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An MD&DI August 1999 Column

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Understanding Wear and Friction in Medical-Grade Stainless Steels

A knowledge of the different types of wear and friction and of current test methods can lead to better-designed stainless-steel devices.

John D. Jarrell and Frank Bejbl

There has been little information—and a good deal of misunderstanding—regarding the subject of wear and friction of medical-grade stainless-steel materials. The following article was prepared on the premise that information addressing the basic behavior of stainless steels would greatly benefit both OEMs and process subcontractors in understanding these important topics.

Devices such as stainless-steel surgical drill bits can be susceptible to wear. The products depicted have been treated with a chromium composition coating (ME-92, from ME-92 Operations) to improve wear resistance.

The article begins with a brief survey of stainless steels—their various categories, uses, and physical and chemical properties. A section on wear introduces the types and causes of wear, reviewing galling, seizure, and abrasion. Several test methods for evaluating wear and galling are presented, along with factors affecting wear and an outline of design considerations for reducing wear and galling. The two classes of friction are then discussed, as are test methods for their measurement and design recommendations for friction applications.

CLASSIFICATION OF STAINLESS STEELS

Stainless steels are regularly used by the medical, chemical, and pharmaceutical industries because of their corrosion resistance, biocompatibility, and ability to withstand elevated temperatures. They offer superior corrosion resistance compared with other steels and aluminum, which is primarily due to the addition of chromium (at least 11% by weight) to the steel and the subsequent presence of chromium oxide on the outer surface. This passive surface layer superficially protects the base metal from further oxidation. Compared with titanium and cobalt alloys, stainless steels are readily available and relatively inexpensive. Some grades have been used for implantation and fixation because of their biocompatibility, while other types can be hardened for use as surgical cutting tools or pharmaceutical molds.

For medical applications, stainless steels can be classified into the following four categories: austenitic, ferritic, martensitic, and precipitation hardening. Tables I and II present the chemical and nominal mechanical properties for stainless-steel materials in each of these classifications.

Austenitic stainless-steel types include both 200- and 300-series materials. They have high concentrations of chromium and nickel and cannot be hardened by a quench-and-temper heat treatment. Hardness is generally in the Rockwell Rb 88 to Rb 95 range for annealed material and can be increased by cold-working. These are normally nonmagnetic grades, but some magnetism can be developed by cold-working. This category offers the highest degree of corrosion protection. Types 304 and 316—the first implanted stainless steels—are the most commonly used varieties from this classification. Both are available in a low-carbon grade that offers superior corrosion resistance in the welded condition.

Ferritic types include some of the 400 series. These are nonhardenable steels, and are typically used for deep-drawn containers and some solenoids. Hardness of annealed material typically ranges from Rb 80 to Rb 100 (Rc 22.8), with only a moderate increase in hardness generally achievable by cold-working. Stainless steels in this classification are magnetic and provide less corrosion resistance than 300-series materials, but are especially resistant to stress corrosion cracking. They are seldom used for medical instrumentation involving contact with internal body fluids or tissues. Type 430 has been used for various medical applications, including operating tables, device cabinets, and trays.

Martensitic steels make up the remainder of the 400 series. These may be hardened via a quench-and-temper process, with hardness after heat treatment ranging from Rc 48 to Rc 56 for type 420 and Rc 52 to Rc 60+ for type 440C. Magnetic in the annealed and hardened conditions, these materials are not as corrosion resistant as the other categories of stainless steels. Types 420 and 440C are often used for surgical cutting instruments, drills, and reamers.

Precipitation-hardening (PH) stainless steels have properties between those of the martensitic and austenitic types. Hardness is developed by a solution heat treatment followed by an aging treatment, through which a second, hardening phase is formed. Hardness for 17-4 PH ranges from Rc 29 to Rc 44. These types have higher corrosion resistance than the martensitic varieties and offer higher bulk hardness than do 200- and 300-series materials. Contrary to what one might expect, this increase in hardness does not provide greater wear or galling resistance. These grades are magnetic and are widely used in orthopedic instruments.

CLASSIFICATION AND DESCRIPTION OF WEAR

When metal mechanical parts or assemblies fail, it is usually the result of fracture, corrosion, or wear. Wear certainly seems to be the least understood of these three factors; however, its importance in medical applications should not be overlooked. It is often assumed that wear and friction are very much the same thing, but lowering friction does not necessarily lower wear. Limited experience has caused some to maintain that a direct relationship exists between higher hardness and lower wear. The fact is that large-scale testing on a variety of materials shows that these are two distinct properties. Wear and friction are quite different.

