Influence of laser surface texturing on the flexural strength of Al2O3 and Si3N4

Laser surface texturing (LST) is well known to be capable of improving the tribological performance and reducing the friction of ceramic surfaces. However, the influence of LST on the flexural strength of ceramics has rarely been researched.In this study, we examine the influence of LST on high purity (> 99.5 wt%) dense-sintered fine Al₂O₃ and hot-pressed fine Si₃N₄ with polished and laser-textured surfaces based on the biaxial ball-on-3-balls (B3B) test. A heat transfer simulation of the LST process is performed to understand the occurrence of residual stress. In addition, the B3B strength of Al₂O₃ and Si₃N₄ texture groups is calculated by adapting the previous formula based on the finite element (FE) simulation. Subsequently, the stress distribution in the FE simulation is used to calculate the effective volume and effective surface to study the size effect on both ceramics. It is found that LST improves the strength of Al₂O₃ and Si₃N₄ due to two reasons: it induces compressive residual stress on the tensile-loaded surface of ceramic specimens; more importantly, it reduces the effective volume and effective surface remarkably, thus improving the component strength significantly.     

A new method for the preparation of ultra-sharp V-notches to measure fracture toughness in ceramics

The preparation of an ultra-sharp V-notch on structural ceramics is a prerequisite for reliable fracture toughness assessment. Femtosecond lasers are used to cut ultra-sharp V-notch on the Si₃N₄ and Al₂O₃ ceramics for fracture toughness testing by single-edge V-notched beam method. The radius of the V-notch tip is smaller than 0.5 μm. The K_Ic of Si₃N₄ and Al₂O₃ ceramics determined by this method is much close to the actual fracture toughness. This method exhibits obvious advantage in good reproducibility, high accuracy and precision for reliable fracture toughness testing of structural ceramics.     

Local Fracture Toughness of Si3N4 Ceramics Measured using Single‐Edge Notched Microcantilever Beam Specimens

Local fracture toughness gives us useful and important information to understand and improve mechanical properties of bulk ceramics. In this study, the local fracture toughness of silicon nitride (Si₃N₄) ceramics was directly measured using single‐edge notched microcantilever beam specimens prepared by the focused ion beam technique. The measured fracture toughness of grain boundary of the Si₃N₄ ceramics is higher than the fracture toughness of SiAlON glass, which exists in the grain boundaries of Si₃N₄ ceramics. It is also shown that the fracture toughness of grain boundary depends on the rare earth oxide added as a sintering aid, which is expected in terms of the difference in the grain‐boundary structure. The fracture toughness of a single β‐Si₃N₄ grains is higher than the grain‐boundary fracture toughness. It was also higher than the value estimated from ab initio calculations and surface energy, which means that any dissipative energy should be included in the fracture toughness of a grain in spite of the brittle fracture in Si₃N₄. The fracture toughness of polycrystals of Si₃N₄ ceramics measured using single‐edge notched microcantilever beam specimens is intermediate between those of grains and grain boundaries, and it agrees with the estimated initial value of the Rcurve, K_I0, in Si₃N₄ ceramics.     

Effect of polystyrene addition on properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics with high strength and high transmittance have been widely used in the field of defense and military. Additive manufacturing (AM) technology is one of the effective means to fabricate porous Si₃N₄ ceramics. Nevertheless, it is difficult to prepare porous Si₃N₄ ceramics by using digital light processing (DLP) because of the large refractive index difference between Si₃N₄ powders and photosensitive resin. In this study, the effects of the amount of polystyrene (PS) powders on the properties of Si₃N₄ ceramic slurries and sintered ceramics were systematically discussed. The addition of PS reduced the overall refractive index of powders and increased the average particle size of powders, thus improving the cure depth of Si₃N₄ ceramic slurries from 11.0 ± 2.0 μm to 55.7 ± 1.8 μm. With the increase of PS content, the shrinkage and porosity of Si₃N₄ ceramics gradually increased, and the bulk density and flexural strength showed the opposite trend. The slurry with low viscosity (2.38 Pa٠s at a shear rate of 30 s⁻¹) and high cure depth (51.2 ± 4.6 μm) was obtained when the content of PS was 15 wt%, which met the thickness requirements for printing. The total porosity of Si₃N₄ ceramics reached the maximum values at 28.21 ± 2.58%. The addition of PS solved the problem of low cure depth of slurries, and PS as a pore-forming agent could help Si₃N₄ ceramics form porous structure. This research provides valuable insights into the fabrication of non-oxide ceramics with high refractive index using DLP technology.     

