A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Other blending <s> Abstract TiN/NbN superlattice hard coatings were deposited using unbalanced magnetron sputtering technology. A gas manifold was used to generate a desired N2 partial pressure for each target, so as to produce stoichiometric TiN and NbN simultaneously. During film deposition, an optical emission monitor (OEM) was used to monitor and control the poisoning status of the targets. Under optimized deposition conditions, the TiN/NbN superlattice coatings deposited on high-speed steel substrates had a well-defined superlattice structure and sharp interfaces as well as stoichiometric composition. These coatings show high toughness, high elasticity and very high hardness, giving rise to significant improvements in tribological performance. The nano-indentation hardness of 48 GPa was achieved in the coatings with an optimized superlattice wavelength of about 7 nm. This super-high hardness coupled with high elasticity implies that TiN/NbN superlattice coatings could be ideal for tools application, especially in the stamping/cutting tooling industry. <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> Other blending <s> Abstract Tungsten disulfide (WS 2 )–zinc oxide (ZnO) composite is a candidate material that exhibits adaptive lubricant behavior. Adaptive lubricants undergo chemical changes with changing environment to provide lubrication in extreme environments. In the current study, the tribological characteristics of WS 2 –nanocrystalline ZnO films have been investigated from ambient to 500°C. The composite films were powder burnished on inconel substrates. Using a ball-on-flat tribometer, friction tests were conducted on WS 2 –ZnO nanocomposite films containing 50% by weight of the oxide. For comparison, measurements were made on pure WS 2 films burnished under identical conditions. The room temperature tests were performed in dry nitrogen, while the elevated temperature tests were run in air. Wear scars and transfer films on the counterface were analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. Results showed that the nanocrystalline ZnO additive resulted in significant reduction in friction coefficient of WS 2 both at 300°C as well as at room temperature. Third-body analyses from 300°C tests revealed that tribooxidation is less prevalent in nanocomposite films. At 500°C, the friction coefficient of pure WS 2 films increased to 0.50 within the first 2000 cycles, whereas the nanocomposite films lasted the entire duration of 10,000 cycles with steady state friction coefficient of 0.22. Raman spectroscopy identified the formation of zinc tungstate (ZnWO 4 ) during the 500°C tribotests, confirming the adaptive lubricant concept in WS 2 –ZnO nanocomposites. <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> Other blending <s> The chameleon's ability to change skin color depending on environment to increase its chances of surviving served as an inspiration in the development of self-adaptive supertough wear-resistant coatings. Surface chemistry, structure and mechanical properties of these thin (0.5 μm) coatings reversibly change with applied load and environment, providing the best wear protection. Coating designs developed in-house are reviewed together with a critical analysis of design reports in the literature. ‘Chameleon’ coatings were prepared using novel nanocomposite structures, consisting of crystalline carbides, diamond-like carbon (DLC), and transition metal dichalcogenides. Various mechanisms were activated to achieve surface self-adaptation and supertough characteristics. They included: transition of mechanical response from hard and rigid to quasi plastic by grain boundary sliding at loads above the elastic limit; friction induced sp3→sp2 phase transition of the DLC phase; re-crystallization and reorientation of the dichalcogenide phase; change of surface chemistry and structure from amorphous carbon in humid air to hexagonal dichalcogenide in dry nitrogen and vacuum; and sealing the dichalcogenide phase to prevent oxidation. These mechanisms were demonstrated using WC/DLC, TiC/DLC, and WC/DLC/WS2 coatings. The hardness of WC/DLC and TiC/DLC composites was between 27–32 GPa and scratch toughness was 4–5 fold above that of nanocrystalline carbides. The WC/DLC/WS2 composites survived millions of sliding cycles in vacuum and air under 500–1000 MPa loading, and exhibited excellent friction recovery in humid↔dry environmental cycling. Their friction coefficients were about 0.1 in humid air, 0.03 in vacuum, and as low as 0.007 in dry nitrogen. The proposed ‘chameleon’ concept can dramatically increase wear-resistant coating applicability, durability, and reliability. <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> Other blending <s> Nanocomposite tribological coatings were designed to respond to changing environmental conditions by self-adjustment of their surface properties to maintain good tribological performance in any environment. These smart coatings have been dubbed “chameleon” because, analogous to a chameleon changing its skin color to avoid predators, the coating changes its “skin” chemistry and structure to avoid wear. The concept was originally developed using WC, diamondlike carbon, and WS2 material combination for adaptation to a humid/dry environment cycling. In order to address temperature variation, nanocomposite coatings made of yttria-stabilized zirconia (YSZ) in a gold matrix were developed with encapsulated nanosized reservoirs of MoS2 and diamondlike carbon (DLC). Coatings were produced using a combination of laser ablation and magnetron sputtering. They were characterized by x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, x-ray