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Drought poses a severe threat to terrestrial ecosystems' carbon sequestration function, which in turn affects regional sustainable development. Understanding its impact on carbon source/sink dynamics is crucial for addressing extreme climate change challenges. This study, based on the Carnegie-Ames-Stanford approach (CASA) model and soil heterotrophic respiration equation, the Net Ecosystem Productivity (NEP) of the middle reaches of the Yellow River Basin (MRYRB) from 2001 to 2019 were estimated and utilized to characterize the vegetation carbon source/sink. Additionally, four factors, namely, average surface temperature (AST), root-zone soil moisture (RM), drought frequency (DF), and drought intensity (DI), were selected and a multiscale geographically weighted regression (MGWR) model was applied to assess the dominant factors influencing vegetation carbon source/sink dynamics at the spatial scale. The results indicated that: (1) MRYRB vegetation acted as a carbon sink, showing an increasing trend. Carbon sink was primarily located in Shaanxi's central region, while carbon source was primarily located in Inner Mongolia and Shaanxi's northern part. (2) Both the extent and severity of drought in the MRYRB showed a mitigating trend, presenting a spatial distribution of high-intensity low-frequency and low-intensity high-frequency. (3) AST, RM, DF, and DI had larger effects on carbon source than carbon sink. AST dominated mainly in northern Shanxi and Shaanxi, RM in central Shaanxi and Gansu, while DI and DF were concentrated in central Shaanxi and northern Shanxi. [ABSTRACT FROM AUTHOR]

Herein, the analytic hierarchy process (AHP) and central composite experimental design in response surface methodology (CCD-RSM) are used to evaluate and optimize the tribological parameters of bionic textured AISI 4140-SnAgCu self-lubricating composite under the conditions of load (L: 20–40 N), velocity (V: 16–32 mm/s), and distance (D: 40–80 m). AISI 4140 biomimetic composites exhibit excellent tribological properties under the conditions of L27.15/V16.43/D40, L27.05/V17.08/D60, and L27.42/V17.85/D80, respectively. The sensitivities of influence factors on the tribological overall score are load, velocity, and distance, in order from high to low. The good antifriction properties of self-lubricating composites under conditions with the optimized tribological parameters benefit from the fact that the solid lubricants are uniformly spread at the contact interface. [ABSTRACT FROM AUTHOR]

Inspired by the surface texture of bivalve shell, bionic wavy grooves were designed and prepared on the bionic coating of TC4 using laser marking technology. A response surface method was used to optimize the parameters of bionic texture structure for improving the tribological behavior. The influence of coupling mechanisms of bionic coating and texture on the tribological performance of TC4 alloy was investigated. The results show that the bionic wavy grooves reduce the fluctuation of loading force. The coupling of bionic texture and coating breaks the continuity of coating interface, and the peeling wear debris is easily captured by the bionic wavy grooves. The repairing effect of solid lubricant is increased, so the complete lubrication film can be formed more easily on the worn surface during the sliding process. [ABSTRACT FROM AUTHOR]

To expand the applications of MXene-Ti3C2 in the field of oil lubrication, rounded groove surfaces filled with Sn-Ag-Cu-Ti3C2 on CSS-42L bearing steel (RGSs-SACT-600) were prepared. Block-on-ring friction and vibration tests under oil lubrication were carried out. The results showed that the synergistic effects of Sn-Ag-Cu-Ti3C2 and grooves improved the variation characteristics of friction forces and reduced friction and vibration on the surfaces. Sn-Ag-Cu-Ti3C2 with excellent adsorption to lubricating oil suppressed the oil film rupture and improved the heat dissipation at the friction interface. The tribochemical reactions were induced by Sn-Ag-Cu and Sn-Ag-Cu-Ti3C2, improving the tribological properties on the worn surfaces. Under low speed and high load, RGSs-SACT-600 with uniform profiles had excellent bearing capacity and antifriction performance. [ABSTRACT FROM AUTHOR]

