INTRODUCTION

The goals of economic efficiency and environmental safety of modern industry require addressing the problems related to the wear of mechanisms, such as increasing their reliability and reducing energy consumption due to friction. The growth of specific loads and speeds and tightening of operating conditions contribute to innovative approaches to the production of modern parts and units, including substitution of materials, alternative constructions, and effective production. Under these conditions, materials engineers continue to develop methods for increasing the durability and reducing the energy consumption of sliding contacts [1, 2].

The production of composite materials with optimal structures that can provide predictive properties increases the reliability and durability of machine parts. As the components of a composite material (CM) affect its characterization, their properties should be carefully studied [3]. The main functions of the matrix are to transfer the forces to the filler and prevent the propagation of fatigue cracks between the components of the CM. This requires that the matrix material be ductile and have high impact toughness and interlaminar shear strength. In addition, a strong interfacial bonding between the filler and the matrix materials is desirable, so the matrix metals must form a mechanical or chemical bonding with the filler. Reinforcing materials usually add stiffness and prevent the propagation of cracks. In particular, they increase the mechanical properties of the matrix and in most cases have higher hardness, strength, and stiffness than the matrix [4].

The increased use of composite materials in the industry depends on the fact that they successfully combine high mechanical characteristics with low weight and required service properties. Composites are produced using many different technologies, sometimes a combination of two or more processes. Production processes are chosen according to the type of matrix or the type of filler used [3]. Among the properties that can be improved by forming a composite material, wear resistance is of importance [5].

Plain bearings used in various industries are made from soft tribological alloys based on tin, lead, and aluminum, mainly through foundry technologies [6]. The data of the failure analysis and unplanned losses of friction units in devices and machines show that the materials produced in this way do not provide the required indicators of fatigue strength and wear resistance [7].

The operational characteristics [8] and high tribological properties [9] of high-tin babbitt B83 have provided its need for the production of sliding bearings for many years. Good compatibility with steel shafts owing to the adaptability to misalignment is a unique property of the babbitt alloy [10, 11]. In addition, recently, this alloy has often been used as a matrix for the production of functional composite materials [12, 13].

The structure of the babbitt alloy consists of a soft matrix of a solid solution of Sn and distributed intermetallic phases (SnSb, CuxSny). The intermetallic phases during the friction process react to the major loads, protecting the surface from wear, and the matrix of Sn acts as an antifriction lubricant. An increase in the content Sb can result in lower wear resistance and mechanical properties and fatigue failure [14]. This is due to the acute-angled geometric shape of the intermetallic phases of the system Sn–Sb, where the vertices and edges are stress concentrators [15, 16].

The use of babbitt alloys has shown good results under mild triboloading conditions, for example, in low-speed diesel engines. However, as speeds and loads increase, babbitt bearings sometimes fail prematurely because their load-bearing capacity is insufficient. In [17], it is shown that the hardness and strength of babbitt Sn–11Sb–6Cu is substantially reduced with the increase in an operating temperature under conditions of high speeds and heavy loads. This was the cause limiting the service life of the bearings. These disadvantages can be eliminated by changing the structure of babbitt alloys through the production process, use of reinforcing fillers of different nature, and a combination of these methods [18].

Wear regimes and wear mechanisms are affected by many friction process variables; their number increases significantly through studies of materials with heterogeneous structure, such as composites. Data on the values of friction coefficients and wear resistance are sometimes insufficient to predict the behavior of materials in a tribounit. Thus, the friction process, including with babbitt-based composites, should be studied and described. The occurring wear mechanisms are as follows: adhesion, plastic deformation, material delamination, strain hardening, and formation of surface and subsurface cracks and associated structural changes [19].

The tin matrix wears out first under friction of babbitt-based materials, where intermetallic phases under loads are pushed into it [20, 21]. After the loss or redistribution of the matrix, the loads begin to react to intermetallic phases, which fill the friction surface during crushing as a result of cleavage stresses, forming a working friction layer [22].

The wear mechanisms during friction should be studied to determine the conditions for a change of friction regimes and the acceptable triboloading range, as well as to understand the mechanisms of material destruction and chemical effects caused by contacts [23].

The object of this paper is to study the wear regimes of babbitt-based composites with different contents of the intermetallic phase Ti2NbAl. The primary attention is given to the processes in the friction zone, which are characterized by changes in the contact surface structure, friction coefficients, wear rate, wear mechanisms, and temperature.

