HARD SURFACING TREATMENTS

 

The treatments discussed below vary in their composition and application method but they each serve a purpose in providing better Wear life for many environments:

 

• Hard Duty

 

• Abrasive Duty

 

• Poorly Lubricated Mechanical Duty

 

Hardness, Bond Strength, and Process Cost are just some of the variables which led to the evolution of these treatments:

 

1.  The FENCR Process

 

1.  The BOCAR Process

 

1. The Tribolite / Abralite Thermal Spray Processes

 

1. The Abralite, Tribolite, and Nicolite Weld Overlays

 

1. The Electrospark Carbide Overlay

 

A common problem with coatings, generally, is misapplication.  Because the world of Wear presents such a wide range of conditions, no single coating or surface treatment, could be expected to function and / or survive all of the possible eventualities.

 

An important part of this presentation, in addition to simple process descriptions, is the application guidelines which identify where the process should not be used, as well as where the process could be employed.

 

Wear can be simply defined as “a loss of surface” which can occur in many and sometimes multiple forms. It is not always possible to tell what type of Wear took place just by looking at a worn part so it is always helpful to know something about the service environment. Because Wear always starts at the surface, the properties of that surface play a big part in the Wear equation.

 

Abrasive Wear is loss of surface due to repeated sliding against something harder.

Hard material such as stones, gravel, sand, sandpaper, steel shot, etc., removes a small amount of metal with each contact transaction. These transactions can be combined with oxidation or rusting but they eventually add up to failure.

 

 

Controlled Abrasive Wear occurs with tires, Mild Abrasive Wear or Polishing occurs with “tooth-brushing”, and Severe Abrasion happens when we sharpen a knife or a mower blade. Note that it was mild abrasion, impact abrasion, and corrosion which dulled these tools in the first place.

 

Another form of abrasion is caused by trapped debris between a hard surface and the softer surface and this “third party” abrasion is frequently present during many conventional abrasion cycles.

 

Adhesive Wear is a little harder to understand because it is invisible when it originates at the atomic scale and grows to visibility only after a large number of interactions in which a small piece of the surface is pulled off  by sticking to an opposed surface. This has been called “micro-welding” but it begins as “nano-welding” long before it becomes visible.

 

Adhesive Wear is always accompanied by friction, because the small Wear particles want to stay home, and the energy required to remove them requires work. Friction is the work of Wear, and heat is one way that adhesive Wear can be made more observable.

 

A car spends about 20% of its fuel in overcoming the friction caused by lubricated adhesive Wear, and the coolant temperature, although also influenced by combustion heating, reflects this frictional heating.

 

Corrosion is a devastating problem on its own.  With plenty of surface loss and component failure totaling 50 billion dollars yearly, but Corrosive Wear is always accompanied by Abrasive or Adhesive Wear when surface contact is present.

 

Corrosion forms a decay film on the steel surface and this film is removed during repeated contact leading to loss of material.  A small amount of water in lubricating oil degrades the oil and corrodes the surface, while Atmospheric Corrosion during abrasive contacts just accelerates the surface losses.

 

There are other forms of wear and surface loss:

 

• Erosion

• Cavitation

• Fretting

 

These mechanisms have a much lower occurrence than the big three: Abrasion, Adhesion, and Corrosion.

 

Surface hardness is the most common way to win the abrasion / adhesion battle, and passive surfaces, such as stainless steel, can retard oxidative/corrosive decay.

In the descriptions below, the surface treatments are explained in the context of the common decay mechanisms.

 

The important parameters which will be evaluated are hardness, bond strength, thickness, cost, lubricity, and passivity, among others.

 

1.  The most well known diffusion treatment is the FENCR process. 

 

A thin layer of carbon and nitrogen is dissolved in the steel surface at an elevated temperature.  This “ceramic-like” skin increases the hardness and fatigue strength of the surface while drastically increasing its Corrosion resistance. The reason that this process works is that the Iron in the base metal  is prevented from doing what it likes to do, which is combining with Oxygen, water, or another Iron-rich metal. So we have an extremely hard skin which won't rust or gall in sliding contact.

 

This treatment is very effective for contacting surfaces with dry sliding wear, sliding contact with water present, and for lubricated bearings which encounter lubricant deficiencies which affect life cycle.

 

This process can be characterized as an Adhesive Wear / Corrosion solution with a ceramic skin hardness of about 70 Rc.