Wear can be defined as both material loss and deformation at contact surfaces. Wear results in particle generation and surface degradation. Therefore, reducing the wear of surgical instruments lowers the production of wear particles and thus the possibility of introducing foreign matter into the body. Extended instrument life and consistent performance are other reasons for instrument designers to consider means of minimizing wear. Many types of wear have been observed and classified, as discussed below.^1,2

Adhesive Wear. Adhesive wear is a common form of metal loss. In surgical instrument applications, adhesive wear and galling are the most frequently occurring forms of wear. Adhesive wear takes place when no outside abrasives are present between two wear surfaces. At low loads, an oxide film usually forms on the contact surfaces. This prevents metallic bonding between the mating materials but results in a low rate of wear referred to as oxidative or mild wear.

As loads are increased, a transition is reached: the oxide film breaks down and metallic bonding (cold welding) takes place between the two surfaces. This is referred to as severe metallic wear, and causes rapid material loss and high wear-particle generation.

Galling is a special case of severe adhesive wear. The wear particles can no longer be accommodated by the surface roughness and contact clearances. At this point, the contact surfaces become cold welded. Further movement causes the surface metal to de-form and tear. In extreme cases, metal seizure occurs. When contact pressures are high, galling and seizure can transpire with minimal amounts of sliding (less than one revolution or fractions of an inch).

Abrasive Wear. Abrasive wear occurs when a hard material scratches or gouges the surface of a softer material. The abrasive material can be either of the sliding surfaces or particles between the two surfaces. When the contact stress is too low to crush the abrasive particles, the cutting action is defined as low-stress scratching abrasion. This usually results in surface scratches with little subsurface deformation. In high-stress grinding abrasion, however, the loads are great enough to crush the abrasive material. This action usually causes permanent plastic deformation of the base metal, along with material removal. Gouging abrasion occurs when high stresses create significantly large grooves on the contact surfaces.

Erosion. Erosion is material loss from the abrasive action of moving fluids on a component. Erosion can be intentional, as when high-pressure liquid streams are used to perform rapid machining operations.

Components that experience large pressure changes and turbulent flow can wear due to cavitation erosion. Cavitation is the formation and collapse of numerous small bubbles during turbulent flow. The ultrasonic shock of the collapsing bubbles scrubs the metal surface, which can cause long-term surface loss. Fluid valves and pumps are examples of applications in which cavitation may occur. Surface destruction is accelerated by the presence of solid particles within a fluid stream.

Wear caused by suspended solid particles is referred to as impingement erosion. A material's resistance to impingement erosion varies with the angle of particle impingement and material hardness.¹

Fretting. Fretting wear is material loss that takes place between tight-fitting surfaces that are subject to vibrational movements (such as riveted or otherwise-fastened joints and electrical connections). Material loss is from a combination of oxidative and abrasive wear. The oscillation of the two surfaces causes the formation of oxide films that are then abraded away by oxidized wear debris. The affected surfaces sometimes look as if they were mechanically deformed, so this wear is sometimes called false brinelling. It is also referred to as fretting corrosion, friction oxidation, chafing fatigue, or wear oxidation.³

WEAR TESTING

Tests are constantly being developed to measure the properties of materials. Some tests are designed to simulate actual use. These often use a large number of variables, such as speeds, loads, and times. For example, one could make a surgical bone cutter using all of the available material options. Each cutter could then be evaluated with a series of simulated-performance tests. This would certainly determine which of the materials was best suited for the particular application—but most likely at a substantial cost. In addition, the results of such tests would only be applicable to selecting materials for devices of similar design and use.

More general tests that measure characteristic properties tend to apply to a larger number of engineering design problems. Their use often allows a designer to rapidly narrow the selection to a few materials. One must use caution when applying general results to a specific design problem—unless the intended use is similar to the test configuration. Even greater caution must be used in applying results from simulated tests to different engineering problems.

Several tests have been used by industry for measuring the wear and galling resistance of materials. Three popular ones—for which much data exist—are ASTM G 83 "Standard Test Method for Wear Testing with a Crossed-Cylinder Apparatus," ASTM G 99 "Standard Test Method for Galling Resistance of Materials," and ASTM G 99 "Standard Test Method for Pin-on-Disk Wear Testing."^4–6 Brief descriptions of each of these tests are provided in the following sections.

