Influence of Processing and Microstructure on Internal Friction of Ceramics (Invloed van de verwerking en de microstructuur op de inwendige wrijving in keramische materialen)

Internal friction or damping is a phenomenon of converting an applied mechanical energy of vibrations by a solid body into internal energy, which dissipates over a period of time. Knowledge of the resonance frequencies and the internal friction is extremely useful in characterising fundamental properties such as elastic modulus, defects in materials, glass transitions, etc. even in materials with complex microstructures. An established characterisation technique called Impulse Excitation Technique (IET) is extensively used in the present study to analyse the internal friction properties of two material systems: crystalline zinc oxides (ZnO) and amorphous ionomer glasses (4.5SiO2-3Al2O3-1.5P2O5-3MO-2MF2, where M is alkaline earth metal). The interpretation of results is further corroborated by carrying out phase and micro structural analysis using other characterization techniques such as high temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).ZnO varistors are used for the protection of electronic circuits against excessive transient voltages. Doping is a crucial step to achieve better electrical and thermal properties of a ZnO varistor because it results in a homogenous and dense microstructure. In the first part, the temperature dependent elastic modulus and damping properties of ZnO varistors doped with bismuth and antimony oxides are investigated. The damping peaks are associated with point defects relaxation and phase transformation mechanisms. Ionomer glasses are widely used in dentistry as a component in dental glass ionomer cements (GIC). The internal friction of ionomer glasses containing alkaline earth ions in the structure are studied in the second part of the thesis. Substitution of calcium by barium or strontium in the glass composition results in a disrupted glass network, whereas magnesium leads to a more packed network. The glass containing magnesium results in high Young s modulus due to its small ionic radius and high field strength. A theoretical model proposed by Makishima and Mackenzie is applied for estimation of Young s modulus at room temperature. By incorporating the structural information in the classical Makishima and Mackenzie model a more accurate estimate can be obtained as compared to the measured Young s modulus. An original approach is presented to describe the temperature derivative of the Young s modulus using an empirical relation based on the Makishima and Mackenzie model. The ionomer glasses showed anelastic relaxation arising from oxygen diffusion and mixed alkaline effect or both, depending on the number of alkaline earth cations (single or double) in the network. The glass transition and crystallization behaviour are interpreted using XRD, DSC and internal friction results. The crystallization studies on ionomer glasses showed apatite phase formation, which has similar structure as bone and can be used for bio-medical applications. The internal friction of calcium- fluoro-aluminosilicate glass annealed at different temperatures exhibits glass transition and crystallization peaks. The impulse excitation technique is used as a characterisation tool for a qualitative analysis of residual glass remains after crystallization.nrpages: 253status: publishe

Analysis of Si addition on phase formation in AlCoCrCuFeNiSiₓ high entropy alloys

In the present work, the effect of Si addition on the structure of AlCoCuCrFeNiSiₓ alloy system is studied. The alloys were prepared by mechanical alloying in high energy ball mill. The phase formation of the present high entropy alloys were analysed by X-ray diffraction to understand the alloying behaviour. The analysis of X-ray diffraction pattern clearly shows that the addition of Si favours the formation of body centered cubic structure. The experimental results were also in accord with the theoretical prediction of structure based upon mixing entropy, atomic size mismatch, mixing enthalpy, valence electron concentration, electronegativity difference and mismatch entropy.status: publishe

Temperature dependent Young's modulus of Si and Ge

© The Electrochemical Society. The vibrational properties of single crystal Si and Ge are studied between room and melting temperature Tm using the impulse excitation technique. From the measurements, the temperature dependent Young's moduli E are extracted in the -, - and -directions. For both semiconductors, the Young's moduli decrease smoothly with increasing temperature and retain high values up to Tm. Using the semi-empiric Wachtman's equation allows an excellent fit to the experimental data for the temperature dependence of E, E and between room temperature and 0.6TmFor higher temperatures, the Young's moduli decrease faster than predicted by Wachtman's equation. In the case of Ge, an apparent enhanced softening is observed starting from about 850 °C which is due to a loss of Ge material, leading to a decrease of sample dimensions and weight.status: publishe

Young's modulus and damping in dependence on temperature of Ti-6Al-4V components fabricated by shaped metal deposition

