Viscoelastic effects on the scratch resistance of polymers: relationship between mechanical properties and scratch properties at various temperatures

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.     

Visco-elastic visco-plastic analysis of scratch resistance of organic coatings

The sensitivity of automotive coatings to scratching, particularly coatings for plastics has been a growing concern among automakers. Scratching may result from such predelivery events as polishing minor defects embedded in the paint, or postdelivery events such as car wash bristles, dirt embedded under a cloth utilized in polishing the car, tree branches, and the like. Warranty cannot separate out which event is the more prevalent (e.g., predelivery or postdelivery to the customer) form of damage on plastics. Data available on coatings for metal, however, does suggest that isocyanate-based crosslinked systems perform more poorly than their melamine-based crosslinked counterparts when exposed to in plant (predelivery) handling. Coatings on plastics, while lower in modulus than coatings on steel, are still subject to scratch events, albeit they have a greater tendency to “self-heal” once scratching events have occurred. This work attempts to correlate the scratch resistance behavior of three different one-component hydroxyl-functional acrylic or polyester-acrylic-melamine-crosslinked systems. Relationships between the tensile and indentation properties of the coatings were found to be related to the initial warranty data of the coating systems. Most importantly, it was apparent that the critical depth to fracture of the flexible coatings may be the relevant indicator of field performance for coatings on plastics. For metals, the relevant indicator to field performance is often only critical load to fracture, negating the influence of indentation depth. This variance is important to consider since coatings on plastic are often softer and more resilient to surface abrasions than their counterparts on metal.     

Scratch durability of automotive clear coatings: A quantitative, reliable and robust methodology

Scratch and mar durability of clear coatings are issues of concern to the automobile manufacturer and paint supplier. Scratching of clearcoats is a consequence of tribological events encountered by painted exteriors during normal service life. Several subjective methods to assess scratch durability have been proposed. These methods offer little insight into scratch mechanisms. More recently, single scratch methods have been proposed to probe clearcoat scratch mechanisms. This paper outlines a reliable and robust scratch methodology for evaluating scratch durability of automotive clear coatings. It is shown that, with appropriate characterization of tip geometry, quantitative and reproducible critical load values can be obtained. A suggested test method for scratch durability is described. Oak Ridge, TN. 401 Southfield Rd., A103L, Dearborn, MI 48121-6231.     

Measurement approaches to develop a fundamental understanding of scratch and mar resistance

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. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2004, in Philadelphia, PA.