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Surface Finishes: Methods and Metrics for Production

METALLIC IMPLANTS

Ed Reitz

September 1, 2007

19 Min Read
Surface Finishes: Methods and Metrics for Production

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Accurate dimensional tolerance and highly refined surface finishes are critical to the function of an orthopedic implant.

There is a wealth of technical data available regarding the tribology of the load-bearing surfaces of orthopedic implants that must slide, roll, pivot, or move by some other action. Tribology, defined as “the science and technology of interacting surfaces in relative motion and embracing the study of friction, wear and lubrication,” has emerged as a primary field in bioengineering.1,2 Implant manufacturers strive to provide working surfaces that offer the least amount of friction and the greatest retention of synovial fluid (a joint's natural lubricant) to maximize the service life of their products. Generating a surface that is free from irregularities is critical to minimizing adhesive and abrasive wear.

Abrasion occurs when a rough hard surface (e.g., steel) slides over a softer one (e.g., plastic) or when particles are trapped between rubbing surfaces. Adhesive wear occurs when local irregularities (asperities) on opposite surfaces weld together.3 Abrasive processes are commonly used to generate both specified dimensional tolerances and required surface finishes. Both parameters are critical to the success of an implant.

The is conclusive evidence that irregular surfaces accelerate the wear of synovial (hips, knees, shoulders) joint-replacement implants. Detailed analysis of retrieved metallic knee implants acquired from revision surgeries reveals surface scratches on femoral implants and resulting high wear to ultrahigh molecular weight polyethylene inserts. Even the surface of a tibial implant can create wear on the bottom of an insert. Revision surgery carries risk and is by no means popular with patients or surgeons. Therefore, for orthopedic implants, generating the highest quality working surfaces is critical.

Virtually all orthopedic implant manufacturers use surface-analysis technology to document the critical final working surfaces of their implants. Sophisticated laboratory equipment is used to measure and document various surface qualities. The scope of this article is to discuss the importance of controlling and measuring implant surface finishes on the plant floor during intermediate abrasive manufacturing processes such as grinding and polishing. If sound abrasive practices are used, not only will the quality of the final surface be enhanced, but the subsurface quality and dimensional tolerance of the implant will be enhanced as well.

The Importance of a Quality Surface prior to Final Finish

Within an abrasive process, several interactions take place. Intermediate stages of implant manufacturing usually call for metal removal. Theoretically, the abrasive tool employs a high number of random cutting points to penetrate the substrate and remove small chips of material. These small chips appear similar to chips produced by a cutting tool, although they are much smaller. Compared with most machine tool cutting processes, abrasive processes produce higher material removal rates and finer surface finishes because of their high speed.

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Figure 1. (click to enlarge) Proper control of grinding interactions ensures that the grinding process achieves optimal productivity and quality.

As mentioned above, intermediate stages of metallic implant production generally call for an interaction resulting in the removal of material. In-house studies performed by the Higgins Grinding Technology Center of Saint-Gobain Abrasives reveal that three basic interactions take place between the abrasive and the workpiece: cutting, plowing, and sliding (see Figure 1).

In grinding implants, plowing and sliding are undesirable interactions between the workpiece and the abrasive. Plowing is undesirable because it results in burnishing and the movement of metal. Movement of metal can actually result in the masking of surface imperfections, e.g., small cracks or voids. Masking of surface flaws due to improper machining or grinding can prevent fluorescent penetrant inspection (FPI) from detecting such flaws. In similar operations, manufacturers of cobalt-chrome jet engine turbine blades and vanes closely control metal-working processes to ensure that potential flaws are not covered by metal moved across the surface rather than removed from the surface.

It is equally important to generate a clean metal surface. The process must remove the amorphous layer of distorted metal, oxides, and any other contaminants that may be on the surface while not burning or otherwise damaging the surface.

Both plowing and sliding produce friction and lead to a rapid buildup of heat on the workpiece. Burn and heat cracks damages the metallurgical properties of the implant. Also, excessive heat often negatively affects the dimensional tolerances of implants, especially knee implants.

The effects of chips (grinding swarf) and abrasive bond can also have a negative effect on surface quality, wheel wear, and the overall process. Abrasive-application engineers can help provide on-site control of these factors to ensure that the problems are not related to other aspects of the process.

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Table I. (click to enlarge) A summary of the effects of different manufacturing processes on the fatigue strength of titanium alloys.

One other aspect of control of grinding and polishing operations is the need to ensure that the process does not have a negative effect on the alloy chosen for the application. In one study of Ti-6-4 alloy, gentle grinding versus abusive grinding produced a 38% higher fatigue-strength limit. A finely polished surface produced a fatigue limit approximately 92% higher than abusive grinding did. Table I summarizes the effects of different manufacturing processes upon the fatigue strength of titanium alloys.