Stress distribution around Fe5Si3 and its effect on interface status and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics with different amount of Fe₅Si₃ were prepared by adding FeSi₂. Residual thermal stress distribution and elastic energy around Fe₅Si₃ particles in various depths were calculated. The interface status between second phase particles and matrix was analyzed in terms of stress and energy. High tangential compressive stresses and low radial tensile stresses were generated along the surface of the ceramics. Elastic strain energy caused by unit interface was high around big particles in deep area of the ceramics. Microcracks are observed around the big Fe₅Si₃ particles. Furthermore, accord to our calculation, microcracks are easily generated around particles in superficial layer of matrix when second phase particles have lower thermal expansion coefficient than the matrix, while microcracks tend to be generated in deep layer of matrix preferentially when the thermal expansion coefficient of second phase particles is higher than matrix. Residual stresses and microcracks around Fe₅Si₃ particles greatly influenced mechanical properties. Fracture toughness of Si₃N₄ ceramics with similar Si₃N₄ particle size distribution increased with the amount of Fe₅Si₃, and fine Fe₅Si₃ particles could enhance the strength of Si₃N₄ ceramics. Si₃N₄ ceramics exceeding 1.2 GPa strength were prepared.     

Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si₃N₄/SiC_w ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si₃N₄/SiC_w ceramics was 13.5 MPa m^1/2, which was almost 1.6 times that of Si₃N₄/SiC_w composite ceramics, namely 8.5 MPa m^1/2. Moreover, the indentation strength of laminated Si₃N₄/SiC_w ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si₃N₄/SiC_w ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si₃N₄/SiC_w ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.     

Influence of the content of polymethyl methacrylate on the properties of porous Si3N4 ceramics fabricated by digital light processing

Porous Si₃N₄ ceramics are highly regarded as ideal materials for radomes due to their unique characteristics. However, the slurry used for the preparation of porous Si₃N₄ ceramics suffers from a low cure depth, making it challenging to fabricate ceramic components using DLP technology. In this study, porous Si₃N₄ ceramics were prepared by combining DLP technology with pore-forming agent method. The addition of polymethyl methacrylate (PMMA) powders with lower refractive index than that of Si₃N₄ powders can improve the penetration depth of ultraviolet light in the Si₃N₄ slurry. A systematic study was conducted to investigate the influence of the addition of PMMA powders on the properties of Si₃N₄ slurries and porous Si₃N₄ ceramics. When PMMA powders were added at 10 wt%, the slurry with a lowest viscosity of 0.13 Pa s (the shear rate is 30 s⁻¹) and cure depth of 40.0 μm (the exposure energy is 600 mJ/cm²) was obtained. With the increase of PMMA content, porous Si₃N₄ ceramics experienced a gradual decrease in both the flexural strength and bulk density, while the porosity increased from 14.41% to 27.62%. Specifically, when 20 wt% PMMA was added, the resulting porous Si₃N₄ ceramics had a lowest bulk density (2.41 g/cm³), a maximum porosity (27.62%), and a flexural strength (435.87 MPa). The study is of great significance in establishing an experimental foundation for fabricating porous Si₃N₄ ceramics by using DLP technology.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

Experimental investigations on surface grinding of silicon nitride subjected to mono and hybrid nanofluids

The grinding of Si₃N₄ ceramics is difficult due to their inherent hardness, brittleness, and machining-induced damages. The effects of environmentally friendly mono and hybrid nanofluids are investigated on grinding forces, specific grinding energy, grinding force ratio, surface integrity and surface/sub-surface damages during grinding of Si₃N₄ ceramics. The experimental results indicated superior lubrication properties of MoS₂ nanoparticles based nanofluids. The reduction in normal grinding force and specific grinding energy for MoS₂ – WS₂ hybrid nanofluids has been 27% and 39% respectively, compared to deionized water base fluid. The surface roughness and chipping layer depth of silicon nitride workpieces using MoS₂ – WS₂ hybrid nanofluid have been reduced by 41% and 86% in comparison to flood grinding. The thermally stable oxides on ground surface attributed to grindabilty improvement.     

Mechanical-property evaluation of thin corroded surface layers of ceramic materials by the microindentation technique

An instrumented depth-sensitive microindentation technique was applied to evaluate mechanical properties of thin corroded layers of some ceramics which were immersed in boiling, highly condensed sulfuric acid up to 1000 h. Indentation load–depth curves were analyzed using a finite element model to systematically investigate the effect on the ceramics of film or corroded layer. The mechanical properties of both the oxide films and the corroded layers were evaluated through the combination between analytical and experimental results on the indentation curves. It was found that the relationship between thickness T of oxide films or corroded layers and the depth d determined by the slope change on the indentation load/depth–depth curve is described as T≈10d. The bending strengths of Si₃N₄ and Al₂O₃ after immersion were evaluated using the d associated with the corroded layer thickness and the K_IC obtained before immersion. It was confirmed that the strength degradation of Si₃N₄ and Al₂O₃ due to the boiling sulfuric acid corrosion can be estimated using K_IC and d-value.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

