Fluidized bed micro-machining and HFCVD of diamond films onto
Co-cemented tungsten carbide (WC-Co) hard metal slabs
Riccardo Polini 1, Massimiliano Barletta 2,*, Michele Delogu 3
1
Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata,
Via della Ricerca Scientifica, 1 Rome, 00133, ITALY
2
Dipartimento di Ingegneria Meccanica, Università di Roma Tor Vergata,
Via del Politecnico, 1 Rome, 00133, ITALY
3
FILMS S.p.A, Via Megolo, 49, 28877 Anzola d’Ossola (VB) ITALY
The effect of fluidized bed (FB) treatment upon Hot Filament Chemical Vapor Deposition
(HFCVD) of polycrystalline diamond films onto WC-Co hard metal substrates was investigated.
Several scenarios to make the substrates ready for HFCVD were, comparatively, evaluated and
the resulting diamond films were examined in terms of their morphology and adhesion. The
diamond grain density was measured by scanning electron microscopy. The adhesion of
continuous diamond film to substrate was evaluated by the reciprocal of the slope of crack
radius-indentation load functions. Surface binder dissolution followed by FB treatment (PF
pretreatment) allowed very high diamond nucleation density and smaller grain size. The
adhesion of films grown on PF-pretreated substrates was found to be very close to that of films
deposited on hardmetal slabs pretreated by Murakami’s reagent followed by Co etching with
Caro’s acid and seeded with diamond suspension in an ultrasonic vessel (MPS pretreatment).
However, diamond coatings on MPS pretreated samples exhibited a rougher surface morphology
as result of both lower diamond nucleation density and larger substrate surface roughening by
Murakami’s etching. Based upon experimental findings, our newly developed PF-pretreatment
was found to be a very promising technique in substrates conditioning as well as in promoting
adherent, uniform and smooth diamond coatings onto hardmetal tools and wear parts.
Keywords: Fluidized Bed Process, Diamond, Chemical Vapor Deposition (CVD), Adhesion
* : Corresponding author (barletta@mercurio.mec.uniroma2.it)
1
1
Introduction
Tungsten carbide (WC) hardmetals are often employed as wear resistant materials for
manufacturing of cutting and drilling tools [1]. The high hardness of the WC grains combined
with the binder toughness leads to outstanding mechanical properties. Moreover, the chance to
coat hardmetals with diamond films allows the development of coated tools with superior cutting
performances, thereby extending the typical domain of hardmetal tools to include applications as
the machining of metal-matrix composite (MMC) materials, Al-Si alloys, non ferrous metals,
wood, fiber-reinforced plastic and graphite [2]. In this respect, an ever-growing interest towards
techniques able to coat WC-Co hardmetal with diamond films exists in several industrial
segments connected with manufacture of parts subjected to adhesive or abrasive wear.
As known, diamond films improve properties like hardness, gliding properties and wear
resistance of hardmetals. Nonetheless, the built-up of diamond films on WC-Co substrates is
quite tricky because of the harmful effects of Co binder, which leads to the formation of a
graphitic carbon layer at the substrate surface during the early stages of deposition, on which
diamond grows later [3]. Mostly, the problem is overtaken by covering the WC-Co substrate with
interlayers, which act as diffusion barrier [4], etching the WC-Co substrate surface to wash
superficial Co out [5], decarburizing the substrate surface after the etching of binder [6]. In each
case, a seeding process with diamond nucleation promoters and a roughening of the substrate
surface are strongly required to improve the built-up of the diamond film and increase the
interface contact area [7], hence speeding up the formation of a continuous film and enhancing
the adhesion between the incoming diamond film and the hardmetal. However, the strict
regulations about the disposal of exhaust etching solutions, the embrittlement of etched and
roughened surface as well as the need for a seeding process with diamond suspension to promote
2
the growth of thin, fine-grained continuous films [8] are pushing towards the definition of
alternative solutions.
In this context, a new technique to pre-treat WC-Co substrates, customizing its surface, and
make it ready for hot filament chemical vapor deposition (HFCVD) of diamond films has been
thoroughly examined. In particular, a fluidized bed (FB) of diamond powders was used to
process WC-Co hardmetal slabs, thereby causing changes of their surface morphology. Two
different experimental scenarios were examined. In the first one, as ground hardmetal substrates
were straightly submitted to FB process. In the second, as ground hardmetal substrates were
previously submitted to etching with Caro’s acid and, subsequently, was exposed to FB process.
