Composite Electroless Nickel Coatings for the Aerospace Industry - Varieties and Performance Advantages

Coatings can be advantageous and in many applications are even essential for proper performance, protection, lifetime, and many other factors.  Selecting the proper coating for each application, therefore, is vital.  But choosing the right coating can be a challenge for two main reasons.  First, there are many coatings available to the aerospace industry.  Second, parts used in the aerospace industry come in a tremendous array of shapes, sizes, base metals, etc. and are utilized in an equally exceptional range of climates, requirements and usage conditions.

One category of coatings that can enhance many applications in the aerospace industry is based on electroless nickel plating.  Electroless Nickel (EN) is a sophisticated yet reliable chemical process with many inherent features well suited to applications in the aerospace industry; including hardness, corrosion resistance, and perfect conformity to even the most complex geometries.  In addition, EN is an exciting coating method as it is possible to add super fine particles into the EN to form composite EN coatings.  These particles can provide hardness, wear resistance, low-friction, release, heat transfer, friction, and/or even phosphorescent properties.

This paper will discuss all varieties, but first a brief background on composite EN coatings.

Electroless nickel has grown to be a mature segment of the metal finishing industry since its discovery in the 1940’s.  EN is generally an alloy of 88-99% nickel and the balance with phosphorous, boron, or a few other possible elements depending on the specific requirements of an application.  It can be applied to numerous substrates including metals, alloys, and nonconductors; with outstanding uniformity of coating thickness to complex geometries.  It is this last point which most commonly distinguishes electroless from electrolytic coatings like chrome plating.

Composite EN is exciting given the synergies possible between the EN and particles that can dramatically enhance existing characteristics and even add entirely new properties.  This makes composite EN coatings especially advantageous for applications in the aerospace industry to:

1. Meet ever more demanding usage conditions requiring less wear, lower friction, etc.
2. Facilitate the use of new substrate materials such as titanium, aluminum, lower cost steel alloys, ceramics, and plastics.
3. Allow higher productivity of equipment with greater speeds, less wear, and less maintenance related downtime.
4. Replace environmentally problematic coatings such as electroplated chromium.

Photograph 1 is one example of a composite EN coating.  It is a cross sectional photomicrograph showing a uniform dispersion of fine diamond within EN.  As you can see from this photograph, composite EN coatings are regenerative, meaning that their properties are maintained even as portions of the coating are removed during use.  This feature results from the uniform manner with which the particles are dispersed throughout the entire plated layer.  Particles from a few nanometers up to about 50 microns in size can be incorporated into coatings from a few microns up to many mils in thickness.  The particles can comprise about ten to over forty percent by volume of the coating depending on the particle size and application.

WEAR RESISTANCE
Coatings designed for increased wear resistance have proven to date to be the most widely utilized composite EN coatings in the aerospace industry.  Particles of many hard materials can be used such as diamond, silicon carbide, aluminum oxide, tungsten carbide, boron carbide, and others.  But the unsurpassed hardness of diamond has made this material the most common composite.  Despite the expensive sounding name, composite EN with diamond is actually comparable to the cost of similar coatings, yet the performance advantages are far greater.

The Taber wear test is the most common test methods have been employed to evaluate wear resistance of different materials and coatings.  It evaluates the resistance of surfaces to abrasive rubbing produced by the sliding rotation of two unlubricated, abrading wheels against a rotating sample.  This test measures the worn weight or volume.  The Taber results in Table 1 demonstrate the wear resistance of various materials including two different composite EN coatings as well as a conventional EN coating.

Table 1 - Taber Wear Test Data
Coating or Material                    Wear Rate - 104 mils3 / 1000 cycles
Composite Diamond Coating*       1.159
Composite EN-Silicon Carbide       1.738
Cemented tungsten carbide,        Grace C-9 (88WC, 12 Co) 2.746
Electroplated hard chromium        4.699
Tool steel, hardened, Rc 62         12.815
*Composite EN containing 25-30% of 3um grade diamond.

In addition to applications in the aerospace industry, composite coatings with wear resistant particles are widely used in the textile, paper, plastics, automotive, molding, petrochemical processing, dental/medical, glass, and other manufacturing industries.

LUBRICITY
Certain particles can be incorporated into EN to produce a coating with all the properties of EN (such as hardness and wear resistance) as well as a low coefficient of friction, dry lubrication, and repellency of water, oil, and/or other liquids.

Most commercial use of such composite lubricating coatings in the aerospace industry has been with 20-25% by volume of sub-micron PTFE particles in EN deposits.  The properties of PTFE are widely recognized, and its enhancement of EN clearly demonstrated in industry applications as well as standardized testing such as those summarized in Table 2.  These results further show that the lowest coefficient of friction is achievable when both mating parts are coated with composite EN-PTFE.