MATERIALS AND METHODS

In order to produce CM, powder of babbitt B83 (<500 nm) was used as a matrix, and powder of intermetallic Ti2NbAl (<100 nm) was used as a reinforcing filler. The Ti2NbAl powder was produced by LLC Metsintez. The hot pressing method described in detail in [24] was used for the preparation of specimens. The powder mixture was prepared in a Retsch PM100 planetary mill. Semifinished products from the powder mixture were produced by hydraulic forging (Pmax = 150 KN) at a pressure of 320 ± 5 MPa. Sintering of semifinished specimens was performed in a muffle furnace until the liquid phase appeared at 260 ± 10°C and held for 30 min at this temperature. Hot pressing (HP) was conducted using a force of 320 ± 5 MPa at an initial temperature of 260 ± 10°C, and then the specimen was cooled in air. The properties of the industrial alloy B83 and specimens produced by pressing powder of babbitt B83 without fillers and with fillers were studied for comparison (Table 1).

Table 1.   Compositions of the test specimens
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Tribological tests of the specimens were performed under dry sliding friction conditions using a CETR UMT Multi-Specimen Test System according to an axial loading scheme of a fixed sleeve and a counterbody (steel hardness >63 HRC) to a rotating disk specimen. Disks were cut from the produced materials and specimens. The diameter of the disk was 20 mm, and its height was 5 mm. The internal and external diameters of the steel sleeve were 12 and 16 mm. The software provided the opportunity of recording the friction coefficient values during tests. The test layout did not allow the thermocouple to be installed directly in the friction zone, so the temperature was measured near the contact surfaces. The temperature values served as an indirect parameter for assessing friction processes.

The tribological characteristics of the specimens were assessed in a wide range of specific loads: 0.5, 1, 1.5, 2, and 2.5 MPa. The sliding speed was 0.5 m/s, and the test duration at each load was 10 min. The range of specific loads varied from 0.5 to 2.5 MPa; the specimens lost their load-bearing capacity at large values of specific loads according to this test diagram. The mass loss Dm of the specimens after testing was recorded on an analytical balance. The wear rate of specimens during dry sliding friction was assessed by the wear rate as the ratio of mass loss to the distance covered.

The morphology of the reinforcing powder, specimen fractures, friction surfaces, and wear products were studied using a LEO 1420 VP scanning electron microscope (Carl Zeiss) fitted with an INCA X-act energy dispersive X-ray microanalysis system (Oxford Instruments). Routine structural studies were performed using a Leica DM ILM optical microscope.

RESULTS AND DISCUSSION

Figure 1 shows an electron micrograph of intermetallic Ti2NbAl powder. It is obvious that the powder particles have a complex geometric shape without sharp vertices and edges. It was suggested that this geometry of the particles contributes to better adhesion of the reinforcing powder to the matrix, and their rounded shape helps avoid stress concentration and destruction of the CM owing to the formation of cracks. This morphology of the reinforcing filler as a whole was expected to improve wear resistance by reducing the loss of the soft matrix owing to an increase in the load-bearing capacity of the produced materials.

Fig. 1.
figure 1

Morphology of Ti2NbAl intermetallic powder.

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The qualitative distribution of the reinforcing phase is achieved using the hot pressing method in the production of CM (Fig. 2). It can be observed that the intermetallic phase retains a rounded geometric shape and is evenly distributed on the surface.

Fig. 2.
figure 2

Distribution and shape of the intermetallic particles after hot pressing.

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The electronic image of the CM fracture (Fig. 3) clearly shows the shape of the intermetallic particle. The results of EDS analysis (Table 2) demonstrated that the composition of the produced CM specimens corresponded to the composition of the initial composite mixture.

Fig. 3.
figure 3

Ti2NbAl particles in the babbitt matrix.

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Table 2.   Data of EDS analysis
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The results of friction tests are given in Fig. 4. It shows the changes in the friction coefficient during testing of specimens 1–5 (Table 1) in the areas with different loads. The nature of the curves helps clearly identify the difference in the processes in the friction zone. The areas of running in, stable friction, and moments of change in regimes and wear mechanisms can be identified.

Fig. 4.
figure 4

Friction coefficients of specimens 1 (a), 2 (b), 3 (c), 4 (d), and 5 (e) (Table 1).

