Microstructural evolution of TiAlN hard coatings at elevated pressures and temperatures

Abstract

LINKÖPING 2014The cover image shows nine two-dimensional x-ray diffractograms of TiAlN merged into one quarter circle. Going counterclockwise from an as-deposited sample, the parts show different stages of annealing. The color scheme is chosen to represent the motto "Stort hjärta, hårt arbete".Niklas Norrby, unless otherwise stated ISBN: 978-91-7519-372-4 ISSN: 0345-7524 Printed by LiU-Tryck, Linköping, Sweden, 2014 i Abstract A typical hard coating on metal cutting inserts used in for example turning, milling or drilling operations is TiAlN. At elevated temperatures, TiAlN exhibits a well characterized spinodal decomposition into coherent cubic TiN and AlN rich domains, which is followed by a transformation from cubic to hexagonal AlN. Using in-situ synchrotron x-ray radiation, the kinetics of the second transformation was investigated in this thesis and the strong temperature dependence on the transformation rate indicated a diffusion based nucleation and growth mechanism. The results gave additional information regarding activation energy of the transformation and the critical wavelength of the cubic domains at the onset of hexagonal AlN. After nucleation and growth, the hexagonal domains showed a striking resemblance with the preexisting cubic AlN microstructure.During metal cutting, the tool protecting coating is subjected to temperatures of ~900 ºC and pressure levels in the GPa range. The results in this thesis have shown a twofold effect of the pressure on the decomposition steps. Firstly, the spinodal decomposition was promoted by the applied pressure during metal cutting which was shown by comparisons with annealed samples at similar temperatures. Secondly, the detrimental transformation from cubic to hexagonal AlN was shown to be suppressed at elevated hydrostatic pressures. A theoretical pressure/temperature phase diagram, validated with experimental results, also showed suppression of hexagonal AlN by an increased temperature at elevated pressures.The spinodal decomposition during annealing and metal cutting was in this work also shown to be strongly affected by the elastic anisotropy of TiAlN, where the phase separation was aligned along the elastically softer <100> directions in the crystal. The presence of the anisotropic microstructure enhanced the mechanical properties compared to the isotropic case, mainly due to a shorter distance between the c-AlN and c-TiN domains in the anisotropic case. Further improvement of the metal cutting behavior ii was realized by depositing individual layers with an alternating bias. The individual bias layers exhibited microstructural differences with different residual stress states. The results of the metal cutting tests showed an enhanced wear resistance in terms of both crater and flank wear compared to coatings deposited with a fixed bias.iii Populärvetenskaplig sammanfattningI den här avhandlingen presenteras forskningsresultat på en keramisk beläggning, TiAlN (titanaluminiumnitrid), som ligger som ett skyddande skikt på svarvskär. Tjockleken på beläggni...

Mechanical and thermal stability of hard nitride coatings

“…The temperatures of cutting tool during the turning process has been measured using various methods including thermocouples/pyrometer [151] or depositing a temperature sensor coating on the tool [153,154]. Norrby et al [82] used an infrared (IR) CCD camera to map the temperature distribution on a cutting insert and found that it increases with cutting speed [155]. The maximum temperature was at a cutting speed of 400 m/min and is around 1000 • C. However, the temperature distribution can also be affected by the cutting feed and the depth-of-cut so that turning processes with different parameters can have varied temperature environment on the cutting tool.…”

Effect of thermal treatments in high purity Ar on the oxidation and tribological behavior of arc-PVD c-Al0.66Ti0.33N coatings

“…Hardness improvement can be induced either by spinodal decomposition or grain refinement. Spinodal decomposition depends on nitride splitting into its single cubic nitrides [3][4][5][6]. This decomposition process is strongly conditional on the coating composition and the temperature of exposure [7,8], e.g.…”

Optical properties of refractory TiN, AlN and (Ti,Al)N coatings

“…TiAlN is well-known as a hard coating for elevated temperature applications. It can be considered as a derivative of TiN, where the additional Al content acts to improve resistance to oxidation and increase hardness 4 . There seems to be some controversy regarding the microstructure of such coatings.…”

“…The decomposition of TiAlN may occur in two ways, by spinoidal segregation into two cubic phases or by precipitation of AlN phase 6 . For example, in 4 it was shown that annealing of TiAlN thin films for 40 minutes at 1000°C led to decomposition of the (Ti,Al)N compound and formation of the c-TiN and c-AlN phases. TiAlN has also started to receive attention as an optical material for sun control applications 9 .…”