The objective of a laboratory test method is to define the approach that will permit an operator to obtain meaningful, reliable data. As materials and decoration techniques evolve, commonly used test methods to measure abrasion resistance of decorated plastics may no longer be sufficient. To ensure a robust product, it is essential to understand how to recreate and measure “real-world” damage. This article presents a process to develop (or improve) test methods intended to quantify the resistance of surface wear on decorated plastics. Emphasis is placed on reducing common sources of test procedure variation and advanced techniques to interpret and quantify the results

For many products, it is easy to identify something that is worn. But understanding how it got to that state is not as simple! The process of wear is a complex phenomenon and trying to replicate it exactly in a laboratory setting is often extremely difficult. ASTM International defines wear as “damage to a solid surface (generally involving progressive loss of material), caused by the relative motion between that surface and a contacting substance or substances ^[1]. In most instances, the material removal is a gradual process and the motion is a repetitive action.

Two common types of wear that occur with plastics are wear abrasion and mar abrasion. Wear abrasion is the removal of a portion of the surface by some kind of mechanical action (e.g., rubbing or sliding back and forth of an object). Mar abrasion is the permanent deformation of a surface but the deformation does not break the surface. Wear and abrasion should not be confused. Although the terms are frequently used interchangeably, wear is “the wearing away of any part of a material by rubbing against another surface ^[2] and abrasion is the action that causes it.

Laboratory wear tests have the potential to provide considerable insight into the various factors that contribute to a material’s performance, but most laboratory techniques do not exactly duplicate the mechanics seen in real-life. This is because there are often multiple influences that impact the rate of wear. It is imperative that the limitations of testing be understood because accelerated wear testing may not always identify potential problems or predict field performance results. Despite this, a controlled laboratory test allows the user to approximate field conditions and eliminate extraneous variables. This enables the life span of a product to be compressed into a much shorter duration, and allows materials to be evaluated in the same manner within a controlled environment. Additionally, the cost of a laboratory test is significantly less than a field study.

Defining an Approach
Attempting to recreate wear damage in a laboratory setting involves determining a complex combination of interrelated properties. The objective of the test is to provide predictive performance under a specified set of criteria and correlate with end-use performance. Yet when evaluating the conditions a product is exposed to during its life, one quickly realizes the task of developing a test methodology is both multifaceted and difficult. A basic principal of establishing new abrasion tests is to use the simplest technique first and to stop once the required information is available. Quite often, the additional information that is obtained by studying additional factors does not justify the additional time and cost.

An engineer concerned with reliability and product life may require precise simulation of the wear system. In contrast, a material developer looking to rank the wear resistance of materials may accept a convenient test that does not exactly replicate intended use. In either case, to generate useful data careful consideration must be given to the wear system and failure mode. The wear system is comprised of the test piece and contacting material(s) along with the relative movement that causes the wear. The failure mode is established by how the system wears and which wear modes are involved. (For additional information on wear modes, see ASTM International, G40 Terminology).

Wear is a response resulting from the conditions to which the whole system is exposed. Resistance to abrasion is affected by the nature of abradant, variable action of the abradant over the area of specimen being abraded, tension of the specimen, the pressure between the specimen and abradant, and the dimensional changes in the specimen. While most standardized test methods specify the parameters that must be adhered to when conducting tests, it should be accepted that all of the influences that create wear conditions probably will not be able to be accurately identified. However, give careful consideration to contact geometry, length of exposure, interacting material surfaces, normal force, sliding speed, environmental conditions, and material composition and hardness. (Note: Do not become distracted when attempting to isolate and replicate the influence of each parameter.)

How does one determine the conditions to which a product might be exposed? The first approach is to consider prior knowledge. If studying a field failure, examine the appearance of the surface wear from an actual application. Keep in mind there may be multiple wear modes occurring at the same time and recognize that matching conditions in a laboratory are usually not perfect. The following parameters are normally associated with sliding wear on plastic materials ^[3]:

1.  Intrinsic parameters relating to the materials involved, such as their nature, surface condition, and finish. These include bulk properties (e.g., chemical composition, physical characteristics, mechanical properties, and hardness) and surface properties (roughness, physico-chemical characteristics).

2.  External parameters relating to the sliding conditions, such as applied load, sliding velocity, characteristics of the motion, mode of contact, ambient conditions (temperature, humidity), and the interstitial substances (lubricant, wear debris).

3.  Parameters depending on both the nature of the materials involved and the sliding conditions, particularly surface temperature of the rubbing surfaces.

Many industries have established test procedures and recommend abrasion instruments that might be used to simulate the wear. Unfortunately, “there are a lot of customer and industry specifications that really are not an indicator of meaningful product performance or durability” [4]. If a method does not exist or it is decided that the industry standard will not be followed, a test modeling the system to be studied should be selected. If unsure of where to start, begin by contacting research industry associations to determine if an accepted abrasion test procedure exists. Other sources of information include organizations that develop test standards, such as ASTM International and ISO.

