**Preface**

Friction and wear occur in all mechanical systems where moving components are in contact with each other. The tribological contact causes the degradation of their performance and reliability due to increased energy consumption and the progressive loss of material. Interactions between solid surfaces depend not only on the properties of the materials in contact but also on the topography of their surfaces. In the majority of cases, the failures of moving components in mechanical systems are surface-initiated. Therefore, most engineering components are enhanced by surface treatments, which improve their tribological properties. One of the most common and effective approaches is the application of coating deposited by physical vapor deposition (PVD) techniques. An important factor influencing the tribological properties of PVD coatings is their surface topography. Different topographical imperfections on the coating surface can degrade its quality; in certain cases, they can even cause a catastrophic failure. The knowledge of the surface topography of the PVD coating is therefore crucial. However, in the literature, the relationship between the surface topography and the functional properties of the coating has only been partially investigated.

This book provides a comprehensive overview of the surface topography of PVD coatings and their role in different tribological contacts. We demonstrate that the coating topography is strongly dependent both on the substrate topography and on the topography induced by the coating deposition process. The former depends on the substrate preparation steps before the deposition of the coating (mechanical pretreatment, ion etching). During the mechanical pretreatment of a substrate, it is impossible to achieve a perfectly smooth surface because each manufacturing technique (e.g. turning, milling, grinding, polishing, electro discharge machining) leaves its own »fingerprint« on the surface. The resulting surface topography is determined by surface roughness (nano- and microroughness) and waviness (macroroughness). The surface roughness is characterized by a series of asperities (local maxima) and valleys (local minima) of a characteristic shape, amplitude, and spacing. According to ISO 14460-1, the real surface of a workpiece has been defined as: »*A set of features that physically exist and separate the entire workpiece from the surrounding medium*«.

All topographical irregularities on the substrate surface form during mechanical pretreatment and ion etching just before the deposition of coating. These irregularities are then transferred onto the coating surface. However, after coating deposition, additional topographic changes occur which are related to the intrinsic micro- and nanomorphology of the coating itself, and especially due to the formation of growth defects. Although the role of growth defects in many thin film applications is crucial, a comprehensive review of this area in the literature is still missing. The present book is intended to fill this void and provide information on the role of defects, especially in the tribological application of hard coatings.

This Special Issue contains ten papers covering a broad range of topics representing the state of knowledge on the recent developments in the area of surface topography of PVD hard coatings. A brief summary of the papers in each category is provided below.

The first review paper »*Review of growth defects in thin films prepared by PVD techniques*« summarizes our studies of the growth defects in PVD coatings. A detailed historical overview is followed by a description of the types and the evolution of growth defects. All topographical irregularities and foreign particles generated during different steps of substrate pretreatment and different coating deposition processes are described because they are seeds for the formation of growth defects. An overview is given on the research related to the influence of growth defects on the functional properties of thin films and coatings.

The second paper »*Surface topography of PVD hard coatings*« presents a study concerning the

surface topography of various PVD hard coatings prepared in various types of industrial deposition systems, which differ significantly regarding both the ion etching method and deposition. The authors show how the coating topography depends on the topography of the substrate surface, intrinsic coating microtopography, and growth defects, formed during the deposition process.

In the third paper with the title »*Microstructure and Surface Topography Study of Nanolayered TiAlN/CrN Hard Coating*«, we focused on the microstructure, surface topography, layer periodicity, interlayer roughness, and formation of growth defects in the nanolayer TiAlN/CrN hard coating. These properties were analyzed with dependence on the substrate rotation mode, the type of substrate material, and the method of ion etching. The multilayer coating was chosen because the growth defect formation and other coating surface irregularities are easier to observe in such a structure. In addition, the nl-TiAlN/CrN coating possesses enhanced mechanical and tribological properties as compared to TiAlN and CrN monolayer coatings. The stresses formed at the interfaces due to different lattice constants significantly contribute to the higher hardness of the nanolayer structure coating in comparison with the corresponding monolayers.

