AC-HVAF as Hard Chrome Replacement

Chrome plating provides a very effective surface treatment for the reduction of wear, and in some cases corrosion. It has been in use for more than 70 years, and has proved to be a relatively cheap, effective solution. However, environmental problems of hard chrome plating are raising its cost and shrinking availability.

Note: The primary problem lies not with chrome itself, but with the hard chrome plating process, which uses a chromic acid solution, released into the air in the form of a fine mist. This mist contains chromium ions in the hexavalent (Cr6+) state, which has long been known to be carcinogenic and to cause a host of other medical problems, including perforated nasal passages and skin rashes.

HVOF and Detonation spray technologies for application of tungsten carbide and chrome carbide based coatings have proved to be cleaner and more effective than chrome plating. Their superior performance is predicted to lead to significant reductions in life-cycle cost of ownership in industry. However, several technical problems are holding back a wide application of these technologies:

1. Remaining porosity in sprayed coatings limits possibility to superfinish coating surface to better than Ra 1.0 microinch (0.025 micron). It is generally not a problem for hard chrome plating.

2. Consistency of coating quality is poor due to quick deterioration of spray gun hardware.
   Low productivity of equipment results in high coating cost.

3. Development of Activated Combustion HVAF technology is expected to become a “break-through” in thermal spray applications for hard chrome replacement since this novel technology overcomes above-mentioned problems.

Optical Mirror Surface

Note: Surface finish is considered of “mirror quality” if Ra is better than 4 microinch. “Optical mirror” surface must exhibit Ra better than 0.5 microinch.

It is a general knowledge that HVOF tungsten carbide based coatings can not be superfinished to better than 1.0 microinch Ra. WC-20Cr-7Ni coatings, sprayed by Super-Detonation process, have been reported superfinished to 0.5-0.7 microinch. However, this is rather brittle but not hard coating compared to WC-based alternatives (WC-CoCr, WC-Co, etc.).

On the contrary, all WC-based and Cr3C2-based coatings, sprayed by AC-HVAF process, are routinely superfinished to optical mirror range. This is considered to be due to superior density of AC-HVAF coatings and absence of brittle phases usually formed due to oxidation and thermal deterioration of carbides.

This WC-10Co-4Cr AC-HVAF coating is applied onto cast iron roll to 30 mil (0.75 mm) thickness and superfinished to Ra 0.40 microinch (0.010 micron) roughness.

Consistency of Coating Quality

Low temperature of air-fuel gas combustion is a primary reason for low heating of the Intelli-Jet gun parts, providing their long-term performance. Since sprayed carbides are heated substantially lower than their melting point and due to large diameter of the gun nozzle, spray particles never deposit on the nozzle wall. Thus, nozzle clogging does not limit its lifetime. As a result, the hardware performance in the AC-HVAF process is significantly better than in HVOF processes, providing an order of magnitude longer lifetime of limiting parts (nozzles, injectors, etc.). In addition, due to production rate, the AC-HVAF process needs 5-10 times shorter time for application of a coating compare to HVOF.

These factors provide a good basis for significant improvement in coating quality consistency for AC-HVAF process.

Productivity as a Key Factor in Cost Efficiency

While consuming about the same amount of spray powder as HVOF processes, the AC-HVAF technology provides 5-10 time faster application rate. This way, application cost is reduced many folds compare to HVOF and Detonation spraying. Insignificant cost of compressed air versus oxygen is another obvious factor of cost reduction. For large applications, the AC-HVAF cost efficiency becomes a breakthrough factor in using thermal spray for hard chrome replacement.