Introduction

Ti and its alloys are key materials in aerospace, automotive and biomedical fields due to their high specific strength, excellent corrosion resistance and superior creep resistance [1,2,3,4,5]. Besides, those alloys keep abreast of the soaring demand for applications in friction field such as glasses shaft, smart wearable case and marine drill pipe [6]. With the rapid development of industrialization, structural materials are required to possess higher performance and longer service life in severe environmental conditions [7, 8]. However, the poor wear resistance restricts the wide application of Ti alloys [9]. In order to overcome the challenge, one of the effective ways is to fabricate high-strength TMCs by introducing ceramic particles. According to the different ways of introducing ceramic particles, the preparation methods can be divided into “in situ” formation and “ex situ” addition [10, 11]. The in situ synthesis method possesses the advantages of effective dispersion of reinforcements, well-bonding interface and low impurity contamination. And it has been successfully used in manufacturing aluminum, nickel and iron matrix composites [12,13,14]. The in situ synthesized reinforcement particles usually include TiC, TiB, TiN, SiC, Si3N4 and their mixtures [15,16,17,18,19,20]. Compared with the reinforcements mentioned above, TiB is one of the most attractive particles due to its high elastic modulus, excellent thermal stability and similar thermal expansion coefficient with Ti matrix [21, 22]. Furthermore, the in situ synthesized TiB particles can play an important role in improving the tribological properties of TMCs. Singh et al. [23] prepared Ti/TiB composites by Ti–TiB2 reaction and found that the in situ formation of TiB makes the specific wear rate of Ti–20 wt.% TiB2 composite reduce by 67.4% than that of pure Ti. Cai et al. [24] fabricated the Ti–6Al–4 V/TiB2 composite and reported that the introduction of 3 wt.% TiB2 promotes the formation of TiB and contributes to decreasing by 20.5% in the coefficients of friction (COF) when compared with the Ti–6Al–4 V alloy. Furthermore, they [19] also fabricated the Ti–6Al–4 V/TiB2 composite by hot isostatic pressing process and found the COF further reduces by 33.8% when the TiB2 content reaches 8 wt.%, accompanying with the wear mass loss 71.4% less compared with the Ti–6Al–4 V matrix. In a nutshell, incorporating TiB ceramic particle is an effective strategy to enhance the wear resistance of Ti materials.

Nevertheless, the interstitial oxygen (O) contamination is a critical issue for Ti materials, which can deteriorate their ductility and toughness seriously. Yan et al. [25] reported that a small increase of O content in Ti–6Al–4 V alloy from 0.25% to 0.49% facilitates the precipitation of acicular α and α2-type nanoscale clusters with an interface-stabilized structure of α-β-α, leading to a remarkable decrease in ductility and toughness. Therefore, seeking an appropriate method to scavenge O impurity in TMCs is of great significance. One of the ways is to add rare earth elements such as La, Ce, Nd and Y into Ti matrix to form rare earth oxides [26,27,28,29]. Liu et al. [29] introduced Y element into Ti–1.5Fe–2.25Mo alloy matrix to prepare composites and found a significant improvement in ductility due to the consumption of O from the matrix by the formation of Y oxides, which promotes the strengthening effect concurrently. Wang et al. [30] fabricated the Ti–35Nb–2Ta–3Zr/(TiB + La2O3) composites and reported that the elongation increases by 33.3% than that of Ti matrix only with 0.1 wt.% LaB6 addition, which is because of the formation of La2O3 resulting in the reduction of O in Ti matrix. Moreover, Yang et al. [31] added NdB6 into Ti powder and found that the NdB6 can effectively react with surface oxide films to form NdBO3 layer, scavenging most of the O impurities and hindering the dissolution of O into Ti matrix. However, the scarcity and high cost remains a great challenge for rare earth elements [32]. Thus, there is a strong demand for a new low-cost O scavenger to fabricate high-performance TMCs. Calcium (Ca), having a high chemical affinity with O, was often used as deoxidizer. For example, Oh et al. [33] studied Ti powder deoxidation behavior using Ca vapor and found that the thickness of oxide layer reduces from 11 to 3 nm as well as the O content within the powder from 2500 to 920 ppm, indicating that the Ca element plays an effective role in scavenging O element. However, it is hard to introduce Ca element into Ti matrix directly due to its low-melting-point and unstable properties. In contrast, the CaB6 compound appears to be one such promising candidate because of its better stability at room temperature, lower cost and higher chemical affinity with O element than that of rare earth elements. In view of published work, our group have added preliminarily CaB6 into pure Ti powder to prepare high-performance TMCs, and found that the ultimate tensile strength and elongation of the Ti–0.2 wt.% CaB6 composite increases by 13% and 91% than that of pure Ti [32]. Nevertheless, there was no open report on the effect of CaB6 addition on the tribological properties of Ti/CaB6 composites.

