Characteristics of Hard Titanium Nitride and Carbon Based Coatings Deposited by Plasma Sources


Student thesis: Doctoral Thesis

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Award date19 Feb 2019


Plasma source deposition and surface modification techniques are widely used to manipulate the surface properties of hard coatings. These techniques rely upon the generation of highly activated gaseous and metallic species by the independent plasma sources in order to alter the surface characteristics. These plasma sources can be used in order to generate the gaseous and metallic plasmas. In gaseous plasma sources, the gaseous plasma is generated and delivered by ion sources, radio frequency (RF) sources, electron cyclotron resonance (ECR) sources, etc., while metallic plasma is generated by cathode vacuum arc sources (CVAs), sputtering sources, metal evaporated vacuum arc source (MEEVA), etc. Metallic plasma sources are quite familiar for deposition of hard coatings like transition metal nitrides and carbides, and the same can be used for carbon-based coatings. A Combination of two plasma sources can modify the structure and properties of these coatings. In this work, we intend to combine the gaseous plasma source, i.e., anode layer ion source (ALIS) and metallic plasma source, i.e., magnetron sputtering to observe the modification in structure and mechanical properties of hard coatings. The system used in this study is titanium nitride (TiN). Furthermore, by using the oxygen plasma by ALIS, the properties of carbon-based coating were also evaluated. Moreover, a cathodic vacuum arc plasma source (CVAs) was used to produce the thick monolayer of ultra-hard carbon coatings for high-speed steel and nickel-titanium shape memory alloy. The first chapter provides a basic introduction of the science and technology of plasma with a focus on sputtered plasma sources, anode layer ion source, and cathode vacuum arc plasma sources. Research related to TiN and carbon-based coatings deposited by plasma or plasma assisted techniques is also overviewed along with their useful mechanical properties.

The next three chapters include the work related to ion-beam-assisted magnetron sputtering (IBA-MS) of TiN. TiN thin films of 250-300 nm thickness were deposited on silicon and stainless steel (SS) substrate by using a hybrid technique where a middle-frequency magnetron sputtering plasma source with the assistance of anode layer ion source (ALIS) were employed. The characteristics of ALIS coupled with a magnetron sputtering source were studied. By coupling and varying magnetron sputtering discharge It, and ALIS discharge current Is, the discharge voltage of two sources were reduced, whereas, the incoming energy of the ions Ei can be controlled to as low as 80 eV and ion flux density Ji as high as 3.5 × 1016−2.s−1. Compositional changes were studied by EDS and electronic states by XPS. Structural and morphological studies were carried out by x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). While mechanical, electrical, and electrochemical properties were also studied. The combination of two plasma sources resulted in the preparation of TiNx films with reduced oxygen concentration up to 95 % as compared to magnetron sputtering alone. Nitrogen contents in the films were increased to 30 %. It was revealed that Ji controls the gaseous concentration in films and is independent of Ei. The preferred orientation evolution seems to depend on Ei and ion to metal particles ratio Ji/JMe. An energy range of 90-100 eV and Ji/JMe of 25-30 led to the formation of a randomly oriented film due to recrystallization by enhanced thermal spikes, thus, altering the original columnar morphology inherited by magnetron sputtering to a dense non-columnar morphology. Further increase in Ei and Ji/JMe resulted in [111] orientation. The mechanism of variations in chemical, structural, and morphological properties has been discussed in relation to Ei, Ji, and Ji/JMe in the second and third chapters.

Chapter 4 covers the relationship between the microstructure, mechanical, and electrochemical properties of the deposited films in detail. A change in microstructure from a columnar to a dense one with a net increase in compressive residual stresses was observed. The stress increases initially and then decreases slightly with the (111) orientation, and the hardness and fracture toughness depend on the residual stresses in the films. The hardness was increased from 14.5 GPa to 27.0 GPa while fracture toughness was improved to 70%. Electrochemical studies revealed reduced corrosion current densities and increased impedances by order of magnitude. The hybrid technique which can be scaled up readily to meet industrial demand enables control of the film composition, microstructure, and properties are quite promising.

Amorphous carbon (a-C) or diamond-like carbon (DLC) coatings are studied in the fifth chapter. An ALIS was used to deposit the hydrogenated amorphous carbon (a:C-H) coating on high-speed steel (HSS). Oxygen gas was doped to modify the characteristics of a-C: H films. Oxygen gas was fed in the ALIS to create the ionised plasma of O2 gas whereas O2 to C2H2 ratio was varied to form O doped a-C: H with different percentage of O2. The coating was characterised for their deposition rate, oxygen incorporation, structure, morphology, roughness, mechanical, and tribological characteristics. It was revealed that with the addition of oxygen in plasma, the deposition rate tends to decrease and no net deposition was achieved with an equal volumetric ratio of O2 to C2H2. The hardness of the a-C: H coating tends to increase up to 55 % and then begins to decrease with the further addition of oxygen, however, it remains higher than native film. Wear rate also decreases from the 5.0 × 10−7 (mm3. (m-N)−1) to 2.0 × 10−7 (mm3. (m-N)−1) and, again, increases. The results indicated that slight oxygen incorporation up to 6 at % seems to be effective to increase the mechanical properties. Bonding within the carbon atoms and carbon to oxygen atoms were studied by the XPS and Raman spectroscopy were employed for their structural disorder, I(D)/I(G) ratio, and FWHM (G). Characteristics of oxygenated a-C: H films were correlated with XPS and Raman analysis.

In Chapter six, a preliminary study to deposit a thick monolayer of tetrahedral carbon (ta-C) is demonstrated by using the filtered cathode vacuum arc plasma source (FCVAs). To deposit a thick ta-C layer on hard, i.e., high-speed steel (HSS), and soft, i.e., Ni-Ti shape memory alloy substrate, a temperature-controlled methodology was adopted. It was revealed that to obtain high sp3 content in the film with excellent adhesion, control of critical transition temperature is necessary. Moreover, a good quality well-adhered ta-C can be achieved at lower arc source discharge currents. ta-C coatings with a thickness higher than 4.5 µm with I(D)/I(G) ratio less than 0.2 and FWHM (G) peak greater than 230 cm−1 was achieved. Hardness was found to be > 50 GPa with a wear rate of 0.9 × 10−7 mm3.(m-N)−1. The same methodology was adopted to coat cutting tools of HSS with high-quality ta-C.