James E. Crocker

Thermal modeling of selective area laser deposition (SALD) and SALD vapor infiltration of silicon carbide

A 3D finite element model was developed that simulates selective area laser deposition vapor infiltration (SALDVI) of silicon carbide. The model predicts the laser input power history needed to maintain constant surface temperature and the distribution of vapor deposited SiC within the powder bed as well as on the surface of the powder bed. The model considers a moving Gaussian distribution laser beam, temperature‐ and pore‐dependent thermal conductivity, specific heat and temperature‐dependent deposition rate. Furthermore, the model also includes closed‐loop control of the laser power to achieve a desired target processing temperature on the surface of the power bed. Effects of laser scanning rates have been investigated. The simulated solid fraction and SALD distributions are also consistent in the trend with the experimental data.

In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. III. Fabrication and Properties of the SiC/C Thermocouple Device

Abstract Fabrication of a SiC/C thermocouple embedded in an arbitrarily-shaped SiC macro-component has been demonstrated using an integrated Selective Area Laser Deposition (SALD) with the closely related Selective Area Laser Deposition Vapor Infiltration (SALDVI) process. SALD is used to make the embedded thermocouple devices in situ and SALDVI to fabricate the macro-components. The thermocouple elements, silicon carbide and carbon lines, and the electrical insulation layer, Si3N4, have been deposited from tetramethylsilane (TMS), acetylene, and a gas mixture of TMS and ammonia, respectively. It is found that the fabricated in situ thermocouples respond sensitively to temperature variation. Furthermore, the electric signal of the embedded thermocouple is very stable and reproducible in response to thermal cycling. This is not the case when the thermocouple is not embedded in the SiC matrix because of the oxidation of the thermocouple elements.

Powder effects in SiC matrix layered structures fabricated using selective area laser deposition vapor infiltration (SALDVI)

Silicon carbide has been deposited by laser-induced chemical vapor infiltration from the gas precursor tetramethylsilane, Si(CH3)4, into loosely packed powder layers of SiC, ZrO2-Y2O3, or Mo. The goal is to produce dense layered structures of arbitrary shape by computer controlled laser scanning where the pore spaces between the powder particles are filled with solid material deposited from the gas phase using the selective area laser deposition vapor infiltration (SALDVI) process. Layered samples were fabricated for each powder material using both single line (bar) and multiple line (slab) laser scan patterns and 10 Torr Si(CH3)4, 2.5 μm/s scan speed, 1000°C target temperature, and 120 μm layer thickness. Samples of SiC and ZrO2-Y2O3 are prone to surface cracking in the bar geometry, and cracking and delamination of layers in the slab geometry. Samples fabricated with Mo powder have no cracks or delamination defects in either bar or slab geometry as well as a better surface appearance.

FRACTURE OF GEOMETRICALLY SCALED, NOTCHED THREE-POINT-BEND BARS OF HIGH STRENGTH STEEL

Abstract The purpose of the experimental work reported in this paper was to provide data that may serve for the development of scaling rules for ductile fracture initiation at blunt notches. Fracture experiments were performed with three sizes of geometrically scaled notched bend specimens of high strength low hardening HY-130 steel using carefully scaled fixturing. Fracture initiation, defined as the appearance of the first sub-millimeter crack with a tensile opening, was reliably detected, using acoustic emission and direct visual inspection with a microscope. Comparison of the normalized load versus load-point displacement curves revealed a significant scale effect on the condition for fracture initiation, with large specimens fracturing at smaller normalized displacement than smaller specimens. The normalized displacement at fracture does not decrease in direct proportion to specimen size. Metallographic cross-sections of the specimens and fractographic observations revealed that at the microscale, fracture initiates by shear localization followed by nucleation and growth of voids under shear and tension in the localization zone. This mechanism profoundly modifies the stress and strain gradients at the notch root so that microstructural length scales become fracture controlling parameters. Both changes are probably responsible for the observed scaling behavior.

