Two distinct properties, wear and friction, are important considerations in the design of medical devices, equipment, and the tooling used for manufacturing surgical instruments.
Many medical applications require special attention to reduce or eliminate unintentional material implantation caused by the generation of metallic debris. Patient exposure to contamination originating from the tooling and manufacturing must also be minimized. Furthermore, doctors demand well-designed, state-of-the-art surgical instruments and medical devices that are reliable, functional, and aesthetically pleasing.
To meet these high expectations, medical designers, engineers, and manufacturers must consider the wear, galling, and friction properties of their materials and the technologies used to enhance those materials.
Overview of factors affecting wear
Wear refers to the surface degradation that occurs when a material experiences frictional forces. In the case of medical-grade stainless steel, wear can take various forms and can be caused by a variety of factors, including the grade of stainless steel, the environment it is used in, and the type of wear it experiences.
Different grades of stainless steel have different levels of corrosion resistance, hardness, and other properties that can affect their wear resistance. For example, martensitic stainless steel is known for high wear resistance, while ferritic stainless steel is not as resistant to wear.
The environment in which stainless steel is used can also affect its wear resistance. Stainless steel that is exposed to abrasive materials or harsh environmental conditions, such as high humidity or saltwater, may experience more wear than stainless steel that is used in a less demanding environment.
Finally, the type of wear that stainless steel experiences also impacts its wear resistance. For example, bearings in robotic surgical devices may experience a different wear rate (rolling contact fatigue) than impellers in medical infusion pumps (abrasive wear). Of course, it’s important to note that the equipment used to make medical-grade devices also needs protection to mitigate foreign debris during the manufacturing process.
Types of stainless steel wear
The most common types of wear affecting stainless steel medical devices include:
Adhesive wear
While stainless steel is known for its excellent corrosion resistance and durability, it isn’t immune to adhesive wear. Also known as abrasive wear or scuffing, adhesive wear occurs when two surfaces rub against each other, tearing or plucking small particles from one surface. These particles then become trapped between the surfaces and continue to cause wear as friction forces act upon them.
Adhesive wear is a common problem in many engineering applications and can be particularly damaging to stainless steel components used in medical devices. Some types of adhesive wear include:
Oxidative Wear
Also known as mild wear, oxidative wear occurs when oxygen comes into contact with the surface of stainless steel, reacts with the metal, and at low loads, forms an oxide layer on the surface. This oxide layer is usually very thin and is known as the “passive layer,” as it helps to protect the stainless steel from further corrosion. However, if the passive layer becomes damaged or if the environment is particularly aggressive, the stainless steel can become susceptible to oxidative wear. Other factors that can contribute to oxidative wear in stainless steel include high temperatures, certain contaminants or impurities, and high humidity or moisture levels.
Metallic Wear
At higher loads, metallic wear occurs. In fact, stainless steel can be more prone to metallic wear than other materials. This is primarily because stainless steel has a high coefficient of friction, which means that it generates a lot of heat when it rubs against other surfaces. This heat can cause the stainless steel to become soft and prone to deformation, leading to the formation of small particles that can cause further wear.
Galling and Seizing
Galling is a form of wear that occurs when two surfaces rub against each other under very high loads, causing a transfer of material from one surface to the other. This can result in the formation of small, smooth, concave indentations on the surface, called galling marks. Galling is often seen in stainless steel and other metals that have a high degree of hardness and strength. Sometimes, seizing occurs. This is an extreme type of wear that occurs when the two surfaces stick together, causing them “seize,” requiring significant force to separate the two surfaces. Both galling and seizure of stainless steel parts are serious problems, as they can lead to damage to the parts, equipment failure, and costly repairs.
Abrasive wear
Abrasive wear can occur when hard particles come into contact with the surface of stainless steel and cause damage. There are several factors that can affect the severity of abrasive wear on stainless steel, including the type and hardness of the abrasive particles, the velocity and pressure at which they come into contact with the stainless steel surface, and the type and condition of the stainless steel. Some types of stainless steel, such as those with high chromium and molybdenum content, are more resistant to abrasive wear than others.
To minimize abrasive wear on stainless steel components, it is important to use materials that are appropriate for the specific application and to maintain them in good condition. This may involve using protective coatings, such as ME-92® biochrome coating, to provide an additional layer of protection against abrasive particles.
Erosion
Stainless steel erosion is the process by which the surface of a stainless steel object is worn away by the action of a flowing fluid or a mechanical force. Erosion can occur due to the presence of impurities or contaminants in the fluid, or due to the presence of abrasive particles in the fluid or on the surface of the object. Erosion can also occur due to mechanical wear and tear, such as friction or impact, or due to the action of corrosive agents. The erosion of stainless steel can lead to a number of problems, including reduced strength and durability of the object, as well as a decrease in its aesthetic appeal.
Fretting
Fretting is a type of corrosion that occurs when tight-fitting stainless steel surfaces experience small, repetitive movements. These vibrations, which can cause small cracks, grooves, and other damaging surface wear, can eventually lead to component failure and greatly reduce product lifespan.
