Surface Enhancement

By using the Metallisation Arc or Flame spray Process, it is possible to remove this major source of spark hazard by applying a thin layer of phosphor bronze onto the offending surface. One of the most common components to be treated in this way is crane hooks, but it is also possible to apply this type of coating to steel fan housings or forklift truck forks. Metal spraying is an economic method of producing a spark resistant surface on any standard manufactured steel component.

Equipment: In this case Arc Spray Equipment was used.

Materials: Phosphor Bronze Arc Wire - Easily machinable material, very good for bearing surfaces and giving an excellent anti-spark coating.

Method - Cleaning

1. Steam clean if equipment available.
2. Degrease by solvent vapour process, if material available.
3. Check all surfaces are free from contamination and debris.

Preliminary Inspection

Check for cracks or surface imperfections taking hooks below the manufacturers recommended operating tolerances.

Preparation

1. Mask surfaces adjacent to area requiring treatment with a heavy duty masking tape.
2. Thoroughly inspect for contamination prior to blasting.
3. Thoroughly blast the area to be sprayed with clean chilled iron grit grade G24.
4. Ensure that areas to be treated are thoroughly blasted A surface profile of between 75μm-100μm should be achieved. It is important that the surface to be sprayed should not come into contact with hands, oil, grease or other contaminants which may cause bond failure after spraying. Delays between blasting and spraying should not exceed 20 minutes.

Application of Sprayed Coating

Bonding and Simultaneous Spraying of Phosphor Bronze.

1. The Arc spray Equipment should be set up in accordance with the MSSA Manual for the spraying of Phosphor Bronze.
2. The area to be sprayed should be cleaned with a vacuum cleaner or a clean, dry air blast to remove any loose particles of dust or grit.
3. The first 75μ-100μm should be applied at close range (typically 100mm) and at lower air pressure to achieve a higher bond strength.
4. The coating should be applied evenly by rotating the component in front of the Arc spray Pistol, keeping the spray-stream at as near as possible to 90° from the surface being treated.

Bond Coat

Spraying parameters for Bond Spraying Phosphor Bronze.

1. Range: 100mm
2. Nozzle Air Pressure: 3.7 bar (55 psi)
3. Volts Before Spraying: 32-34V
4. Volts During Spraying: 28-30V
5. Current: 200A

Note: Parameters may differ in accordance with type and length of power cables and hoses being used.

Main Deposit

1. Apply Phosphor Bronze final deposit to specified required thickness (typically 0.40-0.50mm).
2. The coating should be applied evenly by rotating the component in front of the Arc spray pistol, keeping the spray-stream at as near as possible 90° from the surface being treated.

Spraying parameters for Main Deposit Phosphor Bronze.

1. Range: 150mm
2. Nozzle Air Pressure: 4.3-4.6 bar (62-87 psi)
3. Volts Before Spraying: 32-34V
4. Volts During Spraying: 30-32V
5. Current: 250A

De-Masking

1. Remove all masking tape.
2. Remove all over-spray taking care to prevent coating damage.

Inspection

1. Check dimensions.
2. Check for cracks, defects in sprayed coating, i.e. large pores or protrusions and loose particles.

Finishing

Under normal circumstances, it is possible to use the component in the as-sprayed condition without any problems but for cosmetic purposes, a light polish may be required.

Shaft Coatings with HVAF

Kermetico HVAF & HVOF technology and equipment allows the deposition of impermeable, hard and ductile wear resistant shaft coatings.

The high ductility of our metal ceramic shaft coatings reduces the risk of cracking if flexed and provides loading stress resistance while an optimal surface structure provides improved friction between a shaft and a bearing.

HVAF Spraying of a Wear Resistant Coating onto a Shaft

Compared to traditional electrolytic hard chromium plating, our shaft coatings are characterized by significantly higher wear resistance and the absence of hydrogen embrittlement of high-strength steels. They also have a better quality/price ratio than most other surface engineering methods.

Shaft Wear in Seals and Bearings

Wear of a Shaft Under a Seal

Radial shaft seals must run against a smooth, round shaft surface to seal efficiently. If the shaft becomes worn, the seals will no longer be able to fulfill their function, which is to retain lubricant and exclude contaminants.

Typically, the shaft becomes scored when a contaminant particle is caught under the sealing lip and abrades a track as the shaft rotates. As this continues, the seal will enable more particles to pass or get stuck, and seal efficiency deteriorates, eventually leading to malfunction of the component that the seal is meant to protect.

