3.1 PMMC microstructure
Figure 1 and Table 1 show the morphology and composition of T8N2-850, respectively. Figure 1 shows the nickel matrix (site a), the titanium particle (site e), and the interface layer (site c). The interface layer—with a high concentration of oxygen and titanium was named Ti-oxide-film. Parts of the Ti-oxide were dispersed in the matrix, as shown in site b. The layer (site d) with a thickness of about 3 μm and a high content of titanium, oxygen, and nitrogen was named the Ti-nitride-layer.
The boundary of the titanium particle (site d) in Figure 1 shows that oxidation and nitridation reactions occur on the titanium particles in a high-temperature sintering process under atmospheric conditions. The free energy of the titanium and oxygen reaction is lower than that of the titanium and nitrogen reaction ; therefore, the oxidation reaction takes precedence over the nitridation reaction on the titanium surface. However, the hot forging lubricant covering the titanium particles helped prevent the titanium from making contact with the oxygen. Additionally, the carbon content decreased significantly in the Ti-nitride-layer according to the following carbothermal reduction reaction:
2TiO2 + 4C + N2 → 2TiN + 4CO↑ (1)
The carbothermal reduction reaction converts a portion of the titanium oxide to titanium nitride. Simultaneously, the carbon monoxide gas released from the interface breaks the titanium oxide layer. Consequently, the Ti-oxide-film fragments are dispersed into the matrix around the titanium particles as shown at site b in Figure 1.
In order to investigate the effect of temperature on the carbothermal reduction reaction, the thickness of the Ti-nitride-layer, the titanium particles, and the Ti-oxide-film covering the outer surface of the titanium particles were measured by SEM. The average thickness—shown in Figure 2—was calculated by measuring at least seven sites. The thickness of the Ti-nitride-layer increased as the sintering temperature increased (Figure 2(a)). In addition, the thickness of the Ti-oxide-layer depends not only on the temperature but also on the proportion of titanium in the PMMC (Figure 2(b)). Before sintering, the PMMC ingot will be soaked in the CONDAT solution. The solution with C and O elements can penetrate into the interior of the PMMC through the gaps between the powders. Compared with T5N5 and T8N2, T2N8 has the smallest numbers of Ti particles. For T2N8, each Ti particle obtains relatively higher amounts of CONDAT solution to cover the surface when comparing with T5N5 and T8N2; therefore, T2N8 has a thickest oxide layer during sintering at 1050℃.
Figure 3 shows the SEM micrograph of T2N5, T5N5, and T8N2 sintered at 850, 950, and 1050 ℃, respectively. The T2N8-850 titanium particles were easily detached from the nickel matrix by specimen polishing as indicated by the arrow in Figure 3(a). Most of the T2N8-950 titanium particles remained in the nickel matrix because of the thick Ti-oxide-film that bound the particles and the matrix. However, some pores appeared in the Ti-nitride-layer of the titanium particles as shown in Figure 3(b). In Figure 3(c), the titanium particles remained in the nickel matrix, but fractures were observed within the titanium particles, indicating that Ti-oxide-film bonds well with the nickel matrix, but that the numerous defects in the titanium particles fracture the titanium particles when the specimen is polished. Figure 3(d–f) shows a similar situation for the T5N5 sintered at 850, 950, and 1050 ℃, respectively.
For T8N2-850 (Figure 3(g)), the detachment of the titanium particles is more frequent than for T2N8-850, and parts of the titanium particles inter-diffuse with each other in T8N2 sintered at 950 and 1050 ℃, as shown in Figure 3(h–i), respectively. The detachment of the nickel matrix was observed in Figure 3(i) after polishing, as titanium is harder than the nickel.
3.2 PMMC composition
Figures 4–6 show XRD spectra of the T2N8, T5N5, and T8N2 compacts sintered at 850, 950, and 1050 ℃, respectively. As shown in Figure 4, the Ni phase in the XRD pattern of T2N8-850 is the main phase, and the intensity of the TiN0.3 diffraction peak is very weak. As the sintering temperature was further increased to 950 ℃, the intensity of the TiN0.3 diffraction peak decreased, and the TiN diffraction peak appeared. When the sintering temperature reached 1050 ℃, the intensity of the diffraction peaks of both TiN and TiO2 significantly increased. A SEM-EDS analysis showed that titanium oxide was formed at 850 ℃. However, the titanium content of T2N8 is low, and the titanium oxide forms only on the outer surface of the titanium particles, which results in low titanium oxide content. Therefore, titanium oxide cannot be detected in the XRD pattern of T2N8-850.
The XRD spectra of T5N5 in Figure 5 show why TiN0.3 was formed at 850 ℃. At 950 ℃, titanium is transformed into TiO2 and TiN. As the sintering temperature increased to 1050 ℃, the diffraction peaks of TiN increased, while the diffraction peak of TiO2 decreased, suggesting that the thickness of the titanium nitride layer increases with sintering temperature because of violent carbothermal reduction reactions at 1050 ℃.
Figure 6 shows that the diffraction peaks of the TiO2 and TiN phases appear at 850, 950, and 1050 ℃. Additionally, the intensity of the titanium oxide and nitride peaks of T8N2 is higher than that of T2N8 and T5N5. As the sintering temperature increased from 950 to 1050 ℃, ITiN(200)/I TiO2(111) also increased. Thus, the thickness of the Ti-oxide-film first increases with sintering temperature up to 950 ℃ and then decreases, as measured by SEM observation (Figure 2). The intensity of TiN0.3 and TiN peaks increase significantly with the weight percentage of Ti in MMC under various sintering temperature.
