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Hardmetal compositions for thermal spray coatings

Cobalt and nickel are the most widely used binder metals. In the majority of

commercially available materials, the amount of metallic binder phase lies in

the range 20-0 vol. !. Carbides with very di"erent grain si#es have been used

for feedstock powder and coating preparation, while the available commercial

compositions remained largely unchanged.

$hile for cutting operations hardmetal compositions were modi%ed by the

introduction of a second hard phase with a cubic crystal lattice, for thermal

spray, large additions of chromium carbides &or chromium' are applied. (y

addition of chromium, the melting point of $C-Co hardmetals is reduced by

more than )00 *.

 +hese additions signi%cantly eceed the amounts of chromium carbide added

for grain growth inhibition in bulk hardmetals.

 +he high chromium content reects di"erences in service conditions compared

to sintered hardmetals.

owadays, the composition $C-)0Co-/Cr is widely used for the preparation of

coatings having simultaneously high wear and corrosion resistance.

ccording to the patent, freedstock powders of the composition $C-&$,Cr' 2C-i

can be preferably prepared from 10! $C, 2/! Cr C2 and ! i. +here are

variations in the contents of the components in the range of )-/! from

di"erent manufactures, but feedstock powders of this composition are currently

traded with designations such as $C-3CrC3-i, $C-3Cr C23-i or $C-iCr.

 +he phase composition depends on the progress of the metallurgical reactions

between $C and CrC2, which occurs in most cases already during feedstockpowder preparation but also during the spray process.

 +he corrosion resistance of $C and Cr C2 based coatings with cobalt or nickel

binder is often not satisfactory. +hus, $C and Cr C2 based compositions with

alternative and comple binder alloys have also been introduced to the market.

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Feedstock materials

4ince in thermal spraying the coating is formed from solid feedstock 5uality isessential to the coating properties. +he dominating feedstock for hardmetal

coating are powders consisting of granules commonly in the si#e range )0-/6

um7 however, the particle si#e range is adapted to the needs of each spray

process. +his so called 3cut3 characteri#es the nominal lower and upper particle

si#es, but is not measure for the particle si#e distribution.

8ollowing the tendencies in bulk hardmetal production, where the advantages

of small carbide grain si#es have been shown, there have been many

developments in order to introduce nanosi#ed carbides into feedstocks andcoatings sprayed there of. 9wever, the inuencing factors on carbide grain si#e,

the dependence on alloying carbides and on the carbon content, have been

studied only for $C-Co and much less systematically than for sintered

hardmetals.