Crossed-Cylinder Wear Test. For the crossed-cylinder test (ASTM G 83), two cylindrical specimens are positioned perpendicular to each other in the test machine. (Some commercially available machines have an optional force-measuring system for determining coefficients of friction.) The test equipment allows one specimen to rotate at speeds of up to 400 rpm. The second, nonrotating specimen is pressed against the moving specimen at a specified load by means of an arm and attached weights. Sample surfaces are typically ground to a finish of 16 µin. AA or less. The test duration and rotational speed are varied depending on which of three test procedures is specified. The amount of wear is determined by weighing the specimens before and after the test. Results have been reported as weight loss, but are typically converted to volume loss.

Figure 1. Schematic of a "button and block" galling test arrangement.

Threshold Galling Stress Test. In the threshold galling or "button and block" test (ASTM G 98), a small button and a large block sample are machined to provided two flat surfaces with a 10- to 45-µin. AA finish. The couple is then compressed together at constant load with a hydraulic or screw-feed press. The test arrangement is presented in Figure 1. One specimen is slowly rotated one revolution relative to the other specimen. Sliding surfaces are then examined for the appearance of torn metal, which indicates that galling has occurred. If the specimens have not galled, new samples are tested at higher loads until galling occurs. Couples with higher-threshold galling stresses have greater resistance to galling.

Figure 2. A top view of a test arrangement (ISC-200 PC tribometer) used for a pin-on-disk wear test.

Pin-on-Disk Test. For the pin-on-disk wear test (ASTM G 99), two specimens are required. A pin with a radius tip is positioned perpendicular to a flat disk. A ball, rigidly held, is often used as the pin specimen. The test machine rotates the disk at 60 to 600 rpm. The wear path is a circle on the disk surface. Surface finishes of 32 µin. AA or less are typically used. Test results are usually reported as volume loss per cubic millimeter for the pin and disk separately. The results can be plotted as wear volume versus sliding distance or as friction versus sliding distance. A top view of the test arrangement is shown in Figure 2.

FACTORS AFFECTING WEAR

Wear is generally affected by several factors, among them materials selection, friction, surface load, sliding distance, surface hardness, surface finish, and lubrication. Controlling these factors can contribute to a successful application by helping to prevent wear and premature product failure.

Materials Selection. Wear varies greatly depending on the materials selected. For critical applications, it is crucial that the person responsible for materials and process selection is sufficiently experienced and has adequate information available to make a sound engineering decision. Carelessness in this area can result in poor instrument performance and even field failures. The time and money spent in proper materials and process selection can be vastly less expensive than field failures and product recalls.

Friction. Researchers have not observed a consistent relationship between friction and wear. Materials with low coefficients of friction may have high wear rates (Teflon, PTFE), and some high-friction materials, such as cast iron, have low rates of wear. Again, one must remember that friction and wear are different.

Surface Load. Increasing the surface load between sliding surfaces has been found to cause a proportional increase in material wear rates.

Sliding Distance. Increasing the sliding distance of an assembly will cause a roughly proportional increase in material loss.

Surface Hardness. Wear resistance generally increases proportionally with increasing surface hardness (which may be very different from bulk hardness). Surface deformation during wear causes a localized increase in surface hardness on work-hardened metals. This explains why high-strain-hardening, austenitic stainless steels are more wear resistant than harder precipitation-hardened stainless steels.

Surface Finish. Surface finishes between 10 and 70 µin. AA have been found to offer similar wear resistance for many stainless steels.⁷ Very smooth surfaces promote cold welding (galling) because of high molecular interaction. High finishes also lack adequate clearance to store wear debris. Overly rough finishes increase wear by allowing surface asperities (irregularities) to become interlocked, resulting in what is known as a "stick-slip" phenomenon as these surface irregularities are subsequently worn off when sliding occurs.

Lubrication. Lubrication can be used to prevent or reduce metal-to-metal contact and cold welding. Contact stresses may also be effectively reduced with lubricants. The type of lubrication and delivery system are important considerations for reducing wear and galling.

Many surface modifications and coatings are designed to perform as a "dry" lubricant. These can be used when wet lubrication is not practical.

DESIGNING FOR REDUCED WEAR AND GALLING

The following design recommendations can be helpful in reducing galling and wear with stainless steels:^8,9

• Select mating materials with adequate wear and galling resistance. In unlubricated systems or sporadically lubricated systems, materials selection becomes especially critical. Assemblies used in medical and surgical applications generally lack adequate lubrication. Therefore, it is desirable to:

  (a) Select materials with high-threshold galling stresses for mating surfaces.

