Modeling feedback processes between soil detachment and sediment transport along hillslopes on the Loess Plateau of China.

A dynamic model of soil erosion along hillslopes considering soil detachment and sediment transport is still a major challenge in terms of its applicability to field conditions. Data availability for model calibration and validation is very limited for physically process models. An improved hillslope erosion dynamic model (HED) with a simple structure and strong application on field plots was established based on the sediment feedback mechanism in this study. Observed runoff and sediment data from field plots with slope gradients of 14.1 %-62.5 % and slope lengths of 7.9-64.7 m within runoff events for the Chagagou catchment on the Loess Plateau of China were used to evaluate the HED. We confirmed that the power function can reproduce the soil detachment capacity (Φ) and sediment transport capacity (Tc) under varying field slope conditions (gradients and lengths). The two parameters associated with the power function of Φ or Tc are consistent across the variable conditions. When the HED model simulates the process and event sediment discharge, the unified model parameters could be obtained. The simulation precision of above results ranged from 0.44 to 0.95 for Nash-Sutcliffe simulation efficiency (NSE), from 0.65 to 0.96 for R2 and from -32.37 % to 31.61 % for relative error (RE). The feedback term of HED was close to one as the slope length approached zero. Decreasing of sediment yield as slope length increased was quite consistent with the measured data due to the reasonable sediment feedback term. The critical slope lengths were more easily reached, and the feedback term value along the slope decreased more rapidly at steeper slopes and higher runoff events. HED can be further integrated into distributed watershed models for predicting sediment discharge within runoff events.

Role of tribochemistry reactions of B4C on tribofilm growth at a PEEK-steel interface in simulated body fluids.

Corrosion and wear products of metallic implants can lead to severe adverse tissue reactions. However, there is an absence of effective means to reduce the tribocorrosion of metal. The main purpose of this study is to reveal a mechanism of engineering a barrier layer on metal surfaces via adding functional particles into the polymer counterpart. B4C and BN particles were compounded into a polyetheretherketone (PEEK) matrix and their tribological performance of PEEK-based composites sliding against stainless steel was compared in simulated body fluid. Results demonstrate that the addition of B4C reduces significantly friction and wear. In particular, the addition of only 1 vol% B4C reduces wear of PEEK by up to 94.8%, and tribocorrosion of steel is also obviously mitigated. It is discovered that hydrolysis of B4C particles triggered by friction and deposition of Ca2+ and phosphate ions dominate formation of the barrier layer at the friction interface. The barrier layer endows the PEEK-metal sliding pair simultaneously enhanced anti-wear and anti-corrosion performance.

Tuning the tribofilm nanostructures of polymer-on-metal joint replacements for simultaneously enhancing anti-wear performance and corrosion resistance.

Total joint replacement is currently the most successful clinical treatment for improving the life quality of individuals afflicted with end-stage osteoarthritis of knee or hip joints. However, release of wear and corrosion products from the prostheses is a critical issue causing adverse physiological responses of local issues. ß-SiC nanoparticles were dispersed into polyetheretherketone (PEEK) materials and their role in tribocorrosion performance of PEEK-steel joints exposed to simulated body fluid was investigated. It is demonstrated that ß-SiC nanoparticles increase greatly the wear resistance of the PEEK materials, and meanwhile mitigate significantly corrosion of the steel counterpart. It is revealed that tribochemical reactions of ß-SiC nanoparticles promoted formation of a robust tribofilm having complex structures providing protection and shielding effects. The present work proposes a strategy for developing high-performance polymer-on-metal joint replacement materials of enhanced lifespan and biocompatibility via tuning interface nanostructures. STATEMENT OF SIGNIFICANCE: Adverse tissue responses to metal wear and corrosion products from metal base implants remain a challenge to surgeons and patients. We demonstrated that leaching of metal ions and release of metallic debris are well decreased via tuning interface nanostructures of metal-polymer joint bearings by dispersing ß-SiC nanoparticles into polyetheretherketone (PEEK). It is identified that the addition of ß-SiC greatly improves the tribological performances of the PEEK materials and mitigated corrosion of the steel. Tribo-chemistry reactions of SiC induce the formation of complex structures which provide protection and shielding effects. Nanostructures of the tribofilm were also comprehensively investigated. These novel findings proposed a potential route for designing high performance metal-polymer joint replacement materials.

Switching Brake Materials To Extremely Wear-Resistant Self-Lubrication Materials via Tuning Interface Nanostructures.

Tribological performance of motion components is one of the key aspects that must be considered in a wide range of applications such as vehicles, aircrafts, and manufacturing equipment. This work demonstrates that further addition of only low-loading hard nanoparticles into a formulated nonasbestos organic brake material directly switches its functionality to a self-lubrication material. More importantly, the newly developed nanocomposites exhibit an extremely low wear rate. Comprehensive investigations on the friction interface reveal that the great friction and wear reduction are due to the formation of a nanostructured lubricious tribofilm. Tribofilm formation is continuously fed by complex molecular species released from the bulk nanocomposites, for which nanoparticles digested within the tribofilm greatly enhance its robustness and lubricity. This work gains insight into the crucial role of the interface nanostructure and paves a route for developing extremely wear-resistant self-lubrication composites for numerous applications.

Significance of an in-situ generated boundary film on tribocorrosion behavior of polymer-metal sliding pair.