Based on a functionally graded material (FGM) design concept, the laminated-graded Ag-multilayer graphene/TC4 alloy self-lubricating composites (GTMAC) were prepared to overcome the poor friction and wear behaviors of TC4 alloy and expand engineering applications where friction is involved. Tribological experiments on GTMAC were carried out under different temperatures and loads in contrast with TC4 alloy (TC4) and homogeneous Ag-multilayer graphene/TC4 alloy self-lubricating composites (TMAC). The results show that GTMAC presents better friction and wear behaviors compared with TC4 and TMAC over a broad range of temperatures and loads. This is because of the synergistic effect of Ag and multilayer graphene, which are crushed and spread out in the sliding process, resulting in the formation of a lubricating film on the worn surface. Additionally, the equivalent stress distributions on the friction surfaces of TMAC and GTMAC are investigated using a finite-element method. The simulation results indicate that the GTMAC sample model with a smaller equivalent stress can avoid the occurrence of cracks and peeling of the surface material, so the the lubricating film is formed easily and remains for a longer working time. [ABSTRACT FROM AUTHOR]

To improve the tribological properties and suppress the friction-induced noise of Ti-6Al-4V alloys (TC4), TC4 with smooth surfaces (TSs), groove antifriction surfaces (GAs), Sn-Ag-Cu coating surfaces (SCs), and composite self-lubricating surfaces (CSs) with grooves and Sn-Ag-Cu coating were prepared. Experimental studies on the tribological properties and friction-induced noise were carried out. The results showed that TSs produced high-frequency scream for adhesive wear. GSs suppress the scream noise but induce low-frequency chattering. CSs with uniform profiles and smaller wrinkles showed tribochemical wear characteristics, which produced the various intermetallic compounds and oxides, improving the surface profiles and suppressing the microdefects. The synergistic effects of grooves and Sn-Ag-Cu coating on CSs reduce the high contact stresses of groove edges and suppressed the adhesive wear and friction-induced noise. The friction and noise reduction mechanisms of CSs are beneficial to obtain surfaces with the best antifriction performance and bearing capacity by improving the profiles. [ABSTRACT FROM AUTHOR]

This study aimed to explore the possibility of improving the tribological performance of NiAl matrix composites by graphene addition. Friction and wear experiments of as-prepared specimens were conducted under different conditions using a pin-on-disk wear testing machine. NiAl matrix composites containing graphene showed satisfactory performance in friction coefficient and wear resistance compared to NiAl matrix composites without graphene. For the active effect of graphene, the friction coefficient and wear rate of NiAl matrix composites were maintained at relatively lower values. The beneficial antifriction and antiwear effects of graphene gradually failed when the applied load was above 8 N. Graphene in NiAl matrix composites played an active role in the formation of a friction layer, which was beneficial to the lower friction coefficient and wear rate. In light of this research, graphene plays an active role in reducing the friction coefficient and wear rate. Hence, graphene has great potential in applications as an effective solid lubricant to promote tribological behavior. [ABSTRACT FROM AUTHOR]

A favorable hardness ratio (Hdisk/Hpin = Hd/Hp) could lead to a transition to mild wear during sliding contact. To determine a more appropriate Hd/Hp value for the sliding wear, the dry sliding pin-on-disk wear tests of Ni3Al matrix composites (NMCs) with multilayer graphene (MLG) are undertaken at Hd/Hp values of 0.99, 0.83, 0.42, and 0.35 at sliding speeds of 0.1, 0.3, 0.5, and 0.7 m/s. It is found that the tribological properties of NMCs are strongly affected by the various hardness ratios. At 0.1 m/s, the friction coefficient decreases with a decrease in Hd/Hp value. The low friction coefficient is 0.14 and the wear rate is 0.9 × 10-5 mm3 N-1m-1 under the ceramic counterpart with Hd/Hp of 0.35. At 0.7 m/s, the tribological properties show the opposite trend with a decrease in Hd/Hp. At an Hd/Hp of 0.35, the smooth compact layer on the worn surface could decrease the friction at 0.1 m/s, and the improved hardness in the subsurface by strain hardening would play an important role in the improvement of wear resistance. Under the metal counterpart with Hd/Hp of 0.99, plastic deformation only occurs on the contact surface and the MLG could suppress further shear deformation in the subsurface, leading to a low wear rate (2.4 × 10-5 mm3 N-1m-1) and friction coefficient (0.15) at 0.7 m/s. [ABSTRACT FROM AUTHOR]

