Requirements for Substrate Preparation for BryCoat Thin Film Coatings

BryCoat PVD hard coatings and WS2 Dry Film Lubricant Coatings provide outstanding performance when applied to a properly prepared substrate. BryCoat has extensive capabilities for surface preparation prior to coating. However, parts to be coated must have a surface that is prepared prior to coating application to provide a metallurgical bond for the coating. The following guidelines for substrate part preparation will help ensure a successful coating.

Substrate Materials that can be coated

  • Steels, tools steels, high speed steels
  • Stainless Steels
  • Titanium alloys
  • High temperature alloys, Nickel alloys, Cobalt alloys, NiCr, etc.
  • Copper alloys, Beryllium Copper, Aluminum Bronze
  • Carbide, ferro-tic and cermet
  • Aluminum alloys
  • Ceramics and glass
  • Certain high temperature plastics
  • Beryllium
  • Nickel or Chrome plated parts

Substrate Material Requirements

  • Substrate materials must be compatible with high vacuum conditions at elevated temperatures, and must have a surface that is prepared for a metallurgical bond to the coating.
  • The entire part will be placed inside the vacuum chamber coating. Therefore, even uncoated areas must be compatible.
  • Note that the parts must be held somewhere. Therefore it is impossible to “coat all over” without leaving any kind of fixturing mark. BryCoat has worked with customers to provide creative ways to achieve virtually total coverage.
  • Parts should be free of oxidation to ensure strong adhesion of the coating. BryCoat strongly recommends blasting all heat treat scale off the part after heat treat and before final grinding of finished dimensions. The following oxides can impact adhesion:
    • Heat treat scale
    • Black oxide finish
    • Surface oxides from Nitriding or Carburizing processes
    • Surface oxides from vacuum heat treating (venting too hot).
    • Rust and corrosion
    • Grinding burns
  • Nitrided, Carburized, and NitroCarburized surfaces can be coated. In fact, the added case hardness at the surface can help provide good support for the coating. However, these surfaces often have a surface oxide that must be removed before coating by blasting or polishing. Also, these surfaces sometimes have a brittle, loosely bonded “white-layer” that can interfere with coating.
  • Functional areas should be free of burs and overlapped material caused by aggressive grinding. These imperfections may coat and then break off afterward, leaving an apparent void in the coating.
  • Parts should be free of surface contaminants:
    • Silicone oils
    • Wax or grease
    • Motor oil (the additives can be a problem)
    • Layout die
    • Teflon, or other coatings
    • Glues and adhesives
    • Paint
    • Certain platings, such as Cd plating.
    • Tape or tape residue. Note that tape can leave an invisible film behind that interferes with proper adhesion of the coating.
  • Parts should be preserved for shipment to BryCoat with a light petroleum-based rust-preventive oil. This type of oil can be easily cleaned off at BryCoat and helps to preserve the clean bright rust-free part surface.
  • A fine ground finish is the best for most performance part surfaces. The thin film coating will follow the surface finish of the part. Thin film coatings are generally not recommended on very rough surfaces (where the roughness is much greater than the coating thickness).
  • Materials that outgas in vacuum conditions are not suitable for coating. Cadmium (Cd), lead (Pb), tin (Sn) and other high vapor pressure metals are problems. Porous materials are also a problem.
  • Assemblies, including press-fit components, present a problem because contaminants will be drawn out from the seams when the parts are heated under vacuum conditions. BryCoat recommends that assemblies be disassembled and individual components be coated separately before assembly. If an assembly must be coated, it is important that the parts be extremely clean prior to assembly. Any oil used to assist assembly becomes a contaminant that will outgas in the vacuum coating chamber.
  • Welded parts can be coated if the welds are extremely clean, not oxidized, and free of sealed pockets that can leak in high vacuum.
  • Brazed or silver soldered parts should use a Cadmium-free braze material for vacuum compatibility.
  • Parts should be properly packaged to avoid damage during shipment. BryCoat will usually return the parts using the same packaging they were received in.
  • Previously PVD-coated parts can be recoated over the original coating, or stripped of coating and then recoated.
  • Heat treated parts must be able to withstand the coating process temperatures. The standard coating temperature for BryCoat PVD hard coatings is 400°C (752°F). Lower temperature processes are also available. For high precision parts, with tolerances in the ten-thousandths (0.0001”), there is concern about slight growth/shrinkage or warping at the coating temperatures. In these cases, it is recommended to double-draw or triple-draw the parts at a tempering temperature of 425°C (797°F) or higher to ensure the stability of the part. Aggressive grinding can also induce stresses
  • Used parts often have embedded contaminants in the surface. In some cases these contaminants can create trouble creating a proper bond of the coating. Please advise BryCoat if parts have been used and the conditions.

Design Considerations for Thin Film Coatings

  • All exposed areas will receive coating. To prevent coating in some areas of the part, masking is required. Depending on the complexity of the geometry, masking can be expensive. Please consult with BryCoat on your specific needs. We recommend that you specify these areas on your part drawings:
    • The critical working surfaces where the coating provides a performance benefit
    • Any other surfaces that must be coated
    • Those surfaces that must not receive any coating (i.e. that must be masked).
    • Those surfaces where coating is optional.
  • BryCoat can then determine the best fixturing and masking plan to meet your requirements at an economical price.
  • Surface finish must be appropriate for the design objectives. If the part roughness is much greater than the coating thickness, then the coating cannot work properly. In a sliding wear application, the peaks of the roughness can shear off, exposing substrate material. In some applications, a part should not be too smooth either. In these applications a mirror finish can lead to sticking caused by suction between parts. A micro-rough finish can be advantageous to break the vacuum between parts allowing air to penetrate. BryCoat has finishing capabilities to change surface finish in many cases.
  • Thin film coatings are appropriate for precision parts. When tolerances are too loose, the benefit of a thin film coating is a small fraction of the total service life. The wear resistance of BryCoat thin film coatings is outstanding on close tolerance parts.
  • The parts should be in good working condition prior to PVD coating. BryCoat thin film coatings will not make up for poor quality parts or improper geometry. Thin film coatings are only a few microns in thickness. If the geometry of the parts is not fit accurately enough, or if the surface finish of the parts is too rough (much rougher than the coating thickness), then a thin film coating cannot work.
  • BryCoat needs to know the material/alloy as well as the hardness or tempering temperature in order to advise the best process conditions.
  • The process is not a pure line-of-sight process. Coating can be applied to features and complex geometries. In holes, the coating can be applied at full thickness to a depth approximately equal to the diameter of the hole. In deeper holes, the coating thickness will taper with added depth.
  • Proper call out for coatings includes test requirements and specifications as well as thickness and coverage tolerances. Consult BryCoat for suggested drawing callouts in your industry.
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