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In the analysis of friction processes, it is important to consider not only the values but also the behavior of the friction coefficient. A stability factor, corresponding to the standard deviation of the friction coefficient values measured in the areas for each specific load, should be introduced in order to assess the friction process. The lower the values of the factor, the more stably the friction process occurs. An increase in the stability factor may be indicative of a change in wear mechanisms in the friction zone. Table 3 shows the results of changes in the friction coefficient and stability factor depending on the load.

Table 3.   Friction coefficients and stability factors as a function of the load
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All specimens are characterized by a reduction in the friction coefficient with increasing specific loads. The maximum friction coefficient at the initial stages is present in specimens without the Ti2NbAl phase. Peaks and spikes in the curves show the phenomena of seizure and sliding. Their amplitude and frequency indicate the stability of the friction process.

Thus, an area of running in at a load of 0.5 MPa, which is characterized by an unstable friction process, can be chosen for the initial specimen of babbitt B83AC (Fig. 4a). When the loads increase to 1.0 and 1.5 MPa, the value of the friction coefficient is substantially reduced and a soft wear regime is evident when the stable process occurs. A further increase in loads to 2.0 and 2.5 MPa results in a slight reduction in the average values of the friction coefficient, but as the stability factor indicates, a severe wear regime prevails in the friction process, which shows a change in wear mechanisms.

The duration of the running-in process of the B83HP babbitt specimen produced by hot pressing at a load of 0.5 MPa is distinctly shorter, but the process is less stable (Fig. 4b). When the loads increase to 1.0 and 1.5 MPa, the values of the friction coefficient and stability factor are much lower than those of the initial babbitt specimen. A further increase in loads to 2 and 2.5 MPa results in an even greater reduction in the friction coefficient of the B83HP babbitt specimen and stabilization of the friction process. This may be indicative of a positive effect of the method of production of the specimen on the tribological properties of the material owing to the compaction of the structure and crushing of the SnSb intermetallic phases. Smaller particles are less subject to cleavage and they are better distributed over the surface.

The introduction of reinforcing additives of the intermetallic alloy Ti2NbAl can substantially reduce the friction coefficient in all loading areas compared to unreinforced specimens. In addition, the difference between the values of the friction coefficient at different specific loads is reduced. Changes in the content of the reinforcing phase affects the stability factor of the friction process. The addition of 1 wt % Ti2NbAl (Fig. 4c) provides the most stable friction process. The introduction of 3 wt % Ti2NbAl (Fig. 4d) allows achieving the lowest friction coefficient (Table 3). When 5 wt % Ti2NbAl is added at a specific load of 2.0 MPa (Fig. 4e), there are large fluctuations, which may be associated with chipping of intermetallic particles owing to the accumulation of defects at the filler–matrix boundary.

The data of the wear rate of the specimens are given in Table 4. From comparing the wear rates of specimens without intermetallic additives, one can conclude that the hot pressing method has a positive effect on wear resistance. Unlike molding, the use of the hot pressing method contributes to the fact that the SnSb phase takes a more rounded shape and decreases in size. This reduces the number of internal defects in the material.

Table 4.   Wear rate of specimens
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The introduction of the reinforcing Ti2NbAl phase into the matrix results in a significant improvement in the wear resistance parameter. The addition of 1 wt % Ti2NbAl phase reduces the wear rate by almost two times compared to the initial alloy. An increase in the content of the reinforcing phase to 3 wt % leads to a further reduction of wear. A further increase of the reinforcing intermetallic phase to 5 wt % reduces the wear resistance slightly. This is possibly due to an increase in the number of defects at the filler–matrix boundary. Summarizing the results of the tests, we note that the soft matrix wears out first during the friction of babbitts, since intermetallic compounds are pushed into it under loads [20, 21]. With the gradual loss of the matrix, the loads begin to react to harder intermetallic compounds, which are crushed as a result of cleavage stresses and fill the friction surface, forming a working friction layer [22]. The introduction of the Ti2NbAl phase can be assumed to delay the moment when wear regimes shift into the zone of high loads, which affects the values of wear rate.

One of the factors causing change of wear regimes can be temperature variations of the friction zone. Figure 5 shows the temperature curves obtained during testing in the area of specific loads of 1.5–2.5 MPa. Continuous variations in the temperature (curve 1) indicate the instability of occurring processes and the phenomenon of formation/destruction of a friction body, which corresponds to a severe wear regime. On the contrary, the absence of jumps and changes in direction (curve 3) indicates the stability of the occurring processes. These temperature variations correlate well with the stability factor of the friction process (Table 3).

