Iron-based coatings with the incorporation of solid lubricants have been prepared by means of laser cladding, in an effort to control friction and decrease tool wear at high temperatures during metal forming applications. The choice of a Fe-based powder has been considered advantageous, as it can lead to decreased costs compared to nickel-based claddings previously studied by the authors, in addition to having a lower environmental impact. In particular, the incorporation of transition metal dichalcogenides such as MoS2 as precursors leads to the encapsulation of silver in Fe-based self-lubricating claddings, resulting in a uniform distribution of the soft metal across the thickness of the coating. Subsequent tribological evaluation of the claddings at high temperatures shows that the addition of lubricious compounds leads to lower friction at room temperature and significantly decreased wear up to 600 °C compared to the unmodified iron-based reference alloy, although higher than similar self-lubricating Ni-based claddings. In order to cast light into these observed differences, the corresponding microstructures, phase composition, and self-lubricating mechanisms have been studied and compared for Fe- and Ni-based claddings having both of them the addition of silver and MoS2. The results suggest a key role of the formation of protective tribolayers on the counter body during high temperature sliding contact. Additional simulation of the phase evolution during solidification reveals that the formation of different chromium- and nickel-based metal sulfides in Fe- and Ni-claddings during laser cladding by the decomposition of MoS2 plays a key role in determining their tribological behaviour at high temperatures.

In recent years, the use of aluminium alloys in the automotive industry has gained significant attention due to their specific strength, corrosion resistance and recyclability. However, their forming at high temperature in processes like hot stamping is challenging due to the poor tribological behaviour of aluminium alloys, which is the source of severe adhesive wear and a poor surface quality of the finished product.

In an effort to overcome these tribological problems, iron- and nickel-based self-lubricating laser claddings with the addition of solid lubricants such as silver and molybdenum disulfide have been evaluated under conditions representative of hot stamping against the aluminium alloy AA6082. It has been found that self-lubricating claddings decrease friction and counter body wear at high temperatures compared to alloys commonly used in forming tools such as grade 1.2367 steel. Furthermore, nickel-based self-lubricating claddings have shown a better tribological behaviour than their iron-based counterparts, due to the formation of a nickel-based sulfide layer on the counter body. It is thus expected that the implementation of self-lubricating claddings can improve the quality of the final product while reducing the need for added lubricant during the hot stamping of aluminium alloys.

In recent years, the use of Al/Si coatings has become widespread in hot stamping in order to protect the work piece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperature can lead to unstable friction and damage both the tool and the work piece.

In the present study, self-lubricating coatings with the addition of silver and MoS₂ have been prepared by means of laser cladding deposition, aiming at their use in hot stamping in order to decrease friction and wear. The coatings were evaluated at high temperatures against Al-Si-treated boron steel using two different testing configurations featuring open and closed tribosystems. A significant reduction in friction for the self-lubricating claddings were observed along with decreased material transfer. This could be beneficial for hot stamping applications as it can ensure the stability of the process while preventing surface damage to the work piece. Additionally, closed configuration tribotesting has been found to underestimate friction and wear of the tool/work piece system, thus making it less suited for the lab-scale simulation of hot metal forming compared to open configuration tribometers. This finding has been considered relevant as many references in the available literature still report the use of closed configuration tribometers.

Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.

In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.

In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS₂ and WS₂. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.

During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.

Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS₂ to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS₂ to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.

In this work, laser cladding has been employed for the preparation of nickel-based self-lubricating coatings featuring the addition of different combinations of soft metal solid lubricants such as Ag and Cu. Transition metal dichalcogenides (WS₂, MoS₂) were evaluated as precursors for encapsulating and uniformly distributing the soft metals throughout the microstructure. The tribological behaviour of the resulting claddings was evaluated under high temperature reciprocating sliding conditions, including two different counter body geometries that lead to very different ranges of contact pressures during testing. An improved tribological behaviour was observed for the self-lubricating claddings compared to the unmodified nickel-based alloy up to 600 °C, attributed to the presence of silver and the formation of lubricous sulfides during sample preparation due to the thermal degradation of the transition metal dichalcogenides precursors. Additionally, the role of the contact conditions observed when testing the self-lubricating claddings against flat pins instead of spherical counter bodies are discussed in terms of frictional and wear microstructural mechanisms.

Many high temperature (HT) forming processes require the use of solid lubricants in order to control friction and reduce wear. In an attempt to eliminate the need for solid lubrication in high temperature sliding applications, nickel-based self-lubricating coatings with the addition of Ag and MoS₂ were prepared by means of laser cladding on stainless steel substrates.

The behaviour of the resulting laser claddings was thoroughly evaluated up to 600 °C, including the oxidation behaviour and reciprocating tribotesting using different counter body geometries (ball and flat pin). The self-lubricating coatings showed lower friction than the unmodified reference alloy at all tested temperatures, in addition to a significant microstructural stability after prolonged exposure at high temperatures. The addition of solid lubricants to the claddings was also found to be beneficial in terms of the counter body wear at HT, as no material loss could be measured for the bearing balls after testing at 600 °C against the self-lubricating claddings, despite the significant softening experienced by AISI 52100 bearing steel at HT.

Self-lubricating materials are becoming more widespread in fields like metal forming or power generation due to the inability to use conventional lubricants in high-temperature (HT) applications. In an effort to summarise the progress done in this field, a detailed literature review has been carried out, ranging from micron-thickness thin films to hardfacings and bulk materials, and classified by the reported solid lubricants. Moreover, the most-cited deposition techniques have been reviewed for each lubricant class in addition to their advantages and limitations. HT friction and wear data for self-lubricating materials have also been examined in order to identify effective lubrication ranges and general trends in their tribological behaviour, which is expected to be useful for researchers interested in this field. Finally, several apparent research gaps have been described, with suggestions for new experimental work that could lead to the development of new high-temperature self-lubricating materials.