  The creep mechanism in the compression testing of the tungsten carbide with
different grain sizes has been studied. The WC samples with high density
(96.1-99.2%) were obtained by SPS from nano-, submicron, and micron-grade WC
powders. The samples had a coarse-grained (CG) surface layers of ~0.3 mm in
thickness and ultrafine-grained (UFG) central parts consisting of WC with a
small fraction of W2C. The creep tests were conducted in two regimes: (Mode #1)
holding at different temperatures (1300-1375C) at 70 MPa; (Mode #2) tests at
different stresses (50, 70, 90 MPa) at 1325C. Tests in Mode #1 were done to
determine the effective creep activation energy Qcr while tests in Mode #2 - to
determine the coefficient n in the power law creep equation. The increasing of
the fraction of the W2C particles from 1.7 up to 4% was found to result in a
decrease in the Qcr from 17.5 down to 13 kTm. The coefficient n equals to
3.1-3.7. The Qcr in the WC sintered from nanopowders was shown to be 31 kTm.
This value is 1.5-2 times greater than the Qcr in the UFG samples obtained by
SPS from commercial powders. The increased fraction of the W2C formed when
sintering the nanopowders with increased adsorbed oxygen concentration was
suggested to be one of the origins of the increase in the Qcr when testing the
UFG samples. The mechanical removing of the CG layers from the surfaces of the
tungsten carbide sample was shown to result in an accelerated creep,
insufficient decrease in the Qcr and coefficient n to 2.5-2.6. The creep rate
of the samples was suggested to be determined simultaneously by the creep
process in the CG surface layers and the creep process in the UFG central parts
of the samples. The creep rate in the surface CG layers is determined by
intensity of carbon diffusion in the WC crystal lattice while the creep rate in
the UFG central parts - by the intensity of grain boundary diffusion.
