It by now is apparent that, even if

the TCHP sintering temperature were

to be considerably increased to an amount

sufficient to provide the necessary

amount of liquid-phase for LPS – a thick

WC layer will still be present to protect the

"soluble core" group particles from attack

by cobalt.

The temperature should still

be able to be increased as high as re-

quired to get any additional liquid phase

("lubricant + interstitial filler + capillary

attractor material") needed for full density

with minimal concern about grain growth.

Of course, with nitride core particles (e.g.

TiN, ZrN, HfN, etc.) sufficient N2 over-

pressure would have to be provided to pre-

vent outgassing and associated carbon

depletion and stoichiometry issues.

In fact, on a one-micron core TCHP TiN

particle, with the WC and TiN v% being

equal, the initial WC coating (spherical

model) will be almost 129 nm thick, and will

comprise about 75 wt% of the total particle.

Dissolution of the WC at 1500ºC will

remove only 7.9 nm, or about 6 per cent of

the coating thickness, leaving about 121

nm, or about 94 per cent of the original

coating thickness for core particle protec-

tion, inter-core particle distance uniformi-

ty, and structural toughness.

For metallic systems, pressure reduc-

tions are small and therefore ineffective.

Finally, the "percolation limits" do not

appear to be limitative. If they do become

problematic, unless the core particles com-

prise more than about 80 v% of the total

TCHP volume, higher temperatures and

higher cobalt percentages appear to be safe

solutions, although this will be a difficult

choice at first for veterans of conventional

WC-Co practice.

Final results soon

Liquid Phase Sintering of TCHP was

successfully accomplished at only the sec-

ond attempt in May 2003 at the Center for

Innovative Sintered Products (CISP) at The

Pennsylvania State University. The tests

were conducted on an Al2O3-WC-Co vari-

ant of TCHP with an Al2O3:WC:Co wt%

ratio of about 35.1:57.9:1.5 (WC:Co ratio

≈ 97.5:2.5) from the same Lot C that was

used in the Al2O3-WC-Co variant cutting

and mechanical tests. To learn as much as

possible, designed experiments including

planned process parameter variations,

SEM, chemical evaluation, sintering TGA,

dilatometric evaluation, and mechanical

evaluation were and are being conducted.

Final results will be reported soon.

However, liquid-phase sintering was

attained. There was no open porosity at

all, and the initial Archimedes testing of

the samples indicates that a high density

was achieved.

The average pill shrinkage of 18.9 per

cent plus the SEMs suggest that density

was well in excess of 90 per cent. Cutting

the specimens with a diamond saw was

extremely tedious.

This successful test by no means assures

challenge-free LPS consolidation develop-

ment of TCHP. However, it is a major

milestone in that it virtually assures LPS is

feasible.

In turn, this gives high confidence to the

ease of sintering and to the eventual com-

mercial success of this new family of

TCHP pseudoalloys.

AS READERS will know, I've written about

Rick Toth's "new concept" of tough-coat-

ed hard powders (TCHP) on a number of

occasions and feel that it has very definite

potential. On this occasion, however, the

Editor has asked me to analyse the system

and its future prospects in rather more

depth than hitherto. It should be read in

conjunction with the extract from the lat-

est TCHP technical paper published

alongside.

When evaluating any new material

process, one has to look at a number of

critical factors. These include:

1. Does it offer a new or substantially

new material with novel or enhanced

properties, or offer a novel or enhanced

method of producing an already estab-

lished material?

2 . Is it reliable and reproducible?

3 . What are the competitors?

4 . Are its costs competitive?

5 . What is its unique selling proposi-

tion?

Let's get some basics out of the way.

Though the intended product remains

the same, the production process has

been radically altered. So has the name

of the company - from EnDurAloy to

Allomet.

The powder

product, of course,

consists of fine or

ultrafine particles

of hard, wear-

resistant material,

successively CVD-

coated with tung-

sten carbide and

cobalt. It can be

loosely described as conventional CVD

coating turned "inside out", the hard parti-

cles being of material that would normally

form the coating, and the coating a WC/Co

combination that might normally be the

substrate.

In its previous incarnation, the pro-

duction apparatus consisted of a flu-

idised bed of appropriate powder,

through which passed the CVD gas.

Problems included irregular or absent

coatings on individual particles, and a

tendency to form agglomerates. This

set-up has now been superseded by a

rotary reactor, claimed to be more

efficient and reliable, and equipped

with a "comb" device to prevent

agglomeration.

Ken Brookes takes a hard look at the TCHPfamily and offers some straightforward opinions on its viability…

Let's talk about it

Technical trends

metal-powder.net September 2003 MPR 19

Technical trends

20 MPR September 2003 metal-powder.net

To maintain the initial structure as far

as possible during liquid-phase sintering,

the maximum temperature is kept close to

(though above) the WC/Co pseudo-eutec-

tic temperature, and the sintering time

held to a minimum. A variety of tech-

niques have been evaluated in the full

paper, though conventional cold pressing

and sintering is preferred for economic

reasons. A recent and promising develop-

ment is the use of TCHP powders as a

sprayed coating.

