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Microstructural design of Ca α-sialon ceramics: Effects of starting compositions and processing conditions
Abstract
The microstructural development of Ca α-sialon ceramics has been studied. It was found that the microstructure of these materials could be controlled by the starting compositions and the processing conditions. Consequently, the observed microstructure has been related to the mechanical properties of these materials. It was found that although fracture toughness increased with increasing grain size and aspect ratio, a weakened interface between the α-sialon grains and intergranular glass is essential to further improve the toughness of these materials.
... And for 8Sr-8Mg sample the maximum concentrations of Si, Mg and Sr ions in the released medium were 22 mg/L, 15 mg/L and 24 mg/L, respectively [22]. It was possible that the majority of Al element from sintering additives was incorporated into the lattice of Si 3 N 4 crystals, forming sialon ceramics (M x Si 12−(m+n) Al (m+n) O n N 16−n , where x(=m/v) ≤ 2; v is the valency of the cation M) [42]. Thus, only small amount of Al existed in a glass phase. ...
... The ion concentration of Ti extract was almost identical to cell culture medium (Table 2). It is well known that bioactive ions released from bioceramics, such as Sr, Mg, Si and Zn ions, play an important role in promoting osteogenesis [42][43][44][45][46][47]. Reffitt et al. reported that physiological concentrations of soluble Si (10-20 μM, or 0.28-0.56 ...
Silicon nitride (Si3N4) is an industrial ceramic used in spinal fusion and maxillofacial reconstruction because of its excellent mechanical properties and good biocompatibility. This study compares the surface properties, apatite formation ability, bacterial infection, cell-biomaterial interactions, and in vivo toxicity (zebrafish) of newly developed Si3N4 bioceramics (sintered with bioactive sintering additives SrO, MgO and SiO2) with two standard biomaterials; titanium (Ti) and traditional Si3N4 bioceramics (sintered with standard sintering additives Al2O3 and Y2O3). In general, Si3N4 bioceramics (both the newly developed and the traditional) displayed less in vitro bacterial affinity than Ti, which may arise from differences in the surface properties between these two types of material. The newly developed Si3N4 bioceramics developed lower biofilm coverage and thinner biofilm, compared to traditional Si3N4 bioceramics. The effects of ionic dissolution products (leach) on proliferation and differentiation of MC3T3-E1 cell were also investigated. Ionic dissolution products containing moderate amount of Sr, Mg and Si ions (approximately 4.72 mg/L, 3.26 mg/L and 3.67 mg/L, respectively) stimulated osteoblast proliferation during the first 2 days in culture. Interestingly, ionic dissolution products from the traditional Si3N4 bioceramics that contained small amount of Si and Y ions achieved the greatest stimulatory effect for alkaline phosphatase activity after 7 days culture. The toxicity of ionic dissolution products was investigated in a putative developmental biology model: zebrafish (Danio rerio). No toxicity, or developmental abnormalities, was observed in zebrafish embryos exposed to ionic dissolution products, for up to 144 h post fertilization. These newly developed Si3N4 bioceramics with bioactive sintering additives show great potential as orthopedic implants, for applications such as spinal fusion cages. Future work will focus on evaluation of the newly developed Si3N4 bioceramics using a large animal model.
... Si3N4 ceramics occur in two forms, α-and β-Si3N4, both having hexagonal crystal structures built by corner-sharing SiN4 tetrahedra 58 . α-Sialon and β-Sialon are solid solutions of α-and β-Si3N4, respectively 61 . α-and β-Si3N4 have quite different grain morphology, with α-phase grains typically exist as equiaxed gains, while β-phase grains typically exist as elongated hexagonal prisms. ...
