24 2007 Summer Bulletin Contributed Original Article H ardness is the ability to resist plastic deformation under an applied penetrating load. Historically, hardness of materials was compared by the ability of one material to scratch another, and the 1-10 Mohs scale was established based on a selection of 10 minerals with vastly different hardness. Modern technology uses indentation by a diamond indenter with various standardized shapes (Vickers, Knoop, Berkowich), and defines hardness by the ratio of the indenter load to the area of the plastically deformed indentation that’s left behind after the load is removed, expressed in units of kg/mm 2 , or in GPa. Hardened steels generally have a hardness less that 10 GPa, while typical hard nitride and carbide coatings (e.g. of TiN and TiC applied to cutting tools) might have a hardness of 20-30 GPa. 1 Materials whose hardness exceeds 40 GPa are termed “superhard.” 1,2 Hardness is one of the main properties of a material which deter- mines its ability to resist wear when subjected to frictional forces. However, hard materials are often brittle, expensive, and difficult to form. One strategy to design components which are tough, inexpensive, and easy to produce, and yet wear resistant, is to choose a material for the body of the component which fulfills the first 3 requirements, and to deposit a coating of a second, hard or superhard, material to minimize wear. Of particular interest for low wear are coating materials with a high ratio of hardness-to-elastic modulus, H/E. 3 Deposition of superhard, nano-composite coatings by plasma processing was the subject of DESHNAF, a 35 month “coordination action” which concluded on 31 December 2006. DESHNAF was funded by the European Union and was executed by a consortium comprised of Euro-Consultants, Tel Aviv University, Ben Gurion University, Technical University of Munich, Max Plank Institute of Plasma Physics, Deutsches Zentrum für Luft- und Raumfart, University of Surrey, and University of Sheffield. DESHNAF conducted 5 workshops and several literature surveys – full documentation may be found on the DESHNAF website: www.DESHNAF.net. The culminating public event of the DESHNAF project was the International Conference on Superhard Coatings, held from 27 Feb to 1 March, 2006 at Kibbutz Ein Gedi in Israel. A total of 55 papers were presented on techniques for depositing, characterizing and applying superhard coatings, as well as solving problems connected with interface purity and thermal stability. Abstracts of the papers presented may be found at the conference website, www.hardcoat.org. A special issue of Surface and Coatings Technology devoted to Superhard Coatings and comprised of selected papers from the conference was published recently. 4 Basically there are two types of superhard coatings: 5 (1) coatings comprised of materials which are intrinsically superhard, such as dia- mond, amorphous diamond-like carbon (DLC), cubic BN, and B 4 N, and (2) engineered materials whose superhardness is a result of ion bom- bardment, alloying, nano-layering or nano-structuring hard materials. This latter type can be divided into several groups: (a) Thin coatings in which the hardness is due to a complex, syn- ergistic effect of ion bombardment during their deposition by plasma enhanced chemical or physical vapor deposition (PECVD or PVD). The ion bombardment engenders nano-grain development, densification, point defects and high compressive stresses, and leads to improved hardness, morphology, structure and nanostructure, and has been studied and reviewed by many researchers 6,7,8,9. (b) Multi-component nitride, carbide, and boride coatings, having SUPERHARD COATINGS Raymond L. Boxman, Tel-Aviv University, Tel-Aviv, Israel, Stan Veprek, Technical University Munich, Garching, Germany, Vladimir Zhitomirsky, Tel-Aviv University, Tel-Aviv, Israel, and Avi Raveh, NRC-Negev, Beer Sheva, Israel a single-phase crystalline structure (such as ternary (Ti,Nb)N 10,11,12 ), which have a hardness greater than either material alone, as a result of solid solution hardening. 13 (c) Nanocomposite coatings which are formed by self-organization due to thermodynamically driven spinodal phase segregation. Examples are nanocomposites consisting of transition metal nitride nano-crystals embedded within an X-ray amorphous covalent nitride. 14,15, (d) Multilayer superlattice structures, produced by sequentially depositing up to 1000 alternating layers of two hard nitrides, carbides, and/or borides, (e.g. TiN/NbN, TiN/TaN, TiN/CrN, or TiC/TiB 2 ) and having a layer thicknesses of few nm, and a total coating thickness of few μm. 16,17,18,19 A successful recipe for fabricating group-c nano-structured super- hard coatings is to co-deposit, on a heated substrate, materials which will form under equilibrium conditions at least two phases – a hard crystalline phase such as TiN which will comprise the largest volume of the coating, and a second material which is immiscible with the first phase. Figure 1 shows a transmission electron micrograph (TEM) of nc-TiN/a-Si 3 N 4 nano-composite, 14 showing a nano-structure typical for spinodally segregated binary systems. 14 Under proper conditions there will be a tendency for the hard material to coalesce as nano-crystallites, separated one from the other by a continuous amorphous phase of the second material, such as nc-Me n N/a-Si 3 N 4 (where Me represents a metal such as Ti, Zr, V, W, Ti x Al 1-z and nc and a indicate nano-crystalline and X-ray amorphous phases, respectively). Examples include nc-TiN/a-BN, nc-MeC/a-C, nc-TiN/a-BN/a-TiB 2 , and nc-TiN/a-Si 3 N 4 /nc-TiSi 2 /a-TiSi 2 . The thickness of the continuous phase is only 1-2 monolayers. This interfacial nano-layer (e.g., Si 3 N 4 ) is more stable than bulk Si 3 N 4 , 20 and it is suffi- cient to stabilize the nano-crystallite surfaces, and to retard cracks from propagating from one nano-crystallite to another. The hardness of binary nano-composites, such as nc-Me n N/a-Si 3 N 4 , generally do not exceed 50-60 GPa, whereas that of 70-80 GPa was achieved for ternary nc-TiN/a-Si 3 N 4 /a-TiSi 2 and about 100 GPa – for quaternary nc-TiN/a-Si 3 N 4 /a-TiSi 2 /nc-TiSi 2 . 14 Figure 2 shows the load dependence of the hardness of 3.5 μm thick coatings of the above quater- nary material, in comparison with a 1 μm thick single-phase nc-diamond coating and a single-crystal bulk diamond. 14 The hardness of the quater- Figure 1. TEM micrograph of a nc-TiN/a-Si 3 N 4 nanocomposite coating. 14