The effect of FB treatment was assessed depositing diamond by HF-CVD onto FB machined
slabs, varying FB processing time. A comparison of FB treatment with the “state-of-the-art”
technique, based on etching of WC-Co substrates with Murakami’s reagent and Caro’s agent
followed by a seeding process with an ultrasonic bath of a diamond suspension, was also
performed [8].
The experimental findings show that FB processing allows to significantly improve the
starting surface of hardmetals, making it suitable for next HFCVD of diamond films on cemented
carbide tools and wear parts. The achieved diamond films exhibit a very high grain density with a
concurrent fine-grained continuous film. Besides, the adhesion of diamond films was found to be
very close to that attained onto WC-Co substrate etched with Murakami’s reagent and diamond
seeded, thereby stating once more the effectiveness of FB process.
3
2
Experimental
2.1 Materials
10x10x4 mm3 WC-5.8wt.%Co as-ground samples (supplied by Fabbrica Italiana Leghe
Metalliche Sinterizzate SpA, Anzola d’Ossola, VB, Italy) were used as substrates. The average
grain size of the hard metal substrates was in the range of 5-6 µm, Rockwell A hardness was 90.0
HRA and transverse rupture strength (TRS, according to ISO 3327) was 2900 MPa.
2.2 Fluidized bed processing
A FB, 40x40 mm2 as cross section and 100 mm as static bed height, of diamond powder (120
as mesh size, 0.67 as factor shape, supplied by Poligem Srl, Italy), working in fast regime (3.5-5
m3/h as flow rate), was used to machine the hardmetal slabs (Figure 1). A thorough description of
the experimental apparatus can be found in previous paper [9]. However, it is worth remarking
that the column was made from stainless steel (AISI 304), 2.5 mm in thickness to provide wall
rigidity. It was equipped with glass porthole so that the fluidization process would be visible
during the process. A sliding vane rotary compressor Mapro, model ‘Free-Oil 20’, for flow rates
from 3 to 20 m3/h, pressures up to 1 bar relative and 1.5 kW as maximum power, was used to
feed purified air flux lacking in oil and moisture into the fluidized bed, under strictly monitored
process conditions. In fact, a standard flowmeter with a 24 V output and an inverter Mitsubishi
model FR-S 540E were respectively used to read the current value of air flux, hence keeping the
flow rate constant during the whole treatment. A set of pressure probes, a hygrometer and a set of
thermocouples was also used to monitor the process and environmental conditions. When enough
air flux was fed from the blower to the fluidization column, the diamond powders became
4
individually suspended in the air flow, while on the whole the bed of powder remained
motionless relative to the column walls [10]. As the velocity of the gas flowing across the bed
was slowly raised, the heterogeneous character of the bed first reached its peak and then
gradually changed. As the transport velocity was approached, a sharp increase of powder carryover occurred [11]. The dust collection and solid recycle system of the bed avoided the
dispersion into the environment of elutriated powder, and, in this way, it was possible to
maintain in the fluidization column a relatively large solid concentration typical of the fast
fluidized bed condition.
The hardmetal slabs, entirely dipped in the fluidized diamond powder, were held on the shaft
of a direct current electric motor and, being rotated at 3600 rpm, they were exposed to the
repeated impacts of incoming diamond powders. A system constituted by a digital revolving
counter provided with a 24 V output and an inverter was used to monitor the rotating speed of the
hardmetal slabs and to keep it constant throughout the fluidized bed processing.
2.3 Chemical Vapor Deposition
Diamond syntheses were performed in a stainless steel hot-filament chemical vapor
deposition (HFCVD) chamber previously described [12]. The gas phase, a mixture of hydrogen
and methane with a CH4/H2 volume ratio fixed to 1.0 %, was activated by a tantalum filament
(0.3 mm in diameter) wound in a 1.4 mm internal diameter spiral and positioned at 8 mm from
the substrate surface. The filament temperature (2170 °C) was monitored by a two-color
pyrometer (Land Infrared model RP 12). The total pressure of the gas mixture in the reactor was
4.8 kPa and flow rate 300 standard cm3 min-1. The substrate was placed on a molybdenum ribbon
shaped into a rectangular crucible. The substrate temperature (650 °C) was monitored by a Pt/Pt-
5
10%Rh thermocouple pressed against this ribbon at the center of the sample. Deposition rate,
under these CVD conditions, was around 0.6-0.7 µm/h.
