Friction effect on contact pressure during indentation and scratch into amorphous polymers

doi:10.1016/j.matlet.2009.05.015

Materials Letters

Volume 63, Issue 20, 15 August 2009, Pages 1671-1673

https://doi.org/10.1016/j.matlet.2009.05.015 Get rights and content

Abstract

Using finite element analysis (FEA), for monotonic loadings, variations of the true contact geometry and of the mean contact pressure p_m, defined by the ratio of the applied normal load to the true projected contact area, are described as a function of the testing conditions, the geometrical strain a/R and the local friction coefficient μ_loc, during indentation and scratch experiments with spherical indenter. The estimation of an equivalent plastic strain is also proposed and shown to be a complex function of a/R and μ_loc, especially during scratch experiments. The normalized contact pressure p_m/σ_y, with σ_y, the initial yield stress of the tested material, determined during indentation and scratch tests is compared to an expanding cavity model, recently developed for indentation of elastic-strain hardening plastic materials.

Introduction

Indentation and scratch tests are commonly used as a technique for studying mechanical properties of material near their surface. Indeed, micro and nanoscratch tests appear as a mean to study large strains of materials at high strain rates under quasi isothermal conditions. Though it is not a novel topic for metals, research efforts on the scratch behavior of polymer only began over the last two decades [1]. Understanding of the mechanical process of indentation and scratch tests is essential to make any quantitative predictions in the wear behavior of polymeric surfaces. However, by comparison with indentation, a scratch is a more complex mechanical deformation that involves surface contact, friction interaction, and also material and geometrical non linearity [2]. Scratch testing may be assimilated to a local forming operation of the surface, leading to compressive stresses in front of the indenter and tensile stresses behind the moving tip. Shearing strain caused by friction appears in the contact area. Such complex stress field has been analytically described by Hamilton and Goodman [3] for purely elastic contact. But, for elastic-plastic contact and then, for fully plastic contact, there is not yet an available analytical model to relate geometrical parameters, rheological parameters and surface properties to the stress and strain fields during scratching. Due to the inhomogeneous state of strain and strain rate, interpretation of the test requires at present time a specific effort on numerical simulations [4], [5], [6]. The purpose of this study is to identify key material and surface properties that influence the scratch performance of polymers, as well as define average parameters to describe the scratch behavior of materials. It focuses on studying some characteristic surface response due to spherical scratch, such as the variation of the average contact pressure p_m and the representative strain, as a function of the ratio a/R (with a, the contact radius and R, the tip radius) and the local adhesive friction μ_loc.

Section snippets

Experimental details

The commercial finite element package MSC MARC ® was utilized to perform numerical analysis. As for accurate details of the numerical analysis, we refer to Pelletier et al. [6]. Here, it just should be mentioned that regarding the constitutive specification, a simple linearly elastic, linear strain hardening plastic material for large deformation Von Mises plasticity with isotropic hardening was implanted to reproduce the mechanical material behavior of amorphous polymeric material at room

Representative strain

The level of deformation imposed on the specimen is controlled during indentation by the geometrical shape of the indenter [7]. In the case of scratching with spheres, Gauthier et al. [8] have shown experimentally for a given material with a constant local friction, that the level of deformation is also governed by the geometry of the tip, especially the geometrical strain a/R introduced by Tabor [7] for indentation in metals. In scratch tests, as in indentation, the representative strain can

Conclusions

A detailed study of the effect of the local friction coefficient on indentation and scratch testing was carried out. The results of numerical simulations reveal the influence of the local friction coefficient on the plastic deformation in scratch experiments with a spherical indenter. For contacts with friction the representative strain appears to be described by a complex function of the imposed geometrical strain (a/R)_V and the friction coefficient. The representative strain can be estimated

References (12)

M. Wong et al.
A new test methodology for evaluating scratch resistance of polymers
Wear
(2004)
L. Lin et al.
A new approach to characterize scratch and mar resistance of automotive coatings
Prog Org Coat
(2000)
J.L. Bucaille et al.
The influence of strain hardening of polymers on the piling-up phenomenon in scratch tests: experiments and numerical modeling
Wear
(2006)
H. Pelletier et al.
Viscoelastic and elastic-plastic behaviors of amorphous polymeric surfaces during scratch
Tribol Int
(2008)
C. Gauthier et al.
Scratching of a polymer and mechanical analysis of a scratch resistance solution
Tribol Int
(2006)
S. Jayaraman et al.
Determination of monotonic stress–strain curve of hard materials from ultra-low load indentation tests
Int J Solids Struct
(1998)

There are more references available in the full text version of this article.