In this work, CaB6 was added into Ti powder to achieve high-performance Ti/CaB6 composites by in situ synthesized powder metallurgy method. The microstructure, mechanical and tribological properties of Ti/CaB6 composites were systematically investigated. This work would be useful to extend the application in tribological field of high-performance and cost-effective TMCs.

Experimental procedures

The raw materials used in this work were commercial hydride-dehydride (HDH) Ti powder (99.5% purity, Tian Tai Long Metal Materials Co., Ltd., China), which exhibits an irregular shape with an average size of 35 μm, and coarse angular-shape CaB6 powder with the mean size of ≤ 45 μm (99.9% purity, Shanghai Naiou Nano Technology Co., Ltd., China). The chemical compositions of raw HDH Ti and CaB6 powders are listed in Table 1.

Table 1 Chemical compositions of raw HDH Ti and CaB6 powders (wt.%)
Full size table

Samples with various weight percents of CaB6 in HDH Ti powder were made: Ti–0.1 wt.% CaB6, Ti–0.4 wt.% CaB6, Ti–0.8 wt.% CaB6, Ti–1.0 wt.% CaB6 (hereafter denoted as Ti–0.1CaB6, Ti–0.4CaB6, Ti–0.8CaB6, Ti–1.0CaB6, respectively). The fabrication process contains three steps. Firstly, the designed amounts of coarse CaB6 powder were refined into superfine powder via wet high-energy vibration ball milling (QM-3A) with the n-hexane protective medium for 30 min at 1400 rpm with a ball-to-powder ratio of 5:1. After ball milling, the mixed solution was taken out and heated at 40–60 °C until the hexane was evaporated entirely in a vacuum oven. Secondly, a certain amount of superfine CaB6 and HDH Ti powders were mixed in a tubular mixer for 5 h at 60 rpm. The mixed powders were poured into a cylindrical silicon mold and pressed into green compacts via cold isostatic pressing (CIP) at 250 MPa for 60 s. Finally, these green compacts were sintered in a tube furnace under a high-purity argon atmosphere (99.999% purity) at 1300 °C for 2 h to obtain the sintered samples (Φ30 × 50 mm). The heating rate is 4 °C/min and holding time is 2 h, followed by furnace cooling to room temperature. For comparison, the pure Ti sample was prepared under the same conditions.

Scanning electron microscope (SEM, Zeiss, Germany) equipped with an energy-dispersive X-ray spectrometer (EDS) was used to observe the as-sintered microstructure. The reinforcements were further examined by transmission electron microscope (TEM, Tecnai, USA) operating at 200 kV. Elemental distributions of the sintered samples were determined by a JXA-8530F electron probe microanalyzer (EPMA). The grain size was calculated by line intersect length method using Nano Measure software. The hardness was measured by Vickers hardness tester (430SVD, Shanghai Wobert Measuring Instrument Co., Ltd, China) using a load of 20 kg with holding time of 10 s. Ten points of test were taken for each specimen to obtain the average value. The distance between points was greater than 1 mm. INSTRON 4206 machine was employed to conduct the room-temperature tensile tests, and the strain rate was set at 0.00067 s−1. The testing specimens were cut into M6 tensile bar by electrical discharge machining (EDM) with a diameter of 3 mm and a gauge length of 15 mm. Five samples were carried out for each test.

