Low Friction Properties Research Articles

Nano-multilayered ZrN-Ag/Mo-S-N film design for stable anti-frictional performance at a wide range of temperatures

A multilayer film, composed by ZrN-Ag (20 nm) and Mo-S-N (10 nm) layers, combining the intrinsic lubricant characteristics of each layer was deposited using DC magnetron sputtering system, to promote lubrication in a wide-range of temperatures. The results showed that the ZrN-Ag/Mo-S-N multilayer film exhibited a sharp interface between the different layers. A face-centered cubic (fcc) dual-phases of ZrN and Ag co-existed in the ZrN-Ag layers, whilst the Mo-S-N layers displayed a mixture of hexagonal close-packed MoS2 (hcp-MoS2) nano-particles and an amorphous phase. The multilayer film exhibited excellent room temperature (RT) triblogical behavior, as compared to the individual monolayer film, due to the combination of a relative high hardness with the low friction properties of both layers. The reorientation of MoS2 parallel to the sliding direction also contributed to the enhanced anti-frictional performance at RT. At 400 °C, the reorientation of MoS2 as well as the formation of MoO3 phase were responsible for the lubrication, whilst the hard t-ZrO2 phase promoted abrasion and, consequently, led to increasing wear rate. At 600 °C, the Ag2MoO4 double-metal oxide was the responsible for the low friction and wear-resistance; furthermore, the observed transformation from t-ZrO2 to m-ZrO2, could also have contributed to the better tribological performance.

Synovium friction properties are influenced by proteoglycan content

The synovium plays a crucial role in diarthrodial joint health, and its study has garnered appreciation as synovitis has been linked to osteoarthritis symptoms and progression. Quantitative synovium structure–function data, however, remain sparse. In the present study, we hypothesized that tissue glycosaminoglycan (GAG) content contributes to the low friction properties of the synovium. Bovine and human synovium tribological properties were evaluated using a custom friction testing device in two different cases: (1) proteoglycan depletion to isolate the influence of tissue GAGs in the synovium friction response and (2) interleukin-1 (IL) treatment to observe inflammation-induced structural and functional changes. Following proteoglycan depletion, synovium friction coefficients increased while GAG content decreased. Conversely, synovium explants treated with the proinflammatory cytokine IL exhibited elevated GAG concentrations and decreased friction coefficients. For the first time, a relationship between synovium friction coefficient and GAG concentration is demonstrated. The study of synovium tribology is necessary to fully understand the mechanical environment of the healthy and diseased joint.

Fabrication, characterization and numerical validation of a novel thin-wall hydrogel vessel model for cardiovascular research based on a patient-specific stenotic carotid artery bifurcation

In vitro vascular models, primarily made of silicone, have been utilized for decades for studying hemodynamics and supporting the development of implants for catheter-based treatments of diseases such as stenoses and aneurysms. Hydrogels have emerged as prominent materials in tissue-engineering applications, offering distinct advantages over silicone models for fabricating vascular models owing to their viscoelasticity, low friction, and tunable mechanical properties. Our study evaluated the feasibility of fabricating thin-wall, anatomical vessel models made of polyvinyl alcohol hydrogel (PVA-H) based on a patient-specific carotid artery bifurcation using a combination of 3D printing and molding technologies. The model’s geometry, elastic modulus, volumetric compliance, and diameter distensibility were characterized experimentally and numerically simulated. Moreover, a comparison with silicone models with the same anatomy was performed. A PVA-H vessel model was integrated into a mock circulatory loop for a preliminary ultrasound-based assessment of fluid dynamics. The vascular model's geometry was successfully replicated, and the elastic moduli amounted to 0.31 ± 0.007 MPa and 0.29 ± 0.007 MPa for PVA-H and silicone, respectively. Both materials exhibited nearly identical volumetric compliance (0.346 and 0.342% mmHg−1), which was higher compared to numerical simulation (0.248 and 0.290% mmHg−1). The diameter distensibility ranged from 0.09 to 0.20% mmHg−1 in the experiments and between 0.10 and 0.18% mmHg−1 in the numerical model at different positions along the vessel model, highlighting the influence of vessel geometry on local deformation. In conclusion, our study presents a method and provides insights into the manufacturing and mechanical characterization of hydrogel-based thin-wall vessel models, potentially allowing for a combination of fluid dynamics and tissue engineering studies in future cardio- and neurovascular research.