Cited by (18)

Dynamic mechanical model in grinding C/SiC composites
2024, International Journal of Mechanical Sciences
Grinding force is a crucial component which affects manufacturing processes, machining defects, and material removal rates. Currently, grinding force models for carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are primarily based on the presumption of evenly distributed abrasive grains and indentation fracture mechanics. These modelling techniques ignore the microstructural characteristics for the material and only yield the average grinding force. Therefore, a dynamic grinding force model for C/SiC composites is presented in this study on the basis of the grinding wheel surface morphology, material microstructural characteristics, and brittle removal mode transition. First, the surface topography for the grinding wheel is simulated with a random offset algorithm, and the discriminative mechanisms of active grits in the grinding areas are revealed using a discrete grinding wheel and grit kinematic trajectories. Second, the removal mode of single active grit with different depths of cut on the cutting arc length is analyzed based on the material removal mechanisms (MRMs), and the corresponding cutting force model is presented through the establishment of a material microstructure feature model. Third, a dynamic grinding force model is presented based on the decomposition and summation of the active grit grinding forces in the grinding area. Experimental verification demonstrates that the model is able to precisely predict the detailed information, fluctuation amplitude, and distribution principle of the dynamic evolution in grinding force with fluctuation features. In addition, the model explores the surface morphology and machining defects generated during the grinding of C/SiC composites.
Cutting speed dependence of material removal mechanism for monocrystal silicon
2024, International Journal of Mechanical Sciences
In advanced semiconductor packaging, silicon wafers are typically thinned to tens of microns in thickness using ultra-precision grinding techniques, and the cutting speed greatly determines their surface integrity. However, an in-depth understanding of fundamental material removal mechanism considering the strain rate effect has not been revealed yet. This paper systematically explores the impact of cutting speed on the penetration depth, subsurface damage characteristics and scratch-induced stress field for monocrystal silicon material. First, nanoscratch tests in varied-load and constant-load modes are performed under different scratch speeds on the silicon specimen surface. The surface morphology of generated groove is observed to identify its brittle and plastic features. Next, the geometric relationship and contact pressure between the indenter and workpiece during the scratching process are analyzed. A theoretical penetration depth model is developed considering scratch speed, indenter tip radius and elastic recovery. The corresponding parameters are solved via the particle swarm optimization algorithm and agree very well with the experimental data. Then, the subsurface damage under different scratch speeds is measured by a transmission electron microscope. The atomic scale lattice defects are presented, involving the amorphous layer, stacking faults and phase transformation. Finally, a scratch-induced stress field model is established to interpret the distortion degree variation in damaged regions with scratch speed. The distribution of stress components in the scratch region is presented.
Prediction and measurement for grinding force in wafer self-rotational grinding
2023, International Journal of Mechanical Sciences
The ultra-precision grinding technology based on a workpiece self-rotational principle is extensively used for silicon wafer thinning in the chip post-processing. Nevertheless, owing to the random nature of diamond grains and the unique machining manner of rotating components, accurate prediction and real-time monitoring of grinding forces remain a significant challenge in wafer self-rotational grinding (WSRG) process. This work establishes an improved theoretical model and builds a novel in-situ measurement system to analyze and detect the grinding force. Firstly, the motion trajectory and contact condition of diamond grains are explored. The grain–workpiece interactions are divided into rubbing, plowing, cutting and brittle stages. Secondly, the grinding force prediction model is proposed, which considers the combined effects of grain wear, grain randomness characteristics, brittle-to-ductile transition, elastic rebound, strain rate and grinding marks. Moreover, piezoelectric force sensors are employed to build the in-situ measurement system. The developed system is proven to simultaneously measure grinding forces online with continuous and stable output in the $x$ , $y$ and $z$ directions. The impacts caused by feed rate, wheel speed and wafer speed are further revealed. Experimental results are consistent well with predicted ones, demonstrating a high prediction accuracy of the proposed grinding force model. Finally, a new index, namely the processing factor, is introduced to calculate the subsurface damage depth and residual stress for various machining parameters. This work not only enhances the understanding of predicting and measuring grinding force, but also provides a new method to evaluate the subsurface defects in brittle material grinding.
