YSiAlON Glass Research Articles

Zn–Sr mixing in the Y-sialon glass: Formation, properties and ballistic resistance

The Zn–Sr–Y-sialon glasses were prepared by melting batches under N2 atmosphere in a resistance furnace. The differences in compositions between samples that were amorphous, glassy and those that were partly crystalline are clear. FT-IR spectroscopy was done to find out the basic structural units in some of these glasses. The effects of Zn/Sr substitution for Y on the properties such as density, thermal expansion coefficient, glass transition temperature, crystallization temperature, Vickers hardness, bending strength and fracture toughness were discussed. UV–vis transmission spectra of the samples showed that the transmittance of the obtained glasses can reach up to 80% in the visible region. Ballistic resistance represented by Drel value was assessed by physical and mechanical properties including density, hardness, elastic modulus and fracture toughness. The ballistic resistance values of the obtained oxynitride glasses were calculated and compared.

B-Phase solid solutions in Er- and Y-SiAlON glass–ceramics

Glasses with cation composition of M:Si:Al = 3.45:3:2 (where M = Er and Y) with varying nitrogen contents (0, 5, 8, 15 and 22 eq.%) have been prepared by ball milling in alcohol media. These Er- and Y-sialon glasses were heat-treated at temperatures in the range of 950–1200 °C for 10 h to study the crystal phase(s) obtained. Samples were characterised by SEM and XRD and obtained data from XRD were further analysed to obtain unit cell parameters of some phases. It is shown that it is possible to obtain B-phase (Y2SiAlO5N) from these glass compositions with relatively low nitrogen contents (5 and 8 eq.%N) but with different unit cell parameters (i.e. a = 3.77–3.81 A and c = 9.83–9.97 A) to those reported previously in the literature, suggesting that there are some compositional differences between B-phases obtained from different nitrogen content glasses as well as different modifier cations. Long-term heat treatment studies revealed that the stability range of temperature for Er–B-phase is more extensive than its Y–B-phase equivalent.

Microstructure characteristics of FeMo–Sialon ceramic composite

A new metal–Sialon based ceramic composite of FeMo–Sialon has been processed using Si 3N 4, α-Al 2O 3 and FeMo70 alloy powders as the main raw materials. To explain why this composite has excellent mechanical properties, microstructure characterizations are carried out using the transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The results show that FeMo, FeSi 2 and Mo have formed an alloy mixture of Mo–Fe–Si in the FeMo–Sialon ceramic composite. Elongated β-Sialon crystals, interlacing with each other, vary in size. The Y-SiAlON glass, as an inter-granular phase, surrounds the grain boundaries of β-Sialon crystals. Some of β-Sialon crystals are semi-surrounded by dumbbell-like Mo–Fe–Si particles. The good mechanical properties of FeMo–Sialon composite are greatly dependent on the elongated Sialon grains and Mo–Fe–Si particles with interlocking structures.

Influence of Amount of Nitrogen on Crystallisation of Y-SiAlON Glasses: In Situ XRD Analysis

Y-SiAlON glasses of composition 36.5 Y: 42.3 Si: 21.2 Al with different amounts of N (0, 5, 8, 15 and 22 in e/o) were produced by melting appropriate mixtures of powders under flowing nitrogen at 1715°C. This composition is known to give B-phase (Y2SiAlO5N) on crystallisation at temperatures below 1050°C. In this work, the effect of nitrogen in the starting glass composition on the crystalline phases formed is discussed. High temperature in-situ XRD analysis was performed on powdered glass samples up to 1150°C by using a Philips X’pert PRO MPD (Multi Purpose Diffractometer) with a HTK1200 Oven Camera (Anton Paar, Austria). As expected, the results show that different nitrogen contents affect the crystalline phases formed. In all glasses, yttrium apatite silicate forms first, followed by crystallisation of B-phase. The phase transformation from B-phase to Iw-phase (Y3Si2Al[O,N10] i.e. 10 e/o N) takes place at relatively low temperatures (1050°C) for the lower nitrogen containing samples (5 and 8 e/o), whereas, the transformation does not take place for the glasses with higher nitrogen contents even at the maximum temperature studied (1150°C). This work also confirms that there is a correlation between the temperature where the first crystals appear and the amount of nitrogen in the starting glass.

