1 Introduction

Polycrystalline Cubic Boron Nitride (PcBN) composite materials have high strength, high wear resistance and chemical stability (Ref 1, 2). With the cutting technology entering a new stage of high hardness, high speed, high precision and dry cutting (Ref 3), PcBN composite materials can effectively solve the cutting problems of traditional tools (Ref 4), and occupy an important position in metal manufacturing, energy development and exploration, national defence technology progress and other fields (Ref 5). Cubic boron nitride (cBN) is a kind of superhard material whose hardness is second only to that of diamond (Ref 6, 7). Compared to diamond, cBN has higher thermal stability and lower reactivity with iron (Ref 8,9,10). Therefore, PcBN composites are widely used for cutting and polishing of ferrous metals (Ref 11, 12). The cBN phase has superior mechanical properties. However, it is highly susceptible to tool diffusion and oxidative wear mechanisms. Carbides and nitrides are commonly used as binders for PcBN (e.g. TaC, VC and WC). They have high hardness compared to metallic binders and are less susceptible to oxidation. The use of ceramic binders allows thermal and chemical stability of PcBN cutting tools (Ref 13). Therefore, the development of PcBN composites with ceramic binders is essential. PcBN composites with high volume fraction of ceramic binders are typically used in finishing operations with low mechanical loads and high process temperatures.

Temperature is one of the important factors affecting the performance of PcBN composites, and the sintering temperature has a major influence on the bonding state between the binder and the cBN particles (Ref 14, 15). At a suitable sintering temperature, the binder particles melt without abnormal growth and bind tightly to the cBN particles (Ref 16). In terms of fracture morphology, it is suggested that transgranular fracture and intergranular fracture coexist (Ref 17, 18). The appropriate binder can not only reduce the sintering temperature and pressure but also bond the cBN particles into a compact whole through the binder to improve the performance of the sintered body (Ref 14). The presence of a large number of N vacancies in TiN0.3 can activate the sintering process through the movement of vacancies, and the fracture toughness of the material can be improved by using this vacancy mechanism (Ref 19, 20). The presence of AlN inhibited the conversion of cBN to hBN (Ref 21, 22). Chu et al. (Ref 23). prepared cBN(Al)-Al2O3 composites under high temperature and high pressure conditions. The reaction of cBN with Al produces AlN, where the AlN bridge connects cBN and Al2O3. The hardness of the cBN(Al)-Al2O3 composite reached 29.4 GPa at 1350 °C which is 37.5 % higher than that of the cBN-Al2O3 composite. Sun et al. (Ref 24) prepared B4C-cBN composites by high pressure sintering at 6 GPa and 1700 °C. The phase transition from cBN to hBN was avoided during the sintering process while maintaining hardness and lightweight. The composite material is composed of 50 wt.% cBN, which has excellent comprehensive mechanical properties, relative density, density vickers hardness and fracture toughness of 98.6 %, 2.9 g/cm3, 36.2 GPa and 6.7 MPa m1/2, respectively.

According to our previous research, the best material performance was achieved when the TiN0.3/AlN ratio was 7:3 and hardness reached 22.7 GPa (Ref 25). Therefore, we chose the TiN0.3 to AlN ratio of 7:3 and added TaC, VC and WC as ceramic binders to prepare the PcBN composites. The raw material powders are completely mixed and then sintered under high temperature and pressure to form a dense sintered body. The study investigated the reaction mechanism of using TiN0.3/AlN/TaC/VC/WC as a ceramic binder with cBN at different temperatures. It also examined the phase compositions, microstructures, and mechanical properties of the prepared PcBN composites.

