Steady Friction Research Articles

Effect of B/N dual doping on mechanical and tribological properties of Mo-S-B-N sputtered films

Doping with non-metal element is effective to improve the porous morphology and enhance the mechanical and tribological properties of transition metal dichalcogenides (TMDs) based film. However, the mechanical properties of TMDs based film with single additive should be enhanced further for better wear resistance. In this work, B/N dual-doping was used to further enhance the deformation resistance and toughness of MoS2 based films deposited by magnetron sputtering, while the effect of B/N dopants on the structure, mechanical and tribological properties has been investigated. Mo-S-B-N film with B content of 20.91 at. % and N content of 18.13 at. % consists of MoS2 and amorphous nitride/boride, in which B/N dual doping film forms a higher ordered MoS2 lamellar structure compared to B doping film. With a hardness of 11.6±0.9 GPa and improved toughness, the film achieves a low wear rate and average friction coefficient is achieved. Comparing B/N dual doping film to B doping film, the stable formation of tribolayer with more strengthened deformation resistance results in the steady friction and the improved wear resistance, while the limited TMDs reorientation leads to the slight increase of friction coefficient.

Godunov-type scheme for air–water transient pipe flow considering variable heat transfer and laboratorial validation

A robust comprehensive energy dissipation model is developed to investigate the rapid filling process of a T-shaped bifurcated pipeline with entrapped air pocket, while an experimental system is designed to validate the numerical model. In this work, a self-adapting heat transfer model is proposed to describe the energy exchange during transient event, fully considering the heat transfer of entrapped air pocket and hydraulic losses caused by steady friction and unsteady friction in the section of the filling water column. Importantly, the related heat transfer coefficient is variable and determined by mechanism formula of media characteristics, which is obviously different from the constant heat transfer coefficient in conventional model relying on the trial-and-error method and experimental data. Moreover, the second-order Godunov-type scheme is introduced to solve the governing equations of filling water column, while a virtual plug approach is proposed to track the air–water interface. The resulting predictions are compared to those obtained via a conventional empirical polytropic model and constant coefficient heat transfer model, and to experimental data. The proposed model accurately reproduces the experimental pressure oscillations. The results display that when an air pocket content exceeds a certain threshold (>1.7%), the comprehensive model using forced convection can reproduce the pressure fluctuations. When the air content is lower (<0.9%), the comprehensive model using natural convection can simulate pressure changes more accurately. The conventional empirical polytropic model underestimates the pressure damping. The constant coefficient heat transfer model previously cannot be employed if there is no experimental data to calibrate the constant coefficient, but now the constant coefficient can be obtained from the results of the proposed model. Significantly, the comprehensive model can directly and accurately simulate the energy dissipation during the rapid filling process.

Propagation Characteristics of Initial Compression Wave Induced by 400 km/h High-Speed Trains Passing through Very Long Tunnels

When high-speed trains enter tunnels, an initial compression wave is generated. As the compression wave propagates at the local speed of sound to the tunnel exit, it radiates into the surrounding environment, forming micro-pressure waves (MPWs). MPWs create sonic booms, resulting in significant environmental issues. The magnitude of the micro-pressure waves is directly proportional to the pressure gradient of the compression wave at the tunnel exit. The nonlinear effects of the initial compression wave during propagation lead to a significant increase in pressure gradient. Therefore, the propagation characteristics of the initial compression wave during the tunnel are the crucial factor affecting the amplitude of MPWs. Based on the one-dimensional compressible unsteady non-isentropic flow model and the improved generalized Riemann variable characteristic method, this paper researched the propagation and evolution characteristics of an initial compression wave generated when 400 km/h high-speed trains enter tunnels with three portal shapes: (no tunnel entrance hood (no hood), an oblique, enlarged tunnel entrance hood (type A), an enlarged equal-section non-uniform opening hole tunnel entrance hood (type B)). The results show that when the initial compression wave propagates inside very long tunnels, the pressure gradient of the compression wave exhibits a trend of initially increasing and then decreasing with the increase in propagation distance. When the pressure gradient of the compression wave reaches its maximum value, the corresponding propagation distance is the steepening critical distance. For no tunnel entrance hoods, type A tunnel entrance hoods, and type B tunnel entrance hoods, the steepening critical distances are 5 km, 6 km, and 16 km, respectively. The steepening critical distance shortens with increasing train speed. Steady friction and unsteady friction effects mainly affect the pressure amplitude and pressure gradient during compression wave propagation, respectively. At lower ambient temperatures, the nonlinear effects in compression wave propagation are significantly enhanced. The mitigation effects of type A tunnel entrance hoods and type B tunnel entrance hoods on pressure gradient reduction are mainly concentrated within 4 km and 12 km, respectively. It is necessary to determine the optimal matching relationship between the tunnel entrance hood and tunnel length based on the characteristics of compression wave propagation to ensure their mitigating performance is maximized.

