ICMCTF2016 Session B5-2: Hard and Multifunctional Nano-Structured Coatings

In this work, we will present a novel concept for the design of catalytically active nanocomposite coatings that afford extraordinary wear protection during sliding in methane environment. These nanocomposite coatings are based on strategically-selected metallic nitride and catalytically active metals that can be applied on steel substrates using a dual magnetron sputtering system. The tribological properties of coated samples were evaluated using a high temperature/high vacuum tribometer in the presence of methane over a range of loads, speeds, temperatures and gas pressures. Test results show that the nanocomposite coatings were able to reduce friction by 40% in comparison with the steel-steel test pairs without coatings while wear was reduced to some unmeasurable levels in comparison with the severe wear that occurred on uncoated steel surfaces. The surface analysis of the tested surfaces was performed using Raman microscopy in order to elucidate the mechanisms involved in the superior tribological behavior. A layer of carbon based protective tribofilm was found on the contact are of the coated surfaces due to the decomposition of the methane gas on rubbing surfaces.

Dental implants are usually fabricated using titanium (Ti) based materials due to its biocompatibility and good corrosion resistance. However, the low capacity to form a strong chemical bond with living tissue, known as bioactivity, is one of drawbacks of Ti dental implants. This work deals with tantalum (Ta) as a good alternative since Ta is bioactive presenting high wettability and high surface energy which promotes high osseointegration and good corrosion resistance. On the other hand, the higher surface energy of tantalum oxides stimulates the regeneration process in living tissues, and thus, increases the efficiency of the osseointegration.

In this work Ta based coating were deposited by DC magnetron sputtering onto Ti CP substrates in an Ar+O₂ atmosphere. The influence of the oxygen partial pressure on the chemical bonding, structure and morphology of Ta-based film was analyzed.

X-ray diffraction results show that pure Ta coating revealed a body-centered cubic phase (bcc), typical α-Ta phase; when the oxygen was added the crystal structure changed to a mixture of α-Ta (bcc) and β-Ta (tetragonal) phases, due to the distortion of the α-Ta phase by oxygen incorporation. Increasing the oxygen content, coatings became amorphous due to the precipitation of oxides in the metal grain boundaries, inhibiting the grain growth. Elastic properties of the coatings were measured by surface acoustic waves (SAW) and were also simulated by ab-initio calculations to complete the structural information of the system. The results revealed an increase of Young´s modulus from 195 GPa for pure Ta coating until to 240 GPa for coatings with intermediated oxygen amount. Young´s modulus drastically decrease to 155 GPa with amorphization of the stoichiometric tantalum oxide. Combined experimental structural and mechanical results with theoretical simulations shows that high oxygen content leads to a formation of an amorphous stoichiometric oxide phase (Ta₂O₅) with characteristics of a low energy formation.

A more recent development is to include dopants not to influence the mechanical properties, but to enhance the temperature stability of the hydrogenated DLC coating and to influence the wettability of the hydrogenated DLC coating against different oils.

Non hydrogenated DLC coatings (ta-C) have so far been applied mainly as a single layer. The latest trend is also to apply the complete toolbox of dopants, multilayering, stress tailoring on ta-C coatings. A number of recent examples will be shown of doped ta-C coatings including HR TEM pictures, showing under which conditions dopants form carbide islands.

Determining the causes of the stress state emergence in the coating during its formation, the ability to regulate or reduce the stress value while maintaining superhardness is an actual problem of material science of nanostructured coatings.

The influence of the compressive and tensile stresses appearing in the nanostructured nitride coatings during their deposition on WC-6%Co carbide inserts by arc-PVD method based on Ti-N, Ti-Mo-N systems on physical-mechanical properties is studied in this paper. The modifying influence of introduction into the coating metal components (Ni and Cu), not interacting with nitrogen and having limited solubility with the nitride phase, on the possibility of obtaining these materials with low stresses value while maintaining their superhardness is also under research.

The introduction of Ni and Cu increases the hardness from 24 to 49-53 GPa for Ti-N coatings and from 40 to 60-63 GPa for Ti-Mo-N coatings. The coating composition change leads to the reduction of nitride phases crystallite size in both investigated coatings from 120 to 15-18 nm for Ti-N and from 30 to 10-12 nm for Ti-Mo-N, respectively. However, Ti-Cu(Ni)-N coatings are characterized by tensile stresses about 85-120±40 MPa and Ti-Mo-Cu(Ni)-N – by compressive stresses 130-670±65 MPa against much higher value of compressive stresses in Ti-N and Ti-Mo-N coatings (3,7±0,3 and 2,5±0,2 GPa, respectively). The stresses were determined by sin²Ψ method on Ultima 4 diffractometer (Rigaku, Japan) using CoKα radiation and a diffracted-beam graphite monochromator (in asymmetrical geometry).

Simultaneously, the modification of coatings with Ni and Cu leads to the changing of the destruction mechanism of the coating during scratch-test. The critical loads characterising the emergence of the first cracks in the coatings and complete abrasion of the coating to the substrate (L_c1 and L_c3) were determined. They had the value of 9; 17; 18 N and 53; 64; 90 N for Ti-Cu-N, Ti-Ni-N, Ti-Mo-Cu(Ni)-N coatings respectively. L_c1 parameter for Ti-N coatings was equal to 5 N, along with that this coatings destruct according to the adhesion mechanism when the load L_c2 is equal to about 22-25 N.

The results can indicate the prevailing role of coatings nanostructuring compared to the influence of the stresses in realizing their high hardness and crack resistance.