The influence of fillers such as Si3N4, SiC, Al2O3, TiN, Zirconium-based fillers, cBN, and hBN on α-SiAlON, β-SiAlON, and O-SiAlON composites reveals similarities in their effects across the different structures (Table 2). These fillers generally enhance mechanical properties, including strength, hardness, wear resistance, and thermal stability, regardless of the SiAlON phase. For example, Si3N4 contributes to the formation of SiAlON phases and improves mechanical strength [115, 116], while SiC reinforces the composites and enhances hardness and wear resistance [117, 118]. Al2O3 improves mechanical properties and wear resistance in all composites [111, 119], while TiN improves hardness and wear resistance. [120, 121] Both cBN and hBN enhance lubricity, reduce friction, and act as solid lubricants [38, 122, 123]. However, specific variations may exist depending on the composite’s phase and composition. The choice between nano- and micro-sized fillers depends on specific application requirements and processing considerations, with nano-sized fillers typically offering superior performance in mechanical and thermal properties, while micro-sized fillers provide better control over microstructural features and processing parameters [118, 124, 125]. In general, integrating these fillers enables customization of SiAlON composites’ properties to suit diverse industrial applications, as elaborated in subsequent sections.
The structural and behavioral attributes of SiAlON composite ceramics hinge primarily upon the relative proportions of constituent elements and the employed processing methodologies [64]. Ceramic inclusions significantly alter the microstructural features and properties of resulting composites. The microstructure of β-SiAlON significantly governs its properties, with Si3N4 fillers exerting a pivotal role in shaping it. Introduction of β-Si3N4 into the precursor mix intricately alters the resultant β-SiAlON’s morphology (Fig. 3a-f). For instance, augmenting the molar percentage of β-Si3N4 can transition the structure from fiber-like to rod-like, thereby influencing properties like grain growth, surface area, and oxidation resistance [22, 23].
The optimal molar ratio of β-Si3N4 plays a critical role. At approximately 29.9 mol%, enhanced grain growth and reduced surface area contribute to superior resistance to oxidation. However, excessive Si3N4 content may introduce trade-offs such as diminished mechanical properties or thermodynamic instability in oxidizing environments. Thus, meticulous control over Si3N4 fillers allows tailoring of properties for specific applications, rendering β-SiAlON composites invaluable in high-temperature settings. The incorporation of Si3N4 into SiAlON composites yields enhancements in compressive strength, density, and reduced porosity (Fig. 3g & h). Additionally, it markedly improves mechanical, solar absorptance, and thermal properties [22, 129].
In a research by Li et al. [22], Si3N4 was incorporated into SiAlON and the primary crystal phase was revealed to be β-Si3N4. It was described that the addition of Si3N4 caused phase transformation from α-SiAlON to β-SiAlON. The presence of Si3N4 boosted the densification of the composite, the bulk density of the material and compressive strength increased while its porosity decreased. The authors observed optimum properties for the developed SiAlON-Si3N4 ceramic composite at 30 wt% β-SiAlON content. Furthermore, the embedding of Si3N4 into SiAlON also leads to the discharge of ammonia nitrogen in deionized water, which has a bacteriostatic effect that inhibits the proliferation of bacteria.
Bake et al. [35] investigated the influence of nano-and micron-sized Si3N4 particles on the phase characteristics and properties of SPSed β-SiAlON-based composites fabricated at temperatures between 1400 °C and 1700 °C. They realized that the use of amorphous, nanosized Si3N4 as a precursor in the synthesis process accelerates the reaction kinetics, resulting in higher values of Vickers hardness and fracture toughness. A similar report was presented by Lv et al. [64] while studying the growth mechanism in 1D β-SiAlON nanostructures alongside β-SiAlON-Si3N4 composites fabricated by the reduction nitridation process. However, when micro-sized Si3N4 was used as a precursor, the resulting materials displayed slightly lower hardness and fracture toughness values. Nevertheless, they are still comparable to fully dense β′-SiAlONs, which have been sintered using different techniques at substantially greater temperatures for a longer time [35]. Consequently, samples made from nanoscale, amorphous Si3N4 exhibited hardness values between 13.4 and 13.8 GPa and a fracture toughness range of 3.5 to 4.6 MPa m1/2. In comparison, composites made from microscale β-Si3N4 displayed hardness values between 13.9 and 14.4 GPa and a fracture toughness range of 4.3 to 4.5 MPa m1/2.
