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Spring Coatings and Surface Treatment Options

Various spring coatings and surface treatments are available to enhance the performance of metal springs. Some coatings primarily prevent corrosion, while others improve surface hardness and wear resistance. Additionally, coatings can slightly alter dimensions and modify physical properties such as reflectance and color. Selecting the right spring material, finish, and surface treatment is crucial for optimizing your spring design for its specific application.

Shot Peening
Shot peening is a cold working process that induces compressive stresses in metal surfaces using high-velocity shot streams. Unlike blast cleaning, shot peening is precisely controlled to enhance fatigue strength. Various media such as iron, steel, glass, or cut wire shot are used, with sizes standardized by MIL-S-13165. The effectiveness of shot peening is measured using the Almen strip method, ensuring accurate and reproducible results.

Tumbling, or vibratory finishing, uses an open-topped tub or bowl mounted on springs, lined with polyurethane. A vibratory motor or electromagnetic system creates the action needed to deburr, clean, or brighten metal parts. Bowl vibrators, equipped with a spiral motion, are particularly effective for separating parts from media using a built-in dam and screen system, enhancing efficiency and finish quality.

Electropolishing is a process for polishing metal parts, reversing the principle of electroplating. The workpiece acts as the anode in an electrolyte solution, with a cathode completing the circuit. This method rapidly removes material from raised, rough spots, resulting in a smooth, polished surface. Ideal for stainless steel sheets and parts, electropolishing can achieve a mirror-like finish with a final roughness of less than 0.05 µm, provided the initial surface roughness does not exceed 0.18 to 0.20 µm.

Electroplating involves creating a galvanic cell where the part to be plated is the cathode and the plating material is the anode. This process coats metals like steel, nickel, and copper alloys with protective layers such as tin, nickel, and chromium. Chrome plating increases surface hardness to HRC 70. Zinc electroplating, or galvanizing, serves as a sacrificial anode, protecting steel substrates from corrosion. However, electroplated finishes can cause hydrogen embrittlement, which must be relieved through baking to maintain material strength.

Electroless Nickel Plating
Electroless nickel plating, as specified by MIL-C-26074F, AMS 2404C, and AMS 2405B, deposits nickel without electric current. The process offers uniform thickness and superior corrosion resistance, with two classes: Class 1 (as plated) and Class 2 (heat treated). Thickness grades range from 0.0005" to 0.0015". This plating is ideal for intricate geometries, ensuring even coverage and enhanced durability.

Chemical Coatings
Chemical coatings range from phosphoric acid washes for temporary oxidation resistance to durable paint finishes. Black oxide is a cost-effective option for corrosion protection on steel, stainless steel, and copper substrates. It also reduces light reflection, making it suitable for various industrial applications where surface dullness is desired.

Solid Film Lubrication
Solid film lubrication, defined by MIL-L-46010, reduces wear and prevents galling, corrosion, and seizure of metal components. Available in natural and black colors, this coating provides long-lasting protection in environments where conventional lubricants fail. Solid film lubricants are ideal for aerospace, automotive, and industrial applications, ensuring smooth operation under extreme conditions.

Spring Plating Technical Details

Cadmium Plating
Cadmium plating, specified under QQ-P-416 and AMS-QQ-P-416 standards, is renowned for its exceptional corrosion resistance, primarily protecting steel and cast iron.

The plating is available in three classes based on minimum thickness:

  • Class I (0.0005")
  • Class II (0.0003")
  • Class III (0.0002")

The treatment types include Type I (as plated), Type II (with chromate treatment), and Type III (with phosphate treatment). Available finishes come in various colors such as colorless, iridescent, bronze, brown, olive drab, yellow, and forest green, making it versatile for different aesthetic and functional requirements.

Chrome Plating
Chrome plating, defined by QQ-C-320 and AMS-QQ-C-320, is recognized for its extreme hardness and excellent wear and corrosion resistance.

This plating is categorized into two classes:

  • Class I for corrosion protective plating
  • Class II for engineering plating

It offers two types of finishes: Type I (bright finish) and Type II (satin finish). Chrome plating is ideal for applications demanding superior wear and corrosion resistance, providing both functional and decorative benefits.

Copper Plating
Copper plating, according to MIL-C-14550B and AMS 2418, offers good corrosion resistance and conductivity.

It comes in several classes based on thickness:

  • Class 0 (0.001" - 0.005")
  • Class 1 (0.001" minimum)
  • Class 2 (0.0005" minimum)
  • Class 3 (0.0002" minimum)
  • Class 4 (0.0001" minimum)

This plating is particularly effective for applications requiring good conductivity and moderate corrosion resistance.

Gold Plating
Gold plating, as specified by MIL-G-45204C, is known for its excellent corrosion resistance, high tarnish resistance, and superior conductivity. It is available in several classes with varying minimum thicknesses, from Class 00 (0.00002") to Class 6 (0.0015"). The gold content is categorized into Type I (99.7% gold minimum) and Type II (99.0% gold minimum), with hardness grades ranging from A (90 Knoop maximum) to D (201 Knoop and over). This plating is ideal for applications requiring high durability, solderability, and conductivity.

Nickel Plating
Nickel plating, defined under QQ-N-290 and AMS-QQ-N-290, is extensively used for decorative, engineering, and electroforming purposes. It includes Class I (corrosion protective plating) and Class II (engineering plating) with thickness grades ranging from A to G (0.0016" to 0.0002"). Nickel plating provides a durable, corrosion-resistant finish suitable for various industrial and aesthetic applications.

