3.1. XRD Analysis and Micro-Structure Characterization
Figure 1 shows the XRD patterns of different samples of the alloy. The main components of 10Si-7B, 12Si-8.5B, and 14Si-9.8B alloys were α-Mo, Mo
3Si, and Mo
5SiB
2 phases, as well as a small amount of residual mesophase compound (Mo
5Si
3). The main components of 25Si-8.5B alloy were Mo
3Si and Mo
5SiB
2 phases, and the peak of α-Mo phase was obviously weakened. This finding indicates that the phase composition of the alloys was not affected by doping with 0.3 wt % La
2O
3. Due to the small amount of the doped La
2O
3, the corresponding peak value could not be detected by XRD.
In the process of mechanical alloying, a part of the large impact kinetic energy was converted into the surface energy of the mixed powders. The surface energy of the mixed powder increased after MA. Since MA was conducted in an argon protective environment, Mo, Si, and B powders were not oxidized during MA. Therefore, there was no corresponding characteristic peak for MoO3, SiO2, and B2O3.
Representative micrographs of the alloys doped with 0.3 wt % La
2O
3 are provided in
Figure 2. The bright phase is the α-Mo matrix, which has the highest and the lowest volume fractions in 10Si-7B and 25Si-8.5B alloys, respectively. The other two phases are Mo
3Si and Mo
5SiB
2. They are dispersed within the continuous α-Mo matrix in 10Si-7B and 25Si-8.5B alloys. From
Figure 2, the α-Mo phase became discontinuous and the content decreased gradually with increasing silicon and boron contents.
The micro-structure of 14Si-9.8B alloy is characterized by continuous distribution of the inter-metallic compounds and the intermittent distribution of α-Mo phase. The 25Si-8.5B alloy is comprised almost completely of total inter-metallic compound-phase, and the content of α-Mo phase decreased to the least, which was highly consistent with the results of XRD analysis.
3.2. Density and Vickers Hardness Analyses
Figure 3 shows the Vickers hardness and density of 10Si-7B, 12Si-8.5B, 14Si-9.8B, and 25Si-8.5B alloy specimens after sintering. The density of Mo-Si-B alloys decreased with the increase in Si and B contents due to the lower density of Si and B than that of Mo, which aligns with the changes in the theoretical density. Furthermore, variations in the density of samples prepared by L-L doping with La
2O
3 were similar to those of the Mo-Si-B alloys prepared by solid-solid and solid-liquid doping with La
2O
3 [
15,
16,
17]. It has been shown that the doping mode of La
2O
3 in the alloy has little effect on the density of the material.
The Vickers hardness of the prepared alloy specimens showed that the hardness of all four alloys was increased gradually with enhancing the Si and B contents. The volume fraction of the inter-metallic compounds, such as Mo5SiB2, Mo5Si3, and Mo3Si, increased with the increase in Si and B contents. These inter-metallic compounds are formed by in-situ reaction at a high temperature. A certain amount of Si and B enters the lattice of Mo to form an α-Mo based solid solution, which causes some lattice distortion. The increased lattice distortion improves the ability of the material to resist the Vickers diamond head.
3.3. Effect of Load on Dry Friction and Wear Properties of Mo-Si-B Alloy
Figure 4 shows the friction coefficients and wear rates of plate specimens of Mo-Si-B alloy sliding against the Si
3N
4 ball specimens at 1.96, 5.88, and 10.78 N. As
Figure 4a shows, the friction coefficients of the four different alloys decreased with the increase in the test load because the contact surface of the alloy increased in the elastic-plastic state during sliding friction. Thus, there was a non-linear relationship between the actual contact area and loading of the two pairs of the sliding materials. The progressive interaction between the two friction surfaces was promoted by increasing the loading, that is, the surface asperities were polished and the surface roughness decreased. This led to a decrease in the friction coefficient of the alloy with increasing loading. When the load was 1.96 N, the friction coefficients of 10Si-7B, 12Si-8.5B, 14Si-9.8B, and 25Si-8.5B were 2.12, 2.01, 1.98, and 1.25, respectively. The friction coefficients of 10Si-7B, 12Si-8.5B, 14Si-9.8B, and 25Si-8.5B were 1.94, 1.97, 1.81, 1.07 at 5.88 N and 1.58, 1.43, 1.20, 0.75 at 10.78 N, respectively.