2 Experimental Procedures

The powders used in this experiment were from Qinhuangdao ENO High-tech Material Development Co., Ltd. The mixed particle size of cBN powders (0.5 ~ 1 μm):(2 ~ 5 μm):(5 ~ 10 μm) = 3:5:2 (Ref 25), AlN powders (≤ 0.5 μm, purity > 99.6 %), TaC powders (≤ 1.0 μm, purity > 99.6 %), WC powders (≤ 0.2 μm, purity > 99.6 %), VC powders (≤ 0.3 μm, purity > 99.6 %) and TiN0.3 in the non-stoichiometric ratio were used as starting materials. TiN0.3 powders were prepared by mechanical alloying prior to the experiment (Ref 26, 27). The raw materials used to produce TiN0.3 were Ti powder (40 μm, purity ≥ 99.5%) and urea (CH4N2O, analytically pure). The molar ratio of Ti powder to urea was 6:1. The powder was packed into a canister made of WC carbide and sealed in an argon atmosphere. Additionally, 0.5 ml of alcohol was added as a process control agent to prevent bonding of the powdered material. The grinding media used were tungsten carbide balls made of WC with diameters of 8, 6, and 2 mm, respectively. The mass ratio of the balls was 6:3:1, and the ball-to-material ratio (BPR) was 20:1. The milling process was carried out for 60 h in a planetary ball mill (QM-4, Nanjing, China) at a speed of 450 r/min. The TiN0.3 powder was then processed under vacuum at a temperature of 600 °C for 0.5 h to remove all organic impurities released during the ball milling of the powder. The XRD pattern of the prepared TiN0.3 is shown in Fig. 1(a), which has the crystal structure of FCC. The raw material 50 vol.% cBN is composed with 50 vol.% binder, and the binder ratio is TiN0.3:AlN:TaC:WC:VC = 56:24:10:5:5. The use of mixed particle size of cBN powder can effectively improve the stacking state of the particles and increase the stacking density of the grains. The formulated powder was uniformly mixed in the planetary ball mill for 2 h according to the formulation, then loaded into the mould and pre-pressed into blocks on the high pressure machine at a pressure of 50 MPa (Ref 28). Then, hinge type artificial diamond six-sided top press (CS-IB, China) was used for sintering in the temperature range of 5.5 GPa, 1380-1560 °C, with a holding time was 10 min (Ref 29, 30). The specific sintering process is shown in Fig. 1(b) (Ref 31).

Fig. 1
figure 1

(a) XRD pattern of TiN0.3 powder, (b) Schematic of the sintering process

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The sintered samples were grounded and polished on a diamond grinding and polishing machine. The phase composition of the mixed powders and sintered samples were identified by X-ray diffraction (XRD, D/MAX-2500/PC, Japan) using a Rigaku diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 200 mA. The XRD data were measured from 20 to 80° of 2θ with a step of 0.02° and a counting time of 5 s. The fracture morphology and composition of the composite sinter were analysed by scanning electron microscopy (SEM, FES-4800, Japan). The Vickers hardness tester (THV-5, China) was used to measure the hardness, and the load was maintained at 1000 gf for 15 s. The fracture toughness was measured by indentation method, and the load was maintained at 5000 gf for 15 s. The average value was taken after 6 points were measured for each sample. Vickers hardness and fracture toughness were calculated using Eq (1) (Ref 32, 33) and (2) (Ref 2, 34) respectively:

$$H_{V} = 1.854\frac{P}{{d^{2} }}$$
(1)
$$K{}_{{IC}} = 0.016\left( {\frac{E}{{H_{V} }}} \right)^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}} \frac{F}{{c^{{\frac{3}{2}}} }}$$
(2)

where HV is the Vickers hardness (N/mm2), P is the applied load (N), d is the average value of the diagonal of the indentation (mm), E is the Young’s modulus and c is the half-crack length from the centre of the indentation to the tip of the crack (μm). The bulk density of PcBN composites was calculated using Archimedes principle.The theoretical density of PcBN composites was calculated using the rule of mixtures.The relative density of PcBN composites was derived from the ratio of bulk density to theoretical density. The XRD data were subjected to Rietveld refinement using Jade 6.0 to obtain the relative percentage of each phase composition within the PcBN composite. The abrasion ratio was tested using an abrasion ratio tester (DHM-3, Beijing, China). After the PcBN composite material is fixed, it is intergrated with the green silicon carbide ceramic parallel grinding wheel. Test conditions: cutting load 5 Kg, spindle speed 2000 r/min, loading pressure 800 g, set-up time 600 s. Prior to determining the abrasion ratio, it is essential to ultrasonically clean and dry both the sample and the standard grinding wheel until they reach a constant weight. An electronic balance should be used to weigh the sample and the standard grinding wheel before and after grinding. The abrasion ratio of the sample relative to the standard grinding wheel can then be calculated using Eq 3.

$$Q = \frac{{m_{1} - m_{2} }}{{M_{1} - M_{2} }}$$
(3)

where Q is the abrasion ratio, m1 is the mass of the standard grinding wheel before counter grinding (g), m2 is the mass of the standard grinding wheel after counter grinding (g), M1 is the mass of the sample before counter grinding (g), and M2 is the mass of the sample after counter grinding (g).