Furthermore, the study carried out by Wu et al. [129] on in-situ β-Sialon/Si3N4 ceramic composite prepared at 1580 °C by means of pressureless sintering using Y2O3, calcined bauxite, AlN, α- Si3N4, and La2O3 as sintering additives, showed that the developed composite possesses good mechanical properties, high thermal conductivity, and solar absorptance, alongside excellent thermal shock resistance and oxidation resistance. The composite also exhibits excellent solar heat absorption properties, making it a prospective candidate for use as a solar heat-absorbing material. From the reviewed works, it was generally established that the presence of Si3N4 in SiAlON-based composite considerably enhanced the mechanical and thermal properties of the materials, presenting them as attractive for a widespread variety of potential applications.
Zirconium-based fillers, such as ZrN, play a pivotal role in shaping the composition of β-SiAlON composites. Zirconium augments the stability of the composite and significantly contributes to its high-temperature performance. The formation of a solid solution by incorporating zirconium into the crystal lattice of β-Si3N4 results in enhanced mechanical strength and chemical stability.
The microstructure of β-SiAlON undergoes significant alterations owing to the presence of zirconium-based fillers. During sintering, zirconia (ZrO2) reacts with silicon nitride (Si3N4) and aluminum nitride (AlN), giving rise to zirconium nitride (ZrN). These ZrN particles contribute to the composite’s microstructure, influencing properties such as grain size, porosity, and phase distribution. Precise control over heating temperature and zirconium content, along with the addition of carbon black, enables tailored microstructure development in the synthesized composites.
The optimal quantity of zirconium-based fillers is crucial. Incremental zirconium content enhances nuclear properties, diminishes the coefficient of thermal expansion, and improves mechanical characteristics. However, excessive zirconium incorporation may lead to trade-offs such as reduced mechanical strength or thermodynamic instability [52]. For instance, Yin and Jones [4] studied the properties of SiAlON-ZrN composites developed by a two-step sintering method. The authors discovered that composites sintered under higher pressure displayed better densification and enhanced mechanical properties, with composite reinforced with 20% ZrN exhibiting the maximum hardness of 16.16 GPa. However, a decrease in hardness was observed at ZrN content higher than 20% due to the weak interfacial bond between the SiAlON matrix and ZrN particles. In these cases, the fracture toughness of the composite may increase, reaching a maximum of 5.41 MPa·m1/2 with 40% ZrN. However, the electrical properties of the sintered composites remain unaffected regardless of the adopted sintering techniques. The observed properties of the developed composites were ascribed to the smaller grain size combine with homogeneous microstructure inherited from the processing method since the higher sintering temperatures in the second step can lead to higher relative density, more negligible open porosity, and significantly increased hardness and fracture toughness of the composite.
Yin and Jones [4] and Ma et al. [54] established that the sintering temperature employed affects the structure, composition and properties of the fabricated composite. Ma et al. [54] claim was based on the outcome of their work on SiAlON-ZrN/ZrON composite ceramics using a pressureless sintering technique at temperatures between 1450 °C and 1550 °C. Their study disclosed that increasing the sintering temperature improves the sintering properties and oxidation resistance of the composite. Dense SiAlON-ZrN/ZrON was achieved at a sintering temperature of 1550 °C for 1 hour. This sample displayed excellent oxidation resistance at 900 °C for 6 hours. The microstructure of the developed composites (Fig. 4a-f) typically consists of ZrN, β-Sialon, and ZrON, with evenly dispersed ZrN combined with ZrON particles on the β-Sialon matrix.
Zhang et al. [82] also observed that introducing ZrO2 into the β-SiAlON matrix improves the physicochemical and biological properties of the resulting composite ceramics. In addition, they reported that incorporating a small amount of ZrO2 into the β-SiAlON matrix promotes good adhesion and growth of osteoblast cells on the ceramic surface. Besides, it improves the density, elastic modulus, and flexural strength (Fig. 4g and h). However, it was observed that increasing ZrO2 contents reduces the porosity of β-SiAlON (Fig. 4g), and excessive amounts of ZrO2 can have an adverse effect on the cells’ wound-healing ability. Hmelov [130] synthesized and studied the properties of SPSed mullite-SiAlON-ZrB2 materials. The authors observed that increasing the SiAlON concentration impaired sinterability and worsened the sintered materials’ physicochemical characteristics at temperatures between 1200 °C and 1600 °C.
Therefore, judicious integration of zirconium-based fillers in β-SiAlON composites facilitates customization of properties for specific applications, rendering them indispensable in high-temperature environments.
Boron nitride (BN) has proven to enhance the structure and properties of β-SiAlON composite materials when used as a filler. Cubic boron nitride (c-BN) alongside hexagonal boron nitride (h-BN) are both commonly used in the production of β-SiAlON composites and have been shown to have varying influences on the structure alongside the composite material’s properties [46, 88, 92, 96, 97, 131,132,133,134,135]. The microstructure of β-SiAlON undergoes significant transformations owing to the presence of BN fillers. Fine BN powder leads to improved densification and mechanical properties compared to larger BN particles. The interaction between BN and SiAlON influences grain size, porosity, and phase distribution. Additionally, thermal properties such as thermal conductivity are enhanced in both BN powder-added β-SiAlON composites compared to pristine β-SiAlON. Also, the ability to increase the friction alongside wear resistance of the composite material is one of the main advantages of BN incorporation in β-SiAlON. Yin et al. [131] exhibited that the inclusion of h-BN to SiAlON results in a composite material with a lower friction coefficient and improved lubrication properties. This translates to better friction and wear resistance compared to monolithic SiAlON.