Tribological behaviour of TiAlN and AlCrN coatings on stainless steel

TiAlN and AlCrN coatings are widely used in applications that require high stress resistance. In this work, the tribological behaviour of physical vapour deposition (PVD) TiAlN and AlCrN commercial coatings deposited on AISI martensitic stainless steel is studied. Microstructure of the coatings was analysed by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) and nanohardness was measured. Pin-on-disk and abrasive wear tests were performed. Adhesion was evaluated using Rockwell C Indentation and scratch test. The thickness of both coatings was approximately 3 µm. AlCrN lost 30 and 10 times less volume than TiAlN in pin-on-disk tests, under low and high loads, respectively. The steady friction coefficient value was also lower. This indicated that the AlCrN coating had a better performance under sliding conditions. On the other hand, the mass loss was similar for both coatings under abrasive wear, even under severe conditions. In the scratch tests, TiAlN coating failed at 60 N load and AlCrN coating at 70 N, the latter showing a higher value of critical load. The deformation was similar for both coatings as it could be observed in the profiles obtained by a mechanical profilometer at 60 N, however, AlCrN did not show film delamination. This enhanced performance can be attributed to higher fracture toughness and load carrying capacity, which not only improved the mechanical properties of the coating but also its adhesion to the stainless-steel substrate.

The evolution of isolated cavities and hydraulic connection at the glacier bed – Part 1: Steady states and friction laws

Abstract. Models of subglacial drainage and of cavity formation generally assume that the glacier bed is pervasively hydraulically connected. A growing body of field observations indicates that this assumption is frequently violated in practice. In this paper, I use an extension of existing models of steady-state cavitation to study the formation of hydraulically isolated, uncavitated, low-pressure regions of the bed, which would become flooded if they had access to the subglacial drainage system. I also study their natural counterpart, hydraulically isolated cavities that would drain if they had access to the subglacial drainage system. I show that connections to the drainage system are made at two different sets of critical effective pressure, a lower one at which uncavitated low-pressure regions connect to the drainage system and a higher one at which isolated cavities do the same. I also show that the extent of cavitation, determined by the history of connections made at the bed, has a dominant effect on basal drag while remaining outside the realm of previously employed basal friction laws: changes in basal effective pressure alone may have a minor effect on basal drag until a connection between a cavity and an uncavitated low-pressure region of the bed is made, at which point a drastic and irreversible drop in drag occurs. These results point to the need to expand basal friction and drainage models to include a description of basal connectivity.

Wettability and Frictional Studies of PEEK Composites against Co-Cr Alloys with Surface Textures.