Synthesis and tribological characterization of cold-sprayed Ni-based composite coatings containing Ag, MoS2 and h-BN

The tribo components often fail to perform in a desired way due to absence of adequate lubrication in severe conditions and may cause loss of material. Even under room temperature (RT), it is desirable to maintain the proper lubrication for efficient functioning. In recent years, many efforts have been made to develop a coating material which can facilitate low friction and wear properties in diverse working conditions. The proper use of the solid lubricants can deliver utmost lubricity and, therefore, minimize the loss of materials. Solid lubricating coatings have found various industrial applications in harsh environmental conditions, such as in internal combustion engine, gas turbine and aircraft. As far as development of coating is concerned, cold spray (CS) technology has been observed to develop the coatings at relatively low temperature (avoids decomposition) as compared to conventional thermal spray technologies (plasma spray & high velocity oxy fuel). In the current investigation, synergistic response of the participating solid lubricants viz. silver (Ag), molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) on the tribological properties of the developed coatings were studied in various working regime of loads. The tribological characteristics of different composite coatings, such as NiAl-MoS2-Ag (NB0), NiAl-MoS2-Ag-5 wt. % hBN (NB5), NiAl-MoS2-Ag-7.5 wt % hBN (NB7.5) and NiAl-MoS2-Ag-10 wt % hBN (NB10) against alumina ball were explored at various loads (6, 11, 16 and 21 N) and at a fixed speed of 0.3 m/s under room temperature (RT). The fixed concentration of Ag and MoS2 with varying weight percentage of h-BN (0, 5, 7.5 and 10 wt %) were introduced as solid lubricants for evaluating the lubricating potential of h-BN. During the tests, coefficient of friction (COF) and wear rate were observed to lessen as the load increased from 6 to 16 N. However, increased COF and wear rate were noticed for all the participating coatings at higher load (21 N). The ideal content of hBN in the deposited coatings was ascertained in order to get the utmost favourable lubricating conditions. It was observed that NB7.5 coating exhibited the enhanced tribological characteristics under the aforesaid operating conditions. Coating NB7.5 shows the best friction and wear characteristics during each testing load and reached a COF (0.19) and wear rate (2.1 × 10−5 mm3/Nm) at 16 N load. The observed wear mechanisms were analysed on the basis of the synergy shown by Ag, MoS2, and hBN as well as the optimal hBN level in the coating (NB7.5) which helped in attaining the improved tribological properties.

Friction and wear of Ni alloy-Ag-Ni doped hBN self-lubricating composites from room temperature to 800 °C

Abstract The current investigation explores the potential of Ni doped hBN (hBN-O-Ni) as a solid lubricant in conjunction with Ag in affecting the tribological performance of Ni alloy-Ag- hBN composites containing a fixed amount of silver (10 wt.%) and different amounts (2, 4, 6 and 8 wt.%) of hBN from room temperature to 800 °C by carrying out tests under a fixed load of 5N and speed of 0.5 m/s using a ball-on-disk tribometer. The study also intends to determine the optimum content of addition of hBN requited to attain low friction and low wear properties over the entire range of temperature. The composite having 8 wt. % hBN has shown the lowest coefficient of friction (~ 0.18) at 800 °C due to synergetic action between silver molybdates and hBN. However, the composite containing 4 wt. % hBN has the optimum tribological properties under the conditions used in the present study. At low temperatures, Ag and hBN provided lubrication, whereas at high temperatures lubricious oxides (NiO, NiMoO4 and MoO3), silver molybdates (Ag2MoO4, Ag2Mo2O7) and hBN contributed to lowering the coefficient of friction as well as wear rate.

Cartilage-Inspired, High-Strength, and Heat-Tolerant Lubricating Hydrogels by Macrophase Separation.

