Mechanical andthermal properties of Zeta phase tantalum carbide atelevated temperatures

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Mechanical properties of borothermally synthesized ZrB2

Mechanical properties of borothermally synthesized, highly pure ZrB2 were tested at room and elevated temperatures. Commercially available ZrB2 powder typically contains 1 to 4 wt % hafnium which has been shown to lower thermal properties of dense ZrB2 ceramics. Further, commercial grade ZrB2 contains other impurities (0.6 wt% O, 0.11 wt% N, 0.04 wt% Fe and others) which are also known to decrease its high-temperature mechanical strength. Purer grades of zirconia and boron powders, containing \u3c 75 ppm hafnium and \u3c0.5 wt% of other metal impurities, were reacted to produce ZrB2 for room and elevated temperature mechanical property studies. The zirconia and boron powders were reacted at 1000°C in a graphite vacuum furnace for two hours. The synthesized ZrB2 powder was then rinsed with methanol to remove boria from its surfaces, sieved with a #45 mesh, and hot pressed to near full density with 32 MPa applied pressure in a flowing argon atmosphere at 2100°C. The hot pressed billets were machined to ASTM standard test bars with the flexure surface polished to 1 um. Young’s modulus, Vickers Hardness, fracture toughness, and four-point bend strength were measured, and the results will be discussed

Effects of transition metals on thermal properties of ZrB2

Nominally phase pure zirconium diboride ceramics were synthesized to study their intrinsic thermal properties. Ceramics for this study were synthesized by reaction hot pressing of reactor grade ZrH2 and B to minimize impurities commonly found in commercial powders such as the natural abundance (1-4 wt%) of Hf. Starting powders contained \u3c200 ppm Hf. Previous results showed that Hf impurities present in quantities comparable to commercial powders masked the effect of other transition metal additions. For example, additions of 3 at% Ti and Y had no apparent effect on thermal conductivity of ceramics produced from commercial ZrB2. Lowering the Hf content to 0.4 at% increased thermal conductivity from ~90 W/m•K for ZrB2 ceramics prepared from commercial powders to ~100 W/m•K for low-Hf content ZrB2 at 25 °C. Lowering the Hf content also increased the thermal conductivity at 2000°C from ~70 W/m•K to ~80 W/m•K. For the low Hf ZrB2, adding 3 at% TiB2 decreased thermal conductivity ~15 W/m•K at 25°C while adding 3 at% MoB2 decreased thermal conductivity ~45 W/m•K at 25°C. For the present study, transition metals such as Hf, Ti, Y, Ta, and W were added individually to nominally phase pure ZrB2 to study the effects on thermal diffusivity, thermal conductivity and heat capacity at temperatures from 25°C to 2000°C. These properties will be compared to values obtained for ceramics prepared from commercial ZrB2 powders, which contained the natural abundance of Hf. Most previous reports have relied on heat capacity values from the NIST-JANAF thermodynamic tables to calculate thermal conductivity of ZrB2 ceramics. However, the heat capacity of ZrB2 with low Hf content was approximately 10% greater than widely accepted values. Due to this difference, heat capacity will be measured for each composition, and these values will be used to calculate thermal conductivity. The intrinsic thermal properties of ZrB2will be discussed as well as the effect of transition metal additions on the thermal properties of ZrB2 with low and naturally abundant quantities of Hf

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part II: Microstructural observations

Additions of AlN and Al 2 O 3 to β-SiC hot pressed at 2100°C strongly effect the β- to α-SiC phase transformation and the resultant α-SiC polytypes which are formed. Scanning and transmission electron microscopy were utilized to investigate the microstructural changes occurring in SiC due to these additions and to correlate these observations to their mechanical properties. The results suggest that Al 2 O 3 additions stabilize the formation of the 6H-polytype of α-SiC which grows rapidly into an elongated plate-like morphology, while AlN additions stabilize the 2H-polytype of α-SiC resulting in fine equiaxed 2H-SiC: AlN solid solution grains. It is speculated that the elongated growth of 6H-SiC with Al 2 O 3 additions can be controlled through the simultaneous addition of AlN. The formation of 2H-SiC : AlN solid solution grains inhibits the growth of the 6H-SiC grains since AlN(2H) will not go into solid solution in the SiC(6H) structure, effectively pinning the growth of the 6H-SiC grains.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44759/1/10853_2004_Article_244050.pd

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from β-SiC (3C) to the various α-SiC polytypes in the presence of AlN and Al 2 O 3 additives after hot pressing from 1700 to 2100°C. The 2H- and 6H-polytypes of α-SiC were the predominate polytypes with additions of only AlN or Al 2 O 3 , respectively. The amount of 2H- and 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al 2 O 3 . Improvements in fracture toughness to ∼9 MPa-√m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al 2 O 3 additions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44757/1/10853_2004_Article_244049.pd

Braze for ceramic and ceramic matrix composite components

In some examples, a technique may include positioning a first part comprising a ceramic or ceramic matrix composite and a second part comprising a ceramic or a CMC adjacent to each other to define a joint region at the interface of the first part and the second part. In some examples, the joint region may be heated using at least one of a laser or a plasma arc source to heat the joint region to an elevated temperature. The first and second parts may be pressed together and cooled to join the first and second parts at the joint region. In other examples, a solid braze material including a filler material and a metal or alloy may be delivered to the joint region and locally heated to cause a constituent of the filler material and a constituent of the metal or alloy to react. When reacted, the constituents may form a solid material, which may join the first and second parts

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° led to formation of δ-Y2Si2O7. after 5 h at 1500°, the δ-Y2Si2O7 had transformed to β-Y2Si2O7 with a concurrent dramatic increase in dislocation density within β-Si3N4 grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material

Strength of Functionally Designed Cellular Cemented Carbides Produced by Coextrusion

In an effort to improve the wear characteristics of petroleum drill bit inserts, a series of cemented carbide materials with a functionally designed cellular (FDC) architecture were fabricated by a coextrusion process. The FDC architecture characterized in this study was comprised of cemented carbide cells surrounded by a ductile cobalt cell boundary. Property evaluation employed transverse rupture strength (TRS) testing to characterize their mechanical behavior. It was determined that the presence of Co2 + x W4 − x C in the material greatly affected the bonding of the cell to the cell boundary and therefore the strength of the material. Fractography of the FDC materials supported the hypothesis that the interface between the cell and cell boundary was affected by the Co2 + x W4 − x C phase and the consequential reduction in cobalt content of the cell

Microstructural Evolution in Near-eutectic Yttrium Silicate Compositions Fabricated from a Bulk Melt and as an Intergranular Phase in Silicon Nitride

Near-eutectic composition Y2O3─SiO2 melts were formed as bulk samples or as an intergranular phase in Si3N4. Upon cooling to room temperature the bulk material partially crystallized to γ-Y2-Si2O7 whereas the intergranular phase was glass. on heat-treating at 1500°C the bulk material transformed to γ-Y2Si2O7 whereas the intergranular glass crystallized first to γ-Y2Si2O7 and then to β-Y2Si2O7. Possible reasons for the different behavior are discussed