Size effect on hardness for micro-sized and nano-sized YAG transparent ceramics

Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding.     

Fabrication and characterization of sintered TiC-SiC composites

Some TiC-SiC composites with different SiC volume contents (0, 10, 25 and 50%) are prepared by spark plasma sintering (SPS). The relationship between density, grain growth and temperature is studied in order to fabricate dense and nano-sized TiC-SiC composites. A sintering by SPS at 1800 °C during 5 min allowed to form TiC-SiC composites with relative density above 95% and an homogeneous distribution of TiC (grain size from 270 to 900 μm) and nano-sized SiC. With the increasing of SiC volume contents, Vickers hardness and fracture toughness are improved; thermal conductivity at room temperature is increased whereas at high temperature it is reduced. In future studies, those materials will be irradiated to characterize monolithic TiC and TiC-SiC composites behaviour under irradiation.     

Tribological study of Ni matrix composite coatings containing nano and micro SiC particles

Nickel matrix composite coatings containing micro and nano-sized SiC particles were prepared from an additive-free Watts’ type solution under direct and pulse current conditions, in order to study the correlation between SiC particles embedding and the tribological behaviour of deposits. The wear properties of Ni/SiC composite coatings were shown to depend on the type of current, the size of the embedded particles, the weight fraction of codeposited particles, the microstructural modifications induced by codepositing SiC particles and the plating conditions. It was proved that the presence of SiC particles influences the adsorption-desorption phenomena occurring at the metal-catholyte interface during electrocrystallization and, synergically with the plating conditions, modifies the deposits microstructure thus affecting wear properties.     

Synthesis of centimeter-scale monolithic SiC nanofoams and pore size effect on mechanical properties

By pressure infiltrating pre-ceramic polymer polycarbosilane (PCS) into thermally and mechanically stable silica nanofoam, followed by PCS pyrolysis and silica template removal, synthesis of large-scale monolithic SiC nanofoams has been accomplished. Tailoring of the porosity and cell size of the SiC nanofoam has been realized by dissociating the porosity and pore size of the silica nanofoam. Because of the surface hardening and increased surface volume ratio of deformable nanopores, with the same porosity, the decrease of nanopore size has led to an increase in the quasi-static and dynamic indentation resistance for SiC nanofoams.     

Fabrication and characterization of a new-style structure capillary channel in reaction bonded silicon carbide composites

A new-style structure capillary channel was fabricated by using boron carbide powder mixtures with an appropriate multimodal particle size distribution to promote the liquid silicon infiltration in reaction bonded silicon carbide composites. Two types of core–rim structure were observed and the secondary SiC produced in the siliconisation reaction existed in two forms: nucleating on the original SiC and occupying the original positions of the residual silicon. The size of the secondary SiC in the latter form was in a range of tens to hundreds nanometers. These nano-sized SiC grains and the additive of fine boron carbide particles refined the crystalline structure and broke up the residual silicon phase into small pieces. Using this method, the microstructure was refined and the mechanical properties improved significantly. The lowest residual silicon volume fraction was 4.0% and the flexural strength and fracture toughness reached peak values of 526 ± 21 MPa and 6.2 ± 0.4MPa m^1/2, respectively.     

Densification,microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.     

The relationship between microstructure,fracture and abrasive wear in Al2O3/SiC nanocomposites and microcomposites containing 5 and 10% SiC

Alumina/SiC nanocomposites are much more resistant to severe wear than monolithic alumina. In order to clarify the mechanisms responsible for these improvements, alumina and alumina/SiC nanocomposites with 5 and 10 vol.% SiC and various alumina grain sizes were fabricated. For comparison, a 10 vol.% SiC “microcomposite” was also fabricated using 3 μm SiC particles. The extent of cracking beneath hardness indentations was examined and the specimens were tested in abrasive wear. Quantitative surface fractography of the worn surfaces was carried out. The wear properties depended strongly on the grain size in pure alumina, but were independent of the alumina grain size in the nanocomposites. This is consistent with the idea that much of the improvement in wear resistance when SiC is added to alumina stems from a reduction in the size of the individual pullouts owing to the accompanying change in fracture mode. In addition, crack initiation by plastic deformation during abrasion and indentation was found to be strongly inhibited when 10 vol.% nanosized SiC was added to alumina. The addition of 3 μm “micro-sized” SiC did not have the same effect. The ability of fine SiC particles to suppress cracking is attributed to the blocking of twins and dislocation pileups by intragranular SiC nanoparticles. This reduces the length of the twins or pileups and hence their ability to nucleate microcracks.     

