The Ti3SiC2 sample is composed of Ti3SiC2 particles (marked by A and B in Fig. 1b), tightly combined with some TiC particles (marked by C in Fig. 1b). The relative content of Ti3SiC2 is about 91.37%, calculated by the internal standard method [20] based on Fig. 1a. The Ti3SiC2 particles exhibit irregular and lamellar shape with a particle size of 4 to 5 µm. TiAl alloy is composed of TiAl and Ti3Al. With the Ti3SiC2 addition, the XRD patterns of the composites show the presence of Ti3SiC2, TiC, and Ti5Si3 in the TiAl matrix composites, while TiAl still exists as a major phase (Fig. 1c). The relative content of each phase in the TiAl matrix composites is calculated by the semi-quantitative method [21]. With the increase of the Ti3SiC2 amount, the relative contents of Ti3SiC2, TiC, and Ti5Si3 increase, while the relative contents of TiAl and Ti3Al decrease (Fig. 1d), implying that Ti5Si3 and TiC are mainly formed by the Ti3SiC2 decomposition. Previous studies have reported that the TiC formation may be attributed to the reaction of TiAl and graphite atmosphere generated by the graphite molds during the sintering process [12, 13, 15, 16]. However, in this work, the molten Al can promote the Ti3SiC2 decomposition to form TiC and Ti5Si3 [22–24]. The formation of Ti5Si3 and TiC improves the mechanical and tribological properties of the TiAl matrix composites in this work. The Ti3SiC2 decomposition reaction can be described as follows:
Ti 3 SiC 2 =3TiC+Si
Ti 3 SiC 2 +Al (l) =3TiC+Si(Al)
5Ti+3Si=Ti 5 Si 3
Ti3SiC2 (marked by yellow rhombus in Fig. 2c and Fig. 4) is evenly distributed in the TiAl matrix (Figs. 2-3). The distribution and agglomeration of Cr in the TiAl matrix are indicated by the blue triangle in Fig. 2a and the blue ellipse area in Fig. 3. There are few distinct interstices between Ti3SiC2 and TiAl matrix (Fig. 4a). Ti from TiAl and Si from Ti3SiC2 exhibit mutual diffusion at the interface between Ti3SiC2 and the TiAl matrix. The diffusion distance of Si in Ti3SiC2 of TA2 is about 2 µm, while that of Ti in TiAl is about 3 µm (Figs. 4b-c). The diffusion of elements yields the formation of TiC and Ti5Si3 from the Ti3SiC2 decomposition, which can change the interfacial structure between Ti3SiC2 and the TiAl matrix, improving the mechanical and tribological properties of the composite [25]. With the increase of the Ti3SiC2 amount, the Ti3SiC2 agglomeration becomes more pronounced (Fig. 2), deteriorating the mechanical and tribological properties of the TiAl matrix composites. The volume fraction of Ti3SiC2 in TA1, TA2, and TA3 is approximately 8.22, 16.8, and 26.2%, respectively, Fig. 2, as determined by using Image-Pro Plus (IPP) software [26]. This corroborates the relative content of Ti3SiC2 obtained from the XRD patterns of TA1, TA2, and TA3 (Fig. 1d), which further confirms the result that Ti3SiC2 is partly decomposed in the TiAl matrix composite.
The bulk density, relative density, and hardness of the TiAl matrix composites are in the range of 3.75-3.88 g/cm3, 99.2-99.6%, and 4-5.3 GPa, respectively, while the corresponding values of compression strength, friction coefficient, and wear rate at room temperature and 550°C are in the range of 753-1080 MPa and 298-640 MPa, 0.507-0.581 and 0.423-0.484, (1.35-2.15)⋅10−4 mm3N−1m−1 and (0.21-0.26) ⋅10−4 mm3N−1m−1, respectively (Figs. 5a-e). The values of bulk density and hardness exhibit the same increasing and then decreasing trends with the rise of the Ti3SiC2 amount. The compression strength at room temperature and 550°C exhibits an increasing trend, while the relative density, friction coefficient, and wear rate at room temperature and 550°C exhibits a decreasing trend. In the case of 20 wt.% Ti3SiC2 addition, the bulk density and hardness of the TiAl matrix composites reach the maximum values of 3.88 g/cm3 and 5.3 GPa, respectively. At the 30 wt.% Ti3SiC2 addition, the compression strength at room temperature and 550°C reach the maximum values of 1080 and 640 GPa, respectively, while the friction coefficient and wear rate reach the minimum values of 0.507 and 1.35⋅10−4 mm3N−1m−1, and 0.423 and 0.21⋅10−4 mm3N−1m−1 at room temperature and 550°C, respectively. The synthesized Ti3SiC2-TiAl composites exhibit an extremely low wear rate of about 0.21⋅10−4 mm3N−1m−1 at 550°C compared with the previous studies (Fig. 5f) [12, 13, 15, 16, 27–29]. In addition, the friction coefficient and wear rate at room temperature and the friction coefficient at 550°C are close to the mean values reported in the previous studies.
