You should sign up for the thermal spray society discussion group at:

http://www.asminternational.org/MSTemplate.cfm?Section=Email_Discussion_List&Site=Thermal_Spray_Society&Template=/ContentManagement/HTMLDisplay.cfm&ContentID=3074

You'll get a lot of feedback.

Generally, deposition is a function of deposition method and heat.

Hi MetalMike,
The argon is used for its easy ionizability; the hydrogen increases the heat capacity of the gas-plasma. Your low deposition and losses suggest that the plasma core temperature is too low (cf. 30000^oC according to Praxair).

General process info is given in 'Thermal Spray Coatings,' pp. 497-509 in ASM Handbook, vol. 5 Surface Engineering.
An overview of 'Plasma and Chromium Oxide Coated Rolls' is given by Praxair: http://praxairsurfacetechnologies.com/pst%5Fprinting%5Fweb/plas%5Fchrom%5Fcoated.html

and a bit of detailed information on 'Plasma Torch & Chromium Oxide Coatings Technology' can be downloaded at
http://praxairsurfacetechnologies.com/pst%5Fprinting%5Fweb/PDFs/HF-06.pdf
[It's not really that detailed; suggests contacting Praxair Surface Technologies technical field sales representatives. Phone nos. given for US, UK, Switz., Germany, Italy, Japan & Japan.]

Thanks JimMetalsCeramics and Kenvlach! Good places to start my reading up on this topic.

MetalMike

Metal Mike
Technology Director - Applied Technology Group

Why use chromium?  Industy is substuting HVOF Tunstine Carbine.  

That's true. You do have other options. If you want a wear coating, tungsten carbide-cobalt, MCrAlY and ceramics are an option.

The HVOF process applies a tungsten carbide, nickel base alloy coating to the shaft versus the current chrome/nickel-chrome plating.  The tungsten carbide nickel base alloy is a stronger material and generally regarded as more wear resistant lending itself to the theory that it will not be as susceptible to grooving and wear and therefore will last longer (requiring less re-coating applications).  

We allow nickel and chrome plated surfaces to be reconditioned with a thin layer of Tungsten Carbide powder M5803 (METCO) spray onto the shaft surface.  This process produces a corrosion resistant wear surface with a surface hardness ranging from 68-72 Rc.  The HVOF process can obtain higher density coatings and finer surface finishes compared to other coating processes such as chromium plating.  Other advantages include, a more simplified application process, no part size restrictions and no hazardous waste products.

info see:
 
 http://www.hcat.org/documents/AESF%20Plating%20Forum%20Mar1998.pdf

 and
 http://www.dresser-rand.com/services/coatings/eec-tc.htm

Note: The print rolls (at least those at issue) are being coated with chromium oxide Cr₂O₃, not Cr metal.  Chromium oxide seems to have become the industry standard coating for anilox printing rolls (and rolls for some other types of printing and some lamination, too) due to a combination of properties:  ability to be laser engraved into a hexagonal cell structure* up to 800 ink cells of controlled depth per linear inch (low porosity, fine grain, no edge burning hence little recast), polishability, corrosion resistance and proper ink holding ability,** as well as hardness of 1200-1300 HV.

* see honeycomb cell structure of controlled ink capacity at http://www.harperimage.com/LaserKote/laserkote2.htm

**Praxair coats rolls after engraving with epoxy sealer, which certainly affects some of these properties.

I once had to get info on coatings for rolls used in the paper fabrication industry. They use chrome oxide for some of the key rolls. This contact:

Scott.Lacourse@sulzer.com

was very useful. In any event, the TSS board is for such questions.