Amorphization‐induced volume change and residual stresses in boron carbide

The residual pressure surrounding quasistatic and dynamic Vickers indentations in boron carbide was quantitatively mapped in 3 dimensions using Raman spectroscopy. These maps were compared against similar maps of amorphization intensity and optical micrographs of deformed regions to determine the roles of amorphization and damage upon indentation‐induced residual stress. Stress relaxation was observed near radial cracks, spalled regions, and graphitic inclusions. A positive correlation was found between high levels of residual stress and the number of amorphized sites detected. Finite element simulations were conducted to model the indentation‐induced residual stress fields in the absence of amorphization and cracking. The simulations underpredicted the average residual pressure observed through Raman spectroscopy, implying that amorphization contributes to increased pressure in the material. This pressure is interpreted as potential evidence of volumetric expansion of the amorphized material which is less ordered and hence exerts compressive forces on the surrounding crystalline matrix.     

Raman spectroscopy mapping of amorphized zones beneath static and dynamic Vickers indentations on boron carbide

Three-dimensional models of amorphized zones beneath quasistatic and dynamic Vickers indentations on boron carbide were constructed using micro-Raman spectroscopy. The square of amorphized zone depth varied linearly with load and the maximum amorphized area occurred beneath the indentation imprint in accord with the maximum shear stress under Hertzian contact. Reduced measurements of amorphization intensity at loads above 10 N may be due to a loss of subsurface amorphized material through lateral cracks. Utilizing an expanding cavity model with power-law (n = 0.79–0.80) and linear (E_p = 0.39–0.45) strain hardening responses, finite element simulations were conducted to determine the critical values of stress and strain required to cause amorphization. These simulations suggest that amorphization may initiate at von Mises stresses and equivalent plastic strains above 6.6 GPa and 0.026, respectively. These results may be useful for validating computational models of boron carbide under complex loading scenarios (e.g., ballistic impact).     

Residual stresses and crystalline quality of heavily boron-doped diamond films analysed by micro-Raman spectroscopy and X-ray diffraction

X-ray diffraction analysis and micro-Raman spectroscopy measurements have been used for stress studies on HFCVD diamond films with different levels of boron doping. The boron incorporation in the film varied in the range 10¹⁸-10²¹ boron/cm³. The grain size, obtained from SEM images, showed grains with 2-4-μm average size, which decreases when the doping level increases. The thickness of the films obtained by SEM cross-section view decreased from 8 to 5 μm as the doping level increased from 0 (undoped film) to 10²¹ boron/cm³. The total residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ-values ranging from −60° to +60°, and applying the sin² ψ method. For the micro-Raman spectroscopy the spectral analysis performed on each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which decreases from 99 to 75% as the doping level increases. The type and magnitude of the residual stress obtained from X-ray and micro-Raman measurements agreed well only for undoped film, disagreeing when the doping level increased. We attributed this discrepancy to the domain size characteristic of each technique.     

Measuring the effects of heat treatment on SiC/SiC ceramic matrix composites using Raman spectroscopy

The evolution of residual stresses found within a silicon carbide/silicon carbide (SiC/SiC) ceramic matrix composite through thermal treatments was investigated using Raman microspectroscopy. Constituent stress states were measured before, during, and after exposures ranging from 900 to 1300°C for varying times between 1 and 60 minutes. Silicon carbide particles in the as-received condition exhibited average hydrostatic tensile stresses of approximately 300 MPa when measured at room temperature before and after heat treatment. The room temperature Raman profile of the silicon matrix was altered in both shape and location with heat treatment cycles due to increasing activation of boron within the silicon lattice as heat treatment temperatures increased. By accounting for boron activation in the silicon–boron system, little to no permanent change of any constituent stresses were observed, and the silicon matrix subsequently exhibited a complimentary average hydrostatic compressive stress of approximately 300 MPa at room temperature, measured before and after heat treatment. This result builds upon previous literature and offers increased insight into boron activation phenomena measured through Raman spectroscopy methods.     

Unusual ignition behavior of polyurethane/carbon nanotube composites with a He-Ne laser excitation (632.8 nm) during micro-Raman spectroscopy

We report a spontaneous ignition of carbon nanotubes/polyurethane composite under He-Ne laser. The ignition of the composite is caused by rapid increase of the temperature to at least 660 K due to absorption of light by carbon nanotubes. This temperature was estimated from G band downshift of the carbon nanotube caused due to the bond loosening due to thermal expansion of the bonds. The morphology of the nanocomposite under ignition was studied using SEM and optical microscopy, and revealed craters of dimension on the order of the focused laser beam. This unusual synergistic effect was hypothesized as the result of poor thermal conduction between carbon nanotube and polymer matrix.     

Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities

The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics.     

The effect of residual stresses and dislocations on microwave dielectric loss in rutile-related (Ti0.6Zr0.4)0.8(Zn1/3Nb2/3)0.2O2

Strain and stress play an important role in functional materials, but they are often neglected in electroceramics. Here we report the stress effect on microwave dielectric loss in rutile-related (Ti_0.6Zr_0.4)_0.8(Zn_1/3Nb_2/3)_0.2O₂ solid solutions. The macroscopic, microscopic and nano-scale residual stresses have been systematically investigated, including their relationships. The average Raman spectra have demonstrated considerable macro-residual stresses, especially for the heavily cracked sample. The high-resolution Raman images reveal the inhomogeneity of residual stress distribution at the grain-level, which is reflected by the shifts of Raman-active modes. The high-resolution transmission electron microscope (HRTEM) images and geometric phase analysis (GPA) analysis show the aggregation of dislocations and resulting distortion of crystal. It can be found that the dielectric loss depends strongly on residual stresses, that samples with greater residual stress exhibit much higher dielectric loss because the residual stress increases the vibration anharmonicity of the lattice.     

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

The stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce³⁺ ions (YAG:Ce³⁺)/yttrium oxide partially stabilized zirconia double-ceramic-layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce³⁺ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce³⁺ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.     

Measurement of partial conversions during the solution copolymerization of styrene and butyl acrylate using on‐line raman spectroscopy

The copolymerization of styrene and n‐butyl acrylate in dioxane was monitored by on‐line Raman spectroscopy. The calculation of the individual monomer concentrations on the basis of the individual vinyl peaks is not straightforward for this system, as these bands are overlapping in the Raman spectrum. To tackle this problem, univariate and multivariate approaches were followed to obtain monomer concentrations and the results were validated by reference gas chromatography data. In the univariate analysis, linear relations between various monomer peaks were used to calculate monomer concentrations from the Raman data. In principal component analysis, the main variation in the spectra could be ascribed to conversion of monomer. Furthermore, principal component analysis pointed out that the second‐largest effect in the spectra could be attributed to experiment‐to‐experiment variation, probably attributable to instrumental factors. In the multivariate partial least squares regression approach, single factor models were used to calculate monomer concentrations. Both the univariate and the partial least squares regression approaches proved successful in calculating the individual monomer concentrations, showing very good agreement with off‐line gas chromatography data. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 426–436, 2001     

Effect of TGO thickness,pores, and creep on the developed residual stresses in thermal barrier coatings under cyclic loading using SEM image-based finite element model

Thermal barrier coatings (TBC) allow the metallic internal components of gas turbine engines to operate at elevated temperatures near its melting points. Formation of thermally grown oxide (TGO) layers at the top coat (TC) and bond coat (BC) interface induces cracks in the TC that may lead to complete TBC failure due to spallation. An SEM image-based finite element (FE) model is developed using commercial finite element package ABAQUS to investigate the development of residual stresses resulting from cyclic loading of TBCs. The model includes thermo-mechanical material properties and considers the real interface between the coating layers. The model includes real pores based on an SEM image, taking advantage of image processing techniques. Effect of TC surface roughness and pores on the developed residual stresses during thermal cycling is investigated with respect to different TGO thicknesses. The analysis shows that presence of TC roughness causes stress concentration sites during heating that may force horizontal cracks to initiate and propagate with stress values that are indifferent to the TGO thickness. The pores are found to shift stress concentration regions from the TC/TGO interface to the vicinity of the pores during cooling, and that may cause horizontal cracks to start from within the TC with stresses that increase with TGO thickness. Moreover, the effect of creep for all layers on the generated residual stresses is studied. Considering creep gives lower stresses at the end of cooling, however, stress distribution remains the same with and without creep.     

B‐stage characterization of o‐cresol novolac epoxy resin system using raman spectroscopy and matrix‐assisted laser desorption/ionization mass spectrometry

The B‐stage of the o‐cresol novolac epoxy resin–phenol novolac hardener–triphenylphosphine (TPP) catalyst system was characterized using Raman spectroscopy and matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry. The consistent decreasing intensities of characteristic epoxy resin peaks in MALDI mass and Raman spectra according to the melt mixing time were observed, which is due to the formation of the epoxy–phenol–TPP complex and the propagation reaction between them and with another epoxy resin. Our microscopic analysis method will provide a useful tool to control the optimum condition of the melt mixing process in the B‐stage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1940–1946, 2000