A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> Abstract Nanocrystalline AlMgB 14 containing 0–30 mol% additives are a family of new superhard materials with hardness comparable to that of TiB 2 on the lower end and to that of cubic BN on the higher end. Compared with diamond and cubic BN, AlMgB 14 is an equilibrium material with excellent electrical conductivity, high chemical stability, and lower density. The projected cost of manufacture of the boride is 10% of the cost of diamond and cubic BN. AlMgB 14 materials appear to be congruently melting/evaporating, which would allow them to be processed with techniques such as pulsed laser deposition (PLD). In this work, the feasibility of PLD for synthesizing thin films of baseline AlMgB 14 (0% additive) is demonstrated and compared with TiB 2 . A 248-nm, 23-ns KrF excimer laser was used to prepare baseline boride thin films on cemented carbide (ANSI C-5 and C-2) tool inserts. The films were dark blue, continuous and fairly uniform with few particulates. An impact fracture test showed that adhesion of the films to the substrate was excellent. The deposition rate was 0.08 nm per pulse at an energy density of 7 J/cm 2 . Nanoindentation hardness tests revealed that the films exhibited hardness 60% higher than the carbide substrate. Lathe turning tests with cold-drawn 1045 steel bars indicated that C-5 tools coated with 0.5 μm baseline AlMgB 14 have an average flank wear reduction of 12% compared to uncoated C-5 tools. Further machining tests on C-2 tools showed that the tools coated with baseline boride have much better flank (23% reduction) and nose wear resistance (26% reduction) compared with TiB 2 coated tools. In addition, multilayer composite coating of AlMgB 14 and TiB 2 outperformed single layer boride coating in minimizing the tool wear. This pioneering work sets the stage and serves as a catalyst for rapid and innovative advances in the development of new boride materials for numerous tool and hard coating applications, including bulk cutting tools, hard and erosion-resistant coatings, wear-resistant electrical switch contacts, and conductive thin films for MEMS. <s> BIB001 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> This paper describes the synthesis, structure, and tribological behavior of nanocomposite tungsten disulphide (WS2) solid lubricant films grown by atomic layer deposition. A new catalytic route, incorporating a diethyl zinc catalyst, was established to promote the adsorption and growth of WS2. The films were grown down to 8 nm in thickness by sequential exposures of WF6 and H2S gases in a viscous flow reactor on Si, SiO2, stainless steel, and polycrystalline Si and electroplated Ni microelectromechanical systems structures. Films were studied by cross-sectional transmission electron microscopy (XTEM) with Automated eXpert Spectral Image Analysis (AXSIA) software for X-ray spectral images and X-ray diffraction to determine the coating conformality and crystallinity. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (0 0 2) basal planes. Regardless of orientation to the substrate surface, these basal planes when sheared imparted low friction with a steady-state friction coefficient as low as 0.008 to 50,000 cycles in a dry nitrogen environment. The formation of smooth transfer films during wear provided low interfacial shear stresses during sliding thus achieving low friction and wear. The XTEM combined with AXSIA of the wear tracks identified this mechanism and the effects of vapor phase reaction by-product etching on insulating and native polycrystalline Si and Ni surfaces. <s> BIB002 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> Abstract The binary systems of Mo–N and W–N offer a high potential to enhance the tribological properties of common hard coatings, owing to their ability of forming lubricious oxides (often also referred to as Magneli phases) at elevated temperatures. The aim of this work is to characterize Mo–N and W–N hard coatings prepared by reactive unbalanced magnetron sputtering, and to verify the concept of solid/liquid oxide lubrication. Oxidation of the coatings and possible melting of the oxides are investigated by dynamical thermo-gravimetric analysis and differential scanning calorimetry (DSC) up to 900 °C in air. During heating the samples, the exothermal reactions, detected during the DSC measurements, indicate oxidation in the temperature range between 200 and 750 °C. These exothermic reactions are superimposed by endothermic melting reactions between 650 and 850 °C. The formed oxide phases are investigated by X-ray diffraction after DSC and tribometer testing. To evaluate the effect of formation and melting of the oxides on the coefficient of friction, dry sliding experiments in the temperature range between 25 and 700 °C against alumina and austenitic stainless steel balls are performed using a ball-on-disc tribometer. All coatings investigated show lubricious oxide formation, resulting in a decreasing coefficient of friction at temperatures above 500 °C. For Mo–N, these beneficial properties suffer from the formation of the volatile oxide MoO 3 at temperatures higher than 500 °C. Nevertheless, the lubricious oxide concept within both systems strongly contributes to the reduction of friction, which is important for several industrial applications. We therefore infer that the addition of Mo–N and/or W–N phases to hard coatings can effectively improve their tribological properties, especially in the temperature range between 200 and 700 °C. <s> BIB003 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> Adaptive tribological coatings were recently developed as a new class