The integration of BN into the composite material has also been revealed to improve the machinability alongside the resistance of β-SiAlON to thermal shock. h-BN, in particular, has a significant impact on these properties. According to Tian et al. [92], the addition of h-BN to SiAlON results in in-situ formed BN-SiO2, which immensely contributes to the improved resistance of β-SiAlON composite materials to plasma erosion. The authors further reported that when c-BN is used as a filler in SiAlON, it impacts the improved erosion resistance of β-SiAlON/c-BN ceramic composites.
The incorporation of BN in β-SiAlON composite materials (Fig. 5a) has also been shown to impact the composite’s densification and mechanical characteristics. Ye et al. [47] prepared cBN/β-SiAlON composites by the SPS process at 1550 °C, 50 MPa for 5 min. It was found that the sintered composites exhibited improved density (Fig. 5b), Vickers hardness, fracture toughness, coupled with the flexural strength compared to pure β-SiAlON. This was attributable to the high density, lack of transformation from cBN to h-BN, and strong interfacial bonds between the cBN grains and the β-SiAlON matrix. Conversely, Wu et al. [99] established that the addition of cBN to β- SiAlON impaired the densification alongside mechanical characteristics of the developed composite materials as the aluminum and oxygen content increased. This effect was observed to be more pronounced with higher z values, which represent the amount of Al atoms that Si atoms in the β-Sialon chemical composition have replaced. Barick and Saha [136] also examined the effects of h-BN incorporation alongside its varied particle sizes on the properties of β-SiAlON ceramic fabricated via the pressureless sintering method at 1800 °C for 4 hours. It was established that the incorporation of h-BN into β-SiAlON ceramic significantly influences the mechanical, microstructure, density, dielectric, and thermal properties of the composite material. The bulk density of β-SiAlON obtained drop from 3.22 g/cm3 for pure β-SiAlON to 3.05 g/cm3 and 2.90 g/cm3 for small and large particle sizes, respectively, when the BN content of the initial composition was increased by up to 5 wt%. Pure β-SiAlON was also shown to have greater Young’s modulus, fracture toughness, Vickers hardness, and flexural strength compared to any of the developed composite ceramic materials. Furthermore, improved thermal properties with relatively low dielectric constant were observed in the fabricated β-SiAlON/h-BN composite ceramics than the pure β-SiAlON. Li et al. [27] also reported that adding BN into the β-SiAlON/BN composite improved the flexural strength of the composite material, and consequently obtaining highest flexural strength with 20 wt% BN dosage.
The use of the precursor infiltration and pyrolysis (PIP) process in combination with pressureless sintering was also found to greatly increase the resistance of the composites made of β-SiAlON and h-BN to thermal shock while only slightly decreasing the strength and hardness of the materials. This makes the composite a promising material for high-temperature environments [71, 72]. According to Li et al. [71], adding h-BN to β-SiAlON via a PIP route and pressureless sintered at 1750 °C was found to enhance the machinability and thermal shock resistance of the composite material. In this case, the h-BN particles were observed to be nanoflake-like in shape and distributed homogeneously throughout the β-SiAlON matrix. In a similar work by the authors [72], high strength, remarkable thermal shock resistance and density comparable to that of the pressureless sintered β-SiAlON were obtained in β-SiAlON/h-BN prepared through PIP combined with pressureless sintering.
Overall, the introduction of BN to β-SiAlON composite materials has been shown to have several beneficial effects on the composition and characteristics, including improved friction and wear resistance, machinability, thermal shock resistance, plasma erosion resistance, and mechanical attributes of the developed composite materials. The specific impacts of BN on the structure and properties of SiAlON composite materials will depend on the type, characteristics and concentration of BN utilized, as well as the processing conditions and methods employed to produce the composite material.