Material Removal Processes and Typical Finishes

The values shown in Table I are typical finishes that can be expected from the processes listed. Higher or lower values may be obtained under varying conditions. Comparing typical finishes made from forging and investment casting with finishes obtained by grinding and polishing reveals that initial surface finishes can be quite rough. Ultimately, however, an implant's working surfaces, such as knee condyles, are buffed to a mirrorlike finish.

Most manufacturers use proprietary processes. Still, most processes employ multiple stages of abrasive applications to refine the working surface from an as-cast or as-forged condition to a final functional surface. Reducing the number of process steps only to those necessary to reach the final buffed finish requires an understanding of the finish quality each stage produces.

Measuring Surface Roughness on the Plant Floor

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Figure 2. (click to enlarge) Guide for approximate surface finishes produced via different processes. The typical range is shown in white; atypical range is shown in blue.

Besides form and dimensional tolerance, the other two aspects of a workpiece surface that require control are roughness and waviness (see Figure 2). Roughness is caused by the microstructure of the material and the action of the cutting tool. Waviness is caused by instability of the cutting tool or the machine, or simply by the shape of the cutting tool and the path it takes while removing material.

Surface finishes are generally measured in microinches or micrometers. The conversion factor is 1 mm = 39.37 min. Most desktop measurement devices and many handheld devices are capable of switching between metric and English units.

The following section provides a review of the most common surface-finish parameters that might be measured on the plant floor:

  • Ra = the arithmetic mean of the roughness profile above and below the mean line. Sometimes called average roughness, rms is sometimes used interchangeably with Ra, but they are not the same.

  • Rmax = the maximum peak-to-valley distance within one cutoff length. Cutoff is a method of separating or filtering the wavelengths of a component. The filter determines how much waviness is included in the assessment length. A lower cutoff generally means that the values are less affected by the amount of waviness in the surface of the component. For most general applications, a cutoff of 0.8 mm is adequate.

  • Ry

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    Figure 3. (click to enlarge) Measurement of the surface via Ra or RMS parameters alone may miss significant characteristics of the surface. Additional parameters such as Rmax and Wa are important indicators of surface quality.

    = a value similar to Rmax, but it includes the maximum peak-to-valley distance within the entire sample length instead of within only one cutoff length (see Figure 3).

  • Rv = the maximum depth below the mean line.

  • Rp = the maximum height above the mean line.

  • Rq = the maximum peak-to-valley distance.

  • Rz = the average absolute value of the five highest peaks and the five lowest valleys over the evaluation length.

  • Wa = waviness or the arithmetically averaged roughness, i.e., the average deviation of all points from a plane fit to the waviness data. This is a common parameter for waviness. Handheld profilometers generally do not measure waviness, but some portable benchtop units do.

There are a number of portable and benchtop devices available to measure many of these parameters. Generally, in making measurements, these devices use a probe with a diamond stylus that runs across the surface of a workpiece. The cost of such devices is comparable to other metrological devices and depends upon the complexity and features of the instrument.

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Measuring the contoured surface of a hip joint with an appropriate probe. Photo courtesy of Mahr Federal (Providence, RI).

The approximate price for an entry-level device able to measure multiple parameters is $2000. At a minimum, a surface-quality measuring device should measure both Ra and Rmax or a similar maximum peak-to-valley distance. Ra alone is a poor indicator of surface finish quality. A few deep scratches can skew the Ra value of a surface that otherwise would produce a much smoother finish. In other words, the general surface quality can vary greatly for the same Ra reading. It may also be important to measure waviness. The shape of the grinding wheel or cutting tool and the tool path can generate waviness that must be removed from the surface during the next stage of surface refinement.

Measurement of the surface finish should be conducted against the lay of the scratch or wave pattern. As shown in Figure 4, roughness measurements can vary greatly for different angles of approach.

When taking measurements of the surface, here are a few tips for achieving accurate readings:

  • Fixture (secure) the work or the device, if necessary.

  • Take readings against the lay of the scratch or machining pattern.

  • Ensure that the surface is free from any debris that will create a measurement error. If necessary, use a soft cloth—not fingers or a paper product—to wipe the workpiece.

  • While taking a reading, ensure that the device's probe has constant contact with the surface and does not lift from the surface at any time during the measurement.

  • Ensure that the device probe remains within its up-down travel during the reading.

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    Figure 4. (click to enlarge) Surface finish measurements should be taken across 90° the lay of the grind. Note that the direction of profilometer probe traverse relative to the lay of the grind alters the value of the measurement.