The friction and wear behavior of TiAl matrix self-lubricating composites (TMSCs) with MoO3tabular crystals (MTCs) sliding against a GCr15 steel ball is tested using a constant speed of 0.2 m/s at room temperature under different loads from 6.65 to 16.65 N. The result reveals that TMSCs show a consistently lower friction coefficient in a certain range from 0.2 to 0.6 and less wear rate from 0.29 × 10−4mm3N−1m−1to 0.49 × 10−4mm3N−1m−1compared to TiAl-based alloy. Moreover, the friction coefficient and wear rate of TMSCs decrease with an increase in test load. MTCs in the deformed layer will be refined to produce interfacial shear slip and reduce the shear stress because of the weak binding force of MTCs in the sliding process, which can facilitate the formation of a deformed layer and protect the deformed layer from spalling failure. In addition, MTCs on the worn surface of TMSCs can reduce the shear stress directly. Hence, MTCs can promote antiwear of the deformed layer and reduce the friction on the worn surface of TMSCs. MTCs can play a better role in antiwear and antifriction when the test load is higher. [ABSTRACT FROM AUTHOR]

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti3SiC2 in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti3SiC2-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti3SiC2 as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti3SiC2 is an effective way to achieve and maintain desired tribological properties over a wide temperature range. [ABSTRACT FROM AUTHOR]

In this study, the tribological properties of NiAl matrix composites with 1.5 wt% graphene nanoplatelets (NAG) at elevated temperatures were simulated and tested. NAG exhibits excellent characteristics at 100 and 200°C due to the formation of metal oxides and graphene nanoplatelets lubrication film. Furthermore, the removed layer thickness, the stress distributions, and the high stresses of the affected layer have been estimated theoretically. At 400°C, the friction coefficient increased due to the absence of the lubrication layer. In addition, the wear rate increased due to the excessive stresses, the increased layer thickness removal, and the propagation of subsurface cracks. [ABSTRACT FROM PUBLISHER]

Studies have been carried out to explore the friction and wear behaviors of NiAl matrix self-lubricating composites containing graphene nanoplatelets (NG) against an Si3N4 ball from 100 to 600°C with a normal load of 10 N and a constant speed of 0.2 m/s. The results show that NG exhibits excellent tribological performance from 100 to 400°C compared to NiAl-based alloys. A possible explanation for this is that graphene nanoplatelets (GNPs) contribute to the formation of a friction layer, which could be beneficial to the low friction coefficient and lower wear rate of NG. As the temperature increases up to 500°C, the beneficial effect of GNPs on the tribological performance of NG becomes invalid due to the oxidation of GNPs, resulting in severe adhesive wear and degradation of the friction layer on the worn surface of NG. GNPs could hold great potential applications as an effective solid lubricant to promote the formation of a friction layer and prevent severe sliding wear below 400°C. [ABSTRACT FROM AUTHOR]

The friction layer structure has been proved to be formed during severe plastic deformation and markedly improves the tribological properties of material. The dry friction and wear performance of graphene-reinforced TiAl matrix self-lubricating composites (GTMSC) at different sliding velocities are systematically researched. GTMSC show the best tribological properties and special friction layer structure containing a wear-induced layer and a grain refinement layer with a nanocrystalline (NC) structure under surface after sliding at a sliding speed of 1.1 m/s. Nanoindentation results show that the grain refinement layer has a higher hardness and elastic modulus than the wear-induced layer. This special microstructure of friction layers beneath the surface after sliding leads to a low coefficient of friction and high wear resistance of GTMSC. Moreover, it is deduced that the appearance of an NC structure results in hardening of the material. The formation mechanisms of friction layers are researched in detail. It can be concluded that the formation of a wear-induced layer results from frictional heat and fracture of the counterpart. The formation of a grain refinement layer is due to severe plastic deformation and dynamic recrystallization. Severe plastic deformation results in the formation of an NC structure and dynamic recrystallization leads to grain refinement. [ABSTRACT FROM PUBLISHER]