Figure 4b shows that the wear rates of the four different alloys increased with the test load. The load affected the actual contact area of the friction pairs and the deformation degree of the alloy under the pressure during sliding friction. Therefore, the real contact area of the friction pairs increased, the deformation degree of the alloy surface increased, and a greater friction stress on the surface of alloy was generated. As a result, more La
2O
3 particles detached from the alloy, which increased the three-body abrasion. Thus, further particles of La
2O
3 aggravated the wear.
Figure 4c shows the wear rates of the paired Si
3N
4 ball specimens. The wear rates of Si
3N
4 ball specimens increased with the increase in Si and B contents. The increased wear rates are thought to be due to the increased hardness of the Mo-Si-B alloy.
3.4. Effect of Temperature on Dry Friction and Wear Properties of Mo-Si-B Alloy
Figure 5a shows the friction coefficients of Mo-Si-B alloy plate specimens sliding against the Si
3N
4 ball specimens in the temperature range of 298 to 1173 K and under the experimental load of 5.88 N. With regard to 10Si-7B alloy and 12Si-8.5B alloy doped with 0.3 wt % La
2O
3, the coefficients of friction decreased gradually in the temperature range of 298 to 1173 K. The friction coefficients of the 10Si-7B alloy plate specimens were as low as 0.45–0.98 in the temperature range of 973 to 1173 K. However, the steady friction coefficients of the 10Si-7B alloy plate specimens were as high as 1.98 at 298 K. The friction coefficients of 12Si-8.5B alloy were 0.52–0.85 and 1.97 at the test temperature of 973–1173 K and at 298 K, respectively. However, for the alloys of 14Si-9.8B and 25Si-8.5B doped with 0.3 wt % of La
2O
3, the friction coefficients decreased with temperature in the range of 298 to 1073 K and increased first at 1173 K. The friction coefficients of 14Si-9.8B alloy were 1.01–1.26 and 1.81 at the test temperature from 973 to 1173 K and at 298 K, respectively. For 14Si-9.8B alloy, the lowest friction coefficient of 1.01 was attained at the test temperature of 1073 K. For 25Si-8.5B alloy, the friction coefficients of 0.88–1.05 and 1.07 were determined at the test temperature from 973 to 1173 K and at 298 K, respectively.
Figure 5b,c shows the wear rates for the plate specimens of Mo-Si-B alloy sliding against the Si
3N
4 ball specimens in the temperature range of 298 to 1173 K. The volume wear rates of Mo-Si-B alloy doped with 0.3 wt % La
2O
3 decreased. The results showed that the oxide film produced at a high temperature played a significant role in the lubrication and, therefore, the wear rate was effectively reduced. The Si
3N
4 ball specimens sliding against the Mo-Si-B alloy plate specimens exhibited the smallest specific wear rates at 1173 K. The smallest specific wear rates at 1173 K are thought to be due to the formation of some low friction films, as described in
Figure 5.
Figure 6 shows the friction coefficients of Mo-Si-B alloy doped with 0.3 wt % La
2O
3 plate specimens sliding against Si
3N
4 ball specimens in the temperature range of 973 to 1173 K. At 973 K, the friction coefficients of Mo-Si-B alloys were relatively high at the beginning of friction test and, then decreased to be stable with prolonging the friction process. When the temperature was 1073 K, the initial friction coefficients of the alloys changed considerably; and simultaneously, the average friction coefficients were lower than those obtained at 973 K. At 1173 K, the lowest average friction coefficients were obtained for 10Si-7B and 12Si-8.5B alloys. However, for 14Si-9.8B and 25Si-8.5B, the average friction coefficients were higher than those attained at 1073 K. The real-time friction coefficient of 25Si-8.5B alloy fluctuated the most.