 

 

 

The FENCR samples shown above include sprockets, which are normally lubricated but can be run with no third party lube after treatment.

 

 

The Pillow Block Bearing as shown above, which is self lubricating, can run with water as the only fluid used.

 

Another good use for FENCR involves mechanical systems that use bronze bushings, supporting steel pins, shafts, and plungers.

 

The bronze material is not self-lubricating and is considered sacrificial, so in a bronze / steel sliding system without adequate lubrication, the bronze can wear out quickly leading to a system failure. By replacing the bronze bushing with a self-lubricating, FENCR treated steel bushing, and treating the steel counter-face, dependence on lubrication is reduced and system lives of 4 to 5x can be expected.

There are some variations to the FENCR process which involve the use of synergistic top coatings over the heat treated surface. Although FENCR can be used alone, the use of these secondary  coatings/treatments can add lubricity or enhanced corrosion resistance so they become a complementary treatment.

A) The most common product is called FENCR and sealer. After the heat treating process the parts are sprayed with a liquid polymer and baked in an oven at about 500 deg F. The baking process cures and crosslinks the polymer so that it becomes a self-lubricating plastic skin about .001" thick. This process represents the vast majority of  our treated parts. For large bearings, in the range of 8" diameter or over the FENCR process itself can cause distortion of the races so sometimes we use the sealer only, and because some bearings can de-stabilize at temperatures as low as 300 deg F, we have to use a low temperature sealer which cures at 200 deg F. So the term "sealer" means a cured polymer skin applied by spraying and baking. The sealer can also be modified by adding about 2% solid lubricant to the liquid before curing. The typical solid lubricants are graphite, moly disulphide, and tungsten disulphide. All of these lubes add a transfer film capability to the sealer. Tungsten disulphide has the best lubricity and the highest temperature capability ( approx 1000 deg F) so it is used for furnace bearings and other high temperature applications. Note that for some problems above 600 deg F , the sealer itself can be the weak link and that can lead to the use of "c" below.

B) Fencr can also be followed by a simple "oil dip". The micro-porosity of the Fencr surface can trap some of this oil to become an adsorbed lubricant. Some applications like chains or needle bearings can only be treated by dipping anyway so this thinner film may be the only way to  get extra lubricant to the surface.

c) Fencr can also be followed by a burnished dry film of  tungsten disulphide, which can take the performance limit of the ceramic layer and the burnished layer up to 1000 deg F. In order for this process to work, the steel surfaces have to be extremely clean, because the micro-thin layer of powder particles are held in place by very low "Van der Wals" forces, but after sliding contact occurs the powders can be burnished in place.

 

A  code system for these options would help to prevent confusion as to which process variant would be most suitable

1) Fencr a std streatment with std sealer

2) Fencr b--std fencr with oil dip

3) Fencr with WS2 top coating

4) std sealer only

5) low temp sealer only

6) WS2 treatment only

 

 

1.  The BOCAR Process

 

An abrasive Wear solution is also offered where surface failure caused by impact with hard particles of sand, carbon, concrete, or other abrasive slurries requires something more than the FENCR skin. This solution is the 90Rc Boronizing process. BOCAR, a process of which a thicker and much harder layer of Iron Boride, is created on the surface of a steel part. This Iron Boride, FE2B, is an inter-metallic compound which is formed by the high temperature diffusion of Boron into the steel surface, similar to the FENCR but done at a higher temperature with different constituents.

 

 

 

 

This micrograph shows a section through a BOCAR surface where the upper layer is the treated surface and the bottom layer is the parent metal.

 

The treated layer can be made thicker by using lower alloy steels or longer process times and it becomes thinner and harder with higher alloy materials.

 

Conventional metallurgy for iron based alloys is limited by heat treatment, quenching and tempering of the bulk material, and the creation of Martensite, which is a phase containing iron carbide in the 60Rc range. This is as hard as the steel can get unless we add other carbide formers such as Chromium, Tungsten, Vanadium, etc., which can raise the hardness into the mid / high 60’sRc range.

 

 

There is no combination of melted and heat treated alloys that can achieve the hardness created by the BOCAR process, in fact, adding a significant amount of Boron to a melted alloy would result in an unworkable set of mechanical properties, even though small amounts of less than 1% can be used to add strength to some mild steels because Boron is a better “hardener” than carbon.

 

However, by diffusing a thin layer of Boron into a steel or iron part we can achieve a very high hardness without sacrificing the properties of the substrate material.