The friction coefficient and wear rate of the TiAl matrix composite with the Ti3SiC2 addition decrease by about 11.3% and 29.4% at room temperature, and 10.9% and 19.23% at 550°C, respectively, compared with the TiAl alloy itself. The groove (marked by D and F in Fig. 6a and Table 1) exists on the friction surface of TiAl alloy after friction test at room temperature, while abrasive debris (marked by Q in Fig. 6a and Table 1) and plastic deformation (marked by R in Fig. 6e and Table 1) appear after friction test at 550°C. Besides the grooves (marked by G, K, and N in Figs. 6b-d and Table 1), abrasive debris (marked by U and W in Figs. 6f-g and Table 1) and plastic deformation (marked by T, V, and Z in Figs. 6e-h and Table 1) exist on the friction surface of the TiAl matrix composites with the Ti3SiC2 addition after friction test at room temperature and 550°C, while the peeling (marked by H, J, and M in Figs. 6b-d and Table 1) appears on the friction surface after friction test at room temperature. The difference in tribological properties between the TiAl alloy and the TiAl matrix composites with the Ti3SiC2 addition is attributed to the formation of a lubricating film on the friction surface of the TiAl matrix composites (Figs. 6-8 and Table 1). The thickness of the lubricating film is about 8 µm (Layer 1 and Layer 2 in Fig. 8). There is a mechanical mixture layer with a thickness of about 18 µm connecting the lubricating film and the TiAl matrix composite surface after friction test both at room temperature and 550°C (Layer 3 in Fig. 8). The mechanical mixture layer is mainly composed of Ti3SiC2 and TiAl, which provide Ti, Al, Si that form the lubricating ଁlm and prevent the lubrication film from falling off (Fig. 8). The C content distributed in the mechanical mixture layer is higher than that in the TiAl matrix, implying that the lubricating film of the TiAl matrix composites is formed by squeezing the Ti3SiC2 additive (Fig. 8). In addition, the plastic flow of the Ti3SiC2 lubricant on the friction surface of the TiAl matrix composites also contributes to the formation of the lubricating film. Although the friction test was conducted at room temperature, the instantaneous temperature between the two relatively moving composites in direct contact may reach 3000°C during the friction process [30–32]. Consequently, the lubricating film of the TiAl matrix composites with the added Ti3SiC2 originates from the squeezing and plastic flow of Ti3SiC2 both at room temperature and 550°C.
The friction coefficient and wear rate values of the TiAl composites at 550°C decrease by about 16.6 and 86.6%, respectively, compared with those at room temperature. The groove and peeling appear on the friction surface of the TiAl matrix composites at room temperature, while the abrasive debris and plastic deformation exist at 550°C (Fig. 6). The wear mechanisms of the TiAl matrix composites at room temperature and 550°C mainly belong to abrasive wear and adhesive wear, respectively. The difference in tribological properties of the TiAl matrix composites at room temperature and 550°C is attributed to the composition of lubricating ଁlm on the friction surface of the TiAl matrix composites. The Fe, O, Si, and C contents are about 0.10, 3.62, 3.65, and 6.91%, respectively, at the friction surface of the TiAl matrix composites at room temperature (Figs. 6a-d and Table 1). However, the Fe and O contents on the friction surface of TiAl matrix composite after friction test at 550°C are 639.50 and 9145.45% higher than those at room temperature, respectively, while the Si and C contents are 72.33 and 41.10% lower than those at 550°C, respectively (Figs. 6a-h and Table 1). In addition, the contents of Si and C at the friction surface of the TiAl matrix composite increase at higher temperatures with the Ti3SiC2 amount. Fe2O3, Fe3O4, Fe3Ti0.5O0.3, Fe3Al, and Ti3SiC2 appear in the wear debris of the TiAl matrix composites at 550°C (Fig. 7c). According to the analysis of wear