The impact of multilayer periodicity on texture, morphology, chemical stability and machining performance of titanium aluminum nitride/chromium nitride thin films
Multilayer thin films of TiAlN/CrN were deposited at varying bilayer periods (lambda = 2, 4, 8 and 16 nm), along with a co-deposited (TiAlCr)N film and constituent TiAlN and CrN films, on both [111] silicon substrates and WC-4wt% Co inserts with an ISO SPGN 120308 geometry. The effects of periodicity on grain structure, texture, surface morphology, and roughness were examined, with characterization performed using electron microscopy, atomic force microscopy and x-ray diffraction analysis. An in-air constant temperature oxidation study was also performed at 650 thru 1050°C with stacked XRD line profiles and SEM used to identify the onset and extent of oxidation, respectively. Finally, dry high speed flank wear tool life experiments were completed against A2 tool steel, with the impact of the multilayer period reported in ISO standard wear vs. cutting time format. Machining results were connected back to the previously identified changes in structure. All films exhibited a rocksalt B1-type structure with a columnar morphology and lateral coarsening during film growth; which was more pronounced in the multilayer films with smaller bilayer periods. The surface morphology was found to be dependent on period; exhibiting sharp pyramidal surface features at lambda = 2 nm and an increase in secondary faceting with increasing periodicity. Nearly rounded column tops result at lambda = 16 nm. Surface roughness measurements showed an increasing roughness with decreasing bilayer period and pole figures revealed a small [111] texture component perpendicular to the sample surface in the 2 and 4 nm period films, with the distribution of [111] poles becoming nearly random for the films with larger periodicities (8 and 16 nm). Preferential [200] poles begin approximately 15° tilted away from the substrate normal while further increase in period results in a decrease in the off axis angle. The effect of multilayer period on the surface morphology is explained using an energetic argument whereby the higher interfacial energy density in the smaller period multilayers promotes a higher degree of faceting at the film surface. Oxidation results are comparable for all multilayer films, where oxidation onset was delayed approximately 100°C over the TiAIN film. The co-deposited (TiAlCr)N film performed comparably to the multilayer films with a similar oxidized morphology. The TiAlN exhibited large oxide "rosettes" and through thickness cracking and oxidation in the film. Machining results showed that a multilayer of greater than 2 nm is required for an effective coating, with the 4 nm film outperforming all multilayer and monolithic films due to increased oxidation resistance, hardness and a reduction in coating cracking and spalling. All films exhibited the three documented stages of wear: running in wear, steady state wear and dynamic wear and it was shown that the amount of carbide particles in the workpiece significantly impacts wear rate and tool life.
Engineering, Materials Science, Nanotechnology, Industrial
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