An extended structure zone diagram is proposed that includes energetic deposition, characterized by a large flux of ions typical for deposition by filtered cathodic arcs and high power impulse magnetron sputtering. The axes are comprised of a generalized homologous temperature, the normalized kinetic energy flux, and the net film thickness, which can be negative due to ion etching. It is stressed that the number of primary physical parameters affecting growth by far exceeds the number of available axes in such a diagram and therefore it can only provide an approximate and simplified illustration of the growth condition?structure relationships.

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A structure zone diagram including plasma based deposition and ion etching - eScholarship

Phase stability and oxidation resistance are main objectives when synthesising hard and protective coatings for applications requiring also high thermal stability. Even though TiAlN is a well-studied and nowadays widely used high performance coating, the demand for further optimisation is omnipresent. Recent investigations on quaternary compounds demonstrate that the alloying of Ta to TiAlN films not only results in enhanced phase stability, but also in a significantly increased oxidation resistance. In this study we address thermal investigations of reactive cathodic arc evaporated coatings and elucidate the diverse performance of monolithically grown TiAlN, TaAlN, TiAlTaN, and a multilayered architecture of TiAlN and TaAlN layers. Subtle variations of the bilayer period between 30 and 38 nm were realised by varying the arc current at the TaAl cathode. Our research demonstrates that the quaternary Ti 0.45 Al 0.36 Ta 0.19 N and the multilayered TiAlN/TaAlN coatings feature enhanced mechanical properties and thermal stability as compared with their monolithically grown constituents Ti 0.54 Al 0.46 N and Ta 0.89 Al 0.11 N. All coatings synthesised exhibit as-deposited hardness values of 32 ± 1 GPa, but only the quaternary Ti 0.45 Al 0.36 Ta 0.19 N and the multilayered TiAlN/TaAlN coatings demonstrate pronounced age-hardening with peak hardness values of 37 ± 2 and 35 ± 2 GPa for annealing temperatures of ~ 1000 and 1100 °C, respectively. Complementary X-ray diffraction and differential scanning calorimetry confirm their enhanced phase stability. Even though, also the multilayered design shifts the formation of wurtzite-structured AlN to higher temperatures, only quaternary Ti 0.45 Al 0.36 Ta 0.19 N could withstand ambient air oxidation at 850 °C for 20 h. This is based on the ability of forming a nearly single-phased dense protective mixed oxide scale, having an outermost Al-rich composition. Approximately 70% of this quaternary nitride remained unaffected from oxidation. With the present study we conclusively demonstrate that thermomechanical properties of cathodic arc deposited TiAlN coatings can significantly be enhanced by either forming a quaternary compound or sophisticated architectural design with tantalum, both allow for wide-ranged industrial applications.

Thermal stability and oxidation resistance of arc evaporated TiAlN, TaAlN, TiAlTaN, and TiAlN/TaAlN coatings

Titanium alloys are widely used in aerospace, biomedical, and other engineering areas due to their superior properties. However, machining of titanium alloys has always been a challenge due to the high temperatures and tool wear rates. Dry machining has a limited range of permissible cutting conditions and is hence not suitable for industrial production. As a solution, flood cooling using cutting fluids is conventionally used to reduce the cutting temperatures. However, it is often discouraged in light of the associated environmental and health hazards. In order to achieve sustainable machining, different strategies for applying the cutting fluids are developed. Some of the prominent methods include minimum quantity lubrication (MQL), minimum quantity cooled lubrication (MQCL), and cryogenic cooling. This paper provides a comprehensive review of the available recent literature on such studies. Each of these techniques and results obtained in the studies has been discussed with emphasis on the advantages and limitations of each method. Major conclusions drawn are that coated carbides are better and machinability is greatly affected by the microstructure of the material. MQL certainly improved compared to other methods while cryogenic or super cooled cutting fluid application (MQCL) has been found to be better for specific situations. Use of nanofluids for titanium is not very popular among the researchers.

Application of cutting fluids in machining of titanium alloys—a review

Hard coatings used to protect engineering components from external loads and harsh environments should ideally be strong and tough. Here we study the fracture toughness, K
 IC, of Ti1−xAlxN upon annealing by employing micro-fracture experiments on freestanding films. We found that K
 IC increases by about 11% when annealing the samples at 900 °C, because the decomposition of the supersaturated matrix leads to the formation of nanometer-sized domains, precipitation of hexagonal-structured B4 AlN (with their significantly larger specific volume), formation of stacking faults, and nano-twins. In contrast, for TiN, where no decomposition processes and formation of nanometer-sized domains can be initiated by an annealing treatment, the fracture toughness K
 IC remains roughly constant when annealed above the film deposition temperature. As the increase in K
 IC found for Ti1−xAlxN upon annealing is within statistical errors, we carried out complementary cube corner nanoindentation experiments, which clearly show reduced (or even impeded) crack formation for annealed Ti1−xAlxN as compared with their as-deposited counterpart. The ability of Ti1−xAlxN to maintain and even increase the fracture toughness up to high temperatures in combination with the concomitant age hardening effects and excellent oxidation resistance contributes to the success of this type of coatings.