Titanium-based fillers, such as TiCN, TiO2, and TiN, have significantly improved the properties of SiAlON composites when added in small amounts [53, 137,138,139,140]. Moreover, the microstructure of β-SiAlON is notably influenced by the presence of TiN fillers. TiN particles interact with SiAlON, impacting grain size, porosity, and phase distribution. Proper control of sintering parameters allows for tailored microstructures. Typically, TiN inclusions are found at grain boundaries but may not form a continuous network within the SiAlON matrix. For instance, β-SiAlON/TiC0.3N0.7 composite ceramic comprising 2.0–7.5 wt.% TiC0.3N0.7 and Al2O3–AlN–Y2O3 additives were prepared using pressureless SPS at 1550 °C by Li et al. [53]. The authors observed that the produced composites exhibited homogeneously dispersed TiC0.3N0.7 particles within the closely packed β-SiAlON grains, which culminated in the attainment of high relative density, Vickers hardness alongside fracture toughness of 98%, 14.53 GPa and 7.20 MPa m1/2, respectively, in composite reinforced with 5 wt% TiC0.3N0.7. Fully densified β-SiAlON/5 wt% TiC0.3N0.7 composite with improved Vickers hardness (15.87 GPa) alongside fracture toughness (7.44 MPa m1/2) was reported by further SPS at 1400 °C and 24 MPa. The observed properties improvement of the pressureless SPS compared to the pressurized SPS was ascribed to the absence of non-uniform, uniaxial pressure exerted by the punches during the normal pressurized SPS. Yurdakul et al. [141] examined the electrical properties of α-β SiAlON granules coated with TiCN powder (Fig. 6a & c). Subsequently, they were sintered using the gas pressure sintering (GPS) technique at 1990 °C and 10 MPa nitrogen gas pressure. This process resulted in the creation of a continuous 3D network that conducts electricity. It was reported that the TiCN grains did not retain their original composition after sintering. Also, the migration of Ti from TiCN grains into the α-β-SiAlON and triple junction phases was revealed by the analysis of the Ti:C:N ratios and TEM imaging of the α-β SiAlON/TiCN composite. It was established that the high electrical conductivity of the α-β SiAlON/TiCN composite was due to the presence of Ti in the SiAlON crystal lattice. Canarslan et al. [1] examined the dielectric properties of the initial powders and the characteristics of the microwave sintered α-β SiAlON composite reinforced with 17 wt%TiN at 1300 °C for 30 mins. The authors found that incorporating TiN into α-β SiAlON composites resulted in higher peak sintering temperatures and caused increased microwave absorption characteristics. This led to a decrease in the α:β ratio and an improvement in the mechanical properties, particularly the fracture toughness of the composites (Fig. 6c). Acikbas et al. [142] studied the microstructure, densification, mechanical properties and thermal shock resistance of α:β-SiAlON reinforced with 10, 17, and 25 wt%TiN and sintered by a two-step gas pressure sintering cycle. Enhanced microstructure and fracture toughness were observed in the developed composites due to the presence of TiN in the matrix. Maximum fracture toughness was achieved for composite reinforced with 25wt% TiN. An investigation of the tribological properties of the developed composites was conducted in another study by Acikbas [84]. The significant increase in the wear rate of the composite reinforced with 17 wt% TiN was traced to the low fracture toughness of the sample.
Nekouee and Khosroshahi [49] employed the SPS method to produce fully dense β-SiAlON/TiN composites at 1750 °C, 30 MPa for 12 mins. The results demonstrated the existence of both the cubic TiN phase and the Si4Al2O2N6 phase within the matrix of the sintered composite. The addition of microsized TiO2 as a precursor to the composite improved its mechanical properties, resulting in a fracture toughness of 6.3 MPa m1/2 and a hardness of 14.6 GPa. The enhanced fracture toughness was noticed and ascribed to the toughening effect by the crack deflection mechanism. According to Smirnov et al. [50], β-Si5AlON7-TiN ceramic composites sintering with electric current assistance was also effective in achieving high relative densities (92% and higher) and improved mechanical properties. Using high-frequency induction sintering, the fracture toughness and Vickers hardness of the composite were found to increase with the addition of TiN, reaching values of 6.3 MPa m1/2 and 12.6 GPa, respectively, for a composite containing 5 wt.% TiN. Using pulse electric current sintering, the composite containing 10% TiN has a Vickers hardness of 15.6 GPa and a fracture toughness of 8.3 MPa m1/2. Therefore, the incorporation of titanium-based fillers can drastically increase the densification, hardness, and fracture toughness of SiAlON composite materials, making them suitable for high-performance applications.
