Oxidation Of Free Carbon Research Articles

Influence of Nano-Sized SiC on the Laser Powder Bed Fusion of Molybdenum

Consolidation of pure molybdenum through laser powder bed fusion and other additive manufacturing techniques is complicated by a high melting temperature, thermal conductivity and ductile-to-brittle transition temperature. Nano-sized SiC particles (0.1 wt%) were homogeneously mixed with molybdenum powder and the printing characteristics, chemical composition, microstructure, mechanical properties were compared to pure molybdenum for scan speeds of 100, 200, 400, and 800 mm/s. The addition of SiC improved the optically determined density and flexural strength at 400 mm/s by 92% and 80%, respectively. The oxygen content was reduced by an average of 52% over the four scan speeds analyzed. Two mechanisms of oxygen reduction were identified as responsible for the improvements: oxidation of free carbon and the creation of secondary phase nanoparticles. This study illustrates the promising influence of nanoparticle additions to refractory metals in laser powder bed fusion.

Static Fatigue of SiC/SiC Minicomposites at High Temperatures Up to 1200 °C in Air: Multiscale Approach

The present paper investigates the static fatigue behavior of Hi-Nicalon fiber-reinforced SiC–SiC minicomposites at high temperatures in the 900–1200 °C range, and under tensile stresses above the proportional limit. The stress–rupture time relation was analyzed with respect to subcritical crack growth in filaments and fiber tow fracture. Slow crack growth from flaws located at the surface of filaments is driven by the oxidation of free carbon at the grain boundaries. Lifetime of the reinforcing tows depends on the statistical distribution of filament strength and on structural factors, which are enhanced by temperature increase. The rupture time data were plotted in terms of initial stresses on reinforcing filaments. The effect of temperature and load on the stress–rupture time relation for minicomposites was investigated using results of fractography and predictions of minicomposite lifetime using a model of subcritical growth for critical filaments. The critical filament is the one whose failure by slow crack-growth triggers unstable fracture of the minicomposite. This is identified by the strength–probability relation provided by the cumulative distribution function for filament strength at room temperature. The results were compared to the fatigue behavior of dry tows. The influence of various factors related to oxidation, including multiple failures, load sharing, and variability, was analyzed.

Static Fatigue of SiC Multifilament Tows at Temperatures up to 1200 °C in Air

SiC-based fibers are sensitive to delayed failure under constant load at high temperatures in air. Static fatigue at intermediate temperatures < 800 °C was attributed to slow crack growth from flaws located at the surface of fibers, driven by the oxidation of free carbon at grain boundaries. The present paper examines the static fatigue behavior of SiC-based Hi-Nicalon fibers at high temperatures up to 1200 °C and Hi Nicalon S fibers at intermediate temperatures (500–800 °C). The degradation of stress- rupture time relation of multifilament tows with increasing temperature was investigated. Predictions of tow lifetime based on critical filament-based model of tow failure were compared to experimental stress-rupture time diagrams. Critical filaments are characterized by strength–probability relation. The critical filament-based model was found to describe satisfactorily the static fatigue behavior of fiber tows at these temperatures. The influence of various factors on lifetime as well as the origins of variability is analyzed.

Influence of Oxide Layer on Oxidation of Free Carbon in Si-Ti-C-O Fiber-Bonded Ceramics.

Influence of a silica layer on oxidation of free carbon in Si-Ti-C-O fiber-bonded ceramics has been investigated. The rate of oxidation of the Si-Ti-C-O fibers has been measured by thermogravimetric analysis at temperatures ranging between 773 and 1573K. The relative mass of the fibers increased parabolically and the kinetics of the oxidation followed the parabolic rate characteristic of diffusion controlled processes. The thickness of the silica layer, calculated according to a simple equation, well agreed well with the experimentally observed one. Pores created by the oxidation of free carbon were observed inside the fibers after etching the surface of a heat-treated Si-Ti-C-O fiber-bonded ceramics at 773K. Using the parabolic rate constant and the volume expansion coefficient of the fiber, the time required to close the pores by the formation of silica layer was calculated. The thickness of the free carbon layer and the oxidation rate of the fibers were the key parameters for the analysis of the oxidation behavior of the materials.

Oxidation behaviour of SiC fibre reinforced SiC

The oxidation behaviour of one-dimensional and two-dimensional SiC fibre reinforced SiC was investigated up to 1520°C in air, as well as in water vapour saturated air and argon by calorimetry, DTA and TGA. The oxidation process takes place in three steps: (1) oxidation of free carbon in the carbon coated composites between 530 and 690°C with mass losses up to 7.5%; (2) a fast exothermal process associated with mass gain starting at 800°C and terminated below 1500°C within 1 h; (3) the diffusion controlled oxidation of bulk SiC. The processes follow a logarithmic rate law between 875 and 985°C with an effective activation energy Q = 84 kJ/mol. The amorphous reaction product Si0 2 transforms to cristobalite at (930 ± 50)°C. The oxidation kinetics follows a quadratic rate law above 1000°C with rate constants k = 3.5 × 10 −7g 2/cm 4h for 1D SiC/SiC and k = 5 × 10 −8g 2/cm 4h for 2D SiC/SiC at 1520°C in air. The rate constants are up to three orders of magnitude higher than that for high density monolithic SiC which is explained by the high porosity of the SiC matrix.

Facility for Continuous CVD Coating of Ceramic Fibers

The development of new and improved ceramic fibers has spurred the development and application of ceramic composites with improved strength, strength/weight ratio, toughness, and durability at increasingly high temperatures. For many systems, the ceramic fibers can be used without modification because their properties are adequate for the chosen application. However, in order to take maximum advantage of the fiber properties, it is often necessary to coat the ceramic fibers with materials of different composition and properties. Examples include (1) boron nitride coatings on a ceramic fiber, such as Nicalon silicon carbide, to prevent reaction with the ceramic matrix during fabrication and to enhance fiber pullout and increase toughness when the ceramic composite is subjected to stress[l]; (2) boron nitride coatings on ceramic yarns, such as Nicalon for use as thermal insulation panels in an aerodynamic environment, to reduce abrasion of the Nicalon and to inhibit the oxidation of free carbon contained within the Nicalon[2]; and (3) ceramic coatings on carbon yarns and carbon-carbon composites to permit use of these high-strength, high-temperature materials in oxidizing environments at very high temperatures[3,4].

A conductivity method for combined carbon and free carbon determination in uranium carbide

A conductivity method based on differential temperature oxidation of combined carbon and free carbon has been worked out for their determination in uranium carbide employing purified air for oxidation. The combined carbon determined at 550°C and free carbon at 900°C were found to be 4.58% and 0.27% with a precision of 2% and 11%, respectively. The free carbon determined by the present method and that by dissolution followed by conductivity method agreed within ±5%. Effect of temperature and time on the oxidation of free carbon /taken as graphite/ in air atmosphere was also studied.