Silver Plating
Silver plating, specified by QQ-S-365D and ASTM B700, offers excellent conductivity and solderability, though it tarnishes easily. It is available in finishes such as matte (Type I), semi-bright (Type II), and bright (Type III), with supplementary treatments like Grade A (chromate post-treatment) and Grade B (no supplementary treatment). Silver plating is ideal for applications needing high conductivity and good corrosion resistance.

Tin Plating
Tin plating, defined by ASTM B545 and MIL-T-10727C, provides good corrosion resistance and excellent solderability.

It comes in two types:

  • Type I (electroplated)
  • Type II (hot dipped)

This plating is widely used for its cost-effective corrosion resistance and ease of soldering, making it suitable for various electrical and electronic applications.

Vacuum Cadmium Plating
Vacuum cadmium plating, specified under MIL-C-8837B and AMS-C-8837, is primarily used to provide corrosion resistance without hydrogen embrittlement. It comes in three classes based on thickness: Class I (0.0005"), Class II (0.0003"), and Class III (0.0002").

The treatment types include:

  • Type I (as plated)
  • Type II (with chromate treatment)
  • Type III (with phosphate treatment)

The finishes are available in colors such as colorless, iridescent, bronze, brown, olive drab, yellow, and forest green.

Zinc Plating
Zinc plating, defined by ASTM B633, offers robust corrosion resistance and is categorized by service conditions: Service Condition 1 (Fe/Zn 5, 5μm thickness) for mild conditions, Service Condition 2 (Fe/Zn 8, 8μm thickness) for moderate conditions, Service Condition 3 (Fe/Zn 12, 12μm thickness) for severe conditions, and Service Condition 4 (Fe/Zn 25, 25μm thickness) for very severe conditions.

The plating types include:
Type I (as plated)
Type II (colored chromate conversion coatings)
Type III (colorless chromate conversion coatings)
Type IV (phosphate conversion coating)

Available in finishes such as colorless, blue, olive drab, and yellow.

Spring Chemical Coatings Technical Details
Black Oxide Coating
Black oxide coating, specified under MIL-C-13924, provides a uniform, predominantly decorative black finish with limited corrosion resistance.

The coating process includes several classes:

  • Class 1 (alkaline oxidizing process)
  • Class 2 (alkaline chromate oxidizing)
  • Class 3 (fused salt oxidizing process)
  • Class 4 (alkaline oxidizing process)

Typically, a supplementary oil treatment is applied per MIL-C-16173 to enhance the finish. Black oxide is often chosen for its aesthetic appeal and minimal impact on dimensions, making it suitable for applications where appearance is crucial, though it offers only limited protection against corrosion.

Light Phosphate Coating
Light phosphate coating, as defined by TT-C-490E, is primarily used as a pretreatment prior to painting.

The coating types include:

  • Type I (zinc phosphate spray application and zinc phosphate immersion or dip application)
  • Type II (aqueous iron phosphate)
  • Type III (organic pretreatment coating, as per MIL-C-8514)
  • Type V (zinc phosphate)

Each type serves specific purposes. Type I is a versatile pretreatment for painting, Types II and IV are designed for parts that will be formed after painting, and Type III is used when the size and shape of parts preclude the use of Types I, II, or IV. The light phosphate coating provides a good base for paint, enhancing paint adhesion and corrosion resistance.

Heavy Phosphate Coating
Heavy phosphate coating, specified under MIL-DTL-16232G, is used as a base for supplementary coatings that provide significant corrosion resistance.

The process includes different classes:

  • Class 1 (Type M/Z) with a supplementary preservative treatment or coating.
  • Class 2 (Type M) with lubricating oil
  • Class 2 (Type Z) with a preservative
  • Class 3 (Type M/Z) with no supplementary treatment
  • Class 4 (Type M/Z) chemically converted (and potentially dyed to a specified color)

The heavy phosphate coating uses Type M (manganese phosphate base) or Type Z (zinc phosphate base), making it ideal for medium and low alloy steels. This coating is typically employed to prepare surfaces for additional treatments that will provide enhanced corrosion resistance, such as lubricating oils or other protective coatings.

Spring Electroless Plating Technical Details

Electroless Nickel Plating
Electroless nickel plating, specified under MIL-C-26074F, AMS 2404C, and AMS 2405B, is a process used to deposit nickel onto a substrate without the use of an electric current. This coating technique offers several advantages, including uniform thickness and superior corrosion resistance. The plating is available in two classes: Class 1 (as plated) and Class 2 (heat treated), each tailored to meet specific performance requirements.

The coating is further classified into grades based on thickness:

  • Grade A: 0.001" minimum thickness
  • Grade B: 0.0005" minimum thickness
  • Grade C: 0.0015" minimum thickness

Electroless nickel plating is highly valued in various industries due to its ability to provide a consistent, high-quality finish that enhances the durability and wear resistance of components. It is widely used in applications requiring precise control over coating thickness and where intricate geometries must be evenly covered, ensuring that every surface, nook, and cranny receives the same level of protection and enhancement.

Spring Lubrication Coating Technical Details

Solid Film Lubrication
Solid film lubrication, specified under MIL-L-46010, is a coating used to reduce wear and prevent galling, corrosion, and seizure of metal components. This type of lubrication is particularly useful in environments where conventional lubricants like oils and greases are ineffective or cannot be used. The solid film provides a durable, long-lasting solution that enhances the performance and lifespan of metal parts.

The coating is available in two colors:

  • Color 1: Natural product color
  • Color 2: Black

By forming a protective layer on the surface of the metal, solid film lubrication minimizes friction and wear between moving parts. This makes it an ideal choice for applications in aerospace, automotive, and industrial machinery, where reliable performance under extreme conditions is essential. The use of solid film lubricants ensures that components operate smoothly, even under high loads and temperatures, thereby extending their operational life and reducing maintenance costs.

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