Ti 3 SiC 2 /Inconel 718 alloy tribo-pair were positive at 25-600 • C and decreased with the increase of temperature. Then, its wear rates became negative at 800 • C and the negative wear rate were close to that of the disk (Inconel 718) of the Ti 3 SiC 2 /ZnO-Inconel 718 alloy tribo-pair.  The experimental results showed that the wear resistance of the Ti3SiC2/ZnO composites was worse than that of Ti3SiC2 from 600 °C to 800 °C; however, the friction coefficients of the Ti3SiC2/ZnO composites were lower than that of Ti3SiC2. Therefore, the Ti3SiC2/ZnO composites show good self-lubricating performance. It is feasible to add ZnO to improve the tribological performance of Ti3SiC2 at elevated temperatures. Figure 6 showed the worn surfaces of the Ti3SiC2/ZnO-Inconel 718 alloy tribo-pair from 25 °C to 800 °C, which was the typical morphology of the worn pin and disk below 800 °C. It was seen that the wear mechanism caused by the pulling out and falling off of Ti3SiC2 grains during sliding was abrasive wear. There were some pits on the worn surfaces of the Ti3SiC2/ZnO composites, and the worn surfaces were relatively rough (see Figure 6a). At 25-400 °C, there was plastic flow on the worn surfaces of the Inconel 718 alloy (see Figure 6b), and Ti, Si, and Zn elements could be detected on these worn surfaces (see Table 2b), which indicated that there was a one-way transfer of materials from the Ti3SiC2/ZnO composites to the Inconel 718 alloy during the friction process. At 600-800 °C, Ti3SiC2 grains still fell off, and the Ti3SiC2/ZnO composites also exhibited certain characteristics of plastic flow (see Figure 6c). Similarly, the surfaces of the Inconel 718 alloy still had characteristics of plastic flow (see Figure 6d). Moreover, there was material transfer between tribo-pairs, and the material transfer films became denser after sintering at high temperatures. The experimental results showed that the wear resistance of the Ti 3 SiC 2 /ZnO composites was worse than that of Ti 3 SiC 2 from 600 • C to 800 • C; however, the friction coefficients of the Ti 3 SiC 2 /ZnO composites were lower than that of Ti 3 SiC 2 . Therefore, the Ti 3 SiC 2 /ZnO composites show good self-lubricating performance. It is feasible to add ZnO to improve the tribological performance of Ti 3 SiC 2 at elevated temperatures. Figure 6 showed the worn surfaces of the Ti 3 SiC 2 /ZnO-Inconel 718 alloy tribo-pair from 25 • C to 800 • C, which was the typical morphology of the worn pin and disk below 800 • C. It was seen that the wear mechanism caused by the pulling out and falling off of Ti 3 SiC 2 grains during sliding was abrasive wear. There were some pits on the worn surfaces of the Ti 3 SiC 2 /ZnO composites, and the worn surfaces were relatively rough (see Figure 6a). At 25-400 • C, there was plastic flow on the worn surfaces of the Inconel 718 alloy (see Figure 6b), and Ti, Si, and Zn elements could be detected on these worn surfaces (see Table 2(b)), which indicated that there was a one-way transfer of materials from the Ti 3 SiC 2 /ZnO composites to the Inconel 718 alloy during the friction process. At 600-800 • C, Ti 3 SiC 2 grains still fell off, and the Ti 3 SiC 2 /ZnO composites also exhibited certain characteristics of plastic flow (see Figure 6c). Similarly, the surfaces of the Inconel 718 alloy still had characteristics of plastic flow (see Figure 6d). Moreover, there was material transfer between tribo-pairs, and the material transfer films became denser after sintering at high temperatures.