debris and elemental distribution of the friction surface of the TiAl matrix, the Ti3SiC2 lubricating ଁlm forms on the friction surface of the TiAl matrix composite at room temperature, while the Fe-Ti-Al-Si-oxides lubricating ଁlm forms at 550°C. The variation in the Fe and O contents exhibits similar trends, opposite to the variation trend of Ti, Al, and Si contents (Figs. 7a-b). It is inferred that the Fe-Ti-Al-Si-oxide lubricating film consists of a Fe-oxide layer and a Ti-Al-Si-oxide layer at 550°C (Layer 1 and Layer 2 Fig. 8), while the Ti3SiC2 lubricating film has only a Ti3SiC2 lubricating layer at room temperature. The Fe-oxide layer is located at the top of the Fe-Ti-Al-Si-oxide lubricating film (Fig. 8). Fe2O3 and Fe3O4 exist in wear debris at 550°C, indicating that the Fe-oxide layer on the friction surface of the TiAl matrix composites and the lubricating layer of the counterpart are mainly composed of Fe2O3 and Fe3O4 (Fig. 7c). In addition, Ti-Al-Si-oxides do not appear in wear debris. The wear of the TiAl matrix composites at 550°C is concentrated in the Fe-oxide layer rather than in the whole lubricating film like that at room temperature, which is consistent with the results of lower friction coefficient and wear rate of the TiAl matrix composites at 550°C than at room temperature. Thus, the phase composition of the lubricating film contributes to better tribological properties of the TiAl matrix composites at 550°C than at room temperature.
The variation of the surface characteristics of the TiAl matrix composites shows three stages with the Ti3SiC2 amount (Fig. 6). As the Ti3SiC2 amount increases from 0 to 10 wt.%, the variation trend of groove and plastic deformation on the surface of the TiAl matrix composites increases, while the variation trend of peeling and abrasive debris decreases.
As the Ti3SiC2 amount increases from 10 to 20 wt.%, the variation trend of groove, peeling, abrasive debris, and plastic deformation decreases. When the Ti3SiC2 amount is between 20 and 30 wt.%, the variation trend of groove and peeling increases, while the variation trend of abrasive debris and plastic deformation decreases. On the one hand, the Si and C contents on the friction surface of the TiAl matrix composites after friction test at room temperature increase with the Ti3SiC2 amount (Fig. 6 and Table. 1). On the other hand, although the Si and C contents exhibit no distinct comparison trend after friction test at 550°C, the Fe, Ti, Al, O, Si contents exhibit similar values at different regions in the same composite on the friction surface after friction test at 550°C with the Ti3SiC2 amount (Fig. 6 and Table 1).
It can be concluded that a higher amount of added Ti3SiC2 has a beneficial effect on the integrity of the lubricant film of the TiAl matrix composites on the friction surface both at room temperature and 550°C, decreasing the friction coefficient and wear rate. The peeling region has the lowest content of Si and C compared with other regions at room temperature, indicating that the thin region of the Ti3SiC2 lubricating film tends to form peeling (Figs. 6a-d and Table 1). The plastic deformation region has the highest Cr content and the lowest O content compared with other regions at 550°C (Figs. 6e-f and Table 1), indicating that the plastic deformation is liable to form at the thin region of the Fe-Ti-Al-Si-oxide lubricating film. As the Ti3SiC2 amount increases from 0 to 10 wt.%, the Ti3SiC2 content in the TiAl matrix composite is only 0 - 9% (Figs. 1c-d), so the Ti3SiC2 lubricant on the surface of the TiAl matrix composite is in the form of discontinuous islands. Although the integrity of the lubricating film increases with the amount of added Ti3SiC2, the amount of isolated Ti3SiC2 lubricant on the friction surface increases, decreasing the variation trend of peeling and abrasive debris. As the Ti3SiC2 amount increases from 10 to 20 wt.