of smart materials that were designed to adjust their surface chemical composition and structure as a function of changes in the working environment to minimize friction coefficient and wear between contact surfaces. This paper provides an overview of the current research developments in this field, including: (1) Chameleon nanocomposite coatings which are produced by depositing a multi-phase structure whereby some of the phases provide mechanical strength and others are lubricious; (2) Micro- and nano-textured coatings which consist of hard nitride films with highly ordered micropores and nanopores that are subsequently filled with solid lubricants using various techniques such as lithography, reactive ion etching, laser texturing, pulsed air arc treatment, and ceramic beads as placeholders for sputter deposition; and, (3) Carbon and nitride nanotubes that are filled electrochemically with solid lubricants. The frictional and wear properties of the above three classes of newly developed adaptive structures, tested in various controlled environmental conditions (temperature, humidity), will be discussed in detail. <s> BIB004 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> ) thin ﬁlms and powders from 25 to 600 C. We show, using ab initiomolecular dynamics, high-temperature X-ray diﬀraction, high-temperature Raman spectroscopy and diﬀerential scanning calorim-etry in combination with sliding tests, that the layered atomic structure of silver molybdate facilitates sliding, resulting in a lowcoeﬃcient of friction (<0.2) from 300 to 500 C. 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. <s> BIB005 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> Magneli-type vanadium oxides form the homologous series VnO2 n -1 and exhibit a temperature-induced, reversible metal–insulator first order phase transition (MIT). We studied the change of the adhesion force across the transition temperature between the cleavage planes of various vanadium oxide Magneli phases (n = 3 … 7) and spherical titanium atomic force microscope (AFM) tips by systematic force–distance measurements with a variable-temperature AFM under ultrahigh vacuum conditions (UHV). The results show, for all investigated samples, that crossing the transition temperatures leads to a distinct change of the adhesion force. Low adhesion corresponds consistently to the metallic state. Accordingly, the ability to modify the electronic structure of the vanadium Magneli phases while maintaining composition, stoichiometry and crystallographic integrity, allows for relating frictional and electronic material properties at the nano scale. This behavior makes the vanadium Magneli phases interesting candidates for technology, e.g., as intelligent devices or coatings where switching of adhesion or friction is desired. <s> BIB006 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> Machining difficult-to-machine materials such as alloys used in aerospace, nuclear and medical industries are usually accompanied with low productivity, poor surface quality and short tool life. Despite the broad use of the term difficult-to-machine or hard-to-cut materials, the area of these types of materials and their properties are not clear yet. On the other hand, using cutting fluids is a common technique for improving machinability and has been acknowledged since early 20th. However, the environmental and health hazards associated with the use of conventional cutting fluids together with developing governmental regulations have resulted in increasing machining costs. The aim of this paper is to review and identify the materials known as difficult-to-machine and their properties. In addition, different cutting fluids are reviewed and major health and environmental concerns about their usage in material cutting industries are defined. Finally, advances in reducing and/or eliminating the use of conventional cutting fluids are reviewed and discussed. <s> BIB007 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Potential coatings and solid lubricants for high temperature applications <s> The fundamental mechanisms of solid lubrication are reviewed with examples from well-known solid lubricants like the transition metal dichalcogenides and diamond-like carbon families of coatings. Solid lubricants are applied either as surface coatings or as fillers in self-lubricating composites. Tribological (friction and wear) contacts with solid lubricant coatings typically result in transfer of a thin layer of material from the surface of the coating to the counterface, commonly known as a transfer film or tribofilm. The wear surfaces can exhibit different chemistry, microstructure, and crystallographic texture from those of the bulk coating due to surface chemical reactions with the surrounding environment. As a result, solid lubricant coatings that give extremely low friction and long wear life in one environment can fail to do so in a different environment. Most solid lubricants exhibit non-Amontonian friction behavior with friction coefficients decreasing with increasing contact stress. The main mechanism responsible for low friction is typically governed by interfacial sliding between the worn coating and the transfer film. Strategies are discussed for the design of novel coating architectures to adapt to varying environments. <s> BIB008
