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Thermal Spraying versus Hard Chrome Plating

Topics Covered

Background

Thermal Spraying vs. Hard Chrome Plating Overview

Benefits of Hard Chrome Plating

Limitations of Hard Chrome Plating

Benefits of Thermal Spraying

Limitations of Thermal Spraying

Some Examples From Industry

Conclusion

Background

Hard chrome plating has been a trusted industry solution for wear, erosion, corrosion resistance and dimensional reclamation for many years. It can be applied at a reasonable cost per unit of surface area, but has limitations on thickness build-up, part size, and in some instances performance in service.

Over the past few years, costs have been steadily escalating due to the growing environmental pressures and legislation imposed on the chrome plating process and the disposal of its by-products. It has therefore become critical to industry to find alternative processes that offer similar characteristics to hard chrome plating, but without the consequent risks. Thermal spraying technology is increasingly offering a viable alternative to this technology, and could provide the Chrome Plating industry with complimentary processes for part protection and reclamation.

Thermal Spraying vs. Hard Chrome Plating Overview

When comparing the two processes, a consideration of the economics involved in establishing and maintaining both types of facilities can be made. The following factors make thermal spraying commercially competitive with hard chrome plating.

         Capital Cost - The relative capital expenditure for establishing facilities with the same production capability is much greater for chrome plating than thermal spraying.

         Space - A thermal spray facility requires significantly less floor space than an equivalent plating facility.

         Energy Cost - For plating, approximately 15 watts of energy are required per square inch. As part size increases, so do the energy costs. For thermal spraying, part size affects coating application time, and depending on the process used, energy costs are similar.

         Waste Disposal - Disposing of effluents from the plating process is becoming more and more costly. State and Federal regulations on pollution control require that each facility make substantial investments to adequately provide for waste treatment. Thermal spraying produces hazardous waste in the form of metallic dust, whose disposal is relatively easy.

         Materials Diversity - A chromium plating facility is a total commitment to one coating, whereas a thermal spray facility provides the capability of producing a broad range of coatings.

Hard Chrome Plating utilises more than double the number of process steps compared to thermal spraying. This translates into processing time, giving significantly longer turnaround times for Chrome Plating than Thermal Spray.

Table 1. Comparison of chrome plating vs thermal spraying.

 

Process Step

Hard Chrome Plating

Thermal Spraying

1

Degrease

X

X

2

Alkaline

X

 

3

Grit Blast

 

X

4

Rinse

X

 

5

Etch

X

 

6

Rinse

X

 

7

Plate/Coat

X

X

8

Rinse

X

 

9

Dry

X

 

10

Grind/Polish

X

X

Benefits of Hard Chrome Plating

         High Hardness - Hardness value of Vickers 700-1000.

         History - Well known, well established process with known properties and limitations.

         Surface Coverage - Not a line of sight process, will work on inside diameters, and on complex geometry.

         Excellent Surface Finish - A surface finish, as coated, of ~40 RMS.

         Economical For Thin Deposits - Chrome plating is economical and reliable for very thin deposits, <25 to 100um (0.001 - 0.004”).

Limitations of Hard Chrome Plating

         Adhesion - Two primary reasons cause poor adhesion, especially on iron-based materials such as ductile and cast iron. The first is improper or poor surface preparation and the second is excessive micro cracking throughout the chrome plating. Micro-cracks, which extend from the surface, occur in the plating due to residual stresses (figure 1). When the micro-cracks extend all the way down to the substrate, separation of the plating may occur.

         Slow Throughput - Typically plating requires about one hour to deposit a thickness of 25μm (0.001”) on any size part.

         Nodules/Uneven Buildup - Nodules of excess plating build-up onto corner and edge areas where current density is high. This creates uneven buildup and may cause high residual stresses and therefore adhesion problems. It will also increase finishing time.

         Masking - Because parts must be completely submerged, it is difficult to mask areas that do not require plating.

         Tank Contamination - Contaminants, especially iron, affect the conductivity of the plating solution and the related parameters involved making reproduction of plating quality sometimes difficult.

Figure 1. Typical microstructure of chrome plating shows a network of micro-cracks providing possible routes for corrosion penetration and inherent mechanical limitations.

Benefits of Thermal Spraying

         Low Capital Investment - Compared to a chrome plating installation, a low capital investment in equipment required. Also, installation of a thermal spray system takes much less time and associated installation costs are low.

         Waste Disposal - Wastes from thermal spraying are not toxic, but do contain elements requiring special disposal. Typically they are dry powders, some of which can be reactive.

         Competitive Application Costs - When parts are large enough and the coating thickness requirement is high, thermal spraying becomes very cost competitive with chrome plating.