Hydrogels are considered as a potential cartilage replacement material based on their structure being similar to natural cartilage, which are of great significance in repairing cartilage defects. However, it is difficult for the existing hydrogels to combine the high load bearing and low friction properties (37 °C) of cartilage through sample methods. Herein, we report a facile and new fabrication strategy to construct the PNIPAm/EYL hydrogel by using the macrophase separation of supersaturated N-isopropylacrylamide (NIPAm) monomer solution to promote the formation of liposomes from egg yolk lecithin (EYL) and asymmetric template method. The PNIPAm/EYL hydrogels possess a relatively high compressive strength (more than 12 MPa), fracture energy (9820 J/m2), good fatigue resistance, lubricating properties, and excellent biocompatibility. Compared with the PNIPAm hydrogel, the friction coefficient (COF 0.046) of PNIPAm/EYL hydrogel is reduced by 50%. More importantly, the COF (0.056) of PNIPAm/EYL hydrogel above lower critical solution temperature (LCST) does not increase significantly, exhibiting heat-tolerant lubricity. The finite element analysis further proves that PNIPAm/EYL hydrogel can effectively disperse the applied stress and dissipate energy under load conditions. This work not only provides new insights for the design of high-strength lubricating hydrogels but also lays a foundation for the treatment of cartilage injury as a substitute material.

Study on the tribological properties of hexagonal boron nitride flakes composite hydrophilic/hydrophobic ionic liquids films by self-assembly

Due to the weak out-of-plane van der Waals interaction between layers, hexagonal boron nitride (h-BN) with a stacking structure has been proven to be a promising lubrication material for its friction performance. However, obtaining h-BN composite films with excellent substrate bonding and low friction properties under high humidity remains challenging. Ionic liquids (ILs) composed of anions and cations that are liquid at or near room temperature have distinguished themselves in the field of lubrication (lubricating additives and lubricating films) due to their advantages of low saturated vapor pressure, good stability and high viscosity. Based on the relationship between lubrication performance and hygroscopicity of hydrophilic/hydrophobic ILs, a simple and flexible method for preparing h-BN composite hydrophilic/hydrophobic ILs films on metal substrates has been developed. h-BN has excellent antioxidant and moisture resistance as well as lubrication performance. Consequently, the influence of the hygroscopicity of ILs on the macroscopic tribological performance of h-BN composite hydrophilic/hydrophobic ILs films was investigated. The results showed that PVA/BNOH composite hydrophobic ILs films displayed a lower friction coefficient and outstanding wear resistance than those of PVA/BNOH composite hydrophilic ILs films in both atmosphere and high humidity. The synergies between the film layers, the intrinsic differences between the hydrophilic and hydrophobic ILs, and tribo-chemical reactions at friction interfaces greatly promoted the realization of excellent tribological performance of composite films. Additionally, the tribological properties of these films at high humidity were mainly related to the interaction between water molecules and the ILs.

Recycling Automotive Plastic Waste: Residual Polyvinyl Butyral (rPVB) as Solid Lubricant in Polyoxymethylene (POM) Blends

Polyoxymethylene (POM) is an engineering thermoplastic commonly used in tribological applications due to its low friction and auto-lubrication properties. To improve its performance further and broaden its applications to higher sliding speeds and loads, it is commonly reinforced with different particles, fibers, or solid lubricants. In this study, the effect of adding residual polyvinyl butyral (rPVB) from automotive windshields production lines on the tribological performance of POM during pin-on-disk wear tests at different loads (12, 15, and 18 N) and sliding speeds (0.25, 0.50, and 0.75 m/s) is investigated. It was found that rPVB acts as a solid lubricant, reducing the coefficient of friction (CoF) up to 50%. It is suggested that the formation of a protective layer of adhered material on the wear track is the mechanism behind CoF reduction. Melt flow index, Shore D hardness and elastic properties decrease linearly with rPVB content. However, the reduction in tensile strength, and thus shear strength of the blends, seems to be more significant on their CoF behavior than the reduction in hardness. No trend was found for wear resistance. Under certain sliding conditions rPVB decreased volume loss, but in other cases it increased it. It is suggested that, for the conditions analyzed in this study, adding 10 wt% of rPVB to a POM matrix results in the best balance of tribological performance, with an overall CoF reduction of 36%, and an average volume loss increase of only 1%.

