ABSTRACT
Traditional materials widely used in industry have early met their limits as requirements especially of load-bearing and protective applications became extremely high. In such applications, improved surface properties of the base material enhancing its protecting capability are required. Ever increasing demands on materials performance are moreover coupled with the needs for weight and cost reduction and reduction of energy consumption. Over the years, plasma processing technologies have become to be widely used in the field of surface engineering due to a high potential to satisfy the actual requirements. Fast development in plasma-assisted diffusion treatments, hard and protective coatings, and composite surface layers has covered the most needs put on to new materials with enhanced properties. Above all, thin hard protective films have been successfully used for protection of materials since the 1970s with the aim to prolong the lifetime of working tools. The coating materials developed for wear protection can be classified according to the chemical bonding character as metallic, covalent, or ionic hard materials [1]. Especially, hard ceramic films have attracted considerable interest due to their potential applications in cutting and forming tools, engine parts, bearings, seals, bearing balls, gears, or drills. Protective coatings must, however, meet a set of various
demands depending on the specific applications. In spite of a variety of materials with a high hardness, the choice of possible candidates which would meet all requirements is extremely limited. As material specifications have become stricter, demands for high-performance structural materials have started to call for novel advanced hard and superhard coatings of a new generation having simultaneously a high hardness, stiffness, and a sufficient stability against environmental attack. Plasma processing has been shown to be a powerful tool in preparation of thin films with an ample range of properties that can be tailored to optimize the desired functionality. Recently, a special attention is focused on possibilities to meaningfully improve film properties by optimizing deposition conditions or by synthesizing a new generation of multiphase materials by addition of small amount of alloying elements in order to completely change the film structure and thus mechanical and other functional properties. Since there still exist several drawbacks, which limit the practical application of traditional single phase films of a first generation, such as high residual stresses and coefficient of friction, low thermal stability and oxidation resistance, these new ways might overcome these problems so that the films should be able to perform under extreme conditions. Under various available physical vapor deposition (PVD) processes, sputtering is a deposition technique with the advantage of producing unique fine grained and metastable structures, including amorphous and high-temperature phases. By optimizing the energy and flux of impinging sputtered atoms and bombarding ions, it is possible to effectively alter the crystallographic orientation and size of grains in a large range from hundreds down to a few nanometers. The ever-growing need for superior materials to withstand severe operating conditions has driven the search for new nanocrystalline materials and thermodynamically metastable thin films exhibiting exceptional mechanical properties and enhanced strength and toughness characteristics. One of the design concepts is based on the formation of single phase films with ultra-fine grained microstructures, which consist of grains in the nanometer size level. Another approach utilizes an addition of alloying elements into the films in order to form completely new structures. Films composed of mixtures of various phases, metastable solid solutions, or composite structures in the nanometer range are very promising for variety of
applications due to their new unique functional properties [2-6]. The aim of this chapter is to present a progress in development of thin protective films with superior properties over traditional materials and to show an impact of surface engineering technologies on the performance and lifetime of coated tools. 1.1 Nanocrystalline Materials
One of the very basic findings of the physics of solids is the insight that most properties of solids depend on their microstructure. In particular the arrangement of the atoms and the size of the building blocks of which the solid is consisted in one-, two-, or three-dimensions are the key factors determining functional properties of materials. A huge effort is therefore made by many researchers to prepare materials with exceptional properties through new unique structures. Nowadays, coating technologies attract a great attention since nonequilibrium processes used for the film preparation allow formation of such structures. Moreover, simply by changing the deposition conditions, a wide variety of film microstructures may be generated providing significantly enhanced materials properties. Nanocrystalline and nanocomposite materials are one of those replacing traditional coarse-grained materials in many applications. 1.1.1 CharacteristicsIn general, materials properties are strongly related to their structure and have been shown to vary considerably with the size of grains of which they are composed. A novel way of improving material properties has been found in reduction of grain sizes to the nanometer scale that significantly distinguishes these materials from conventional coarse-grained polycrystalline materials which have grains of several micrometers in size. Materials with a nanometer-scale microstructure where the atomic arrangement, crystallographic orientation, chemical composition, and the size of grains vary on a length scale of a few nanometers throughout the bulk are referred to as nanocrystalline materials. These materials are single or multiphase materials with the grain size typically below 100 nm in at least one-dimension and may be composed of crystalline, quasicrystalline, or
amorphous phases. Unique properties of nanocrystalline materials are given mainly by a combination of rare compositions and novel microstructures. These presumptions ensure an exceptional potential of the nanocrystalline materials in many technological applications. However, a nanometer-sized microstructure is not the only feature responsible for improved mechanical, electrical, or magnetic properties of these materials. Another characteristic feature of nanometer-scale materials is the presence of incoherent or coherent interfaces formed between individual crystallites. The atomic structure, thickness, and chemical composition of the boundary regions are equally crucial for the material properties. It means that materials composed of grains with comparable size and composition may markedly differ from one another due to the various interfacial structures [7]. Figure 1.1 shows a schematic depiction of a typical nanocrystalline material composed of one kind of atoms forming equiaxed grains with long range order and grain boundaries exhibiting a broad spectrum of interatomic spacings.
