Abstract
The performance of transition metal nitride based coatings deposited by magnetron sputtering, in a broad range of applications including wear-protective coatings on cutting tools and components in automotive engines, is determined by their phase content. The classical example is the precipitation of thermodynamically-favored wurtzite-AlN while alloying TiN with Al to obtain ternary single phase NaCl-structure films with improved high-temperature oxidation resistance. Here, we report on reactive high-power impulse and direct current magnetron co-sputtering (HiPIMS/DCMS) growth of Ti0.31Al0.69N and Zr0.48Al0.52N thin films. The Al concentrations are intentionally chosen to be higher than theoretically predicted solubility limits for the rock salt structure. The goal is to investigate the effect of the incident Al+ energy EAl+, controlled by varying the amplitude of the substrate bias applied synchronously with the Al+-rich portion of the ion flux from the Al-HiPIMS source, on the crystalline phase formation. For EAl+ ≤ 60 eV, films contain predominantly the wurtzite phase. With increasing EAl+, and thus, the Al subplantation depth, the relative fraction of the NaCl structure increases and eventually for EAl+ > 250 eV, Ti0.31Al0.69N and Zr0.48Al0.52N layers contain more than 95% of the rock salt phase. Thus, the separation of the film forming species in time and energy domains determines the phase formation of Ti0.31Al0.69N and Zr0.48Al0.52N layers and enables the growth of the cubic phase outside of the predicted Al concentration range. The new film growth concept can be applied to the entire family of multinary transition metal aluminum nitrides, where one of the metallic film constituents is available in the ionized form while the other arrives as neutral.
Highlights
The performance of transition metal (TM) nitride-based coatings deposited by magnetron sputtering, successfully used for wear-protection on cutting tools and components in automotive engines, is to a large extent determined by their phase content [1,2,3,4]
The substrate bias V s was applied in a form of pulses that were synchronized to the Al+ -rich portions of the ion fluxes generated during the HiPIMS phase, as determined by the ion mass spectrometry analyses conducted at the substrate plane [28]
Further proof for a controlling role of Al+ ion irradiation utilized here for manipulation of the crystalline phase content by means of synchronized substrate bias pulses, is presented in Figure 6, which shows a set of θ–2θ X-ray diffraction (XRD) scans acquired as a function of the sample tilt angle ψ from Zr0.48 Al0.52 N
Summary
The performance of transition metal (TM) nitride-based coatings deposited by magnetron sputtering, successfully used for wear-protection on cutting tools and components in automotive engines, is to a large extent determined by their phase content [1,2,3,4]. The flagship example is the precipitation of thermodynamically-favored wurtzite-AlN (w-AlN) phase in the widely used. Coatings 2019, 9, 17 presents a great challenge in the design of generations of TM(N)-based coatings, where control over the phase formation is required. Growth of (TM)AlN by conventional reactive DC magnetron sputtering (DCMS), with irradiation of the film surface by inert-gas ions with energies lower than the lattice displacement threshold (depending on the material system from 20 to 50 eV), typically results in significantly lower solubility levels.
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