This thesis focuses on elaboration and characterization of two types of bioceramics: one
is ZrO2-SiO2 nanocrystalline glass ceramic (NCGC) for dental application. The goal is to
develop new ZrO2-SiO2 NCGCs with a combination of high strength and high translucency; the other is biodegradable Si3N4 ceramics for spinal fusion. This project aims to improve the osteointergration property of Si3N4 ceramics. Translucent glass ceramics typically suffer from impaired mechanical properties, compared to full-ceramics. We presented a method of obtaining ZrO2-SiO2 NCGCs, with a microstructure of monocrystalline ZrO2 nanoparticles (NPs), embedded in an amorphous SiO2 matrix. Raw powders containing different ZrO2 contents were prepared by the sol-gel method, followed by the spark plasma sintering (SPS). The NCGC with a composition of 35%ZrO2-65%SiO2 (molar ratio, 35Zr) was transparent. Tetragonal ZrO2 NPs were spherical with a diameter of 20–40 nm. The average flexural strength of 35Zr NCGC was 234 MPa. To improve the flexural strength, NCGCs with compositions of 45%ZrO2-55%SiO2 (45Zr), 55%ZrO2-45%SiO2 (55Zr), 65%ZrO2-35%SiO2 (65Zr) were also elaborated. All NCGCs showed high translucency. The flexural strength of the NCGCs significantly increased with the increase of ZrO2 content, achieving as high as 1014 MPa for 65Zr NCGC. ZrO2 NPs in 65Zr NCGC were ellipsoidal and had a core-shell structure with a thin Zr/Si interfacial layer as the shell. Some of the ZrO2 NPs were connected and formed
ZrO2 nanofibers. Moreover, the ZrO2 nanofibers were orderly stacked in short-range to form the 3D nano-architecture. The high flexural strength of the 65Zr NCGC mainly originates from synergistic strengthening effects of the thin Zr/Si interfacial layer and 3D stacked nanoarchitecture. Regarding biodegradable Si3N4 bioceramics, we used a ternary sintering additive of SrO, MgO and SiO2. The mechanical properties of the developed Si3N4 bioceramics were comparable to those of traditional Si3N4 ceramics. Sr2+, Mg2+, and Si4+ ions released from the intergranular glass phase after immersion in solution, indicating that the developed Si3N4 bioceramics showed certain biodegradable ability. These ions enhanced the proliferation and differentiation of preosteoblasts. Meanwhile, the ionic dissolution products did not show any toxic effects to the development or physiology of zebrafish embryos.
... The density values were in the range of 3.12 g/cm 3 to 3.18 g/ cm 3 [26]. Even though it is challenging to calculate the theoretical densities of the sintered samples due to the presence of more than one phase in all samples, however, the density values were similar to those reported for sialon samples sintered at high temperatures (greater than 1700°C) [26,37,38]. The densification of silicon nitride-based materials is influenced by the formation of oxi-nitride eutectic liquid phase(s) formed in the presence of the oxide layer on the surface of starting nitride powder and the oxide additives. ...
Several compositions of calcium stabilized sialon ceramics were synthesized by using nano-size starting powder precursors and spark plasma sintering technique at a relatively low temperature of 1500 °C. The formation of calcium alpha/beta-sialons was investigated for the compositions represented by Cam/2Si12-(m+n)Alm+nOnN16-n where m and n values were varied from 0.6 to 1.6 and 0.4–1.6, respectively. Phase analysis of the selected compositions helped in developing the phase boundary between alpha and alpha/beta phase regimes. Effect of m and n values on the evolution of the final phase(s), densification and mechanical properties were evaluated. All samples yielded densified ceramics with density values (3.13–3.19 g/cm³) comparable with those reported in the literature for similar compositions and synthesized at temperatures greater than 1700 °C via conventional sintering techniques. Vickers hardness value HV10 of 20 GPa was measured for the Ca0.5Si10.6Al1.4O0.4N15.6 composition (with m = 1.0 and n = 0.4) synthesized at 1500 °C. An increase in n value (higher oxide content) was observed to facilitate the formation of beta-sialon and AlN polytype phases leading to an increase in fracture toughness but a decrease in Vickers hardness.
... In the present study, relatively low values of translucency were observed for all three sintering protocols tested, with a slightly better performance of the SS group ( Table 1). However, the SS and S groups exhibited a slightly lower hardness value relative to group LT, due probably to a larger grain size observed in group LT [28]. The surface views of wear craters for zirconia and steatite are shown in Fig. 2. Images in Fig. 2a show that the glazing layer has been completely removed, exposing the underlying zirconia in the wear crater. ...
... In the present study, relatively low values of translucency were observed for all three sintering protocols tested, with a slightly better performance of the SS group (Table 1). However, the SS and S groups exhibited a slightly lower hardness value relative to group LT, due probably to a larger grain size observed in group LT [28]. The surface views of wear craters for zirconia and steatite are shown in Fig. 2. Images in Fig. 2a show that the glazing layer has been completely removed, exposing the underlying zirconia in the wear crater. ...
