Α-sialon Grains Research Articles

Improvement of cutting performance of high x value α/β-SiAlON ceramic cutting inserts via tailoring microstructure and oxidation behavior

Despite the well performance of SiAlON ceramic cutting tools in machining nickel-based superalloys, there is still the problem of rapid wear due to high cutting temperatures and high cutting forces. In this study, novel high x value Y-α/β-SiAlON ceramic cutting inserts were fabricated by microwave sintering. The microstructure and oxidation behavior were tailored by varying the designed α/(α + β) ratios to enhance the cutting performance. The oxidation behavior was studied by static oxidation tests at 1000–1400 °C in air, and the cutting performance was tested by milling Inconel 718 superalloys. The results indicated that increasing α/(α + β) ratio under high x value composition could promote the elongation of α-SiAlON grains thus improving the mechanical properties. However, this also increased the volume of intergranular phase (IGP), which gradually weakened the oxidation resistance. At temperature above 1200 °C, the oxidation of IGP was much faster than α’ and β’ phases. This resulted in the decline of surface hardness and grain boundary strength, which in turn affected the cutting performance. When controlling the designed α/(α + β) ratio to 50%, a balance was achieved between the microstructure and oxidation resistance that the tool life was promoted.

Effect of rare-earth oxide additives on transparency and fluorescence of α-SiAlON ceramics

Recently, novel transparent and fluorescent materials are in demand for various optical applications such as lasers, scintillators, and solid-state lighting. α-SiAlON, which has excellent thermal and mechanical properties, also exhibits photoluminescence depending on the stabilized doped rare-earth ions. Its transparency and fluorescence depend on the rare-earth oxide added as a raw material, particularly in conventional powder processing. In this study, we fabricated α-SiAlON ceramics by adding various rare-earth oxides to elucidate their effects on the transparency and fluorescence of these ceramics. High-transparency α-SiAlON ceramics were fabricated by adding rare-earth oxides whose rare-earth ions have small ionic radii: Y2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, and Lu2O3. Because the fraction of α-SiAlON was high, the relative density was high, and the microstructure was composed of fine grains. In particular, α-SiAlON ceramics prepared by adding Ho2O3 showed lower light scattering than the other fabricated α-SiAlON ceramics because of the smaller α-SiAlON grains, resulting in higher in-line transmittance (48% at 600 nm). Furthermore, these transparent α-SiAlON ceramics exhibited fluorescence corresponding to the activated rare-earth ions: Ho3+, Er3+, Tm3+, and Yb3+ or Yb2+.

Preparation of Ca-α-sialon Ceramics with Compositions along the Si3N4-1/2 Ca3N2:3AIN Line

Abstract Starting from a powder mixture of α-Si3N4, A1N and Ca3N2, calcium α-sialon ceramics with extensive compositions on the line of Si3N4 – l/2Ca3N2 : 3A1N (with a nominal composition of Ca m/2Si12 – m Al m N16) ranged from m = 0.5 up to m = 6.0 were hot pressed at 1700 °C for 1 h under 20 MPa. The sintering behavior of the sample was found to be significantly composition-dependent, and full densification was achieved for all compositions. The solubility of Ca2+ in the present α-sialon was defined in the range of 0.5 ≤ x ≤ 1.7, which was wider than that in the Ca-α-sialon prepared from CaO. The single-phase samples revealed a similar microstructure comprising the matrix α-sialon grains and a grain boundary phase as the usual α-sialons. The energy dispersive X-ray spectroscopy analysis showed that 70 – 85 % Ca2+ ions entered the α-sialon structure accompanying the Si substitution by Al.

