Coating medical components and devices is often an afterthought for designers, rather than part of the early-stage discussion. Ignoring surface treatments in early phases means risking longer product cycle times because design properties may be incompatible with a hastily selected coating. Each type of surface treatment, for example, anodizing, electroplating, and thermal spray, provides its own set of properties and benefits, along with limitations that must be discussed as early as possible.

A frequent design goal for medical equipment is to extend its life by protecting parts from corrosive attacks by test specimens and analytical reagents. Protective coatings guard these parts against corrosion and provide wear resistance, lubrication, and structural integrity. The key factors affecting surface treatment include process limitations, the choice of base metal, part configuration and design, and surface finish.

Process limitations

‘Line-of-site,’ a frequent process limitation can be restated as ‘what you see is what you can coat.’ It is most commonly encountered with electroplating, anodizing, spray coatings, and physical-vapor-deposition (PVD) methods. So consider a more suitable immersion process for parts with ‘hidden’ surfaces.

Line-of-site applications are usually limited to 1 to 1½ times the diameter for the thickness of the coating to uniformly penetrate down the hole because of the limited ‘throw’ of a line-of-site process. This means that parts with deeper holes will not have a uniform coating thickness, which may be a significant issue for parts with tight tolerances.

Air pockets, or pocketing, is an issue more commonly found with immersion methods such as anodizing and plating. If a critical surface must be coated, ensure access to that surface so pockets can be removed by the coating vendor. Pocketing also reduces the efficiency of the coating because the part often has to be orientated (in the case of immersion processes) at different angles and then maneuvered to remove the air. Complex designed parts are more susceptible to this issue so consider designing in relief holes to let the air escape.

Base metal choices

Choosing the right base metal for the part can eliminate issues later, especially if the part has tight tolerances because high processing temperatures can cause problems. For instance, the part can deform if surface loads are high and process or post-coating temperatures exceed the substrate heat treatment operating temperatures. In addition to deformation, some metals suffer from low structural stability. That is, the part moves or actually changes size. For example, machining causes a lot of internal stresses in the part and as coating processes are applied (which may involve heat) the internal stress may be relieved, therefore triggering these changes.

When considering high-strength alloys or materials such as titanium or tool steels, be aware that hydrogen embrittlement can make the part crack and fail under loads. During a cleaning step, which often involves acids, certain alloys will impart hydrogen to the metal's surface, which causes the embrittlement. If the coating company is alerted of this, it can post-treat the part to avoid embrittlement or use alternative cleaning processes to expel the hydrogen from the surface.

Sometimes a part has more than one material. In this case the part may not be conducive to plating because cleaning each metal calls for a different process. One cleaning process may activate the surface of a specific metal for coating while passivating the surface of the second material. When both surfaces cannot be coated at the same time, special masking is required. The labor-intensive nature of this work drives the cost up. Adhesion issues crop up on the boundary areas between the two metals. Exposing them to electrolytes may generate a galvanic reaction, corroding one of the base materials.

Additionally, an extremely hard base metal can cause surface tensions that prevent a strong adhesion between base material and coating. A weak bond between part and coating leads to chipping.

Part configuration

In many plating processes, a sharp corner is the designer's greatest enemy. Sharp corners are subject to chipping, therefore it is always advisable to radius the parts, depending on the required coating thickness. A greater radius provides more support for the coating and minimizes chipping, but if a sharp corner is absolutely necessary then the coating thickness should be kept to a minimum.

Some processes that require electric currents to generate the coating produce the unwanted effect of a greater coating thickness on sharp edges and in corners. The problem is less so in other areas. If the part has tight tolerances, subsequent machining operations will be required, which adds cost and time. Designing parts with sufficient corner and edge breaks helps avoid this issue.

A part with thin areas of metal is prone to burn in those areas. When high voltages are required for some coatings, thin sections of metal generate high amperages, and hence high temperatures which can cause those areas to burn. The good news is that a metal-finishing expert can adjust the process to minimize this effect.

Size also matters for coatings. Immersion-coating methods are limited by tank size, so designers must ask the vendor what size parts they can physically accommodate. In the case of heavy parts, also discuss material handling capabilities.

Coating considerations

Because parts must be placed in racks for coating, it is important to discuss how they will be racked early in design. Racking means areas of the part may be left uncoated where they contact the rack, or the part may end up marked by the racking process. When large parts cannot be suspended by a small hole, significant rack points must be considered. In certain processes, particularly anodizing and electroplating, the racks carry current so their correct positioning is critical, as is the number of racks points required for a uniform coating.

Some applications require areas of the part to be free of coating. This leads back to the labor-intensive issue of masking. Don't underestimate how much this process adds to costs. When cost is an issue, it is best to design a part that can be coated all over.

Tight part tolerances

Each coating process has a specific thickness and holds certain tolerances. Depending on the process, parts may or may not require machining after coating. So when possible, pre-size parts to accommodate coating thickness and tolerances.

Coating thickness should be a primary consideration, especially where tight tolerances are required. Certain processes, such as plating, are described in terms of surface growth. For example, a 0.001 mm coating equals a 0.001-mm surface growth. Other methods, such as anodic processes, are described in terms of a combination of penetration and surface growth, where a 0.001-mm coating may only equal a 0.0005-mm surface growth because the coating penetrates the metal's surface.

Threads often have tight tolerances, especially on pitch diameters where two parts engage each other. On a 60° thread, every unit of coating thickness applied to the surface affects the pitch diameter by four times that amount. So if threads must be coated, pre-size them to accommodate the coating thickness to ensure proper engagement.

Surface finish

Some coatings replicate the surface finish, but only to a certain degree, measured in units of Ra. For instance, plating a surface replicates as low as 16 to 32 Ra finish. For lower Ra values (a better finish) post grinding or polishing may be required.

Required radius to reach a nominal coating thickness

Nominal coating thickness (in.)	Approximate radius of curvature on edge and inside corner (in.)
0.001	0.031
0.002	0.063
0.003	0.094
0.004	0.125

Ratio of change after coating

Thread	Change ratio after coating
60° thread	4 to 1
14 ½° ACME	8 to 1
14 ½ to 5° buttress	11.9311 to 1
10° square thread	23 to 1