Optical and Mechanical Properties of α/β Composite Sialons

Two-phase α/β composites have been produced with a combination of high hardness, fracture toughness, and strength. Compared with a single-phase α-sialon, the composite showed around a twofold increase in both fracture toughness and bending strength, with only minimal reduction in hardness. Despite being a two-phase material, the optical properties of the composite were very good, showing transparency in sections of around 0.5 mm thickness. The optical properties were in fact better for the composite than for the single-phase α-sialon. Work to date on transparent sialons has focused on single-phase α-materials, which have inherently low fracture toughness unless elongated microstructures are developed. However, this microstructural development appears to adversely affect optical transparency. In this work it has been shown that good combination of mechanical properties can be achieved while maintaining optical transparency in two-phase composite sialons. The development of such materials should widen their range of application.     

Effect of the Amount of Additives and Post-Heat Treatment on the Microstructure and Mechanical Properties of Yttrium–α-Sialon Ceramics

The yttrium–sialon ceramics with the composition of Y_0.333Si₁₀Al₂ON₁₅ and an excess addition of Y₂O₃ (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y₂O₃ could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K _IC= 6.3 MPa·m^1/2) via grain debonding and pullout effects. Although the addition of 5 wt% Y₂O₃ could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness.     

Translucent α-Sialon Ceramics by Hot Pressing

The single-phase α-sialon ceramics with high optical transmittance have been prepared by hot pressing. The maximum transmittance reached 65.2% and 52.2% in the infrared wavelength region, 58.5% and 40% in the visible region for the samples 1.0 and 1.5 mm thickness, respectively. The material also exhibited good mechanical properties of high hardness (20 GPa) and better fracture toughness (5.1 MPa·m^1/2). Both high optical transmittance and improved toughness of α-sialon ceramics were attributed to the less-grain-boundary glassy phase and the homogeneous microstructure, which was obtained by a proper process and confirmed by SEM and TEM observation, compared to that prepared by ordinary sintering. It is, therefore, expected that the translucent α-sialon ceramics could be a promising optical window material.     

Yttrium α-Sialon Ceramics by Hot Isostatic Pressing and Post-Hot Isostatic Pressing

Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Y _x (Si_{12–4.5 x} , Al_{4.5 x} )-(O_{1.5 x} ,N_{16–1.5 x} ) with 0.1 ≤ x ≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon ( x = 0.1) to large amounts of α-sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness ( K _IC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values.     

Effect of Seeding on the Microstructure and Mechanical Properties of α-SiAlON: III, Comparison of Modifying Cations

Single-phase in situ toughened SiAlON ceramics containing various modifying cations and single-crystal seeds were studied. The modifying cations include rare-earth cations from the smallest to the largest allowed in the α-SiAlON structure (Yb to Y, to Nd), and from monovalent to trivalent (Li to Ca, to rare earths). At low seeding levels, the aspect ratio of grains increases with the size of modifying cations, giving rise to rather different appearances of the microstructure in different SiAlONs. A one-to-one correspondence between seed crystals and large grains at low seeding levels is also observed. An optimal amount of seeds is required to maximize the fracture toughness, which is controlled by grain pullout with the fracture energy that scales with the fraction of elongated grains, their width, and their aspect ratio. The optimal amount of seeds required to reach maximal toughening increases with the aspect ratio of grains and is the lowest (1%) in Y- and Yb-SiAlONs.     

Control of Microstructures in α-SiAlON Ceramics

The effects of sintering cycles and doping elements on the microstructures of Ln-α-sialon were studied. The results showed that microstructures with an elongated α-sialon morphology could be obtained through high-temperature post-heat treatment (1800–1900°C) or by prolonging soaking times during sintering. Different rare-earth elements had a profound effect on the microstructure of the resulting α-sialon. The Ln-α-sialon doped with low- Z -value elements could easily develop elongated grains with higher aspect ratio.     

Plasma Etching of α-Sialon Ceramics

Plasma etching of β-Si₃N₄, α-sialon/β-Si₃N₄ and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si₃N₄ grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si₃N₄ grains. The size, shape, and distribution of β-Si₃N₄ grains in the α-sialon/β-Si₃N₄ composite ceramics were revealed by the present method.     

Microstructure Control of In-Situ-Toughened α-SiAlON Ceramics

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.     

R-Curve Behavior of In Situ Toughened α-SiAlON Ceramics

R -curves of single-phase Y- and Ca-containing α-SiAlON ceramics have been measured. They range from flat ones for fine-grain ceramics to pronounced rising ones when large elongated grains are present. The highest toughness measured reached 11.5 MPa·m^1/2 over a crack extension of about 1000 μm.     

Self-Reinforced α-SiAlON Ceramics with Improved Damage Tolerance Developed by a New Processing Strategy

Alpha-SiAlON ceramics with a refined self-reinforced microstructure, i.e., containing acicular grains with dimensions much smaller than those obtained in previous studies, embedded in a matrix consisting of submicrometer-sized isotropic grains, were prepared by applying a rapid one-step sintering procedure. To suppress the overabundant formation of α-SiAlON nuclei, a combination of stabilizing cations, Y + Yb, was used; to encourage formation of acicular α-SiAlON grains, a small amount of an extra liquid (∼3 vol%) was introduced; to avoid abnormal grain coarsening resulting from dynamic ripening, the final sintering temperature was set to just slightly above the minimum temperature threshold for activating grain growth (1700°C). The fully dense compacts obtained exhibited excellent thermal-shock resistance, and hardness and fracture toughness values of 20 GPa and 5.1 MPa·m^1/2, respectively.     

Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2O3 Additions

Addition of Y₂O₃ as a sintering additive to porous β-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y₂O₃ were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y₂O₃-free ceramics of the same chemical composition. Although this level of Y₂O₃ addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y₂O₃-free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y₂O₃ addition the supposed formation of Y-α-SiAlON.     

α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.     

Scratch Damage in Ceramics: Role of Microstructure

Scratch tests were conducted using a standard pyramid indenter against α-SiAlON ceramics with different microstructures: (i) fine equiaxed grains and (ii) large elongated grains. The formation and propagation of cracks were investigated via focused ion-beam milling, with an emphasis on the effect of microstructure on material removal. The fine equiaxed microstructure exhibited high resistance to material removal at low loads, because of its high hardness and homogeneous structure. As the load increased, radial and lateral cracks developed, resulting in large-scale chipping. In contrast, the large elongated microstructure showed a propensity to form microcracks and microabrasion, which is characteristic of partial grain removal, at low loads. With increasing loads, however, the large elongated grains suppressed the propagation of radial and lateral cracks, and, consequently, no large-scale chipping occurred. Implications for material design in abrasive-wear conditions have been discussed.     

Liquid-Phase Growth of Small Crystals for Seeding α-SiAlON Ceramics

Single-phase small crystals of Li-, Mg-, Ca-, Y-, Nd-, and Yb-α-SiAlONs have been obtained by liquid-phase sintering for various compositions and processing conditions. These crystals are suitable for seeding grain growth in α-SiAlON ceramics. The influence of chemical and processing parameters (starting composition and powders, green density, liquid content, heating schedule, nitrogen pressure, and temperature) on the size and morphology of seed crystals has been investigated. The results are compared with those for β-Si₃N₄ crystal formation, and the differences are discussed in terms of nucleation and growth kinetics during liquid-phase sintering.     

Homogeneity Region and Thermal Stability of Neodymium-Doped α-Sialon Ceramics

Dense sialon ceramics along the tie line between Si₃N₄ and Nd₂O₃·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T_cool) of ∼50°C/min) or quenching (T_cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd_xSi_{12–4S x} Al_{4.5 x} O_{1. 5 x} , N_{16–1.5 x} . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si_6-zAl_z)N_10-zO_z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K _lc= 4.4 MPa·m^1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.     

Duplex α,β-Sialon Ceramics Stabilized by Dysprosium and Samarium

Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R₂Si_3−xAl_xO_3+xN_4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H _V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam^1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam^1/2).     

β-Sialon Ceramics Prepared at 1700°C by Hot Isostatic Pressing

Dense, single-phase β-sialon ceramics were sintered at 1700°C and 200 MPa using the glass-encapsulated hot isostatic pressing technique. The materials were very hard, 1500 to 1700 kg / mm² (98 N load), but were fairly brittle, with an indention fracture toughness of about 3 MPa · m^1/2. The addition of 1 wt% Y₂O₃ before sintering had a positive effect on the toughness, especially at the low x compositions of Si_3-xAl_xO_xN_4-x, where K_IC∼4 MPa · m^1/2.     

Microstructural Tailoring and Characterization of a Calcium α-SiAlON Composition

Three calcium α-SiAlON microstructures—namely, fine-grained, bimodal, and large elongated—were developed using powders of the same composition and then characterized. The evolution of grain size and morphology was determined to be a process of nucleation and growth that could be controlled with a two-step sintering technique. The extent of texture was identified in the as-hot-pressed materials as a function of sintering conditions. Samples with different microstructures exhibited different hardness and fracture toughness. The true hardness was derived from the intrinsic relation between applied loads and indent sizes. The effect of microstructure on hardness and fracture toughness was analyzed.     

Processing and Properties of in Situ-Reinforced α-SiAlONs Stabilized with Y2O3 and Lu2O3

α-SiAlONs with equiaxed and elongated microstructures stabilized with Y₂O₃ and Lu₂O₃ were produced by hot pressing, and the phase structure and room- and high-temperature mechanical properties were assessed. Additional liquid added to the starting composition in the form of 5 wt% rare-earth monosilicate resulted in the formation of elongated microstructures and improvements in room-temperature strength and fracture toughness. The elongated grain growth was promoted by the additional liquid phase, which crystallized to form a secondary grain-boundary phase thought to be J ' (Re₄Si_{2– x} Al _x O_{7+ x} N_{2– x} ). For the equiaxed and the elongated samples, those sintered with Lu₂O₃ showed higher hardness than the comparable Y₂O₃-sintered materials, and, at elevated temperature, the strength retention of the elongated Lu₂O₃ SiAlON was much higher than that of the Y₂O₃ sample, which was attributed to properties of the residual grain-boundary phase associated with the difference in the cationic radius of the stabilizing cation.     

Optical Properties of Gd–α-Sialon Ceramics: Effect of Carbon Contamination

Thick translucent and luminescent Gd–α-sialon ceramic disks (0.7–1.06 mm in thickness) were prepared by hot pressing. The effect of carbon atmosphere on their optical properties during sintering was explored by change packing methods. The results show that the sample with a lower carbon contamination has a higher translucence in the visible band and IR band (450–3500 nm), increasing transmission around 10% even if it is thicker. When excited at 350 nm, Gd–α-sialon with the lower carbon contamination can produce a visible light at 450–500 nm bands, but the luminescence is very weak in the sample containing more carbon contamination. These indicate that carbon contamination causes a severe degradation of the optical properties of α-sialon ceramics, and reduction of carbon contamination of α-sialon ceramics is very important for the optical property improvement.