2.1 Results and Discussion

Figure 2 shows the XRD pattern and phase content variation of PcBN composites. Four temperatures of 1380, 1440, 1500 and 1560 °C were selected to investigate the phase composition of PcBN composites prepared at different temperatures. The main phases of the composites at different temperatures were cBN, WC, TaC and TiC0.3N0.7 respectively. The peak intensities of TaC, WC, cBN diffraction peaks all decreased to different degrees and the peak intensity of TiC0.3N0.7 gradually increased with increasing temperature. The XRD data were subjected to Rietveld refinement and the content of each phase TaC, WC, cBN, TiC0.3N0.7 in the composites was calculated by semi-quantitative analysis. The results showed that the relative content of TaC phase decreased from 14.4 to 8.4%, the relative content of WC phase remained the same, the relative content of cBN phase decreased from 50.3 to 43.4 %, and the relative content of TiC0.3N0.7 phase increased from 29.4 to 42 % as the temperature increased from 1380 to 1560 °C. This is because the addition of TiN0.3 introduces N atomic vacancies and facilitates the exchange and transfer of atoms at high temperatures. The solid solution occurs between carbonitrides and TiN0.3, and C and N atoms diffuse into the vacancies in TiN0.3 to form TiC0.3N0.7, resulting in a gradual increase in the content of TiC0.3N0.7. The Al atoms react with the O atoms on the surface of the powder to form Al2O3.

Fig. 2
figure 2

(a) XRD patterns of PcBN composites at different temperatures, (b) Phase content of each phase in PcBN composites at different temperatures

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According to the above analyses, the material may react in the ways described in Eqs 4 and 5 during high temperature and high pressure sintering.

$$4{\text{Al}} + 3{\text{O}}_{2} \to 2{\text{Al}}_{2} {\text{O}}_{3} + 4{\text{N}}^{ * }$$
(4)
$$10{\text{TiN}}_{0.3} + 3{\text{C}}^{*} + 4{\text{N}}^{*} \to 10{\text{TiC}}_{0.3} {\text{N}}_{0.7}$$
(5)

The addition of AlN can play a role in inhibiting the phase transition of cBN. cBN undergoes a phase transition at high temperature to form hBN dissolved in AlN, which is inhibited by the fact that cBN precipitates out of AlN due to its extremely low solubility in AlN (Ref 21,22,23). The N atoms in cBN also diffuse to the N vacancies in TiN0.3 and form covalent bonds with them, resulting in stronger bonds between them. The N atoms in AlN diffuse to TiN0.3, and the remaining Al atoms react with O atoms on the powder surface to form Al2O3. Fig. 2 indicates that the peak of VC disappears after 1440 °C with increasing temperature, indicating that the solid phase reaction between TiN0.3 and VC occurs during the sintering process to solid-solve the two phases together.

Figure 3 shows the SEM images of the fractures of the PcBN composites at four sintering temperatures of 1380, 1440, 1500 and 1560 °C. At a temperature of 1380 °C, only a small portion of the binding agent particles react with the cBN particles, as shown in Fig. 3(a). This is because of the lower sintering temperature. The particles are only stacked on top of each other, and there are gaps in the loose particles due to mutual extrusion, overlap and different particle sizes (Ref 35). The hardness of PcBN composites sintered at low temperatures is lower according to the hardness calculation formula. The encounter of voids between the particles during crack propagation dissipates the crack propagation stress, resulting in a reduction in crack propagation dynamics, preventing further crack propagation and increasing the toughness of the material. During the sintering process, fewer cBN particles react with the binder due to the low sintering temperature. As a result, most of the cBN particles are only embedded in the binder. Grinding resulted in low wear resistance of the PcBN composites prepared at 1380 °C due to the dual effect of inter-particle voids and weak bonding.