Polymer composites have a high potential for applications as tribo-materials exposed to sea water owing to their self-lubrication characteristic and high chemical stability. In the present work, tribological behaviors of polyetheretherketone (PEEK) composites rubbing with stainless steel in sea water were explored using a pin-on-disc tribometer integrated with a potentiostat for electrochemical control. It was demonstrated that further adding 5 vol% hexagonal boron nitride (h-BN) nanoparticles into PEEK reinforced with short carbon fibers (SCF) significantly enhanced the wear resistance. Moreover, the stainless steel exhibited significantly enhanced tribocorrosion resistance when rubbing with the hybrid nanocomposite, in comparison to the sliding against PEEK filled only with SCF. Nanostructures of the boundary films formed on the steel surface were comprehensively investigated. It was manifested that tribo-chemistry products of h-BN, i.e. H3BO3 and B2O3, were arrayed in a closely packed boundary film. It seems that inclusion of layer-structured H3BO3 and B2O3 improved the resilience of the boundary film. The continuous boundary film covering the steel surface provided a lubrication effect and strengthened the passivation layer. A new route for enhancing simultaneously tribological and corrosion resistance of polymer-metal pairs by controlling in-situ tribo-chemistry was thus proposed.

Soft/Hard-Coupled Amphiphilic Polymer Nanospheres for Water Lubrication.

Amphiphilic polymer nanospheres of poly(3-sulfopropyl methacrylate potassium salt- co-styrene) [P(SPMA- co-St)] were prepared by a simple soap-free emulsion polymerization method and used as efficient water lubrication additives to enhance the antiwear behaviors of the Ti6Al4V alloy. The monodisperse and flexible P(SPMA- co-St) bicomponent copolymer nanospheres were synthesized with a controllable manner by adjusting the mass fraction ratio of the monomers, with the hydrophobic polystyrene (PSt) as the hard skeletal carrier component and the hydrophilic PSPMA with a hydration layer structure as the soft lubrication layer in the course of friction. The influences of the monomer concentration, the copolymer nanosphere additive content, the load, and the frequency of the friction conditions on their tribological properties were studied in detail, and a probable antiwear mechanism of the soft/hard-coupled copolymer nanospheres under water lubrication was also proposed. The results show that compared with pure PSt, the P(SPMA- co-St) polymer nanospheres exhibited better antiwear property as an additive for water lubrication, and the friction coefficient and the wear volume first decreased and then increased with the increase of the SPMA content, indicating that the hydrophilic SPMA has a significant effect on lubrication properties owing to its hydration performance. Furthermore, with the increase of polymer nanosphere concentration, the friction coefficient and wear amount also decreased to a stable and low value at a saturation concentration of 1 wt %. The flexible polymer nanospheres with a hydrophilic soft SPMA shell and a rigid PSt core exhibited good friction-reduction and antiwear performance as lubrication additives, indicating their promising and potential applications in water lubrication and biological lubrication.

Comparative study of tribochemistry of ultrahigh molecular weight polyethylene, polyphenylene sulfide and polyetherimide in tribo-composites.

HYPOTHESIS: Tribochemical reactions of polymer matrix with steel counterpart can exert an important role in tribofilm's structure, and thereby affect the tribological performance of its composites. EXPERIMENTS: In the present work, tribochemistry of ultrahigh molecular weight polyethylene (UHMWPE), polyphenylene sulfide (PPS) and polyetherimide (PEI) in tribo-composites was comparatively studied. Two kinds of formulations, i.e. conventional composites filled with carbon fibers and graphite and hybrid nanocomposites containing additional silica (SiO2) nanoparticles, were investigated. FINDINGS: It was revealed that when rubbing with steel, molecular chains of UHMWPE were broken and free radicals finally chelated with the counterface. Whereas, PPS molecules underwent thermal decomposition, oxidation and finally ferrous sulfide (FeS) and ferric sulfate [Fe2(SO4)3] were generated. Chelating reaction of PEI was identified only for sliding of PEI-based nanocomposite. Tribochemistry of polymer molecules played an important role in tribofilm formation and tribological performance of the conventional composites. When sliding took place with the hybrid nanocomposites at low pv conditions, tribochemistry of polymer molecules played a similar role as for the conventional composites. Nonetheless, at high pv conditions, independent on the polymer matrices, robust tribofilms containing high fraction of silica were generated on the steel counterface, minimizing direct rubbing of the friction pair.

Comparative study on tribological mechanisms of polyimide composites when sliding against medium carbon steel and NiCrBSi.

Tribological behaviors of various polyimide (PI) composites when rubbing with medium carbon steel (MCS35) and NiCrBSi, were comprehensively investigated. When the conventional PI composite filled with carbon fibers and graphite was concerned, the carbon-based tribofilm formed on NiCrBSi surface resulted in obvious friction- and wear-reduction. However, no lubricating tribofilm was formed on MCS35 surface. Chelation of polymeric molecular radicals with the metallic counterparts was identified on the worn surfaces. Theoretical calculations corroborated that the Ni-based metal-organic compound showed a higher stability than the Fe-based one. With respect to the sliding of the hybrid nanocomposites containing silica or h-BN nanoparticles, the nanoparticles released onto the interface significantly mitigated tribo-oxidation of metallic counterparts, and were finally tribo-sintered into a compact layer after being mixed with remnant polymer and tribo-oxidation products. The effect of counterpart material was less pronounced for the tribological mechanisms of the nanocomposites than for the conventional composite.