A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Duplex system: the application of textured surface and Nanotube <s> Abstract We are now close to the new millennium and on the threshold of an era of rapid change. Limitations to the further advance of manufacturing industry in the 21st century are most likely to be surface-related. Many mechanical systems will operate under ever more severe application conditions, such as intensive loads, high speeds and harsh environments, in order to achieve high productivity, high power efficiency and low energy consumption. Consequently, many challenging complex design situations have emerged where the combination of several properties (such as wear resistance, load bearing capacity, and fatigue performance) are required. These new challenges can be met only through realising the potential of duplex surface engineering. Indeed, there are thought to be great technical and economic benefits available through the application of duplex surface engineering technologies in many new market sectors. The present paper is a synthesis of several strands of recent surface engineering research at the University of Birmingham, including the duplex ceramic coating-nitrided steel system and the duplex DLC coating–oxygen diffusion treated titanium system. The prediction of the performance of duplex systems based on advanced contact mechanics modelling is also discussed. <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> Duplex system: the application of textured surface and Nanotube <s> Composite coatings consisting of hard compounds and soft solid lubricant phases have been actively investigated as tribological coatings for reducing friction while improving wear resistance. In most cases structural modifications in these coatings have been achieved by either natural phase separation or sequential layer deposition. In this study, we explore the concept of creating a three-dimensional structure in a tribological composite coating by using well-established semiconductor patterning methods. Using photolithography, we have patterned the substrate surface with a regular array of micron-size islands. The substrate was then coated with various metal and carbide coatings, including Cr and TiC, using sputter deposition. The substrates were sonicated in a solvent to remove the photoresist islands leaving an array of holes. MoS 2 was then deposited onto the substrates. A pin-on-disk friction and wear test was conducted on a TiC/MoS 2 sample to examine the effects of these islands on wear morphology. The results showed that the MoS 2 -filled holes can act as lubricant reservoirs, and the areas of the sample coated with TiC/MoS 2 provided better wear protection than areas coated with only MoS 2 . <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> Duplex system: the application of textured surface and Nanotube <s> Solid lubricant systems were developed to provide friction and wear reduction in variable environmental conditions. The mechanisms employed to achieve cross-environment capability include “on demand” solid lubricant supply from reservoirs in hard wear protective coatings. This research explores recent advances in a precision laser machining to generate arrays of micrometer sized dimple reservoirs with controlled size, location, and density. These micro-reservoirs were machined by a focused UV laser beam on the surface of hard TiCN coatings produced by a cathodic vacuum arc deposition. Different dimple diameters and spacing were used to achieve area coverage with micro-reservoirs between 0.5% and 50% of the entire tribological contact surface. Solid lubricants based on MoS2 and graphite were then applied by burnishing and sputtering to such laser textured surfaces. Sliding friction tests were performed against steel balls in humid air and dry nitrogen environments. The results indicate that there is an optimum dimple surface coverage of about 10%. The life of the solid lubricants on dimpled surfaces was an order of magnitude longer than on the unmodified TiCN coating surface. The laser-processed micro-reservoirs also helped to renew lubricant supply for tests with multiple cycling between humid air and dry nitrogen. Tribological surface adaptation to the test environment was investigated by micro-Raman, which demonstrated a repeated change from hexagonal MoS2 to graphite carbon in the wear track with each dry/humid environment cycle. Solid lubricant composition inside the micro-reservoirs did not change. <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> Duplex system: the application of textured surface and Nanotube <s> Abstract Surface texturing has been recognized as an effective means to improve the tribological performances of sliding surfaces. Usually, generation additional hydrodynamic pressure to increase the load carrying capacity is regarded as the most significant effect of surface texture. In the case of silicon carbide sliding against identical material in water, the experimental results indicate that surface texture is also helpful to improve the running-in progress to smooth the contact surfaces, showing another reason to result in low friction. Based