Stress measurement and development of zirconium diboride-silicon carbide ceramics

Research presented in this dissertation focused on the production of ZrB₂-SiC composites and their characterization; in particular their mechanical properties and thermally generated residual stresses. Thermally generated stresses were measured using Raman spectroscopy as well as both neutron and x-ray diffraction. For 70 vol% ZrB₂ - 30 vol% SiC composites neutron diffraction revealed that the SiC phase was under ~880 MPa compressive stress and the ZrB₂ phase was under ~450 MPa tension at room temperature. It was also discovered that stresses began to accumulate at ~1400 ⁰C upon cooling from the processing temperature (1900 ⁰C -2000 ⁰C). Raman spectroscopy and x-ray diffractions agreed well with one another and showed the stresses in the SiC phase on the surface of the samples to be ~350 MPa; lower than that in the bulk as measured by neutron diffraction. It has also been shown that annealing composites at temperatures below 1400 ⁰C, particularly while under pressure can partially relieve these stresses leading to increases in mechanical strength of as much as 30%. The role of the particle size of the SiC phase was also investigated. For SiC particle sizes smaller than 11.5 µm, the failure strengths of the composites followed a 1/c1/2 type relationship as predicted by Griffith. Above that particle size however, strength, modulus, and hardness all decreased rapidly. It was discovered that for SiC particle sizes larger than 11.5 µm microcracking occurred resulting in the decrease of the measured mechanical properties --Abstract, page iv

Mechanical Properties Of ZrB2 Ceramics Determined By Two Laboratories

The mechanical properties for zirconium diboride (ZrB2) were measured at two laboratories and compared. Two billets of ZrB2 were prepared by hot-pressing commercial powder. The relative densities of the billets were \u3e99% and with an average grain size of 5.9 ± 4.5 µm. Both laboratories prepared American Society for Testing and Materials (ASTM) C1161 B-bars for strength and ASTM C1421 bars with notch configuration A for fracture toughness. Specimens were machined by diamond grinding at the Army Research Laboratory (ARL) and electrical discharge machining (EDM) at Missouri S&T. Strength bars tested at Missouri S&T were polished to a.25 μm finish while the bars were tested as-ground at ARL. Strengths were 473 ± 79 MPa for the Missouri S&T bars and 438 ± 68 for the ARL bars while the fracture toughness values were 3.9 ±.7 MPa•m1/2 for the Missouri S&T bars and 4.4 ±.6 MPa•m1/2 for the ARL bars. Vickers hardness was measured by both laboratories over a range of indentation loads. The resulting hardness values were on the low end of previously reported values and were quite different from each other especially at indentation loads ≤20N. The study demonstrated that the properties of materials tested to ASTM standards at different laboratories can be compared directly. In addition, strength and fracture toughness were nearly identical for bars prepared by conventional diamond grinding or EDM

Room-Temperature Mechanical Properties of a High-Entropy Diboride

The mechanical properties of a (Hf,Mo,Nb,Ta,W,Zr)B2 high-entropy ceramic were measured at room temperature. A two-step synthesis process was utilized to produce the (Hf,Mo,Nb,Ta,W,Zr)B2 ceramics. The process consisted of a boro/carbothermal reduction reaction followed by solid solution formation and densification through spark plasma sintering. Nominally, phase pure (Hf,Mo,Nb,Ta,W,Zr)B2 was sintered to near full density (8.98 g/cm3) at 2000°C. The mean grain size was 6 ± 2 µm with a maximum grain size of 17 µm. Flexural strength was 528 ± 53 MPa, Young\u27s modulus was 520 ± 12 GPa, fracture toughness was 3.9 ± 1.2 MPa·m1/2, and hardness (HV0.2) was 33.1 ± 1.1 GPa. A Griffith-type analysis determined the strength limiting flaw to be the largest grains in the microstructure. This is one of the first reports of a variety of mechanical properties of a six-component high-entropy diboride

Ceramic Welds, and a Method for Producing the Same

A method of producing a ceramic weld, including identifying a ceramic first surface and a ceramic second surface to be bonded together, maintaining a non-oxidizing atmosphere over the first and second surfaces, and engaging the first and second surfaces to define a joint. An arc is generated between an electrode and the joint to create a liquid phase, and the liquid phase is cooled to yield a solid fusion layer, wherein the first and second surfaces are joined in the fusion layer

Ceramic-Ceramic Welds

A method of producing a ceramic weld, including identifying a ceramic first surface an a ceramic second surface to he bonded together, maintaining a non-oxidizing atmosphere over the first and second surfaces, and engaging the first and second surfaces to define a joint, An arc is generated between an electrode and the joint to create a liquid phase, and the liquid phase is cooled to yield a solid fusion layer, wherein the first and second surfaces are joined in the fusion layer