Fig. 3
figure 3

SEM patterns of fracture surfacesfor PcBN composites at different temperatures (a) 1380 °C, (b) 1440 °C, (c) 1480 °C, (d) 1540 °C.

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The surface of the binder particles is partially molten at 1440 °C. The particles bonded together by filling the gaps with molten binder (Fig. 3b). The bonding state between particles at 1500 °C is further improved in Fig. 3(c). Due to the migration of N atoms, chemical reactions occur on the surface of binder particles and cBN particles to form covalent bonds. Since covalent bonds have a higher bond energy than metallic bonds, it takes more energy to break them, and the strength of the material is improved by this strong chemical bond (Ref 30). As the binder partially melts to fill the gap between the particles and the particles, binding them into an organic whole, there is not only physical engagement between the particles, but also strong chemical bonds to reinforce the bond strength. Therefore, the penetration fracture can be observed in Fig. 3(b), which indicates that the binder is more firmly bonded to the cBN grains. At high temperatures, the various elements fully flow and transfer completely, so that the material is uniformly organised in different places inside, and the sintered body has high hardness. During the wear process, the bond and cBN wear simultaneously as they are intimately bonded. Due to the physical embedding and chemical bonding, the sintered body grinds both the bond and cBN particles, resulting in a high abrasive ratio of the sintered body (Ref 36).

Figure 3(d) shows that sintering at 1560 °C resulted in high temperature recrystallisation and grain growth, which led to extrusion of the particle bond and high stress concentration in the sintered body. The material is subjected to an external force and fractures through the crystal along the flat surface of the material. The grain arrangement at the grain boundary is irregular, the energy is highest, the energy required to destroy the grain boundary is higher, and the grains are bonded into sheets after coarsening, as shown by the white dashed line in Fig. 3(d). The ability of the material to resist external forces is reduced due to the reduction in the number of grain boundaries per unit area, resulting in a reduction in hardness.. It is very difficult for the crack to propagate under these conditions because of the compressive stress between the grains. The abnormal grain growth is accompanied by the formation of pores, which can hinder the propagation of cracks. Therefore, the fracture toughness of PcBN composites is higher. The density of the ceramic sintered body greatly affects the internal particle combination state and the tissue sintering state (Ref 37). The binder does not form a strong bond with the cBN at high temperatures. As a result, it expands and cracks when stress is suddenly released during wear. The rapid consumption of the bond causes the cBN particles to flake off without any wear or trace wear, and the wear resistance is poor (Ref 38), which is the same result as shown in Fig. 5(c).

Figure 4 shows the EDS patterns of the PcBN composites that were prepared at 1500 °C and 1560 °C. Both Fig. 4(a) and (b) show a uniform distribution of fine particles inside the material. The particles are composed of TaC, WC, VC, TiC0.3N0.7, Al2O3 and small grain size of cBN. The cBN particles of different sizes were bonded using TaC, WC, VC, TiC0.3N0.7 and Al2O3 as binders to improve the overall properties of the materials. Larger size AlN grains are present in both Fig. 4(a) and (b). The grain size increases with higher sintering temperature, which contributes to the decrease in hardness of PcBN composites between 1500 and 1560 °C.