With the aim of promoting the qualities for total hip joint replacement, the wettability and tribological behaviors of PEEK composites pins with two sets of different fillers (PEEK/CF or PEEK/CF/PTFE/graphite) against Co-Cr alloy discs with five categories of surface textures (polished, orthogonal, spiral, r-θ, and orthogonal combined with spiral) were explored. It is revealed that the existence of CF in PEEK matrix increases the hydrophilicity in addition to the strength of PEEK, while the addition of PTFE increases the hydrophobicity of PEEK. The Co-Cr alloy discs with hydrophilic properties can be adjusted as hydrophobic, with the depth of textured grooves exceeding the critical sag height determined by the contact angle and the groove width. It can be concluded that PEEK/CF/PTFE/graphite composite has a lower wear rate than PEEK only reinforced with CF against Co-Cr alloy, both without surface texture and with shallow or deep grooves. The existence of shallow grooves on the disc surface could help the PEEK blends to achieve a steady friction against Co-Cr alloy in addition to collecting the worn debris. PEEK blend pins with 10 vol% CF, 10 vol% PTFE and 10 vol% graphite can achieve a lower friction coefficient of no more than 0.2 against Co-Cr alloy discs with shallow grooves around 3.5 μm in orthogonal or spiral textures.

Finite Volume Method for Transient Pipe Flow with an Air Cushion Surge Chamber Considering Unsteady Friction and Experimental Validation

In various water transmission systems such as long-distance water transfer projects and hydropower stations, accurate simulation of water hammer is extremely important for safe and stable operation and the realization of intelligent operations. Previous water hammer calculations usually consider only steady-state friction, underestimating the decay of transient pressure. A second-order Finite Volume Method (FVM) considering the effect of unsteady friction factor is developed to simulate the water hammer and the dynamic behavior of air cushion surge chamber in a water pipeline system, while an experimental pipe system is conducted to validate the proposed numerical model. Two unsteady friction models, Brunone and TVB models, were incorporated into the water hammer equations, which are solved by the MUSCL–Hancock method. One virtual boundary method was proposed to realize the FVM simulation of Air Cushion Surge Chamber. Comparisons with water hammer experimental results show that, while the steady friction model only accurately predicts the first pressure peak, it seriously underestimates pressure attenuation in later stages. Incorporating an unsteady friction factor can better predict the entire pressure attenuation process; in particular, the TVB unsteady friction model more accurately reproduces the pressure peaks and the whole pressure oscillation periods. For water pipeline systems with an air cushion surge chamber, energy attenuation of the elastic pipe water hammer is primarily due to pipe friction and the air cushion. The experimental results with the air cushion surge chamber demonstrate that the proposed FVM model with the TVB unsteady friction model and the air chamber polytropic exponent near 1.0 can well reproduce the experimental pressure oscillations.

On the computation of compressible multiphase flows with heat and mass transfer in elastic pipelines

The present work is motivated by the engineering need to simulate the multiphase flows in the course of the start-up of a long-distance crude oil pipeline. We propose a model for compressible multiphase flows with heat and mass transfer and diffusion processes in elastic pipelines. The model is derived with two approaches, i.e., (a) the spatial averaging procedure of a single phase model and (b) the Arbitrary Lagrangian Eulerian (ALE) formulation of the Baer-Nunziato model with quasi-1D approximation. The diffusion processes include the viscous dissipation, wall friction, heat conduction, and heat exchange with external environment. In particular, the wall friction consists of steady friction term calculated by Darcy-Weisbach formula, and the unsteady friction determined by the instantaneous-acceleration-based (IAB) model. The unsteady friction modifies the characteristic structure of the hyperbolic part of the model. For the solution of the hyperbolic part, we propose a three-wave approximate Riemann solver incorporating the unsteady friction term. Mass and heat transfer are realized via instantaneous relaxations of the chemical potential and temperature, respectively. Efficient iterative relaxation procedures for N-phase flows have been proposed. We have validated the proposed model and numerical methods against some benchmark multiphase problems and applied the model to calculate the start-up of a realistic liquid pipeline with intermediate pump station boundary condition.

