title: Single-source-precursor derived additive-free bulk SiHf(B)N ceramics with excellent mechanical properties and oxidation resistance
Creators: name: Li, Wei
Publisher: name: UNSPECIFIED
Year: 2023

description: In the last few years, numerous efforts have been explored to add metal-based (nano)powders into Si3N4 to prepare particle-reinforced Si3N4-based composites using hot pressed sintering, pressureless sintering or hot isostatic pressing. However, all of these methods generally require sintering temperature up to 2000 °C, long hold times and/or high applied loads as well as sintering aids due to its high melting point and low self-diffusion coefficient. Until now, most of the Si3N4-based composites are fabricated by using traditional powder techniques, but the grain sizes of the composites are limited to the micrometer range, and the dispersion of metal-compound particles is not homogeneous. In recent years, it has been proven that polymer-derived ceramic (PDC) approach can prepare ceramic composites to reach nano scale, showing outstanding behavior at (ultra)high temperatures. Moreover, single-source precursors (SSPs) can be easily tailored by the design of the molecular precursor, which provides the possibility of designing ceramic nanocomposites with unique phase compositions, microstructures, properties. Therefore, the PDC route is considered to be the most promising approach in fabrication of homogeneous ceramic nanocomposites with unique nanostructures by pyrolysis of suitable SSPs at low sintering temperatures. The motivation of this thesis is to further develop the concept for fabrication of SiHf(B)N ceramic nanocomposites with versatile properties by molecular design of their SSPs and to gain a better understanding of the manyfold “composition-structure-property” interrelationship. With this motivation, additive-free amorphous bulk SiHfN ceramic was fabricated using SSP synthesis plus warm pressing. The densification mechanism of warm-pressing in-situ consolidation of preceramic polymer powders was identified based on cross-linking reactions monitored by TGA/DTA and FT-IR measurements. The critical problems concerning gas evolution and crystallization inducing bloating and cracking are addressed through controlled thermolysis and pressure. Then the microstructural evolution of the polymer-to-ceramic transformation was characterized using XRD, TGA, elemental analysis, SEM and TEM. The results indicate that the incorporation of Hf in perhydropolysilazane (PHPS) not only increases the ceramic yield (97.4 wt%) and crystallization resistance (1300 ºC), but also suppresses the transformation from α-Si3N4 to β-Si3N4 at high temperatures (1700 ºC). The high-temperature microstructural evolution of the amorphous SiHfN ceramic demonstrates that α- and β-Si3N4 were obtained during the high-temperature treatment (1500 ºC, 1700 ºC) and form a matrix, in which nano-sized HfN crystallites were homogeneously dispersed. Furthermore, the nanohardness and microhardness of the obtained additive-free amorphous bulk SiHfN ceramic are up to 17 GPa and 19.6 GPa, respectively, which shows a significant improvement in comparison to most of the reported amorphous and polycrystalline Si3N4-based ceramics. The second focus of this Ph.D. work is the synthesis of γ-Si3N4/Hf3N4 ceramic nanocomposites as well as the investigation of the relationship between mechanical properties and nanostructures. The phase evolution of single-phase amorphous SiHfN ceramic was studied in-situ at HP-HT conditions with energy-dispersive X-ray diffraction (ED-XRD) using synchrotron radiation. The results show that the amorphous SiHfN phase starts to separate into γ-Si3N4 and Hf3N4 from 1090 °C under 19.5 GPa. There are no further structural changes in the XRD patterns up to ~1570 °C, while rock salt-type HfN was observed at temperature far beyond 1570 °C, indicating that Hf3N4 decomposes into rock salt-type HfN and N2 at that temperature. Therefore, the optimal HP-HT conditions for the formation and stability of the γ-Si3N4/Hf3N4 ceramic nanocomposites was determined as 20 GPa and 1500 ºC. Furthermore, the investigation of mechanical properties reveals that the fracture toughness of the resultant γ-Si3N4/Hf3N4 (6.98 MPa m1/2) ceramic nanocomposite exhibits a significant improvement over pure γ-Si3N4 (3.5 MPa m1/2) without sacrificing the hardness of the material, making it a competitive ceramic candidate for technological applications in harsh conditions. The third focus of this Ph.D. work is placed on the synthesis of novel polymer-derived SiHfBN ceramics. They were prepared via the heat treatment of a series of B/Hf-containing SSPs which were synthesized by the modification reaction of PHPS with borane dimethyl sulfide complex (BMS) and tetrakis(dimethylamido) hafnium(IV) (TDMAH). The chemical reaction to synthesize the SSPs was confirmed by FT-IR and XPS, in which both Si−H and N−H groups of PHPS react with BMS and TDMAH. The polymer-to-ceramic conversion was characterized using TGA/TDA, FT-IR as well as XPS. The SiHfBN precursors synthesized using BMS and TDMAH lead to high ceramic yield (≈ 100 wt.%) upon pyrolysis at 1000 °C under ammonia atmosphere, which is higher than those of the pristine PHPS (78 wt%), boron-modified PHPS (92 wt%) and hafnium-modified PHPS (97 wt%). The resultant SiHfBN ceramic exhibits high-temperature resistance against crystallization up to 1500 °C. The conversion of the amorphous SiHfBN ceramics into Si3N4/HfBxN1-x ceramic nanocomposites was observed by XRD after annealing at 1700 °C in N2 atmosphere, and extensive TEM characterizations reveal the homogeneous dispersion of HfBxN1-x in the Si3N4 matrix. Furthermore, the oxidation behavior of warm-pressed bulk Si3N4/HfBxN1-x ceramic nanocomposites was investigated at 1500 °C, indicating that the SiO2-B2O3 glass (i.e., borosilicate) formed between low-viscous B2O3 and high-viscous SiO2 glass leads to a dense and continuous protective barrier against inward diffusion of O2. In summary, the present Ph.D. work deeply investigated the single-source-precursor synthesis, polymer-to-ceramic conversion, the microstructural evolution of final SiHf(B)N ceramics as well as densification of SiHf(B)N ceramic nanocomposites without sintering additives. The main findings are as follows: (1) Novel single-source precursors are successfully synthesized via chemical reactions; (2) Additive-free bulk SiHf(B)N ceramics could be fabricated using the PDC route plus warm pressing; (3) Si3N4/X (X = HfN, Hf3N4) ceramic nanocomposites with a homogeneous microstructure can be obtained by further annealing at higher temperatures. Moreover, the correlations between molecular design, compositions, microstructure and properties of the SiHf(B)N ceramic nanocomposites were carefully discussed, which provides new insights into the design and synthesis of metal-compound-modified Si-based ceramic nanocomposites via the PDC approach.