Different fillers, including aluminium oxide (Al2O3), have been used to modify the structure and properties of SiAlON in the literature [5, 56, 107, 109, 111, 143, 144]. Al2O3-C refractories are a type of advanced ceramic material used in high-temperature applications, such as furnace linings, burner nozzles, and heat exchangers. They are known for their excellent thermal stability, chemical resistance, and high melting point, making them suitable for use in harsh environments. The microstructure of β-SiAlON is profoundly impacted by Al2O3-C refractories. These refractories provide a framework for SiAlON formation through interactions between Si3N4, Al2O3, and carbon to improve their mechanical and thermal properties [145,146,147]. In a study by Yin et al. [111], they found that the addition of Al2O3 to β-SiAlON in Al2O3-C refractories improved the thermo-mechanical performance of the materials. It was observed that diverse morphologies of β-Sialon, such as a hexagonal prism, columnar, and plate-like structures, provide materials with high mechanical strength (Fig. 7a-c). The specimen with the plate-like β-SiAlON had a slightly greater cold modulus of rupture (CMOR) and cold crushing strength (CCS) compared to those with columnar and hexagonal prism structures, with highest values of 105.2 MPa for CCS and 27.2 MPa for CMOR, respectively. Moreover, the development of β-SiAlON resulted in strong thermal shock resistance, and the specimen with columnar β-SiAlON still had a residual strength ratio of 78% after five thermal shock cycles [110, 111]. Lan et al. [148] reported that the presence of Al2O3 significantly increased the hardness, density, and fracture toughness of SiAlON composites prepared by two-stage spark plasma sintering. The authors found that using Al2O3 as a filler in SiAlON composites leads to the production of completely dense ceramics with a density ranging between 3.22-3.24 g/cm3. The inclusion of Al2O3 resulted in the formation of β-SiAlON with interlocking microstructures and corundum and silicon carbide phases. Improved hardness (15.68-15.95 GPa), and fracture toughness (6.38-7.03 MPa m1/2) due to the presence of the hard phases within the microstructure of the developed composites were also reported. The introduction of Al2O3 was also found to increase SiAlON composites’ thermal conductivity, with values ranging from 13.5-19.6 W/m-K [148]. According to Kovziridze et al. [149] and Deng et al. [143], the use of Al2O3 as a filler in SiAlON composites prepared by aluminothermic and nitro-aluminothermic processes leads to the formation of composites with improved corrosion resistance [5, 143]. Zhang and co-researcher [110] found that the utilization of Fe2O3 as a catalyst to Al2O3-C refractories resulted in the formation of β-SiAlON with a one- or two-dimensional shape, which significantly improved the CMOR and heat modulus of rupture (HMOR) of the developed materials by 45.07% and 60.47%, respectively. The findings of Yin et al. [109] and Zhang et al. [112] also revealed that the addition of Fe2O3 to Al2O3-C refractories resulted in the formation of plate-like β-Sialon, which improved the CMOR and CCS of the materials.
Sintering additives such as Sm2O3 and La2O3 also improve the sintering behaviour and densification of SiAlON composites. In a study on the preparation of SiC-SiAlON and Al2O3-SiAlON composites using alumothermal processes, the addition of Sm2O3 [29] and La2O3 [150] was found to make the density higher and hardness of the resulting composites. The addition of Sm2O3 resulted in a density increase of up to 4.1% and a hardness increase of up to 9.8% [29], while the addition of La2O3 resulted in a density increase of up to 5.5% and a hardness increase of up to 22.8% [150]. Hence, the structure and properties of SiAlON composites can be significantly influenced by using Al2O3 fillers, producing enhanced mechanical, thermal, and thermal shock resistant qualities. These properties can be tailored by controlling the various parameters, including the type and amount of catalyst used, the Al2O3/Si3N4 ratio, and the sintering temperature.
Metallic inclusions such as Mg, Ti, Ca, and Fe are added to SiAlON composites to significantly influence their structure and properties and catalyze the reaction process [26, 40, 113, 120, 126, 151]. The addition of Mg has been shown to improve the IR transparency of SiAlON ceramics. Joshi et al. [40] fabricated IR Mg-α/β-SiAlON: Ba2+ ceramics by the hot pressing technique. Their results showed that BaCO3 addition increases the α-SiAlON phase in the sintered bodies due to the extra liquid phase that permits Mg2+ migration in the α-Si3N4 lattice. According to the authors, this resulted in ceramics with a light transmission of about 78% in the IR region (2500 nm) and a hardness of 21.4 GPa [40]. Yurdakul and Turan used transmission electron microscopy to determine if it was possible to include Ti transition metal into the crystal structure of β-SiAlON in β-SiAlON-TiN composite material. [120]. The study demonstrated that Ti (transition metal) may penetrate the β-SiAlON crystal structure, creating Ti-doped SiAlON-based materials for different engineering applications. Ahmed et al. [122] studied the influence of particle size of α-SiAlON precursor and the amount of cubic boron nitride (cBN) reinforcement on the microstructure and mechanical properties of cBN-reinforced α -SiAlON composites fabricated using SPS at 1500 °C for 30 mins. The use of a ball milling technique with high energy was shown to cause a phase change from cBN to hBN, as well as an α-to β-Sialon phase change. The developed composites displayed greater mechanical strength than pristine SiAlON, with a Vickers hardness (HV10) value of up to 24.0 GPa as opposed to pure alpha SiAlON’s 21.6 GPa. According to Adeniyi et al. [152], the addition of Ni to α-SiAlON ceramics enhanced the corrosion resistance and mechanical properties of the resulting composites. The authors achieved the α-SiAlON/Ni composites via SPS at 1500 °C for 30 min. It was found that the inclusion of Ni resulted in an improvement in the densification up to 15 wt% Ni, corrosion resistance, as well as an increase in the hardness and fracture toughness of the sintered composites. The improvement in mechanical properties was attributed to Ni particles’ strengthening effect and the establishment of a Ni/SiAlON interpenetrating phase composite.
