Georg Rixecker

Liquid phase sintering and microstructure-property relationships of silicon carbide ceramics with oxynitride additives

Abstract SiC ceramics incorporating sintering additives from the system AlN–Y2O3 and having variable α-SiC/β-SiC, AlN/Y2O3 ratios and additive contents were gas-pressure sintered to theoretical density. Post-sintering heat treatments were performed in order to induce phase transformations from β-SiC to α-SiC and anisotropic grain growth, leading to platelet-strengthened microstructures. The indentation fracture toughness of the platelet materials reaches values in excess of 6 MPa m 1/2 after annealing at 1950°C. Four-point bending strengths were measured at room temperature and at high temperatures, and were correlated with microstructural data obtained by scanning electron microscopy and X-ray powder diffraction. Both the kinetics of the phase transformation/platelet formation and the strength retention at high temperature were found to exhibit maxima as a function of the AlN/Y2O3 ratio in the sintering additive.

Fracture properties of SiC ceramics with oxynitride additives

Abstract Silicon carbide ceramics incorporating sintering additives from the system AlN–Y 2 O 3 can be gas-pressure sintered to theoretical density. While commonly a combination of sesquioxides is used such as Al 2 O 3 –Y 2 O 3 , oxynitride additives offer the advantage that only a moderate nitrogen overpressure is required instead of a powder bed for thermochemical stabilization at the sintering temperature. In the present study aspects of the fracture behavior of these materials are addressed, namely the influence of anisotropic grain growth and processing flaws, additional toughening at high temperatures and thermal shock characteristics. They are correlated with microstructural data obtained by scanning electron microscopy.

Compositional identification of the intergranular phase in liquid phase sintered SiC

Abstract A model system consisting of coarse SiC (32–160 μm) as starting powder and Y 2 O 3 and AlN as sintering additives was liquid phase sintered. Coarse-grained starting powder led to large intergranular phase regions which allowed an accurate determination of the chemical composition by wavelength-dispersive X-ray microanalysis (WDS). When N 2 was used as sintering atmosphere, a N-rich amorphous phase (about 44 at.% N) was identified by WDS to be the main triple-junction phase in the sintered SiC ceramics, while three further crystalline intergranular phases were AlN, Y 2 SiN 4 O 3 and an O-rich phase (Y 10 Al 2 Si 3 O 18 N 4 ). The overall O content was found to be reduced in comparison to the initial powder composition. The incorporation of N from the sintering atmosphere into the intergranular phase and a subsequent carbothermal reduction are believed to be responsible for the removal of O and the formation of the N-rich amorphous phase.

Mössbauer spectroscopic studies of defect structure and alloying effects in nanostructured materials

Mössbauer spectroscopy provides spectral information from both ordered and structurally disordered regions of a solid and is therefore well suited for the atomic-scale characterisation of materials with very high defect concentrations. This applies especially to nanocrystalline materials where 5–50% of the atoms may be located at planar defects such as grain boundaries. In this paper, the range of Mössbauer spectroscopy in exploring the structure of nanostructured materials will be discussed in the form of case studies dealing with (i) nanometer-sized antiphase domains in the intermetallic compound Fe3−xSi1+x, (ii) ball-milling induced structural changes and alloying effects in dilute Al(57Fe) and Y(57Fe) alloys, and (iii) the Mössbauer signature of grain boundaries in nanocrystalline W(57Fe).

The difficulty of isolating grain boundary components in the Mössbauer spectra of ball-milled materials: iron and silver–iron alloys

Abstract Although 57 Fe Mossbauer spectroscopy is well known to be a particularly suitable method for the characterization of nanostructured solids, attempts to isolate spectral features that can be associated with grain boundaries encounter unexpected difficulties in the case of ball-milled or mechanically alloyed metallic materials. In this paper, the examples of ball-milled α-Fe and Ag(Fe) alloys prepared by mechanical alloying are discussed in order to show what kinds of structural information can still be extracted from the hyperfine parameters, in spite of the presence of large concentrations of lattice defects and long-range magnetic interactions.