 

As shown above there are three separate layers in this process, which are the Boron rich surface layer, the diffusion boundary layer, and the un-alloyed substrate. Because there is no sharp boundary between layers, as there is with plating or thermal spray, there is no sudden change in physical properties which could lead to de-bonding or fracture.

 

The BOCAR process temperature is about 1800 deg F, which is high enough to anneal any heat treated base metal, so for some applications it is necessary to re-heat treat the part to restore core hardness. The BOCAR layer then provides excellent starting wear resistance which is followed later by normal wear life so that the total life cycle is compounded.

 

High impact abrasion such as rock crushing is not suitable for BOCAR, but abrasive slurry pump impellers, carbon conveying equipment for electric furnaces, paper handling guides, shot-blasting vanes, forming dies, and many other sliding abrasion applications would be suitable for this process.

 

The BOCAR layer is passive, in that the surface molecular layers have no free constituents which want to bond with any opposing surfaces such as metals or environmental materials such as water or air.

 

A new potential application is to use BOCAR on Tungsten Carbide parts and inserts because the 72Rc hardness of the Carbide can be improved and the Cobalt binder used in the Carbide parts, which can sometimes be the weak link in these systems, can be hardened and made more inert.

 

1.  The Tribolite and Abralite thermal spray processes

 

These processes involve using specially formulated powders in a thermal spray procedure that deposits semi-molten powder particles on a prepared substrate which then solidify to create a thin mechanically bonded layer with superior resistance to abrasive, adhesive, and corrosive wear environments.

 

Because this layer only has a mechanical bond, with a bond strength of around 10,000 psi., the applied loads cannot involve heavy impact or severe thermal expansion excursions.

 

The highway of thermal spray mis-applications is paved with bond strength failures. Another limitation of thermal spray is that it is “line of sight”, so the treated surfaces must be open and accessible to a sprayed particle stream. Once these limitations are recognized, thermal spray is hugely successful and cost effective in the modification of working surfaces. Every aircraft jet engine contains thousands of thermal spray coated parts.

 

 

This is a thermal spray coating on a roll used in a Stainless Steel Strip Annealing Furnace.

 

The Tribolite coating has a 72Rc micro-hardness, and the glossy surface of the roll indicates a mild wear, or polishing wear of about .001” per year.

 

The life of the thermal spray coating over uncoated ceramic is about 30x improvement over the previous technology.

 

 

1.  The Abralite, Tribolite, and Nicolite Weld Overlay Systems.

 

These claddings use a Metallurgical Bond created when the cladding and the host material are melted and mixed together.

 

The surface properties of the claddings are reinforced by a welded bond to the substrate materials.  This means that the cladding is less likely to chip or de-bond under heavy load because it adopts the strength of the parent material.

 

A drawback with these overlay systems is that more heat is involved because molten metal is created.  This melting and shrinkage can cause distortion and possible cracking, and the welded surface needs to be machined if surface accuracy is required.

 

The good news is that the weld overlays can be made much thicker than the other coatings and they can be adjusted to provide Abrasion, Adhesion, and Corrosion resistance.

 

 

 

This is a semi-automated welding overlay process which applies the Nicolite

Weld Overlay for high temperature sliding wear resistance.

 

The Tribolite, Abralite, and Nicolite welding families can use this mechanized overlay, or hand overlay to achieve welded bond strength and unique surface wear properties.

 

 

1.  The Electrospark Tungsten Carbide Overlay

 

 A welding process in which a metallurgical bond creates a deposit of Tungsten Carbide / Cobalt on the work surface and the texture can be varied by a weld frequency adjustment.

 

 

 

This is a cladding option which combines the wear properties of Tungsten Carbide with the bond strength of a weld overlay.  This makes it unique in the surface protection business.

 

These coatings, in addition to providing wear resistance, can also create an increase in friction coefficient where it is helpful. The photo above shows the surface of a tension bridle roll being coated to increase traction and reduce wear.

 

CONCLUSION

 

The only mechanical decay mechanisms that do not begin at the surface involve contact fatigue or Hertzian Contact Stress where the subsurface stress creates delamination.

 

This can be fixed but the surface is usually the weak link. That is why we usually just provide treatments to enhance surface properties because this is the low hanging fruit.

 

We have explained some treatments which can control friction and wear at the surface and the variety of coating and duty parameters that must be evaluated before a successful Wear Life Cycle can be achieved. The key for these treatments is to apply the correct treatment to the problem.