Dynamic Consolidation of TaC and NanoYSZ Powders

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Dynamic Consolidation of TaC and Nano-YSZ Powders
SAMPE 2006 Long Beach April 30 May 4, 2006
Presenter Bhaskar Majumdar

• L. Xu, B.S. Majumdar
• New Mexico Institute of Mining and Technology,
  Socorro, NM
• D. Merchant
• A.F. Research Laboratory, Edwards AFB,CA
• L. Matson
• A.F. Research Laboratory, WPAFB, OH

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Introduction

• Tantalum carbide (TaC) has the second highest
  melting point (3880C) of known solids, offering
  strong promise in ultra high temperature
  applications. Also, it is electrically
  conductive, and can be electric discharge
  machined (EDM) to complex shapes.
• However, the high temperatures needed for powder
  consolidation (1900 - 2500C) leads to rapid
  grain growth. This has significant detrimental
  effect on fracture toughness and strength of TaC.
  Therefore, there is a need for an alternate
  processing method for consolidating TaC.
• Dynamic or shock wave consolidation has the
  advantage that high pressures (5 - 60 GPa) can be
  generated reasonably easily, although for a few
  microseconds. This high pressure, combined with
  temperatures that can reach 1500C (due to
  friction), can be used to consolidate ceramic
  materials.
• Previous work has shown that the residual strain
  induced during consolidation also has the
  beneficial effect of reducing the temperature
  required for any after-shock sintering treatment,
  aimed at densification and removing
  porosity/defects.

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Research Objective

• The objective of this research is to evaluate
  shock wave consolidation as a method to fabricate
  TaC with fine grain size.
• The high temperature generated during shock wave
  has created strong interparticle bonding so that
  fracture surface is featured with intergranular
  fracture. Therefore, the shock-wave consolidated
  materials may have comparable fracture toughness
  with other structural ceramics, such as SiC and
  Si3N4.
• Nano-size yttria stabilized zirconia (YSZ) is
  also studied in this investigation, to evaluate
  the potential of shock consolidation to produce
  nano-bulk ceramics. In particular, nano-YSZ
  offers the potential of shaping by plastic
  deformation (like metals) at relatively low
  temperatures.
• It may also be noted that the consolidation of
  nano-size particles is difficult since they have
  more surface areas and higher friction in
  packing, therefore, unable to reach high green
  density. Shock wave consolidation offers a way
  to obtain high green density for such materials.

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Determination of the Pressure Need for the
Consolidation
TaC H 15 GPa YSZ H 12 GPa
TaC P 20 GPa YSZ P 18 GPa
1 kg/mm20.01GPa
From Meyers et al., in Shock Waves for Industrial
Applications, Eds Murr et al., (1988)
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Experimental Materials

• TaC
• Micron-size TaC, purity, 99.95, diameter, d
  1 µm. Supplier Inframat Advanced Materials LLC.
  Most of the results in this presentation are for
  this material.
• YSZ
• Yttria Stabilized Zirconia (YSZ), d 30 - 60 nm,
  Supplier Inframat

Left image Inframat TaC As-received, SEM image
Right image Inframat TaC After Ball Milling
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Experimental Setup

• A double-tube setup was selected for the shock
  consolidation process. This method provides a
  longer duration pulse.
• A lower pressure reduces the magnitude of
  reflected tensile pulses, thus reducing chances
  of cracking.
• The cylinders containing the powders were
  evacuated from the bottom, through pre-drilled
  hole, prior to shock consolidation.

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Experimental Procedures

• Powders were packed in double-wall steel
  containers, evacuated at room temperature and in
  some case at 250 C, then subjected to explosive
  consolidation.
• Some samples were heat-treated at 1100 C after
  shock wave consolidation.
• The microstructure and fracture surfaces of
  samples were examined using FESEM and optical
  microscopy.
• Vickers hardness measurements were used to assess
  the magnitude of consolidation. A value of 15
  GPa is characteristic of fully dense TaC.
• Results showed that the extent of consolidation
  depended on axial position along the cylinder.
  Crack free material, combined with fairly high
  density, was observed in the mid-length region of
  the cylinders.

