Research On The Direction Related Frictional Properties Of Multi-pit Surface Texture

The wear of mechanical parts is the most common failure form of construction machinery,which will seriously impair the performance and service life of the whole equipment.Surface texture(ST)technology,preparing some regular distributed units(including pits,grooves or vein,etc.)with certain processing methods(such as laser surface remelting,laser cladding,laser ablation)on the surface of mechanical parts,can significantly change the tribological properties of the friction pair,improve the thermodynamic performance and the friction and wear properties of the contact surface,and further prolong the service life of the parts.Therefore,ST technology has very important engineering application value in modern industry.In this work,using the automobile disc brake system as the original model,based on the tribology and material mechanics theory,the comprehensive study on the textured surface with linear distributed pits in the radial direction(LDRD)or curve distributed pits in the radial direction(CDRD),including pattern design,thermodynamic behavior,wear failure form and anti-wear performance etc.,was conducted through theoretical analysis,numerical simulation and experimental methods.The aim of this work was to reduce the heat generation of friction pair,alter the temperature and thermal stress distribution,and further improve the auxiliary heat stability and wear resistance of the friction pair.The specific contents were listed as follows:1.Taking the disc brake system as the prototype,a three-dimensional finite element(FE)model of pin-on-disc friction system was established through ANSYS/LS-DYNA module,APDL(ANSYS Parameter Design Language)programming and direct coupling method(DCM).The numerical simulations of the friction process of the LDRD textured surfaces(with different diameter of pits(BOP)and different distance between pits(DBP))were conducted.The thermal stress variation curves of the nodes of different depth were obtained,as well as the contours of surface temperature and thermal stresses.Under the same conditions as in the simulations,the tribology tests of the pin-on-disc friction system were carried out,and the friction coefficient curves and the wear losses of specimens with different texture parameters obtained.Through the observation and characterization of worn surfaces,based on the classical material strength theory,the influence of thermal behavior of varied texture parameters on the tribological properties were discussed,including temperature,von Mises stress,the first principal stress and so on.Furthermore,the crack conditions in different directions,the radial direction and the frictional direction(circumferential direction),were also analyzed,and compared with the force of those nodes in different directions of the pit.In the end,the reasons of the wear failure performance in different directions were analyzed.2.The pattern design,finite element modeling and analysis,wear tests and characterization of CDRD textured surfaces were conducted.Taking the pit center fitting curve of CDRD textured surface as its characteristic curve,the influence of different initial deflection angle of the first pit(IDAFP,0°,30°,45°,60°,75°)and different deflexion angle of adjacent pits(DAAP,0°,5°,10°,15° and 20°)on the thermal behavior of the pin-on-disc friction system was researched.Through the experimental analysis of wear performance and the observation of worn surfaces using the ultra deep field 3D microscopy,the relationship between tribology behavior and texture characteristic parameters of CDRD surfaces was also researched.The surface texture pattern with the best wear behavior was obtained,which would provide a solid foundation for the study of the frictional orientation of the CDRD textured surface.3.Based on the obtained optimal CDRD textured model,the distributions and variation of the thermal behavior were analyzed,including the 1st principal stress,the von Mises stress,surface temperature and the thermal structural coupling strain etc.The thermodynamic performance of the smooth disc was also compared.Some thermal characteristics of the textured surface were obtained,including the temperature creeping phenomenon of textured surface,the stress and strain difference in the radial and circumferential(friction direction),as well as the distribution tendency of the high stress-high strain zone inclined to the direction of friction.Furthermore,two kinds of paths(rectilinear and curvilinear)of the textured surfaces(LDRD and CDRD)were established,and the differences of thermal results,such as the temperature and stress of those nodes on the paths,were also compared and analyzed.The influence of pit group on the multi-axial stress of the textured surface was explored.According to the comprehensive analysis of the wear affecting factors of textured surfaces(temperature,stress,strain and films)and the observed macro-and micro-wear phenomena,based on the stress field intensity theory and the thermal elasticity theory,The wear mechanism of pit textured model was proposed.4.Based on the obtained optimal CDRD texture pattern(IDAFP of 30°,DAAP of 10°),the friction and wear process of pin-on-disc system was modeled,analyzed and tested,when the pin rotated in two different directions(clockwise and counterclockwise).The effects of different rotating directions on the tribological behavior of textured surfaces,such as the thermodynamic behavior,the anti-wear properties and the micro-characteristics etc.,were investigated under two different rotating speeds of 25 rad/s and 50 rad/s.At 25 rad/s rotation speed,the friction and wear testing data of grey cast iron and 30#steel showed that:when the pin rotates counter clockwise to the textured disc,the friction coefficient and the wear loss of the sample were smaller than the data when the pin rotates clockwise relative to the disc,and the surface grinding mark was also slightly lighter.5.Based on the macro-and micro-tribology theory,the wear patterns(adhesive wear,abrasive wear,fatigue wear and so on)of the LDRD/CDRD textured surface were analyzed.The anti-wear mechanism of pit textured surface was analyzed and verified,from the point of view of temperature,thermal stress,surface contact state,micro-structure of materials as well as wear pattern.Based on the dry wear and friction theory of metal materials,the problem,how does the orientation of ST affect the surface anti-wear properties of the material,was put forward,from the aspects of theory,preparation,simulation,experiment and characterization.Through the analysis of the relation between the characteristic distribution curve of the CDRD textured surface and the motion path of the pin,the frictional orientation mechanism of the CDRD textured surface was revealed.The research on the friction and wear properties of LDRD/CDRD textured surfaces in this work,especially the study on the friction orientation of the CDRD texture pattern,was helpful to enrich the pattern design of the pit textured surface of friction pair parts.It also provided a new way to improve the thermal stability of friction pairs,as well as the analysis of the friction and wear properties of friction systems.