Nondestructive techniques for evaluating and characterizing coatings were extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or to develop life prediction models of thermal spray coatings used in various applications (e.g., wear, fatigue, and high temperatures). In the current study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser-ultrasonics is a noncontact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was used to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the current study was to focus on evaluating the elastic modulus of very uniform, dense, and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by air plasma spray (APS) and high-velocity oxyfuel (HVOF) and two types of WC-Co coatings, conventional and multimodal (nanostructured and microsized particles), deposited by HVOF.

/pdf/elastic-modulus-measurements-via-laser-ultrasonic-and-knoop-36pma75u9n.pdf

Elastic modulus measurements via laser-ultrasonic and knoop indentation techniques in thermally sprayed coatings

Ti2AlC and Ti3AlC are both considered as candidate materials for structural applications in harsh environments. In this paper, we adopt density functional theory to study the electronic, elastic, acoustic and defect properties of Ti2AlC and Ti3AlC. It was found that the mechanical properties of Ti2AlC and Ti3AlC vary little; the bulk modulus at equilibrium of Ti3AlC is 10% higher than that of Ti2AlC. However, we found that their defect properties were very different. The migration barrier of an Al vacancy in Ti2AlC is 0.834 eV compared with 4.518 eV in Ti3AlC. The difference by a factor of six in the migration barriers is explained by the bond angle variance of the C-centred octahedral. Using the calculated elastic constants, the slowness surface of the acoustic waves is obtained using the Christoffel equation. Since Ti2AlC and Ti3AlC are often presented as a second Ti–Al–C phase in each other, the individual properties calculated can be used to assess their characterizations in experiments.

https://iopscience.iop.org/article/10.1088/0022-3727/47/21/215301/pdf

A comparative first-principles study of the electronic, mechanical, defect and acoustic properties of Ti2AlC and Ti3AlC

The goal of this paper is to determine the bulk acoustic wave (BAW) propagation velocities (quasi-longitudinal, quasi-shear vertical and quasi-shear horizontal) in two important piezoelectric smart materials, Lithium Niobate (LiNbO3) and Lithium Tantalate (LiTaO3). To determine the BAW propagation velocities, the BAW elemental equations are deduced. The BAW velocities are calculated for each direction by solving the Christoffel's equation systematically base d on the theory of acoustic waves in anisotropic solids exhibiting piezoelectricity. The modification of the BAW velocities by the piezoele ctric effect are calculated and graphically compared with the velocities in the corresponding non-piezoelectric materials. Further more, the electromechanical coupling factors are defined and investigated. The results obtained in this study can be applied to signal pr ocessing, sound systems and wireless communication in addition to the improvement of surface acoustic wave (SAW) devices and military defense equipment.

/pdf/calculation-of-bulk-acoustic-wave-propagation-velocities-in-2y1a75erzd.pdf

Calculation of Bulk Acoustic Wave Propagation Velocities in Trigonal Piezoelectric Smart Materials

MION, CHRISTIAN. Investigation of the thermal properties of gallium nitride using the three omega technique. (Under the direction of Dr. J. F. Muth). Gallium nitride based devices suffer from undesirable heating effects that significantly limit the performance of high electron mobility transistors and laser diodes thereby reducing device lifetime and reliability. An accurate knowledge of the gallium nitride thermal conductivity is crucial to understanding thermal effects. This work addresses issues related to both the thermal limitations and thermal characterization of gallium nitride alloys and devices. First, theoretical developments concerning the three omega technique applied to a film-on-substrate system are considered when the film-to-substrate thermal conductivity ratio is larger than one. The case of thermal boundary resistance between film and substrate is included. In the case of high thermal conductivity films, it is found that neglecting the presence of thin thermally insulating buffer layer would lead to large uncertainties in extracted film thermal conductivities. Next, very thick, free standing gallium nitride layers grown by hydride vapor phase epitaxy were precisely measured from 300K to 450K using the three omega method. By comparing the measured values with dislocation density measurements the dependence of the gallium nitride thermal conductivity on dislocation density was obtained and compared with theory. In addition, the thermal conductivity of iron doped semi-insulating gallium nitride was measured to be as high as 230 W·K·m. In this study, a 2mm thick iron doped gallium nitride substrate was measured to have an average dislocation density of 5.10 cm which represents the present state of the art. Finally the modeling of the thermal resistance of multifinger AlGaN/GaN HEMTs was examined using the experimentally determined values of the thermal conductivity. Using the three omega method as the starting point, an original accurate closed-form compact expression for the thermal resistance of single and multifinger HEMT device structures was derived. It was found that the thermal performance of a single finger HEMT composed of homoepitaxial GaN on a GaN substrate is superior to that of single finger GaN HEMT heteroepitaxially grown on silicon carbide. INVESTIGATION OF THE THERMAL PROPERTIES OF GALLIUM NITRIDE USING THE THREE OMEGA TECHNIQUE

Investigation of the thermal properties of gallium nitride using the three omega technique

Under high pressure, the long believed single-phase material CaB6 was latterly discovered to have a new phase tI56. Based on the density-functional theory, the pressure effects on the structural and elastic properties of CaB6 are obtained. The calculated bulk, shear, and Young's moduli of the recently synthesized high pressure phase tI56-CaB6 are larger than those of the low pressure phase. Moreover, the high pressure phase of CaB6 has ductile behaviors, and its ductility increases with the increase of pressure. On the contrary, the calculated results indicate that the low pressure phase of CaB6 is brittle. The calculated Debye temperature indicates that the thermal conductivity of CaB6 is not very good. Furthermore, based on the Christoffel equation, the slowness surface of the acoustic waves is obtained.

https://iopscience.iop.org/article/10.1088/1674-1056/22/7/077101/pdf

Density-functional theory study of the effect of pressure on the elastic properties of CaB6

Acoustic anisotropic properties of PbB4O7 crystals are investigated and discussed. The slowness curves of three bulk waves as well as the surface waves for the plates with different cut directions are calculated by considering piezoelectric effects. The theoretical calculations are in agreement with experimental results determined by laser ultrasonic technique. On the other hand, the excitation efficiency of surface waves of PbB4O7 crystals is obtained by calculating the change of the velocities of acoustic surface waves get from the plates bare and deposited by a thin metal film. The results of this paper can be used for selecting PbB4O7 crystal orientations for designing the related devices.

Acoustic anisotropy of piezoelectric PbB4O7 crystals studied by laser ultrasonics

Extraction of electromechanical coupling coefficient of piezoelectric thin films deposited on substrates

Based on one-dimensional theory of thermal wave reconstruction, a two-dimensional theory of thermal conductivity profile reconstruction is developed. The important developments in the two-dimensional (2D) inverse problem are that a line source is used instead of the source with a large cross section in one dimension, and a finite difference method is adopted for the numerical solution of the 2D heat equation. Numerical simulations demonstrate that this approach is effective and stable even with 5% random noise disturbance in the signal.

Qing-biao Zhou

Papers

Acoustic anisotropy of piezoelectric PbB4O7 crystals studied by laser ultrasonics

Extraction of electromechanical coupling coefficient of piezoelectric thin films deposited on substrates

Reconstruction of thermal properties of inhomogeneous materials from one dimension to two dimensions