The fabrication of zirconia dental restorations is a time-consuming process due to traditional slow sintering schemes; zirconia (Y-TZP) produced by these conventional routes are predominantly opaque. Novel speed sintering protocols have been developed to meet the demand for time and cost effective chairside CAD/CAM-produced restorations, as well as to control ceramic microstructures for better translucency. Although the speed sintering protocols have already been used to densify dental Y-TZP, the wear properties of these restorations remain elusive. Fast heating and cooling rates, as well as shorter sintering dwell times are known to affect the microstructure and properties of zirconia. Thus, we hypothesize that speed sintered zirconia dental restorations possess distinct wear and physical characteristics relative to their conventionally sintered counterparts. Glazed monolithic molar crowns of translucent Y-TZP (inCoris TZI, Sirona) were fabricated using three distinct sintering profiles: Super-speed (SS, 1580 °C, dwell time 10 min), Speed (S, 1510 °C, dwell time 25 min), and Long-term (LT, 1510 °C, dwell time 120 min). Microstructural, optical and wear properties were investigated. Crowns that were super-speed sintered possessed higher translucency. Areas of mild and severe wear were observed on the zirconia surface in all groups. Micropits in the wear crater were less frequent for the LT group. Groups S and SS exhibited more surface pits, which caused a scratched steatite surface that is associated with a greater volume loss. Tetragonal to monoclinic phase transformation, resulting from the sliding wear process, was present in all three groups. Although all test groups had withstood thermo-mechanical challenges, the presence of hairline cracks emanating from the occlusal wear facets and extending deep into the restoration indicates their susceptibility to fatigue sliding contact fracture.
... It is known that the decomposition of Si 3 N 4 ceramics and SiAlONs cannot be fully suppressed by 0.9 MPa of nitrogen at 2000 o C. 17) The maximum relative density through the current experiment was close to 90%, which is lower than the reported density of Ca-α-SiAlONs for engineering applications. 16,[18][19][20][21][22] In spite of the deviation from the full density condition, useful knowledge regarding the compositional design of the Ca-α-SiAlON:Eu 2+ plate phosphor was acquired. First, the increase in the 'm' value for the compositions in the Si 3 N 4 -CaO· 3AlN tie line, m=1.5 for α1 and m = 3.0 for α2, promoted densification. ...
Plate-type phosphor is a promising substitute in overcoming the issues related to the powder phosphor paste mixed with resin. In this research, plate phosphor () was investigated for the varied compositions (m,n) of the host crystal with the fixed Eu content (y). Densification was promoted for the compositions with increasing 'm' values for the m=2n relationship. Dictated by the Eu concentration inside the phosphor crystal, photoluminescence intensity was stronger in specimen (m = 3.0, n = 1.5) containing the second phases when compared to specimen (m = 1.5, n = 0.75) comprising a single-phase -SiAlON. The concentration of Eu in the non-emitting amorphous interfacial glass phase was 2~4 times of the designed Eu concentration inside the -SiAlON crystal.
The present study reports for the first time the synthesis and evaluation of magnesium (Mg)‐doped nitrogen rich (N‐rich) sialon ceramics exploring the possible existence of Mg‐stabilized single phase alpha sialon (on the Mg‐alpha sialon plane). Mg stabilized N‐rich sialons, with the general formula of MgxSi12‐2xAl2xN16 having x value in the range of 0.2−2.2 for the composition laying along the Si3N4 − ½Mg3N2:3AlN line, were synthesized at 1500°C, using nano‐size starting powder precursors and field‐assisted (or spark plasma) sintering technique. Consolidated sialon ceramic samples were characterized for their microstructure, phase stability regime, and physical and mechanical properties. Although a relatively low synthesis temperature was adopted, well densified sialon samples were achieved; however, densification of the samples became difficult with a higher x value (containing high Mg3N2/AlN content). Contrary to what was expected to take place, a single phase Mg‐doped sialon was not obtained near the N‐rich line (on Mg‐alpha sialon plane). Such distinctive behavior in Mg‐doped sialons was supposed to be due to the formation of a highly stable Mg‐containing aluminum nitride polytype phase which consumed most of the high temperature transient liquid phase. Mg‐doped N‐rich sialon sample having the maximum amount of alpha phase depicted a remarkable hardness (HV10) of 21.4 GPa and a fracture toughness of 3.5 MPa m1/2. This article is protected by copyright. All rights reserved
Mg-doped sialon ceramics with the composition of M0.4Si10.2Al1.8O1N15 were fully densified by hot pressing at 1850 °C for 1 h, using 0.5 wt.% MgF2 or CaF2 as a sintering additive. Densification behavior, phase assemblage, microstructure, and mechanical and optical properties were investigated in detail. The addition of fluorides, especially MgF2, not only resulted in more high-temperature liquid by promoting the dissolution of more N and other constituents but also reduced the viscosity of liquid due to the terminal effect of fluorine. Consequently, the densification was effectively improved. Additionally, the fluoride addition facilitated the formation of a small amount of β-sialon. Both the samples possessed high hardness (∼20 GPa) and fracture toughness (∼4.2 MPa m1/2). The CaF2-added sample exhibited higher infrared transmittance than its counterpart due to less residual glass phase. The present work implies that fluorides are also very effective sintering additives for densifying α-sialon.