Morphology and Composition of the Worn Surfaces
In conclusion, there was a one-way transfer of materials from the Ti 3 SiC 2 /ZnO composites to the Inconel 718 alloy in the friction process at 25-400 • C, and a mutual transfer of materials between the Ti 3 SiC 2 /ZnO composites and the Inconel 718 alloy in the friction process at 600-800 • C.  In conclusion, there was a one-way transfer of materials from the Ti3SiC2/ZnO composites to the Inconel 718 alloy in the friction process at 25-400 °C, and a mutual transfer of materials between the Ti3SiC2/ZnO composites and the Inconel 718 alloy in the friction process at 600-800 °C.   Figure 7 shows the element chemical states after friction and wear test from 25 • C to 600 • C. From the experimental results, it can be seen that ZnO and SiO 2 were formed on the worn surface of the Ti 3 SiC 2 /ZnO composites at 25 • C. At 200-600 • C, the oxide films consisted of ZnO, SiO 2, and TiO 2 . As shown above, tribo-oxidation reactions occurred for the Ti 3 SiC 2 /ZnO composites to generate SiO 2 and TiO 2 .
When the generation rate of oxide was greater than its consumption rate, an oxide layer was formed on the friction surface. The progress of the tribo-oxidation reaction depended on the frictional heat. At 25-400 • C, the oxidation reaction induced by friction heat was not enough to form an oxide film on the friction surface. Therefore, the tribological and wear properties of the Ti 3 SiC 2 /ZnO composites was not improved from 25 • C to 400 • C. However, when the temperature exceeded 600 • C, the friction heat provided enough energy to generate oxides continuously. At this point, the formed oxides, such as ZnO, SiO 2 , and TiO 2 , had obvious characteristics of plastic flow and could be sheared easily. In addition, the oxide films avoided direct contact between the Ti 3 SiC 2 /ZnO composites and the Inconel 718 alloy, and also decreased the material transfer and dropping of grains of Ti 3 SiC 2 . Therefore, the friction coefficients of the Ti 3 SiC 2 /ZnO composites decreased significantly. With the increase of temperature, the oxide films would be softened, making it easier to shear.  Figure 7 shows the element chemical states after friction and wear test from 25 °C to 600°C. From the experimental results, it can be seen that ZnO and SiO2 were formed on the worn surface of the Ti3SiC2/ZnO composites at 25 °C. At 200-600 °C, the oxide films consisted of ZnO, SiO2, and TiO2. As shown above, tribo-oxidation reactions occurred for the Ti3SiC2/ZnO composites to generate SiO2 and TiO2. When the generation rate of oxide was greater than its consumption rate, an oxide layer was formed on the friction surface. The progress of the tribo-oxidation reaction depended on the frictional heat. At 25-400 °C, the oxidation reaction induced by friction heat was not enough to form an oxide film on the friction surface. Therefore, the tribological and wear properties of the Ti3SiC2/ZnO composites was not improved from 25 °C to 400 °C. However, when the temperature exceeded 600 °C, the friction heat provided enough energy to generate oxides continuously. At this point, the formed oxides, such as ZnO, SiO2, and TiO2, had obvious characteristics of plastic flow and could be sheared easily. In addition, the oxide films avoided direct contact between the Ti3SiC2/ZnO composites and the Inconel 718 alloy, and also decreased the material transfer and dropping of grains of Ti3SiC2. Therefore, the friction coefficients of the Ti3SiC2/ZnO composites decreased significantly. With the increase of temperature, the oxide films would be softened, making it easier to shear.

Conclusions
Dense Ti 3 SiC 2 /ZnO composites were successfully prepared by SPS from the mixture of Ti 3 SiC 2 and ZnO powders. The friction coefficients and wear rates of the composites were relatively high at low and medium temperatures (at 25-400 • C). Fortunately, the friction coefficients of the composites decreased significantly at higher temperature (at 600-800 • C), due to material transfers of tribo-pairs and the presence of SiO 2 and TiO 2 . These oxide films had obvious characteristics of plastic flow and avoided direct contact of tribo-pairs. Therefore, Ti 3 SiC 2 /ZnO composites could be used as a solid lubricant in a wide-temperature range.