48 research outputs found
Microstructural changes in Beta-silicon nitride grains upon crystallizing the grain-boundary glass
Crystallizing the grain boundary glass of a liquid phase sintered Si3N4 ceramic for 2 h or less at 1500 C led to formation of gamma Y2Si2O7. After 5 h at 1500 C, the gamma Y2Si2O7 had transformed to beta Y2Si2O7 with a concurrent dramatic increase in dislocation density within beta Si3N4 grains. Reasons for the increased dislocation density is discussed. Annealing for 20 h at 1500 C reduced dislocation densities to the levels found in as-sintered materials
Thermal properties of zirconium diboride ceramics
This presentation will focus on the thermal conductivity of zirconium diboride ceramics. Previous reports of thermal conductivity values for ZrB2 vary from as low as about 30 W/m•K to over 100 W/m•K without any direct evidence to identify the reasons for the variations. Our group systematically investigated the effects of transition metal impurities, which led to the discovery that the size of the dissolved impurity species was directly related to the decrease in thermal conductivity. Analysis of the electron contribution to thermal conductivity utilizing the Wiedemann-Franz methodology led to the conclusion that both the phonon and electron contributions were affected by dissolved metallic impurities. Further, the effects of some transition metals including Ti and Y were masked by other impurities in ceramics produced from commercial ZrB2 powders.
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Mechanical properties of zirconium diboride ceramics
This presentation will focus on the mechanical properties of zirconium diboride ceramics. Diboride ceramics offer a combination of properties that include high elastic modulus, hardness, strength, and moderate fracture toughness to elevated temperatures. However, like all structural ceramics, their mechanical properties are controlled by microstructure wherein grain size, dispersion and size of second phases, and impurities limit their potential use at elevated temperatures, particularly for proposed extreme environment applications at temperatures exceeding 2000°C. As an example, the flexure strength of nearly phase pure ZrB2 ranges from 300 to \u3e600 MPa at room temperature but retains a strength of \u3e300 MPa at temperatures \u3e1500°C. Further, the fracture toughness of ZrB2 ceramics is generally low, typically in the range of 3 to 4 MPa-m1/2, at both room and elevated temperatures.
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Freeform Extrusion of High Solids Loading Ceramic Slurries, Part II: Extrusion Process Control
Part I of this paper provided a detailed description of a novel fabrication machine for high solids
loading ceramic slurry extrusion and presented an empirical model of the ceramic extrusion
process, with ram velocity as the input and extrusion force as the output. A constant force is
desirable in freeform extrusion processes as it correlates with a constant material deposition rate
and, thus, good part quality. The experimental results in Part I demonstrated that a constant ram
velocity will produce a transient extrusion force. In some instances the extrusion force increased
until ram motor skipping occurred. Further, process disturbances, such as air bubble release and
nozzle clogging that cause sudden changes in extrusion force, were often present. In this paper a
feedback controller for the ceramic extrusion process is designed and experimentally
implemented. The controller intelligently adjusts the ram motor velocity to maintain a constant
extrusion force. Since there is tremendous variability in the extrusion process characteristics, an
on-off controller is utilized in this paper. Comparisons are made between parts fabricated with
and without the feedback control. It is demonstrated that the use of the feedback control reduces
the effect of process disturbances (i.e., air bubble release and nozzle clogging) and dramatically
improves part quality.Mechanical Engineerin
Processing and Mechanical Properties of Hot-Pressed Zirconium Diboride – Zirconium Carbide Ceramics
ZrB2 was mixed with 0.5 wt% carbon and up to 10 vol% ZrC and densified by hot-pressing at 2000 °C. All compositions were \u3e 99.8% dense following hot-pressing. The dense ceramics contained 1–1.5 vol% less ZrC than the nominal ZrC addition and had between 0.5 and 1 vol% residual carbon. Grain sizes for the ZrB2 phase decreased from 10.1 µm for 2.5 vol% ZrC to 4.2 µm for 10 vol% ZrC, while the ZrC cluster size increased from 1.3 µm to 2.2 µm over the same composition range. Elastic modulus was ~505 GPa and toughness was ~2.6 MPa·m½ for all compositions. Vickers hardness increased from 14.1 to 15.3 GPa as ZrC increased from 2.5 to 10 vol%. Flexure strength increased from 395 MPa for 2.5 vol% ZrC to 615 MPa for 10 vol% ZrC. Griffith-type analysis suggests ZrB2 grain pullout from machining as the strength limiting flaw for all compositions
Pressureless Sintering of Zirconium Diboride with Carbon and Boron Carbide Nanopowder
