A comparative study of silicon nitride and SiAlON ceramics against E. coli

In recent decades, due to some limitations from alumina (Al₂O₃) and zirconia (ZrO₂), silicon nitride (Si₃N₄) has been investigated as a novel bioceramic material, mainly in situations where a bone replacement is required. Si₃N₄ ceramics and its derivative form, SiAlON, possess advantages in orthopedics due to their mechanical properties and biologically acceptable chemistry, which accelerates bone repair. However, biological applications require additional properties, enabling stronger chemical bonding to the surrounding tissue for better fixation and the prevention of bacteria biofilm formation. Therefore, two commercial Si₃N₄ and SiAlON ceramics were investigated in this study and compared to each other according to their material properties (like wetting angles and surface chemistry) and their antibacterial behaviors using E. coli. Results provided evidence of a 15% reduction in E. coli colonization after just 24 h on Si₃N₄ compared to SiAlON which is impressive considering no antibiotics were used. Further, a mechanism of action is provided. In this manner, this study provides evidence that Si₃N₄ should be further studied for a wide range of antibacterial orthopedic, or other suitable biomaterial applications.     

Progress in SiAlON ceramics

In view of the considerable progress that has been made over the last several years on the fundamental understanding of phase relationships, microstructural design, and tailoring of properties for specific applications of rare-earth doped SiAlONs, a clear review of current understanding of the basic regularities lying behind the processes that take place during sintering of SiAlONs is timely. Alternative secondary phase development, mechanism and full reversibility of the α′ to β′ transformation in relation with the phase assemblage evolution are elucidated. Reaction sintering of multicomponent SiAlONs is considered with regard of wetting behavior of silicate liquid phases formed on heating. Regularities of SiAlON's behavior and stability are tentatively explained in terms of RE element ionic radii and acid/base reaction principle.     

Scratch behavior of SiAlON ceramics

The mechanical response of four different types of hot pressed yttrium stabilized α/β composite SiAlON ceramics was investigated by scratch testing in order to interpret their severe wear behavior. A progressive loading scratch tester with both rough and smooth spherical diamond styluses was used to analyze the variation in mechanical response with roughness of stylus. The extent of subsurface damage produced by the different styluses on a 50% β-content composite was also investigated using a visualization technique with plasma etching, which has been previously developed by one of the authors. When scratched by the rough stylus, quasi-plastic deformation recognized in the form of grain release after plasma etching was present predominantly in the surface, and the use of this stylus was regarded to be more appropriate for comparing the results of scratch tests with those of severe wear due to similarity of crack formation between grain dislodgement in severe wear and subsurface damage in the scratch tests. The work required to produce damage and groove formation (W_dg) when scratched by the rough stylus was evaluated for all test materials and it was found that the worn volume under severe wear follows the reverse order of W_dg.     

Spark plasma sintering of SiAlON ceramics

Investigated was the spark plasma sintering (SPS) of sialon ceramics from SHS-produced powders. Experimentally established were (a) sintering temperatures that ensure a required density, phase composition, and microstructure of sintered multicomponent sialon ceramics, (b) individual stages of the SPS process, and (c) the effect of starting powder composition on the phase composition and microstructure of sintered sialon ceramics.      

Molecular-level composition design for efficient synthesis of SiAlON ceramics

SiAlONs are a class of liquid-phase sintered ceramics with excellent room-temperature strength and toughness, but whose residual grain boundary glass softens at high temperatures, limiting use in extreme environments. For this reason, efforts are made to minimize the volume of the grain boundary glass while still facilitating full densification. This work describes a potential route for the densification of SiAlONs with very low concentrations of liquid-phase sintering additive (e.g., rare-earth oxides such as yttria) by using an organometallic precursor. Solid solution of Al and O in the Si₃N₄ lattice was accomplished through the incorporation of solute atoms via liquid organic precursor aluminum sec-butoxide (ASB). Al₂O₃ powder is conventionally used for this purpose, and the subsequent lattice softening associated with the solid solution helps to facilitate densification. However, a liquid-phase additive is still essential for the full densification of SiAlONs. Higher densities were obtained from SiAlON powder blends utilizing organometallic ASB than those utilizing alumina powder, allowing for greater densification at very low Y₂O₃ concentrations. The thermal decomposition of the organic precursor was investigated by high-temperature scanning electron microscopy, thermogravimetric analysis, and various X-ray diffraction experiments. Immersion density measurements and lattice parameter refinements were performed for samples sintered with varying Y₂O₃ concentrations and/or dwell times. Results indicate that ASB-containing powder blends favor SiAlON formation more strongly than Al₂O₃-containing powder blends and favor densification at very low Y₂O₃ concentration.     