%, the Ti3SiC2 content (Figs. 1c-d) is high enough, 9-17.9%, to form continuous Ti3SiC2 lubricant on the TiAl matrix composites’ surface. Thus, the peeling and plastic deformation on the friction surface of the TiAl matrix composites increase. When the Ti3SiC2 amount is from 20 to 30 wt.%, Ti3SiC2 gradually agglomerates (Fig. 2). It induces detaching of the Ti3SiC2 lubricant, enhancing the peeling region on the friction surface of the TiAl matrix composite (Figs. 6a-d and Table 1). However, the Ti3SiC2 agglomeration has little influence on the formation of plastic deformation on the friction surface at 550°C. It is attributed to the phase composition of the Fe-Ti-Al-Si-oxide film and is affected by the double oxide layer of Fe-Ti-Al-Si-oxides film (Figs. 6e-f and Fig. 8). Consequently, the peeling on the friction surface of the TiAl matrix composites increases with the Ti3SiC2 amount (20-30 wt.%), while the plastic deformation decreases. Fe exists on the groove, while other regions do not exhibit the Fe presence at room temperature (Figs. 6a-d and Table 1). Moreover, the variation trend of groove depth is consistent with the value variation trend of the hardness of the TiAl matrix composites. Thus, the hard phase of the counterpart sinks into the friction surface due to the applied load and then, as the counterpart moves along the surface, forms a groove on the friction surface at room temperature [33]. However, the groove does not appear on the friction surface at 550°C since the direct friction between the TiAl matrix composites and the counterpart is transformed into the friction between the Ti-Al-Si-oxide film of the TiAl matrix composites and the oxide film of the counterpart. The abrasive debris region has the lowest Ti, Al, and Si contents compared with other regions at 550°C, while the O and Fe contents are higher than in the other regions (Figs. 6e-h and Figs. 7a-b). In addition, the Fe content in the abrasive debris region decreases with the Ti3SiC2 amount. It can be inferred that the abrasive debris tends to form at the thick region of the Fe-oxide layer located at the top of the Fe-Ti-Al-Si-oxide film, which can be inhibited by the increase in the amount of added Ti3SiC2 (Figs. 6e-h and marked by Layer 1 in Fig. 8).
Table 1
EDS elemental analysis (wt.%) of the regions marked in Fig. 6.
EDS measured points
|
Ti
|
Al
|
O
|
Fe
|
Cr
|
Si
|
C
|
D
|
62.41
|
31.73
|
4.42
|
0.2
|
1.24
|
--
|
--
|
E
|
61.91
|
32.38
|
4.16
|
--
|
1.55
|
--
|
--
|
F
|
62.72
|
31.78
|
3.78
|
0.11
|
1.61
|
--
|
--
|
G
|
58.37
|
29.41
|
3.65
|
0.05
|
1.93
|
2.47
|
4.12
|
H
|
60.28
|
30.16
|
4.13
|
--
|
1.90
|
1.14
|
2.39
|
I
|
58.68
|
29.39
|
3.66
|
--
|
1.88
|
2.38
|
4.01
|
J
|
58.09
|
29.63
|
3.52
|
--
|
0.98
|
2.00
|
5.78
|
K
|
55.07
|
29.23
|
3.24
|
0.02
|
1.21
|
4.55
|
6.68
|
L
|
55.38
|
28.62
|
3.18
|
--
|
0.89
|
4.52
|
7.41
|
M
|
55.58
|
28.49
|
3.29
|
--
|
0.87
|
4.18
|
7.59
|
N
|
51.85
|
26.29
|
3.24
|
0.13
|
0.92
|
5.73
|
11.84
|
O
|
49.68
|
28.02
|
3.11
|
0.1
|
0.85
|
5.89
|
12.35
|
P
|
46.41
|
18.78
|
25.5
|
8.93
|
0.38
|
--
|
--
|
Q
|
12.18
|
10.12
|
37.44
|
40.13
|
0.13
|
--
|
--
|
R
|
40.63
|
24.51
|
25.01
|
9.37
|
0.48
|
--
|
--
|
S
|
42.24
|
22.04
|
22.4
|
8.57
|
0.34
|
0.83
|
3.58
|
T
|
39.61
|
24.83
|
21.46
|
8.74
|
0.51
|
0.91
|
3.94
|
U
|
22.81
|
15.13
|
30.46
|
29.73
|
0.4
|
0.34
|
1.13
|
V
|
40.21
|
20.61
|
23.7
|
8.51
|
1.08
|
1.27
|
4.62
|
W
|
32.02
|
18.06
|
30.74
|
15.38
|
0.22
|
0.48
|
3.10
|
X
|
42.38
|
15.49
|
28.31
|
8.11
|
0.25
|
1.39
|
4.07
|
Y
|
44.07
|
22.34
|
20.3
|
6.51
|
1.15
|
1.43
|
4.20
|
Z
|
37.38
|
14.06
|
29.16
|
8.05
|
2.02
|
1.45
|
7.88
|