The addition of Fe-containing compound has been found to improve the mechanical characteristics and microstructure of β-Sialon ceramic. Sun et al. [126] demonstrated this by reinforcing β-SiAlON composites with Fe3Al. The addition of Fe3Al was found to improve the sinterability of the composites and resulted in an increase in the flexural strength and hardness of the composites. Furthermore, enhanced composites’ resilience to wear was also reported. Zhang et al. [113] introduced Fe, Co and Ni transition metals to aid the catalytic reaction during the nitridation of β-SiAlON. The authors concluded that the addition of transition metals greatly improved the amount of vapour generated, which significantly enhanced the nitridation process of Si and eased β-SiAlON nanofibers formation. Also, the addition of molten Al as a heating medium in a fast heating thermal shock test (FHTST) for β-SiAlON has equally been demonstrated by Li et al. [26]. The authors stated that with a temperature differential (T) up to 900 °C, Vickers cracks did not spread in melted metal in the FHTST, and the retained strength did not diminish. However, when the temperature exceeded 600°C, the maintained strength for use with room temperature water in the quenching thermal shock test (QTST) dramatically decreased, and the indentation fractures substantially spread. The FEM calculation demonstrated that the variable stress state (compressive or tensile stress) on the surface might be the source of the discrepancies between the FHTST and QTST findings. The distribution of stress and temperature in the β-SiAlON specimens at 0.01 s for FHTST compared to the quench thermal shock test (QTST) is shown in Fig. 8a-c. They suggested that a useful thermal shock evaluation technique for the ceramics utilized in the business of nonferrous metals would be FHTST with molten metal. In general, the incorporation of metallic components, such as Mg, Ti, Ca, and Fe can significantly impact the composition and characteristics of SiAlON composites, including their IR transparency, microstructure, mechanical properties, and wear resistance. Understanding these relationships can help in the design and optimization of SiAlON composites for specific applications.
α-SiAlON is a ceramic material with excellent hardness, high temperature stability, and good chemical resistance. It is extensively utilized in many different applications, such cutting tools, wear-resistant coatings, and ball bearings. It can be reinforced with various inclusions by combining α-SiAlON with other materials, such as cBN, hBN, Si3N4, Ca, SiC, Y2O3 (yttrium oxide), Ni, WC (tungsten carbide), and TiCN (titanium carbonitride). Several reports in the literature [6, 30, 38, 74, 103, 104, 114, 117, 123, 153,154,155,156,157,158] revealed that these inclusions can improve The α-SiAlON ceramics’ mechanical characteristics and resistance to wear, as well as enhance their performance in various applications. The addition of cBN has been found to improve the hardness and fracture toughness of α-Sialon composites [38, 123, 153, 154]. In a study conducted by Gareth et al. [123] on α-SiAlON–cBN composites using FAST/SPS–sintering at 1575 °C -1625 °C, the hardness of the materials increased up to 21 GPa with the inclusion of 10 vol.% cBN, while with 30 vol. % cBN, the fracture toughness improved to over 8 MPa m0.5. Due to the cBN grains’ poor matrix bonding, crack deflection at those grains was thought to cause an increase in fracture toughness.
However, as the sintering temperature increased, the mechanical properties of these composites started to degrade because of the presence of hBN (hexagonal boron nitride) near the junction of cBN and the matrix. (Fig. 9a & b). In another related work by Gareth et al. [154], the influence of cBN grain size (10 μm and 100 μm) on the properties of SPS produced α-SiAlON ceramics at 1550 °C was studied. Conversion of cBN to hBN was reported on the surface of the smaller grain size, while transformation occurred both on the surface and at the internal grain boundaries inside the cBN grains of the larger grain size. The authors established that the higher the hBN formation, the higher the fracture toughness and the lower the final product’s hardness. Hence, the properties of the composite ceramic reinforced with 100 μm cBN grain size are greatly influenced by the sintering temperature.
