Report on microfocus XRD studies on the diamond metal interfacial area in diamond tools

A. Steffen1, K. Rüster1, J. Möller1, S. Bieder1, M. Paulus1, M. Ferreira2,W. Tillmann2, and M. Tolan1

1Fakultät Physik/DELTA, TU Dortmund, Maria-Goeppert-Mayer-Str. 2, 44227 Dortmund, Germany 2Institute of Materials Engineering, TU Dortmund, Leonhard-Euler-Str. 2,44227 Dortmund, Germany

Diamond tools have widely established their usage in machining, especially in grinding and cutting, of hard to process materials as natural stone and concrete [1-5]. Diamond metal composites are mainly fabricated powder metallurgically in a vacuum sintering or in a hot pressing process [1,2,6]. The sintered metal especially serves as a boundary matrix for the embedded diamond. The size of the synthetic diamonds varies between 280 µm and420 µm, the size of the interfacial area is much smaller and depends on the composition of the metallic matrix and the sintering process. The diamonds are primarily responsible for the grinding process and therefore the bonding type of the diamonds in the metal matrix is of essential relevance because the interfacial region has to bear up the developing forces at each diamond particle [7]. The existence of a chemical bonding between matrix and diamond in tools is preferred in contrast to the always given mechanical bonding [2,6]. The chemical bonding type results in higher durability, better grinding performance and longer lifetimes [5]. Thus, it is of fundamental interest to gain information about this interfacial area, e.g. if it consists of metal carbides, solid solutions of carbon in metal, or even graphite.

Even if there is a big variety of matrix systems today, cobalt is often used as a constituent of diamond tools. This is due to the fact that cobalt offers a good combination of ductility, compatibility, abrasion resistance and hardness associated with a stable embedding of diamonds [7]. Studies on the chemical bonding between matrix metals and synthetic diamonds in metal diamond composites are rare. Cobalt carbides were only detected after thermally induced reactions between cobalt and amorphous carbon [8,9].Taking account of the high price and the toxic effect of cobalt nowadays many alternative materials as iron, copper, and nickel are used as matrix material. First XRD studies on diamond cobalt composites do show a graphitization of the diamond due to the sintering process. Cobalt carbides could not be detected but cannot be excluded.

At beamline P07 x-ray diffraction (XRD) studies were performed in order to investigate the interfacial area between diamonds and metal matrices in diamond tools. Using a photon energy of 60 keV, and a large image plate detector(PerkinElmer)with 2048 x 2048 pixel of 200 x 200 µm2

size we were able to gather all Bragg reflection caused by parallel lattice planes with dhkl >= 1 Å. In order to achieve high spatial resolution the beamsize was chosen to 3 µm in vertical and 30 µm in horizontal direction.As all measurements were performed at room temperature. It was adequate to fix the single diamond grains on a plastic bar.

Samples Co mech

Co chem

Cr mech

Cr chem

Ni mech

Ni chem

Fe mech

Cu mech

Quantity of analysis

9 5 4 4 4 4 5 2

Table 1:Quantity of measurements of different metallic matrices.

In table 1 all investigated sample systems and the quantity of analysis of each sample system are listed. Co, Cr, Ni, Fe, and Cu indicate which matrix material was used. 'mech' indicates that the diamond grains were extracted out of the metallic matrix by mechanical treatment, 'chem' that chemical treatment in form of nitric acid washing was used. Each analysis consists of different XRD measurements. Between these single measurements the height of the sample was variied in order to scan the interfacial area in vertical direction.

In figure 1 some diffraction patterns are shown exemplarily. This raw data shows the characteristics of the different matrix materials used. In contrast to the strong scattering signals if chromium or

iron is used as matrix material the scattering signal from cobalt-diamond and nickel-diamond interfacial area is weak. As the investigated scattering volumesshould be nearly identical for each measurement this may be a hint that chromium and iron have a stronger ambition to form polycrystaline structures (instead of amorphous ones).

The data analysis is still in progress.

Figure 1: Typically raw data diffraction patterns of different metallic matrices.

All in all it can we were able to perform several XRD analysis within each sample system using cobalt, iron, copper, chromium and nickel as matrix materials. Therefore the data taken at beamline P07 at PetraIII at Hasylab/DESY will reveal the structural information about the composition of the interfacial area between diamond and metallic matrices in diamond tools, e.g. if the interfacial areas consists of solid solutions of carbon in metal, pure metal matrices, graphite or metal carbides.

References [1] Y.S. Liao and S.Y. Luo, J. Materials Science 28 1245 (1993). [2] A. Molinari et al., Materials Science and Engineering A 130 257 (1990). [3] A. Neto and E. Pereira, Diamond and Related Materials 15 465 (2006). [4] W. Tillmann, Int. J. Refractory Metals and Hard Materials 18 301 (2000). [5] Y.K. Vohra et al., Diamond and Related Materials 5 1159 (1996). [6] M. Zeren and S. Karagz, Materials and Design 28 1055 (2007). [7] A. Romanski, Powder Metallurgy 50 115 (2007). [8] O. Nittono et al., Materials Science and Engineering A 312 248 (2001). [9] H. Wang, M.F. Chiah, W.Y. Cheung, S.P. Wong, Physics Letters A 316 122 (2003).