Sintering of refractory metal based materials by field assisted sintering technology (FAST)

Open Access
Author:
Chanthapan, Sinthu
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 12, 2011
Committee Members:
  • Jogender Singh, Dissertation Advisor
  • Anil Kamalakant Kulkarni, Committee Member
  • Gary Lynn Messing, Committee Member
  • Douglas Edward Wolfe, Committee Member
  • Todd Palmer, Committee Member
Keywords:
  • tungsten
  • tantalum
  • field
  • assisted
  • sintering
  • TiC
  • WC
  • grain growth
  • inhibition
  • refractory
  • metals
  • nano
  • submicron
  • strengthening
Abstract:
Sintering of tungsten (W; Tmelt=3400oC), tantalum (Ta; Tmelt=3000oC), and tungsten carbide (WC; Tmelt=2800oC) based materials was investigated by field assisted sintering, where electric current, uniaxial pressure, and relatively high heating rate were applied during sintering. A relative density of around 99% was achieved in each material without a need of additives. For a tungsten powder with an average particles size of 0.6 μm , a temperature of 2000oC with a pressure of 85 MPa and a 30 minute holding time was used to achieve 98.5% relative density. However, the sintered grain size was 33.3 μm, which is around 50 times that of the starting powder size. In order to reduce grain growth in the sintered material, sintering additives were introduced into the materials to reduce sintering temperature and inhibit grain growth. The effects of ultrafine tungsten carbide (WC, particle size of 220 nm) and nano titanium carbide (TiC, particle size of 60 nm) additives on sintering, microstructure, and hardness of tungsten were studied. A noticeable enhancement in densification and reduction in grain growth were observed in the tungsten materials with the carbide additives. The relative density of around 99% was achieved in W-10vol.%WC and W-10vol.%TiC powders sintered at temperatures of 1700oC (85 MPa for 5 minutes) and 1800oC (75 MPa for 5 minutes), respectively. WC phase in W-WC materials was transformed into W2C at elevated temperatures (>1250oC), while TiC in W-TiC was decomposed partially with a formation of W2C during sintering, which agreed well with the phase diagrams. The W-W2C system exhibits eutectic reaction at the temperature around 2700oC, which is much lower than the melting point of W. The formation of the eutectic composition at the W-W2C interface increases a homologous temperature for sintering, leading to enhanced mass transport at the interfaces, which would contribute to the improved densification observed. Additionally, the solubility of W in TiC could promote densification by dissolution and precipitation processes of W through the TiC phase. The sintered grain sizes achieved were 4.8 μm for the sintered W-10vol.%WC, and less than 1 μm for the sintered W-10vol.%TiC, which are smaller than the sintered tungsten with equivalent relative density (33.3 μm), or sintered at the same sintering condition. It is expected that grain boundary migration was inhibited by the presence of the carbide phases along the grain boundary. Since TiC additive has a smaller size and higher thermal stability than WC additive, a more effective grain boundary pinning effect was observed in the W materials containing nanosized TiC additive. The hardness values of the sintered materials was increased from 415 +/- 6 HV in pure tungsten to 500 +/- 8 HV in sintered W-10vol.%WC, and 1000 +/- 10 HV in sintered W-10vol.%TiC. The presence of hard carbide phases and smaller grain size, as observed in microstructure images and X-ray diffraction data, would be responsible for the increase in hardness values of both materials. Additionally in sintered W-TiC material, Ti and W could form a substitutional solid solution, which could further strengthen the material. Similar results were observed in the sintering of tantalum. A relative density of around 99% was achieved at sintering temperatures between 1850oC and 2050oC, depending on the applied pressure (35, 55, and 75 MPa) at a holding time of 5 minutes. The applied pressure could enhance densification by initiating plastic deformation and diffusional creep. Although an almost fully dense tantalum material was obtained, the grain size was increased from 5 μm in the starting powder to more than 100 μm in the sintered material, leading to a need for a sintering additive to control grain growth. Ultrafine WC and nanosized TiC at levels of 5 vol.% and 10 vol.% were investigated as an additive to improve densification and reduce grain growth in the sintering of tantalum. The relative density of around 99% was achieved at a lower sintering temperature, 1650oC (75 MPa and 5 minutes holding time), in both sintered Ta-10vol.%WC and Ta-10vol.%TiC materials, while the sintering temperature of 1850oC (75 MPa and 5 minutes holding time) was required for pure tantalum. This densification improvement could be due to the solubility and reactivity between the carbides and the tantalum matrix, and the formation of eutectic compositions of the Ta-WC and Ta-TiC systems, having a melting point of around 2840oC, which is lower than the melting point of pure tantalum (2996oC). The grain sizes of the sintered tantalum with carbide additives were around 10 μm or below, which is much smaller than the pure tantalum sintered to the same relative density (>100 μm), or under the same conditions. In this investigation, the sintered tantalum exhibits a relatively high oxygen content (~0.3 wt.%), which would originate from the starting powder, which contained around 0.4 wt.% of oxygen. As a result, the sintered tantalum materials showed a brittle fracture mechanism during bending tests. In contrast to the sintered Ta and Ta-WC materials, the sintered Ta-TiC underwent plastic deformation and exhibited ductile failure. The yield strength and fracture strength of the sintered Ta-10vol.%TiC were very high, 820 MPa and 1200 MPa, respectively, while the fracture strength of sintered Ta and Ta-10vol.%WC were 350 MPa and 450 MPa, respectively. It is suspected that the sintered Ta-10vol.%WC materials failed prematurely due to the presence of oxide phases. The ductility in the sintered Ta-TiC materials is expected to be a result from Ti atoms purifying the tantalum matrix by reducing tantalum oxide and taking away the solute oxygen. Traces of titanium oxide were observed in the sintered Ta-TiC materials. Pure tungsten carbide powders with various particle sizes (0.2, 0.5, and 3 μm) were sintered to above 97% relative density for the 0.2 μm powder, and around 99% relative density for the 0.5 and 3 μm powders. The sintering temperature was 1500oC with the pressure of 65 MPa and the holding time up to 20 minutes. Limited grain growth (around 50% for 0.5 and 3 μm powder, and negligible for 0.2 μm powder) were observed in all three powders after sintering. The hardness values of the materials showed a trend according to Hall-Petch relation. The hardness values of 95.8 +/- 0.1, 94.6 +/- 0.1, and 90.0 +/- 0.3 HRA were obtained from the 0.2, 0.5, and 3 μm powder, respectively. WC-6wt.%Co with the average particle size of 4 μm were sintered to 99% relative density at 1250oC (30 MPa, for 5 minutes). The hardness obtained was 90.5 +/- 0.1 HRA. By adding the 0.2 μm pure WC powder at a level of 25 wt.% to the WC-6wt.%Co powder, the hardness was increased to 91.6 +/- 0.1 HRA, while fracture toughness remained at 14.5 MPa.m-0.5.