In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. I. Thermochemical Modeling

Abstract To fabricate macro-structural SiC components containing an In Situ SiC/C thermocouple using an integrated SALD and SALDVI technique, thermodynamic analyses on the involved reactant gases have been performed with the CET89 code based on the minimization of the system free energy. The gaseous precursors considered include tetramethylsilane (TMS) and methyltrichlorosilane (MTS) for the deposition of silicon carbide, and methane, ethylene and acetylene for the deposition of carbon. Reactions between disilane and acetylene and between TMS and ammonia have also been thermodynamically calculated for the deposition of silicon carbide and silicon nitride (for use as an insulation layer between the thermocouple and the matrix), respectively. Based on these analyses, four characteristic temperature zones have been defined for the decomposition of silicon carbide from TMS. A silicon nitride deposition map has been built for the TMS and ammonia system. The deposition temperature range of silicon nitride is fo...

Using Small Cracked Round Bars to Measure the Fracture Toughness of a Pressure Vessel Steel Weldment: A Feasibility Study

The research objectives were to demonstrate the feasibility of using small fatigue pre-cracked round bars to measure the initiation fracture toughness of ductile nuclear pressure vessel steels and weldments and to refine and validate experiment and analysis procedures. Initiation fracture toughness values were measured for a duplicate of HSSI Weld 72W, unirradiated, in the temperature range -150° to 50°C, using small cracked round bar (CRB) specimens. The results were compared with the values obtained with 1T-CT specimens. The good agreement between the toughness values measured with CRB and 1T-CT specimens indicates that using small CRB specimens (possibly cut from Charpy bars) to measure fracture toughness is feasible. A relationship between J and the displacement due to the crack δ c r , where δ c r is obtained from extensometer measurements, was established experimentally. Fracture initiation in CRBs of the size investigated here occurred at or near maximum load, with the crack growth prior to maximum load being less than 200 μm. This observation, together with the unique relationship between J and δ c r , open the possibility of greatly simplified testing and data reduction procedures for fracture experiments with CRB.

In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. II. Evaluation of Processing Parameters

Abstract In order to fabricate well-control led in situ SiC/C thermocouples embedded within macro-structural SiC components using an integrated selective area laser deposition (SALD) and the selective area laser deposition and vapor Infiltration (SALDVI) technique, the major processing parameters affecting the crystal structure, the deposition rate, surface morphology of deposits, and shapes and sizes of the cross section of deposited lines are evaluated. It is found that the growth rate of SiC deposits increases with temperature and tetramethy Isilane (TMS) gas pressure over the temperature and pressure range studied. The apparent activation energy for depositing SiC from TMS is 61 kJ/mole in the temperature range from 700 to 1200°C and independent of the TMS gas pressure ranging from 20 to 60 torr. The shape and size of the cross section of SiC lines depend strongly on the deposition temperature. XRD examination indicates that the deposition product using a C2H2 precursor at 900°C is crystalline graphit...

Gas Phase Solid Freeform Fabrication of Saldvi of SiC Cermets

In this work, the solid freeform fabrication of cermets was explored. Using a laser-based approach, SiC was deposited by chemical vapor deposition from tetramethylsilane gas into powder layers of Cu, Mo, or Ni. The resulting structures were examined to observe the extent of reaction between the metal powders and the vapor deposited SiC. Silicide formation was observed, most readily with the Ni powder. The thermal expansion of the metals compared to that of the vapor deposited SiC affected the interfacial stresses generated in the cermets during fabrication.

Investigation on Morphology and Microstructure of the SALD SiC

In this work, the deposition of silicon carbide lines using a tetramethylsilane (TMS) precursor was investigated. Effects of target temperatures on the morphology and crystal structure of the deposits were examined. It was found that the morphology of the SALD SiC depends strongly on the target temperature. The contour of the cross section of the SiC deposits changes from a triangle to trapezoid to volcano shape and the surface morphology of the deposited lines changes from smooth to rough to porous as the target temperature increases. A critical target temperature was found to be about 700°C to initiate deposition of SiC under the current experimental configurations. X-ray diffraction analyses show that the SALD SiC formed at 1000°C contains both crystalline and amorphous phases. The results are briefly discussed.