Microstructure formation of α/β-sialons prepared via an aqueous processing route and from cost effective silicon nitride powders

High quality α/β-Sialon materials were reproducible prepared using cost effective silicon nitride powders and an aqueous processing route. The influence of the powder quality (SN-E10, Silzot HQ, SicoNide P95H) on sintering and phase formation was investigated. With all 3 powders dense Sialon materials with nominal composition RxSi12-(m + n)Alm + nOnN16-n and m = 0.5 and n = 1 and 4 wt% excess rare earth additives could be prepared. The material based on Silzot HQ exhibits the highest α-Sialon content and subsequently the highest hardness due to the lower oxygen content of the starting powder.The microstructural analysis using XRD, FESEM including EDX- and EBSD-mapping reveal, that the α-Sialon grains are formed on existing α-Si3N4 grains of the starting powder. Anisotropic grain growth of the α-Sialon takes place therefore the differences in the microstructure depend strongly on the oxygen content but not from the α/β-content of the starting powders.

High porosity Ca-α-SiAlON ceramics with rod-like grains fabricated by freeze casting and pressureless sintering

Ca-α-SiAlON ceramics with high porosity and fine rod-like grains were fabricated using unidirectional freezing and pressureless sintering. The high porosity of green body was maintained during sintering, which was mainly controlled by solid loading of slurry. Phase analysis showed that α-SiAlON was major phase while small amount of AlN’ was inevitable. The anisotropic growth of α-SiAlON was controlled by sintering temperature, fine rod-like α-SiAlON grain morphology was obtained after sample was sintered at 1810℃ for 1.5 h. The aligned channel became more uniform with higher solid loading, gave rise to better mechanical properties. The fabricated samples demonstrated remarkably comprehensive properties including low thermal conductivities, low dielectric constant and dielectric loss.

Hot-pressed sintered Ca-α-Sialon ceramics with grains from short prismatic to elongated morphology synthesized via carbothermal reduction and nitridation

Ca-α-Sialon powders was initially synthesized by carbothermal reduction-nitridation (CRN) of SiO2, AlN, CaF2 and carbon black at 1700 °C, subsequently was used as the raw material to prepare Ca-α-Sialon ceramic by hot-pressed (HP) sintering at 1900 °C. The main intermediate nitridated products were Si2N2O and O-Sialon formed at 1400 °C–1600 °C. The resultant Ca-α-Sialon just had hexagonally short prismatic structure at 1700 °C. In order to obtain its elongated crystals, α-Sialon seeding was introduced during CRN process. The results revealed that the seeding facilitated crystal refinement of Ca-α-Sialon and the increase in its aspect ratios from 2 to 5.45. The resultant HP-sintered Ca-α-Sialon ceramic with seeding at 1900 °C had compact structure with fine and elongated grains, forming a highly interlocked microstructure. They had a Vickers indentation hardness of ∼21 GPa independent of the microstructure. The flexural strength and fracture toughness of the Ca-α-Sialon ceramics drastically increased with increasing the amounts of seeds, which was in accordance with that in grains aspect ratio of α-Sialon crystals synthesized by CRN as function of amounts of seeds. The improved flexural strength and fracture toughness should be ascribed to the development of elongated α-Sialon grains, which could enhance crack deflection and grain pullout.

Effect of Al metal precursor on the phase formation and mechanical properties of fine-grained SiAlON ceramics prepared by spark plasma sintering

This paper reports the effects of using nano-precursor powders, including α-Si3N4 or amorphous-Si3N4, and particularly the partial replacement of AlN by Al metal precursor, on the properties of a fixed Ca-α-SiAlON composition, consolidated by spark plasma sintering at 1450°C and 1600°C. The observed changes in mechanical properties are related to the phases present in the microstructures. In addition to allowing the consolidation of dense and fine ceramics at lower temperatures, the partial replacement of AlN by Al metal precursor has shown remarkable improvement in the mechanical properties of samples sintered at 1600°C containing α-Si3N4, a result of fine α-SiAlON grains and nitrogen-rich grain boundary phase. Another key contribution of Al metal precursor is found in lowering the sintering temperature while keeping, or even improving, the mechanical properties of the sintered samples, as observed in samples sintered at 1450°C.