Mg-doped sialon (Mgm/2Si12−m−nAlm+nOnN16−n) ceramics with different compositions of m = 2n = 0.6, 0.84, 1.0, 1.2, 1.6 were hot pressed at 1850 °C for 1 h. Phase assemblage, microstructure, mechanical and optical properties of these samples were investigated. All samples achieved/approached full densification. However, the densification of Mg-doped sialon ceramics with higher MgO/AlN content becomes more difficult. Additionally, the anisotropic growth of β-sialon grains was significantly inhibited. The unique characteristics of Mg-doped sialon ceramics intrinsically derive from the formation of Mg-containing AlN polytypoids, which consumed most of the high-temperature liquid. Furthermore, their high stability at high temperatures accounts for the difficulty in preparing single-phase Mg-α-sialon., The hardness of these samples gradually increases while indentation fracture toughness gradually decreases with increasing m = 2n value. Due to little residual glassy phase, high infrared transparency/translucency was more readily achieved in Mg-doped sialon. The m = 1.2 sample possesses the maximum transmittance of ∼50% at ∼2 μm.
Microstructural homogeneity is crucial in determining the performance and mechanical behavior of ceramic materials. Nondestructive characterization techniques have traditionally been employed to ascertain microstructural uniformity by looking for variations in material properties. Methods developed to ascertain homogeneity from characterization results have often been qualitative or subjective in nature. Information entropy offers a distinct solution as an objective representation of the frequency and distribution of data depicting unique microstructural states present in the material. In this study, a methodology for determining information entropy from characterization results was developed and applied to three different silicon carbide materials. Entropy values for each measured property were used to assess the homogeneity of a variety of microstructural features, including inclusion concentration and size distribution, grain size distribution, relative phase concentration. Use of information entropy enabled an objective comparison of samples with fundamentally different microstructures and allowed for quantitative ranking based of microstructural homogeneity throughout the sample volume.
Dy doped alpha-SiAlON ceramics prepared by the hot-pressing method show a high optical transmittance value, >70%, in the infrared region of 1.5-4.5 mum. First principles calculations have been carried out to reveal the underlying transparency mechanism. It is found that the valence shell of doped Dy atoms interacts strongly with the doping states of alpha-SiAlON, resulting in the increase in the optical gap from 0.4 to 1.1 eV, which suppresses the photoabsorption in the wavelength region longer than 1.0 mum and leads to the good transparency property. The calculated optical transmission spectra are in good agreement with the corresponding experiments.
With boron-rich slag, silica fume, bauxite chalmette and carbon black as starting materials, alpha-sialon powder was prepared by carbothermal reduction nitridation(CRN). The mechanical properties of hot-pressed sintered slag a-sialon ceramic, which was started from slag alpha-sialon powder, were measured. The hardness, toughness and bend strength of hot-pressed slag alpha-sialon ceramic reached 17GPa, 3.74MPa.m(1/2) and 333.5MPa, respectively. These properties were close to that of chemical a-sialon ceramics prepared by expensive chemical agents as starting materials.
Ca-alpha-sialons with low oxygen content were fabricated at 1800 degrees C by hot pressing. The phase and microstructure were characterized by XRD and SEM. The calculated lattice parameter reveals that 20-30% Ca(2+) ions still exist in the grain boundary phase of Ca-alpha'. A large amount of Ca-containing liquid phase efficiently promotes the anisotropic growth of alpha' grains in Ca-rich composition. Reinforced only by elongated alpha' grains, the fracture toughness of Ca-alpha' can reach 4.96 MPa.m(1/2).