3.3 PMMC porosity
Table 2 shows the porosity of pure nickel, T2N8, T5N5, and T8N2. As the sintering temperature increased from 850 to 950 ℃, the porosity of the pure nickel decreased from 9.02% to 2.01%. The porosity of T2N8-950 is lower than that of T2N8-850 because the pores are eliminated by the thermal diffusion of the nickel powders. However, the carbothermal reduction reaction occurs violently at high temperatures (1050 ℃) and forms numerous pores. Hence, the porosity of T2N8 increased to 10.43% when the T2N8 compact was sintered at 1050 ℃. The porosity of the sintered T5N5 and T8N2 compacts increased with increased sintering temperature, indicating that the degree of pore formation is greater than that of compact shrinkage from diffusion. This is because the high titanium content in the compacts fuels the carbothermal reduction reaction, which produces numerous pores.
The porosity of T5N5 is lower than that of T2N8 and T8N2 sintered at 850, 950, and 1050 ℃. According to the literature, a wider powder particle distribution range, stable sintered compact shrinkage rate, and smaller pores produce a more uniform crystal phase distribution . In this study, the average particle sizes of titanium and nickel powders are 75 and 45 μm, respectively. When the weight ratio of these two powders is equivalent, a larger particle size distribution range is obtained; therefore, the porosity of Ti5N5 is lower than that of T2N8 or T8N2.
3.4 PMMC hardness
Figure 7(a) shows the Rockwell hardness of T2N8, T5N5, and T8N2 sintered at 850, 950, and 1050 ℃. The hardness increased with an increased sintering temperature and number of titanium particles. Figure 7(b) shows the hardness of the centre of the T2N8 titanium particle as measured by the Vickers microhardness tester. The hardness of the PMMC is greater than that of than pure titanium (150 HV) and nickel (123.6 HV) [20,21]. The transformation of titanium into TiN and TiN0.3 reinforces the PMMC and increases the amount of titanium particles.
Figure 8 shows the indentation location of the Vicker’s microhardness of T2N8-950 observed by an optical microscope. The centre of the titanium particle is HV1033.0 in Figure 8(a). The hardness of the Ti-oxide-film is HV318.2, which is between that of the nickel matrix (HV79.7) and the titanium particles (HV1033.0) in Figure 8(b).
3.5 PMMC wear resistance
The factors affecting the wear resistance of a composite include (1) the number of reinforcing materials in the matrix, (2) the combination of reinforcing materials in the matrix, and (3) the porosity of the composite. In Figure 9(a), T2N8 sintered at 850 and 950 ℃ has the highest and lowest weight loss, respectively. The thickness of the Ti-oxide-film makes strengthen the bonds between the reinforcing material and the matrix. However, the porosity generated by the carbothermal reduction reaction increases rapidly at 1050 ℃ and lowers the wear resistance of T2N8-1050 more than that of T2N8-950. T2N8-850 has the lowest wear resistance because the thickness of its Ti-oxide film is very small. T2N8-850 has the lowest hardness among the T2N8 compacts. T5N5 shows the same relationship between wear resistance and sintering temperature as does T2N8 (Figure 9(b)).
Figure 9(c) shows the wear resistance of T8N2 sintered at 850, 950, and 1050 ℃. T8N2-1050 and T8N2-850 have the lowest and highest weight loss, respectively. The T8N2 matrix consists primarily of titanium particles, some of which with each other. Therefore, the bonding ability of titanium particles increased with the sintering temperature, leading to a significant reduction in weight loss. However, since the nickel content is low, the nickel powder is discontinuously distributed in the titanium matrix; therefore the nickel peels off easily, as shown in Figure 3. Furthermore, multiple carbothermal reduction reactions cause poor interfacial adhesion between the titanium and the nickel and allow titanium particles to be easily worn away. Hence, the T8N2 compacts have the lowest wear resistance among the sintered compacts.
T8N2 has the highest hardness but the lowest wear resistance of the sintered compacts. Previous studies found that the hardness and the wear resistance of composites are not directly related. During wear processing, shear stress occurs on the PMMC, and cracks may appear in the matrix . As a result, even if the hardness of the PMMC is enhanced by particle reinforcement, the PMMC may be easily peeled off from internal cracks.
3.6 PMMC compressive strength
Figure 10(a–c) show the compressive stress-strain curves of T2N8, T5N5, and T8N2 sintered at 850, 950, and 1050 ℃, respectively. A sintering temperature of 950 °C provides a higher compressive strength for T2N8 and T5N5 than does 1050 ℃ (a–b). The worse compressive strength is from the high porosity of the compacts sintered at 1050 °C. The position around the pores is likely to cause stress concentration and become the starting point of deformation. The compressive strength of T2N8 and T5N5 sintered at 1050 °C is higher than that of T2N8 and T5N5 sintered at 850 °C. This result is consistent with the wear test. Furthermore, T8N2 exhibited the lowest compressive strength of the sintered compacts (Figure 10(c)), because T8N2 has high amounts of hard material (titanium) and low amounts of soft material (nickel). This combination produces brittle fractures when the compacts are under compressive loads.