ICMCTF1998 Session B3/F4: Microstructural Characterization - Structure & Properties of Hard Coatings

Multilayer coatings have been the subject of intense interest over the last few years. When the layer thickness is on the nanometre scale, techniques with high spatial resolution are required for their detailed microstructural characterisation. The micro-structure of the coating is determined by the deposition conditions and in turn determines the properties of the coating in use. The long term aim of such microstructural studies is to help to determine the relationship between deposition conditions, microstructure and coating performance.

Among the microstructural features of interest in a multilayer are the thickness of the individual layers, their uniformity, their composition, their crystallography, the defects present, the sharpness of the interfaces both crystallographically and chemically, the texture and its development, the internal stress and its variation through the film, the nature of the substrate coating/interface and the effects of any pre-treatment of the substrate prior to deposition of the coating. Many of these parameters are of interest in any coating whether multilayered or not.

Transmission electron microscopy of a cross-section through the coating and substrate is one well established way of gaining such information particularly when combined with x-ray and electron energy loss spectroscopy. The latter technique provides information about the composition, chemistry and crystallography on a nanometre scale and may even provide information on local stress.

Results will be presented for a range of multilayer coatings deposited in a commercial arc evaporation system. The results obtained suggested that further information could be obtained from such systems if the techniques in use were developed further. Progress in this area will be reported.

In previous work we confirmed that the long range effect reviewed by Sharkeev ¹ is also found in TiN coatings deposited by CVD methods ² and in uncoated ground cemented carbide ³. The magnitude of the effect recorded in the TiN coating implanted with yttrium ions ⁴ is sufficiently great that it seemed likely that the residual stress in the cemented carbide substrate should also be impacted.

In the present work, first, the residual stress in cemented carbide following PVD and CVD coating processes is reported and, second, the effects of post ion implantation of the CVD coating on the stress in the substrate is studied. The coating process is found to change the compressive stress found in the WC phase to tensile in the plane of the surface, which is regarded as detrimental as it should enhance crack propagation. The subsequent ion implantation appears to have a beneficial effect by reducing the stress in the WC phase to near zero which should counteract the effect of the coating process.

¹ Y.P. Sharkeev et al, Surf. Coat. Technol., in press.

² D.E. Geist et al, Adv. X-ray Anal., 38 (1995) 471.

³ A.J. Perry et al, Surf. Coat. Technol., 86-87 (1996) 364.

⁴ A.J. Perry and D.E. Geist, Nucl. Instr. Meth., B 127/128 (1997) 967.

The increasing importance and use of surface coatings in tribological applications bring with them a need for fundamental understanding of their properties if the optimum coating for a particular purpose is to be selected. Furthermore, in order to achieve a coating tailored for a particular task, a knowledge of the interrelationship between their physical, structural and mechanical properties needs to be studied in some detail. Within the frame of this work we produced Ti_1-xSi_xN hard coatings, with 0 ≤ x ≤ 0.3 by r.f. reactive magnetron sputtering. The films, with thicknesses ranging from 1 to 3 micrometers, were produced with and without Ti interlayer. Some of the physical properties, such as oxidation resistance, structural properties such as texture and grain size and mechanical properties such as hardness, adhesion and residual stress were analysed. The atomic composition of the samples was obtained by Rutherford Backscattering Spectrometry (RBS). Texture and grain size were characterised by X-ray diffraction (XRD). The deflection method was used to evaluate the residual stress. The hardness and adhesion were characterised using a ultarmicrohardness tester and a scratch tester, respectively.

All properties will be characterised and discussed as a function of the Si content in the (Ti,Si)N matrix, bias voltage and thickness of the adhesion layer. Regarding the preliminary results, and although all samples with x > .05 present good results, the Ti_0.70Si_0.30N sample presented not only an oxidation resistance several orders higher than that of TiN and similar to that of (Ti,Al)N, but also good results in the mechanical properties, presenting a hardness value of about 36 GPa and a critical load for total failure around 115 N.

Ti_0.43Al_0.52Cr_0.03Y_0.02N coatings have been grown in a multiple target PVD coater using the combined steered arc evaportion / unbalanced magnetron deposition technique. Coating properties and yttrium distributions have been found to change significantly when 3 targets with equal yttrium content (Ti_0.495Al_0.495Y_0.01) or one yttrium containing target (Ti_0.48Al_0.48Y_0.04) neighboured by 2 pure Ti_0.5Al_0.5 targets to produce a non-homogenous distribution were used.

The structural characteristics and residual stress of the coatings were determined by x-ray diffraction using both Bragg-Brentano and glancing angle parallel beam (sin² U method) geometries. The residual stress state for all cases was found to be compressive, with -6.5GPa using a single 4 atomic %yttrium containing target and -3.25GPa deposited using three TiAlY targets each containing 1 atomic % Y. Line broadening measurements demonstrated that the domain size within the in-homogeneously grown TiAlCrYN coatings was also significantly smaller than the other films examined.

The surface chemistry of as-deposited and thermally treated films was further analsed by high resolution monochromatic XPS. Large area (700x200 um) and small spot (55um) spectroscopic analysis was used to examine the role of low concentrations of Cr and Y during the oxidation process whilst 0 1s imaging has been used to elucidate the spatial distribution of the oxides formed. High resolution analysis of the yttrium 2p core level identified two chemical species for the as-deposited TiAlCrYN films at positions y 2P_3/2 156.4eV and 158.3eV respectively, whilst only single species were identified in heat treated films at Y 2p_3/2 157.9eV.

Wear resistant CVD coatings on cemented carbide inserts are widely used in metal cutting operations. The wear on cutting tool inserts during metal machining might result in microstructural changes in the coatings and the cemented carbide substrates. In this investigation, these changes in microstructure during turning operations were studied.

Experimental multilayer coatings were deposited in a hot wall CVD reactor on TiC coated cemented carbide cutting tool inserts. The coatings consisted of eight κ-Al₂O₃ layers separated by thin intermediate layers of TiC. After coating, the inserts were used in turning tests of steel. The influence of the turning operation on the microstructure of the cutting tool inserts were characterised using a combination of SEM, TEM and EDX.