A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools <s> Summary and conclusions <s> Abstract The aim of the present work was to investigate and compare different hard coatings as to the tendency for work material adhesion and galling properties when applied on forming tool steel and sliding against different work materials. The surface coatings included were PVD deposited TiN, TiB 2 , VN, TaC and DLC coatings. They were all applied to cold work tool steel. Tribological evaluation was carried out in a load-scanning test rig, with the normal load being gradually increased during each test from 100 to 1300 N (1–3.5 GPa). The coated steel was tested against austenitic stainless steel and alloys of aluminium and titanium. This investigation clearly indicates that work material adhesion and galling performance of coated forming tool steel greatly depends on the type of work material. In the case of stainless steel, carbon-based coatings provide the best protection against the work material transfer, while forming of aluminium and titanium alloys, requires nitride type coatings, such as TiN. <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> Summary and conclusions <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> BIB002
The last decades have seen progress in applying existing solid lubricants and hard and wear resistant materials on different tool surfaces to meet an increasing demand for high-performance and long-life forming tools in hightemperature environments. Monolithic nitrides such as CrN and TiN have been extensively investigated by adding such alloying elements as Al, Si and V to mainly enhance their high temperature wear resistance and to some extent to reduce their coefficient of friction (CoF). In order to further reduce CoF, other refractory metal nitrides VN, WN, MoN/Mo 2 N, NbN and TaN have been studied due to the possibility of forming Magnéli phase at high temperature, and among them, VN seems most promising. In the meantime, Magnéli oxides including V 2 O 5 , MO BIB002 and WO 3 have also been explored for their friction and wear properties, but they can only perform well at temperatures ranging from 500 to 700°C. Therefore noble metals such as silver were added into the oxides and nitrides to form composites thus providing extra solid lubrication at temperatures below 500°C and it is also beneficial for lubrication up to 1000°C; other solid lubricants like MoS 2 , carbon-based materials were also introduced into the composite coatings to reinforce its adaptability. The composite coatings either with monolithic nanocomposite or incorporated with multi-elements or blended with other lubricants or composites have a layer structure as shown in Figure 10a . Multi-layer structured coating is another option to increase the adaptability and longevity of coatings for demanding environment. Figure 10b is one of the typical designs of multilayer system consisting of alternating solid lubricant layer and hard nanocomposite/barrier layer, which could confer the system with low-friction and high wear resistance. More recent development is to increase the bonding or retention of coatings by texturing the hard coating surfaces (such as TiAlN) before applying lubricant containing nanocomposite coating (Figure 10c) . Selection of proper coating or surface treatment plays an important role in the process of obtaining the best performance of forming tools. This selection depends greatly on the type of material to be formed. For very soft metals (aluminium and aluminium alloys) with a high degree of plastic deformation during forming, nitrided forming tool steels and nitride-type coatings show a good resistance against galling and a relatively stable friction. For materials that show a high tendency to galling and transfer layer reception (titanium and titanium alloys), only plasma nitriding and VN coatings give some improvement in galling performance BIB001 .