Shaft Wear in its Bearings

Under normal running conditions of full fluid lubrication wear of a shaft in its bearings is due primarily to the presence of grit, etc., which is imported into the bearing by the lubricating oil. Particles of grit, too small to be caught by any filter, become embedded in the surface of a bearing material but may still project sufficiently to span across the oil film when it is at its thinnest and so to lap the shaft. To a secondary degree it is due to attrition, when owing to the thinness of the oil film, the high spots on both members of the bearing come into metallic contact, but with highly finished surfaces, and an ample supply of lubricant, it is doubtful whether wear by attrition is an important factor provided that the materials are compatible and are not such as will readily weld together. Clearly, the higher the mean loading, the thinner the oil film will be on the loaded side of the bearing, and therefore the greater the intensity of the lapping process.

The harder the surface of the shaft or the softer the bearing material the particles will more readily be driven home into the latter and so out of harm’s way. Other things being equal, the rate of wear of the shaft will depend on the difference in surface hardness between itself and the material of the particles.

That is why different techniques of surface treatment are being used to protect a shaft surface.

The Features of Our HVAF Coating Technology

We recommend the application of Fe-based materials to protect shaft surfaces. Depending on the shaft base material, environment, sealing and bearing conditions we recommend using SS350, SS410, SS431 or harder 6AB coatings. For the most demanding applications, we use tungsten carbide coatings.

Just 100 µm of our coating is gas-tight, impermeable to gas or liquid without additional sealing.

Kermetico High Velocity Air Fuel (HVAF) technology has been shown to be very competitive as an environmentally friendly alternative to electrolytic hard chromium (EHC).

The ecological aspect and shorter processing time reduce processing costs.

The option to apply thicker layers provides a way to repair heavily worn components.

Kermetico HVAF metal and carbide coatings are also superior to conventional HVOF counterparts regarding wear resistance, corrosion protection and production cost. The higher velocity of the Kermetico HVAF in-flight particles (over 1,000 m/s) enables the production of coatings with high bond strength and low porosity.

Moreover, the low combustion spraying temperature (1,960-2,010 °C / 3,560-3650 °F) and gentle particle heating lead to minimal feedstock phase transformation and almost nonexistent elemental depletion/decomposition of the in-flight particles.

Furthermore, the replacement of pure oxygen necessary for HVOF with air in our HVAF process significantly reduces the oxide content of the coatings, which is desirable for high-performance coatings.

The result – gas-tight metal or carbide coatings, impermeable to gas or liquid.

The Kermetico HVAF System Spraying a Repair Coating onto a Shaft

The Kermetico HVAF Shaft Coating Method

The traditional thermal spray coating approach is to melt and atomize the feedstock, propel it to the surface of the target part whereupon contact ‘splat cooling’ builds up a coating.

The Kermetico HVAF process operates on a different principle “heat slowly, spray faster.“

We heat the feedstock material to near its liquid phase temperature without exceeding it.

Then we accelerate the particles to an optimized high velocity, and when the particles impact the substrate, there is a rapid conversion of kinetic to thermal energy that allows for plastic deformation of the particle and a bond that we cannot accurately measure.

In the ASTM 633C bond test, the only result we get is broken glue, even with 0.040“ (1 mm) of the WC-based coating.

A Polished Shaft with a Kermetico HVAF Coating

Blasting and Spraying Shafts with Kermetico High Velocity Equipment

Usually, we deposit coatings using robotic blast and spray operations.

We blast a shaft surface with a Kermetico HVAF gun (it is extremely fast and uniform) and spray with the same gun after switching the powder feed hose and perhaps changing the nozzle.

It is much faster, more accurate and needs much less grit than manual blasting.

HVAF grit blasting also produces very even surface preparation and induces less stress into the base metal of a shaft.

The multi-purpose HVAF AK systems are recommended for this application - AK7 for large shafts, AK6 for medium shafts and AK5 for small shafts.

Flash Carbide Coating of Titanium Parts for Hard Chrome Replacement

Hard Chrome Replacement with HVAF Thermal Spray Equipment and Coatings

There is an increasing demand for a cheap alternative to electrolytic hard chrome coatings in corrosive environments. The market needs environmentally friendly long-lasting tungsten carbide coatings or inexpensive stainless steel coatings.

