Thin Films for Extreme Conditions by HiPIMS

The need for materials with special functionalities is particularly felt in forefront domains. In the field of refractory hard compounds, the development of new materials is restricted by various metallurgical criticisms, which force the use of sophisticated and expensive production methods. The film deposition from vapor phase enables the fabrication of new materials, avoiding problems connected with liquid phase formation and/or machining. Thin coatings are designed and applied to modify the surface properties of bulk materials, thus protecting them from mechanical, physical or chemical damages. In this study, an investigation on protective films for harsh conditions, produced in CNR-IENI laboratories is presented. Thin films were deposited by High Power Impulse Magnetron Sputtering (HiPIMS) which is an emerging Physical Vapor Deposition (PVD) technology, combining advantages of magnetron sputtering with several forms of energetic deposition of films such as Ion Plating and Cathodic Arc Deposition. Indeed, the HiPIMS process produces an ultra-dense plasma, which leads to a very high ionization degree of the sputtered material with respect to conventional magnetron sputtering techniques. The growth of ultra-dense coatings is promoted, with enhanced adhesion, in particular for complex-shaped surfaces, improved toughness, reduced columnar structure, residual stress and deposition temperature. Moreover, HiPIMS includes the possibility to deposit alloy compounds, multilayer compositions and structures, and the ability to obtain functionally graded coatings. Numerous applications were identified for the coatings with such improved properties: high temperature stage of gas turbine engines, internal combustion engines, pipeline for extreme corrosive environments, first wall and divertor coverage in thermonuclear fusion devices. Several coatings (TiAlN, MoN, Ta, TaN and W) were deposited on innovative and traditional structural materials, as ?-TiAl, martensitic stainless steel (T91), AISI 304, AlSi alloy and graphite, Depending on coating specific application, appropriate characterization techniques were selected to investigate films properties. Morphologies, compositions and structures of the films were analyzed by FIB, SEM, EDS, XRD and optical microscopy. The mechanical and tribological characterization included hardness measurements by nano-indentation, adhesion evaluation by scratch tests, wear and friction tests under different loading conditions, tribocorrosion tests. Experimental results presented in this work show that while optimizing deposition parameters like power, pressure and sputtering gas composition, pulse length and frequencies, bias voltage, temperature and system geometries it is possible to customize final properties of coatings. Indeed in this context it was possible to drive the desired properties which are peculiar for each selected application: for example density was varied by acting on power, pressure, and substrate polarization; adhesion was improved by pretreating the substrate prior to deposition and reducing the mismatch of thermal expansion coefficient through the insertion of an interlayer; wear resistance was increased by modifying the H/E ratio.