Ultrasonic study of the elastic and nonlinear acoustic properties of ceramic aluminum nitride

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of aluminum nitride (AlN) ceramic samples as functions of temperature in the range 100–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Aluminum nitride is an elastically stiff but light ceramic: at 295 K, the longitudinal stiffness (C _L), shear stiffness (), adiabatic bulk modulus (B ^S), Young's modulus (E) and Poisson's ratio () are 373 GPa, 130 GPa, 200 GPa, 320 GPa and 0.234, respectively. The temperature dependences of C _L and B ^S show normal behaviour and can be approximated by the conventional model for vibrational anharmonicity. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Grüneisen parameters. The values determined at 295 K for the hydrostatic-pressure derivatives (C _L/P) _P=0, (/P) _P=0 and (B ^S/P) _P=0 are 4.7 ± 0.1, 0.22 ± 0.03 and 4.4 ± 0.15, respectively. The adiabatic bulk modulus B ^S and its hydrostatic-pressure derivative (B ^S/P) _P=0 are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (_L), shear (_S), and mean (^el) acoustic-mode Grüneisen parameters of AlN are positive: the zone-centre acoustic phonons stiffen under pressure. The shear _S (=0.006) is much smaller than the longitudinal _L (=1.09) accounting for the low thermal Gr¨neisen parameter ^th (=0.65) obtained for this ceramic: since the acoustic Debye temperature _D (=980 ± 5 K) is so high, the shear modes play an important role in acoustic phonon population at room temperature. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic AlN.     

Measurements of elastic and anelastic properties of reaction-formed silicon carbide ceramics

The piezoelectric ultrasonic composite oscillator technique (PUCOT) was used to measure dynamic Young's modulus, damping, strain amplitude, and the strain-amplitude dependence of damping on three reaction-formed silicon carbide (RFSC) specimens. The data were compared with results found on NC 203 SiC. The temperatures used to conduct the tests ranged from 595–1049°C, and this information complemented data collected earlier. The main results are as follows: the modulus for RFSC specimens decreases with increasing temperature over a range of approximately 350–260 GPa from room temperature to 1049°C, the modulus for NC203 ranges from 450–415 GPa over the same temperature range, and damping of RFSCs is independent of strain amplitude and has little temperature dependence.     

Ultrasonic determination of the temperature and hydrostatic pressure dependences of the elastic properties of ceramic titanium diboride

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of titanium diboride (TiB₂) ceramic samples as functions of temperature in the range 130–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. TiB₂ is an elastically stiff but light ceramic: at 295 K, the longitudinal stiffness (C _L ), shear stiffness (), adiabatic bulk modulus (B ^S ), Young's modulus (E) and Poisson's ratio () are 612 GPa, 252 GPa, 276 GPa, 579 GPa and 0.151, respectively. The adiabatic bulk modulus B ^S is in good agreement with the results of recent theoretical calculations. All elastic moduli increase with decreasing temperature and do not show any pronounced unusual effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Grüneisen parameters. The values determined at 295 K for the hydrostatic-pressure derivatives (C _L /P) _P=0, (/P) _P=0 and (B ^S /P) _P=0 are 7.29 ± 0.1, 2.53 ± 0.1 and 3.91 ± 0.1, respectively. The hydrostatic-pressure derivative (B ^S /P) _P=0 of the bulk modulus of TiB₂ ceramic is found to be larger than that estimated previously from uniaxial shock-wave loading experiments. The longitudinal (_L), shear (_S), and mean (^el) acoustic-mode Grüneisen parameters of TiB₂ are positive: the zone-centre acoustic phonons stiffen under pressure in the usual way. Since the acoustic Debye temperature _D (=1190 K) is very high, the shear modes provide a substantial contribution to the acoustic phonon population at room temperature. Knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic TiB₂.     

Measuring the nonlinear elastic properties of tissue-like phantoms

A direct mechanical system simultaneously measuring external force and deformation of samples over a wide dynamic range is used to obtain force-displacement curves of tissue-like phantoms under plain strain deformation. These measurements, covering a wide deformation range, then are used to characterize the nonlinear elastic properties of the phantom materials. The model assumes incompressible media, in which several strain energy potentials are considered. Finite-element analysis is used to evaluate the performance of this material characterization procedure. The procedures developed allow calibration of nonlinear elastic phantoms for elasticity imaging experiments and finite-element simulations.     