The tribological behaviors of the Ni3Al-WS2-Ti3SiC2composites and synergistic effect of composite solid lubricants are investigated from room temperature to 800°C. The results show that the Ni3Al matrix composites (NASC) exhibit excellent tribological properties throughout the test temperatures compared to Ni3Al-based alloy. The excellent tribological properties of NASC are attributed to the synergistic lubricating effect of WS2and Ti3SiC2. A lubricating film could be formed below 400°C, and an oxidation protection film is formed above 400°C on the worn surface of NASC during the sliding process, leading to low coefficients of friction (0.18–0.39) and wear rates (1.5–3.7 × 10−5mm3N−1m−1). [ABSTRACT FROM AUTHOR]

Ni3Al-MoS2-TiC composites (NMTC) were fabricated using spark plasma sintering. The effect of TiC content on tribological properties and antiwear mechanisms of NMTC at high temperatures was investigated. The results showed that NMTC containing 10 wt% MoS2 and 6 wt% TiC exhibited the excellent tribological properties from 25 to 600°C. At 400°C, NMTC had the lowest friction coefficient of 0.28 and the considerable lower wear rate of 3.02 × 10−5 mm3 N−1 m−1. The excellent tribological properties of NMTC were attributed to the synergetic action of the lubricant-phase MoS2 and reinforcing-phase TiC. [ABSTRACT FROM AUTHOR]

The tribological properties of Ti3SiC2/TiAl composite (TTC) slid against a GCr15 steel counterface were investigated at sliding speeds in the range of 0.2–0.8 ms−1and loads that ranged from 2 to 8 N. The results showed that the tribological properties of TTC strongly depended on the covering percentage of tribofilm on the TTC worn surface, which varied with changes in sliding speed and load. The tribofilm mainly consisted of Al-Ti-Si oxides, which provided a self-antifriction action that resulted in a reduction in the friction coefficient and an increase in the wear rate. [ABSTRACT FROM AUTHOR]

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS2 (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti3SiC2 (a high-temperature lubricant, above 600°C) designated as MhT against an Si3N4 counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS2, hBN, and Ti3SiC2. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range. [ABSTRACT FROM AUTHOR]

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite. [ABSTRACT FROM PUBLISHER]

The tribological behaviors of NiAl-Ti3SiC2composites (NMC) against Si3N4at elevated temperatures were investigated by carrying out dry sliding wear tests at the condition of 10 N–0.2 m/s. The results showed that NMC without Ti3SiC2had a high friction coefficient and wear rate at elevated temperatures, and the addition of Ti3SiC2could improve the high-temperature tribological properties. Both the friction coefficient and wear rate of NMC obviously decreased at 600 and 800°C with the addition of Ti3SiC2. The excellent tribological behaviors of NMC with 5 wt% Ti3SiC2at elevated temperatures implied that 5 wt% Ti3SiC2could be the optimal content. [ABSTRACT FROM AUTHOR]

TiAl matrix self-lubricating composites (TMC) with various weight percentages of Ti3SiC2and MoS2lubricants were prepared by spark plasma sintering (SPS). The dry sliding tribological behaviors of TMC against an Si3N4ceramic ball at room temperature were investigated through the determination of friction coefficients and wear rates and the analysis of the morphologies and compositions of wear debris, worn surfaces of TMC, and the Si3N4ceramic ball. The results indicated that TMC with 10 wt% (Ti3SiC2-MoS2) lubricants had good tribological properties due to the unique stratification subsurface microstructure of the worn surface. The friction coefficient was about 0.57, and the wear rate was 4.22 × 10−4mm3(Nm)−1. The main wear mechanisms of TMC with 10 wt% (Ti3SiC2-MoS2) lubricants were abrasive wear, oxidation wear, and delamination of the friction layer. However, the main wear mechanisms of TMC without Ti3SiC2and MoS2lubricants were abrasive wear and oxidation wear. The continuous friction layer was not formed on the worn surfaces. The self-lubricating friction layer on the frictional surface, different phase compositions and hardness, as well as density of TMC contributed to the change in the friction coefficient and wear rate. [ABSTRACT FROM AUTHOR]