         No Limitation On Size 0f Part - The thermal spray process has no real limitations on part size because there is no need for immersion in plating tank.

         High Deposition Rates - Most of the thermal spray processes have a higher deposition rate than chromium plating.

         On-Site Capability - Thermal spray equipment is portable and has on-site capability.

         Fewer Process Steps - Thermal spray coatings require fewer process steps to apply than chrome plating.

         Thicker Coatings - The thermal spray process has the ability to economically apply thicker coatings, in some cases up to 10mm.

         Denser Coatings - Coatings can be applied with almost theoretical density with no crack patterns (figure 2).

         Uniform Coating Thickness - The coating build-up with thermal spraying is much more uniform over the entire part, depending on automation and part geometry.

         Materials Choice - This is probably the single most important benefit. Individual applications can be matched to materials depending upon the specific requirements for corrosion, wear, service temperature or other factors.

Figure 2. A HP/HVOF coating of tungsten carbide 17% cobalt showing very few pores, no cracking and a clean interface between substrate and coating.

Other benefits include:

         No Hydrogen Embrittlement - No base metal hydrogen embrittlement problems.

         No Post Heat Treatment - No post heat-treatment or stress relief required.

         No Fatigue Debit - Chrome plating reduces the fatigue life of parts it is applied to, whereas, some thermal spray coatings can increase it.

         Finish With Conventional Grinding - Thermal spray coatings can be finished with conventional grinding and polishing technologies, some can even be single point machined.

         Process Time - The decreased number of process steps, and high application rate can dramatically reduce turnaround time, which is increasingly a factor in repair applications.

Table 2. Performance comparison between Chrome Plate and various thermal spray coatings.

Coating Type

Wear Performance

Corrosion Performance

Cost/Unit

Chrome Plate

Good

Good

Low

Arc Sprayed 420SS

Fair

Poor

Low

Plasma Al0/Ti0

Very Good

Excellent

Low

HP Plasma CrO

Excellent

Excellent

Medium

HP/HVOF NiCrBSi

Good

Very Good

Medium

HP/HVOF WC/Co

Excellent

Good

High

HP/HVOF CrC/NiCr

Excellent

Excellent

High

HP/HVOF WC/Co/Cr

Excellent

Very Good

High

Limitations of Thermal Spraying

         Line of Sight Process - Some capability to coat inside diameters, and difficult geometry can mean significant automation programming.

         Noise - Thermal spray processes vary in their sound output, some are as high as 135dB, this requires noise attenuation.

         Dust - All of the thermal spray processes produce dust particles as waste or over-spray which needs to be collected by dust collection for disposal, and also as a means of keeping the spray area clean.

         Process Parameters - A thorough understanding of the thermal spray process is required to produce the optimum results. As with any technology, there is an associated learning curve.

Some Examples From Industry

The bearing and seal ends of this gas turbine shaft were previously reclaimed by the application of chrome plating. They are now thermally sprayed with HP/HVOF which provides a coating with superior wear resistance. The coating can also be applied to the part when it is partially assembled saving turnaround time.

Figure 3. A gas turbine shaft previously chrome plated, now coated using thermal spray techniques.

The main axle of a Boeing 767 aircraft being sprayed with the HP/HVOF process with Tungsten Carbide, 10% Cobalt, 4% Chrome. Boeing are at the leading edge of utilising thermal spray to replace chrome plate on critical components.

Figure 4. The main axle of a Boeing 767 aircraft being thermally sprayed.

Conclusion

Thermal spraying is not about to completely replace Chrome Plating the world over. However, what it will do, is to offer an alternative solution where coating performance, part size, coating thickness, turnaround time or environmental issues create challenges for Chrome Plating.

Various industry sectors are carefully and methodically moving forward with using thermal spraying in areas where Chrome Plating was the trusted solution for many years. Both processes have their limitations, hence it is highly unlikely that this technology will do anything more than supplement Chrome Plating and other surface treatments in industry. This will provide engineers in industry an additional solution to the many problems encountered as higher levels of performance are demanded.

The lead taken in evaluating and utilising thermal spraying on previously plated components by the major airlines in the US and Europe, and other high volume, high profile users of Chrome Plating, should reinforce the confidence level of other industries enough to evaluate what thermal spraying can provide. The advances in thermal spraying over the past 10 years, both in terms of product and process, and also in the level of understanding, now positions this technology to move forward on a broader front as an established, and thoroughly proven industrial tool.

 

Primary author: Michael Breitsameter

Source: Materials Australasia Vol. 32, no. 6, pp. 11-13, November/December 2000

 

For more information on this source please visit The Institute of Materials Engineering Australasia.

 

Date Added: Aug 15, 2002 | Updated: Jun 11, 2013
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