Enzymatic digestion does not compromise sliding-mediated cartilage lubrication

Articular cartilage's remarkable low-friction properties are essential to joint function. In osteoarthritis (OA), cartilage degeneration (e.g., proteoglycan loss and collagen damage) decreases tissue modulus and increases permeability. Although these changes impair lubrication in fully depressurized and slowly slid cartilage, new evidence suggests such relationships may not hold under biofidelic sliding conditions more representative of those encountered in vivo. Our recent studies using the convergent stationary contact area (cSCA) configuration demonstrate that articulation (i.e., sliding) generates interfacial hydrodynamic pressures capable of replenishing cartilage interstitial fluid/pressure lost to compressive loading through a mechanism termed tribological rehydration. This fluid recovery sustains in vivo-like kinetic friction coefficients (µk<0.02 in PBS and <0.005 in synovial fluid) with little sensitivity to mechanical properties in healthy tissue. However, the tribomechanical function of compromised cartilage under biofidelic sliding conditions remains unknown. Here, we investigated the effects of OA-like changes in cartilage mechanical properties, modeled via enzymatic digestion of mature bovine cartilage, on its tribomechanical function during cSCA sliding. We found no differences in sliding-driven tribological rehydration behaviors or µk between naïve and digested cSCA cartilage (in PBS or synovial fluid). This suggests that OA-like cartilage retains sufficient functional properties to support naïve-like fluid recovery and lubrication under biofidelic sliding conditions. However, OA-like cartilage accumulated greater total tissue strains due to elevated strain accrual during initial load application. Together, these results suggest that elevated total tissue strains—as opposed to activity-mediated strains or friction-driven wear—might be the key biomechanical mediator of OA pathology in cartilage. Statement of SignificanceOsteoarthritis (OA) decreases cartilage's modulus and increases its permeability. While these changes compromise frictional performance in benchtop testing under low fluid load support (FLS) conditions, whether such observations hold under sliding conditions that better represent the joints’ dynamic FLS conditions in vivo is unclear. Here, we leveraged biofidelic benchtop sliding experiments—that is, those mimicking joints’ native sliding environment—to examine how OA-like changes in mechanical properties effect cartilage's natural lubrication. We found no differences in sliding-mediated fluid recovery or kinetic friction behaviors between naïve and OA-like cartilage. However, OA-like cartilage experienced greater strain accumulation during load application, suggesting that elevated tissue strains (not friction-driven wear) may be the primary biomechanical mediator of OA pathology.

Comparison of lubrication mechanism and friction behavior of graphene on stainless steel substrate at different scales

Graphene at both nano and macro scales reduces friction in stainless steel, but there are differences in the lubrication mechanisms of graphene at the two scales. In macroscopic friction, graphene achieves lubrication through the combined effect of low friction properties of the material itself and relative sliding at the friction interface. In nano-friction, the relative sliding of graphene at the friction interface does not occur, and the lubrication is realized only by the low friction property of graphene. In nano-friction, the bulge structure of graphene on a stainless-steel substrate causes a two-stage increase in friction. At the same time, friction strengthening behavior and stick–slip behavior occur in graphene during nano-friction.

Ternary low-friction coatings on thermoplastics by plasma spraying: Investigations on the process-structure

Thermally sprayed MoS2/graphite/Zn coatings were developed by the atmospheric pressure plasma jet technique to investigate the process-structure-relationship of the low-friction coatings. The fine-tuning of the powder carrier gas flow to the plasma jet was supported by computational fluid dynamics and revealed the impact on the powder particles within the coating process i.e. ideal powder particle temperature for transferring the solid Zn powder into the liquid state as main requirement for a successful embedding process of the dry lubricants. MoS2/graphite/Zn composite coatings were deposited on polyamide 4.6 (PA4.6) substrates to evaluate the adhesion/cohesion properties and tribological performance by tuning the current of the atmospheric pressure plasma jet (APPJ) from 125 A to 150 A. In particular, the plasma spraying distribution of the composite feedstock and the resulting coating architecture are strongly dependent on the plasma process setting, which are investigated by scanning electron microscopy and transmission electron microscopy in combination with an energy-dispersive x-ray spectrometer. Tribological characterisation indicates that coating composition and thickness influence the coating performance significantly. According to the 125 A plasma current setting, the low frictional compounds MoS2 and graphite are embedded in a Zn matrix in contrast to plasma current settings of 150 A, where Zn is embedded in a MoS2 and graphite matrix and demonstrates excellent low frictional properties.