energy dispersive spectroscopy, and micro-Ram... <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> Other blending <s> Abstract Nanocomposite materials demonstrating multiple temperature-adaptive mechanisms including diffusion, oxidation and/or catalysis mechanisms to yield low friction coefficients of 100 nm R a ) failed after relatively few sliding cycles. The utility and application of adaptive coatings materials providing lubrication over multiple thermal cycles is discussed. <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> Other blending <s> Abstract The tribological conditions of hot forming processes are much different from those at room temperature. Thus classical wear-resistant coating like CrN, CrAlN, TiAlN in most cases are not sufficient for hot forming tools in industrial applications. Additionally the use of lubricants is limited, thermal shock conditions and the increase of sticking work piece material are leading to severe wear. This paper presents examples for ternary boron based Ti–B–N gradient coatings in specific multilayer designs obtained through plasma enhanced chemical vapor deposition. After a thorough characterization of the chemical composition by Auger electron spectroscopy, the microstructure by X-ray diffraction and the mechanical properties by microindentation and small-angle cross section nanoindentation, the coatings were applied onto hot forming tools made from DIN 1.2367 tool steel. The coated tools were tested in hot forging of AISI 1043 raw parts in an automatic press and have been compared in terms of adhesion and wear resistance. In this study an optimum design has been found which significantly reduces sticking of work piece material and wear. This allows an efficient production without interruptions for a reworking of the tools and enables to increase the process reliability paired with a longer tool life. <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> Other blending <s> Abstract The objective of this study is to investigate the tribological and thermal properties of the recently developed Ag–MoS 2 nanoparticle-based multi-component lubricating system. To obtain greater tribofilm durability and to enhance the tribological and thermal properties of MoS 2 , a chemo-mechanical processing method has been developed for the synthesis of composite silver nanoparticles (Ag NPs) incorporated into MoS 2 nanoparticles (nMoS 2 ). In order to characterize and investigate thermal and tribological behavior of the Ag–MoS 2 hybrid system, four different Ag compositions were studied, ranging from 2 to 25 wt. % in MoS 2 . Different characterization techniques have been used to examine structural properties, silver content, particle size, and particle size distribution. The characterization results showed that the Ag NPs were successfully embedded into nMoS 2 with particle sizes lower than 300 nm for 90% of agglomerated particles and 100 nm for most of de-agglomerated particles with no phase change of the system. The increase of internal strain of nMoS 2 from the order of 0.002 to 0.02 was confirmed by XRD after the successful embedding of Ag NPs into nMoS 2 . In addition, the chemical and thermal analysis showed that silver molybdate was formed at temperatures above 450 °C, which demonstrates that this multicomponent system should be very effective for high temperature applications. The tribological tests revealed that nMoS 2 – 2 wt. % Ag results in 15–20% reduction in friction under boundary lubricated sliding conditions and 30–37% reduction in wear. Therefore, the addition of Ag nanoparticles at an optimum concentration can significantly enhance the thermal and tribological performance of the developed Ag–MoS 2 hybrid system. <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> Other blending <s> article i nfo Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbal- anced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000 °C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T>700 °C. Post-wear testing structural and chemical char- acterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750 °C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates. <s> BIB008 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Other blending <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> BIB009 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Other blending <s> Abstract The lifetime of forming tools can be increased by surface functionalization using novel, nanostructured coatings. A special centre of focus of this is the tribological requirements on the coating. Within the framework of the BMBF project, the friction and wear behaviours of 3 newly developed nanostructured composite tool coatings: VAIN, VAlN/VCAlN and VAlN/VCAlN with a carbon-enriched top layer, were examined and compared to each other. Because a conventional wear test would have required a large number of tests (⪢10,000) with a concomitantly high consumption of sheet metal, the examinations were run in a model trial at an increased surface pressure of 150 MPa, to accelerate the wear of the tool. Wear analysis was carried out by considering the changes in topography and hardness of the tool surface. <s> BIB010
