3.2. High-Temperature Friction and Wear Properties of NiCr/hBN Composites
The change in friction coefficient of the NiCr/hBN composites containing 10% hBN with friction time at different temperatures is shown in
Figure 3. At 25 and 600 °C, the material friction coefficient was high in the initial friction phase and declined to a steady level after about 20 min. The stable friction coefficient at room temperature w lower than that at 600 °C. At 700 and 800 °C, the material friction coefficient was low in the initial friction phase, taking about 10 min to reach the stable friction phase in the test; after the stable phase, the friction coefficient of the composite increased and became less stable than that at 25 and 600 °C. The stable friction coefficients of NiCr/10% hBN composites were about 0.28, 0.35, 0.56, and 0.63 at 25, 600, 700, and 800 °C, respectively.
The schematic of the formation of NiCr/hBN self-lubricating composite films during sliding friction is shown in
Figure 4. The NiCr/hBN composite is regarded as a whole, whereas the solid lubricant hBN is uniformly distributed throughout the material. Before the friction phase starts, the contact surface of friction pairs was the contact surface between the metal matrix and mating material, as shown in
Figure 4a. At the same time, it was in a nonsolid lubrication state, so friction coefficients in the initial friction phase at all temperatures were large (
Figure 3). When the initial friction phase was started, the surface of the composite experienced deformation due to extrusion, and a large quantity of friction heat was generated. The hBN solid lubricant that was uniformly distributed throughout the material gradually flowed toward the friction surface after extrusion, and the friction surface transformed into a mixed surface of solid lubricant and metal matrix, as shown in
Figure 4b. Thus, the friction coefficient between friction pairs gradually declined. As the extrusion-induced deformation continued, the hBN content on the friction surface continuously increased. Given the small shear strength of hBN, it was sheared and spread out on the external surface under the effect of frictional force, and a lubricating film with complete coverage formed, as shown in
Figure 4c. This time, the friction coefficient between friction pairs reached a stable state.
According to the formation mechanism of the solid lubricating film, Blau et al. [
17] proposed that when the solid lubricating film consists of metal matrix and solid lubricating film, the friction coefficient between friction pairs can be expressed as follows:
where
μm is the friction coefficient between the metal matrix and mating material, and
μe is the friction coefficient between the solid lubricating film and mating material. When
n = 1, the friction surface reaches a complete solid-lubrication state and
μ =
μe; thus, the friction coefficient is equal to that of pure hBN (about 0.25) [
10]. When
n = 0, the friction surface is in a nonsolid lubrication state and
μ =
μm; thus, the friction coefficient is equal to the friction coefficient of pure NiCr matrix (about 0.46) [
18]. Accordingly, if the Ni-Cr/hBN composite does not undergo oxidation, then the lubricating film consists of solid lubricant and metal matrix and its friction coefficient is between 0.25 and 0.46 (
Figure 3) at room temperature and 600 °C. However, with increased friction temperature to 700 and 800 °C, the friction coefficient is much higher than that of NiCr matrix, indicating the presence of components with a high friction coefficient in the lubricating film.
To investigate the friction and wear behavior of NiCr/hBN composite at high temperature, the friction coefficients and wear rates of the materials with different hBN contents at various friction temperatures were tested in this study.
Figure 5 shows the change curves of the friction coefficient of NiCr/hBN composites with different hBN contents at different friction temperatures. When the friction temperature was room temperature and 600 °C, the friction coefficient initially decreased and then increased with increased hBN content and reached the minimum value at 10% hBN content. When the friction temperature was 700 and 800 °C, the friction coefficient increased with increased hBN content. When the hBN content was constant, the friction coefficients of the NiCr/hBN composites increased with increased friction temperature.
Figure 6 shows the change curves of NiCr/hBN composite with different hBN contents at different friction temperatures. At room temperature, the wear rate of NiCr/hBN composite initially decreased and then increased with increased hBN content, reaching the minimum value when the hBN content was 10%. At 600 °C, the wear rate of the NiCr/hBN composite initially increased gradually and then increased abruptly when the hBN content exceeded 10%, which indicates that the wear mechanism of the material changes when the hBN content exceeds 10%. At 700 °C, the wear rate of the NiCr/hBN composite linearly increased, but at 800 °C, the wear rate of the material abruptly increased with increased hBN content from 8% to 9%. When hBN content exceeded 9%, the wear rate increased slowly and remained unchanged.
The wear-surface morphologies of NiCr/hBN self-lubricating composite under different frictional conditions are shown in
Figure 7. At room temperature, the lubricating-film coverage on the wear surface of NiCr/hBN composite containing 8% hBN was incomplete, and the boundary of NiCr particles was vague, as shown in
Figure 7a; the friction coefficient was the maximum. When the hBN content was 10%, the lubricating-film coverage was complete and the friction coefficient reached the minimum value. With increased temperature to 600 °C, the lubricating film basically covered the entire wear surface of NiCr/hBN composite containing 10% hBN, as shown in
Figure 7c, so the friction coefficient remained low. However, the wear surface loosened with peeling pits forming in local parts, and then the wear rate slightly increased. When the hBN content was increased to 12%, the lubricating film fell off and its coverage on the specimen surface was incomplete, as shown in
Figure 7d, so both friction coefficient and wear rate obviously increased. At 800 °C, obvious furrows appeared on the wear surface of NiCr/hBN material containing 10% hBN with peeling pits forming in local parts, as shown in
Figure 7e. A large area on the wear surface of 12% hBN material fell off and almost no lubricating film existed, as shown in
Figure 7f, so its wear rate was large at 800 °C.