The tribological properties were tested on a ball-on-plate tester (UMT-II, Bruker, USA) at room temperature. The specimens with a size of 20 mm (length) × 6 mm (width) × 2 mm (thickness) were cut by EDM, then polished with 1500-grit emery paper and carefully cleaned in an ultrasonic bath to reach a surface roughness of 0.5 μm. A ZrO2 ball with a diameter of 10 mm and hardness of 76 HRC was acted as the counterpart material. And the wear tests were carried out for 30 min at a sliding velocity of 0.03 m/s and a load of 3 N. The dynamic friction coefficients of pure Ti and Ti/CaB6 composites were recorded automatically by the machine. The width and depth of the wear track were conducted by a white light interference microscope (Contour GTK Bruker, Germany). The morphologies of worn surfaces and wear debris were observed by EPMA and SEM.

Results

Microstructure analysis

Figure 1 shows the SEM images of pure Ti and Ti/CaB6 composites. It can be seen that the CaB6 addition refines the average gain size from 163 μm for pure Ti (Fig. 1a) to 74 μm for Ti–1.0CaB6 composite (Fig. 1e). It is clear that the in situ synthesized needle TiB particles appear only with 0.1 wt.% CaB6 addition (Fig. 1b) and the amount of TiB particles increases with the CaB6 content increases (Fig. 1b–e). Furthermore, the TiB particles with a length ranging from 4 to 40 μm and width of approximately 4 μm are uniformly distributed at the grain boundaries or inside the grains.

Figure 1
figure 1

SEM images of pure Ti and Ti/CaB6 composites. a Pure Ti; b Ti–0.1CaB6; c Ti–0.4CaB6; d Ti–0.8CaB6; e Ti–1.0CaB6

Full size image

Figure 2 shows the SEM image and EPMA elemental mapping results of Ti–0.8CaB6 composite. It can be seen that the needle TiB particles are dispersed randomly in Ti matrix (Fig. 2a). Figure 2b–e shows that the fine particle in yellow boxes is rich in Ti, O and Ca elements, implying the formation of Ca–Ti–O phase. Similarly, the positions of Ti and B elements are overlapped in the yellow ellipses, determining as TiB phase. These two products come from the in situ reaction between CaB6 and Ti(O) matrix.

Figure 2
figure 2

SEM image and EPMA mapping results of Ti–0.8CaB6 composite. a SEM image; b Ti; c O; d B; e Ca

Full size image

In order to further analyze the reinforcement characteristics, TEM observations were performed. Figure 3 displays the STEM image and EDS mapping results of Ti–1.0CaB6 composite. It can be seen that the spherical reinforcement particle with a diameter of 500 nm is embedded in matrix, which means a good interfacial bonding with matrix (Fig. 3a). The reinforcement particle in Fig. 3a is cubic CaTiO3, as evidenced in Fig. 3b. Furthermore, the EDS mapping results reveal that there exists an enrichment area of elementals Ti, O and Ca, illustrating that O has been scavenged to form CaTiO3 compound (Fig. 3c).

Figure 3
figure 3

STEM image and EDS mapping results of Ti–1.0CaB6 composite. a STEM image; b associated SAED patterns; c EDS mapping results of Ti, O, B, Ca elements

Full size image

Mechanical properties

Figure 4 shows the Vickers hardness of pure Ti and Ti/CaB6 composites. Obviously, the Ti/CaB6 composites with not more than 0.8 wt.% CaB6 exhibit a higher hardness than that of pure Ti. The hardness increases continuously until the CaB6 content reaches 0.8 wt.%. However, a further increase CaB6 addition to 1.0 wt.% results in a considerable decrease. Therefore, the Ti–0.8CaB6 composite possesses the highest hardness of 321 ± 13 HV, which increases by 68.1% than the 191 ± 15 HV of pure Ti.