Fusion Welding of Refractory Metals and ZrB2-SiC-ZrC Ceramics

Molybdenum and a molybdenum alloy were fusion welded to ZrB2-based ceramics to determine if the electrical and thermal properties of the metals and ceramics affected their weldability. Commercial ceramic powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics to commercial metals using plasma arc welding. Weldability varied as indicated by the range of porosity observed within the fusion zones. Measured thermal and electrical properties appeared to have little to no effect on the weldability of metal-ceramic welds despite the large range of values measured across each property. Differences in melting temperatures between metal and ceramic coupons did affect weldability by changing the weld penetration depth into ceramic coupons. Future studies on metal-ceramic welds are suggested to investigate the effect that work function, melt viscosity, wetting, or other properties have on weldability

Vacancy Ordering In Zirconium Carbide With Different Carbon Contents

Zirconium carbide (ZrCx) ceramics with different carbon contents were prepared by reactive hot-pressing. The rock-salt structure of ZrCx was the only phase detected by x-ray diffraction of the hot pressed ceramics. The relative densities of ZrCx decreased as carbon content increased, in general. The actual carbon contents were measured by completely oxidizing the ZrCx ceramics to ZrO2. For most compositions, the actual carbon contents were higher than nominal batched compositions, presumably due to carbon uptake from the graphite furnace and hot press dies. Selected area electron diffraction and neutron powder diffraction revealed the presence of carbon vacancy ordering in ZrCx for 0.6 \u3c x \u3c 0.75. Rietveld refinement of the neutron diffraction patterns determined that the crystal structure of the ordered phase was hexagonal, and the carbon site occupancies were higher than nominal batched carbon stoichiometry

Relating Detonation Parameters to the Detonation Synthesis of Silicon Carbide

Detonation synthesis of silicon carbide (SiC) nanoparticles from carbon liberated by negatively oxygen balanced explosives was evaluated in a 23 factorial design to determine the effects of three categorical experimental factors: (1) cyclotrimethylene-trinitramine (RDX)/2,4,6-trinitrotoluene (TNT) ratio, (2) silicon (Si) additive concentration, and (3) Si particle size. These factors were evaluated at low and high levels as they relate to the detonation performance of the explosive and the solid Si-containing phases produced. Detonation velocity and Chapman-Jouguet (C-J) detonation pressure, which were measured using rate stick plate dent tests, were evaluated. Solid detonation product mass, silicon carbide product concentration, and residual silicon concentration were evaluated using the x-ray diffraction analysis. The factors of Si concentration and the RDX:TNT ratio were shown to affect detonation performance in terms of detonation velocity and C-J pressure by up to 10% and 22%, respectively. Increased concentration of Si in the reactants improved the average SiC concentration in the detonation products from 1.9 to 2.8 wt. %. Similarly, increasing the ratio of RDX to TNT further oxidized detonation products and reduced the average residual Si remaining after detonation from 8.6 to 2.8 wt. %. A 70:30 mass ratio mixture of RDX to TNT loaded with 10 wt. % \u3c 44 μm silicon powder produced an estimated 1.33 g of nanocrystalline cubic silicon carbide from a 150-g test charge. Using a lower concentration of added silicon with a finer particle size reduced SiC yield in the residue to 0.38 g yet improved the SiC to residual Si ratio to 1.64:1

Detonation Synthesis of Nanoscale Silicon Carbide from Elemental Silicon

Direct reaction of precursors with the products of detonation remains an underexplored area in the ever-growing body of detonation synthesis literature. This study demonstrated the synthesis of silicon carbide during detonation by reaction of elemental silicon with carbon products formed from detonation of RDX/TNT mixtures. Continuum scale simulation of the detonation showed that energy transfer by the detonation wave was completed within 2–9 μs depending on location of measurement within the detonating explosive charge. The simulated environment in the detonation product flow beyond the Chapman-Jouguet condition where pressure approaches 27 GPa and temperatures reach 3300 K was thermodynamically suitable for cubic silicon carbide formation. Carbon and added elemental silicon in the detonation products remained chemically reactive up to 500 ns after the detonation wave passage, which indicated that the carbon-containing products of detonation could participate in silicon carbide synthesis provided sufficient carbon-silicon interaction. Controlled detonation of an RDX/TNT charge loaded with 3.2 wt% elemental silicon conducted in argon environment lead to formation of ∼3.1 wt% β-SiC in the condensed detonation products. Other condensed detonation products included primarily amorphous silica and carbon in addition to residual silicon. These results show that the energized detonation products of conventional high explosives can be used as precursors in detonation synthesis of ceramic nanomaterials

Mechanical Activation and Cation Site Disorder in Mgal2o4

The synthesis and crystallographic site occupancy were investigated for MgAl2O4 with and without mechanical activation of the precursor powders. Heating to 1200 °C or higher resulted in the formation of a single spinel phase regardless of whether the powders were mechanically activated or not. Neutron diffraction analysis was used to determine cation site occupancy and revealed that mechanical activation resulted in a lower degree of cation site inversion compared to the nonactivated materials, which indicated that the powders were closer to thermodynamic equilibrium. This is the first study to characterize the effects of mechanical activation on crystallographic site occupancy in magnesium aluminate spinel using neutron diffraction