稀土氧化物在SiAlON陶瓷材料中的应用研究进展

简要论述了稀土氧化物在SiAlON陶瓷材料中的应用研究概况;介绍了Y_2O_3、Sm_2O-3、Dy_2O_3和Nd_2O_3等单一稀土氧化物以及复合掺杂稀土氧化物在SiAlON陶瓷材料中的应用进展,并比较了它们对SiAlON陶瓷材料性能的影响;最后对稀土氧化物在SiAlON陶瓷材料中的应用前景作了扼要的分析和展望。     

Role of organic additives on non-aqueous tape casting of SiAlON ceramics

Suspensions consisting of precursor α/β SiAlON forming powders, azeotropic solvent mixture of 60 MEK/40E, dispersant, binder, and plasticizer were optimized for tape casting by rheological measurements and tape properties. Sodium tripolyphosphate (STPP) was introduced as a dispersant for low temperature applications of α/β-SiAlONs. Optimum STPP amount was determined as 0.012 g/m² (of the particle surface) for stable α/β-SiAlON suspensions. Different amounts of binder/plasticizer mixtures were added to the slurries and the effects on rheological and green tape properties were investigated. Green tapes with dibutyl phthalate (DBP), and plasticizer mixture, poly(ethylene glycol) (PEG) and DBP, exhibited centered cracks with high plasticity, on the other hand, polyvinyl butral (PVB) and PEG showed no crack but low plasticity. Therefore, many different parameters were found to be effective on final tape properties. In addition, tapes were prepared with 6 vol% PVB + PEG, sintered at 1800 °C for 2 h and exhibited almost 97%TD in room temperature applications of α/β SiAlONs.     

Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

An attempt was made to prepare various F-doped β-, O-, X-, and -SiAlONs from a mixture of Si₃N₄, SiO₂, Al₂O₃, AlN, or Y₂O₃ using AlF₃ or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and -SiAlONs by X-ray diffraction. When AlF₃ was used, a single phase ceramic (O-SiAlON) was produced from a mixture of -Si₃N₄ and SiO₂ at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of -Si₃N₄, AlN, and Y₂O₃ with AlF₃ produced β-/Y--SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of -Si₃N₄ and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from -Si₃N₄ and SiO₂ at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y--SiAlON mixture showed the growth of O- and β-SiAlON and Y--SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.     

Understanding the mechanical properties of hot pressed Ba-doped S-phase SiAlON ceramics

In this contribution, we report the analysis and interpretation of the mechanical property measurements for a new class of SiAlON ceramic. The hardness and indentation fracture toughness were measured on the hot pressed Ba-doped S-SiAlON ceramic using Vickers indentation at varying loads (up to 300 N). An important observation was that all the investigated S-SiAlON exhibited the characteristic rising R-curve behavior with a maximum toughness of up to 10–12 MPa m^1/2 for ceramics, hot pressed both at 1700 and 1750 °C. Crack deflection by large elongated S-phase grains and crack bridging by β-Si₃N₄ needles has been found to be the major toughening mechanisms for the observed high toughness. Theoretical estimates, using a toughening model based on crack bridging and deflection by platelet shaped ‘S’-phase grains and β-Si₃N₄ needles, reveal the interfacial friction of around 200 MPa. Careful analysis of the indentation data reveals the average (apparent) hardness modestly increases with indent load in all S-SiAlON samples, with more significant effect for S-SiAlON, hot pressed at 1600 °C. This effect has been analyzed in the light of the established model of ‘indentation-induced cracking’ phenomenon. Our experimental results suggest that a modest combination of average hardness of 15 GPa and indentation toughness of around 12 MPa m^1/2 could be achieved in Ba-S-SiAlON ceramic and further improvement requires microstructural tailoring.     

Effect of SiC addition on the thermal diffusivity of SiAlON ceramics

Despite the fact that thermal conductivity is a crucial parameter for SiAlON ceramics with respect to their suitability in various applications, including high-temperature structural components, wear parts, and cutting tools, studies on SiAlON ceramics reported thus far mainly focus on the improvement of their mechanical properties. In view of the lack of sufficient studies on the thermal conductivity of SiAlON ceramics, this study investigates the improvement in the thermal diffusivity behaviour of SiAlON ceramics by the addition of highly conductive SiC particles. As solid-solution SiAlON ceramics exhibit complex crystal structures typically composed of defects, the phonon scattering increases, subsequently decreasing diffusivity. In particular, the improvement in the thermal diffusivity of both α- and β-SiAlONs was investigated by the addition of 0.25 wt% SiC. In addition, the effect of the SiC particle size on the thermal diffusivity of β-SiAlON was examined. Using inverse diffusivity data, intrinsic and extrinsic scattering parameters were determined, and compared to intrinsic scattering, extrinsic scattering was a dominant factor. Furthermore, transmission electron microscopy (TEM) images of SiC_p-reinforced α and -β-SiAlON ceramics were recorded to examine the SiC particle distribution.     