Kermetico HVAF technology and equipment provide a way for hard chrome replacement with impermeable, hard and ductile coatings that are inexpensive and easy to apply. Numerous researchers have pointed out that HVAF coatings work several times longer than electrolytic hard chrome does having the same or lower cost.

How do we thermal spray coatings as cheap as hard chrome plating?

• We spray thin gas impermeable coatings with roughness low enough to avoid grinding.
• We spray it fast.
• We have the systems to spray it onto outer and internal surfaces.
• We have developed the technology to do it without failures.

Features of Kermetico HVAF Coatings for Hard Chrome Replacement

The Kermetico High Velocity Air Fuel (HVAF) process has been shown to be very competitive as an environmentally friendly alternative to electrolytic hard chromium (EHC).

Kermetico HVAF carbide coatings are superior to HVOF and EHC rivals in corrosion protection, wear resistance and production cost. The high velocity of the in-flight particles (faster than 1,000 m/s | 3,280 ft./sec.) in our HVAF process enables the production of very dense coatings with high bond strength.

Moreover, the low combustion spraying temperature (1,960-2,010°C | 3,560-3,650°F depending on fuel gas) and gentle particle heating lead to minimal feedstock phase transformation and almost non-existent elemental depletion/decomposition of the in-flight particle.

Furthermore, the replacement of pure oxygen in the HVOF process by using air in the HVAF process significantly reduces the oxide content in the coatings, which is desirable for high-performance coatings.

Background

A manufacturer had a mission-critical titanium part in a new product which contained two OD seal surfaces. The use condition subjected the part to fully reversing hydraulic pressure cycles of 5 KSI (with occasional spikes to 8 KSI).

Note: An NDA covers this application and our relationship with this customer, so, for this reason, details are intentionally vague. This report has been reviewed, edited, and approved by the customer for our use.

Customer Initial Requirements

• The seal surfaces should not experience excessive wear over the service life of the part.
• The seals themselves should not be negatively affected due to the reciprocating motion while in contact with the part.
• Application of the coating should not affect the substrate material properties. In particular, it should not cause heating which would result in the formation of beta phase within the Ti crystalline structure.
• The coating should not be cost prohibitive.

Initial Customer Testing

Customer performed testing which simulated operational conditions over a period of several months.

Preliminary Customer Results

Parts exhibited longitudinal cracks through the coating which formed a stress concentration resulting in crack propagation through the titanium alloy substrate. Once cracks progressed through the substrate, this created a passage between two isolated fluid circuits, rendering the equipment unfit for continued use. Fatigue life was on the order of 2e5 cycles – far below the expected infinite fatigue life. These failures prompted the customer to specify an additional performance requirement.

Additional Customer Requirements

Application of the coating should not cause a fatigue debit.

Kermetico Recommendation for Coating Thickness

After discussions with the customer, Kermetico recommended reducing the layer thickness from 250 microns (.010”) to 65-80 microns (0.0025”-0.0035”).

Customer Development Efforts

In addition to Kermetico’s recommendation, the customer pursued the addition of a shot-peening process prior to the application of the WC-Co-Cr coating as well as modifications to the machining and finishing processes.

Kermetico Responsiveness

The customer ordered some variants in preparation to test the effect of modifying dimensional and process parameters. Knowing the criticality of the parts, we completed the turnaround time of our processing in a matter of days.

Customer Development Testing

Fatigue testing under worst-case loading conditions, as well as seal endurance testing, was performed over the course of roughly a 12-month period.

Customer Hard Chrome Replacement Results

Fatigue testing exhibited a fatigue life more than 10e7 cycles. As this number of cycles is more than the published endurance limit for the substrate material at the stress ratio experienced by the parts, testing was halted with the expectation of infinite fatigue life. One of these two designs was adopted for production and subsequently placed into service. As of this writing, no field failures using the updated part design have been reported.

Other Hard Chrome Plating Alternatives

Numerous studies of Kermetico HVAF sprayed Fe-based coatings have shown their high corrosion resistance in different environments such as acid, alkaline, and chloride solutions. High-quality microstructures with low oxide content, high retention of the powder chemistry and low porosity have been reported and make this family of coatings suitable as a low-cost hard chrome replacement in many applications.

For more information on HVAF systems, Click Here.

