October 4-9, 2015
Supererlastic hard carbon particles up to 1 mmin size were produced by fullerene collapse upon high-pressure high-temperature treatment with simultaneous sintering of metal-matrix composite materials (CM) reinforced by such particles. The hardness of carbon particles can be varied in a wide range by changing the parameters of their structure, which consists of curved graphene planes or their packets of different sizes. Such carbon phase was called “nanoclusterd graphene phase” (NGP) . The properties of the carbon particles were controlled by changing treatment pressure (5 and 8 GPa) and temperature (1100-1800 K), composition of parent fullerites (C60 or C60/70), and pre-treatment (ball milling) of parent fullerites. The carbon particles formed from fullerites under pressure are close to diamond-like carbon coatings in mechanical properties (combination of high hardness and high elasticity), but they are sufficiently large for their microhardness testing at rather high loads. The mechanical characteristics of the carbon particles were tested with a DUH 211S (Shimadzu) tester according to ISO14577 with a Vickers indenter at loads of 10-1970 mN in load-unload regime at a strain rate of 70 mN/sec. The Martens hardness HM measured at a load of 500 mN on the carbon particles of 28 CMsamples was varied from 2700 to 13600 N/mm2; the corresponding indentation hardness HIT changes from 8100 to 42400 N/mm2, i.e., for the carbon material under consideration, HIT/HM = ~3. Such great difference between the above hardness parameters is due to a great contribution of the elastic deformation to the total deformation upon indentation. The ratio between the corresponding deformation works hIT = Welast/Wtotal (%) with increasing hardness decreases from 87 to 78%, but still remains very high, exhibiting the superelastic behavior of the NGP carbon particles. All samples are characterized by the indentation size effect (ISE), which manifests itself as decreasing HM, HIT, and EIT with increasing Fmax. The ISE becomes more pronounced with increasing hardness of the carbon particles, for example, HM of the least and most hard particles with increasing Fmax decreases by a factor of 1.7 (from 3900 to 2300 N/mm2) and 5.4 (from 40200 to 7400 N/mm2), respectively. The intensity of hardness reduction in the range of small loads (10-250 N) is significantly higher than in a range of 250-1970 N. The elastic recovery upon indentation expressed as hIT at Fmax ranging between 250 and 1970 N is virtually unchanged while, at lower loads, the dependence of hIT on Fmax is non-monotonic, with a small peak at Fmax = 50 N in all cases. Indentation creep CIT (%) was measured at Fmax = 500, 1000, and 1970 N, the holding times at Fmax were 60, 300, and 600 sec. For all samples, CIT decreases with increasing Fmax and increases with holding time. The time dependence curves of CIT tend to saturation with increasing Fmax. CIT increases with increasing hardness of the superelastic hard carbon particles.
 O. Chernogorova, I. Potapova, E. Drozdova, et al. Applied Physics Letters 104, 043110-1 - 043110-4 (2014); doi: 10.1063/1.4863470.