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Solid lubricants offer a promising approach for the targeted reduction in friction and wear. Additional to the external optimization of friction pairings, solid lubricants can be structurally incorporated into metallic matrix by designing self-lubricating composite materials. Microstructural characteristics of these composites such as the amount, chemical composition and distribution of solid lubricants distinct influence the property profile, especially the tribological behavior of bulk materials. Besides the bulk material composition, the lubrication effect is related to the contact condition during wear testing. Molybdenum disulfide (MoS₂) was investigated as promising solid lubricant for the Ni-based alloy NiCrBSiFe. Microstructure, hardness and wear behavior of the spark-plasma sintered composites with MoS₂ contents up to 10 wt.% were determined in this study. The best friction behavior was observed for the composite containing 7.5 wt.% MoS₂. The coefficient of friction for this composite is more than halved compared to the NiCrBSiFe reference. Simultaneously, the wear rate is reduced from 0.41 to 0.15 × 10⁻⁴ mm³/Nm due to MoS₂ incorporation at higher wear loads of 26 N. Based on the overall tribological results, the composite with 10 wt.% MoS₂ was laser surface hardened using a high-power 10 kW diode-pumped solid-state laser. An adherent Fe/Ni sulfide top-coat and CrS precipitates were formed. Compared to the unfused condition, the laser hardened composite exhibited 200 HV0.5 higher hardness and a similarly high wear resistance. Hence, the incorporation of MoS₂ and implementation of laser surface hardening for self-lubricating composites is a promising concept for improving the surface properties.

The development of multi‐principal‐element alloys (MPEAs) with unique characteristics such as high work hardening capacity similar to well‐known alloy systems like Hadfield steel X120Mn12 (ASTM A128) is a promising approach. Hence, by exploiting the core effects of MPEAs, the application range of conventional alloy systems can be extended. In the present study, work‐hardening MPEAs based on the equimolar composition CoFeNi are developed. Mn and C are alloyed in the same ratio as for X120Mn12. The production route consists of cast manufacturing by an electric arc furnace and surface functionalization via mechanical finishing using ultrasonic‐assisted milling (USAM) to initiate work hardening. The microstructure evolution, the hardness as well as the resulting oscillating wear resistance are detected. A pronounced lattice strain and grain refinement due to the plastic deformation during the USAM is recorded for the MPEA CoFeNi‐Mn12C1.2. Consequently, hardness increases by ≈380 HV0.025 in combination with a higher oscillating wear resistance compared to the X120Mn12. This shows the promising approach for developing work‐hardening alloys based on novel alloy concepts such as MPEAs.

Eutectic high-entropy alloys (EHEAs) with a fine-lamellar structure and homogenous property profile are of particular interest for wear and corrosion protection coatings. High cooling rates in the laser metal deposition (LMD) process can induce microstructure refinement and allow the formation of a supersaturated solid solution in EHEAs. A subsequent solution annealing can create the equilibrium state. In the present study, LMD coatings with an ultrafine-grained Widmanstätten structure were produced from the EHEA Al_0.3CoCrFeNiMo_0.75 gas atomized powder. High cooling rates during deposition led to a supersaturated solid solution with face-centered cubic (FCC) structure. The LMD coating exhibits the highest average hardness of 734 HV0.5, which drops to approx. 200 HV0.5 due to an increased microstructural domain size after heat treatment. Under oscillating wear, the formation of oxidized wear debris promotes material removal in the heat-treated condition. Corrosion tests reveal a deterioration of the passivation behavior. LMD processes exhibit great potential to create supersaturated solid solutions with refined structure in EHEAs to enhance the property profile.

Multi-Principal-Element or High-Entropy Alloys (MPEAs/HEAs) have gained increasing interest in the past two decades largely due to their outstanding properties such as superior mechanical strength and corrosion resistance. However, research studies on their processability are still scarce. This work assesses the effect of different machining conditions on the machinability of these novel alloys, with the objective of advancing the introduction of MPEA systems into industrial applications. The present study focuses on the experimental analysis of finish-milling conditions and their effects on the milling process and resulting surface finish of CoCrFeNi, Al_0.3CoCrFeNi and Al_0.3CoCrFeNiMo_0.2 alloys fabricated via Spark Plasma Sintering. Ball-nose-end milling experiments have been carried out various milling parameters such as cutting speed, feed per cutting edge, and ultrasonic assistance. In situ measurements of cutting forces and temperature on the tool edge were performed during the experiments, and surface finish and tool wear were analyzed afterwards. The results exhibited decreasing cutting forces by means of low feed per cutting edge and reduced process temperatures at low cutting speed, with the use of ultrasonic-assisted milling. It was shown that the machinability of these modern alloys through conventional, as well as modern machining methods such as ultrasonic-assisted milling, is viable, and common theories in machining can be transferred to these novel MPEAs.