Effect of Annealing Environment on the Crack Healing and Mechanical Behavior of Silicon Carbide-Reinforced Alumina Nanocomposites

The crack healing and strength behavior of an alumina-silicon carbide (Al₂O₃-SiC) nanocomposite (Al₂O₃+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al₂O₃ (SiC) nanocomposites will be discussed.     

氧化气氛下热处理对SiC/h-BN复相可加工陶瓷性能的影响

采用热压烧结制备h-BN体积含量分别为30%和40%的可加工SiC/30%h-BN和SiC/40%h-BN复相陶瓷,并在不同温度和氧化气氛下对复相陶瓷样品进行热处理。测试了预制Vickers压痕的复相陶瓷样品在热处理前后的强度和表面硬度,并通过X射线衍射和扫描电子显微镜等研究了复相陶瓷表面成分和显微结构的变化。结果表明:氧化气氛下的热处理可恢复陶瓷在加工时由表面损伤降低的强度,1100℃热处理2h的陶瓷效果最佳。经1100℃热处理2h后,带压痕的SiC/40%h-BN复相陶瓷的强度恢复到389.81MPa,表面硬度从6.11GPa提高10.48GPa。SiC和h-BN的高温氧化行为是强度恢复和表面硬度升高的主要原因。     

Nano- versus macro-hardness of liquid phase sintered SiC

Silicon carbide polycrystalline materials were prepared by liquid phase sintering. Different rare-earth oxides (Y₂O₃, Yb₂O₃, Sm₂O₃) and AlN were used as sintering additives. The final microstructure consists of core–rim structure owing to the incorporation of AlN into the rim of SiC grains by solid solution. Nano- versus macro-hardness of polycrystalline SiC materials were investigated in more details. The nano-hardness of SiC grains was in the range of 32–34 GPa and it depends on the chemical compositions of grains. The harness followed the core–rim chemistry of grains, showing lower values for the rim consisting of SiC–AlN solid solution. The comparison of nano- and macro-hardness showed that nano-hardness is significantly higher, generally by 5–7 GPa. The macro-hardness of tested samples had a larger scatter due to the influence of several factors: hardness of grains (nano-hardness), indentation size effect (ISE), microstructure, porosity, and grain boundary phase. The influence of grain boundary phase on macro-hardness is also discussed.     

Fabrication and characterization of SPS sintered SiC-based ceramic from Y3Si2C2-coated SiC powders

The Y₃Si₂C₂ coating was in-situ synthesized on the surface of SiC powders to form SiC-Y₃Si₂C₂ core-shell structure by using a molten salt technique. Phase diagram calculations on Si-Y-C ternary phase at different temperatures well illustrated that the Y₃Si₂C₂ phase can be stable with SiC but will be in liquid state at 1560?°C. The liquid Y₃Si₂C₂ explained the enhanced consolidation of SiC ceramics and its disappearance after spark plasma sintering. Such Y₃Si₂C₂ coating could not only effectively improve the sintering, but also their mechanical and thermal properties of resultant ceramics. Typically, at 1700?°C, the bulk SiC ceramic presented a mean grain size of 2.5?um and relative density of 99.5% when the molar ratio of Y to SiC is 1:4 in molten salts; the Young’s modulus, indentation hardness and fracture toughness measured by indentation test were 451.7?GPa, 26.3?GPa and 7.9?M?Pam^1/2, respectively; the thermal conductivity is about 145.9?W/(m?K). Excellent thermal and mechanical properties could be associated with the fine grain size, optimized phase composition and improved grain boundary structure.     