Y-α-SiAlON (Y1/3Si10Al2ON15) ceramics with 5wt.%BaAl2Si2O8 (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600°C without holding and using a heating rate of 100°Cmin−1, but the transformation from α-Si3N4 to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800°C for 5min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2GPa and fracture toughness of 6.8MPam1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300°C.
Refractory Y-α-SiAlONs with elongated microstructure were obtained by hot pressing and post-heat treatment using BaAl2Si2O8 (BAS) as an additive. The results showed that BAS not only served as a liquid phase sintering aid to promote densification, but also facilitated the development of elongated α-sialon grains, resulting in excellent room and high-temperature mechanical properties. Increasing the amount of BAS addition and post-heat treatment could further effectively promote the anisotropic growth of α-SiAlON grains, the room-temperature flexural strength and fracture toughness of 10 wt% BAS/Y-α-SiAlON could reach 634 MPa and 7.2 MPa m1/2, respectively. The resultant materials could maintain the room strength up to 1400 °C due to the extensive crystallization of BAS.
Dense Y-, Yb- and (Y + Yb)-doped α-sialons containing 2 wt.% extra liquid phase were fabricated by hot pressing. The results show that the elongated grains morphology can be obtained by partially substituting Yb with Y as a modifying cation, and hence improve the toughness of the materials. An additional intermediate holding step during sintering has no obvious effect on the microstructures and the mechanical properties. Post-heat treatment can further facilitate the growth of elongated α-sialon grains and increase the toughness of the materials. All the materials exhibited very similar high hardness of over 20 GPa.
α′-Sialons are a relatively new class of ceramics that promise excellent high-temperature mechanical properties and thermal shock resistance. This report reviews the current status of research on α′-sialons, including phase equilibria, formation, sintering, and properties.
αâ²-Sialons are a relatively new class of ceramics that promise excellent high-temperature mechanical properties and thermal shock resistance. This report reviews the current status of research on αâ²-sialons, including phase equilibria, formation, sintering, and properties.
In this work, the oxygen rich compositions near the intersection of the α-SiAlON plane with the Si3N4-AlN-CaO plane were investigated. The pressureless sintering behaviour was explained in terms of the phase evolution and stability of secondary phases. It was possible to produce low porosity α-SiAlON by pressureless sintering where excess second phase for the formation of α-SiAlON was present. Preliminary mechanical property investigations for pressureless sintered material resulted in a MOR of 550 MPa, an elastic modulus of 240 GPa, a Vickers hardness of 14.4 GPa and a fracture toughness of 4.5 MPa.m1/2.
As ceramists we apply the term ‘recrystallization’ to changes in microstructure that occur in crystalline or largely crystalline bodies when atoms move to positions of greater stability. In other words, we include phase changes in the solid state, sintering, grain growth, and precipitation or exsolution phenomena. It is important to remember that metallurgists use the term recrystallization in a much more restricted sense: the nucleation and subsequent growth of a new generation of strain-free grains into the deformed matrix of cold-worked material.
Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated beta-Si(3)N(4) seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R-curve and extensive interfacial debonding between the elongated beta-Si(3)N(4) grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the beta'-SiAlON growth layer formed on the elongated beta-Si(3)N(4) grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the beta'-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct beta'-SiAlON growth layer forms, It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.
A review on the present state of liquid phase sintering in ceramic and metallic systems is given. It is shown how the densification phenomena i.e. rearrangement and center to center approach can occur by the elementary mechanisms of particle disintegration, Ostwald ripening and shade accommodation as well as directional grain growth. The influence of driving forces provided by chemical reaction on the kinetics of mass transfer is discussed.
Single-phase α-SiAlON with elongated grains is obtained from α-Si3N4 powder for a broad range of compositions of practical interest. Following the concept of nucleation and growth, two-step firing is used for microstructure control. This method takes advantage of the slow transformation reaction from α-Si3N4 to α-SiAlON at low temperature when the composition is near the α-SiAlON phase boundary and, hence, is marginally stable. For more-stable compositions, the seeding of α-SiAlON crystals is more effective, because it allows elongated grains to grow onto the seed crystals. The fracture toughness is strongly correlated with the microstructure and is enhanced greatly in the optimized materials.