Porous Si3N4 ceramics with surface roughness for bone repair

The repair and functional reconstruction of bone defects had become an urgent need and an important research topic. To repair bone defects, artificial bone materials must have an interconnected microporous structure to match the defect site with sufficient mechanical strength and good biocompatibility. To fulfill these requirements, a continuous porous Si3N4 ceramic with high strength and toughness, low friction and good biological properties was prepared. The particle-stabilized foam provided a controlled hierarchical porous structure, whereas the in situ growth of β-Si3N4 grains during sintering not only enhanced mechanical strength to meet practical applications in tissue engineering scaffolds but also changed the surface chemistry of pores to improve biocompatibility. These attractive properties promoted porous Si3N4 ceramics with advanced applications in bone defect repair.

Construction and Properties of High-Toughness Soft-Soft Interfaces Based on the Adhesion of Natural Polyphenols.

Artificial joint replacement is the most effective way to treat osteoarthritis. However, these artificial joints are too stiff with high interfacial contact stress and poor surface lubrication, resulting in stress shielding and severe wear and tear lead to an extremely high failure rate. At present, hydrogels are considered the most promising substitute for artificial joint prostheses owing to their good biocompatibility, adjustable mechanical properties, and excellent flexibility. Nevertheless, a traditional single-layer hydrogel has poor bearing capacity and lubrication, which are far from the properties of natural articular cartilage. The high strength and low friction properties of natural articular cartilage are based on its own multilayer fibrous structure. Therefore, by simulating the multilayer structure of natural cartilage, a bilayer bionic cartilage hydrogel was prepared; that is, the upper hydrogel realized excellent lubrication and the lower hydrogel realized high load-bearing capacity. However, the interface binding of bilayer hydrogels is a challenge at present. Therefore, the interfacial adhesion of the bilayer hydrogel is improved by adding tannic acid (TA) based on the adhesion of the natural polyphenol structure. The average interfacial toughness reaches 3650 J/m2, and the average interfacial shear force reaches 800 kPa. In the preparation of the bilayer hydrogel, taking advantage of the coordination reaction between TA and metal cations, Fe3+ is further added to endow the bilayer hydrogel with excellent mechanical properties and good sliding friction performance. Therefore, this work opens up a new way to construct cartilage-like materials with high toughness and a soft-soft interface.

Microgel-Modified Bilayered Hydrogels Dramatically Boosting Load-Bearing and Lubrication.

Hydrogel-based articular cartilage replacement materials are promising candidates for their potential to provide both high load-bearing capacity and low friction performance, similar to natural cartilage. Nevertheless, the design of these materials presents a significant challenge in reconciling the conflicting demands of the load-bearing capacity and lubrication. Despite extensive research in this area, there is still room for improvement in the creation of hydrogel-based materials that effectively meet these demands. Herein, a facile strategy is provided to realize simultaneously high load-bearing and low friction properties on the proposed hydrogel by modifying the surface of mechanically strong annealled PVA-PAAc hydrogel with a high hydration potential PAAm-co-PAMPS microgel. Consequently, a bilayer hydrogel with a porous surface and a compact substrate has been obtained. Compressive experiments confirmed that the bilayer hydrogel exhibited excellent mechanical strength with a compressive strength of 32.23 MPa at 90% strain. A high load-bearing (applied load up to 30 N), extremely low friction coefficiency (0.01-0.05) and excellent wear resistance (COF low to 0.03 after a 4 h test at 10 N using a steel ball as the contact pair) are successfully achieved. These findings provide new perspectives for the design of articular cartilage materials.

Preparation and Properties of PDMS Surface Coating for Ultra-Low Friction Characteristics.