on the consideration of enhancing the generation of hydrodynamic pressure and improving running-in progress, a surface texture pattern, which was combined with large (circle, 350 μm in diameter) and small (rectangular, 40 μm in length) dimples, was designed to maximize the texture effect on the load carrying capacity of SiC surfaces sliding in water. The friction coefficient of such textured surface was evaluated and compared with that of untextured and those only with large or small dimples only. The friction reduction mechanisms of the patterns with different dimples in size are discussed. <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> Duplex system: the application of textured surface and Nanotube <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> 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> Duplex system: the application of textured surface and Nanotube <s> Measured friction coefficients of carbon nanotubes vary widely from μ 1.0 [1, 2, 3, 4, 5, 6], while theoretical studies suggest intrinsically high friction coefficients, approaching unity [7]. Here we report that measured friction coefficients of MWNT films are strong functions of surface chemistry and temperature, but are not dependent on the presence of water vapor. We hypothesize that the origin of the temperature dependence arises from the interaction of the surface chemical groups on the nanotubes [8, 9, 10, 11, 12] and rubbing counterface. The friction coefficient of individual films can be easily tuned by changing the surface temperature and chemistry of either the countersurface or the nanotubes, we have demonstrated the ability to create and control high and low friction pairs through plasma treatments of the nanotube films with argon, hydrogen, nitrogen, and oxygen. This behavior is completely reversible, and when coupled with the superior strength, thermal, and electrical properties of nanotubes, provides a versatile tunable, multifunctional tribological system. <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> Duplex system: the application of textured surface and Nanotube <s> Abstract Texturing of contact surfaces has a remarkable influence on their tribological properties. Various texturing techniques such as machining, ion beam texturing, etching techniques and laser texturing are employed for preparation of micrometer-sized valleys. Modern modification of surface layers for friction applications combines surface texturing and filling of textured layers by solid lubricant MoS 2 /WS 2 or other compositions. In this work pulsed air arc treatment (PAAT) was used to texture the surface layers. The textured surfaces were obtained and then filled by solid lubricant. Some burnishing methods were used to deposit solid lubricant films on steel surfaces. The tribological properties of solid the lubricant films on textured surfaces were compared with the behavior of solid lubricant films burnished on ground surfaces. The wear rate and wearability of solid lubricant films were assessed. It was shown that application of PAAT and filling of micro-reservoirs by solid lubricant particles provide longevity under dry friction conditions. <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> Duplex system: the application of textured surface and Nanotube <s> Individual multiwalled boron nitride nanotubes of different diameters (40−100 nm) were bent inside a 300 kV high-resolution transmission electron microscope (TEM) using a new fully integrated TEM−atomic force microscope (AFM) piezodriven holder under continuous recording of force−piezodisplacement curves. The tubes were gently compressed in situ (i.e., inside the electron microscope) between a piezomovable aluminum wire and a silicon cantilever. Typically, bending stress values ranging from ∼100 to ∼260 MPa, and corresponding to elastic moduli of 0.5−0.6 TPa, were estimated. Tube gross failures were absent up to very large bending angles (in excess of 115°). Extending the bending angles beyond 30−40° resulted in the elastic deformation of BN nanotubes, which proceeded through the propagation of consecutive momentary kinks. These had the effect of accumulating a bending curvature rather then uniformly curl the tube under the compression load. These kinks were found to be entirely reversible on reloading wi... <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> Duplex system: the application of textured surface and Nanotube <s> Abstract The production of surface micro-patterns in steel substrates for solid lubricant (SL) reservoirs was previously shown to provide improved wear and frictional properties in sliding contact. We report on the fabrication of micrometer-sized dimple patterns of different sizes on TiAlCN hard coatings produced by cathodic arc vacuum evaporation. These dimples were produced by reactive ion etching in a mixed Ar/CF 4 plasma. An overlayer of MoS 2 or Mo/MoS 2 /Ag SLs was deposited on the micro-textured surfaces using unbalanced magnetron sputtering. The tribological properties of the coatings were investigated against Si 3 N 4 in dry sliding at 25 °C and in air at 570 °C. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy (OM), optical profilometry (OP), energy dispersive x-ray spectroscopy (EDX), and micro-Raman spectroscopy. The frictional and wear properties of textured and flat surfaces were compared. A significant decrease in friction and wear coefficients was achieved at 25 °C and 570 °C for the textured films because the dimples acted as reservoirs that provided a new supply of SLs to the contact surfaces. The chemistry and phase analysis of the coating wear tracks showed temperature adaptive behavior with the lubrication being provided primarily by MoS 2 at 25 °C and by silver and silver molybdate compounds at high temperature. <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> Duplex system: the application of textured surface and Nanotube <s> Abstract The concept of incorporating microscopic reservoirs within a hard coating for the purpose of solid lubricant storage and supply during wear of interacting surfaces has been investigated in this study. A novel method was devised using ceramic beads (1.5–10 μm diameter) as placeholders during the deposition of a TiN coating by reactive sputter deposition. A pin-on-disk wear test was used to test these coatings using graphite and sputter-deposited carbon as the solid lubricant, and an alumina counterface. When tested without any lubricant, the presence of the microreservoirs in the TiN coating appeared to degrade the mechanical integrity of the coating leading to rapid failure. With the graphite lubricant present, the frictional behavior ranged from levels similar to the TiN coating alone, to that of graphite alone. Tests of the TiN coating made using 10 μm beads running against an aluminum counterface showed substantial improvement when the microreservoirs were present. Optical microscopy examination of the wear tracks showed the microreservoirs were generally successful at trapping the graphite lubricant during wear. With a sufficient density and appropriate distribution of the microreservoirs significant improvements in tribological performance can be realized. <s> BIB010 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Duplex system: the application of textured surface and Nanotube <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> BIB011 </s> A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Duplex system: the application of textured surface and Nanotube <s> Abstract There are several manufacturing processes, in which the employment of solid lubricants is limited. In addition, ecological damage and higher production costs are further consequences of using such solid lubricants. This work aims at using the great potential of thin film technology to deposit adaptive, self-lubricating coatings as an alternative to conventional solid lubricants. Using magnetron sputtering process several titanium aluminum vanadium nitride coatings (TiAlVN) were developed in this study. These quaternary coatings possess the ability of forming lubricious oxides, known as Magneli phases, at elevated temperatures, which significantly reduces the friction coefficient and surface wear. <s> BIB012
Duplex system, which combines two or more established surface engineering techniques together, has been used to fight against challenging working condition BIB001 . High speed steel (HS6-5-2c) material was pre-treated with plasma nitriding first, and then deposited with a hard and lubricious TiAlVN coating via magnetron sputtering BIB012 . The friction coefficient at 500°C was lower than that of the room temperature which was due to the forming of lubricious oxide. The combination of solid lubricants with nanocomposites has provided a versatile option to extend the working temperature range. However, due to the consumption and diffusion of solid lubricants at elevated temperature, it is difficult to preserve their effect for extended operation. Therefore, many attempts have been reported to modify the coating structure using textured surfaces and nanotubes as reservoirs for lubricants. Many different techniques have been used to introduce surface textures and create favourable architectures, which can accommodate the solid lubricants, such as: laser texturing BIB003 , lithography BIB004 BIB002 , reactive ion etching BIB009  , pulsed air arc treatment BIB007 and sprayed ceramic beads BIB010 . All these techniques are used to produce well-defined circular or rectangular depressions or dimples on the surface of hard coatings (either single or multiphase). The size of these defects/pores can be controlled and their distribution should be uniformly spread on the surface, as they will serve as lubricant reservoirs over the tribological contact. Solid lubricant coatings can then be applied by PVD, especially by sputter deposition and evaporation, pulsed laser ablation, burnishing/ impingement or metal organic chemical vapour deposition (MOCVD) BIB011 BIB005 . The lubricant layer can then be replenished, thus minimising the amount of deformation during the sliding process and increasing the lifetime of the contacting surfaces. In Voevodin and Zabinski's research BIB003 , powerful lasers with short wavelengths and pulse durations were used to create 10-20 lm reservoirs on TiCN coatings, causing