Computational estimation of elastic properties of spark plasma sintered TaC by meshfree and finite element methods

In this study, the overall elastic modulus of spark plasma sintered TaC composite has been estimated using a novel engineering analysis technique, called Scan-and-Solve, that makes it possible to perform completely automated stress analysis directly from the segmented micrographs. The computed results have been compared with object oriented finite element technique (OOF), which also makes use of the microstructure. In contrast with the traditional mesh based engineering analysis methods, Scan-and-Solve uses spatial meshes that may or may not conform to the shape of the geometric model. This makes Scan-and-Solve computational technology essentially meshfree, and it makes it possible to eliminate error-prone and time consuming data conversion and spatial meshing. The presented method guarantees exact treatment of the prescribed boundary conditions. In the paper, we compare the stress simulation results in porous TaC ceramic obtained by the Scan-and-Solve and object oriented finite element methods. It is shown that the effective elastic modulus predicted from the microstructure by the two methods is very similar (266 vs. 270 GPa) provided the porosity coefficients are measured close to each other.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Measurement and analysis of elastic and anelastic properties of alumina and silicon carbide

Measurements of dynamic Young's modulus, E, and damping as a function of temperature, T, were made for alumina and silicon carbide. The Young's modulus data were compared with some from the literature, and analysed in terms of a theoretical framework relating the Debye temperature, θD, with the elastic constants. For both materials this analysis yielded a ratio T₀/θ_D which was near 0.4, where T₀ is an empirical fitting constant for the plot of (E(0)−E)/T versus 1/T (E(0) is the value of E at 0 K). The analysis of the damping data in terms of an Arrhenius type dependence led to effective activation energies near kT, where k is Boltzmann's constant. This revised version was published online in November 2006 with corrections to the Cover Date.     

Elastic properties of carbide, nitride, and boride ceramics with WC-type structures

The elastic constants (C _ij ) of boride, carbide, and nitride (RuB, WC, WN, and TaN) ceramics with WC-type structures have been calculated using the full-potential linearized augmented plane wave (FLAPW) method with an exchange-correlation potential in the generalized gradient approximation (GGA). Numerical estimates of elastic parameters of the corresponding polycrystalline ceramics (bulk compression modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Lamé’s coefficients) of these ceramics are obtained and analyzed for the first time.     

Enhanced microwave absorption properties of polymer-derived SiC/SiCN composite ceramics modified by TiC

Porous SiCN(Ti) composite ceramics with good microwave absorbing performance were fabricated by pyrolysis of solid polysilazane modified by tetrabutyl titanate. The introduction of Ti not only acted as active filler to react with free carbon in the matrix to form TiC, but also played the role as catalyst to promote the formation of SiC nanowires. Finally, SiCN(Ti) composite ceramics formed a microstructure containing multi-nanophases and multi-nano heterogeneous interfaces when annealing temperature reached 1500 °C. The complex microstructure annealed at 1500 °C made composite ceramics have good matching impedance, as well as greatly increase the interfacial polarization loss and dipole polarization loss. As a result, the TiC/SiC/SiCN composite ceramics showed the excellent performance of electromagnetic wave absorption in X band. The minimum reflection loss (RL) of samples was ??17.1 dB at the thickness of 1.9 mm, and the maximum effective absorption bandwidth (EAB) of composite ceramics was 3.2 GHz when the thickness of sample was 2.1 mm, which exhibited a promising prospect as a structural and microwave absorbing integration material.

     

A study of the mechanical properties of highly porous ceramics using acoustic emission

In this paper the results of indirect tensile tests on highly porous ceramics are presented. A relation between the mechanical strength of the highly porous ceramic materials and Acoustic Emission (AE) has been established. We have shown that the amplitude distribution of the AE events depends on the crack velocity, which itself depends on the stress intensity of the crack. Apart from the Brazilian (side crushing strength) tests also multi-point loading experiments were carried out. The AE results show the additional damage accumulation due to compressive/shear stresses.     

添加固体润滑剂的Al2O3/TiC陶瓷材料力学性能和显微结构

采用热压工艺制备了添加固体润滑剂MoS2、BN、CaF2的Al2O3/TiC陶瓷材料,测量了其力学性能和分析了其显微结构.结果表明,添加固体润滑剂的Al2O3/TiC陶瓷比未添加时的力学性能有大幅下降,其中Al2O3/TiC/CaF2陶瓷材料的力学性能最好,当CaF2含量为10%时,Al2O3/TiC/CaF2陶瓷材料的强度和硬度最高,分别达到了589MPa和HV1537;而添加BN的Al2O3/TiC陶瓷材料的力学性能最差.XRD衍射结果和微观结构显示,添加MoS2的Al2O3/TiC材料中的MoS2发生分解,基体中存在较多的气孔;添加BN的Al2O3/TiC材料中的BN与Al2O,反应生成AlN,造成大量裂纹的产生,致使材料的强度和硬度都大幅下降;Al2O3/TiC/CaF2陶瓷材料中的CaF2在烧结过程中没发生化学反应,复合材料晶粒大小均匀,基体组织成网状结构,有利于提高材料的强度.     