    Do not rely upon Ra or rms alone to characterize a surface. These parameters will provide a general idea of the finish quality, but they do not provide enough data to allow definitive conclusions about the uniformity or quality of a surface.

  • In grinding applications, use this general rule of thumb: that an Rmax reading within 10 times an Ra reading usually indicates a uniform finish. For example, the following readings would indicate a relatively uniform finish: Ra = 35 min., Rmax = 275 min. By contrast, the following reading would indicate a surface finish that may have deep scratches or other irregularities: Ra = 12 min., Rmax = 250 min.

  • Do not take multiple readings at the same location when measuring with a device that uses a contact point (e.g., a diamond stylus). The stylus may blend some of the surface features.

  • The performance of abrasive processes can be significantly affected by the amount of force (unit pressure) applied in any part of the grinding and polishing cycle. Take readings from different sections of the implant to ensure that the finish is uniform throughout the entire component.

  • Handheld profilometers usually lack the resolution to accurately measure a mirrorlike surface finish below 1 or 2 min. Ra. Employ benchtop or lab units to measure final mirrorlike surfaces.

  • Do not manually check the surface finish of each in-process component. This is especially true for high-volume operations. It does make sense to check the surface finish periodically, especially following any process change.

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Measurement of the surface finish should be conducted against the lay of the scratch or wave pattern. As shown here, roughness measurements can vary greatly for different angles of approach.

It is important to know what type of surface is produced at each manufacturing stage to determine whether the next process is capable of effectively improving upon the finish produced by the prior process. It is also possible to evaluate whether one can shorten or even eliminate some surface-refinement steps.

While complete details cannot be provided in a single article, there are a few pointers about available products and their uses for finishing implants that this space allows.

Orthopedic Implant Grinding with Conventional and Superabrasive Wheels

Speed, wheel type, and other factors must be considered when grinding implants. The following section reviews a number of important guidelines for selecting and using conventional and superabrasive wheels.

High Speeds. Within the maximum operating speed of a wheel, high speeds can improve surface finishes. However, high speeds may be limited by machine integrity (static stiffness) or by a machine's ability to supply adequate coolant velocity and flow. A grinding wheel at high speeds acts as an air pump and creates a barrier that can divert coolant from the grinding zone. An air dam or shroud fitted close to the wheel and prior to the grind zone breaks up this air barrier.

Coolant. Coolant flow rate and placement are more important than velocity. You must first have an adequate volume of dynamic coolant, directed to the proper location, to dissipate the grinding energy. A good rule of thumb is to have 2 gpm of coolant per grinding horsepower. Coolant velocity should then be set to reach or slightly exceed the velocity of the wheel. Adequate coolant velocity is necessary to ensure that the coolant reaches the grinding zone and is not deflected by the air barrier.

Wheel Balance. Conventional grinding wheels must be mounted as described on the wheel and properly dressed before grinding. Often a mount-up arrow is indicated on the side of a grinding wheel. Following these mounting instructions enhances the balance of a wheel. Poor wheel balance degrades surface finish.

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Table II. (click to enlarge) Approximate surface finish generated via single-layer cubic boron nitride superabrasive grinding wheels.

Wheel Types. Most operations have converted from conventional vitrified-bonded grinding wheels to superabrasive wheels that feature a single layer of cubic boron nitride (CBN) abrasive nickel-plate bonded to a precision metal core. Individual abrasive suppliers experience significant differences in wheel dimensional tolerances, grain type and quality, grain concentration (the amount of grain bonded to a given area on the wheel face), and the quality of the bonding to the metal. Lowest-cost CBN-plated bond wheels rarely produce the lowest operational cost. Neither do they produce a quality surface. Table II provides a quick reference chart as a rough guide for possible finishes produced by single-layer, plated-bond CBN superabrasive wheels.

Using CBN wheels and a computer numerical control (CNC) grinder, a manufacturer of implants can take workpieces that may include some gate materials from as-cast or as-forged finishes to very refined surfaces of less than 30 min Ra.

Complex Shapes. CNC grinding of complex shapes using grinding wheels will leave toolpath scallops or grooves. If the implant features any degree of symmetry, it may be possible to use a form wheel that matches the form of the implant to eliminate or match the tool path.

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Figure 5. Example of a form wheel (upper left) and a 1FF1 shape peel grind, or single point, of a contact wheel.

If the implant is asymmetrical, a single point-of-contact (peel grinding) wheel will be effective. A point-of-contact wheel's face can feature a radius that allows it to grind the various convex and concave surfaces of the implant. So long as the wheel creates correct dimensions within the concave areas of the implant, a larger radius will lessen the tendency of the wheel to leave toolpath grooves on the implant surface (see Figure 5).

