Assessing the Damage Resistance of a Coating

Coil coated metal is used in many applications. In each of its uses, some fabrication of the painted metal coil is necessary. After fabrication, additional processes might be used as the painted product is brought to market. During these processes, the coated metal must be designed to withstand forces and matter that can create physical and aesthetic damage to the product. All damage is at the least an aesthetic problem, or—at worst—presents a defect that could lead to a shortened lifetime for the product. This tool kit has been created in an effort to introduce the reader to the current science associated with the mechanics of damage, and the tools and methods available to assess a coating’s resistance to damage.There are many opportunities for damage to occur to a coated metal system, beginning at the point when the painted metal coil is wound-up after the painting and curing operation. Too much rewind tension could create pressure mottling; too little tension might lead to abrasion as one wrap of the coil moves against another. Many times these conditions are referred to as imprinting, flecking, picking or scuffing. If the painted coil is transported “eye horizontal,” pressure mottling can occur, since the weight of the entire coil rests on a small surface area. Coil coatings are routinely roll formed, brake formed, drawn, punched, notched, and processed in countless ways.Each of these processes represents an opportunity for damage to occur. In the case of building products, nested stacked panels produced at a fabricating plant are transported to a job site and once at the job, are now handled under the usual conditions seen at a construction site. During the transportation step, the nested stack of panels will experience some amount of vibratory movement. The rubbing of the back side of a panel to the front side of another panel is an opportunity for wear. While at the job site, panels from the nested stack could be dragged—rather than lifted—from the stack as they are installed onto the building. When one panel is dragged over another, there is an opportunity for the edge of one to cut into the face of another; therefore, abrasive wear may take place.Many building panels are nailed or screwed to a support frame. During this process the coated metal system is pierced. In addition to resisting the damage of a piercing operation, coatings may be damaged from errant hammer strikes, a dropped tool, or a person walking on a panel with stones embedded into the tread of their work boots. There are countless opportunities for a coating to be damaged from the time that it is coil coated until it is actually placed into service. Coil coated metal has been designed to resist this damage, but assessing the physical properties needed for a paint film to resist the various opportunities for damage is a daunting task. The branch of science that studies friction, lubrication and wear is known as Tribology. For coil coatings, we are primarily concerned with two types of wear—Abrasive Wear and Adhesive Wear. Adhesive wear is concerned with a transfer of material from one surface to the other, or the formation of a particle that can then create wear. Abrasive wear is the result of two surfaces rubbing against each other, but where no material transfer is evident. Wear, however, cannot be easily categorized. In addition to adhesive and abrasive wear, there are other types of wear that have an impact on coil coatings (such as erosion wear from wind-blown particles of sand), or represent complex combinations of wear (such as fretting wear that results from small oscillating motion, which might occur, for example, as nested building panels, banded loosely to a skid, are trucked to a job site). This tool kit, therefore, introduces the reader to the science of Tribology, while, at the same time, limiting the discussion to those types of common modes of potential damage that a coil coated metal system experiences. Tribology： Tribology is a science that studies the interaction of surfaces. These surfaces might be the piston in an internal combustion engine, or the lubricant that attempts to minimize the influences of friction between the piston and cylinder wall. Tribology is the study of friction, lubrication and wear. These cannot be neatly separated into individual issues. Interactions between surfaces are not trivial. They involve complex relationships among myriad factors, but this tool kit attempts to minimize the complexity. Abrasive Wear: Abrasion resistance is the ability of a material to withstand wear. Although wear and abrasion are often used interchangeably, there is a difference. Abrasion is the action (or cause); wear is the result (or effect). ASTM defines abrasion as “the wearing away of any part of a material by rubbing against another surface”. Abrasion wear is caused by mechanical action of surfaces rubbing against each other, resulting in the loss of material. There are many types of wear which are defined by the type of motion between the surfaces. Wear is often a natural consequence of two surfaces moving in relative motion to each other which interact in a largely unpredictable manner.