Silicon nitride (Si₃N₄) ceramics are considered to be one of the most promising ceramic substrate materials due to their good insulation, low coefficient of thermal expansion and high mechanical properties. However, the thermal conductivity of Si₃N₄ ceramics prepared by conventional methods is low (< 50 W·m^-1·K^-1), which greatly affects the reliability of chips and even causes the failure of electronic devices. New methods such as gas pressure sintering (GPS), and sintered reaction-bonded silicon nitride (SRBSN) have been employed to fabricate Si₃N₄ ceramics with high thermal conductivity and high bending strength. Nevertheless, the sintering temperature of both GPS and SRBSN is high and the holding time is too long, which lead to the high cost and limit its applications. Spark plasma sintering (SPS) can produce ceramics with high density in a relatively short time, while the short soaking time is not suitable to improve the thermal conductivity. High-temperature heat treatment can promote the growth of Si₃N₄ grains, and thus improves the thermal conductivity of Si₃N₄ ceramics. Therefore, it is expected that Si₃N₄ ceramics with high bending strength and high thermal conductivity can be prepared at a lower temperature and a shorter time by the combination of SPS and high-temperature heat treatment processes.

Si₃N₄ ceramics were prepared by SPS at 1 400, 1 500, 1 600 ℃ and 1 700 ℃ for 5 min using α-Si₃N₄, Y₂O₃ and MgSiN₂ as raw materials. The obtained samples with the best overall properties were then selected and put into a gas pressure furnace to be heated at different temperatures (1 600, 1 700 ℃, and 1 800 ℃) for 3 h. Density and open porosity of the sintered samples were tested by Archimedes method. The phase composition of samples was analyzed by X-ray diffractometer (XRD). Samples were polished and then etched in molten NaOH at 500 ℃ for 20 s to observe the microstructures using a field emission scanning electron microscope (FESEM). The diameters of at least 200 grains in the SEM images were measured by Nano measure software and statistically analyzed to obtain the average grain size. The bending strength was examined via three-point bending test using an electronic universal testing machine. The thermal diffusion coefficient of Si₃N₄ ceramics was determined by a laser thermal conductivity analyzer.

As the sintering temperature increased from 1400 ℃ to 1 700 ℃, the relative density of samples increased rapidly from 80.43% to 97.81% at first and then slowly to 99.09%. With the increase of sintering temperature, the viscosity of liquid phase produced by the reaction of sintering additives gradually reduced, and the rate of material transport improved during sintering process, which led to a higher densification. When sintering temperature reached to 1 600 ℃, the content of β-Si₃N₄ in sample reached 100%, indicating that the phase transition from α-Si₃N₄ to β-S_i3N₄ has been completed. The average grain size of samples gradually grew from 0.31 μm to 1.35 μm with the increase of sintering temperature from 1 400 ℃ to 1 700 ℃, and the thermal conductivity of the samples rapidly increased from 9.88 W·m^-1·K^-1 to 61.80 W·m^-1·K^-1, being attributed to the phase transition of α→β-S_i3N₄. Besides, the process of phase transition could purify the crystal lattice, while the increase of densification and grain growth reduced the scattering of phonons by defects such as pores, grain boundaries, and so on. Therefore, the thermal conductivity of the Si₃N₄ ceramics was improved.

After the heat treatment of sample sintered by SPS at 1 600 ℃, the relative density reduced in a certain degree compared with that of the untreated samples. The average grain size of β-Si₃N₄ in samples before heat treatment was 1.16 μm, and grows to 1.33 μm after heat treatment at 1 600 ℃. Rising the heat treatment temperature to 1 700 ℃ and 1 800 ℃, the average grain size of the samples grew to 1.35 μm and 1.51 μm, respectively. when heat treatment temperatures increased from 1 600 ℃ to 1 800 ℃, the thermal conductivity of the samples rosed from 50.87 W·m^-1·K^-1 to 57.36 W·m^-1·K^-1, 0.45% and 13.27% higher than that of the un-treated samples. Bending strength of samples decreased with the increase of heat treatment temperature. When heat treated at 1 800 ℃, the bending strength of sample reduced to 743 MPa. which still meet the requirement of bending strength >700 MPa for Si₃N₄ ceramic substrates.

The relative density, thermal conductivity, and β-Si₃N₄ content of the Si₃N₄ ceramics increases with the increase of SPS temperature. When SPS temperature was 1 600 ℃, the bulk sample was composed of β-Si₃N₄ with a high relative density of 98.78% and a thermal conductivity of 50.64 W·m^-1·K^-1. After heat treated at the same temperature, the thermal conductivity of the samples was improved, while the bending strength displayed the opposite trend. The thermal conductivity of samples heat-treated at 1 800 ℃ achieved 13.27% higher than that of the untreated sample, while the bending strength still exceed 700 MPa. Therefore, the combination of SPS and high-temperature heat treatment is a promising process to produce Si₃N₄ ceramics with high thermal conductivity and bending strength.