Surface Quality. Vibration or chatter can degrade the surface quality of a component.

Fixture Integrity. Fixture integrity is an important component of a CNC grinding system. The fixture must accurately locate the part and must remain rigid throughout the entire grind cycle.

Ceramic Grain. Newer grinding wheels featuring an advanced ceramic grain abrasive have produced outstanding results on stainless steel components. This new technology will soon be tested on cobalt chrome materials.

Belt Polishing of Orthopedic Implants

A number of factors affect the finish of an implant. Grit size, grain, and buffing compounds are among the critical elements to consider. This section reviews these factors and provides some guidance for belt polishing of orthopedic implants.

Surface Refinement. Surface refinement of implants generally begins with 80-grit or finer abrasive belts.

Grit Size. Selection and substitution of abrasive belts must consider the vendor's method of grading the grit size of belts. Some belt manufacturers overgrade belts by using coarser grit at their belts' surfaces. Although this practice can yield gains in productivity and belt life, workpiece surface quality is often compromised. One vendor's 80-grit belt may not produce the same finish as another vendor's 80-grit belt.

Ceramic Grain. Newer generations of ceramic grain have produced quantum leaps in belt life and processing speed. A high-technology abrasive can produce superior results in productivity, finish, and ultimately the total cost of an operation. Stainless steel and cobalt chrome alloys are considered difficult-to-grind materials. High-technology abrasive grain belts are usually cost-justified for such applications.

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Figure 6. Robot processes may employ staged feed-pressures. As the belt begins to wear, more pressure is applied to maintain belt performance and increase belt life.

Robotics. Robotic processes may employ staged feed-pressures. As the belt begins to wear, more pressure is applied to maintain cut rates and belt performance (see Figure 6). The process should be controlled to avoid excessively heating the implant. Aside from potential metallurgical burn, dimensional tolerance of the implant may be altered via heat-generated warping.

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Figure 7. (click to enlarge) Engineered abrasive belts provide dimensional control and can produce down to single-digit micro-inch Ra. Shown here is an example from Saint-Gobain Abrasives's Metal Finish Guide. Note: Finish number 19 not shown here.

Engineered Abrasive Polishing Belts. Whether using a robotic or an off-hand polishing method, the industry has widely adopted the use of engineered abrasive polishing belts. These belts feature a multilayered section of abrasive grain and grinding aids. The abrasive is bonded to the belt in patterns specially engineered for specific applications. This type of belt is available with various backings from stiff to flexible, and in grit sizes ranging from approximately 200 mm to 5 mm (or less). Engineered abrasive belts are capable of producing Ra microinch finishes well into single digits. Figure 7 shows an example.

Polishing the implant to the finest finish possible using an engineered abrasive belt produces consistent and superior results compared with a cut-buff process using coarse buffing compounds and stiff sisal-composition buffing wheels. Ultrafine engineered-abrasive belts feature outstanding dimensional control and also achieve finer finishes by means of a clean-cutting metal-removal process. Excessive buffing, however, may tend to move metal and thereby mask surface imperfections.

Buffing Compounds. Lubricants or grease-stick-type buffing compounds are sometimes applied to abrasive belts to further reduce surface-finish roughness caused by the belt. The lubricant must be approved for use on implants.

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This is a highly magnified view of a metal surface and the diamond stylus tip used to measure surface roughness. Photo courtesy of Mahr Federal (Providence, RI).

Prior to cleaning or using any chemical process on an implant, the final surface-conditioning stage is usually color buffing with a soft cotton buff wheel and very fine, relatively soft, cutting compounds. If a component leaves this stage with surface imperfections, the depth of the imperfection can be measured to determine just how much reprocessing must be done to successfully rework the component. Proper abrasive-belt polishing lowers cycle times within the color-buff stage.

Conclusion

In the grinding and polishing of orthopedic implants, there are many methods to achieve and maintain superior surface finishes and surface integrity. It is important to ensure that each stage of a process efficiently refines an implant's surface and prepares it for the next stage without creating any flaws. It is possible to achieve the optimum final finish using proper control of processes and the measurement of intermediate results.

Ed Reitz is a senior application engineer at Saint-Gobain Abrasives (Worcester, MA). He can be contacted at [email protected].

References

1. JH Dumbleton, Tribology of Natural and Artificial Joints, Tribology Series 3. (Amsterdam: Elsevier Scientific Publishing, 1981).

2. IM Hutchings, Tribology: Friction and Wear of Engineering Materials, (Boca Raton, FL: CRC Press, 1992).

3. Vaughan Kippers, PhD, Joints, (Queensland, Australia: University of Queensland, 2000).

Copyright ©2007 Medical Device & Diagnostic Industry

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