Ultra-precision finishing of low expansion ceramics by compliant abrasive technologies: A comparative study

Advanced ceramics have many attractive features such as high stability and wear resistance that find broad applications in various fields, e.g. optics, aerospace, etc. However, the accompanying difficult-to-machine property with complex geometry brings great challenges to the commonly used laser machining and rigid wheel based grinding in industry. To achieve optical surface quality with surface roughness below 10?nm Ra, three promising ultra-precision compliant machining technologies using adaptive elastic tools are presented in this paper, including bonnet polishing, compliant pitch polishing and shape adaptive grinding with fine grain size. A comparative study was conducted by machining three different low thermal expansion ceramics while continuously increasing attack angle, spindle speed and tool offset across rectangular regions. Material removal rate (MRR) and surface roughness (Ra) with respect to different process conditions are compared. With sufficient data, the processing ability using above three compliant machining technologies is summarized based on the MRR-Ra plots for different ceramics. In addition, microscopic observation and X-ray diffraction analysis are conducted to characterize differences in material behavior.     

SiAlON ceramics from preceramic polymers and nano-sized fillers: Application in ceramic joining

Commercial polysiloxanes filled with alumina nano-particles have been employed for the preparation of β-SiAlON-based ceramics in the temperature range 1450–1550 °C in nitrogen atmosphere. The formation of β-SiAlON was found to be preceded by the formation of intermediate alumino-silicate phases. The SiAlON yield was affected by the occurrence of phase separation in the oxycarbide ceramic residue (SiOC) derived from the silicones and by the partial vaporization of silica, by reduction into gaseous SiO, leading to products with an oxide contamination, consisting of corundum. Filled silicones finally found a promising application in the ceramic joining, sandwiched between two pre-existing α–β (Yb-)SiAlON pieces and treated at high temperature (1550 °C): with a proper formulation, a significant inter-diffusion was observed between the joining layer and the SiAlON parts, causing the evolution of a homogeneous joint region, matching the microstructure and the mechanical properties of the parent ceramics. The pre-oxidation of the SiAlON, generally aiding the wetting of the joining media prior to thermal treatment, showed no significant benefit on the microstructure. On the contrary, the addition of a small load during the thermal treatment allowed the formation of strong joints, not exhibiting any significant difference in mechanical properties with the parent material.     

A comparative study on in vitro behavior of calcium silicate ceramics synthesized from biowaste resources

Calcium silicate ceramics have received significant attention for biomedical applications for their excellent bioactivity and osteoconduction properties. Sol-gel process is extensively used for the fabrication of calcium silicates. In sol-gel process, calcium nitrate tetra hydrate (Ca(NO₃)₂·4H₂O) and tetraethylorthosilicate (TEOS) are used as precursors. However, these precursors are expensive. The objective of this work was to compare in vitro behavior of calcium silicate (CaSiO₃) produced using biowaste such as rice husk ash (RHA) and eggshells (coded; NCS) with CaSiO₃ prepared using TEOS and Ca(NO₃)₂·4H₂O (coded; CCS). Thermal investigation results revealed that the crystallization temperature for NCS is relatively lower (772°C) than for CCS (870°C). Bioactivity was studied in vitro using simulated body fluid (SBF) with respect to mineralization rate of hydroxyapatite. Mineralization of a greater hydroxyapatite was observed on NCS ceramics than CCS ceramics after incubation for 3, 7, 14 days in SBF solution, which was confirmed using X-ray diffractometer, Fourier transform infrared spectroscopy, scanning electron microscopy-energy dispersive spectroscopy. Degradation studies were conducted in Tris-HCl solution and the test results revealed that NCS ceramics has lower dissolution rate than CCS ceramics. The antimicrobial assay has shown that NCS samples exhibit significant zone of inhibition against Escherichia coli and Staphylococcus aureus which confirmed that the CaSiO₃ prepared from RHA and eggshell can prevent bacteria from adhering to the surface. In addition cell culture studies revealed that NCS ceramics possess good cytocompatibility with MG-63 cells and significantly promoted cell proliferation.     

Relationship between color changes and ternary eutectic reaction in Yb/Y-co-doped SiAlON ceramics

This study investigated the relationship between the green/brown colors and Yb/Y-co-doped ternary eutectic reactions. A color difference was initially observed at 1600 °C. The green color indicates a relatively highly porous area compared to the brown color, and it was confirmed that the green color was spherical in all samples. This difference in color was analyzed through photoluminescence (PL), and it was confirmed that the reduction of Yb²⁺ increased at temperatures higher than 1550 °C owing to the reducing atmosphere generated by the carbon ambient. The reaction sequence of the phase formation of SiAlON exhibited a difference resulting from the reduction in Yb ions, and the sequence of phase formation improved with an increase in the content of Y substituted for Yb.     