  (b) Use dissimilar materials and/or those with differential hardnesses on the sliding surfaces.

  (c) Select high-work-hardening austenitic stainless steels for improved wear resistance (but not galling resistance) at temperatures below 350°F.

  (d) Select alloys for mechanical properties alone and apply surface modifications to obtain the desired wear and corrosion resistance.

• Lubricate when possible.

• Keep load, temperature, and speed as low as possible.

• Start with mated surfaces having a finish between 10 and 70 µin. AA. Surfaces below 10 µin. increase a material's susceptibility to galling, whereas those above 70 µin. increase susceptibility to wear.

• Increase the part contact area:

  (a) To lower contact stress below threshold galling stress.

  (b) To lower the depth of wear by spreading the wear volume over a greater area.

• Specify acceptable part dimensional tolerances, proper clearances, and fits. Good materials selection and lubrication cannot be expected to make up for poor mechanical design.

• Consider the use of a reliable wear-resistant coating to extend the capabilities of bare contact materials.

FRICTION AND FRICTION COEFFICIENTS

Friction is the resistance to movement encountered when one surface slides or attempts to slide over another. Friction is generally described as either static or dynamic. The resistance to the initiation of sliding is called static friction and is generally greater than the dynamic friction that continues once sliding has begun. Friction is expressed in terms of static and dynamic coefficients.

The coefficient of friction, f, is considered to be one of the general physical properties of a material, and is defined as the ratio of the sliding force to the total force pressing the two surfaces together. It is described by the following formula:

where f is the coefficient of friction, F is the force required to initiate sliding, and N is the total (normal) force between the two surfaces. Figure 3 illustrates a block sliding on a flat surface, showing that the normal force (N) is equal to the weight of the block.

Figure 3. Diagram of block sliding on plane. The normal force (N) is equal to the block's weight.

FRICTION TESTING

A variety of testing instruments have been developed to measure the force required to initiate or sustain sliding. Both sliding and rotational setups are commonly used. For example, a gauge may be employed to measure the force required to initiate and sustain the sliding of a block on a horizontal surface. These force measurements are then divided by the block weight to determine the static and dynamic coefficients of friction. Such a setup is referred to as a flat-on-flat (FOF) arrangement.

Torsional load cells have also been used to measure the resistance to rotation caused by pressing blocks of one material against round samples of another. In these cases, the measured resistance is divided by the normal contact load to calculate the frictional coefficients. Organic solvents are routinely used for final cleaning in unlubricated friction tests, so as to remove surface films. Oils on the contact surfaces generally alter test results by giving unusually low friction values.

Figure 4. Diagram of the inclined-plane test.

The Inclined-Plane Test. A simple way of determining the static coefficient of friction for materials is by measuring the largest angle of inclination before sliding. A block of one material is placed on a flat surface, which is slowly inclined until the block begins to slide. This arrangement is depicted in Figure 4. The resultant force (R) is equal and opposite to the force (P) of the sliding body. The resultant force can be resolved into a normal component (N) and a frictional component (Fr), which resists sliding. The coefficient of static friction is given by the following equation:

where a₀ is the angle of rest or repose immediately prior to block sliding. Once sliding begins, the frictional force, Fr, typically drops to a lower resistance value, F. The coefficient of dynamic or sliding friction, f, is commonly expressed as F = Nf.

Much research has been done to characterize friction between similar metals and various couples. Friction testing results can vary greatly depending on test configuration and conditions, and only those results collected under the same or similar circumstances should be compared. There is still a great need for standardized friction databases.

DESIGNING FOR REDUCED FRICTION

Surface conditions are critical when measuring friction. Friction is sensitive to many factors, such as atmospheric dust and humidity, oxide films, surface finish, sliding velocity, load, temperature, vibration, surface contamination, and lubrication.¹⁰ Testing is often necessary to establish how each factor affects a particular assembly. To make a proper selection, each material couple should be evaluated with the following variables in mind.

Sliding Configuration. It is important to know what type of sliding is taking place (rotational, translation, etc.) and which material is the moving member. Friction can change depending on which of the two surfaces is the sliding member. For example, Teflon sliding on titanium alloy has a static friction coefficient of 0.17, whereas titanium alloy sliding on Teflon has a value of 0.23.¹¹ Unlubricated assemblies are especially sensitive to variations in test configuration.