Zirconium diboride ceramics with and without carbon and boron carbide nano powder additives were prepared by ball milling with ZrB2 grinding media and pressureless sintering. Additions of up to 1 wt% nano-B4C and 0.5 wt% C were made to the ZrB2 powder. The materials were then sintered between 1800 and 2300 °C for between 90 and 360 min in an Ar/10H2 atmosphere. After sintering at 2200 °C for 90 min, densities ranged from 88.3 to 90.7% for the ZrB2 with 0–1.0% nano-B4C addition. Carbon additions of 0.5 wt% and nano-B4C additions from 0 to 1.0 wt% resulted in densities ranging from 90.9 to 91.9% after sintering at 2100 °C for 90 min. Grain size ranged from 16.6 to 21.7 μm for ZrB2 with nano-B4C content increasing from 0 to 1.0 wt%, sintered at 2200 °C. For the ZrB2 with 0.5 wt% C, increasing the nano-B4C content from 0 to 1.0 wt% resulted in a decrease in grain size from 25.4 to 18.5 μm. The densities achieved in this study were lower than previous pressureless sintering studies of ZrB2 that used WC-6Co grinding media, presumably due to the absence of WC and Co that can also act as sintering aids
Optimization of Selective Laser Sintering Process for Fabrication of Zirconium Diboride Parts
Selective Laser Sintering (SLS) was investigated to fabricate Zirconium Diboride
(ZrB2) parts for ultra-high temperature applications. Experiments were conducted to
determine values of SLS process parameters (laser power, scan speed, line spacing, and
layer thickness) that can be used to build ZrB2 parts with high integrity and sharp
geometrical features. A sacrificial plate with a proper number of layers (determined from
experimentation) separated from the main part was built in order to reduce thermal
gradients when building the main part. The sacrificial plate was found to assist in
eliminating cracks in the bottom of the main part. The fabricated green parts then went
through post processing steps including binder burnout and sintering at proper
temperature schedules, to remove the binder and sinter the ZrB2 particles. The test bars
after sintering had an average relative density of 87% and an average flexural strength of
250 MPa.Mechanical Engineerin
Freeform Extrusion of High Solids Loading Ceramic Slurries, Part I: Extrusion Process Modeling
A novel solid freeform fabrication method has been developed for the manufacture of
ceramic-based components in an environmentally friendly fashion. The method is based on the
extrusion of ceramic slurries using water as the binding media. Aluminum oxide (Al2O3) is
currently being used as the part material and solids loading as high as 60 vol. % has been
achieved. This paper describes a manufacturing machine that has been developed for the
extrusion of high solids loading ceramic slurries. A critical component of the machine is the
deposition system, which consists of a syringe, a plunger, a ram actuated by a motor that forces
the plunger down to extrude material, and a load cell to measure the extrusion force. An
empirical, dynamic model of the ceramic extrusion process, where the input is the commanded
ram velocity and the output is the extrusion force, is developed. Several experiments are
conducted and empirical modeling techniques are utilized to construct the dynamic model. The
results demonstrate that the ceramic extrusion process has a very slow dynamic response, as
compared to other non-compressible fluids such as water. A substantial amount of variation
exists in the ceramic extrusion process, most notably in the transient dynamics, and a constant
ram velocity may either produce a relatively constant steady-state extrusion force or it may cause
the extrusion force to steadily increase until the ram motor skips. The ceramic extrusion process
is also subjected to significant disturbances such as air bubble release, which causes a dramatic
decrease in the extrusion force, and nozzle clogging, which causes the extrusion force to slowly
increase until the clog is released or the ram motor skips.Mechanical Engineerin
Boro/carbothermal Reduction Co-Synthesis of Dual-Phase High-Entropy Boride-Carbide Ceramics
Dense, dual-phase (Cr,Hf,Nb,Ta,Ti,Zr)B2-(Cr,Hf,Nb,Ta,Ti,Zr)C ceramics were synthesized by boro/carbothermal reduction of oxides and densified by spark plasma sintering. The high-entropy carbide content was about 14.5 wt%. Grain growth was suppressed by the pinning effect of the two-phase ceramic, which resulted in average grain sizes of 2.7 ± 1.3 µm for the high-entropy boride phase and 1.6 ± 0.7 µm for the high-entropy carbide phase. Vickers hardness values increased from 25.2 ± 1.1 GPa for an indentation load of 9.81 N to 38.9 ± 2.5 GPa for an indentation load of 0.49 N due to the indentation size effect. Boro/carbothermal reduction is a facile process for the synthesis and densification of dual-phase high entropy boride-carbide ceramics with both different combinations of transition metals and different proportions of boride and carbide phases
Superhard Single-Phase (Ti,Cr)B2 Ceramics
A nominally pure and dense (Ti0.9Cr0.1)B2 ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 μm and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB2 ceramic obtained under the same processing conditions, the (Ti0.9Cr0.1)B2 ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 μm), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB2 ceramics. This is the first report for dense and single-phase (Ti,Cr)B2 ceramics as superhard materials