Relationship between fracture toughness determined by surface crack in flexure and fracture resistance measured by indentation fracture for silicon nitride ceramics with various microstructures

The reliability of the Vickers indentation fracture (IF) method for various types of silicon nitride (Si₃N₄) ceramics was assessed by comparing the fracture resistance, K_R obtained from the IF test with the fracture toughness, K_Ic from the surface crack in flexure (SCF) technique in the same crack depth region. The K_R of a fine-grained and equiaxed Si₃N₄ matched with the K_Ic from the SCF test when Miyoshi's equation was used, while the K_Ic of a bearing-grade Si₃N₄ was found to lie between K_R values calculated with Niihara's equation (higher side) and Miyoshi's equations (lower side). In the case of coarse Si₃N₄ with elongated grains, the K_R determined using Niihara's equation gave the best fit with K_Ic. The inconsistent outcomes were explained by the probable mechanisms, indicating that the K_R from the IF test cannot be correlated directly with the K_Ic unless the effective crack length for the IF test was clarified.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

In this study, the mechanism of the effect of ZrB₂ on phase transformation of Si₃N₄ at a low temperature and the influence of its content on Si₃N₄-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B₂O₃ and ZrO₂ on ZrB₂ particles, alone cannot contribute to phase transformation of Si₃N₄ at a low temperature. But, the introduction of 0.5 vol% ZrB₂ into Si₃N₄ ceramics can promote the α-β phase transformation of Si₃N₄, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB₂ content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si₃N₄-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB₂ increases to 5 vol%, the Si₃N₄-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si₃N₄-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m^1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB₂.     

Degradation evaluation of Si3N4 ceramic surface layer in contact with molten Al using microcantilever beam specimens

Although Si₃N₄ ceramics are often utilized as structural components in the Al casting industry due to their excellent properties, they occasionally suffer breakage after long-term use. In this study, the bending strength, fracture toughness, and Young’s modulus in the vicinity of the Si₃N₄ ceramic surfaces after contact with molten Al were evaluated using microcantilever beam specimens, which were fabricated using a focused ion beam technique. Fracture testing of the specimens was carried out by nanoindentation. The bending strength of the ceramic surface before and after contact with molten Al was 5.89 ± 1.33 and 3.03 ± 0.28 GPa, respectively. The fracture toughness of the corroded layer in Si₃N₄ ceramics also decreased compared to that of the polished surface. Using fractography by observation with scanning electron microscopy, it was shown that changes in the grain boundary glassy phase resulted in the degradation of strength and fracture toughness.     

Artificial Intelligence-based determination of fracture toughness and bending strength of silicon nitride ceramics

Two mechanical properties, fracture toughness (K_IC) and bending strength (σ), of silicon nitride (Si₃N₄) ceramics were determined from their microstructural images via convolutional neural network (CNN) models. The Si₃N₄ samples used for database were fabricated using various kinds of sintering additives under different process conditions. In total, 330 data sets were prepared and used for building the CNN models for artificial intelligence-bassed determination of the two mechanical properties and testing the determination accuracy of the trained models. The determination coefficients (R²), which were used as accuracy indices, were approximately 0.85 for K_IC and 0.92 for σ. Although both the R² values were relatively high, the lower value for K_IC suggests that it is influenced more by what is little obtained from the microstructural information, such as grain-boundary characteristics. Furthermore, gradient-weighted class activation mapping, which can visualize which parts of the image the CNN models focus on, showed that the trained models determined the two mechanical properties based on correct recognition of the microstructural difference among the images.     

Toughening of laminated ZrB2-SiC ceramics with residual surface compression

Laminated ZrB₂-SiC ceramics with residual surface compression were prepared by stacking layers with different SiC contents. The maximum apparent fracture toughness of these laminated ZrB₂-SiC ceramics was 10.4 MPam^1/2, which was much higher than that of monolithic ZrB₂-SiC ceramics. The theoretical predictions showed that the apparent fracture toughness was strongly dependent on the position of the notch tip, which was confirmed by the SENB tests. Moreover, laminated ceramics showed a higher fracture load when the notch tip located in the compressive layer, whereas showed a lower fracture load as the notch tip within the tensile layer. The toughening effect of residual compressive stresses was verified by the appearance of crack deflection and pop-in event. The influence of geometrical parameters on the apparent fracture toughness and residual stresses was analyzed. The results of theoretical calculation indicated that the highest residual compressive stress did not correspond to the highest apparent fracture toughness.