The effect of the structural parameters on the friction and wear properties of some FCC metals

Friction and wear are very complex thermo-mechano-chemical processes of contact interaction. For a better understanding of the mechanisms of plastic deformation affecting friction and wear, four face-centered cubic (FCC) metals (Ag, Cu, Ni, and Al) were studied using a pin-on-disk rig. The topmost surface layer presents nanocrystalline (NC) and ultrafine grain (UFG) structures with the transition to a sublayer with a lamellar structure parallel to the direction of friction. The thickness of the topmost layers is mainly determined by stacking fault energy (SFE), hydrostatic pressure, and shear stress. The thermo-mechanical control of plastic flow during the steady friction state was considered. The effective shear stress as the activation barrier, the strain rate sensitivity, and the contact temperature were calculated. The values of the strain rate sensitivity, m, indicate that NC and UFG surface layers for Ag, Cu, and Ni, exhibit low capacities for strain hardening. The friction of FCC metals in lubricated conditions is characterized by a switch from dislocation-mediated plasticity to grain boundary (GB) plasticity in the topmost surface layers. The similarity between friction with low and high coefficients of friction during dry and lubricated conditions was considered. The wear resistance of studied materials can be compared to strength improvements and easy shearing of the topmost surface layer under dry and lubricated conditions. Dislocation glide and grain growth play a key role in the hardening/softening of surface layers both in dry and lubricated conditions.

Effect of Ambient Air Temperature on the Compression Wave Propagating along a Railway Tunnel

The parameters of the compression wave propagating in a railway tunnel are significantly influenced by the large ambient air temperature variation throughout the year. High-speed train entering a railway tunnel produces a wave of finite amplitude to propagate at sonic speed. The wave attenuates while propagation through viscous dissipation and inertial forces nonlinearly steepen the wave. As a result of the dependence of sound speed on air temperature, the wave characteristics are altered with changing temperature. Therefore, it is crucial to comprehend the impact of ambient air temperature on the properties of the compression wave in order to construct an aero-acoustically ideal railway tunnel system. The method of characteristics (MOC) has been used to solve Euler equations with steady and unsteady friction parameters in the current study. According to the findings, wave attenuation ratio is reducing along the tunnel length, and gradient is rising as train speed increases. The case study illustrates the key distance within a tunnel where the steepening ratio is at its highest point. This critical tunnel length is estimated to be 65 times the tunnel hydraulic diameter (300 km/h) for a particular air temperature (T = 323 K), and it decreases by 15% for a 70 K reduction (323K to 253K) in temperature. Similarly, the critical length falls by 40% for greater train speeds (500 km/h).

Tribological Properties of Ti6Al4V Alloy Composite Texture Fabricated by Ultrasonic Strengthening Grinding and Laser Processing

The Ti6Al4V alloy has been widely used in aerospace equipment and medical devices. However, the poor wear resistance of the Ti6Al4V alloy hinders its further engineering application. In this study, the ultrasonic strengthening grinding process (USGP) and laser texturing process were employed to enhance the wear resistance of Ti6Al4V alloy. The frictional behavior of all samples was determined via a ball-on-disc friction and wear tester under dry conditions. The worn surface morphology, cross-sectional hardness, surface roughness, and microstructure were analyzed. The results demonstrated that the USGP induced high hardness, high dislocation density, and grain refinement, as well as improvements in the wear resistance of Ti6Al4V. Moreover, laser texture could enhance the capacity to capture wear debris and reduce wear probability. When combining the USGP and laser texturing process for the surface treatment of Ti6Al4V alloy, the lowest and most stable friction coefficients were obtained, as well as the best wear resistance. Compared to the polished sample, the steady stage friction coefficient of the sample treated by USGP and laser texturing process was remarkably decreased by 58%. This work demonstrates that combining the USGP and laser texturing process could be a promising solution for improving the wear resistance properties of Ti6Al4V alloy, which makes it more suitable for various engineering applications.