minimum damage to the areas surrounding the reservoirs. A multifold increase in the wear life of burnished and sputtered MoS 2 on textured TiCN coatings was demonstrated in comparison with that of monolithic TiCN films. The introduction of MoS 2 / graphite/Sb 2 O 3 in the pores helped to keep a layer of solid lubricant on the surface, which worked reliably under cyclic humid and dry conditions. Different dimple diameters and spacing were used to cover an area with micro-reservoirs between 0.5% and 50% of the entire tribological contact surface and the best tribological effect was attained with an optimum dimple surface coverage of about 10%. Basnyat et al. used reactive ion etching to fabricate 80 lm sized dimple patterns on TiAlCN films produced by cathodic arc vacuum evaporation BIB009 . Single-phase MoS 2 and multi-phase MoS x -Ag solid lubricants were subsequently deposited on the microtextured TiAlCN film, via unbalanced magnetron sputtering. At 570°C, the addition of Ag, Mo and MoS 2 was found to significantly decrease the CoFs as a result of the migration of Ag to the surface and the formation of lubricious silver molybdate Z. Zhang and H. Dong: Manufacturing Rev. 2014, 1, 24 phases. The presence of pores containing solid lubricant materials reduced the coefficient of friction and improved the longevity of the coating by a factor of 2, as seen in Figure 8 . Reactive ion etching (RIE) was also used to create arrays of micro-scale dimples on sputtered VN coatings on Inconel alloy substrates, and the pores were subsequently filled with Ag 3 VO 4 powders by burnishing  . It was found that the textured films with burnished powders had a low CoF (against Si 3 N 4 counterface) of 0.2 at 750°C, and were more effective in enhancing the tribological properties when compared to burnished/un-burnished monolithic VN coatings. Moskovitch et al. applied the pulsed air arc treatment to produce dimples on the surface of stainless steel specimens and MoS 2 was then burnished with stearin on the textured surface, which significantly extended its lifetime BIB007 . Zimmerman et al., utilised ceramic beads (1.5-10 lm diameter) as placeholders during the deposition of a TiN coating by reactive sputter deposition. The beads were removed by sonication, creating micro-pores which were finally filled with a solid lubricant, like graphite BIB010 . The authors claimed a substantial improvement on the tribological properties of TiN with a textured surface formed by using 10 lm beads. Krzanowski et al. used photolithography, a well-established method for patterning semiconductors, to create a regular array of micron-size islands on the surface of the substrate. Various coatings were applied on the substrate, including Cr, TiC and MoS 2 , via sputter deposition, to create a three-dimensional structure in a tribological composite coating BIB002 . Carbon nanotubes (CNT) with superior strength, thermal, and electrical properties provided the chance to create a versatile tuneable, multifunctional tribological system BIB006 . The CoFs of carbon nanotubes varied widely from less than 0.1-1.0. A high CoF was obtained for vertically aligned CNTs while a very low value was obtained for the CNTs dispersed flat on substrates. Self-lubricating CNT composites were created by electrodepositing molybdenum disulfide (MoS 2 ) on vertically aligned multi-walled carbon nanotubes (MWNTs)  . These composite films displayed much better performance than the monolithic MoS 2 coatings under the same operating conditions without compromising CoF values (0.04) as seen in Figure 9 . The film exhibit low CoF ($0.03) and wear rate ($10 À13 mm 3 /N mm) even at 300°C, which were about twofold better than those of nanoparticulate MoS 2 -based coatings. The CNTs, with high load-bearing capacity, provided a strong enduring support for the MoS 2 nanoclusters and were responsible for their ultralow wear. The MWNT/MoS 2 coating displayed a low CoF both in humid (~0.16) as well as in dry (~0.05) air and was able to sustain its frictional properties under cyclic non-humid/humid environments. Similarly, the tribological properties of CNT/Ag composites were improved in comparison to their constituent materials both at 25°C and at 500°C  . The improved performance of the self-lubricating CNT composites could be due to (1) the high strength and fracture toughness provided by the nanotubes; (2) the self-lubricating nature of the CNTs as they turn into graphite when worn; (3) the lateral-support provided by the solid lubricants i.e. MoS 2 or Ag to the CNTs preventing them from breaking under relatively high loads  . Boron nitride nanotubes (BNNTs) have a similar structure to CNTs and they have low CoF and good thermal stability. Under a compression load, the bending angles of BNNTs can be extended beyond 30À40°resulted in the elastic deformation, which proceed through the propagation of consecutive momentary kinks, therefore they have the effect of accumulating a bending curvature rather than uniformly curling as CNTs behave. BNNTs were also used to incorporate solid lubricants to form a Nanotube/self-lubricating coatings for tribological applications BIB008 .