Eutectic high-entropy alloys (EHEAs) are characterized by a fine lamellar microstructure. This allows for homogeneous functional surface properties. Furthermore, the risk of coarse precipitate formation during coating processes from the liquid state is avoided. However, the influence of the size and the texture of the local microstructure domain on functional properties is still unknown. The present work is devoted to the microstructural evolution of the EHEA Al_0.3CoCrFeNiNb_0.5. Inert gas atomized powder was processed by laser metal deposition (LMD) and spark plasma sintering (SPS). Both specimens were heat treated near their liquidus temperatures. The different production routes have a decisive influence on the orientation of the lamellar structure. The SPS bulk material has a statistically distributed orientation of the lamellae defined by the microstructure of the powder. However, the remelting of the powder during the LMD process causes a directional vertical solidification of the lamellar structure. Based on these differences, positive effects on their functional properties were detected for directionally solidified LMD coatings. As a result of the heat treatment, the influence of the lamellar orientation on their tribological properties is reduced, whereas the influence of the lamellar size on their property profile can be derived.

Compositional alterations to high-entropy alloys (HEAs) allow further evolution of these materials by adjusting their property profiles. This way, they can be used for coating technologies and surface-protection applications. In the present work, minor quantities of the non-metallic alloying constituents, BSiC, were added to the CrFeNi base system. The alloy development was carried out in an electric arc furnace in comparison with the nickel-based alloy Ni-600. With regard to the BSiC-free variant, the wear resistance can be significantly increased. The powder was manufactured by inert gas atomization and characterized, followed by processing via high-velocity oxy-fuel spraying (HVOF) and high velocity laser metal deposition (HS-LMD). Depending on the manufacturing conditions, the proportion and shape of the precipitates within the microstructure differ. Compared to both the reference system and the as-cast condition, the coating systems demonstrated comparable or improved resistance to wear. The evaluation of the process–structure–property relationships confirmed the great potential of developing load-adapted HEA systems using non-metallic alloy constituents in the field of surface engineering.

The addition of refractory metals represents a promising approach for development of future high-entropy alloys (HEAs). Niobium and molybdenum are particularly suitable additives for increasing hardness as well as wear and corrosion resistance. In the context of surface protection applications, eutectic high-entropy alloys (EHEAs) with their homogeneous property profile are of specific interest. In the present work, two EHEAs were developed starting from the alloy Al_0.3CoCrFeNi using electric arc melting. Following mechanical and microstructural characterization, the two alloys were found to have the compositions Al_0.3CoCrFeNiMo_0.75 and Al_0.3CoCrFeNiNb_0.5. For thermal spray processing, powders of the above alloys were prepared by inert gas atomization. The coatings produced by high-velocity oxy-fuel spraying (HVOF) were characterized and evaluated compared with castings, allowing process–structure–property relationships to be derived. Based on the results, statements on possible application potential can be made.

The processing of high-entropy alloys (HEAs) via laser metal deposition (LMD) is well known. However, it is still difficult to avoid chemical intermixing of the elements between the coating and the substrate. Therefore, the produced coatings do not have the same chemical composition as the HEA feedstock material. Single-layer CrFeCoNi and AlCrFeCoNi HEA coatings were deposited using high-speed laser metal deposition (HS-LMD). Elemental mapping confirmed a good agreement with the chemical composition of the powder feedstock material, and revealed that chemical intermixing was confined to the immediate substrate interface. The coatings are characterized by a homogeneous structure with good substrate bonding. The machining of these coatings via turning is possible. Subsequent diamond smoothing results in a strong decrease in the surface roughness. This study presents a complete manufacturing chain for the production of high-quality HS-LMD HEA coatings.

Austenitic high-manganese steels (HMnS) offer very high wear resistance under dynamic loading due to their high work hardening capacity. However, resistance to static abrasive loading is limited. Various approaches to increasing abrasion resistance are known from traditionally manufactured metallurgical components. These confirm the high potential for surface protection applications. In this work, the powder of the Hadfield HMnS X120Mn12 is prepared and processed by high-velocity oxy-fuel (HVOF) spraying and spark-plasma sintering (SPS). A good correlation was observed between the results of the HVOF and SPS specimen. Different surface conditions of the coatings and the sintered specimens were prepared by machining. Compared to the polished state, turning and diamond smoothing can increase the surface hardness from 220 HV to over 700 HV significantly. Regardless of the surface finish condition, similar good wear resistance can be demonstrated due to strong work hardening under sliding and reciprocating wear loading. In contrast, the finish machining process clearly influences abrasion resistance in the scratch test with the best results for the diamond smoothed condition. Especially against the background of current trends toward alternative coating systems, the presented results offer a promising approach for the development of HMnS in the field of coating technology.