Nano-to-microscale ductile-to-brittle transitions for edge cracking suppression in single-diamond grinding of lithium metasilicate/disilicate glass-ceramics
2023, Journal of the European Ceramic Society
Citation Excerpt :
Elastic and plastic deformation, and brittle fracture are consecutively incurred when the induced stresses reach respective critical values [27]. Controlling factors of the contact stresses in grinding comprise not only the mechanical properties, but also the diamond grit profiles as well as the process variables [23,28,29]. The degrees of grinding-induced damage are associated with characteristic impression depths/lengths produced by diamond grits [28,30].
To suppress grinding-induced edge cracks in dental lithium metasilicate/disilicate glass-ceramics (LMGC/LDGC), this paper established a contact stress model for single-diamond grinding (SDG) to relate their crack generation and ductile-to-brittle transition (DBT) thresholds with the mechanical properties, diamond tool profiles and process variables. Nanoindentation, friction test and SDG were conducted to unravel material responses and dynamic diamond grit-workpiece interactions to determine the DBT thresholds. The nanoindentation revealed significant indentation size effects (ISEs) on the hardness and elastic moduli of the ceramics. SDG clearly elucidated their DBT behaviors, wherein edge cracks initiated at diamond peripheries when the concurrent contact stresses reached the DBT thresholds. Accordingly, the indicated critical cutting depths for DBT may be changed from the nanoscale to the microscale by increasing the tool radius and reducing the machining speed. This research contributes to edge crack suppression for ductile machining of brittle materials at large removal rates.
Monitoring of ductile–brittle transition mechanisms in sapphire ultra-precision grinding used small grit size grinding wheel through force and acoustic emission signals
2023, Measurement: Journal of the International Measurement Confederation
Progressive ultra-precision grinding experiments used small grit size grinding wheels were performed to investigate the ductile–brittle transition process of sapphire, force and AE signals were employed to monitor the material removal behavior. Precision dressing and measurement of circular arc grinding wheels were performed to meet the minimum grinding depth for ultra-precision grinding. A grinding force model was proposed to interpret the grinding forces in different materials removal modes. A continuous complete ductile–brittle transition process was observed and monitored, the progressive grinding was divided into three stages: ductile dominant, ductile–brittle transition and brittle eruption. The grinding force, surface profile, micro morphology and brittle fracture percentage were discussed. Brittle fracture will not entirely occur, but rather brittle fracture and ductile grooves were coexisted on the surface in the brittle eruption stage, which was attributed to the grinding characteristics of small grit wheels. In addition, raw AE signals and multiple transformation forms were utilized to monitor the material removal behavior and their respective roles were analyzed. The Fourier transform, discrete wavelet, continuous wavelet and wavelet coefficients in different stages can all establish a mapping relationship with material removal modes.
A grinding force predictive model and experimental validation for the laser-assisted grinding (LAG) process of zirconia ceramic
2022, Journal of Materials Processing Technology
Laser-assisted grinding (LAG), as a potential machining method, is expected to achieve high-efficiency machining without any surface damage or sub-surface damage. However, grinding force tends to exert serious impact on the surface damage during LAG process. In this paper, a grinding force predictive model for the LAG process was established, which has taken the combined effects of temperature-dependent mechanical properties of the material, statuses of grit-material micro interaction, and stochastic shapes and random distributions of abrasive grits into consideration. This model also reveals the mechanism for the reduction of grinding force during LAG. In the meantime, the simulative grinding force distributions of workpiece surface with different laser powers were obtained. LAG experiments of zirconia ceramic were carried out to validate this model. It is found that the modelled forces are in good agreement with the measured forces and the error rates can be confined within 12 %. In addition, the effect of grinding parameters on grinding force has been investigated. It is demonstrated that the grinding force can be reduced by a certain percentage ranging from 29.4%–60.1% using the optimal machining parameters. Within a certain threshold, higher laser power can improve the surface integrity and decrease the depth of damage. This work is expected to provide significant guidance for promoting the development of the laser-assisted machining technologies.

View all citing articles on Scopus

View full text

Friction effect on contact pressure during indentation and scratch into amorphous polymers

Abstract

Introduction

Section snippets

Experimental details

Representative strain

Conclusions

Wear

Prog Org Coat

Wear

Tribol Int

Tribol Int

Int J Solids Struct