So it passes Test 1. We seem to have a

new class of materials, confirmed by the

issue of a patent to its inventor. And it

certainly offers a range of interesting

properties. According to Rick Toth, it

also has the potential to pass Test 2 on

reliability and reproducibility, though

many more experiments and much practi-

cal experience will be needed to confirm

this.

Test 3 ("What are the competitors?") is

particularly interesting. It is easy to make

a material look better than it is by a care-

ful (or even accidental) choice of com-

petitors and test conditions. Makers of

ceramic cutting inserts have been doing

so for years, but they still haven't

replaced hardmetals, for excellent but sel-

dom-mentioned reasons.

Carried away by hype

So I'm sorry to see Allomet being car-

ried away, to some extent, by commercial

hype. A draft (I stress draft) for its new

website, for example, claims "sintered

composite pseudoalloys that combine

hardness approaching that of diamond

with fracture toughness greater than

tungsten carbide and weight approxi-

mately that of titanium."

The weight (strictly density) depends

on the composition of the hard central

particle, and of course will be low if that

of the particle is low, as with alumina or

TiN. The hardness claim is frankly

ridiculous unless the central particle

is itself diamond, since the latter is

so much harder than anything else

(even alumina and TiN are soft by

comparison).

And the fracture toughness claim does

not refer to a specific composition, grain

size and microstructure of WC/Co. Whilst

some TCHP products may well have bet-

ter fracture toughness than some WC/Co

compositions (say 96WC/4Co, fine

microstructure), they may offer far worse

fracture properties than other composi-

tions and structures (like 70WC/30Co,

coarse structure).

We have to look further and in greater

detail for real competitors, existing or

potential - polycrystalline diamond or

cubic boron nitride (PCD or PCBN) for

example, with perhaps the range of

ceramic and metallic binders pioneered

by such companies as Sumitomo. Then

we could look at a similar TiN powder

but using a conventional binder (say

NiMo) and regular high-energy ball-

milling, or perhaps particles of

WC/TiC/Ta(Nb)C solid solution with

cobalt binder. And, of course, in cutting-

tool applications, conventional PVD- or

CVD-coated crater-resistant hardmetals

with functional gradient (FG) substrate

layers, which were Rick Toth's original

target.

I'm personally doubtful (though I

could be wrong) about the possibilities of

TCHP in mass-production steel machin-

ing, where maximising productivity is the

name of the game.

Tool inserts are nowadays one of the

cheapest items in the metalcutting equa-

tion (a small fraction of the costs of cut-

ting fluids, for example) and it is general-

ly less expensive to exchange them when

worn than to reset or resharpen.

Thus a saving due to the same struc-

ture being present throughout the insert

will be nullified if the potential cutting

speed or depth of cut are less, or if the

machine must be stopped more often to

regrind or reset the insert. That would

take us back to the early days of throw-

away inserts, before coatings were intro-

duced. Unfortunately, if one compares the

cutting capabilities of a layer of pure TiN

against a layer which is mainly TiN but

partly faster-wearing WC/Co, the former

should win every time. Things get more

complicated with multiple coatings and

other factors, but the same general princi-

ple applies.

Wider applications

I therefore applaud Allomet's decision

to widen the originally targeted applica-

tions to take on some that seem currently

to be monopolised by simple WC/Co. In

my opinion, the hardmetals industry is a

little too fascinated by this basic alloy,

rather like using carbon steel for every-

thing and ignoring the possibilities of

stainless and alloy tool steels. I suspect

that, if TCHP catches on, hardmetals

manufacturers may suddenly become

more "innovative", which to some extent

means looking back at what was being

done 40 to 60 years ago.

The next test (4) is the big one - "Are

its costs competitive?" We just don't know.

Claims of 10s to 100s of times existing

service lives are often an accompaniment

to massive unit costs.

The statistics (and the massive upfront

prices) were true in the case of PCD and

PCBN tools, as were the ultimate

economies, but it still took around 10

years for these genuinely superhard mate-

rials to catch on. Diamond coatings are

still at an early stage of commercialisa-

tion and PCBN coatings have yet to

become commercial in any way. Widia or

Kennametal at EMO Milan

may announce the first production FG

coatings in October of this year, with

potentially large savings to both producer

and user.

Important unknowns

But we don't yet know enough about

the economics of Allomet's materials.

Indeed, we don't know whether the com-

pany intends to manufacture finished

products (from hardfacing powders to

wear parts and metalcutting tools) or

to supply intermediates for other

companies to process. We don't know

about raw material costs, processing

costs, scrap values, inventory segregation

and similar matters, though presumably

they all have a place in Mr Toth's mar-

keting, development and investment

plans. If Allomet can deliver finished

products at prices that closely compare

with those of its competitors, whilst

offering many times the current operating

life, it will indeed take the industry by

storm. We'll have to wait and see.

Finally, let's look at our test number

5, the unique selling proposition or USP.

Initially, this was the 'inside-out' nature

of the particles when used in steel-

cutting tools, but I think I'd take the

emphasis off this - always the most

difficult of applications - and concen-

trate instead on the versatility of the

patented process, which of course had its

origins in Ludvig Mond's celebrated

CVD production method for pure nickel.

Let's see what Rick Toth chooses when

his commercial production finally hits

the marketplace.