Frictional factor and flow characteristics in pulsating flow with high amplitude of pulsations

Pulsating flow as a means of achieving higher heat transfer rates are of interest in many engineering applications such as industrial, environmental and biological flows. These flows are mainly characterised by superimposition of unsteady flow over a steady flow ensuring a net flow along the pipe. The forced pulsation is found to greatly affect the flow characteristics, depending upon the frequency and amplitudes of pulsations. The annular effect in which the flow near the wall is increased is explicitly observed for amplitude of pulsation greater than 0.6, resulting in the heat transfer enhancement up to 40 % for amplitude ratio of 1.8 (Sonawane and Sewatkar, 2019) [11]. The flow characteristics exhibited due to variation in amplitude ratio is absent for variation in Reynolds number and pulsation frequency. In case of heat exchangers used in the Stirling engines, the enhancement in heat transfer can bring a noticeable reduction in the size, thereby saving in the cost and space requirement. However to produce the pulsations by artificial mechanism, like the use of reciprocating pump or pulsators asks for additional amount of input energy as compared to steady flow energy. Hence it becomes necessary to know the energy lost in friction and acceleration and retardation of the fluid due to induced pulsation. Therefore, observations of effect of different amplitudes on the instantaneous and time averaged friction factor is discussed with the sole purpose of obtaining an optimised combination of the governing parameters, namely Reynolds, frequency and amplitude of pulsation for enhancing convective heat transfer for flow through pipe. It has been observed that for amplitude ratios up to 0.6, the friction factor values show a marginal decrease as compared with steady laminar flow, however for higher amplitudes, there is steep decrease in time averaged friction factors. It is also noticed that the instantaneous friction factors show high values in the latter half of pulsation cycle. The friction factor values are greatly influenced by the induced pulsation, due to which the correlations developed for steady laminar flow friction factor calculations needs to be modified.

Comprehensive application analyses of elastic models and viscoelastic models in transient flows in polymeric pipelines

Abstract Based on the MOC (Method of Characteristics), the applicabilities of both the elastic models of the SF (Steady Friction), the CB-UF (Convolution-Based Unsteady Friction) and the MIAB-UF (Modified Instantaneous Acceleration-Based UF), and the viscoelastic models of the SF-VE (SF-Viscoelasticity), the CB-UF-VE (Convolution-Based Unsteady Friction-Viscoelasticity) and the MIAB-UF-VE (Modified Instantaneous Acceleration-Based UF-Viscoelasticity), are investigated for hydraulic transients induced by a downstream rapid valve closure in a polymeric pipeline. The predicted results by the elastic models are very different from the experimental data, whereas the predicted pressure peaks and phases by the viscoelastic models agree well with the experimental data because considering the pipe–wall creep effect, the CB-UF-VE and the MIAB-UF-VE generate better results than the SF-VE. The creep effects near the first pressure peak are captured well by the viscoelastic models. The analyses of the contributions of different factors to pressure attenuation show that for transient flows in polymeric pipelines, the effect of VE is greater than that of the CB-UF and MIAB-UF. The spectrum analyses show that the pressure amplitudes and harmonic frequencies by the elastic models match badly with the experimental data, whereas those by the viscoelastic models match well with the experimental data. The harmonic frequencies by the MIAB-UF-VE are the best, followed by the CB-UF-VE, and are worst by the SF-VE.

How to improve superlubricity performance of diketone at steel interface: Effects of oxygen gas

Achievement of steady and reliable super-low friction at the steel/steel contact interface, one of the most tribological systems applied for mechanical moving parts, is of importance for prolonging machine lifetime and reducing energy consumption. Here we reported that the superlubricity performance of the steel/steel sliding interface lubricated with tiny amounts of diketone solution strongly depends on the oxygen content in surrounding environment. The increase of oxygen not only significantly shortens the initial running-in time but also further reduces the stable coefficient of friction in superlubricity stage due to the enhancement of tribochemical reactions. On the one hand, more severe oxidation wear occurring at higher oxygen content facilitates material removal of the contact interface, lowering the contact pressure and the corresponding initial friction. On the other hand, the growth of iron ions during the shear process in high oxygen environment promotes the formation of chelate which acted as an effective lubricated film chemisorbed at the steel/steel friction interface to further lower the interfacial friction. The results provide a new opportunity to further optimize the tribological performance of diketone superlubricity system, especially towards the lubrication of mechanical engineering materials.