Young's modulus and damping behavior is investigated by the impulse excitation technique in vacuum up to 1100 A degrees C for Ti-6Al-4V components, fabricated by shaped metal deposition (SMD). This is a novel additive manufacturing technique where near net-shape components are built layer by layer by tungsten inert gas welding. The Young's modulus decreases linearly from 118 GPa at room temperature to 72 GPa at 900 A degrees C, followed by a stronger decrease up to 1000 A degrees C and during the first heating a plateau thereafter. The damping exhibits an exponential increase with temperature superimposed by two peaks around 700 and 900 A degrees C during the first heating. During cooling and follow-up cycles only the damping peak around 700 A degrees C appears. The change in Young's modulus and the damping behavior is interpreted by different processes like α/ß transformation, O alloying and grain boundary sliding. These results indicate that components fabricated by SMD contain a non-equilibrium alpha phase which transforms to the ß phase at higher temperatures than the equilibrium α phase. Furthermore, the vacuum between 2.4 and 5.3 x 10(-4) mbar proved at high temperatures to be not good enough to rule out the contamination by O, which leads to alpha casing, stiffening, and hardening.status: publishe

Determination of the Si Young's modulus between room and melt temperature using the impulse excitation technique

Stress induced by the thermal gradients near the meltsolid interface affects the intrinsic point defect properties and the quality of single crystal Si grown from a melt. Also during device processing, stress in the Si substrate influences point defect behavior during thermal treatments. To be able to simulate and control the stress distribution one needs to know the elastic constants of single crystal Si at high temperatures. In the present study, the vibrational properties of single crystal Si samples are studied between room and melt temperature using the impulse excitation technique. From the measurements, the temperature dependent Young’s moduli Eijk of moderately doped Czochralski-grown Si samples are extracted in the 100, 110 and 111 crystallographic directions. Close to the melt temperature, very high Young’s moduli values between 110 and 160 GPa are obtained, depending on the crystallographic direction. Empiric expressions are derived for the temperature dependence of E100, E110 and E111 and of the elastic compliances s11 and s12 +s44/2, useful for application in process simulation.status: publishe

Diamond dispersed Si3N4 composites obtained by pulsed electric current sintering

30 vol.% 2 and 30 µm diamond dispersed Si3N4 matrix composites were prepared by pulsed electric current sintering (PECS) for 4 min at 100 MPa in the 1550–1750 °C range. The densification behaviour, microstructure, Si3N4 phase transformation and stiffness of the composites were assessed, as well as the thermal stability of the dispersed diamond phase. Monolithic Si3N4 with 4 wt% Al2O3 and 5 wt% Y2O3 sintering additives was fully densified at 1550 °C for 4 min and 60 MPa. The densification and α to ß-Si3N4 transformation were substantially suppressed upon adding 30 vol.% diamond particles. Diamond graphitisation in the Si3N4 matrix was closely correlated to the sintering temperature and grit size. The dispersed coarse grained diamonds significantly improved the fracture toughness of the diamond composite, whereas the Vickers hardness was comparable to that of the Si3N4 matrix ceramic. The Elastic modulus measurements were found to be an excellent tool to assess diamond graphitisation in a Si3N4 matrix.status: publishe

Thermal expansion and lattice parameters of shaped metal deposited Ti-6Al-4V

Thermal expansion and lattice parameters are investigated up to 1100 degrees C for Ti-6Al-4V components. fabricated by shaped metal deposition. This is a novel additive layer manufacturing technique where near net-shape components are built by tungsten inert gas welding.status: publishe

Temperature dependence of the dynamic Young's modulus of ZrB2-MoSi2 ultra-refractory ceramic composites

The dynamic Young's modulus is studied by the impulse excitation technique (IET) on ZrB2-based composites containing MoSi2 as a secondary phase up to 1700 K under a continuous flow of Ar (5% H-2). The Young's modulus of the composites shows three different decreasing rates, the first two of which are very similar to the temperature trend of the constituent phases. At 1700 K, the Young's modulus of the composites is still more than 75% of the room temperature value.status: publishe

Ultrafine Al2O3-B4C composites consolidated by pulsed electric current sintering

Al2O3-60 vol.% B4C composites with 0-5 wt% SiC addition were consolidated by pulsed electric current sintering (PECS) during 4 min at 1650 °C. The influence of the SiC additive on the densification behaviour, microstructure and mechanical properties of the Al2O3-B4C composites were investigated. Microstructural analysis revealed the presence of Al18B4O33 grains, formed by chemical reaction of Al2O3 with surface oxides on B4C, in agreement with thermodynamic calculations. The Al2O3 grains were found to coarsen from 0.22 μm in the starting powder up to about 0.60 μm during PECS, whereas the size of the ∼0.6 μm B4C grains hardly changed. The Al2O3 and B4C grain size slightly decreased upon SiC addition. The addition of 3 or 5 wt% SiC hardly influenced the hardness and toughness of the Al2O3-B4C composites, whereas the strength and stiffness were reduced. The composite with 1 wt% SiC combined a hardness of 26 GPa, an E-modulus of 448 GPa, a bending strength of 600 MPa and a fracture toughness of 6.2 MPa m1/2. The influence of the loading condition and dwell time during PECS of the 5 wt% SiC doped composite was assessed. © 2010 Elsevier B.V. All rights reserved.status: publishe