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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.
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