Microstructural effect of α-Sialon ceramic on the resistance to cavitation erosion in deionized water

The resistance to cavitation erosion of one Yb-α-Sialon ceramic and two Dy-α-Sialon ceramics (Dy-α-Sialon-1 and Dy-α-Sialon-2) was investigated in deionized water using an ultrasonic vibratory apparatus. The incubation periods of Dy-α-Sialon-1, Dy-α-Sialon-2 and Yb-α-Sialon ceramic were 210min, 300min and 360min, respectively. Surface roughness and area fraction of cavities were found good indicators for the evolution of ceramics erosion. The microstructural features, including main phase (α phase), inhomogeneities of the microstructures and type of rare earth oxide, were correlated to the cavitation erosion behavior. It was found that the elongated α-Sialon grains in Yb-α-Sialon and Dy-α-Sialon-2 produced a self-toughening effect to increase the incubation period. The aggregation of Dy2O3 in Dy-α-Sialon-1 had an adverse effect on the resistance to cavitation erosion. Impurity phases and type of rare earth oxide had impacts on resistance to cavitation erosion. The findings in this work indicate that the cavitation erosion resistance of α-Sialon ceramics could be improved by microstructure optimization.

Influence of Composition on Self-toughening and Oxidation Properties of Y-α-Sialons

Y-α-sialon ceramics with different compositions were hot-press sintered. Varying the m or n value in the α-Sialon general formula, both the α-sialon grain morphology and phase assemblages present diversification, therefore, affecting the toughness and hardness. Meanwhile, varying the compositional parameters also influenced the composition and structure of the intergranular glass phase, as well as its oxidation behavior at 1100–1200 °C. The oxidation of the intergranular phase M′ in the Y-α-sialon ceramics was significantly enhanced with further rising the oxidation temperature to 1300 °C.

Light transmittance in α-SiAlON ceramics: Effects of composition, microstructure, and refractive index anisotropy

Abstract Translucent α-SiAlON ceramics were synthesized by hot pressing and spark plasma sintering. The in-line transmittance of the visible-infrared light through the α-SiAlON ceramics was discussed in correlation with the SiAlON compositions, rare earth dopant types, microstructures, and the refractive index anisotropy of the α-SiAlON grains. The results showed that the α-SiAlON ceramics had a refractive index about 2.17 and a refractive index anisotropy about 0.014. The birefringent losses due to the index anisotropy were the main reason for the severe transmission degradation in the visible light regions in the transmission curves. Accordingly, smaller grain sizes could be used to counteract the large birefringent losses to improve the in-line transmittance of the short wavelength light.

Effect of α-Si3N4 Addition on Sintering of α-Sialon Powder via Carbonthermal Reduction Nitridation of Boron-rich Slag-based Mixture

With boron-rich slag, silica fume, bauxite chalmette, carbon black and α-Si3N4 as starting materials, α-Sialon powders were prepared by carbothermal reduction-nitridation(CRN). Different contents of α-Si3N4 addition were added to investigate the α-Sialon formation as a function of α-Si3N4 addition from boron rich slag based mixture fired at 1480 °C for 8 h under N2 flowing of 0.4 L/min. Phase assembly, microstructure of reaction products were determined by X-ray Diffraction and Scanning Electron Microscope. The results showed that the main phases of the samples were a-Sialon, h-BN, AlN and small quantity of SiC. With the increasing amount of the α-Si3N4 addition, the h-BN content remained in a constant and AlN content was running down steadily, while the α-Sialon content increased gradually. The aspect ratio and the amount of elongated α-Sialon grains could be tailored by using different amounts of the α-Si3N4 addition.

Composition controlled microstructure and mechanical properties of Yb/Lu co-doped SiAlON

A series of Yb3+ and Lu3+ co-doped SiAlON ceramics with different Al content were hot pressing sintered using same process. The phase composition, microstructure and mechanical properties of the sintered samples were studied to find the tailoring effects of composition variation on mechanical properties of SiAlON. The starting composition of the SiAlON ceramics was demonstrated to be effective in the final phase formation and the grain shape and size in the bulk. The grain morphology and grain boundary phase of the SiAlON ceramics thus prepared also played an important role in the mechanical properties of the Yb/Lu co-doped SiAlON. It has been found that the fracture toughness of the samples with a lower Al replacement and more elongated grains is higher, while the hardness values of the samples with more α-SiAlON grains and less grain boundary phase is higher.

