The invar property of the metallic inclusions in synthetic diamond crystals grown in the FeNiC system

Materials &@nee and Engineering, A I 50 (1992) L9-L 10

L9

Letter

The invar property of the metallic inclusions in synthetic diamond crystals grown in the Fe-Ni-C system E. Pavel, Gh. Bfilutfi and C. Giurgiu Institute of Scientific Research aml l~,chnological Engineering Jot Fine Mechanics attd Tools, P.O. Box 58-52, Bucharest (Romania)

D. Barb, D. R Lazfir and M. Morariu Institute of Atomic Physics', P.O. Box 52-06, Bucharest (Romania) (Received June 10, 1991 : in revised form September 16, 1991 )

Abstract Thermal and magnetic properties of metallic inclusions in synthetic diamond crystals grown in the Fe-Ni-C system were investigated by means of metallographic study, X-ray microprobe analysis, scanning electron microscope and magnetic measurements. An increased thermal stability was found for crystals exhibiting metallic inclusions with the invar composition (at.%): 65Fe-35Ni.

Nowadays, great amounts of synthetic diamond crystals are produced by the high pressure-high temperature method by means of a suitable catalyst (group VIII metals, as well as manganese, chromium and to a lesser degree tantalum, and also alloys of these metals). Ternary systems such as Fe-Ni-C are more important to high quality diamond synthesis than the binary systems Fe-C or Ni-C, because of their interesting properties. During the growth of synthetic diamond crystals, some metal catalyst is retained into the crystals. The existence of a great difference in the expansion coefficient of diamond and metallic inclusions (e.g. group VIII metals) implies that high temperatures (greater than 1000 °C) could generate destructive forces inside crystals. A solution to decrease this effect is to choose an alloy catalyst that yields metallic inclusions with expansion coefficients similar to that of diamond. In the present study, we investigate the link between the thermal stability of crystals and the nature of Fe-Ni metallic inclusions. 0921-5093/92/$5.00

For the alloy Fe~ _,Ni, there is a minimum value of the expansion coefficient of about x = 0.35. This invariance, observed in many alloys, was named "invar". Diamond also has a low thermal expansion. Similar behaviour has been observed in invar Fe-Ni and diamond in the range 300-700 K [1]. In order to investigate some correlations between the thermal stability of synthetic diamond crystals and the nature of metallic inclusions, we carried out high pressure runs using a modified belt-type high pressure apparatus and a reaction cell like that described in ref. 2. Fe-Ni alloys were used as catalysts. Synthetic diamonds which have been synthesized at a pressure of about 5.5 GPa and a temperature of about 1800 K retain a small amount of metal and metallic compounds, the nature of which was studied in recent papers [3-6]. The metallic inclusions in synthetic diamond crystals produced at high pressure and high temperature in the Fe-Ni-C system consist of (FeL ,Ni~)3C in a mixture with f.c.c. Fe-Ni. According to the experiments and data from thermodynamic calculations in ref. 7 relating to the constitution diagrams for the Fe-Ni-C system at 5.7 GPa, it was possible to control the composition of the metallic inclusions. Crystals labelled A, B, C and D were obtained by varying the composition of Fe-Ni catalysts. Table 1 shows some features of these specimens. The composition and phases of metallic inclusions were investigated by X-ray microprobe analysis on mechanically polished cross-sections of crystals, after etching by 1% nital solution. Magnetic susceptibility curves of A, B and C samples vs. temperature are presented in Fig. 1, confirming the composition of the inclusions. A, B, C and D crystals were heated at 1200 °C for 1 h, in vacuum. Optical analysis of these crystals reveals that the C crystals--which have metallic incluTABLE 1. Some features of A, B, C and D crystals Sample

A B C D

Composition (wt.% Fe)

Curie temperature

Phases

80-90 75-80 61-68 50-57

228 +_4 239_+6 395 _+6 --

(Fel_ ~Ni~)3C Ledeburite eutectic Ec.c. Ni-Fe invar F.c.c. Ni-Fe

(oc)

© 1992 - Elsevier Sequoia. All rights reserved

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sions with invar composition--present smaller damage than the others. Damage consists in graphite flakes grown inside the diamond crystals. T h e relative measurements of the crystal toughness after heating were performed with a Friatester [8], which is a special ball mill adapted for diamond crystals. Results are given in Fig. 2. Sample C has minimum friability, confirming its good thermal stability. In conclusion, synthetic diamond crystals grown in the F e - N i - C system and exhibiting metallic inclusions with the invar composition (at.°/,,) 6 5 F e - 3 5 N i have increased thermal stability.

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References

Fig. 1. Magnetic susceptibility of A, B and C crystals.

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Composifion of inclusions, % Fe

Fig. 2. The dependence of the relative friability on the composition of metallic inclusions.

1 Y. S. Touloukian, R. K. Kirby, R. E. Taylor and T. Y. Lee, Thermophysical Properties of Matter, Vol. 13, Plenum, New York, NY, 1977. 2 H.M. Strong, U.S. Patent4,340,576, 1982. 3 J. Wong and F. W. Lytle, J. Appl. Phys., 51 (1980) 280. 4 J. Wong, E. F. Koch, C. I. Hejna and M. E Garbauskas, J. Appl. Phys., 58(1985) 3388. 5 D. Barb, D. R Lazfir, M. Rogalski, M. Morariu, E. Pavel and Gh. Bfilu[fi,Rev. Roum. Phys., 34(1990) 247. 6 E. Pavel, Gh. Bfilu[fi, D. Barb, L. Gheorghe, Gh. Ilie, D. E Lazfir, M. Morariu and C. Giurgiu, Mater. Lett., 10 (1990) 62. 7 E. Pavel, L. Pintiliescu, Gh. Bfilu[fi,C. Giurgiu, D. Barb, D. R Lazfir and M. Morariu, The ternary constitution diagrams for Fe-Ni-C and Fe-Co-C systems at 5.7 GPa, Physica B, in the press. 8 N. G. Belling and H. B. Dyer, Impact Strength Determination of Diamond Abrasive Grit, Industrial Diamond Information Bureau, London, 1964.