Fig. 4
figure 4

EDS mapping of PcBN composites. (a) 1500 °C, (b) 1560 °C

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Figure 5 shows the Vickers hardness, fracture toughness, theoretical density, relative density and abrasive ratio curves of PcBN composites at different temperatures. As the sintering temperature increases, the hardness of the material initially increases before decreasing, reaching its maximum value at 1440 °C. Conversely, the toughness of the material initially decreases before increasing, as shown in Fig. 5(a). Both relative density and theoretical densities showed a pattern of increase followed by decrease, Fig. 5(b). The C atoms in TaC and the N atoms in AlN form covalent bonds with the vacancies in TiN0.3 as the temperature increases. Since it contains a small amount of VC, it can play a role in grain refinement which can improve the hardness, fracture toughness, wear resistance, chemical stability and service life of the material to a certain extent. As shown in Fig. 2(a), TiN0.3 in solid solution reacts with all VCs at 1500 °C. The addition of VC can play a role in grain refinement which can improve the hardness, fracture toughness, wear resistance, chemical stability and service life of the material to a certain extent (Ref 39,40,41). As the temperature continues to rise, the grain grows abnormally resulting in a decrease in hardness. The graph of toughness versus temperature is also in line with materials science theory where toughness and hardness properties are opposite to each other. At 1500 °C, when the hardness is 26 GPa, the toughness also reaches 8.8 MPa m1/2, and the abrasive ratio also reaches the peak. Between 1500 and 1560 °C, the grains inside the sintered body experience abnormal growth, resulting in a significant decrease in density and increased susceptibility to damage. Expansion and cracking due to rapid release of stress during abrasion leads to easy stripping of cBN particles and deterioration of wear resistance.

Fig. 5
figure 5

The Vickers hardness, fracture toughness, theoretical density, relative density and abrasive ratio of the PcBN composites

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This study used non-stoichiometric ratios of TiN0.3 and AlN as binders, followed by the addition of three transition metal carbides (WC, TaC, and VC) with the expectation of improving the fracture toughness of the prepared PcBN composites while maintaining high hardness compared to other PcBN composites. Yuan et al. (Ref 42) prepared cBN-Ti-Al composites using SPS in the temperature range of 50 MPa, 1200-1700 °C. The PcBN composites showed the best performance at 1400 °C with hardness and toughness of 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa m1/2, respectively. The hardness was lower than the 27.8 GPa of this study due to sintering under non-high pressure and the use of ceramic binders in this study resulted in a higher increase in toughness. Mo et al. (Ref 43) prepared cBN-Ti-Al-W composites at 5.5 GPa and 1350-1600 °C. TiB2 has a major influence on the hardness of PcBN composites due to the production of TiB2 during the sintering process and its gradual increase with increasing temperature (Ref 44).

In conclusion, the Vickers hardness first increases and then decreases, and the fracture toughness first decreases and then increases with increasing sintering temperature of the carbon-nitride ceramic-bonded PcBN composites. The relative density, bulk density and abrasive ratio follow the same trend as the hardness. The best overall performance was achieved at 1500 °C, with Vickers hardness, fracture toughness, abrasive ratio and relative density of 26.0 GPa, 8.8 MPa m1/2, 271.3 and 99.6% respectively. The introduction of N atomic vacancies by the addition of TiN0.3 promotes the exchange and transfer of atoms at high temperatures. The solid solution phenomenon occurs between the carbon nitride and TiN0.3 and the C and N atoms diffuse into the vacancies in TiN0.3 to eventually form TiC0.3N0.7 as shown in Fig. 6.

Fig. 6
figure 6

The schematic images of the diffusion of C, N atoms into vacancies

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As the sintering temperature increases, the reaction between the binder and cBN within the composite material becomes more complete, resulting in a denser sintering and significantly improved performance of the sintered body. However, high temperatures can lead to abnormal grain growth, porosity, and stress concentration defects, which can cause a sharp deterioration in the overall material performance. Under the same experimental conditions, an appropriate increase in VC can play a role in grain refinement, which can improve the hardness, fracture toughness, wear resistance, chemical stability and service life of the material to a certain extent.

3 Conclusions

The physical phase composition, mechanical properties and microstructure of PcBN composites in combination with carbon-nitride ceramics have been investigated and analysed in detail in this paper. The results show that the addition of TiN0.3 can activate the sintering process, accelerate the exchange and transfer of atoms at high temperatures, promote the formation of internal covalent bonds and improve the properties of the material. As the temperature increases, C and N atoms continue to diffuse into the vacancy in TiN0.3, resulting in a gradual increase in the TiC0.3N0.7 content. The bonding state in the PcBN composites reached the optimum and the best overall performance of the material at 1500 °C. The Vickers hardness, fracture toughness, relative density and abrasive ratio reached 26.0 GPa, 8.8 MPa m1/2, 99.6% and 271.3, respectively.