Similarly. the inclusion of hBN has also been found to affect the properties of α-SiAlON/BN composite ceramics. Shan et al. [38] examined the effect of hBN on the flexural strength of these ceramics. It was found that the inclusion of hBN contributed to the powder body’s densification but had little effect on the phase transformation of α-SiAlON. The authors found that the highest flexural strength was obtained with 20 wt.% hBN and the formation of β-SiAlON was suggested as a significant explanation for the increased strength [153, 154]. The use of Y2O3 as a dopant has been found to improve the sinterability and mechanical properties of α-SiAlON ceramics. Choi et al. [127] examined the impact of Y2O3 on the microstructure and mechanical properties of α -SiAlON-based composite ceramics using a gas pressure sintering furnace at 1820 °C for 90 min under 1 MPa nitrogen. It was found that the addition of Y2O3 improved the sinterability and density of the ceramics and increased the fracture toughness but reduced hardness (Fig. 9c). The inclusion of Y2O3 further led to the creation of yttrium-containing oxide phases, which were suggested to act as a strengthening phase and improve the bonding between the grains.
Several authors [9, 30, 103, 104, 114, 117] have posited that the inclusion of SiC or WC in α-SiAlON significantly improves the tribo-mechanical properties of the resulting composites. SiC particles are typically present as micron-sized grains and form a homogenous distribution within the matrix. Kushan Akin et al. [117] synthesized α-SiAlON ceramic composites by adding SiC particles using SPS at 1500 °C for 30 min. The addition of SiC resulted in improved hardness and fracture toughness compared to monolithic α-SiAlON ceramics. The thermal diffusivity of α- and β-SiAlON ceramics was also improved by the addition of 0.25 wt% SiC. Liu et al. [30] studied the effects of temperature on the phase development, microstructures, and mechanical properties of SPSed (1800–2000 °C in an environment of 0.6 atm nitrogen) α-SiAlON composites reinforced with 80 wt% α-SiC. Their results showed that the addition of SiC enhanced flexural strength and fracture toughness and produced self-reinforcing microstructures. Biswas et al. [103] consolidated WC-reinforced α-SiAlON composites by SPS at 1750 °C, 40 MPa for 25 min. The authors reported that as the indentation load increases, the KIC values of the pure α-SiAlON and 40 wt % WC/α-SiAlON composite increased, but those of the pure WC fluctuated within a narrow range of 4.16-4.50 MPa-m0.5. This behaviour was attributed to the naturally brittle character and the equiaxed grain shape of WC. The changes in KIC of the developed WC/α-SiAlON composites as a result of the indentation load are depicted in Fig. 9d.
Hence, the addition of nano- and micro-inclusions of cBN, hBN, Si3N4, Ca, SiC, WC, and TiCN significantly influence the tribomechanical attributes of the composite, like hardness, toughness, and wear resistance. However, the effect of these inclusions on the structure and properties of the composite may vary depending on their concentration and distribution within the matrix.
O-SiAlON composite is a type of ceramic material that is composed of silicon, aluminum, oxygen, and nitrogen. It is known for its high strength, high temperature resistance, and excellent corrosion resistance. The structure and composition of the O-SiAlON composite are closely related to its properties and can be tailored by incorporating various inclusions such as Si3N4, SiC, Al2O3, Gd2O3, Yb2O3, TiN, and TiO2 [19, 32, 118, 155, 159, 160]. The X-phase of SiAlON composite materials has a crystal structure similar to that of mullite, with columns of octahedral aluminum oxide units bridged by tetrahedral units of aluminum oxide and silicon oxide. Its stoichiometry can vary from SiAlO2N to Si16.9Al22.7O48.8N11.6. Hot-pressed X-SiAlON composites containing 28% volume fraction of Al2O3 platelets have been found to possess a bulk density of 3.18 g/cm3, a fracture toughness of 4.16 MPa m0.5, and a hardness of 1270 kg/mm2 [34]. Dayan et al. [159] produced an O-SiAlON composite in a ball milling process by combining silicon carbide fines, Si, Al, and alumina micro-powders having different activities. These micro-powders were then wet mixed with silicon carbide particles and a binder, shaped into specimens, and fired at a temperature of 1450 °C in a nitrogen atmosphere. The resulting O-SiAlON composite has a SiAlON content of up to 23.5%, a reduction in the amount of unreacted alumina and silicon nitride from 8.78 mass% to 1.79 mass%, and an increase in the percentage of micropores with sizes under 1 μm from 30% to 68%. The CO resistance of the O-SiAlON composite is also improved to level A.
Liu et al. [155] modified the structure and composition of the O-SiAlON composite by adding h-BN nanosheets as a reinforcement phase. Their results showed improvement in the mechanical and thermal properties, including the damage tolerance and toughness of the O-SiAlON composite. The thermal diffusivity and conductivity of the developed O-SiAlON composite significantly increased due to the ultra-high thermal conductivity of the h-BN nanosheets. Consequently, the O-SiAlON composite exhibits good thermal shock resistance, with a water absorption of only 14.4% and a residual strength ratio of 73% after 15 thermal cycles at a temperature difference of ΔT=1100 °C.
