Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 13, pp. 289-290. Pergamon Press 1976. Printed in Great Britain

Technical Note Environment-Sensitive Wedge Indentation Behavior of Granites

J. J. MILLS* R. D. HUNTINGTON* A. R. C. WESTWOOD*

That the hardness of non-metallic solids is a maximum in media which produce a condition of zero surface charge now appears to be well established [1, 2], and related work has shown that the drilling rates of either rotary diamond coring bits, or carbide wedge bits in a rotary-percussive mode, through hard rocks and minerals are also maximized at their zero point of charge (z.p.c.) [3-7]. However, the mechanisms by which environmentally-induced changes in surface hardness, termed chemomechanical effects, cause these increases in drilling rates are still not fully understood. To provide a better basis for the development of such understanding, therefore, this Note reports the results of studies on the influence of surface active environ- ments on the energy to disintegrate unit volume of rock with a loaded wedge indenter (the specific energy, Es). The cationic surfactant selected (aqueous dodecyltri- methyl ammonium bromide---DTAB), and one of the rocks (Westerly granite), were used in earlier drilling studies [4, 5, 7].

Two tungsten carbide wedges were used, each having an included angle of 75 ° and a radius of curvature of 40 ram. One of them, however, referred to as the blunt wedge, was prepared such that the load was applied to the granite through a 1.5 mm flat ground on its apex. The wedges were mounted on a test machine crosshead, and driven into the test rocks at a constant speed of 5 x 10- s m/s for a I mm total penetration. Wedge load and penetration were recorded continuously. The en- vironments were poured onto the smoothly ground (120 grit) rock surfaces to form site of indentation.

Values of Es were calculated done--determined from the area

a puddle around the

by dividing the work under the load penet-

ration curve--by the volume of rock removed---deter- mined by filling the cleaned out indentation with a low surface tension solution from a calibrated microsyringe.

The Es of the Westerly and Seattle granites are given as a function of DTAB concentration in Table 1. A t-test showed that the difference between the mean value of Es for each DTAB solution and that for water

* Martin Marietta Laboratories, Baltimore, MD 21227, U.S.A.

a.M.M.S. 13 10

is significant at the 5% level, with the exception of that for the combination 10 -3 mol/l solution and Seattle granite. In addition, the differences between the mean Es for the 10-4mol/l DTAB solution and those for the other environments are also significant at the 5% level with two exceptions: both granites with the sharp indenter and 10 -5 mol/l DTAB. The data indicates, therefore, the existence of a minimum centered around 10 -4 mol/1 DTAB with a maximum decrease in Es rela- tive to water of -,- 40% (sharp wedge) and -,- 45% (blunt wedge) for Westerly granite and -,-20% for Seattle granite (sharp wedge).

The DTAB concentration (10-4-+°5mol/1) for which Es for both types of rocks appears to be a mini- mum is close to that previously found to produce a z:p.c, condition on polycrystalline quartz (10 -3.8 mol/l) and on Westerly granite (10 -3.9 tool/l) [7], a maximum in the Knoop hardness of quartz (10-3"5mol/1) 1"8] -- the dominant phase in both of the granites tested-- and the same in the penetration rates with diamond coring bits (10 -a'2 mol/1) and with carbide wedge bits used in a rotary-percussive mode (10 -3"s mol/l) [7], in Westerly granite. This agreement between, on the one hand, the drilling rate maxima and the z.p.c, and, on the other, the minima in Es and the z.p.c, suggests that chemomechanical effects do have a direct role in enhancing drilling performance, and do not influence cutting primarily via effects such as improved cooling or muck removal.

A point of special interest in the data of Table 1 is that, for each rock-indenter combination, the energy consumed in penetrating I mm remains approximately constant, but the volume of rock removed substantially increased in the active environment, being 40-75% greater in DTAB than in water. This observation pro- vides a useful clue to the manner in which active en- vironments enhance drilling rates in hard rocks.

Presumably, since such environments increase hard- ness, they reduce the ability of the solid to relax stress concentrations beneath the loaded indenter by plastic flow processes. Consequently, larger amounts of elastic energy can be stored in the solid. On unloading, when the compressive stress field beneath the indenter becomes tensile, the energy stored is dissipated via

289

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extensive brittle fracturing. Hence a greater volume of chips is produced--both beneath an indenter and in drilling.

In drilling, however, a further possibility exists. Since environmentally-induced increases in hardness are also likely to lead to reductions in the coefficient of friction between the bit and the solid [9], the amount of mechanical energy converted to heat at the interface will be reduced, and the bit will run cooler. Since tem- perature is considered [10] to be a prime factor in determining the wear of cutting tools, bit life should be improved. Accordingly, other things being equal, the instantaneous drilling rate should always be higher than that in water [3-8]. It should be appreciated, how- ever, that this beneficial influence is not due to im- proved cooling of the bit by the fluid, as has been sug- gested [10], but rather to a chemomechanically- induced reduction in the amount of heat generated.

Acknowledgements--The authors are pleased to acknowledge the early contributions of M. V. Swain to this work, which was supported by the National Science Foundation under Grant Number APR 73-07787. The views expressed herein, however, are those of the authors, and not necessarily those of the Foundation.

Received 26 March 1976

REFERENCES

1. Westwood A. R. C. & Macmillan N. H. Environment-sensitive hardness of non-metals. In The Science of Hardness Testing, pp. 377-417. Am. Soc. Metals, Metals Park, OH (1973).

2. Macmillan N. H. & Westwood A. R. C. Surface charge depen- dent mechanical behavior of non-metals. In Surfaces and Inter- faces in Glass and Ceramics, Vol. 7, pp. 493-513. Plenum Press, N Y 0 9 7 3 ) .

3. Westwood A. R. C., Macmillan N. H. & Kalyoncu R. S. Environ- mental-sensitive hardness and machinability of alumina. J. Am. Ceram. Soc. 56, 258-262 0973).

4. Westwood A. R. C., Macmillan N. H. & Kalyoncu R. S. Chemo- mechanical effects in hard rock drilling. In SME/AIME Trans. 256, 106-111 (1974).

5. Jackson R. E., Macmillan N. H. & Westwood A. R. C. Chemical enhancement of rock drilling. Adv. in Rock Mechanics, Proc. Third Cono. IRSM, ham. Acad. Sci. Washington, D.C., IIB, pp. 1489-1493 (1974).

6. Westwood A. R. C. Control and application of environment- sensitive fracture processes. Proc. 3rd Tewkesbury Symp. Fracture, Univ. of Melbourne, pp. 1-60o (June 1974). J. Mat. Sci. 9. pp. 1871-1895 (1974).

7. Macmillan N. H., Jackson R. E., Mularie W. M. & Westwood A. R. C. Optimization of fluids for diamond core drilling of sili- cates. SME/AIME Trans. 258. pp. 278-280 (1975).

8. Jackson R. E., Huntington R. D. & Wcstwood A. R. C. Report to NSF on Grant No. GI-38114 (June 1974).

9. Macmillan N. H., Huntington R. D. & Wcstwood A. R. C. Che- momechanical control of sliding friction behavior in non-metals. J. Mat. Sci. 9, pp. 697-706 (1974).

10. Joris A. C. T. & McLaren G. Additives to coolants used in dia- mond drilfing and sawing in Australia. Min. mineral Engng 3, p. 190 (1969).