Experimental and FEM analysis of mar behavior on amorphous polymers

Highlights

• Experimental observation and FEM modeling have been carried out to study the mar behavior of amorphous polymers.

• The ${\epsilon }_1^p$ and total $E_p$ on mar path are shown to be criteria for ranking mar visibility resistance of amorphous polymers.

• Mar resistance can be improved with lower modulus, higher yield stress, higher hardening slope, and lower softening slope.

Abstract

Mar is a type of subtle surface damage caused by a sliding object barely visible to human eyes. This minor damage phenomenon has rarely been systematically studied. Significant research efforts for the fundamental understanding of mar behavior in polymers are still needed. In this study, the mar behavior of a series of model amorphous polymers, i.e., polymethylmethacrylate (PMMA), polycarbonate (PC), and polystyrene (PS), were investigated based on a modified ASTM/ISO scratch testing methodology and a corresponding finite element method (FEM) modeling. Furthermore, the mar-induced visibility and material parameter relationships were established through a systematic FEM parametric study. Experimental results show that PMMA has the highest mar visibility resistance, indicated by lower surface roughness variation and low contrast between marred region and the background. The numerical analysis showed that the maximum principal plastic strain $\left({\epsilon }_1^p\right)$ and total dissipated plastic energy $\left(E_p\right)$ can be considered for evaluating mar visibility resistance. Higher mar visibility resistance corresponds to lower ${\epsilon }_1^p$ and $E_p$ values. Based on these two criteria, the parametric analysis shows that mar visibility resistance increases with lower modulus, higher yield stress, higher hardening slope, and lower softening slope. The usefulness of the present study for the preparation of mar resistant polymers is discussed.

Introduction

Retention of the surface properties of polymeric materials is a significant indicator of the quality of many products such as cellular phone casings, packages, and automotive components. Thanks to large volumes of research activities in recent years, single path tribological polymer surface damage features have been well recognized and categorized as ‘scratch’ and ‘mar’ [[1], [2], [3], [4]]. Scratch is a significant surface deformation caused by sliding a sharp asperity. Mar is a subtle surface damage caused by sliding objects on the material surface barely visible to human eyes.

Previous studies quantified scratch resistance based on the scratch groove dimensions, namely its depth, width, and shoulder height [[5], [6], [7], [8]]. Furthermore, distinctive damage transitions are commonly observed along the scratch path as the applied scratch load increases according to the ASTM-7027/ISO-19252 standards. For example, Browning found that periodic micro-cracks developed in the scratch groove for brittle styrene-acrylonitrile (SAN) random copolymers when the scratch normal load reached a critical value [9]. For higher scratch loads, continuous plowing with massive material removal is observed. For ductile polymers, periodic fish-scale type damage dominated by stick-slip phenomenon was observed [10,11]. These scratch behaviors were not observed in the case of mar damage. Unlike scratch, this subtle damage feature cannot be easily quantified because of the gradual evolution of mar damage severity upon increased loading. Also, no clear damage transitions could be observed on the mar path. Instead, only progressive changes in surface roughness and possible wavy or proto-craze types of surface damages were found [12]. This makes mar quantification and analysis considerably challenging.

Mar damage can be caused by one or a combination of the following mechanisms: meniscus wrinkling, uneven viscoelastic deformation, localized molecular orientation, crazing, shear banding, and micro-cracking. The underlying physics of most of these mechanisms are still unclear and cannot be easily modeled. Depending on the surface properties, mar damage can be induced by ironing or roughening mechanisms [13]. Ironing process is usually formed on rough surfaces. Surface asperities are suppressed by the smoother mar tip and the mar area becomes smoother than the background of the sample. In contrast, roughening mar is usually formed on smooth surfaces. The damaged area is roughened by the mar tip.

Mar tests on polymer surfaces were performed using either a smooth barrel head or a rough sandpaper surface [1]. A smooth barrel head usually causes ironing mar. However, it can also induce roughening damage when the tip roughness of the mar head is higher than that of the polymer surface [13]. Given its significant roughness, the sandpaper surface can only generate roughening mar damage. Recently, the smooth barrel tip was used to conduct mar tests on textured thermoplastic olefin (TPO) used for automotive interior parts. This study found a good correlation between the obtained mar damage performance and the results of initial quality surveys (IQS) filled out by consumers [14]. Therefore, this testing condition will be considered in our study because of its simplicity and practicality.

Limited studies were conducted to investigate the behavior of mar damage [8,13]. A fundamental understanding of how the constitutive behavior influences the mar resistance is needed. Previous research has shown that finite element methods (FEM) modeling is effectively in investigating the effect of polymer constitutive parameters on scratch behavior [[15], [16], [17], [18]]. The same methodology was also used to quantitatively predict scratch damage features in ductile amorphous polymers [16]. FEM modeling has been shown to be an efficient research tool for understanding the surface deformation and damage in polymers. In this study, a similar FEM approach was chosen to investigate mar behavior. Given that mar damage feature dimensions are considerably low and cannot be used as a quantification parameter, other alternative criteria should be explored to correlate between mar performance and material parameters. For simplicity, only the elastic-plastic smooth surfaces without asperities will be considered for the present FEM mar behavior simulation.

Also, since it is extremely difficult to experimentally choose polymers with systematic change in their constitutive property without altering other properties, a parametric analysis will be performed to separately determine the impact of the constitutive properties on mar behavior. For instance, previous studies showed that the elastic modulus and yield stress are usually coupled [[19], [20], [21]]. Similarly, the softening and hardening slopes are interconnected as in the case of PS modified with poly(2,6-dimethyl-1,4-phenylene oxide) or di-(ethyl glycol)-dimethacrylate cross-linking agent [22]. Therefore, parametric studies will be conducted using FEM modeling.

The main objectives of the present research are twofold: (1) establish the correlation between the mar behavior and the material's constitutive properties and (2) propose useful criteria for the quantitative assessment of mar performance. For experimental validation, mar damages were created on a set of model amorphous polymethylmethacrylate (PMMA), polycarbonate (PC), and polystyrene (PS) samples following the ASTM-7027 standard, and their performance was assessed using a contrast-based approach. Meaningful correlation between FEM modeling and experimental results will be established. It is hoped that the present study can assist in the design of polymeric materials with improved mar visibility resistance.