Synthesis of diamond in cast iron

Abstract

An account of the synthesis of diamond in cast iron by the use of detonation waves was published more than a quarter of a century ago [i]. Detonation synthesis of diamond in cast iron requires pressures greater than 60 GPa, depending mainly on the density and velocity of the impacting bodies (i.e., the specimen and the detonation device). The most widely used scheme for detonation treatment of cast-iron specimens is by compression with a planar detonation wave (Fig. i). Methods for calculating the detonation adiabats of the media under investigation, taking into account the elements entering into their composition, have been worked out in order to determine the basic kinetic and thermodynamic parameters. Analysis of experimental results and calculated detonation adiabats for cast iron and graphite make it possible to specify the range of pressures necessary for the synthesis of diamond as 60i00 GPa. A pressure variation in graphite inclusions from 23 to 50 GPa lies within this range, wherein the lower value represents the beginning of transformation of the graphite lattice into that of diamond, and the higher the complete transformation of graphite into diamond. Gray cast irons with flake and spheroidal graphite, alloyed with Si, Ni, and Mn, were studied (see Table i). The parameters for detonation treatment of the cast-iron specimens were as follows: pressure 90 GPa, deformation rate i00 sec -l, pressure action time i0-~-I0 -7 sec. A portion of the specimens were forged at 950°C to 50% reduction at a rate of 10-30 sec -I before detonation treatment. Furthermore, after detonation treatment (without preliminary forging) specimens were thermal cycled following the procedure: heat at 150°C/min to 900-950°C, hold for 20-30 min, furnace cool (five cycles). The cast-iron microstructures were examined using the optical microscope "Neophot-21" and the scanning electron microscope JSM-35. One of the unique features of detonation loading is heating of the specimen under impact. The temperature field in cast-iron specimens at the moment of impact and during release of the load is inhomogeneous, due to the different wave impedances of the graphite and matrix. This results in the effect that at the detonation wave front graphite is heated to higher temperatures than the surrounding matrix. The higher the pressure, the higher the graphite temperature [2]. Consequently, there is a pressure threshold above which the synthesized diamond reverts to graphite. In order to avoid this reversion a load of minimal amplitude and maximal duration is usually selected. The heat in the graphite is rejected to the matrix of the cast iron. However, the pressure decreases more rapidly than the temperature. Insufficiently rapid temperature decrease is a major obstacle to preserving the diamond polycrystals. The shaded region IV in the graphite-diamond diagram shown in Fig. 2 is that in which the phase transformation occurs. Depending upon the point (a or b) at which the transformation to diamond occurred, the residual temperature of the specimen might lie in temperature field I or II. In temperature field I the probability of retaining the 4iamond is satisfactorily high. If the final temperature of the specimen lies in field II the reverse transformation diamond + graphite will most likely occur. The detonation loading time amounts to i-2 ~sec. The transformation of graphite to diamond may occur not only by a martensitic transformation [4], but also via fluctuations within the body of the graphite inclusions [5].