Fig. 5.
figure 5

Variations in the temperature during friction of the specimens: (1) B83AC; (2) B83HP; (3) B83HP + 1 wt % Ti2NbAl; (4) B83HP + 3 wt % Ti2NbAl; (5) B83HP + 5 wt % Ti2NbAl.

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The friction surface analysis of test specimens and wear products provides an assessment of the conditions in which friction occurs and helps determine wear regimes and the prevailing wear mechanisms.

Figure 6 shows the micrographs of the surface of the specimens after testing. The rough lines and marks of plastic deformation and particles of wear products of various morphologies can be seen clearly in the surfaces of unreinforced specimens 1 and 2 (Figs. 6a and 6b), which indicates a severe wear regime [25]. At the same time, the surface of specimen 2 is less rough with fewer wear products, which indicates the positive effect of the hot pressing method. In addition, cracks were found in the surface of specimen 1 (Fig. 7), which indicates fatigue wear mechanisms [26].

Fig. 6.
figure 6

Friction surfaces of the specimens: (a) B83AC; (b) B83HP; (c) B83HP + 1 wt % Ti2NbAl; (d) B83HP + 3 wt % Ti2NbAl; (e) B83HP + 5 wt % Ti2NbAl.

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Fig. 7.
figure 7

Crack on the surface of the B83AC specimen.

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Fig. 8.
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Wear products of the smples: (a) B83HP + 3 wt % Ti2NbAl, (b) B83DS.

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The surfaces of specimens with different amounts of reinforcing additives (Figs. 6c–6e) are smoother with single tracks of adhesive wear, which indicates a mild wear regime. The number and size of such surface damage characterizes the stability of the friction process. The condition of these surfaces shows the positive effect of the reinforcement with Ti2NbAl intermetallic particles.

The type, size, and shape of wear products serve as additional parameters for determining wear regimes and mechanisms [27, 28]. Figure 8 shows the wear products of specimens B83HP + 3 wt % Ti2NbAl (a) and B83AC (b) after the entire triboloading cycle. There is a significant difference in their morphology depending on changes in wear regimes and mechanisms. Wear products in Fig. 8a represent fragments common to abrasive and adhesive wear, which corresponds to a soft wear regime. The size of wear products is related to an increase in wear rate when specific loads increase.

In addition to the fragments, large particles of material formed as a result of delamination processes and ribbon-shaped particles, the occurrence of which is associated with the formation of fatigue striations in the friction surface, are given in Fig. 8b [29]. This type of wear products is caused by the plasticity of the working layer of unreinforced babbitt owing to the frictional heating and change of the wear regime to hard. The reinforcement with Ti2NbAl intermetallic particles helps avoid deformation of surface layers and maintain a mild wear regime with increasing specific loads.

CONCLUSIONS

The effect of the method of producing hot-extruded specimens of B83 babbitt and Ti2NbAl intermetallic additives on the friction processes of this material were studied. The use of hot pressing showed a positive effect on the stability of the friction process. The reinforcement with intermetallic phases reduced the friction coefficient and wear rate of the specimens. The changes in wear regimes and mechanisms were judged by differences in the behavior of the friction coefficient and differences in friction surfaces and wear products. The positive effect of the method of production and reinforcement on the wear mechanisms occurring in the friction zone was shown; the occurrence of a severe wear regime was prevented. Selection of the amount of the reinforcing phase requires a detailed approach to designing the optimal characteristics of the produced antifriction composite material.

Studies have demonstrated that only the use of hot pressing reduces the wear rate by ≈2.5–3% and the friction coefficient by 17.5% relative to the as-received material. The friction process can be stabilized at specific loads of 1.5 MPa and above. Further introduction of the intermetallic phase Ti2NbAl has a positive effect on the wear resistance of the material. The wear rate of the specimen of B83HP + 3 wt % Ti2NbAl is reduced by ≈60%, and its friction coefficient is reduced by ≈38% relative to the as-received material. When adding 5 wt % Ti2NbAl, the stability factor of the friction process improved by ~43% relative to the as–received material at specific loads of 1.5 MPa and above.

These data can help determine and recommend the regimes for increasing the service life of tribounits based on B83 alloy as volumetric liners and plain bearings (or sliding bearings), as well as produce new functionally structured layer compositions having enhanced tribological properties, which are based on structural steels and surface coatings using not only B83 babbitt alloy but also its composite materials.