An experimental refinement of solid–solution relationship in Ca-α-Sialon ceramics by analytical electron microscopy

Local dopant compositions within individual α-Sialon grains were measured by analytical electron microscopy (AEM) in hot-pressed CaxSi12−3xAl3xOxN16−x (x = 0.3–1.4) ceramics. The reduction of local x values from the nominal dopant compositions is about 40% in general, and it reaches 60% for the end member (x = 1.4) which contains inclusions of AlN-based 21R phase. This results exhibit stronger departures from x than the previous report of 30% dopants missing in α-Sialon phase by electron probe micro-analysis (EPMA) [J Eur Ceram Soc 19:1637, 1999]. Amorphous films of ~1 nm thick were commonly found at grain boundaries (GBs), which could only take a small fraction of undetected dopants while the film composition exhibits a quite different behavior. The general presence of GB films can rationalize the discrepancy between AEM and EPMA results by their differences in probe size and detection geometry, while the much larger gap in the end member suggests the existence of Ca-rich glasses in the intergranular regions. By excluding this end member, a linear relation between dopant solution and lattice expansion is restored in α-Sialon structure, which leads to 20 and 80% increases of the expansion coefficients from those given in the previous and original reports, respectively. This study not only demonstrated the necessity of solubility study in ceramics by AEM refinement, but also opens a new front to correlate the solution behavior with the intergranular glass/amorphous structures, both were regarded so far as largely independent.

Synthesis of Ca-α-SiAlON phosphors by a mechanochemical activation route

Fine Eu2+-doped Ca-α-SiAlON phosphors have been synthesized by a novel mechanochemical activation route from stoichiometric mixtures of β-Si3N4, AlN, Al2O3, CaO and Eu2O3. Mechanical milling mostly transformed the starting powder mixture into an amorphous phase that contained a large number of well-dispersed nanocrystalline β-Si3N4 grains. Homogeneity of elements at the atomic level was achieved in amorphous phase. The formation mechanism of α-SiAlON grains in unmilled powders was through a solution–diffusion–reprecipitation process, leading to an inhomogeneous distribution of Eu2+ ions in the host phase. Mechanical activation significantly promoted the reaction among the starting powders during post-annealing, and α-SiAlON grains composed of nanocrystals with the intended composition formed directly through rapid precipitation from the homogeneous metastable amorphous phase. This novel thermodynamically unfavorable approach leads to the homogeneous distribution of Eu2+ in α-SiAlON crystals resulting in improved photoluminescence. The mechanochemical activation route is expected to be applicable to the synthesis of a wide range of nanosized (oxo)nitridosilicate phosphors with improved homogeneity and photoluminescence efficiency.

Effects of α-Si 3N 4 and AlN addition on formation of α-SiAlON by combustion synthesis

Preparation of Yb α-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from α-Si 3N 4- and α-Si 3N 4/AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si 3N 4/Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si 3N 4/Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si 3N 4/Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb α-SiAlON was achieved in the sample with Si 3N 4/Si = 0.5. Moreover, the growth of elongated α-SiAlON grains was enhanced in the samples with high contents of Si 3N 4. In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce α-SiAlON grains in a fine equiaxed form.

Fast bonding α-SiAlON ceramics by spark plasma sintering

Yb- and Y-doped α-SiAlON ceramics were effectively bonded by spark plasma sintering in less than 20 min without using any interlayer materials at 1650–1700 °C. The microstructures and mechanical properties of the joints were investigated by means of SEM, STEM, EDX, and three-point bending tests. The results showed that α-SiAlON joint formation was accompanied by fundamental rare earth diffusion. The intergranular liquids in the α-SiAlON ceramics at the bonding temperatures provided the diffusion paths for the species. α-SiAlON grain growth across the joining interfaces modified the joint microstructures to secure high bonding strength.