In the fourth paper »*Contamination of substrate-coating interface caused by ion etching*«, we described the problem of target surface contamination in an industrial magnetron sputtering deposition system with the residual products from the etching process. Such contamination can be prevented by a movable shutter located close to the targets, but in order to achieve reasonable economics of the deposition process, complicated installations (including shielding and shuttering) are usually avoided. In the initial stage of deposition, this material is re-deposited back on the substrate and causes the formation of a contamination layer at the substrate-coating interface. We also found that many seed particles that cause the formation of nodules are covered with a similar contamination layer. We believe that these weakly bonded particles were formed on the target surface outside of the racetrack, and that they were transferred to the substrate surface immediately after starting the deposition process by the self-repulsion effect.

In the fifth paper »*Comparative study of tribological behavior of TiN hard coatings deposited by various PVD deposition techniques*«, the authors correlate the tribological behavior of TiN hard coatings, prepared using different deposition methods, to their surface topography, microstructure, and mechanical properties. The surface topography of PVD hard coatings is an important factor influencing their tribological performance under sliding contact conditions because the real contact area strongly depends on the roughness of the interacting surfaces. In this paper, it is analyzed how tribological properties depend on roughness and the surrounding atmosphere (ambient air, nitrogen, oxygen).

The next paper »*Influence of growth defects on the oxidation resistance of sputter-deposited TiAlN hard coatings*« discusses the influence of growth defects on the oxidation resistance of sputter-deposited TiAlN coatings. The formation of an oxide scale at temperatures of 800 °C and 850 °C and different oxidation periods was investigated. The authors found that intensive local oxidation takes place at sites of pinholes and pores that are formed at the rim of the nodular defects. During oxidation, they provide direct paths between the coating surface and the substrate for the transport of oxygen inwards and substrate elements towards the surface.

The surface topography and surface roughness of the orthopedic and dental implants have a decisive influence on their integration and biological response in soft and hard tissues. The paper »*Laser-assisted surface texturing of Ti/Zr multilayers for mesenchymal stem cell response*« deals with the surface functionalization of the Ti-base alloy in terms of improving the osteoblast cell response. The authors performed the irradiation of the Ti/Zr multilayer structure via femtosecond laser irradiation in order to form the laser-induced periodic surface structure and intermixing between the titanium and zirconium layers. They found that cell adhesion and growth improve on these modified surfaces.

In the paper »*Metallurgical soldering of duplex CrN coating in contact with aluminum alloy*«, the performance of CrN duplex coatings with different roughness was evaluated using an ejection test performed with conventional (CS) and delayed (DS) casting solidification. They observed that the roughness strongly affected the ejection force in the CS experiment, where the ejection force increased with the decreased roughness. On the other hand, an almost equal ejection force was measured in the DS experiments for samples of different roughness. These observations along with the above provided discussion suggest that in DS experiments, the effects of surface chemistry are more dominant than the effects of surface topography, i.e., metallurgical mechanisms are more dominant than mechanical mechanisms. The decrease in the ejection force, observed in DS tests, is attributed to the formation of a thick chromium oxide layer on the CrN coating which reduced soldering and sliding friction against a thick aluminium oxide casting scale.

The paper »*Properties of tool steels and their importance when used in a coated system*« deals with correlations between different tool steel properties, including fracture toughness, hardness, compressive and bending strength, wear resistance and surface quality and how these substrate properties influence the coating performance. In the case of coated applications, steel substrates must provide sufficient load-carrying capacity and support for the coating. Surface roughness and topography have a major influence on galling resistance during the forming operation, with smoother surfaces and a plateau-like topography providing better results. In the case of typical hard ceramic coatings, the post-polishing of the coated surface and use of a smoothened substrate give about two times better galling resistance.

The last paper »*Distribution of the deposition rates in an industrial-size PECVD reactor using HMDSO precursor*« deals with the problem of non-uniform deposition rates in commercial plasma reactors for the preparation of thin films from organic precursors using the PECVD technique. The plasma was maintained via asymmetric capacitively coupled radiofrequency (RF) discharge using a generator with a frequency of 40 kHz and an adjustable power of up to 8 kW. They found that the deposition rates of hexamethyldisiloxane far from the powered electrodes dropped by more than an order of magnitude for a fully loaded chamber.

The editors are grateful to all authors that contributed to this book. We also acknowledge the Multidisciplinary Digital Publishing Institute (MDPI) in Basel, and especially Ms. Flora Ao for her assistance with the publication of this Special Issue of the journal *Coatings* in book form.

> **Peter Panjan and Aljaž Drnovšek** *Editors*