It is clear that the CoF can be reduced at medium and high temperatures by doping nanocomposite coatings with silver. However, the CoF is persistantly high at low temperature due to the lack of solid lubricants acting in this tempearature range. Therefore, additional lubricious phases, such as graphite/DLC or MoS 2 , have been introduced in the nanocomposite to generate dual-phase lubricating materials which provide adaptive lubrication over an extended temperature range. An AgMoS 2 nanoparticle-based multi-component lubricating system was developed to improve the durability and to enhance the tribological and thermal properties of MoS 2 BIB007 . The silver and MoS 2 in the coating provided adequate lubrication at lowtemperature. A silver molybdate phase formed at moderate temperature (350-600°C) and it decomposed into MoO 3 at higher temperature, providing lubrication between 600°C and 700°C. In a separate study, nano-composite films consisting of tungsten disulfide (WS 2 ) with 50 wt.% zinc oxide (ZnO) were powder burnished on Inconel substrates BIB002 . The nanocrystalline ZnO additive resulted in a significant reduction in the CoF of WS 2 against stainless steel counterface up to 500°C from 0.50 to 0.22, due to the formation of zinc tungstate (ZnWO 4 ). Other solid lubricant composites based on oxides and dichalcogenides (PbO/MoS 2 , ZnO/MoS 2 ) which could operate in a broad temperature range, were also explored in earlier research   . In the case of molybdenum disulfide (MoS 2 ) and lead oxide (PbO) adaptive lubricating system, MoS 2 provides lubrication at ambient and intermediate temperatures, and a lead molybdate compound (PbMoO 4 ) is formed as the temperature increases, providing continuous lubrication  . The addition of MoS 2 to the NbN/Ag coating system, further reduced the coefficient of friction of the coating up to 750°C, as seen in Figure 3 BIB008  . 4. Multi-layerd structure and duplex system 4.1. Multi-layered structure The use of a large number of alternating thin layers, such as TiN/NbN, forming a multilayer/superlattice coating can reduce the propagation of cracks, and yield finer grain sizes, leading to higher hardness and improved tribological properties BIB001 . The modern trend is to use multilayer coatings tailored to combat different types of wear and provide a synergistic tribological effect. In a typical TiC/Al 2 O 3 /TiN multilayer coating produced by CVD, TiC resists abrasive flank wear, Al 2 O 3 provides good chemical protection for the rake face, and TiN provides lower friction as well higher hardness  . Three nanostructured coatings, namely VAlN, VAlN/ VCAlN and VAlN/VCAlN with a carbon-enriched top layer were deposited by high-frequency magnetron sputtering to improve the life time of forming tools BIB010 . All coatings showed a stable frictional behaviour (CoF < 0.1) within the examined range of surface pressure up to 150 MPa. However the coatings' response to temperature was not discussed. Multilayered coatings consisting of ceramic-metal nanocomposite adaptive lubricating layers separated by diffusion barriers allowed adaptation to occur only upon exposure of the lubricant layer by wear, which resulted in prolonged wear life at static and cycled temperatures. The number and the thickness of the individual layer are important because diffusion and oxidation-based adaptation in these particular coatings required a minimum volume of solid lubricant material and hard wear resistance materials BIB005 . By carefully designing the multilayer system, CoF (against silicon nitride counterface) below 0.2 can be sustained over multiple thermal cycles from room temperature to 700°C. A multilayer system based on Mo 2 N/MoS 2 /Ag was deposited on Inconel via unbalanced magnetron sputtering with individual targets of Mo, MoS 2 and Ag  . The objective was to improve the tribological properties of these materials in the temperature range from 0°C to 700°C, by three different adaptive mechanisms. From 0°C to 300°C, the lubrication was provided by MoS 2 because of its lamellar structure. From 300°C to 500°C, the lubrication was provided by the migration of the noble metal to the surface. In this temperature range, the migration of silver to the surface would be dramatically reduced by its strong affinity to sulphur. From 500°C to 700°C, the sulphur in MoS 2 acted as a catalyst for the formation of lubricious compounds as the result of the reaction with silver (MoS x + Ag ! AgMoS x ), with the sulphur atoms being replaced by oxygen atoms at temperatures above 500°C. The system performed well at 600°C, maintaining a low CoF against silicon nitride counterface of 0.1 for 300,000 cycles due to the formation of three types of silver molybdate phases, namely, Ag BIB009  . However, the reduction of CoF at RT (0.6-0.4) and at 350°C (0.3) was not pronounced. In the YSZ/Au/MoS 2 /DLC system BIB004 produced by a combination of laser ablation and magnetron sputtering, load bearing was provided by the yittria stabilised zirconia (YSZ) and the DLC phases, while the lubrication was provided by the other three phases: (1) Au for temperatures above 300°C (CoF 0.10-0.20), (2) hexagonal MoS 2 for temperatures below 300°C in vacuum or in dry environments (CoF 0.02-0.05) and, (3) graphite, formed as a by-product of the DLC phase during sliding contact, for temperatures below 300°C in humid environments (CoF 0.10-0.15). Other multilayer nanocomposites including WC/DLC/ WS 2 , and Al 2 O 3 /Au/MoS 2 /DLC have also been discussed BIB008 . The WC/DLC/WS 2 composites exhibited excellent friction recovery in humid and dry environmental cycling. Their CoFs were about 0.1 in humid air, 0.03 in vacuum, and as low as 0.007 in dry nitrogen BIB003 . A ternary boron based Ti-B-N gradient coating in specific multilayer designs, which minimise the high internal stress of TiB 2 , obtained through plasma enhanced chemical vapour deposition (PECVD) was applied onto hot forming tools made from DIN 1.2367 tool steel BIB006 . After 3000 cycles stamping of AISI 1043 raw parts at 1150°C, the coating with special designed 25 graded Ti(N!B)/Ti(B!N) layers greatly reduced tool's adhesion and abrasive wear in comparison with that of untreated and plasma nitrided tools.