Polydimethylsiloxane (PDMS) has excellent physical-chemical properties and good biocompatibility. Thus, PDMS has been widely applied in biomedical applications. However, the low surface free energy and surface hydrophobicity of PDMS can easily lead to adverse symptoms, such as tissue damage and ulceration, during medical treatment. Therefore, the construction of a hydrophilic low-friction surface on the PDMS surface could be helpful for alleviating patient discomfort and would be of great significance for broadening the application of PDMS in the field of interventional medical catheters. Existing surface modification methods such as hydrogel coatings and chemical grafting suffer from several deficiencies including uncontrollable thickness, surface fragility, and low surface strength. In this study, a hydrophilic surface with ultra-low friction properties was prepared on the surface of PDMS by an ultraviolet light (UV) curing method. The monomer acrylamide (AM) was induced by a photoinitiator to form a coating on the surface of the silicone rubber by in situ polymerization. The surface roughness of the as-prepared coatings was regulated by adding different concentrations of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to the monomer solution, and the coating properties were systematically characterized. The results indicated that the roughness and thickness of the as-prepared coatings decreased with increasing AMPS concentration and the as-prepared coatings had good hydrophilicity and low-friction properties. The Coefficient of Friction (CoF) was as low as 0.0075 in the deionized water solution, which was 99.7% lower than that of the unmodified PDMS surface. Moreover, the coating with a lower surface roughness exhibited better low-friction properties. The results reported herein provide new insight into the preparation of hydrophilic, low-friction coatings on polymer surfaces.

A Janus hydrogel material with lubrication and underwater adhesion

Double-layer hydrogel materials combine excellent hydrophilicity, outstanding lubricating properties, high load-bearing modulus, and good biocompatibility, and are often used to achieve high load-bearing (high modulus) and low frictional (low modulus) properties of articular cartilage, but which also makes it difficult for them to form efficient, stable and long-term adhesion underwater. Herein, we constructed a kind of novel underwater Janus hydrogel patches with strong adhesion performance by robustly anchoring mussel-protein-inspired poly (dopamine methacrylamide-co-methoxyethyl acrylate) p(DMA-co-MEA) copolymer onto the surface of poly(acrylic acid-co- acrylamide) p(AAc-co-AAm) hydrogel with bilayer modulus structure analogous to articular cartilage. Due to the coupling of the top lubrication layer and the bottom load-bearing layer, the constructed Janus hydrogel patch not only has fatigue resistance and low friction but also high load-bearing [coefficient of friction (COF) ∼0.0076, 5 N; COF∼0.048, 30 N]. Moreover, the Janus hydrogel also exhibits excellent bonding strength underwater (bonding strength ∼41.6 kPa, Fe substrate). Our strategy for constructing a novel underwater Janus hydrogel patch will provide valuable guidance for the realization of interface bonding of soft material of bionic articular cartilage and promote the practical application of tissue engineering.

Friction and wear of Ni3Al-based composites containing Ag and Cu modified hBN at elevated temperatures

Ni3Al-based composite containing Ag/Cu modified hBN nanosheets (Cu-hBN)/Ag-Cu-hBN have been fabricated by vacuum hot press sintering and their tribological performance has been evaluated at RT, 200, 400, 600 and 800 °C at a constant load of 10 N and a sliding speed of 0.2 m/s using a ball-on-disc rotary tribometer against a Si3N4 ball. The main aim of the study is to examine the occurrence of a synergetic action between Ag and Cu-hBN in attaining low friction and anti-wear properties from room temperature to 800 °C. The results indicated a significant reduction in the coefficient of friction by the addition of a combination of Ag and Cu-hBN in comparison to either Ag or Cu-hBN. Ni3Al-Ag-Cu-hBN attained the lowest coefficient of friction (∼0.19) and wear rate (1.85 x 10-5 mm3/Nm) at 800 °C, which has been ascribed to the supportive lubrication provided by Ag and Cu-hBN. The investigation also revealed that Ag provided lubrication at temperatures below 500 °C, whereas Cu-hBN and other lubricious oxides like Ag2MoO4, Ag2Mo2O7, MoO3, CuO and NiMoO4 provided low friction and anti-wear properties beyond 500 °C. Self-lubricating Ni3Al-Ag-Cu-hBN composites offer themselves as a potential candidate for high-temperature applications where the components work under relative sliding motion.