3.3. Analysis of the High-Temperature Friction and Wear Mechanism of NiCr/hBN Composites
According to the formation mechanism of the lubricating film, it forms on the friction surface of NiCr/hBN composite accompanied by processes such as material deformation and hBN flow, so the formation of the lubricating film is related to factors such as the strength of the composite material, magnitude of load, and hBN content. In other words, decreased material strength, increased magnitude of load, or increased hBN content can all promote the formation of the lubricating film and reduce the friction coefficient, thereby improving the lubrication properties. However, according to
Figure 5, the friction coefficient of NiCr/hBN composite does not gradually decrease with increased hBN content and decreased material strength because the favorable tribological properties of the material depends on the formation capacity of the lubricating film and on the wear rate of the lubricating film.
The wear rate of the lubricating film is influenced by the strength of the composite and the repair ability and wear resistance of the solid lubricating film. First, if the strength of the composite is too low, then the shear force during the friction process goes deeply into the material such that the matrix is torn and peeled off. Thus, the lubricating film difficultly retains its integrity because it loses the support from the matrix; consequently, the lubrication properties of the composite decline. Second, after the solid lubricating film is peeled off, the solid lubricant in the matrix continues to be extruded outside. At this time, the solid lubricant must guarantee sufficient content and favorable mobility to form a new lubricating film and fill the void caused by wear and, thus, guarantee good lubrication properties. Furthermore, the matching between solid lubricant and matrix, the matrix-surface conditions, and the composition and thickness of the solid lubricating film directly affect the wear resistance of the lubricating film.
At room temperature and 600 °C, the strength of the NiCr/hBN composite increases with increased hBN content from 8% to 10%. The main wear mechanism of the composite is abrasive wear, and its abrasive dusts mainly present a flocculent shape with fine and uniform particle sizes, or a small quantity of massive abrasive dusts appear as shown in
Figure 8a,b. To identify the physical phases of abrasive dusts, XRD analysis was carried out, and results are shown in
Figure 9. As the physical phase at 25 °C is basically identical with that at 600 °C, the analytic curve of abrasive dusts at room temperature is neglected in the figure. As shown in
Figure 9, abrasive dusts are mainly hBN and Ni (Cr) alloy at 600 °C. Combined with the wear-surface morphologies shown in
Figure 7a–c, plastic deformation on the material surface is found to be slight at room temperature and 600 °C. Bulges on the material surface fall off and are separated under the effect of shear force, and abrasive particles are formed. This wear form usually occurs at the surface layer of the solid lubricating film and can be repaired through the material self-consumption effect with flat wear surface, so the wear process is relatively stable. When the hBN content reaches 12%, the sintering degree of the composite is lowered, and the strength abruptly declines. As the supporting effect of the matrix declines, the wear resistance of the lubricating film decreases, and adhesive wear occurs locally, as shown in
Figure 7d.
At 700 and 800 °C, the strength of the NiCr alloy matrix significantly declines at high temperature, leading to phenomena such as increased plastic deformation of the material, enlarged frictional contact area, and crushing of the lubricating film. Thus, the tribological properties of NiCr/hBN composite decline, as shown in
Figure 5 and
Figure 6. At 700 and 800 °C, increased hBN content gradually increases the friction coefficient and wear rate of NiCr/hBN composite. At 700 °C, abrasive dusts become granular and their particle sizes enlarge. At 800 °C, these abrasive dusts are mainly large particles, as shown in
Figure 8c,d.
Figure 9 shows that at 700 and 800 °C, three spinel-type oxides (Cr2O3, CrBO3, and NiCr2O4) appear as abrasive dusts in addition to hBN and Ni (Cr). This finding indicates that oxidation occurs on the wear surface and that oxides with high hardness are generated, so the sliding friction on the friction interface is aggravated. Hence, the friction coefficient of NiCr/hBN composite is larger than that of the matrix metal at 700 and 800 °C, as shown in
Figure 3 and
Figure 5. According to the analysis of wear-surface morphologies shown in
Figure 7e,f, after oxides appear on the material surface at high temperature, the main wear mechanism of the composite transforms into scratching and gluing forms, which are the main wear forms of adhesive wear. During the wear process, shear failure occurs at the lubricating film layer with soft material texture and further develop toward the deep hard-metal layer accompanied by a large quantity of furrows, as shown in
Figure 7e. This furrow effect continues to degrade the friction and wear properties of the composites. When plastic deformation at the contact peak point is large and the surface temperature is high, the stress of the adhesive bonding point enlarges. If the stress exceeds the shear strength of the composite itself, shear failure occurs at the deep metal part and gluing phenomenon occurs, as concretely manifested by the overall peeling off and large-scale peeling pits shown in
Figure 7f.