Liquid-phase sintering of alumina with YSiAlON oxynitride glass

A single-phase alumina ceramic with elongated grains was obtained by pressureless sintering of alumina powder at 1500 °C nitrogen gas, using YSiAlON oxynitride glass as a sintering additive. The green compacts were sintered for various times to determine the optimal conditions for liquid-phase sintering of alumina with a YSiAlON glass additive. The relative densities of samples sintered at 1500 °C for 3 h reached as high as 99.2%, but further increasing the holding time resulted in decreased densities. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) with energy-dispersive analysis indicated a Y 3Al 5O 12 crystalline phase and a small amount of amorphous phase, containing nitrogen and oxygen, at the grain junctions. The high-temperature mechanical properties of A1 2O 3/5 wt.%YSiAlON ceramic are much higher than monolithic alumina. The bending strength and fracture of the materials at 800 °C were 420±17 MPa and 5.3±0.2 MPa m 1/2, respectively. The YSiAlON additive was found to be useful for the liquid-phase sintering of alumina.

Diffusive Phenomena at Si<sub>3</sub>N<sub>4</sub>/Glass Interfaces

The chemistry of glass-ceramic interface was studied in two different diffusion couples, YSiAlO/Si 3 N 4 and SiO 2 Si 3 N 4 . Experiments were performed at 1500°C for 2 hours in argon atmosphere. The couples were analyzed through SEM and EDS to draw the concentration profiles across the interface. It was observed that aluminosilicate glass infiltrates within the ceramic secondary phase and detaches the silicon nitride grains. The free grains move toward the glass surface either by solution-reprecipitation or floating. The glass also dissolves silicon nitride grains and the nitrogen is absorbed, forming a broad zone of YSiAlON glass. In the SiO 2 /Si 3 N 4 couple there was extensive cation diffusion from the silicon nitride grain boundary to the silica glass, forming an aluminosilicate intermediary phase. It was observed crystallization of some yttrium-rich phase at the interface.

The Formation of B and I<sub>w</sub> Phases in the YSiAlON System: Influence of Nitrogen Content

YSiAlON glasses have been produced using a cation ratio of Y:Si:Al=3.5:3.38:2 with a nitrogen content of 17 e / o and Y:Si:Al=3.45:3:2 with varying nitrogen contents (N=8, 10, 12, 14, 17, 20 e / o ). Crystal phase formation occurred after heat treatment of the glasses under nitrogen atmosphere for 10 hours in the temperature range 1050-1150°C. Glass-ceramics were characterised by X-ray diffraction (XRD) and certain mechanical properties measured. Depending upon heat treatment temperature and/or nitrogen content, either a single crystalline phase glass-ceramic or a multi-phase glass-ceramic containing B and I w was produced. Low nitrogen contents (8 e / o N) favour the crystallisation of I w -phase over B-phase, whereas B-phase is formed more readily for higher nitrogen contents (17 e / o N).

Deformation mechanisms and constitutive modeling for silicon nitride undergoing laser-assisted machining

Material deformation behavior has been studied for silicon nitride containing 10 wt% of YSiAlON glass under laser-assisted machining (LAM). Material removal mechanisms were inferred from scanning electron microscopy observations of the chips, and the shear zone stress was determined using three-dimensional machining theory with a new approach to determining shear angles for segmented chips formed in LAM. The effects of operating conditions on the shear zone stress were determined from an experimental parametric study, and the results were used to develop a constitutive equation for the deformation process.

Transient, three-dimensional heat transfer model for the laser assisted machining of silicon nitride: I. Comparison of predictions with measured surface temperature histories

Laser assisted machining (LAM), in which the material is locally heated by an intense laser source prior to material removal, provides an alternative machining process with the potential to yield higher material removal rates, as well as improved control of workpiece properties and geometry, for difficult-to-machine materials such as structural ceramics. To assess the feasibility of the LAM process and to obtain an improved understanding of governing physical phenomena, experiments have been performed to determine the thermal response of a rotating silicon nitride workpiece undergoing heating by a translating CO 2 laser and material removal by a cutting tool. Using a focused laser pyrometer, surface temperature histories were measured to determine the effect of the rotational and translational speeds, the depth of cut, the laser-tool lead distance, and the laser beam diameter and power on thermal conditions. The measurements are in excellent agreement with predictions based on a transient, three-dimensional numerical solution of the heating and material removal processes. The temperature distribution within the unmachined workpiece is most strongly influenced by the laser power and laser-tool lead distance, as well as by the laser/tool translational velocity. A minimum allowable operating temperature in the material removal region corresponds to the YSiAlON glass transition temperature, below which tool fracture may occur. In a companion paper [1], the numerical model is used to further elucidate thermal conditions associated with laser assisted machining.