Non-Slip Coatings

Steel floors, decks and panels are used in many applications from train steps to oil platforms. Untreated, this steel can become very slippery, especially in wet conditions. In many applications, corrosion is also a hazard. Safe walking and industrial operating conditions are vital to personal safety and corporate productivity. Now there’s a durable solution for both of these problems. The MSSA 28E coating is used for non-slip applications. The incredibly durable non-slip metallic coating has been used on baggage handling ramps and walkways in airports, as well as ferry loading ramps and walkways in a new sporting stadium in the UK. Untreated steel surfaces can become very slippery, especially in wet conditions, and are prone to corrosion. To ensure safe walking and industrial operating conditions, vital to personal safety and corporate productivity, the MSSA 28E coating provides a durable non-slip anti-corrosion coating.

Image Provided Courtesy of Metal Spray Hungary

What Normally Happens?

Traditionally, steel structures are hot dip galvanised, or painted, to protect against corrosion. The disadvantage of hot dip galvanising is that the surface can become slippery and it does not easily accept paint without the need for special primers. Painting this type of surface, which is sometimes applied with grit inclusions, also has its disadvantages. The surfaces can degrade quickly in heavy use, resulting in corrosion and an increased slip hazard.

Many large steel structures, including oil platforms, refineries and bridges, have been routinely protected against corrosion by thermal spray aluminium (TSA), zinc or an alloy of the two.

Why Isn't this Good Enough?

While providing unrivalled corrosion protection in very aggressive corrosive environments, pure TSA is not durable enough to prevent long term wear on floor plates.

What Should Happen?

Ideally, steel structures need a durable coating that protects against both slip and corrosion and that’s exactly what the MSSA 28E coating does.

• Provides a suitable level of grip to avoid personal slips or industrial skidding.
• Provides comparable corrosion protection to aluminium as used in extremely aggressive environments.
• Provides easy application by a long-standing process covered by international standards.

28E is applied using the arc spray process with the ARC 145 system. In the arc spray process the raw material, in the form of a pair of metallic wires, is melted by an electric arc. This molten material is atomised by a cone of compressed air and propelled towards the work piece. Upon contact, the particles flatten onto the surface, freeze and mechanically bond, firstly onto the roughened substrate and then onto each other as the coating thickness is increased. Coating thickness can range from around 50 microns up to several hundred microns or even millimetres for some metals. Typically, metal sprayed corrosion protection coatings vary from 100 to 350 microns.

A 28E coating is a thermally sprayed coating that can be applied with a rough texture and has excellent non-slip properties, while being extremely hard and resistant to wear.

The resulting coating is corrosion resistant and because of its durability, site owners can be confident that once applied, they can forget about rust or slipping for many years.

28E Non-Slip Durability Testing

28E coatings have been tested with a Pendulum Coefficient of Friction (CoF) tester, in accordance with independent British Standards and UK Health and Safety Executive guidelines. The HSE guidelines suggest that a floor coating with a Pendulum Test Value (PTV) greater than 36 will have a low slip potential in pedestrian areas. To prove the durability of the coating, steel plates were coated and tested as above. The coated plates were then walked on by a standard shoe on a robotic walking machine. The plate was rotated after each step to simulate walking in a straight line and around corners. The PTV was checked before walking and at 250,000, 500,000, 750,000 and 1 million steps. Sealed and unsealed plates were tested in wet and dry conditions.

Pendulum Test Values obtained in accordance with BS7976-2:2002

The coatings tested were sprayed to produce a surface texture that would wear well but not be too rough for pedestrian surfaces. If the surface has too much grip on pedestrian areas, it can cause a trip hazard. However, in some industrial applications, it is desirable and possible to spray a coating that has a rougher texture and more grip.

28E Corrosion Testing

28E has undergone a range of accelerated corrosion testing. This offers a comparison of the performance between the 28E coating and 99.5% aluminium that has offered corrosion protection for several decades in harsh, corrosive environments around the world. Two tests were undertaken by an independent research laboratory:

• Galvanic corrosion tests for 1 week.
• Neutral salt spray corrosion tests for 1000 hours.

The latter was performed in accordance with ASTM B117. It was concluded that in both tests, the corrosion protection performance of the 28E was comparable to 99.5% aluminium, enabling confidence that the coating will provide excellent corrosion protection in harsh environments.

This article was published on the Offshore Technology website and is Available Here.