Traditionally, thermo-chemical surface treatments, i.e. hardening, nitriding, carbonitriding, are used to improve the wear resistance of forming tools. Hard and corrosion-resistant coatings like nitrides and carbides of titanium and chromium (TiN, TiCN, TiAlN and CrN) are frequently used to protect the tools and extend their lifetime. These coatings are suitable to avoid galling problems, to prevent forming tool wear and to lower tool maintenance costs. More recently, solid lubricating coatings such as DLC and metal-MoS 2 composites are being used to reduce friction. Thin films and coatings based on hard and/or solid lubricant materials have been widely used on tribological components to reduce friction, wear and debris generation, and they can be classified into five categories: (1) carbon based materials e.g. graphite, DLCs and nanocrystalline diamond; BIB007 refractory materials e.g. titanium and chromium based nitrides and carbides, oxides (Al 2 O 3 ) and cubic boron nitride (cBN), (3) transition metal dichalcogenide compounds (MoS 2 and WS 2 ); (4) polymer (e.g. PTFE) and (5) soft metals like silver, copper, lead, tin, indium and gold BIB008 . Tin, indium, lead and PTFE have very low melting temperatures which make them inherently unsuitable for high temperature use. On the other hand, silver, copper and gold perform well as solid lubricants at moderately temperature (300-500°C). Molybdenum disulphide is the first choice of lubricant in vacuum or dry air environments, but it oxidises rapidly in moist air and acts as an abrasive at room temperature. Furthermore, it is not lubricious at temperatures beyond 350°C. Compared with MoS 2 , WS 2 exhibits higher thermal stability and provides a 100°C increase in the maximum operating temperature. In a typical WS 2 structure, a layer of tungsten atoms is sandwiched between two layers of hexagonally packed sulphur atoms. The bonding between W and neighbouring S atoms is covalent, while consecutive layers of S are held together by weak van der Waals forces, resulting in inter-lamellar mechanical weakness. Thus, under a shearing force, the basal planes slide over one another by intra-crystalline slip and material is transferred to the counterpart rubbing surface, imparting low CoF of 0.02-0.06 in either dry inert gas or in ultrahigh vacuum. Conversely, when sliding in humid air, dangling or unsaturated bonds on the edge of basal planes react with moisture and oxygen in the environment forming WO 3 , which has a higher CoF (>0.15) BIB002 . For high temperature applications (>500°C), a few oxides (PbO and B 2 O 3 ) or fluorides (CaF 2 and BaF 2 ) are generally used as solid lubricants  . However, the shear strength of these materials is relatively high at low temperature, and the incorporation of these phases in a composite generally leads to high coefficient of friction throughout the temperature range used. More recently, oxides of transition metals like vanadium BIB006 , tungsten BIB003 and molybdenum  have attracted much research interests. Double oxides incorporated with noble metals such as silver are particularly interesting solid lubricants BIB005 . Carbon-based coatings, while providing notable improvements in operating efficiency, are prone to oxidation at operating temperatures exceeding 400°C. Graphite is an excellent lubricant in dry and humid air but it loses effectiveness in vacuum as it needs condensable vapours (e.g. water) to terminate dangling bonds that are otherwise adhesive in nature BIB005 . Although diamond coatings, offer several benefits with regard to surface finish, built-up edge, cutting forces and tool life, they do not adhere well to tool substrates, and they can undergo undesirable chemical reactions upon contact with ferrous alloys. Furthermore, their coefficient of thermal expansion is much lower than that of most metals, thus inducing stresses between coating and substrate as the temperature changes. Ceramic coatings provide thermal stability and higher resistance to abrasive wear but suffer from brittleness and poor bonding to the tool substrate. Cubic BN, which is relatively inert with respect to ferrous metals, is difficult to deposit by PVD (Physical Vapor Deposition) at thicknesses exceeding 300 nm because of residual stresses that build up in the films  . Nanocrystalline AlMgB 14 is highly resistive to wear and has a low coefficient of friction, reaching a record value of 0.02 against AISI 52100 steel counter-face in lubricated sliding of AlMgB 14 -TiB 2 composites BIB001 . In summary, it is interesting to note that materials that provide lubrication at ambient temperature oxidise at elevated temperatures, while high temperature solid lubricants typically exhibit high friction at room temperature. The drawback of using solid lubricants as monolithic layers is that they are soft, and hence, don't provide enough wear resistance. In addition, no single phase solid lubricant displays a low coefficient of friction in a broad range of environment conditions BIB004 . Therefore, there has been a continuous driving force to develop better coatings or surface modification solutions to reduce both wear rate and coefficient of friction at high temperature and/or over a wide range of working conditions. Different approaches have been reported extensively, and these coatings can be categorised as follows: d Nanocomposite coatings based on transition metal nitrides, oxides and the blending of solid lubricants such as noble metal; d Multi-layer structured coatings and duplex systems with different combinations of hard and/or lubricating coatings. In this paper, the authors review the recent developments on modified surfaces and coatings with application in a wide temperature range, from room temperature to 1000°C and beyond. The working mechanisms and the effect of different coating designs are analysed, in order to find a path to improve the tribological properties especially the reduction of friction during service over a broad range of environmental conditions.