Experimental Evaluation of the Laser Assisted Machining of Silicon Nitride Ceramics

To assess the feasibility of the laser assisted machining (LAM) process for the machining of difficult-to-machine materials such as structural ceramics, experiments were performed on silicon nitride workpieces for a wide range of operating conditions. Data for cutting forces and surface temperatures indicate that the lower bound of the material removal temperature for avoidance of cutting tool and/or workpiece fracture corresponds to the YSiAlON glass transition temperature (920–970°C). As temperatures near the cutting tool increase to values above the glass transition temperature, the glassy phase softens, facilitating visco-plastic flow and, correspondingly, the production of semi-continuous or continuous chips. The silicon nitride workpiece machined had a surface roughness of Ra=0.39 μm at the nominal LAM operating condition. Examination of the machined surfaces and chips reveals no detectable sub-surface cracking or significant changes in microstructure, respectively. Relative to grinding, the most significant advantage of LAM is its ability to achieve much larger material removal rates with high workpiece surface quality and reasonable levels of tool wear. [S1087-1357(00)00704-8]

Classical and Differential Thermal Analysis Studies of the Glass‐Ceramic Transformation in a YSiAlON Glass

Nucleation and crystallization studies were conducted on a YSiAlON glass that contained 17 equiv.% nitrogen (7.5 at.%) by using a two‐stage nucleation‐and‐growth treatment. Classical and differential thermal analysis (DTA) techniques were both used to study the crystallization process, to ensure that the optimum heat‐treatment schedule that yielded a fine microstructure and minimum residual glass was applied. The optimum nucleation and crystallization temperatures were determined from DTA traces that were recorded from isothermal heat treatments at different nucleating temperatures, ranging between Tg ‐ 40°C and Tg+ 100°C for 1 h and crystal‐growth temperatures in the range of 1170°‐1310°C for 0.5 h, respectively. The activation energy for the crystallization process was determined, based on the analysis of the variation of peak temperature at five different heating rates. Specimens heat treated in a tube furnace under nitrogen gas were subjected to microscopical investigation, and results showed variations in the volume fraction of crystalline phases and crystal size with nucleation temperature. The nucleation temperature, Tg+ 40°C (1025°C), which corresponded to the maximum volume fraction of crystalline phases and minimum crystal size, was consistent with the optimum nucleation temperature, Tg+ 35°C (1020°C), as determined from DTA. The time and temperature of nucleation and crystal growth dictated the nature and size of the crystalline phases. Properties such as hardness and density were assessed and correlated to the nucleation temperature. The influence of sample specific surface on the devitrification mechanisms was estimated, and bulk nucleation was observed to be the dominant nucleation mechanism.

Preparation of Pure B-Phase Glass-Ceramic in the Yttrium SiAION System

The present study aims to determine controlled crystallisation heat treatments and thermal stability for pure B phase glass-ceramic in the Y-Si-Al-O-N system. Experiments using differential thermal analysis and the ultrasonic techniques were performed on a YSiAlON glass to define the optimum nucleation temperature of B phase according to Marotta's method. Changes in elastic properties allowed to determine the optimum nucleation time and to follow the glass to glass-ceramic transformation on massive glass samples. Maximum bulk nucleation occurs after one hour in the range 960-970°C and the grain growth ends after 10 hours at the crystallisation temperature of 1050°C. XRD analyses confirm B phase as the unique crystalline phase thermally stable up to 1150°C and SEM observations show a very fine and homogeneous microstructure with a high crystal volume fraction. By determining optimum conditions for nucleation and crystal growth of B phase, good quality nitrogen glass-ceramics have been produced.

Joining of ceramics using oxide and oxynitride glasses in the Y-Sialon system

Glasses in the Y-SiAlO and Y-SiAlON systems, of standard cation composition (28:Y, 56:Si, 16:Al) have been investigated as potential adhesive materials for silicon nitride-based ceramics. Glass transition and crystallisation temperatures are reported. Hardness and density have also been measured. These glass materials have been used to bond hot-pressed and sintered β-sialon ceramics containing an intergranular glass composition similar to that of the adhesive material. Joints were formed via two processes: (1) hot-pressing at temperatures from 1130 °C to 1600 °C for 1h at a pressure of 5 MPa using the Y-sialon glass; and (2) pressureless sintering at 1600 °C for 1h, using both the Y-sialon and Y-sialo glasses. At a temperature of 1600 °C, SEM analysis illustrates that both processes yield visibly good joints. Hardness measurements indicate that there is no degradation in value between the parent ceramic and the bonded region. Joining was dominated by the flow of glass across the ceramic interfaces, dissolution of the individual grains within the glass and reprecipitation of the Si 3N 4 across the interface reinforcing the joint.

Mechanical Spectroscopy of a YSiAlON Glass

A YSiAlON glass has been studied by mechanical spectroscopy, through the glass transition. The α-relaxation peak observed in the YSiAlON glass can be interpreted well by a formalism of hierarchically correlated molecular mobility through the glass transition. In particular this formalism accounts for the observation of unusually high apparent activation energy. The viscoelastic properties of the glass are compared to the mechanical behavior of a silicon nitride ceramic.