Mechanical Properties and Thermal Shock Resistance of Refractory Self-Reinforced α-SiAlONs Using Barium Aluminosilicate as an Additive

Yb-, Y-, Yb/Y-, and Yb/Nd-doped α-SiAlON ceramics with 5 wt% barium aluminosilicate (BAS) were synthesized by hot pressing. Typical self-reinforced microstructures were obtained in all investigated α-SiAlONs in spite of the type of doped cations. This is attributed to the incorporation of BAS, which could supply suitable liquid phase to promote the anisotropic growth of the α-SiAlON grains. All the composites exhibited excellent high-temperature mechanical properties and thermal shock resistance due to the formation of a self-reinforced microstructure and the complete crystallization of BAS additive.

Novel mixed α/β-SiAlONs with both elongated α and β grains

Yb-, Y- and Dy-doped mixed α/β-SiAlON ceramics were fabricated by hot-pressing using 10 wt.% BaAl 2 Si 2 O 8 (BAS) glass ceramic as an additive. The results showed that BAS could effectively facilitate the anisotropic growth of both α- and β-SiAlON grains. The α-SiAlON grains as well as the β-SiAlON grains were highly elongated. The obtained mixed α/β-SiAlONs possess much better mechanical properties than those of conventional mixed α/β-SiAlONs containing equiaxed α-SiAlON grains in their microstructure.

Effect of post heat treatment on microstructure and mechanical properties of Lu-doped α-SiAlONs

The effects of the excess Lu 2O 3 addition and post heat treatment on the phase composition, morphology of α-SiAlON grains and mechanical properties of Lu-α-SiAlON were investigated. The results show that incorporation of excess Lu 2O 3 facilitates the formation of elongated α-SiAlON grains. Post heat treatment not only further promotes the growth of α-SiAlON grains with a larger aspect ratio, but also results in the adjustment of phase compositions and eliminates the residual glass phases between α-SiAlON grains.

The Valance Determination of Cerium Ions in α-SiAlON by Electron Energy Loss Spectroscopy Analysis

SiAlON ceramics have found applications in many different areas due to their excellent engineering properties such as high hardness, fracture toughness, good thermal shock and oxidation resistance. SiAlON exist mainly in two different polymorphs: a (MxSi12-(m+n)Al(m+n)OnN16-n; M: metal and rare earth cations, x≈0,35 and n≤1,35) and β (β-Si6-zAlzOzN8-z; 0≤z≤4). In general, stable alpha and beta phases separately as well as in combination of α and β are obtained by incorporation of metal and rare earth cations as sintering additives. The metal cations such as Li, Mg, Ca, Y, and most lanthanide cations with the exception of La, Ce, Pr and Eu are able to stabilise α-SiAlON structure. Ekstrom et al. 1991 found that cerium can not occupy interstitial sites in α-SiAlON structure due to the fact that ionic radius of Ce3+ (0.103 nm) is too large, whereas ionic radius of Ce4+ (0.080 nm) is too small to stabilise α-SiAlON structure. After this work, several studies carried out to incorporate cerium cations into α-SiAlON structure. It was shown that cerium cation alone can be incorporated into α-SiAlON if the samples are either fast cooled after sintering, or when the samples are spark plasma sintered. On the other hand, cerium can also be incorporated into the α-SiAlON structure when it is used as a sintering additive together with a smaller α-SiAlON stabiliser cation such as Yb or Ca. Similar results were observed in other multi-cation doped SiAlONS that non α-SiAlON stabiliser cations like Sr2+ (0.112 nm) and La3+ (0.106 nm) are able to stabilise α-SiAlON when used together with α-SiAlON stabiliser cations such as Ca or Yb. Although it was shown that cerium existed in mixed valance state at domain boundaries in Ce-doped and spark plasma sintered α-SiAlON, there is no work on the valance determination of cerium in sintered α-SiAlON which has no domain boundaries. Therefore, in this study; it was aimed to incorporate cerium into α-SiAlON structure by combining with Yb3+ and the determination of possible cerium valence states (Ce3+/Ce4+) in both α-SiAlON grains and secondary phases.