A novel friction weakening-based dynamic model for landslide runout assessment along the Sichuan-Tibet Railway

The Sichuan-Tibet Railway traverses the Qinghai-Tibet Plateau in China, where giant landslides occur frequently and densely. Accurately assessing the velocity, runout distance, and influenced area of the potential landslides is one of the nonnegligible scientific and technical issues. This study establishes a dynamic model for the landslide runout assessment along the Sichuan-Tibet Railway. The proposed model is based on the shallow water theory and takes into account the mechanism that the friction is weakening with the increase of velocity, the finite difference method is used to solve the governing equations, and the ideal one-dimensional dam break experiment was used to verify the shock-capturing capacity of the numerical methods. The Lagang ancient landslide, only 500 m away from the Jiacha Station of the Sichuan-Tibet Railway, represents the typical environment and development pattern of landslides in this region. Therefore, the proposed model is applied to discuss its effects on the landslide dynamic process and deposit patterns. Besides, 75 sets of numerical experiments were further carried out to explore the sensibility of the three parameters including peak friction coefficient μp, steady friction coefficient μs, and critical velocity Vc in this model. The results indicate that μs determine the lower limit of the friction coefficient, which significantly impacts the landslide runout distance; μp is mainly acting at the initial and final stage and has less influence on the landslide dynamic. By contrast, Vc is the most significant. When the Vc is small, the landslide can quickly reach the threshold velocity after the initial, and the dynamic process has a small correlation with the μp; when the Vc is large, the landslide is difficult to reach the threshold, and the mobility will be limited. It should be noted that the materials, lithology, strength, and micro-cracks of landslides mass formed under the special geological, geomorphic, and climatic conditions in this region may affect the input values when using the friction weakening model on landslides risk assessment.

Tribological characterizations of bio-polymer based ecofriendly copper-free brake friction composites

PurposeThis study aims to discuss the impact of using bio-polymer (kraft lignin) in the formulation of passenger vehicle disc brake pads (as a substitute for cashew nutshell liquid [CNSL]-based friction dust) and investigate the characteristics of the pads.Design/methodology/approachWithin the scope of this investigation, three different brake pads were generated by altering the biopolymer-lignin content in conjunction with the friction dust from CNSL without modifying the other components. The brake pads were created in accordance with industry-standard practices. Industrial standards were used to evaluate the newly created brake pad’s thermal, physical and mechanical qualities. The tribological properties of the materials were determined using a full-scale inertia brake dynamometer. The scanning electron microscope examined the worn surfaces in conjunction with elemental mapping.FindingsThe test findings suggest that the brake pads filled with biopolymer-lignin and CNSL-based friction dust (as a partial replacement 50%) exhibited excellent thermal, physical, mechanical characteristics, as well as steady friction and low wear rate.Originality/valueA bio-polymer (kraft lignin) in friction composites has the potential to produce eco-friendly brake pads and improve the tribological performance of its copper free-composition, which might be used to replace CNSL-based friction dust in friction composites by addressing the issues raised in this work.

Numerical Study on the Turbulent Structure of Tsunami Bottom Boundary Layer Using the 2011 Tohoku Tsunami Waveform

In this study, the tsunami-induced bottom boundary layer was investigated based on actual waveforms obtained by the GPS buoys along the coast of the Tohoku region during the 2011 Great East Japan Earthquake tsunami. The k-ω model was utilized for the numerical analysis in this study. As a result, the tsunami boundary layer thickness was found to be extremely thin compared to the water depth. The velocity distribution was similar to that of the bottom boundary layer under wind-generated waves. The flow regime is located in the transition from smooth turbulence to rough turbulence. Because of this, the gradient of the flow across the layer is much greater than the gradients in the steady flow direction. Therefore, the bottom friction is underestimated if the steady friction factor, such as in the Manning formula, is used. This study proposes a new simple method for calculating the bottom shear stress due to an irregular tsunami based on the wave friction law, and the k-ω model results are used to validate the proposed methods.