Influence of zirconia addition on the crystallization kinetics of a Y-Si-Al-O-N glass

The crystallization behaviour of YSiAlON glasses with and without ZrO 2 additives, prepared by melting at 1700 °C, were studied by means of differential scanning calorimetry, scanning microscopy and X-ray powder diffraction analysis. The optimum nucleation temperatures were determined to be T g + 65° C and T g + 50 ° C for the ZrO 2-free and the ZrO 2-containing glass, respectively. The crystallization of the glasses was found to be dominated by the surface mechanism of nucleation and growth. The added zirconia acted as a growth modifier rather than a nucleating agent.

Oxidation behaviour of yttrium and neodymium oxynitride glasses

The oxidation behaviour of oxynitride glasses in M-Si-Al-O-N system (M = Nd, Y) has been studied in air and under various oxygen pressures. The influence of glass composition on the oxidation resistance was studied at 1075 °C by varying Y Al or Si Al ratios and also by substituting Y by Nd and by varying the nitrogen content. The reactivities in air of the YSiAlON system is studied in the temperature range of 1000–1150 °C. The oxidation resistance of YSiAlON glasses is better than for glasses containing Nd. A pressure law is determined for the YSiAlON system based upon data from oxidation at 1050 and 1075 °C at oxygen pressures in the range of 15–100 kPa. The nature and morphology of oxidised samples were characterised by optical microscopy, XRD and SEM. These investigations reveal that the oxidation of the studied oxynitride glasses is governed by a reaction process where the progress of the internal interface is the limiting step.

Heat Treatment of Controlled Composition Y-Sialon Ceramics

A systematic study of the devitrification behaviour of the grain boundary phase in Y-Sialon ceramics has been carrled out in an attempt to optimize the devitrification products by compositional control and heat treatment schedules. Three serles of Y-Sialon compositions with varying amounts of additivles were sintered to near-theoretical density to achleve a two-phase system consisting of β-Si3N4 or β’Sialon and a Y-Sialon glass. Optimum heat treatment conditions to crystallize the glassy grain boundary phase to Y3Al5O12 (YAG) were determined to be 1400°C and 100 h for most compositions. However, devitrification of the glass to the garnet had a detrimental effect on the room temperature strength. This decrease is attributed to the volume shrinkage associated with the formation of YAG which results in microcracking and debonding of the garnet crystals from the surrounding grains. Résumé Une étude systématique a été entreprise pour élucider le comportement des céramiques Y-Sialon quant à la dévitrification des phases aux points de grains, dans le but d'optimiser les produits de dévitrification par des contrôles de composition et des horaires de traitement thermique. Trois séries de Y-Sialon de compositions différentes et ayant différentes quantités d'additifs, ont été frittées jusqu'à une densité proche de la densité thémique, produisant ainsi un système biphasé formé de β-Si3N4 ou β'Sialon et un verre Y-Sialon. Les conditions optimales du traitement thermique nécessaire à la cristallisation en Y3Al5O12 (YAG) des phases viteuses aux points de grains, sont de 1400°C et 100 h pour la plupart des compositions. Cependant, la dévitrification du verre en grenat a un effet néfaste sur la resistance a température ambiante. Cette perte de resistance est attribuée à la contraction du volume associée à la formation du YAG qui induit de la microfissuration et une perte de cohésion entre les cristaux de grenat et les grains les entourant.

15N MAS NMR in the YSiAlON system

15N MAS NMR has been used to characterise an YSiAlON glass and its crystallisation products. In the glass twofold and threefold coordinated nitrogen was identified. In addition in the crystallised material a peak of large chemical shift is ascribed to singly coordinated nitrogen substituted into the Y 3Al 5O 12YAG structure.

Grain‐Boundary‐Phase Crystallization and Strength of Silicon Nitride Sintered with a YSlAlON Glass

Densifying silicon nitride with a YSiAlON glass additive produced 99% dense materials by pressureless sintering. Subsequent heat‐treating led to nearly complete crystallization of the amorphous intergranular phase. Transmission electron microscopy revealed that for heat treatments at 1350°C, only β‐Y2Si2O7 was crystallized at the grain boundaries. At a higher temperature of 1450°C, primarily YSiO2N and Y4Si2O7N2 in addition to small amounts of Y2SiO5 were present. Al existed only in high concentrations in residual amorphous phases, and in solid solution with Si3N4 and some crystalline grain‐boundary phases. In four‐point flexure tests materials retained up to 73% of their strengths, with strengths of up to 426 MPa, at 1300°C. High‐strength retention was due to nearly complete crystallization of the intergranular phase, as well as to the high refractoriness of residual amorphous phases.