The high temperature friction and wear behavior of Mo-Si-B alloy should be related to the surface oxidation and contact deformation of the alloy at a high temperature.
Figure 7 shows SEM images of 10Si-7B alloys obtained after friction test at 973 K (
Figure 7a), 1073 K (
Figure 7b), and 1173 K (
Figure 7c,d).
Figure 7e,f depicts the responding EDS analyses of P1 and P2 in
Figure 7d, respectively. According to the corresponding EDS results, the brightest appearing phase particles (labeled as P1) were mainly Mo, Si, O, and La, which indicates that the second phase particles exist in the form of oxide. The existence of MoO
3, SiO
2, and La
2O
3 can be detected by calculating the content ratio of Mo, Si, and O atoms. The slightly darker gray phase (labeled as P2) are assigned as Mo3Si and oxide.
The friction coefficient depends on the surface contact between the Mo-Si-B alloy and the Si3N4 ceramic ball. The low friction coefficients were thought to be due to the formation of low friction MoO3 films. When the alloy was rubbed at 973 K, molybdenum oxide (MoO3) gradually formed on the surface of the alloy. Although MoO3 is a volatile compound, it was constantly pressed into the surface of the alloy by the ceramic ball under the pressure and volatilization of MoO3 was postponed. MoO3 acted as a lubricating film. Therefore, a lower friction coefficient for the alloy was observed at 973 K compared to 298 K.
When the test temperature was 1173 K, the low melting MoO3 (1068 K) and B2O3 (723 K) began to melt partially to form a liquid phase and the contact state between the friction pairs changed again. For 10Si-7B and 12Si-8.5B alloys, the friction coefficients were further reduced with the increase in friction temperature. Since the viscosity of a liquid phase with a high content of SiO2 in 14Si-9.8B and 25Si-8.5B alloys is higher than the low content of SiO2 in 10Si-7B and 12Si-8.5B alloys, the liquid phase not only played the role of liquid lubrication but also caused the adhesion of friction pairs and led to an increased friction coefficient. In addition, the high viscosity of the liquid phase could not cover the friction surface evenly and thus resulted in the fluctuation in the friction coefficient of the 25Si-8.5B alloy.
Figure 8 shows the laser confocal microscope images of the worn surfaces on the Mo-Si-B alloy plate specimens after sliding at 298 K. With increasing Si and B contents, the volume fraction of the inter-metallic compounds increased and the content of α-Mo phase with low hardness and good plasticity gradually decreased. The ploughing wear of the alloy surface was reduced by the hard particles as characterized by the decrease in wear volume, the number, and depth of the furrows on the surface.
According to the laser confocal images of the wear tracks in
Figure 8i–l, obvious furrows were observed on the surfaces of Mo-Si alloys, as highlighted by arrows in
Figure 8. The depth and width of the furrow on the 10Si-7B alloy were large and the furrows were continuous in the parallel direction and no discontinuity was observed in the middle. With enhancing Si and B contents, the width and depth of the furrow clearly decreased and the number of convex peaks (hard inter-metallic compounds) in the grinding mark increased. However, the wear tracks became shallow and narrow and the shape was no longer continuous and parallel but curved or even interrupted, as highlighted by the rectangles in
Figure 8. These phenomena indicate that when the hard abrasive particles encountered more brittle inter-metallic compounds, the ploughing behavior was hindered, the ploughing effect was interrupted, and the wear resistance of the alloy was increased.
Related research has shown that the hardness is one of the main parameters used to characterize the wear resistance of a material, and in the present study, the wear resistance of Mo-Si-B alloy increased with the increase in the hardness of the material [
18,
19]. This is consistent with the findings of Archard who found that the wear resistance and hardness of materials are linearly related [
20].