Thermal shock resistance of rare-earth doped in-situ SiAlON reinforced h-BN matrix ceramics under vacuum thermal cycling

In-situ SiAlON reinforced BN-matrix ceramics were prepared by hot pressing sintering, and the effects of different rare earth oxides on the thermal shock resistance of the materials were investigated. The effects of rare earth oxides on the phase composition, microstructure, bending strength, thermal properties and thermal shock resistance of the composites were studied. The results show that the phase composition and bending strength of ceramics with different rare earth oxides had no obvious change. However, the influence on the thermal expansion coefficient of the material was notable. The thermal expansion coefficient of the ceramics with CeO₂ increased by 24.6% compared with Sm₂O₃ in the test temperature range. After 50 cycles of thermal shock at Δt = 1150 °C, the residual strength of ceramics with CeO₂ was down to 157.1 MPa, decreased by 40.6% compared with the one tested in room temperature. And the Sm₂O₃-added ceramics reduced by 34.7%–167.1 MPa after thermal shock. The decrease of the residual strength of ceramics is mainly caused by the internal stress generated by the mismatch between the growth of quartz and SiAlON phase in the matrix and the thermal expansion coefficient of the matrix. However, no macro cracks were observed on the surface of the samples after thermal shock.     

The effect of the wet-milling process on sintering temperature and the amount of additive of SiAlON ceramics

In this study, nano-sized SiAlON powders were produced by wet milling at elevated speeds as a top-to-bottom process. Before the milling process, different milling times and mediums were performed for the determination of the most efficient milling system. The milled powders were characterized by BET and X-ray diffraction (XRD) measurements and the results were compared to standard samples. The standard powders were produced using a conventional process (the ball to powder ratio was 1:1.5, at 300 rpm, for 1.5 h) having a few hundred nanometer particle size. The nano powders were milled using a wet-milling process in an optimum medium so that the particle size was decreased down to ≈70 nm. The samples, produced from the nano powders, were densified at 150 °C lower degrees than the sintering temperature of samples which were produced by a conventional method (185 nm). However, the phase transformation of α → β-SiAlON was also observed related to the amount of additives. This transformation affected the mechanical properties of the SiAlON ceramic. The results were discussed using the relationship between density, phase composition, microstructure and mechanical properties.     

Kinetics of phase transformations in SiAlON ceramics: I. effects of cation size,composition and temperature

Kinetics of direct α/β-Si₃N₄→α-SiAlON transformations, reverse α′-SiAlON→β′-SiAlON transformation, and the formation of intermediate phases were investigated for SiAlON ceramics with rare earth stabilizing cations. It was determined that smaller cations (Yb), and α-Si₃N₄ starting powders lead to faster Si₃N₄→α′-SiAlON transformations. Using the knowledge of phase stability of α′-SiAlON, these observations have been correlated to the overall driving force for the transformation, and the correlation is further extended to the reverse α′-SiAlON→β′-SiAlON transformation. Formation of intermediate phases during Si₃N₄→α′-SiAlON transformation is shown to depend on the α′-SiAlON formation and to be inversely correlated to the stability of α′-SiAlON. A thermodynamic interpretation thus emerges to account for the kinetics of SiAlON transformation that encompass the effects of the size of the stabilizing cations, type of starting powder, overall composition, and the reaction temperature.     

Superplastic Forming of SiAlON Ceramics

Superplastic SiAlON's of the nominal composition Y _{m /3}-Si_{12-( m + n )}AL _{m + n} O _n N_{16– n} are reported in this study using a transient-phase-forming approach. They encompass the fields of single-phase α'-SiAlON, single-phase β'-SiAlON, and their two-phase mixtures. Excellent formability is obtained at 1550°C for the β'and α'+β'materials, and at 1600°C for the α'material. Typically in the nonequilibrium state before deformation, these fine-grained materials undergo dramatic phase and microstructure evolutions during superplastic forming. In particular, the stress-biased α-Si₃N₄→β'-SiAlON reaction is found to result in elongated and aligned grains with fiber-strengthening effect and excellent formability.     

From high-entropy ceramics to compositionally-complex ceramics: A case study of fluorite oxides

Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf_1/3Zr_1/3Ce_1/3)_1-x(Y_1/2X_1/2)_xO_2-δ (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young’s modulus (∼210 GPa) and nanohardness (∼18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.