O-SiAlON composite can also be prepared by a repeated sintering method. Ma and Bao [116] investigated the oxidation behaviour of Si3N4/O′-SiAlON composite sintered in a furnace under 3 MPa N2 gas pressure at 1,750 °C, 5 °C/min, for 2 h. The authors observed that repeated sintering resulted in an increasing fraction of the O′-SiAlON phase and densification of the composite, leading to improved oxidation resistance of the developed composite. A reduction in the thickness of the oxide layer on the O-SiAlON composite after oxidation at high temperatures of 1100-1500 °C for 30 h was also reported. In addition, when Si3N4/O′-SiAlON composite ceramics are oxidized at 1500 °C for 30 h, the oxidation weight gain is decreased by 43.3%, from 0.432 mg/cm2 to 0.025 mg/cm2, compared to the oxidation weight increase after one-time sintering [116, 160].
Further work conducted by Ma and Bao [160] showed that the structure and composition of the O-SiAlON composite can be modified by the SiO2 particle size. It was found that increasing the size of SiO2 particles leads to an increase in bulk density and bending strength, as well as a decrease in oxidation weight gain. However, at higher SiO2 sizes, these properties may begin to decrease. The 3D morphology of composite ceramics made of Si3N4/O’-SiAlON with various SiO2 contents and oxidized for 30 hours at 1500 °C is shown in Fig. 10i. A steady decrease in the distance between the highest and lowest peaks could be seen. To differentiate the phase of the oxidized surface of the sample, Fig. 10ii displays the oxidized surfaces of samples with 0 wt.% and 7.5 wt.% SiO2 oxidized for 30 h at 1500 °C. The presence of equiaxed grains and dispersedly thin rod-like grains were observed on the sample’s oxidized surface (Fig. 10ii(a) and (b)). Both were recognized by the EDS; the thin rod-like grains included Y, Si, and O elements, while the equiaxed grains had Si and O elements. In addition, the elements Al, N, O, Si, and Y were evenly dispersed on the oxidized surface according to the EDS mapping images of the analyzed samples. The authors reported that a composite with 2 μm SiO2 grain size displayed a maximum bulk density of 2.86 g/cm3, an ideal bending strength of 305.38 MPa, and an oxidation weight increase of 0.025 mg/cm2.
In addition to the inclusions mentioned above, other factors can also affect the structure and composition of the O-SiAlON composite. For instance, the employed sintering temperature and time can impact the microstructure and densification of the composite, as well as its mechanical properties, for example, flexural strength and hardness, but may also result in the formation of defects or grain growth. The addition of a second phase, such as Al2O3 or SiC, can also influence the structure and properties of the O-SiAlON composite. According to Shao et al. [119], the addition of Al2O3 to O-SiAlON improves the composite’s hardness and wear resistance, while Kaya et al. [118] found that the incorporation of SiC into O-SiAlON ceramic increases the resultant material’s fracture toughness and flexural strength. The microstructure of O-SiAlON composite can also be influenced by the type and size of the silicon carbide particles used in its preparation. For instance, utilizing fine SiC particles leads to a more homogeneous microstructure and improved properties like hardness and flexural strength. On the other hand, using coarse SiC particles result in a more porous structure with lower mechanical properties [118, 124, 125].
The inclusion of TiN filler in O-SiAlON composite can affect its oxidation behaviour and phase compositions. Jiang et al. [121] subjected TiN/O′-Sialon ceramics to high-temperature oxidation of 1200 – 1300 °C in air. The oxidation of TiN and O′-Sialon was found to occur at approximately 500 °C and 1050 °C, respectively. The weight gain by the material dramatically rises along with increasing TiN concentration. Likewise, Yang et al. [128] used a selective oxidative process in the air at 800-1000 °C to produce in-situ TiO2/O′-Sialon multiphase ceramics from TiN-reinforced O′-SiAlON composite ceramic material using a logarithm rule. According to the authors, in-situ TiO2/O′-SiAlON multiphase ceramics cannot be made without an apparent activation energy of 56.1 kJ/mol. There is also no protective scale growth on the materials’ surfaces. It was concluded that gas diffusion through the closed pore spaces is gradual and follows a rate-limiting step. Also, the addition of AlN to 27R-SiAlON polytype has been found to improve the densification, microstructure, and tribomechanical properties of the composite by Biswas et al. [94]. Sintering in the pulsed direct current mode (PM) was reported to produce better densification and wear resistance. Therefore, the structure and composition of the O-SiAlON composite can be tailored by incorporating various inclusions and manipulating sintering conditions, as well as the type and size of silicon carbide particles deployed. These modifications can significantly impact the mechanical, thermal, and corrosion resistance properties of the composite.