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Fracture toughness and structural evolution in the TiAlN system upon annealing.

Relationship of microstructure, mechanical properties and titanium cutting performance of TiAlN/TiAlSiN composite coated tool

Alloying Ti–Al–N coatings with Ta has proven to enhance their hardness, thermal stability, and oxidation resistance. However, especially for arc-evaporated Ti–Al–Ta–N coatings only limited information on the detailed influence of the elements on various properties is available. Therefore, we have developed arc-evaporated Ti 1−x−y Al x Ta y N coatings with various Al (x = 0.50–0.65) and Ta (y = 0.00–0.15) contents. While the thermal stability of our coatings during annealing in inert He atmosphere increases with increasing Ta content, best results are obtained for specific Ta–Al ratios during oxidation. Single phase cubic Ti 0.32 Al 0.60 Ta 0.08 N yields a mass-gain of only ~ 5% after 5 h at 950 °C in synthetic air, whereas Ti 0.35 Al 0.65 N is completely oxidized after 15 min. This is in part based on the suppressed anatase and direct rutile TiO 2 formation at a defined Ta–Al content. Consequently, the anatase-to-rutile transformation, generally observed for Ti 1−x Al x N, is absent. This reduces the generation of pores and cracks within the oxide scale and especially at the nitride–oxide interface, leading to the formation of a protective rutile and corundum based oxide scale. This is also reflected in the pronounced decrease in activation energy for the protective scale formation from 232 kJ/mol for Ti 0.35 Al 0.65 N down to 14.5 kJ/mol for Ti 0.32 Al 0.60 Ta 0.08 N. Based on our results we can conclude that especially phase transformations within the oxide scale need to be suppressed, as the connected volume changes lead to the formation of cracks and pores.

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Origin of high temperature oxidation resistance of Ti–Al–Ta–N coatings

Thermal expansion of Ti-Al-N and Cr-Al-N coatings

Ti-Si-N nanocomposite thin films usually consist of a few nm sized crystals embedded in an amorphous matrix. Here we study the influence of the Si content in Ti-Si-N films – prepared by reactive magnetron sputtering using Ti targets alloyed with intermetallic TiSi2 (to obtain Si contents of 0, 10, 15, 20, 25 at.% in the targets) – on fracture toughness (KIC) and hardness. Single cantilever bending experiments show that Ti-Si-N exhibits KIC values of up to 3.0 ± 0.2 MPa√m, which are significantly higher than those for TiN with 1.9 ± 0.2 MPa√m. Complementary nanoindentation experiments reveal also a higher hardness for Ti-Si-N with 34 ± 1 GPa as compared to 26 ± 1 GPa for TiN. The film structure is carefully studied by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy.

Fracture toughness of Ti-Si-N thin films

Substoichiometry and tantalum dependent thermal stability of α-structured W-Ta-B thin films

Superlattice films are generally known for their exceptional high hardness compared to their monolithic constituents. Recently, we have shown that CrN/TiN superlattice films exhibit a peak in fracture toughness for a bilayer period of 6.0 nm, similar to the former reported peak in hardness. We propose that a dominating factor for obtaining such favourable material properties is the interface constitution between the individual layers. To proof this notion, we have intentionally modified the interface sharpness by post-deposition vacuum annealing of the samples at different temperatures. This promotes interdiffusion of Ti or Cr into its adjacent layers and gradually changes the interfaces to interphases (because TiN and CrN form a solid solution). In order to obtain reliable KIC fracture toughness values as a function of the annealing temperature, in-situ micromechanical cantilever bending tests on ex-situ vacuum annealed freestanding films were performed. High temperature loads take also place during machining processes like dry cutting or high-speed cutting, and are thus of high practical relevance.

M. Arndt

Papers

Origin of high temperature oxidation resistance of Ti–Al–Ta–N coatings

Thermal expansion of Ti-Al-N and Cr-Al-N coatings

Fracture toughness of Ti-Si-N thin films

Substoichiometry and tantalum dependent thermal stability of α-structured W-Ta-B thin films

Annealing effect on the fracture toughness of CrN/TiN superlattices