High temperature tribological behaviour of PVD coated tool steel and aluminium under dry and lubricated conditions

Aluminium alloys are commonly used as lightweight materials in the automotive industry.This non-ferrous family of metallic alloys offers a high versatility of properties and designs.To reduce weight and improve safety,high strength-to-weight ratio alloys(e.g.6XXX and 7XXX),are increasingly implemented in vehicles.However,these alloys exhibit low formability and experience considerable springback during cold forming,and are therefore hot formed.During forming,severe adhesion(i.e.galling)of aluminium onto the die surface takes place.This phenomenon has a detrimental effect on the surface properties,geometrical tolerances of the formed parts and maintenance of the dies.The effect of surface engineering as well as lubricant chemistry on galling has not been sufficiently investigated.Diamond-like carbon(DLC)and CrN physical vapour deposition(PVD)coated steel have been studied to reduce aluminium transfer.However,the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood.The present study thus aims to characterise the high temperature tribological behaviour of selected PVD coatings and lubricants during sliding against aluminium alloy.The objectives are to first select promising lubricant-coating combinations and then to study their tribological response in a high-temperature reciprocating friction and wear tester.Dry and lubricated tests were carried out at 300℃ using a commercial polymer lubricant.Tests using DLC,CrN,CrTiN,and CrAIN coated tool steel were compared to uncoated tool steel reference tests.The initial and worn test specimen surfaces were analysed with a 3-dimensional(3D)optical profiler,scanning electron microscope(SEM)and energy dispersive X-ray spectroscope(EDS)as to understand the wear mechanisms.The results showed formation of tribolayers in the contact zone,reducing both friction and wear.The stability of these layers highly depends on both the coatings'roughness and chemical affinity towards aluminium.The DLC and CrN coatings combined with the polymer lubricant were the most effective in reducing aluminium transfer.

Super-low friction of ta-C coating in presence of oleic acid

Friction tests have been carried out on bearing steel friction pairs coated with a-C:H and ta-C under lubricated conditions with 1-hexadecene and pure oleic acid,respectively.Main results show that ta-C leads to the super-low regime with oleic acid(friction coefficient below 0.01)and that a-C:H is quite well lubricated by 1-hexadecen.Surface analyses by coupling X-ray photoelectron spectroscopy(XPS)and secondary ion mass spectrometry(SIMS)have been carried out in the case of oleic acid with ta-C and a-C:H coatings.Results show that tribochemical reactions have occurred.OH groups terminate the ta-C surface whereas a-C:H is covered by a thin tribofilm and is terminated by oleic acid monolayers.The superlow regime in the case of ta-C is mainly attributed to the presence of the OH-termination.

Hysteresis and the nonlinear equivalent piezoelectric coefficient of MFCs for actuation

The hysteretic behavior and nonlinearity of the equivalent material coefficient of macro fiber composites（MFC） under staircase input conditions are investigated using the Preisach model.Based on a database of first order reversal curves, formulas are derived to predict the hysteresis of strain output and nonlinearity of the equivalent piezoelectric coefficient of MFCs. Formulae are verified by comparing the predicted strains with the measured strains of three MFC specimens,which are driven by a random sequence of staircase voltage inputs. The coefficients obtained by the formulae and experimentation coincide. Further results indicate that the equivalent piezoelectric strain coefficient depends greatly on the value of drive voltage across the entire input range, and the coefficient is asymmetric across the negative and positive input ranges. Deflection testing of an MFC composite cantilever demonstrates the importance of taking the nonlinearity of the equivalent piezoelectric coefficient into consideration in the application of actuation.

Wave propagation analysis for a second strain gradient rod theory

In this work,an enriched model describing the longitudinal wave propagation is established based on Mindlin’s Second Strain Gradient(SSG)theory,which can describe the heterogeneity caused by the micro-structure interactions in the frame of continuum mechanics.The governing equation and associated boundary conditions are derived based on Hamilton’s principle,then the dispersion relation of non-classical longitudinal wave together with the extra-waves appearing exclusively in SSG theory model are investigated.The investigations are based on the modal density,energy flow,and forced response of the rod.Wave transmission and reflection through planar interfaces based on the proposed model have been calculated.Finally,the results of the enriched model are well interpreted by comparing with the classical theory results,and some useful conclusions are derived on the SSG theory based model in the wave propagation characterization.

Comparative study of abrasion via microindentation and microscratch tests of reinforced and unreinforced lamellar cast iron

This study compares micro-abrasive wear in two kinds of grey cast iron. Classical lamellar cast iron with fully pearlitic matrix(FGL1) and lamellar micro-alloyed cast iron with phosphorus and boron(FGL2) are used. FGL2 has a fully pearlitic matrix reinforced by the hard phosphorus eutectic phase. The microstructures of these two types of iron are mechanically characterized using nanoindentation tests. Vickers microindentation and microscratch tests are also performed on these iron samples. The indent and scratch images obtained via scanning electron microscopy were used to compare the scratch damage to the two kinds of iron. The friction coefficient is discussed in terms of applied load, indenter attack angle, and scratch damage. Nanoindentation tests show an improvement in graphite’s mechanical properties and an increase in the matrix hardness of the FGL2, relatively to FGL1. The same damage forms for both microindentation and microscratch testing were observed for the two iron samples. However, cracking of the hard phase is observed in FGL2. The results show that the scratching of the micro-alloyed iron(FGL2) leads to less matrix damage and to an extended microploughing wear mechanism. However, at low normal load, the reinforcement of the matrix can increase the friction coefficient.

Evaluation of phase change materials for a light-weight building in Morocco:Effect of building’s volume,window orientation&infiltration

In this study,we will model a light-weight building made of phase change materials(PCMs)to analyze the impact of the building volume,window orientation,and air infiltration on the PCM performance.This is done by calculating the energy savings attained by the use of PCM across all of Morocco.We’ll use the commercial Rubitherm panels with RT28HC as a phase change material in this work.Typically,the EnergyPlus simulation engine is chosen to perform the modeling.The impact of building volumes is also evaluated on the PCM activation for light-weight square buildings with different side lengths of 10 m,9 m,8 m,and 7 m.Also,we looked at how well the PCM performed in terms of energy savings and thermal regulation at different window orientation placements(south,north,west,and east)and various air infiltration rates(0.5,1,1.5,and 3 ACH).This paper’s primary objective is to determine the energy savings for the PCM-enhanced building in Morocco,as well as the effect of building volume,window orientation,and infiltration on the PCM capabilities for stabilizing the indoor room temperature.The results show good indoor temperature stabilization during the summer,for the light-weight square building with a south-facing window and no air infiltration.This configuration was able to achieve a total fluctuation reduction of 1303.3℃for the 10 m building in a semi-arid environment.Besides,a high energy savings percentage of 69.56%was achieved for the PCM-enhanced building with the south-oriented window and air infiltration of 0.5 air change per hour.