As mentioned above, the mechanical and tribological properties of the TiAl matrix composites improve with the Ti3SiC2 amount. The change in the density, hardness, and compression strength values with the amount of added Ti3SiC2 is achieved by the joint contribution of three factors. Firstly, the addition of Ti3SiC2 introduces high density and hardness Ti3SiC2 (4.53 g/cm3 and 4 GPa), TiC (4.93 g/cm3 and 28.5 GPa), and Ti5Si3 (4.32 g/cm3 and 9.5 GPa) phases (Fig. 1c) into the TiAl matrix composites, yielding the increase of bulk density and hardness of the TiAl matrix composites. Secondly, Ti5Si3 and TiC formed at the interface of the TiAl matrix and Ti3SiC2 induce a pinning effect (Fig. 4b), which is conducive to the stress transfer from the TiAl matrix to Ti3SiC2 with higher pressure, and it also impedes grain boundary sliding and increases the resistance to dislocation movement, ultimately increasing the hardness and compression strength. Finally, the Ti3SiC2 agglomeration is especially aggravated when the Ti3SiC2 amount exceeds 20 wt.% (Fig. 2). It leads to the stress concentration and the increase of number of pores, resulting in a notable decrease of relative density, bulk density, and hardness (Figs. 5a-c). However, the Ti3SiC2 agglomeration has little effect on the compression strength with 0-30 wt.% of added Ti3SiC2. The compression strength is mainly affected by the increase in the relative content of Ti3SiC2, TiC, and Ti5Si3 hard phases. Although these factors also affect the trends of friction coefficient and wear rate, they are mediated by the lubricant film formed on the friction surface of the TiAl matrix composites. The composition and integrity of the lubricating film play important roles in influencing the tribological properties of the TiAl matrix composites. The composition of the lubricating film is determined by the friction temperature and the composition of the TiAl matrix composites, resulting in different tribological properties after friction tests at room temperature and 550°C. Ti3SiC2 in the TiAl matrix composites gradually accumulates on the composite’s surface during the friction process due to the squeezing and plastic flow of Ti3SiC2, and then it forms a Ti3SiC2 lubricating film at room temperature or a Fe-Ti-Al-Si-oxide film at 550°C (Figs. 6-8). The direct friction between the Ti3SiC2 lubricating film of the TiAl matrix composites and the counterpart at room temperature is transformed to the friction between the Ti-Al-Si-oxides film of the TiAl matrix composites and the oxide film of the counterpart. In addition, the wear of the TiAl matrix composites at 550°C is mainly concentrated on the Fe-oxide layer of the Fe-Ti-Al-Si-oxide film rather than on the whole lubricating film at room temperature during the friction process (Figs. 6-8). Thus, the Fe-Ti-Al-Si-oxide film yields better tribological properties of the TiAl matrix composites than the Ti3SiC2 lubricating film (Figs. 5-6). The integrity of the lubricating film is affected by the synergy of the Ti3SiC2 content in the TiAl matrix composites and the interfacial bonding between Ti3SiC2 and the TiAl matrix. The more compact the lubricating film, the better tribological properties of the TiAl matrix composites. The increase of the Ti3SiC2 content in the TiAl matrix yields a higher content of the Ti3SiC2 lubricant on the surface of the TiAl matrix composites, resulting in better integrity of the lubricating film (Figs. 1c-d and Figs. 5d-e). However, in excess of Ti3SiC2 (20-30 wt.%) in the TiAl matrix, the Ti3SiC2 agglomeration is aggravated, decreasing the interfacial bonding between Ti3SiC2 and the TiAl matrix (Figs. 1c-d and Fig. 2-4). The decrease in the interfacial bonding strength between Ti3SiC2 and the TiAl matrix leads to higher consumption than the production of the lubricating film on the friction surface of the TiAl matrix composites, decreasing the integrity of the lubricating film. This is why the decreasing trends of the TiAl matrix’s friction coefficient and wear rate moderate when the relative Ti3SiC2 content in the TiAl matrix composites increases from 17.9 to 26.7% (Figs. 1c-d and Figs. 5d-e). Moreover, an unprecedented low wear rate of the TiAl matrix composites at 550°C compared with previous studies (Fig. 5f) is attributed to the Ti3SiC2decomposition. The formation of TiC and Ti5Si3 hard phases effectively pin Ti3SiC2 so that it prevents falling off and improves the integrity of the lubricating film [34].