Ca sialon compositions rich in Al and Ca were fabricated and characterised in this work. Sintered compositions are found to be dominated by AlN′ and AlN-polytypoid phases. Large quantities of grain boundary glass are also present. Analysis of crystalline phases provided phase assemblage information for compositions with high m and n values and led to the refinement of an existing phase behaviour diagram. A modified phase behaviour diagram incorporating the current work has been proposed. Microstructural and fracture surface features are examined by a scanning electron microscope (SEM). a Sialon grains in these compositions have been observed with an elongated morphology.
Silicon nitride has two polymorphous structures, α-Si3N4 and β-Si3N4. In this study three different Si3N4 starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca1.8Si6.6Al5.4O1.8N14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si3N4 resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si3N4 remained until 1800° in samples prepared with ∼100%β-Si3N4 powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si3N4 starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.
The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPa·m¹² and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPa·m¹² coupled with a reduced fracture strength.
Ca-α-Sialon (CaxSi12-(m+n)Alm+nOnN16-n,) ceramics with extensive compositions on the line of Si3N4–CaO:3AlN (where m=2n) ranged from x=0·3 up to x=2·0 were fabricated by hot-pressing. An exploration for Ca-α-Sialon involving reaction sequences, phase compositions, cell dimensions, microstructure and mechanical properties was carried out in the present work.
Calcium α-sialons, containing elongated grains, were developed by pressureless sintering. Preliminary electron microscopy analyses revealed the high aspect ratio of the α′ grains resulted from preferred crystal growth in the [001] direction. Such grain morphologies offer hope for in-situ toughening of α-sialon materials via the promotion of crack deflection and bridging during failure.
Silicon nitride (Si3N4) is a light, hard and strong engineering ceramic,. It can withstand harsh environments and support heavy loads at temperatures beyond those at which metals and polymers fail. It can also be manufactured reliably at a reasonable cost and in large quantities. There are two forms of silicon nitride: alpha-Si3N4 and beta-Si3N4. The former is harder, but only the latter is currently used in engineering applications, because only this form can be given a microstructure resembling a whisker-reinforced composite,,, which gives it the necessary toughness and strength. Here we report the synthesis of a tough alpha-Si3N4 solid solution with this kind of microstructure. This material is 40% harder than beta-Si3N4 and is equally strong and tough. Its hardness (22GPa) is exceeded only by boron carbide and diamond (which are both brittle). These properties mean that this form of alpha-Si3N4 should be preferred over beta-Si3N4 for all engineering applications, and it should open up new potential areas in which the ceramic can be applied.
The introduction of elongated silicon nitride grains during densification in the presence of a liquid phase can impart considerable improvement to the fracture toughness. This toughening is not universally attained but depends on the activation of intergranular rather than transgranular fracture. This is reminiscent of the requirement of interfacial debonding in whiskerreinforced ceramics. In fact, additional observations such as bridging in the crack wake by elongated grains and pullout of some of these grains further suggest that the crack wake mechanisms that contribute to the toughening of whisker-reinforced ceramics can also operate in silicon nitrides containing elongated grains. Various investigators have found that, consistent with crack wake mechanisms, the fracture toughness of silicon nitrides increases with increase in the diameter of the larger elongated grains. However, little is known about the effects of the grain boundary phase(s) and their properties on the interfacial debonding/intergranular fracture in such silicon nitrides. This is critical as observations show that crack propagation in some systems exhibiting larger elongated grains occurs transgranularly and no toughening occurs.
A computer-controlled microdensitometer has been designed and built in this laboratory for measuring Guinier powder diffraction photographs. It consists of a microcomputer-controlled single-beam microdensitometer using as a measuring unit an IR emitting diode ( lambda =940 nm), a collimator system and a photodiode. The scan process is controlled by a microcomputer, Zilog MCZ 1/05, with two floppy disc units. The microcomputer is also used for data reduction. A direct connection with a large scale computer, Amdahl 470/V7, makes it possible to run indexing, search-match or profile refinement programs within 15 minutes from the moment the film is mounted in the scanner.
Silicon nitride materials containing different mixtures of sintering aids were studied with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted to coarsen the respective microstructure. It was found that fracture toughness is influenced by altering the residuals glass network structure, affecting the secondary-phase crystallization at triple pockets, and changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid.
Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß-Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R-curve and extensive interfacial debonding between the elongated ß-Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´-SiAlON growth layer formed on the elongated ß-Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.
The deformation behavior of a hot-pressed, fine-grained β-Si3N4 ceramic was investigated in the temperature range 1450°—1650°C, under compression, and the results for strain rate and temperature dependence of the flow stress are presented here. The present results show that the material is capable of high rates of deformation (∼10−4—10−3 s−1) within a wide range of deformation temperatures and under a pressure of 5—100 MPa; no strain hardening occurs in the material, even at slow deformation rates, because of its stable microstructure; Newtonian flow occurs, with a stress exponent of approximately unity; and the material has activation energy values for flow in the range 344—410 kJ·mol−1. Grain-boundary sliding and grain rotation, accommodated by viscous flow, might be the mechanisms of superplasticity for the present material.
A systematic set of samples with compositions in the Ca alpha-SiAlON (α′) system has been fabricated using pressureless sintering to evaluate the phase relationships on, and near, the Ca α′ plane. A phase behavioral diagram for the Ca α′ plane within the Jänecke prism has been determined and found to be similar to those for the Y and Sm α′ systems. However, distinct differences were also observed; two additional regions (α′+ 27R + liquid) and (α′+ 33R + liquid) have been identified. Investigation of the microstructures revealed elongated Ca α′ grains in the samples prepared by pressureless sintering. The aspect ratio of the elongated grains of α′ increased as the concentration of stabilizing cation was increased. Single crystalline phase Ca α′ samples of comparable density to hot-pressed material were fabricated successfully using pressureless sintering.
Silicon nitride (Si3N4) and SiAlONs can be self-toughened through the growth of elongated β-Si3N4/β-SiAlON grains in sintering. α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs. The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering. alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ-toughened α-SiAlON ceramics.
A Ca α-sialon (α′) ceramic with an overall composition of Ca0.8Si8.8Al3.2O1.6N14.4 was fabricated from α-Si3N4, AlN, Al2O3 and CaCO3 starting powders using pressureless sintering (PLS) at 1800°C for 1 h. The microstructural morphology of the resultant Ca-α′ depended greatly on firing patterns, i.e., heating rate. Typical fine and equiaxed α′ grains could only be found in samples fired at a regular heating rate of 20°C/min. In contrast, aciculate grains with a high aspect ratio of 5 to 10 dominated the Ca-α′ sample sintered at a heating rate of up to 60°C/min. The number of α′ nuclei in the Ca-α′ sample during heating was found to be significantly reduced by rapidly increasing the temperature because the yield of α′ through reactions of Si3N4, AlN, Al2O3 and CaO was somewhat retarded. Fewer nuclei could then grow into elongated grains during the subsequent isothermal holding process at higher temperature. Furthermore, much liquid phase existed in the system until higher temperatures were reached due to rapid heating, and this was another key factor in enhancing the elongation of α′ grain.
The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPam1/2 and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPam1/2 coupled with a reduced fracture strength.
A novel shear-thickening phenomenon has been observed in superplastic silicon nitrides compression tested between 1500° and 1600°C. Liquid-enhanced creep of SiAlONs undergoes a transition from Newtonian behavior to shear-thickening behavior at a characteristic stress, with the strain rate sensitivity increasing from unity to around 2. The transition stress is always around 20 MPa, even though the Newtonian flow stress is very sensitive to temperature, grain size, and phase composition. Rheopexic hysteresis, manifested as a slow stress relaxation to a steady-state value after a strain rate decrease, was also observed in the shear-thickening regime. We attribute the cause for shear thickening to a repulsive force between initially wetted SiAlON grains, which form a “dry” and “rigid” bridge in between when pressed above a characteristic stress, possibly due to the contact of the residue Stern layers on the opposing grain/liquid interfaces. A micromechanical model, which takes into account the stress variation among differently oriented grain boundaries, has been formulated to assess the effect of “rigid” grain boundaries. A continual stochastic rearrangement of grain configurations and a relatively thick Stern layer are suggested as the necessary prerequisites for shear thickening in liquid-enhanced creep.
The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.