Figure 4
figure 4

Vickers hardness of pure Ti and Ti/CaB6 composites

Full size image

Figure 5a presents the values variation of ultimate tensile strength, yield strength and elongation for pure Ti and Ti/CaB6 composites. It can be seen that the ductility of pure Ti has been dramatically improved with a small amount of CaB6 addition and meanwhile the strength displays a slight rise. With an increase in CaB6 content, the strength of Ti/CaB6 composites is enhanced gradually. However, the variation tendency of elongation is adverse. Among all the Ti/CaB6 composites, the Ti–0.1CaB6 composite obtains the best ductility with the elongation of 26.1 ± 1.8% and the Ti–0.8CaB6 composite achieves the highest ultimate tensile strength of 740.3 ± 23.8 MPa with the elongation of 10.6 ± 1.5%, which is consistent with the corresponding Vickers hardness. And a further increase in CaB6 content (1.0 wt.%) leads to the apparent deterioration of mechanical properties. And Fig. 5b compares the room-temperature tensile properties of something like the composites here analyzed with those published reports. It can be found that the Ti–0.1CaB6 composite possesses the superior ductility and Ti–0.8CaB6 composite owns the better tensile properties.

Figure 5
figure 5

Room-temperature tensile properties of pure Ti and Ti/CaB6 composites. a Variation curves of tensile properties with different CaB6 contents; b comparison of tensile properties of TMCs. Data taken from Refs. [34,35,36,37,38,39,40,41,42]

Full size image

Friction and wear behavior

According to the results above, we found that the Ti–0.8CaB6 composite possesses the highest strength and Ti–0.1CaB6 composite owns the best ductility. Furthermore, the mechanical properties of Ti–1.0CaB6 composite are deteriorated due to the excessive addition of CaB6. In fact, the friction and wear behavior of materials is directly related to the mechanical properties. Therefore, the pure Ti, Ti–0.1CaB6, Ti–0.8CaB6 and Ti–1.0CaB6 composites were selected to further study their tribological properties.

Figure 6 presents the friction coefficient curves of pure Ti and Ti/CaB6 composites (3 N, 0.03 m/s). For all of the samples, there is an initial running-in period prior to the friction coefficient reaching a quasi-steady state value. Different from other materials, the pure Ti displays large fluctuation range during steady stage. In addition, the friction coefficients of Ti/CaB6 composites are lower than that of pure Ti. The average friction coefficient values reduce continuously from approximately 0.80 in pure Ti to about 0.45 in Ti–0.8CaB6 composite, indicating a pronounced improvement in tribological properties.

Figure 6
figure 6

Friction coefficient-time curves of pure Ti and Ti/CaB6 composites

Full size image

Figures 7 and 8 show the wear depth curves and surface profiles of pure Ti and Ti/CaB6 composites after the tribological test. As compared with pure Ti, the Ti/CaB6 composites have a shallower wear depth (Fig. 7). With the increase of CaB6 content, the wear depth becomes smaller until the CaB6 content reaches 0.8 wt.%. However, a further increase the CaB6 content to 1.0 wt.%, the wear depth increases to 20.0 μm. The variation tendency of wear width is consistent with the wear depth (Fig. 8). Among the pure Ti and Ti/CaB6 composites, the Ti–0.8CaB6 composite obtains the minimal width of surface profile, corresponding to the shallowest wear depth. The blue color region represents the wear depth. The surface profile of Ti–0.8CaB6 composite displaying considerably light blue color reveals smoother than that of pure Ti (Fig. 8b), which could be due to the improvement of hardness. In terms of the results above, the Ti–0.8CaB6 composite has the highest wear resistance.