Materials Selection. Coefficients of friction depend on both mating materials. Friction can vary greatly when one of the mating materials is changed. Reducing the coefficient of friction reduces the force required to operate an instrument. This translates into easier instrument operation and greater operator sensitivity.

Lubrication. There are several types of lubricants used to reduce friction. Graphite, molydisulfide, PTFE, and chromium composition coating are examples of dry lubricants. Molydisulfide performs excellently under vacuum conditions, whereas graphite can become abrasive under the same conditions. Various water-soluble and oil-soluble liquid lubricants are also available. The operating parameters and biocompatibility of solid and liquid lubricants must be considered for each application.

Surface Finish. Excessive surface roughness generally increases friction between unlubricated materials. The shape of the surface asperities is important. Surfaces with the same average arithmetic surface finish may have very different surface profiles. More jagged profiles increase friction by biting into the mating material. Polishing of surfaces can be used to lower friction. The friction of bearings often becomes lower during use from a mutual polishing that takes place, assuming that wear debris is removed.

Available Oxygen. It is helpful if the operating conditions maintain an oxide film on the sliding surfaces. Because some materials perform poorly in environments that lack oxygen, special consideration must be given to systems operating under inert gases, reducing environments, or vacuum.

Contact Load, Sliding Velocity, and Temperature. Increasing the contact load can cause an initial increase in friction that is related to changes on the contact surfaces. Increasing the sliding velocity can increase the operating temperature, which also usually increases friction. However, if one of the two surfaces liquefies as a result of heating, friction may decrease.

Contamination. The presence of surface contamination can either increase or lower friction. Dust, wear particles, and debris tend to increase resistance to sliding. On the other hand, some films—such as body oils—can act as lubricants. Oils picked up during handling, however, can accelerate surface corrosion.

CONCLUSION

Both wear and friction are important considerations in the design of medical devices, equipment, and the tooling used for instrument manufacture. Many medical applications require special attention to reduce or eliminate unintentional material implantation caused by the generation of metallic debris. Patient exposure to any contamination originating from the tooling and manufacturing must also be minimized.

End-users are demanding devices that are well designed, functional, and consistent in performance and appearance. Devices need to perform reliably each time they are used: it is unacceptable, for example, for instruments to bind, seize, or stick. In order to meet the high expectations placed on medical devices, designers, manufacturers, and engineers must consider the wear, galling, and friction characteristics of their materials and should be familiar with the available means of enhancing these critical properties.

REFERENCES

1. ASM Metals Handbook, 7th ed., vol. 6, Welding, Brazing, and Soldering (Materials Park, OH: ASM International, 1982), 772–773.

2. WJ Schumacher, "Wear and Galling Can Knock Out Equipment," Chemical Engineering (May 9, 1977): 155–160.

3. ASM Metals Handbook, 9th ed. (Materials Park, OH: ASM International, 1992), 772–773.

4. "Standard Test Method for Wear Testing with a Crossed-Cylinder Apparatus," ASTM G 83-90, Philadelphia, American Society for Testing and Materials (1990).

5. "Standard Test Method for Galling Resistance of Materials," ASTM G 98-91, Philadelphia, American Society for Testing and Materials (1991).

6. "Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus," ASTM G 99-90, Philadelphia, American Society for Testing and Materials (1990).

7. Unpublished report by WJ Schumacher, October 31, 1977.

8. Review of the Wear and Galling Characteristics of Stainless Steels (Washington, DC) Committee of Stainless Steel Producers, American Iron and Steel Institute, April 1978, 15–16.

9. ASM Metals Handbook, vol. 18, Friction, Lubrication, and Wear Technology (Materials Park, OH: ASM International, 1993), 723.

10. Marks' Standard Handbook for Mechanical Engineers, 9th ed. New York, McGraw-Hill, 1987, 11, 25, 151.

11. Friction Data Guide (Linden, NJ: General Magnaplate Corp., 1988).

John D. Jarrell is president of Materials Science Associates (Providence, RI), a metallurgical and materials consulting, research, and testing firm for the medical industry. Frank Bejbl is president of ME-92 Operations (Providence, RI), which manufactures a biocompatible Rc 80–hard chromium coating developed specifically for use with medical-grade stainless steels for improving corrosion and wear resistance, and high-temperature and friction performance. Portions of this article were excerpted from a larger study of medical device wear and friction, from which more detailed test results for different materials can be obtained by contacting ME-92 at 401/831-9200 or bejbl@ME-92operations.com.

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