A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Oxide based coatings <s> Abstract The tungsten and vanadium oxides are known to be usable as solid lubricants at elevated temperatures. So far they have been used mainly as powdery materials for tribological applications or as a component of ceramics. Metal oxides are interesting for tribological insets at atmospheric conditions because of their oxidation stability. This study reports on investigations to find stable process windows for the deposition of tungsten and vanadium oxides in a reactive and non-reactive mode by the magnetron sputter ion plating–PVD-process. One focus of the study is the metal oxide phases generated in the MSIP-process under various conditions and their mechanical and metallographic properties. Coatings were deposited and analyzed by various testing methods to characterize the mechanical and structural properties, such as SEM, scratch-testing, Rockwell test, nanoindentation and XRD. Stable process windows will be presented for the reactive mode as well as the influence of different process parameters to phase generation, microstructure and the material properties. The influence of process-parameters on the deposition rate (d.c. versus r.f. mode, reactive versus non-reactive mode) is of great interest to evaluate the economic relevance. It will be shown that it is possible to deposit vanadium and tungsten oxides despite target contamination effects in wide ranges by adapting the reactive gas-flow, the total gas pressure and the target power. <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> Oxide based coatings <s> Abstract The tungsten and vanadium oxides are promising to be usable as solid lubricants at elevated temperatures because of their ability to form oxygen deficient Magneli-phases. As a matter of fact, metal-oxides are interesting for tribological insets at atmospheric conditions because of their expected oxidation stability and low adhesion. The study reports about the deposition of tungsten and vanadium oxides in a reactive d.c. mode by the MSIP- (Magnetron Sputtering Ion Plating) PVD process and especially about the influence of the oxygen content in the sputtering atmosphere as well as the deposition temperature on the phase generation. A simplified ‘sputtering phase diagram’ of the binary systems V–O and W–O as a function of the deposition temperature (378–650 K) and the oxygen content (0–50%) was determined. Furthermore, it was shown that the tested vanadium-oxides are phase stable up to 878 K and the tungsten-oxides up to 1100 K (measured in a high-temperature XRD facility). Additionally tribological properties of the deposited oxide coatings, like the friction coefficient vs. steel, will be presented. For polished and WO x coated samples a friction coefficient of μ≈0.2 against steel was measured at room temperature. The coatings were analyzed by various testing methods to characterize the tribological, mechanical and structural properties, like SEM, nanoindentation, (high-temperature)-XRD and pin-on-disk. <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> Oxide based coatings <s> Abstract Thin films of pure molybdenum oxide MoO 3 , vanadium oxide V 2 O 5 as well as phases originating from Ag 2 O–MoO 3 , Cu–V 2 O 5 and Ag–V 2 O 5 systems were deposited on alumina substrates by means of reactive magnetron sputtering. Structure and phase composition of deposits were characterised by X-ray diffraction (XRD) and Raman spectroscopy. High temperature (100–700 °C) friction tests were performed using ball-on-disc configuration in air of normal humidity. Among studied phases, the best results were observed for silver molybdate (Ag 2 MoO 4 ) and vanadium oxide bronze (β-Ag 0,33 V 2 O 5 ) coatings, which were chemically stable and allowed to decrease friction coefficient to the level of 0.2 at 450 and 600 °C, respectively. <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> Oxide based coatings <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> 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> Oxide based coatings <s> The mechanisms for frictional self-adaptation of advanced hard coatings with optimum tribological properties are based on the formation of thin reaction films in sliding contacts. The aim of this work was to characterize VN and V coatings deposited by unbalanced magnetron sputtering and to determine the influence of oxide phase formation on their friction and wear properties during high-temperature exposure in ambient air. Dry sliding experiments were performed in the temperature range between 25 and 700 °C. Structural changes and oxide phases were identified by X-ray diffraction and Raman spectroscopy after tribometer testing. The friction coefficient of the V coatings decreases from about 1.05 at RT to 0.27 at 700 °C, while the VN coating has a lower coefficient of friction (∼0.45) at RT which further decreases at temperatures above 400 °C. The onset of rapid oxidation of V and VN coatings, as evidenced by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA), occurred at 400–450 °C with the formation of vanadium oxides, which were identified by XRD as V2O5, VO2 and V6O13. For both coatings evaluated, a significant decrease of the friction coefficient at temperatures above 400 °C was observed reaching a value of 0.25–0.27 at 700 °C, due to the formation of V2O5, giving rise to solid lubrication, and subsequent melting of this phase at temperatures above 600 °C, leading to liquid lubrication. <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> Oxide based coatings <s> Abstract The high-temperature tribological properties of a cathodic arc ion-plated (V,Ti)N coating have been investigated with a SRV high-temperature friction and wear tester. After heating in air at temperatures above 500 °C, V 2 O 5 and TiO 2 phases were identified on surfaces of (V,Ti)N coating with V 2 O 5 -type platelets as the major oxidized product. (V,Ti)N