To achieve toughening by the crack bridging process the introduction of large elongated grains by fracture resistance is necessary
but not sufficient. While increasing the diameter of the elongated grains can increase the toughening effect, this requires
that fracture occur along grain interfaces rather than through the grains. This interface debonding process appears to be
modified by the chemistry of the oxynitride glass at the grain boundaries. Experiments show that increasing the yttria to
alumina ratio or decreasing the ntirogen content of Si-AI-O-N glasses promotes interfacial debonding. The crack bridging contributions
to the R-curve behavior is also a function of the content and size of the bridging reinforcement as noted in whisker-reinforced
ceramics. Thus, control of micrstructure and interfacial phases is critical to the development of toughened silicon nitiride
ceramics.
Grain boundary devitrification was carried out on three Ca -sialon ceramics with different grain sizes and morphologies and various amounts of grain boundary glass. The devitrified product was gehlenite in all samples, indicating that the crystallization of the Ca oxynitride glass was accompanied by a volume reduction. The volume reduction upon crystallization and the thermal expansion mismatch between the devitrified product and -sialon grains would result in tensile residual stresses located at multi-grain junctions. These residual tensile stresses were expected to promote the crack deflection and bridging mechanism and thus to improve the material toughness. However, indentation fracture toughness measurement and scanning electron microscope study showed that there was no significant difference in fracture toughness and the fracture mode in present samples prior to and post heat treatment. This may be attributed to a change in the chemistry of the residual glass as a result of the grain boundary devitrification, which could enhance the bonding strength between the adjacent -sialon grains. The enhanced bonding strength could have to some degree hindered the crack deflection and bridging mechanism.
The microstructural evolution in Si3N4 ceramics is analysed by studies of the growth of isolated Si3N4 grains dispersed in oxinitride glasses as well as model experiments of sintered materials. The experiments with the supersaturated
oxynitride glasses offer the possibility to study the grain growth in the absence of steric hindrance and show the influence
of the additive composition on the development of the grain morphology. The differences in grain growth behaviour is discussed
with respect to the cation radius of the glass forming rare earth oxides. Investigations of polycrystalline Si3N4 ceramics show that the grain size and morphology are controlled by the properties of the Si3N4 starting powder, the sintering temperature and the additives. Different Si3N4 powders and various additive combinations were used in order to explore the dominant growth mechanisms.
The investigated microstructures are also related to strength as well as toughness measurements at room temperature. It is
shown that fine-grained gas pressure sintered materials can exhibit strength values > 1100 MPa. Although strength decreases
with increasing grain size, a higher fracture toughness and a significant improvement in reliability could be achieved by
controlled grain growth. Furthermore, it is demonstrated that coarse grained ceramics exhibit greater thermal shock resistance
than do fine-grained ones.
The driving force leading to densification during sintering in the presence of a liquid phase and the material transport phenomena have been analyzed and relationships for the densification rate during the rearrangement process, the solution‐precipitation process, and the final coalescence process have been determined. These relationships allow an experimental determination of the mechanism of sintering in the presence of a liquid phase on the basis of the time, particle size and temperature dependence of the densification rate. In addition, they allow direct calculations of densification rates to be made for certain simple systems for which property data are available.
The most intriguing recent development in the field of silicon nitride ceramics has undoubtedly been the discovery of a cubic form of silicon nitride. Major advances were made in α-SiAlON ceramics, including the development of thermally stable, in situ reinforced grades. Significant achievements were reported in tailoring the mechanical properties of silicon nitride ceramics through control of secondary phase chemistry and grain morphology.
The erosion behaviour of three pressureless sintered Ca α-sialon ceramics with different grain sizes, morphologies and various amounts of grain boundary glass was investigated using a gas-blast type erosion rig. The erodent particles used were SiC grits. The effects of grain size and morphology on the erosion mechanism were studied. It was found that the dominant material removal mechanism for the fine equiaxed-grained sialon was grain dislodgment, while that for the elongate-grained material was transgranular fracture. The effect of different amounts of intergranular glass on the erosion resistance of these materials was also studied. It was found that an optimum amount of grain boundary glass had a beneficial effect on erosion resistance. Finally, the effect of post-sintering heat treatment on the erosion behaviour of these materials was investigated. The results showed that heat-treated samples exhibited a higher erosion rate than their as-sintered counterparts.
Analysis of microstructural development and mechanical properties of Si 3 N 4 ceramics. In Tailoring of Mechanical Properties of Si 3 N 4 Ceramics The Neth-erlands
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A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements
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Microstructure and fracture toughness of Si3N4 ceramics
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