Figure 7
figure 7

Wear depth curves of pure Ti and Ti/CaB6 composites (3 N, 0.03 m/s)

Full size image
Figure 8
figure 8

Surface profiles of pure Ti and Ti/CaB6 composites (3 N, 0.03 m/s). a Pure Ti; b Ti–0.1CaB6; c Ti–0.8CaB6; d Ti–1.0CaB6

Full size image

Characterization of worn surface

Figure 9 illustrates the worn surface and corresponding wear debris morphologies of pure Ti and Ti/CaB6 composites after the tribological tests. The worn surface of pure Ti is characterized by many craters, wear debris, delamination and grooves with severe plastic deformation (Fig. 9a). And these features manifest the preponderance of adhesive wear mechanism [43]. For the Ti/CaB6 composites, the worn surfaces are dominated by the grooves and debris (Fig. 9c, e and g), indicating that the wear mechanism transforms from adhesive wear to abrasive wear. In addition, the worn surface of Ti–0.8CaB6 composite is relatively smooth and has many shallow grooves, demonstrating a significant improvement in wear resistance. As for wear debris, the pure Ti displays many large irregular wear debris with an average size of 7.9 μm (Fig. 9b). In comparison, with increase in the CaB6 addition, the wear debris size of Ti/CaB6 composite changes into fine particles with the mean size of less 6.0 μm accounting for the existence of TiB reinforcements (Fig. 1), showing that the dominated wear mechanism is abrasive wear mechanism. Furthermore, the Ti–0.8CaB6 composite shows the smallest wear debris compared with the Ti–0.1CaB6 and Ti–1.0CaB6 composites, indicating its best wear resistance.

Figure 9
figure 9

Worn surfaces (a, c, e, g) and corresponding wear debris morphologies (b, d, f, h) of pure Ti and Ti/CaB6 composites after the tribological tests. a, b Pure Ti; c, d Ti–0.1CaB6; e, f Ti–0.8CaB6; g, h Ti–1.0CaB6

Full size image

To explore the element distribution of worn surface, the EPMA mapping results of pure Ti and Ti–0.8CaB6 composite are displayed in Figs. 10 and 11. As shown in Fig. 10a and c, the O element is mainly accumulated on the black region, implying that the oxidation wear occurs on the worn surface. The reason is that the wear process can result in continuous heat accumulation, which will rise the surface temperature of materials. Then, the Ti matrix is ready to react with O2 in air to generate the oxides [44]. By contrast, the slighter black color appears (Fig. 11a) and Fig. 11c shows a lower oxygen content on the worn surface of Ti–0.8CaB6 composite, suggesting that the CaB6 addition can relieve the oxidation wear behavior. Moreover, the needle TiB and nano-size CaTiO3 particles shown in Figs. 2 and 3 are not found in Fig. 11 due to the lower magnification images.

Figure 10
figure 10

EPMA mapping results in the wear surface of pure Ti. a Worn surface; b Ti; c O

Full size image
Figure 11
figure 11

EPMA mapping results in the wear surface of Ti–0.8CaB6 composite. a Worn surface; b Ti; c O; d B; e Ca

Full size image

Discussion

Improvement mechanisms in mechanical properties

The reasons for enhanced strength can be mainly explained by two strengthening mechanisms: (1) grain refinement strengthening and (2) dispersion strengthening. As shown in SEM images (Fig. 1a–e), the CaB6 addition refines the grain size from 163 μm for pure Ti to 74 μm for Ti–1.0CaB6 composite. The grain growth is restricted by the in situ synthesized ceramic phase particles locating at grain boundaries via pinning effect [45]. The refined grain size promotes the increase of strength and hardness according to the Hall–Petch equation [46, 47]. As seen from the SEM images (Fig. 1b–e) and EPMA mapping results (Fig. 2b–e), the in situ synthesized TiB particles are uniformly distributed in matrix accompanying with a few CaTiO3 particles. In addition, the interface of TiB particles and matrix displays no microcracks (Fig. 2a) and the nano-size CaTiO3 particles are embedded in matrix tightly (Fig. 3a), indicating the strong interface bonding between reinforcements and matrix. Therefore, both TiB and CaTiO3 particles play a dispersion strengthening role in Ti matrix.