Effects of powder bed conditions on the liquid-phase sintering of Si3N4

The effects of the active and passive protection mechanisms of powder beds on the sintering of Si 3 N 4 were investigated. Shrinkage, density, and coloring behavior of sintered samples were analyzed using different compositions and packing conditions of powder beds based on BN and Si 3 N 4 with different additives. Y 2 O 3 additive in the powder bed influences the weight change and phase formation behavior of the samples, although it has a very low vapor pressure at the sintering temperature. When MgO/Y 2 O 3 was used as sintering additives, the packing density and thickness of the powder bed had a much stronger effect than in the case of Al 2 O 3 /Y 2 O 3 . For the optimization of the powder bed conditions, the vapor pressure and chemical stability of sintering additives at the sintering temperature has to be considered.

Analytical TEM study of microstructure-property relations in liquid-phase-sintered SiC with AlN-Y2O3 additives

The microstructures of liquid-phase sintered SiC with AlN-Y 2 O 3 additives are systematically investigated by using transmission electron microscopy and analytical electron microscopy. Pure α-SiC as starting powder leads to fine, equiaxied microstructure. Introduction of oc-SiC seed crystals into β-SiC powder accelerates the β-to-α-SiC phase transformation through a solution-precipitation process and promotes anisotropic grain growth, which results in a plate-like microstructure. Core/rim structures were found in both cases as a result of AlN dissolution into the re-precipitated part of SiC grains. This changes the liquid composition during sintering and induces crystallization of Y 1 0 Al 2 Si 3 O 1 8 N 4 and Y 2 O 3 in the triple-pockets. Amorphous films were observed to wet both grain boundaries and two-phase interfaces. A low ratio of AlN to Y 2 O 3 in the sintering additive accelerates the devitrification of triple-pockets. Additional annealing can further devitrify the triple-pockets as well as the amorphous GB films, leading to a microstructure with potentially higher creep resistance.

Influence of Impurities on the High-Temperature Water-Vapour Corrosion of Environmental Barrier Rare-Earth Silicates

Corrosion experiments on a number of rare earth di-silicates containing Y, Yb and Lu have been conducted in flowing air with 30 vol.% H2O at 1500°C. Nominally the corrosion rate is slower by a factor 5 to 10 compared to silica. However, alumina impurity incorporation has a profound influence on the process of corrosion as it masks not only the gravimetric results but also changes the surface phase assemblages to contain rare earth garnets and liquid phases. It is presumed that silica loss occurs under those conditions via the liquid phase and does not necessarily follow the same kinetics as direct silicate corrosion. Furthermore, atmospheric silicon hydroxide saturation effects contribute to the corrosion process. Currently all determined corrosion rates of rare earth silicates are viewed as system specific only.

Fe16Nb6Si7 and Fe16Ta6Si7: New D8a phases synthesized by the crystallization of mechanically alloyed amorphous powders

Abstract Ternary phases with the cubic D8 a structure (isostructural to Mg 16 Cu 6 Si 7 ) were identified in both the Fe–Nb–Si and Fe–Ta–Si systems. They form during the crystallization of mechanically alloyed amorphous materials and are thermally stable during heat treatments up to 90 h at 1050°C. The X-ray powder diffraction data were evaluated (i) by local line fit, and (ii) by Rietveld analysis. Mossbauer spectroscopy was used to complement the XRD results and to show the absence of magnetic ordering above 77 K.

Bipolar Electric Fatigue in Ferroelectric Nb-Doped PZST Ceramics

Bipolar electric fatigue in ferroelectric niobium-doped lead zirconate titanate stannate ceramics was investigated. Variations in the polarization and strain hysteresis loops as well as microstructural modifications of the material due to the electric cycling were analyzed. Compared with ferroelectric PZT ceramics cycled under similar conditions, the material exhibited a higher resistance to electric fatigue. Properties of the specimens showed significant decays above 106.5 cycles. Asymmetric suppression of strain hysteresis loop was monitored. At 108 cycles, the remanent polarization, the right maximum strain and the left maximum strain decreased to 70%, 58% and 44% of their initial values, respectively. The color of the material was changed from pale yellow to dark grey with cycling. The properties and color of the fatigued samples were recovered almost fully to those of the virgin state after a heat treatment at 500oC for 1 h. The pinning of domain walls, probably by oxygen vacancies, was concluded to be the predominant fatigue mechanism for the material.