An experimental investigation of the scratch behaviour of polymers – 2: Influence of hard or soft fillers (original) (raw)
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In this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting timetemperature superposition and Eyring's model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier's model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic).
Progress in Organic Coatings, 2003
Scratch durability of polymer surfaces and coatings is becoming critical for the increasing use of these materials in new applications, replacing other materials with harder surfaces. Scratch resistance of polymers has been the subject of numerous studies, which have led to specific definitions for plastic deformation characterization and fracture resistance during scratch testing. Viscoelastic and viscoplastic behavior during a scratch process have been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, the understanding of the origin of the fracture process of a polymer during scratch remains approximate. Parameters like tip shape and size, scratch velocity and loading rate, applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established. The goal of this work is to define and analyze scratch parameters that relate to mechanical properties. The evolution of scratch resistance parameters as a function of temperature and strain rate, compared to the evolution of dynamic mechanical properties obtained from indentation and uniaxial tensile tests over a range of temperature for poly(methyl methacrylate) (PMMA) helped in identifying a correlation between the tensile stress-strain behavior and scratch fracture toughness. This correlation brings a new understanding of the origin of the fracture mechanisms during a scratch process. In particular, it is demonstrated that the characteristic strain applied by the indenter is a most relevant parameter to describe the fracture resistance during a scratch process, independently of the indenter geometry.
Scratch evaluation on a high performance polymer
2013
The scratching process is a well know concept and is usually defined as a kind of surface abrasion, where plastic deformation is promoted by relative friction between soft phase and a hard intender. It is necessary to reduce material loss to minimum or even to reach zero to have an efficient and effective functionality of the materials. Polymers being highly sensitive to wear and scratch damage, their various modes of deformation such as, tearing, cracking, delamination, abrasive and adhesive vary with a narrow range of contact variables like applied normal load, sliding velocity, interfacial lubrication and testing temperature. This is particularly important when these materials are used to improve the tribological performance by adding various types of fillers such as, carbon fibers, graphite, PTFE, TiO2, and ZnS are added. The polymers with nanocomposites have the advantages over micro- composites from the viewpoint of wear and scratch damage, the underlying mechanism of damage i...
Wear, 2006
The aim of this study was to relate the scratching behaviour of polymers to their mechanical properties. A thermosetting resin (CR39) and a thermoplastic polymer (PMMA) were studied using a microscratch tester allowing in situ observation of the contact area. These two polymers exhibit different elastic and viscoplastic properties, the main difference being the large ability of CR39 to strain harden, whereas PMMA softens. A spherical indenter was used to vary the level of deformation imposed on the samples. The response was initially elastic, then viscoelastic and finally mainly viscoplastic with increasing penetration of the indenter into the material. The two polymers displayed the same response for small levels of deformation, while at larger strains PMMA showed more pronounced plastic behaviour. The origin of this difference in behaviour was investigated by means of a three dimensional finite element analysis. The rheology of PMMA and CR39 was simplified and modelled by assuming linear elastic behaviour and a viscoplastic law taking into account their strain hardening capacity at high strains. Strain hardening was found to be a key factor to correctly model the material flow around the indenter. The response of the polymers was governed by the ratio between the plastic and elastic strains involved in the deformation in the contact region. In first approximation, the representative strain was imposed mainly by the geometry of the indenter, while the elastic deformation was controlled by the mechanical properties of the material, a larger strain hardening leading to a greater elastic deformation and a lower plastic strain thus a better scratch resistance of the specimen.
Journal of Tribology, 2004
An experimental and numerical study of the scratch test on polymers near their surface is presented. The elastoplastic response of three polymers is compared during scratch tests at large deformations: polycarbonate, a thermosetting polymer and a sol-gel hard coating composed of a hybrid matrix (thermosetting polymer-mineral) reinforced with oxide nanoparticles. The experiments were performed using a nanoindenter with a conical diamond tip having an included angle of 30 deg and a spherical radius of 600 nm. The observations obtained revealed that thermosetting polymers have a larger elastic recovery and a higher hardness than polycarbonate. The origin of this difference in scratch resistance was investigated with numerical modelling of the scratch test in three dimensions. Starting from results obtained by Bucaille (J. Mat. Sci., 37, pp. 3999–4011, 2002) using an inverse analysis of the indentation test, the mechanical behavior of polymers is modeled with Young’s modulus for the ela...
Scratch resistance of high performance polymers
Tribology International, 2011
Scratch tests were carried out on various high performance polymers, including (1) polybenzimidazole (PBI), (2) polyparaphenylene (PPP), (3) polyetheretherketone (PEEK), and (4) polyimide (PI). The scratch damage features were characterized using laser confocal and scanning electron microscope. Scratch resistance at room temperature decreased in the same order as the materials are listed above. It was attempted to correlate the scratch depth with basic mechanical properties, such as Young's modulus, tensile strength, and scratch hardness. Also, the scratch coefficient of friction was considered as a possible measure to differentiate between the various materials tested.
Wear, 2011
Three-dimensional finite element method (FEM) parametric study was performed by simulating linearly progressive load scratch process on a polymer substrate with a spherical-tip indenter, following the established ASTM D7027-05 and ISO 19252:08 testing protocol. The scratch depth and shoulder height of the groove formed during scratch, which has been shown to be directly related to the scratch visibility resistance of polymers, is simulated by varying a set of material constitutive parameters, namely, yield stress, strain softening slope, strain hardening slope, strain at stress recovery, and strain before hardening. The simulation results indicate that yield stress and strain at stress recovery are two most important parameters that affect scratch depth and shoulder height, thus scratch visibility resistance, of a polymer. Implication of the present findings for designing scratch resistant polymers is discussed.
Measurement approaches to develop a fundamental understanding of scratch and mar resistance
Journal of Coatings Technology and Research, 2004
Instrumented indentation and confocal microscopy were used to characterize the surface mechanical response of polymeric materials. Viscoelastic behavior was measured using instrumented indentation. A model based on the contact between a rigid probe and a viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two polymeric materials, epoxy and poly(methyl methacrylate) or PMMA. Scratch testing was performed on these materials with various probes under a variety of conditions, and confocal microscopy was used to characterize the resulting deformation. Relationships among viscoelastic behavior, scratch damage, and appearance are currently being explored using these methods along with finite element modeling.
Mechanical study of polymers in scratch test
International Journal of Material Forming, 2008
The numerical study of the scratch test on glassy polymers needs a well adapted mechanical behaviour law both viscoelastic and viscoplastic. To test different models, an algorithm which allows to assembly in series two models has been developped. In our cas, it is used to associate a viscoelastic model with viscoplastic one. The simulations carried out lead to some phenomenological results showing the importance to account for the time dependency of the stiffness modulus.