The improvement in ductility of Ti/CaB6 composites mostly contains the following two factors: (1) The CaB6 addition makes the grain refine (Fig. 1a–e). More grain boundaries due to the grain refinement are beneficial for the coordinate deformation between adjacent grains. (2) The formation of in situ synthesized CaTiO3 oxide particles (Fig. 3) means that the O in Ti powder is consumed, contributing to the decrease of dissolved O content. Thus, the lattice expansions become smaller owing to the less O interstitial solution atoms [48]. And the interaction between stress field caused from the expansions and dislocations becomes weak in blocking the development of deformation by slip. The two factors above simultaneously promote the improvement of ductility for Ti materials.

Wear mechanisms

The reasons for the improvement in wear resistance of Ti/CaB6 composites are mainly due to the formation of in situ synthesized TiB particles. Firstly, the in situ chemical reaction provides a strong interfacial bonding strength between TiB particles and Ti matrix (Fig. 2a), which increases the tolerance in peeling off the reinforcements during mechanical friction, obtaining the wear resistance surface. Secondly, the hardness test (Fig. 4) reveals that the Ti/CaB6 composites have remarkably higher hardness than pure Ti. According to the Archard's wear equation [49], the wear resistance raises with hardness increase. The higher hardness can prevent the initiation and propagation of cracks and thus reducing the size of wear debris (Fig. 9). The wear mechanism is transformed from the adhesive wear to abrasive wear. Thirdly, the TiB particles can relieve the surface oxidation wear of Ti matrix (Figs. 10 and 11). Surface oxidation wearing is a combined result of both mechanical friction and chemical reaction [19, 50]. The generated thin and brittle oxides fall off easily and cannot support protection for Ti matrix. Thus, the surface oxidation wear will continue to occur. Nevertheless, the stable TiB particles existing on the surface of TMCs can prevent the direct contact of grinding ball and Ti matrix, limiting the reaction of Ti matrix with O2 in air. Therefore, the oxidation wear degree is decreased for the Ti/CaB6 composites.

Conclusions

The Ti/CaB6 composites are successfully prepared via powder metallurgy technique. The microstructure, mechanical properties and tribological properties of the Ti/CaB6 composites are investigated. The following conclusions can be drawn:

  1. (1)

    In situ synthesized needle TiB particles and nano-size CaTiO3 oxide particles are formed by the CaB6 addition. The TiB particles with the length ranging from 4 μm to 40 μm and width of approximately 4 μm are distributed uniformly in Ti matrix and have a strong interfacial bonding. The CaB6 addition makes the grain size refine from 163 μm for pure Ti to 74 μm for Ti–1.0CaB6 composite.

  2. (2)

    The Ti/CaB6 composites with not more than 0.8 wt.% CaB6 have superior mechanical properties. The Ti–0.8CaB6 composite obtains the highest hardness of 321 ± 13 HV and good comprehensive mechanical properties with the ultimate tensile strength of 740.3 ± 23.8 MPa, yield strength of 598.4 ± 23.1 MPa and elongation of 10.6 ± 1.5%.

  3. (3)

    The Ti/CaB6 composites possess the excellent wear resistance. The in situ formed TiB particles can provide the wear resistance surfaces, increase the hardness of materials and possess a high oxidation resistance, hence resulting in excellent wear resistance. And, the Ti–0.8CaB6 composite with the best wear resistance reveals a low COF of 0.45, 50% less than pure Ti, under constant conditions (3 N, 0.03 m/s).