镧对碳化硅超细粉合成温度与粒度的影响

众多研究表明,提高反应物起始原料的均匀混合程度、添加适量的添加剂是改善碳热还原法制备碳化硅超细粉体的有效途径。由于稀土元素对众多的化学反应都能起到有效的促进作用,于是,以纳米二氧化硅和活性炭为起始原料,以稀土镧为添加剂,采用碳热还原法制备碳化硅超细粉体,并借助X射线衍射仪和激光粒度仪分别对合成的粉体进行物相和粒度分析,实验结果表明:添加一定量的稀土镧有助于降低碳化硅超细粉的合成温度及粉体粒度。     

Highly Creep-Resistant Silicon Nitride/Silicon Carbide Nano–Nano Composites

A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/silicon carbide composite with a nano–nano structure (nanosized SiC and Si₃N₄ in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si–C–N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nano-sized SiC included in sub-micron Si₃N₄) to nano–nano type. Nanocrystalline silicon nitride/silicon carbide ceramic composite with 30–50 nm grain size was synthesized without using sintering additive.     

Transition metal diboride-silicon carbide-boron carbide ceramics with super-high hardness and strength

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B₄C. Vickers’ hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB₂ and HfB₂ were ~30% and 20% lower than the corresponding TiB₂ containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB₂, SiC and B₄C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called “superhard” ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m^1/2, decreasing to ~0.9 GPa and 4.5 MPa·m^1/2 for a ceramic containing equal volume fractions of the constituents.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Carbide-derived-carbons with hierarchical porosity from a preceramic polymer

Synthesis of carbon by extraction of metals from carbides has been successfully used to produce a variety of micro-porous carbide-derived carbons (CDCs) with narrow pore size distributions and tunable sorption properties. This approach is of limited use when larger mesopores are targeted, however, because the relevant synthesis conditions yield broad pore size distributions. Here we demonstrate the porosity control in the 3-10 nm range by employing preceramic polymer-derived silicon carbonitride (SiCN) precursors. Polymer pyrolysis in the temperature range 600-1400 °C prior to chlorine etching yields disordered or graphitic CDC materials with surface area in the range 800-2400 m² g⁻¹. In the hierarchical pore structure formed by etching SiCN ceramics, the mesopores originate from etching silicon nitride (Si₃N₄) nano-sized crystals or amorphous Si-N domains, while the micropores come from SiC domains. The etching of polymer-derived ceramics allows synthesis of porous materials with a very high specific surface area and a large volume of mesopores with well controlled size, which are suitable for applications as sorbents for proteins or large drug molecules, and supports for metal catalyst nanoparticles.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

Si3N4 with comparable permeability to SiC

Porous sintered reaction-bonded silicon nitrides (SRBSNs) with comparable permeability to SiC were fabricated using presintered Si-additive mixture granules. By increasing the granule strength through the adjustment of the presintering conditions, the strengthened sturdiness of the granules led to an increase in the pore channel size. Porous SRBSNs with ≥60% porosity were achieved without employing a pore former due to the formation of intragranular narrow pore channels as well as intergranular wide pore channels. As the specific pore area of the developed SRBSN is nearly 26 times that of SiC with similar permeability, superior filtering efficiency for nano-sized particulate matter is expected.     

Miniaturization of micrometric SiC from a detonation process of highly energetic material

Submicron- and, in a low quantity nano-sized carbide particles were successfully prepared through a dynamic process of energetic material detonation. For that, a commercial micrometric SiC powder, homogeneously coated by a polymer layer, was dispersed in an energetic 2,4,6-trinitrotoluene (TNT) matrix and then detonated. After detonation, the resulting carbide material was characterized by several techniques such as X-ray diffraction, infrared spectroscopy, electron microscopies, and nitrogen adsorption. The results showed a decrease in the initial micrometric SiC size with particle sizes reaching, in some cases, less than 20 nm. The specific surface area was also considerably increased since an approximate value of 7.8 m² g^− 1 was obtained for a detonated SiC sample as opposed to 0.12 m² g^− 1 for the initial micrometric SiC powder.