Phase Formation, Microstructure and Properties of Nanostructured
Hard Coatings of the Material System Cr-Al-N-O
M. Stüber, U.
Albers, H. Leiste, C. Ziebert, S. Ulrich
International Conference on Metallurgical Coatings and Thin Films,
Hard Coatings and Vapor Deposition Technology, Hard and Multifunctional Nano-Structured Coatings, 27.04.2007
The phase formation and microstructure evolution of magnetron-sputtered
coatings was examined for the Cr-Al-N-O material system. A combinatorial
material science based approach by sputtering from a segmented target with a Leybold Z 550 machine was applied
for the deposition experiments. The target was composed of two pieces, one made
of bulk, hot pressed (Cr, Al)N and the other of a
commercial Al2O3 plate. Both non-reactive and reactive deposition was applied.
In each experiment six coatings of different composition and microstucture were obtained simultaneously by placing 6
substrate samples in individual positions below the target. The coatings,
as-deposited and thermally annealed in vacuum up to 750°C, were characterized
by Electron Probe Micro Analysis, X-Ray Diffraction, and Transmission Electron
Microscopy. The microhardness was measured by the
Vickers method. Non-reactively as-deposited coatings exhibited a nanocrystalline structure independent of the chemical
composition and moderate hardness values between 1000 and 1200 HV0.05. A strong
increase in the crystallite sizes of these coatings was observed at annealing
temperatures above 600°C. This crystallization was accompanied by a significant
increase in the microhardness up to 1500 HV0.05 at
lower Cr:Al and higher O:N
concentration, and up to 2000 HV0.05 at higher Cr:Al
and lower O:N concentration. The brittle, ceramic behaviour
of these coatings increased clearly with increasing annealing temperature. The
reactive deposition in nitrogen atmospheres resulted in the synthesis of nanocrystalline coatings with a significantly higher
hardness and ductility. The Vickers microhardness of
as-deposited films was in the range of 2250 to 2500 HV0.05 and no brittle
cracking of the coatings was observed. Annealing in vacuum however did not
result in a remarkable change of the hardness values but in a change of the
crystalline structure of the coatings.