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, San Diego, California, 23.-27.04.2007

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.