DENG Chao 1, CHEN Song 1, ZHANG Weiru 2, 3, LIN Yan 1, LIN Jiayuan 1, WANG Weiguo 1
(1. School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, Fujian, China; 2. Sinoma high-tech Nitride Ceramics Co., Ltd., Zibo 255022, Shandong, China; 3. Sinoma high-tech Materials Co., Ltd., Beijing 100021, China)
Extended abstract:
[Background and purposes] The rapid advancement of modern aerospace vehicles, hypersonic aircraft and high-tech equipment has raised higher demands on structural ceramic materials for load-bearing capacity, wear resistance and stability under complex friction and wear conditions. Si3N4 has been widely used due to its excellent properties. However, they are still prone to fatigue cracking and wear failures under severe operating conditions. These failures are closely related to the materials microstructure. In this study, Si3N4 ceramics with different contents of TiN were prepared by using pressureless sintering, with Al2O3–Y2O3 as sintering additives. The effects of TiN content on phase composition, lattice parameters, grain structure and interfacial structure of silicon nitride ceramics were systematically examined, thus providing experimental evidence for the design and performance optimization of Si3N4.
[Methods] Si3N4 ceramics were prepared by using pressureless sintering with α-Si3N4 as the main raw material. Al2O3 and Y2O3 were used as sintering additives at contents of 3.0 wt.% and 6.0 wt.%, respectively, while 5.0 wt% β-Si3N4 seed crystals were introduced to promote the phase transformation during sintering. The raw powders were ball-milled with absolute ethanol as solvent. After drying, the ceramic powder mixture was ground and sieved through 200-mesh screen to ensure uniform particle size distribution. 50 g powder was put into a 40-mm-diameter mold and compressed using a two-step compression process. The raw powder was first pre-pressed at 30 MPa for 3 min to form a preform, which was then placed into a 20 mm-diameter mold for cold isostatic pressing at 200 MPa for 5 min, yielding ceramic green bodies with uniform thickness and consistent dimensions. The compacted samples were placed in pressureless sintering furnace, purged three times with high-purity nitrogen and then sintered at 0.1 MPa high-purity N2, using two-step heating to 1600 ℃ at 10 ℃·min−1 and holding for 2 h, followed by heating to 1800 ℃ at the same rate and holding for 8 h to achieve densification of Si3N4 ceramics. After sintering, the samples were furnace-cooled to room temperature. The sample density was measured by using Archimedes method. XRD was used to analyze the phase composition and lattice parameters of Si3N4. The grain structure, phase and elemental distribution were characterized by using CBS, SEM and EDS. The Si3N4/TiN interfaces were characterized by using aberration-corrected TEM to analyze their interface and lattice mismatch.
[Results] The addition of TiN addition improved densification of the pressureless-sintered Si3N4 with the Al2O3-Y2O3 sintering-aid system, increased relative density and reduced residual porosity and microstructural inhomogeneity, which are caused by insufficient driving force during the liquid-phase sintering. With increasing content of TiN, the grain-refinement effect becomes more pronounced. TiN promoted the diffusion and solid-solution reaction of Al and O in Si3N4, thereby facilitating the formation of β-SiAlON solid solution, effectively consuming free oxygen and suppressing the formation of the harmful Si2N2O phase. According to the high-resolution TEM results, TiN refined the Si3N4 grain through two primary mechanisms. Firstly, TiN particles, which have a high melting point of 2930–2950 ℃, pinned the Si3N4 grain boundaries and effectively suppressed grain growth. Secondly, low lattice mismatch, between TiN crystal planes [such as (200) and (011)] and Si3N4 crystal planes [such as () and (0001)], provideed conditions for the heterogeneous nucleation of β-Si3N4, increasing the nucleation density of β-Si3N4. The resultant Si3N4 grains form stable semi-coherent interfaces with TiN. Due to grain refinement and stable interfacial structure, the worn surfaces become smoother, the wear tracks become shallower and brittle spalling is effectively suppressed, resulting in a marked improvement in wear resistance of the Si3N4 ceramics.
[Conclusions] To address the problems of insufficient densification, coarse grains and limited wear resistance of pressureless-sintered Si3N4 ceramics prepared with conventional Al2O3-Y2O3 sintering additives, TiN with varying content was used. The effect phase composition, microstructure and interfacial structure was systematically studied. The addition of TiN addition significantly increased the densification of the material and suppressed the formation of the harmful Si2N2O. The addition of TiN also enhanced the diffusion and solid solution of Al and O within Si3N4, thereby facilitating the formation of the β-SiAlON solid solution. Furthermore, the addition of TiN effectively refined the Si3N4 grains and promoted the formation of stable semi-coherent interfaces by enhancing heterogeneous nucleation and grain boundary pinning. As a result, the worn surfaces of the Si3N4 ceramics became smoother, the wear tracks became shallower, the brittle spalling was significantly reduced and the wear resistance was markedly increased.
Key words: silicon nitride ceramics; heterogeneous nucleation; pinning; transition metal compounds; microstructure