Effect of a new additive on mechanical properties of hot-pressed silicon carbide ceramics

The mechanical properties and microstructure of SiC ceramics, hot pressed by simultaneously adding nano-SiC and oxides (MgO+Al₂O₃+Y₂O₃) or nitrate salts (Mg(NO₃)₂+Al(NO₃)₃+Y(NO₃)₃) as additives, were evaluated. The oxide additives system slightly influenced the mechanical properties of the ceramics, while the addition of nano-SiC lead to finer microstructure, and 5 vol.% nano-SiC changed the fracture mode from intergranular type to transgranular type. The ceramics with nitrate salts had fine, equiaxed grains with an average grain size larger than that of the system added oxides, thus inducing lower Viker’s hardness and flexural strength, while the presence of crystalline YAG phase improved the fracture toughness by 54.7%. Also, an observed increase in grain growth—with decreasing weight fraction of liquid and the grounded grain morphology in this system—confirmed a diffusion-controlled growth mechanism. Although the sample with the least amount of additives has the lowest relative density and largest grain size, its flexural strength did not drastically decrease. The influence of nano-SiC on the fracture toughness in the nitrate additive system was negligible.     

Composition and temperature dependence of the dielectric, piezoelectric and elastic properties of pure PZT ceramics

Pure (undoped) piezoelectric lead zirconate titanate (PZT) ceramic samples at compositions across the ferroelectric region of the phase diagram were prepared from sol-gel-derived fine powders. Excess lead oxide was included in the PZT powders to obtain dense (95-96% of theoretical density) ceramics with large grain size (>7 mum) and to control the lead stoichiometry. The dielectric, piezoelectric, and elastic properties were measured from 4.2 to 300 K. At very low temperatures, the extrinsic domain wall and thermal defect motions freeze out. The low-temperature dielectric data can be used to determine coefficients in a phenomenological theory. The extrinsic contribution to the properties can then be separated from the single-domain properties derived from the theory.     

Temperature and pressure dependences of the elastic properties of ceramic boron carbide (B4C)

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic boron carbide (B₄C) as functions of temperature in the range 160–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. B₄C is an elastically stiff but extremely light ceramic: at 295 K, the longitudinal stiffness (C _L), shear stiffness (), adiabatic bulk modulus (B ^S ), Young's modulus (E) and Poisson's ratio () are 498 GPa, 193 GPa, 241 GPa, 457 GPa and 0.184, respectively. In general, the adiabatic bulk modulus B ^S agrees well with both experimental and theoretical values determined previously and is approximately constant over the measured temperature range. Both E and increase with decreasing temperature and do not show any unusual effects. The values determined at 295 K for the hydrostatic-pressure derivatives ( C _L/P) _P=O , (/P) _P=O and (B ^S /P) _P=O are 5.7 ± 0.3, 0.78 ± 0.4 and 4.67 ± 0.3, respectively. The hydrostatic-pressure derivative (B ^S /P) _P=O of the bulk modulus is found to be comparable with that estimated previously from dynamic yield strength measurements. The effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Grü neisen parameters. The longitudinal (_L), shear (_S), and mean (^el) acoustic-mode Grüneisen parameters of B₄C are positive: the zone-centre acoustic phonons stiffen under pressure in the usual way. Knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic B₄C. Since the acoustic Debye temperature _D (=1480 K) is very high, the shear modes provide a substantial contribution to the acoustic phonon population at room temperature.     

A study of the tool life of TiC and TiC plus Al2O3 chemical vapour deposited tungsten carbide tools

Tool life characteristics were investigated for tungsten carbide cutting tools coated with TiC and with TiC plus Al₂O₃. A low carbon steel workpiece was turned on a lathe at a feed rate of 0.206 to 0.410 mm rev^–1 and a depth of cut of 0.1 to 0.5 mm for cutting velocities between 100 and 250 m min^–1. Data were analysed using both Taylor's tool life equation and Wu's tool life method. Results were similar for both methods but Wu's method seemed to give more consistent results. Compared to an uncoated tungsten carbide tool, the tool life of both the coated tools were from 5 to 7 times longer and the improvement was greater at higher cutting speeds. The TiC plus Al₂O₃ coated tool was slightly superior to the TiC coated tool. The wear mechanism and a possible explanation of increased tool life for the coated tools are discussed.     

Multi-layer phase analysis: quantifying the elastic properties of soft tissues and live cells with ultra-high-frequency scanning acoustic microscopy

Scanning acoustic microscopy is potentially a powerful tool for characterizing the elastic properties of soft biological tissues and cells. In this paper, we present a method, multi-layer phase analysis (MLPA), which can be used to extract local speed of sound values, for both thin tissue sections mounted on glass slides and cultured cells grown on cell culture plastic, with a resolution close to 1 μm. The method exploits the phase information that is preserved in the interference between the acoustic wave reflected from the substrate surface and internal reflections from the acoustic lens. In practice, a stack of acoustic images are captured beginning with the acoustic focal point 4 μm above the substrate surface and moving down in 0.1-μm increments. Scanning parameters, such as acoustic wave frequency and gate position, were adjusted to obtain optimal phase and lateral resolution. The data were processed offline to extract the phase information with the contribution of any inclination in the substrate removed before the calculation of sound speed. Here, we apply this approach to both skin sections and fibroblast cells, and compare our data with the V(f) (voltage versus frequency) method that has previously been used for characterization of soft tissues and cells. Compared with the V(f) method, the MPLA method not only reduces signal noise but can be implemented without making a priori assumptions with regards to tissue or cell parameters.