Microstructure of Functionally Graded Materials and Thin Films Studied by X - ray Diffraction

M. Dopita¹, D. Rafaja¹ and V. Ucakar²

¹ Faculty of Mathematics and Physics, Charles University

Ke Karlovu 5, CZ – 121 16 Prague, Czech Republic

² Institute for Chemical Technologies and Analytics, Vienna University of Technology

Getreidemarkt 9/161, A-1060, Vienna, Austria

The most general effort in production of cemented carbides and carbonitrides is devoted to the increase of their service time. One of the most successful achievements in this task was the deposition of thin films by CVD or PVD techniques. The first generation of such layers were single-phase layers such as TiN and TiC, followed by the second generation multiphase layers where a variety of compounds such as TiN, TiC, Ti(C,N), (Ti,Al)N and Al₂O₃ was combined. The third generation of materials consists of functional-gradient cemented carbonitrides. Such materials have the benefit of thermal and mechanical load dissipation (because of a smooth variation of constituents in their microstructure) avoiding piling up of stresses at distinct interfaces. The surface roughness is only slightly higher than for coated materials and the residual stresses in the surface layers are in general compressive or zero.

In this work, basically two sets of samples were studied: 1) materials coated with thin layers, prepared by CVD methods. These samples consist of combination of three layers - TiN\TiC\TiN. The bulk material (substrate) is made from the cemented tungsten carbide. 2) functionally graded tools prepared by reaction diffusion process (diffusion of nitrogen into the mixture of TiC, TiN and WC).

The main interest was to obtain information about parameters that has primary influence to advantageous physical, chemical and mechanical properties of these kind of materials. Especially, it means crystalitte size, microstrain, residual stress, lattice parameters and in case of functionally graded tools the concentration profile too.

The samples were analyzed using the glancing angle X-ray diffraction performed with the parallel beam optics and with classical Bragg-Brentano geometry. To distinguish properties changing from sample surface into the bulk, the angle of incidence of the primary beam (g) used for asymmetrical geometry varied from 2 to 10 deg. It corresponds to the penetration depth up to 10 mm, depending on the real composition of the samples, of course.

In the functionally graded tools, the microstructural and physical properties follow the changes in concentration. Simple diffusion model was used to describe the concentration profile in the sample under the following assumptions: one-dimensional diffusion, concentration independent diffusion coefficients, crystallites with the same size. The concentration profile was described by the error function. This work compares the results of the X-ray tensometry, with the concentration profile fitting.