Generating Reliable Data
Numerous instruments exist to evaluate a material’s resistance to surface wear damage. Because the results for each apparatus are based on that tester’s unique system, data is generally not comparable between different instruments. And occasionally, materials may not exhibit the same relative order of resistance to abrasion when tested by different methods.

The primary elements involved in simulating a wear system include apparatus design, specimen preparation, test protocol, and measurement. Whether or not a particular type of abrasion test correlates with end-use performance depends not only on a similarity of abrading mechanisms but also, on the extent to which that mechanism is maintained during the course of the abrasion test. The following describe the important features that should be considered:

Motion. The type of relative motion is often used to define the wear that is generated. Because of its complexity, a number of different wear modes have been recognized and include rubbing, sliding, rolling, and scuffing. Wear can occur in combination or on different areas of the same component.

Apparatus. The test apparatus should be of a rugged design to provide repeatable and reproducible results. Parameters such as load, speed, rigidity of apparatus construction, alignment, and supply of abrasive require adequate control to ensure stable wear conditions. The most commonly used testers for decorated plastics involve a reciprocating movement; rotating abrasive disc/wheel; or point contact.

Materials involved. The structure of the wear system includes the specimen and counter-body (usually an abradant of some sort). Basic properties such as elasticity, hardness, strength (including cohesive, tensile, and shear strength), toughness and especially in the case of wear resistance, thickness can all influence wear resistance of the materials. Be aware that a material can wear differently when exposed to different situations, or may be influenced by the wear of the other contacting body.

Abradant (Wear Agent). The mechanism of wear depends upon the topography of the counterface abradant. Popular types of abrasives include textiles, engineered abrasives, and sandpaper. Although abrasive particles may not be the primary cause of actual wear, they are often used to accelerate the test. Abrasive particles, regardless if they are embedded in a binder material or are loose, have a strong influence on the rate of wear. It is preferable in most cases to use an abrasive only once, unless it can be refreshed. Consider the following:

- Shape – Particles that are angular or “blocky” in shape can cause up to ten times the wear rate as compared to rounded particles.

- Size - The size of the particle is critical, as smaller particles cause proportionally less wear than larger particles. Particles responsible for abrasion or erosion are typically between 1 µm and 500 µm in size.

- Type – Popular abrasive particles include silicon carbide and aluminum oxide. With sandpaper, silicon carbide creates a thinner scratch pattern due to being a sharper grain than aluminum oxide and will typically cut faster.

- Friability – How easy the abradant breaks down and fragments under localized heat and pressure, creating new sharp edges.

Contact Geometry. This includes the shape of the abrading head or abradant, and contact between it and the specimen. Some systems may require the specimen and abradant to “wear-in”, thus establishing a uniform and stable contact geometry. Although point-contact eliminates many alignment problems associated with other contact geometries, stress levels may change as wear progresses, requiring more complex data analysis and comparison techniques.

Contact Pressure (Applied Load). With an accelerated test, the load may exceed what is actually seen in the field. This parameter usually involves the amount of force used to push the abrading material against the specimen during the rubbing action. Do not use a load that exceeds the ultimate strength of a material.

Sliding Speed (Sliding Velocity). This is the speed of the abradant as it moves over the specimen. While acceleration in a test is desirable, if the speed is too fast for the material (abradant), the precision of the test may be compromised by introducing different phenomena. Excessive speeds can cause a typical thermal condition on the test specimen.

State of lubrication. Lubrication will affect the frictional characteristics of a material. Usually involved with metals, many plastics formulations also include a lubricant additive.

Specimen Preparation. Specimen preparation and the details of test control vary with the test and materials involved. For example, surface roughness, geometry of the specimens, homogeneity, and hardness should be controlled for reproducible test results. Similar controls also are necessary for the counter-face and the wear-producing mediums. When evaluating multilayered systems, the substrate plays an important role.

Environment. Many materials are sensitive to changes in temperature and humidity, and changing the test environment may influence results.

A well thought out wear test can provide valid data without exactly replicating the application. Before attempting to recreate surface damage, a step that is often overlooked is to establish the purpose of the test and how the data will be used. Taking the time to state the objective(s) before conducting any tests will help keep focus and minimize distractions. 

The majority of companies conduct testing only because they have a customer or industry specification they must satisfy in order to sell their product. Others utilize testing to better understand their product/process. Regardless of whether the decoration is for cosmetic appeal or functional performance, the primary reason companies conduct surface damage tests on decorated plastics is to ensure that they are producing a quality finish that will endure throughout the product’s life cycle. The goal is to make certain the product maintains a minimum performance over its estimated life and withstands deterioration or wearing out in use. This becomes challenging because customers have a habit of using products in applications for which they were not originally designed or tested. A common problem facing the industry today is the chemical attacks that household cleaning agents and personal care products have on coatings. Due to the many different formulations of hand creams, sunscreens, insect repellants, etc., available to consumers, it is not feasible to test the effects of each. In this case, having a stated test objective will help keep testing on track.