Time-reversal of water-hammer waves

Time reversal (TR) of waves is being used in many fields and underpins the foundation of practical equipment that, for example, is used to destroy brain tumour and kidney stones, to detect defects in solids, to pinpoint underground mineral deposits, and to assist in long-distance communication. Theoretically, the TR property of waves is lost in the presence of damping. However, techniques which are based on TR of waves are found to be robust to damping. In particular, the TR property of water-hammer waves was recently used to develop computationally fast and noise-tolerant methods for pipeline defect detection in the presence of viscous and visco-elastic damping. The objective of this paper is to understand how TR-based methods remain applicable when applied to damped water-hammer waves. The problems being investigated are (i) an unbounded pipe with and without damping, and (ii) a bounded pipe with and without damping. Steady friction, unsteady friction, and visco-elastic damping effects are considered. The results show that water-hammer waves remarkably refocus at the correct position and timing of the source. The spatial resolution of the waves in both unbounded and bounded pipe systems is about half the minimum probing wavelength . However, damping reduces the amplitude of the refocused wave which may affect the resolution in the presence of large noise.

Effect of Arc Currents on the Mechanical, High Temperature Oxidation and Corrosion Properties of CrSiN Nanocomposite Coatings

In methods for multi-arc ion plating technology, the behavior and characteristics of the arc spot determine the physical characteristics of arc plasma and the properties of the subsequent deposited coatings. In this paper, the effect of arc currents on the hardness, friction coefficient, high temperature oxidation, and corrosion properties of the CrSiN coatings was studied. According to the XRD and SEM results, with the increase of arc currents, the coatings grew preferentially to the CrN (111) crystal direction, and the CrN (220) crystal phase appeared at high currents of 90 A. In addition, the number of large particles increased when the current exceeded 70 A. The HR-TEM results confirmed the formation of nanocomposite structure of nanocrystalline of CrN embedded into the amorphous phase of Si3N4 as explored by XRD. The maximum hardness was achieved at 3120 Hv when the coatings were deposited under currents around 70 A. However, the hardness values decreased with further increase of arc currents. From the contact of ceramic balls with the wear of coatings, the surface of coatings gradually produced friction marks, and the friction force increased from a steady friction force to a dynamic friction force. The high temperature oxidation results showed that fewer oxides were formed on the surface of the coatings when oxidized at 800 °C. It was also found that CrSiN nanocomposite coatings prepared at an arc current of 70 A had a larger corrosion potential and polarization impedance, which could effectively protect the tool matrix.

Unsteady Linearisation of Bed Shear Stress for Idealised Storm Surge Modelling

The modelling of time-varying shallow flows, such as tides and storm surges, is complicated by the nonlinear dependency of bed shear stress on flow speed. For tidal flows, Lorentz’s linearisation circumvents nonlinearity by specifying a (steady) friction coefficient r based on a tide-averaged criterion of energy equivalence. However, this approach is not suitable for phenomena with episodic and irregular forcings such as storm surges. Here, we studied the implications of applying Lorentz’s energy criterion in an instantaneous sense, so that an unsteady friction coefficient r(t) adjusts to the temporal development of natural wind-driven flows. This new bed-stress parametrisation was implemented in an idealised model of a single channel, forced by time-varying signals of wind stress (acting over the entire domain) and surface elevation (at the channel mouth). The solution method combines analytical solutions of the cross-sectionally averaged linearised shallow-water equations, obtained in the frequency domain, with an iterative procedure to determine r(t). Model results, compared with a reference finite-difference solution retaining the quadratic bed shear stress, show that this new approach accurately captures the qualitative and quantitative aspects of the surge dynamics (height and timing of surge peaks, sloshing, friction-induced tide-surge interaction) for both synthetic and realistic wind forcings.