Molybdenum disulfide/carbon multilayer films achieve ultra‐low wear in vacuum

Although diamond‐like carbon (DLC) films are known for their low friction and wear properties in atmospheric environments, they commonly experience failure in vacuum environments. On the other hand, MoS2 exhibits a low friction coefficient under vacuum conditions, but its columnar structure limits its load‐bearing capacity and results in high wear rates. In this study, we prepared MoS2/DLC multilayer films using a high‐power impulse magnetron sputtering (HIPIMS) technique and examined the composition, bonding structure, mechanical properties, and frictional wear of the resulting films. The study findings revealed that the multilayer film exhibits a significantly low coefficient of friction (0.04), particularly in vacuum conditions (5 × 10−3 Pa). Remarkably, compared to the pure MoS2 film, the wear rate of the multilayer film is reduced by two orders of magnitude, wear rate as low as 3.6 × 10−9 mm3/Nm. Additionally, the DLC component enhances the hardness and reduces the wear rate of the multilayer film. Furthermore, the use of nanometer thickness (17 nm) allows for the incorporation of more MoS2 and DLC layers, which promotes the formation of graphene bands and further reduces the friction coefficient and wear rate. Our findings open new avenues for the application of MoS2 and DLC in vacuum environments.

Impact Resistance of CVD Multi-Coatings with Designed Layers

Coated cutting tools are widely used in the manufacturing industry due to their excellent properties of high heat resistance, high hardness, and low friction. However, the milling process is a dynamic process, so the coatings of milling tools suffer severe cyclic impact loads. Impact resistance is important for the life of milling tools. Multi-coatings with different layer thickness may influence their impact resistance, but few studies focus on this topic. In this study, CVD coating with a structure of TiN layer, Al2O3 layer, and TiCN layer was selected as the research objective. Four different CVD coatings with different layer thicknesses were designed and prepared. The impact resistance test method was proposed to simulate the impact due to cut-in during down the milling process. We obtained the load by setting an impact depth of 25/30/35 µm, recording the impact force during the impact process, and calculating the contact stress; it was found that, at the impact depth of 25/30/35 µm, the download loads were around 9/11/13 N, while the contact stresses were all around 1 GPa. The failure morphology of the coating surface was investigated after the impact process. By comparing the contact stress and the surface morphology of the designed coatings, the impact resistance of four kinds of designed CVD coatings were evaluated. Experiments have shown that an increase in coating thickness and total coating thickness reduces impact resistance by about 10%. The impact resistance of coating samples without a TiN surface layer also decreased by about 10%. When the surface layer of TiN was thinner than 1 µm, the surface layer of TiN was prone to chipping and peeling off. Decreasing the thickness of the middle layer of Al2O3 and increasing the thickness of the inner layer of TiCN obviously lowered the impact resistance.

Effects of deposition temperature on the wear behavior and material properties of plasma enhanced atomic layer deposition (PEALD) titanium vanadium nitride thin films

This work investigates the effects of deposition temperature on the wear behavior and material properties of recently developed plasma enhanced atomic layer deposited (PEALD) TiVN films. ∼50–100 nm thick TixV1-xN (x ~ 0.5, hereto referred to as TiVN) films were deposited using PEALD on Si substrates with thermal oxide at a range of deposition temperatures (150 °C, 200 °C, 250 °C, 300 °C, 350 °C). Wear testing was performed on each film using a linearly reciprocating tribometer in a controlled humidity environment. Wear rates of the TiVN films varied with deposition temperature, with the 250 °C sample achieving ultralow wear(5.4 × 10−7 mm3/Nm), while the 150 °C sample wore through the film completely in early sliding cycles. Along with their low wear properties, these PEALD TiVN films were found to have low electrical resistivity when deposited above 150 °C. Film density and crystallite size increased with increasing deposition temperature up to 250 °C, where these properties plateaued. High compressive residual stresses were measured, ranging from 2.7 to 7.7 GPa. XRD Bragg peak intensities showed that films with increasing deposition temperatures had higher degrees of crystallinity. XPS indicated that above 150 °C deposition temperatures, impurities in the film were reduced by ∼4x. Higher deposition temperatures allow for increased adatom mobility, which can lead to higher densities and crystallinity, which are typically associated with lower wear rates. At deposition temperatures below 200 °C, precursor reactivity is sluggish and there is insufficient energy for complete surface reactions to occur, leading to film contamination and less dense films. The subtle differences in wear rates above 200 °C deposition temperature indicated that there are competing material properties that can either contribute to or detract from the wear behavior of the film, and a combination of desirable mechanical, physical, structural, and chemical properties are required for ultra-low wear rates to be achieved. The low wear and friction properties and low electrical resistivity of these films combined with the low deposition temperature, conformality, and atomic layer thickness control of PEALD make them potential candidates for applications such as thermally sensitive microelectronics and MEMS/NEMS.