Reducing Common Sources of Variation
As stated in the introduction, the process of replicating wear in a laboratory setting is a complex phenomenon. There are multiple sources of variation that can influence test results. Being aware of each allows the development of an approach that can provide a means to generate repeatable and reproducible data.

If developing a new protocol, keep in mind that test development is dependent on the capability of the developer. It may take some trial and error to establish a test procedure that provides useful information. An exact simulation is generally neither practical nor possible and some differences will have to be accepted. This is because wear involves two or more bodies, one or more materials, and is dependent on a wide range of influences. Once a procedure is identified, it must be documented. Test methods that lack critical procedural information could introduce problems with reproducibility, as much of the variation that occurs with abrasion tests result from operator error.

Depending on the apparatus, any of the aforementioned parameters may introduce variation into test results. This is normally seen with companies that do not follow an established test method or are utilizing a procedure that is missing critical information.  Review the test set-up parameters to ensure they are the same for each test. For example, operators often overlook the importance of sample positioning. If the apparatus being used does not secure specimens in the same position, consider a fixture or other device. Furthermore, the rate of abrasion may change as debris adheres to the body that is generating the wear. Failure to change or refresh the abradant may cause a decrease in the abrasion rate. Another issue may be the age or condition of the abradant, especially if it has a shelf life or is adversely impacted by environmental/storage conditions. Finally, do not overlook the importance of providing effective training for laboratory personnel. Variation also can be introduced if the technician does not understand the proper usage of the device and is utilizing it incorrectly.

Advanced Techniques to Interpret and Quantify the Results
Abrasion resistance is normally calculated using one of three methods: loss in weight for a specified number of abrasion cycles (mass loss); number of abrasion cycles required to wear the coating through to the substrate material (wear cycles per mil); or a visual change in the appearance of the specimen (amount of coating removed compared to predetermined standards). Other methods that have been used include volume loss, depth of wear, haze measurement (for transparent materials), and strength testing.

Most of the recognized abrasion test methods provide a comparative measurement of wear resistance and the results are used to rank materials. For decorated plastics, results are normally interpreted by a subjective assessment of the appearance or condition of the specimen after a fixed number of abrasion test cycles. For repeatable results, a standardized grading system (e.g., 1 – 5 visual scale) should be used to measure the change in appearance and rank performance. Reference photographs along with an associated verbal description are often provided to indicate an evenly spaced ranking. Another popular option is to determine the number of cycles required to generate a specified level of destruction (e.g., change in gloss, color, thickness, wear through).

As technology advances, magnification of the test sample can be accomplished in-house at a relatively modest cost. With the electronic industry striving to continue miniaturizing components, tools are available that can be used to magnify parts in all cost ranges. A 10x enlargement using a reticle eyepiece or microscope may continue to be sufficient if the application is for decoration. But a magnification of 200x or 300x may be necessary if the application is performance-based. In these cases, software may be utilized for conducting analysis of surface damage or for generating a 3D surface mapping of the part.

Whatever the method of evaluation, wear rate may not always be a linear function of time or number of contact cycles – it depends very much on the materials, type of wear, and the contact conditions.  Extrapolate results only with great care. The primary reason that companies conduct abrasion tests is to ensure that they are producing a quality product that is free from defects, consistent in characteristics and quality, and will endure throughout its life cycle. Through testing, a company can monitor quality assurance of the manufacturing process; conduct product research and development; demonstrate its product conforms to industry standards; develop new products; provide information to buyers; establish criteria for warranties; etc. Employing a meaningful test program is a necessary step to validate product quality and to ensure that the specified material or surface finish meets the customer’s expectations.

Unfortunately no laboratory abrasion test can guarantee success in the field; there are just too many influences to be modeled in the lab. The effect of abrasion is generally only one of several factors that contribute to product durability, and the relationship may vary with different end uses. It is not recommended to rely solely on abrasion results to predict wear-life, unless there is data showing a specific relationship between laboratory abrasion tests and actual wear in the intended end-use. While it is advisable to establish a predictive wear model for design and component life estimation, no model is universally satisfactory.  But with proper consideration, abrasion test results can provide meaningful, reliable data.

References

1.    ASTM International, G40 Terminology Wear & Erosion
2.    Ibid.
3.    International Standard ISO 6601, Plastics – Friction and wear by sliding –identification of test parameters
4.     A.F. Zielnik, Test of Time: Will Your Finish Last? Finishing Today,  p. 28 (June 2007)

Alan Jaenecke is VP of marketing; materials test and measurement/press divisions for Taber Industries and plays a critical role in the company’s Materials Test and Measurement division. In charge of new product development and strategic planning, he has given presentations on physical property testing of numerous materials and has written new test methods and coordinated numerous reviews for existing methods. For over 65 years, Taber^® Industries has been helping companies understand wear resistance. Taber instruments have been utilized in diverse applications including paints and coatings, textiles, paper, laminate flooring, plus many more. For more information on Taber Industries, call (716) 694-4000, email [email protected] or visit www.abrasiontesting.com.