Testing methods for evaluating wear
Stainless steel coating and metallurgical consultants at ME-92® can help identify and execute tests applicable to your medical device design requirements. Some commonly used tests to measure the wear and galling resistance of materials in the medical industry include:
Pin-On-Disk Test
Also known as the pin-on-disc test or the pin-disk test, this test measures a material’s wear and friction properties, particularly its resistance to wear and its coefficient of friction (COF).
In the pin-on-disk test, a small cylindrical pin made of the tested material is placed on the surface of a rotating disk made of a harder reference material. The pin presses against the disk with a specified normal load and rotates against the disk at a specified sliding speed. To evaluate the wear and friction properties of the material, we measure the wear on both the pin and the disk, as well as the COF between them.
ASTM G99, a standardized version of the pin-on-disk test, evaluates the wear and friction properties of materials under sliding contact.
Threshold galling stress test
Also known as the threshold galling load test or the threshold galling force test, this test measures a material’s galling resistance when subjected to sliding contact.
In the threshold galling stress test, a specimen made of the tested material slides against a harder reference material under a specified normal load. The load increases incrementally until galling occurs. The threshold galling stress is the minimum load at which galling begins.
Several standardized versions of the threshold galling stress test exist, including ASTM G98, which evaluates the galling resistance of materials under sliding contact, and ASTM G133, which evaluates the galling resistance of stainless steel materials.
Four-ball Test
Also known as the Four-ball Wear test or the Four-ball EP (extreme pressure) test, this test measures a lubricant’s lubrication properties, particularly its ability to withstand EP conditions.
In the Four-ball test, a lubricant is applied to three steel balls placed in a fixture called a “four-ball tester.” The fourth ball rotates against the other three balls under a specified load and temperature, simulating the conditions a lubricant might encounter in an actual application. After a specified number of revolutions, we measure the wear scar diameter of the fourth ball to evaluate the lubricant’s ability to reduce wear and prevent corrosion under extreme pressure conditions.
Standardized versions of the Four-ball test include ASTM D4172, which evaluates the EP properties of lubricating oils and greases, and ASTM D2783, which evaluates the EP properties of hydraulic fluids.
Scratch Test
Also known as the scratch adhesion test or the scratch resistance test, this test measures a coating’s or surface’s resistance to scratching or abrasion. The coatings industry uses this standard test method to evaluate the performance of coatings, especially their ability to withstand scratching or abrasion during handling, transportation, or use.
In the scratch test, a standard scratch tool, such as a stylus or diamond scribe, applies a specified force to a coating or surface while moving across the surface in a controlled manner. To evaluate the coating’s or surface’s resistance to scratching or abrasion, we measure the depth or width of the scratch or assess the visual appearance of the scratched surface.
ASTM D3363, a standardized version of the scratch test, evaluates the scratch resistance of organic coatings.
Abrasion Test
Also known as the abrasion resistance test or the abrasive wear test, this test measures a material’s resistance to abrasion or wear due to sliding contact with a harder reference material.
In the abrasion stress test, a specimen made of the tested material slides against a harder reference material under a specified normal load. The load increases incrementally until the specimen fails due to abrasion. The abrasion stress is the maximum load the specimen can withstand without failing.
Several standardized versions of the abrasion stress test exist, including ASTM G65, which evaluates the abrasive wear resistance of materials under sliding contact, and ASTM G105, which evaluates the abrasive wear resistance of metals.
Fatigue Test
Also known as the fatigue strength test or the endurance test, this test measures a material’s resistance to failure due to cyclic loading or stress.
In the fatigue test, a specimen made of the tested material undergoes a series of cyclic loading or stress cycles at a specified frequency and amplitude. To determine the material’s fatigue strength, we measure the maximum stress or load the specimen can withstand without failing due to fatigue.
ASTM E466, a standardized version of the fatigue test, evaluates the fatigue strength of materials under cyclic loading.
Design recommendations for stainless steel wear prevention
In conclusion, the ME-92® team offers the following medical device design recommendations for reducing stainless steel galling and wear.
- Select mating materials with adequate wear and galling resistance.
In unlubricated systems or sporadically lubricated systems, materials selection becomes even more critical. Assemblies used in medical and surgical applications, generally lack adequate lubrication. Therefore, it is desirable to:
- Select materials with high threshold galling stresses for mating surfaces.
- Use dissimilar materials and/or those with differential hardnesses on the sliding surfaces. (coating both surfaces with ME-92® is an exception to this rule)
- Select high work-hardening austenitic stainless steels for improved wear resistance (but not galling resistance) at temperatures below 350F.
- Select alloys for mechanical properties alone and apply surface modifications to obtain the desired wear and corrosion resistance.
- Lubricate where possible.
- Keep load, temperature, and speed as low as possible.
- Start with mated surfaces having a finish between 10 and 70 micro inches (AA). Below 10 increases susceptibility to galling; above 70 increases wear.
- Increase contact area to:
- lower contact stress below threshold galling stress, and
- lower the depth of wear, by spreading the wear volume over a greater area.
- Specify acceptable 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. Coatings from ME-92® extend the capabilities of bare contact materials.
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As a metallurgical and materials consulting, research, and testing firm, Armoloy is here to help OEMs and medical engineers, design engineers, and manufacturing engineers reach beyond the capabilities of bare stainless steels.