Table 2 - Friction Coefficient and Wear Data
for Electroless Nickel-PTFE Composite
Coating              Coating          Coefficient              Relative
On Pin                On Ring         Friction Relative    Wear Rate

EN                   Cr steel             0.6-0.7                      35
EN + PTFE        Cr steel              0.2-0.3                      40
EN + PTFE        EN + PTFE          0.1-0.2                        1
EN + PTFE        Cr steel              0.2-0.5                      20

In addition to EN-PTFE coatings, newer low-friction coatings have been developed and are being increasingly adopted in the aerospace industry.  As beneficial as PTFE is, there are certain limitations that have been overcome by the incorporation of materials other than PTFE into EN.  For example, composite EN-PTFE does not always provide optimal wear resistance and lubricity. This is often due to the fact that PTFE is relatively soft and cannot withstand high temperatures.  By contrast, particles of certain ceramics such as boron nitride provide lubricity, are significantly harder than PTFE, and can withstand temperatures above 850°C.  This tolerance for heat allows such coatings to be heat-treated after coating to achieve maximum hardness, which is a standard post treatment for most electroless nickel coatings.

Hardness of the composite is critical in applications, as is often the case in the aerospace industry, for greater wear resistance and in situations where there is a higher “loading”, or force, between the coated part and the mating part or materials.  When the coating is harder, it is less prone to “give way” under pressure, and if the coating does not give the friction will not increase as the loading increases.  Think of the difference in friction between the point of a pencil and the eraser as they move across a piece of paper.  Table 3 demonstrates this effect in the coefficients of friction for a variety of coatings under different load conditions.  As you can see, the coefficient of friction of EN-PTFE and chrome coatings increase as the load in increased, but the coefficient of friction of EN-BN and conventional EN actually decreases as the load in increased.  This shows the incompatibility of the soft PTFE particles in higher load applications.

Table 3 - Friction Coefficients For Various Composites and Materials
Coating           Load kg/cm2          Friction Coefficient
EN-PTFE                0.1                          0.12 
EN-BN                   0.1                          0.13 
EN (No particles)     0.1                          0.18
Chrome                  0.1                         0.25

EN-BN                   0.3                          0.09
EN-PTFE                0.3                          0.13
EN (No particles)     0.3                          0.16
Chrome                  0.3                          0.40

EN-BN                   0.5                          0.08
EN-PTFE                0.5                          0.13
EN (No particles)     0.5                          0.15
Chrome                  0.5                       150.00 

INDICATION
This category of composite EN coatings is a more recent and novel development in the field.  These coatings have all the inherent features of EN, and appear normal under typical lighting; but when these phosphorescent coatings are viewed under an ultraviolet (UV) light, they emit a constant lighted glow.  This is a feature that can be used in two ways.

First, the presence of a colored light emission from the coating can be valuable in authenticating parts from a distinct source.  This is especially promising for the identification of genuine OEM parts which otherwise can be routinely counterfeited.  Its value also extends to the identification of specific manufacturing lots where conventional methods of marking are not sufficient or durable.

Second, the light can serve as an indicator layer, warning when the coating has worn off and replacement, or recoating, is necessary.  This feature permits the avoidance of wear into the part itself that may cause irreparable damage to a potentially costly part, or the production of inconsistent product from a worn manufacturing device.  Such a layer can be employed in one of two options to achieve this feature.  Option one is to have a light emitting indicator layer applied to a part prior to (or under) another functional coating to signal when the functional coating has worn through to expose the indicator layer.  In this case, it is the appearance of light that signals wear to the functional layer and exposure of the indicator layer.  Option two is to use the light emitting coating by itself; whereby the disappearance of the light following periodic inspections indicates wear.  Fortunately, hand-held, battery operated UV lights are readily available, and make inspection for the indicator layer at the operating site fast and convenient.

GENERAL FEATURES
All varieties of composite EN coatings share some additional general features that make them further suited for applications in the aerospace industry.

For example, as with any conventional EN coating, these composite coatings can be chemically stripped, leaving the substrate ready for recoating. This can be a very cost effective alternative to disposing of overly worn parts and replacing them with entirely new parts.

For certain applications, customized composite coatings have been developed to satisfy unique requirements:

• Coatings with particles of two or more materials into the same layer to provide multiple properties.
• Overcoating with a conventional EN layer for greater smoothness, cosmetics, or other priorities.
• An underlayer of conventional (often high phosphorous) EN can be applied to insure maximum corrosion resistance.

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