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Macroscopic View of the Billets

• The caving in of the double wall region
  corresponds to the densification of the billets
• The powders were nearly fully dense at the bottom
  of the billets

Consolidation
Densified Region
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Microstructures of Consolidated TaC

• The hardness near the bottom of the sample is
  14.5 GPa (fully dense TaC, 15 ? 0.5 GPa).
• This bottom region also contains cracks.
• The fracture toughness was measured as 4.8 MPavm
  (structural SiC and Si3N4 have a value between 4
  and 6 MPavm).

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Middle Part of the Billets

• The density at half around the consolidated
  billets is 11.6 g/cm3, 83 of theoretical
  density. Hardness test gave a value of 4.8 GPa.
• The region is fracture free. Three bend bars have
  been machined from the billets of this region.
• Residual strain has been generated inside powders
  in the billets. The observed XRD line broadening
  has verified this effect. This middle region can
  be sintered or HIPed at much lower temperature.

As received
After shock
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Calculations related to the residual stain stored
in the TaC powders

• From GSAS software analysis on XRD data, the
  residual stain stored in the TaC powders can be
  obtained as 2.4 x 10-3.
• The Faulkners formula gives the energy stored in
  the powders,E, Youngs modulus, ?, Poissons
  ration. e, residual stain. The energy is
  calculated as 320 J/mol.
• The activation energy of TaC is 380 kJ/mol. Even
  though the strain induced energy is small, the
  diffusion coefficient will increase largely
  because of its exponential form.

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After Consolidation Heat-treatment

• Heat-treatment was conducted at 1100 C for the
  middle part of the billets. A thin strip of high
  reflectivity with a golden color could be
  observed around the periphery of the sample.
• The width of the strip is 50 µm and 130 µm for
  the 6 hr and 24 hr heat-treatment sample,
  respectively. The hardness at the strip of the 24
  hr sample is 11 GPa and interior is 6.4 GPa.
• The densification at such a low temperature
  indicate that diffusion controlled sintering is
  easier after shock-wave consolidation processing.

Sample
Mount
Layer
100 mm
24 hr sample
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Characterization of the YSZ billet

• Microstructure of at the bottom of the YSZ billet
  shows no features and a smooth surface (left
  image).
• the fracture surface revealed transgranular
  fracture that spanned across many particle sizes.
  The surface exhibits typical cleavage type
  fracture (right image).
• Hardness of the region is 9.7 GPa, quite
  comparable to the theoretical value, 12 GPa.

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Summary

• Dynamic consolidation trials of TaC powders (from
  Inframat) indicate promise, and confirm that the
  double-tube method is a good approach for
  consolidating TaC powders.
• Cracking has been absent in the mid-length of the
  billets, although the density there is only 83
  of theoretical density. Heat treatments in argon
  at 1100C suggest that sintering may be active at
  this low temperature around the periphery of the
  sample. The hardness there reached approximately
  11 GPa, comparable to a hardness of 15 GPa of
  fully dense TaC.
• The consolidation trial with YSZ was quite
  successful, in spite of a low tap density. The
  hardness of 9.7 GPa HV in the bottom region was
  comparable to literature values of 12 GPa.
  Fracture surfaces revealed transgranular fracture
  that spanned many individual YSZ particles, and
  confirmed consolidation of the nano YSZ.

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Acknowledgement

• This work has been funded by the Air Force
  Research Laboratory at Edwards Air Force Base,
  contract number FA9300-05-M-T013.
• The shock wave explosive consolidation test was
  accomplished at Energetic Materials Research and
  Testing Center (EMRTC) of New Mexico Institute of
  Mining and Technology. Mr. Tony Zimmerly offered
  generous help for the experimental setup.

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Questions?