coatings exhibit a friction coefficient in the range of 0.53–1.08 and a wear rate in the range of 1.69 × 10 −7 –1.31 × 10 −5 mm 3 /Nm, depending on test temperature used. At low temperatures, the presence of oxide debris induced by tribological stressing on the load-bearing regions reduces direct contact between (V,Ti)N coating and Al 2 O 3 , and generates a mild wear, however, adherent and compacted wear-protective oxide layers are not developed. At temperature of 500 °C or above, the wear-protective oxide layers containing thermally-oxidized V 2 O 5 platelets deform plastically to produce a lubricious and smooth appearance to reduce friction. Raman spectroscopy was used to identify the structures of wear debris and coating surfaces after wear tests. Tribochemical reactions are the dominating wear phenomena at low temperatures. The wear debris generated during the wear process consists mainly of the V 2 O 5 -type tribo-oxides. <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> Oxide based coatings <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> 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> Oxide based coatings <s> Abstract The current study has been undertaken to determine friction, adhesion and wear mechanisms of tribological coatings for elevated temperature applications of Ti–6Al–4V alloy. Sliding wear characteristics of diamond-like carbon (DLC) coatings and polycrystalline diamond (PCD) was studied and compared with those of conventional N-based coatings, which exhibited a high coefficient of friction (COF) due to titanium adhesion to the coating surface. Hydrogenated DLC (H-DLC) and W containing DLC (W-DLC) coatings offered low and stable COF values of 0.11–012 at 25 °C while the PCD had the lowest COF of 0.05 at 25 °C. At temperatures > 200 °C H-DLC's COF increased rapidly accompanied by high coating wear. At 100 °C W-DLC had a COF of 0.06 which increased to 0.46–0.54 between 200 °C and 300 °C similar to that of H-DLC. However, at 400 °C W-DLC's COF decreased to 0.07, and a low COF of 0.08 persisted at temperatures as high as 500 °C. The governing mechanisms of the low friction of W-DLC observed at elevated temperatures were revealed by studying the compositions of the coating surfaces and the transfer layers formed on Ti–6Al–4V. Raman spectroscopy indicated that at 25 °C the transfer layers were rich in carbon, whereas at 500 °C they consisted mainly of tungsten trioxide (WO 3 ) that formed on W-DLC's surface. <s> BIB008
Oxidation plays an important role in the friction mechanism at high temperature in the presence of humid air. Much work has been done to understand the role played by oxides such as ZnO, PbO, NiO, TiO 2-x , B 2 O 3 , Re 2 O 7 and WO 3 and to describe their frictional behaviour  BIB003 . It was first recognised by Magnéli et al., that oxides of titanium and vanadium, as well as those of molybdenum and tungsten, formed homologous series with planar faults of general formulae (Ti, V) n O 2nÀ1  or (W, Mo) n O 3nÀ1  . The oxygen deficient phases of tungsten and vanadium oxides are known to crystallise in the form of Magnéli phases, which offer a wide range of structures generated by the presence of ordered patterns of edge-sharing octahedral within the ReO 3 like cornersharing octahedral network. The disordered phase containing crystallographic shear planes with reduced binding strength enables its use as a solid lubricant BIB001 . In a simplified way, the Magnéli phase structure can be derived from a perfect V 2 O 5 crystal, which has one missing oxygen layer, i.e. the (1 2 1) plane, which is called the crystallographic shear (CS) plane and compensates for the non-stoichiometry of the compounds BIB007 . Due to their oxidation stability and their low tribooxidation sensitivity, some lamellar oxides of these transition metals (WO 3 , MoO 3 and V 2 O 5 ) have been considered to be good candidates for low friction coatings BIB003 . Tungsten and vanadium oxides were deposited by Lugscheider in a reactive DC magnetron sputtering rig and their thermal stability was studied in a high temperature XRD facility BIB001 . The vanadium oxide phases were stable until 605°C and the tungsten oxide phases were stable until 827°C. A low CoF of 0.2 was measured on a polished WO x coated sample sliding against steel at room temperature, while a slightly higher CoF (0.2-0.4) was measured for the VO x coated sample BIB002 . The authors attributed the low CoF to the high hardness of the oxides, but they did not test the tribological behaviour of the coatings at high temperature. Gulbiński et al. found that the CoF of magnetron sputtering deposited vanadium oxide (V 2 O 5 ) and molybdenum oxide (MoO 3 ) was reduced at high temperature and they followed a similar trend during both heating and cooling ( Figure 2 ) BIB003 . These oxides or their derivatives can be deposited as coatings on the contact surfaces using reactive PVD techniques, especially magnetron sputtering  . However, the poor adhesion resulting from the reactive oxidation process seriously compromises their applications. Another approach is to deposit hard and wear resistant coatings containing VN BIB005 , TiVN BIB006 , MoN and WN BIB004 which, upon tribo-chemical oxidation reactions act as a source of elements for the creation of the desirable lamellar oxides and oxide-derivative phases at elevated temperature. The decrease in friction is a result of the softening or even the micro-melting of appropriate phases generated through tribo-contact, and thus the lubrication effect appears at relatively high temperatures BIB003 . Tungsten was added to DLC coatings to modify its microstructure via a cathodic arc PVD system. The resulting W-DLC coating offered high wear resistance and low coefficient of friction up to 500°C due to the formation of tungsten trioxide (WO 3 ) on the surface BIB008 . However there were some fluctuations in the CoF at intermedium temperatures (200-300°C).