Table 1 reports the scheduling of the stages involved during hardmetal slabs preparation
before diamond film deposition in order to enhance diamond nucleation, to suppress the
deleterious effects of the binder and to increase the contact area at the interface. Following each
pretreatment, the substrates were washed with acetone and deionized water in an ultrasonic
vessel. The first three hardmetal slabs (PF samples) were in turn submitted to binder etching with
Caro’s acid (3 ml 96wt.% H2SO4 + 88 ml 30 % w/v H2O2) for 10 s and to fluidized bed
processing for 3, 8 or 16 h. A part of samples surface was masked using an epoxy resin to keep
the substrate untouched by FB treatment. After the FB treatment elapsed, the hardmetal slabs
were submitted to HFCVD. The fourth hardmetal slab (FP samples) was exposed to FB treatment
as ground, prior to binder etching. Also in this case, a part of the sample surface was masked to
keep the substrate untouched by FB treatment. Then, an etching with Caro’s acid was performed
to evaluate the difference in morphology between PF and FP samples. The fifth hardmetal slab
was submitted to a well-known ‘state-of-the-art’ two-step chemical pretreatment before being
submitted to HFCVD. The sample was first etched with Murakami’s reagent (10 g K3[Fe(CN)6]
+ 10 g KOH + 100 ml of water) 15 min long. Then, it was subjected to a further etching with
Caro’s acid for 10 s. At last, prior to diamond deposition, the substrate was ultrasonically seeded
for 15 min in a ¼ µm diamond suspension (Struers, DP-Suspension, HQ), followed by ultrasonic
cleaning in ethanol.
6
2.4 Characterization tests
WC-Co substrates were characterized before diamond HFCVD by grazing incidence (1°) XRay Diffraction (XRD) with a Philips X’Pert Pro diffractometer, equipped with a graphite filter
using Cu K radiation ( = 1.5418 Å). Field Emission Scanning Electron Microscopy (FE-SEM,
LEO mod. Supra 35) was employed to study the morphology of substrates after the different
pretreatments. Energy Dispersive X-Ray Spectroscopy (EDS, Oxford Instruments Ltd., model
Inca 300) was used to check the presence of superficial Co after each pretreatment. The surface
roughness of as-ground, FP, PF and MPS substrates was measured by using a Taylor-Hobson
instrument, model Form Talysurf Intra. In particular, 1000 points (1 per µm) along stylus axis
and 1000 roughness profiles (1 per µm) along the normal axis were used to reconstruct the
surface morphology of all the investigated WC-Co substrates. TalyMap software was used for
data processing. The spatial parameter Sa, as included in regulation EUR 15178 EN, was
considered to have an estimation of average surface roughness.
After HFCVD, the achieved films were examined in terms of their morphology using SEM
analysis. Roughness parameters were measured using the same measurement technique employed
for WC-Co substrates. The grains density was measured by using the data image processing
software Leica MW. In particular, the brightfield algorithm with grey processing wizard of
QMW grain sizing wizard was used to detect each diamond grain onto coated substrates. The
adhesion of the diamond films was assessed by Brale indentation tests. The tests were performed
using a Wolpert Rockwell hardness tester with a Brale diamond indenter having a cone of 120 °
and a tip radius of 0.2 m. According to Jindal et al. [13], when the applied load overcomes the
so-called critical load, Pcr, the plastic deformation of the substrate induces the delamination of
the film and cracks propagate between the film and the substrate. In this study, lateral crack
diameter, X, was measured by SEM according to the procedure described in Ref. [2]. The results
7
are plotted as a function of the loads, P, applied at the indenter tip. The reciprocal of the slope
obtained from the crack diameter-indentation load plot, (dX/dP)-1, was used as an estimation of
the adhesion. Continuous films were characterized by Raman spectroscopy. Raman spectra were
recorded with a Spex Triplemate spectrograph, equipped with a liquid nitrogen cooled (512x512
pixels) EG&G Princeton Applied Research CCD optical multichannel analyzer (OMA) detector.
The 514 nm line of an Ar+ laser was used for excitation in a backscattering geometry with 100
mW on the sample (spot size 0.2 mm as diameter).
8
3
Results and discussion
3.1 Substrate pretreatments
Figure 2 shows an as-ground sample after 16 h FB pretreatment. A certain difference in
surface morphology between treated and masked zone exists as stated by spatial roughness
values, which pass from Sa of 0.12 m measured on as-ground surface to 0.10 m of FB treated
surface. In addition, a smoothing of grinding stripes can be also observed.
Figure 3 shows the WC-Co substrate after etching with Caro’s acid and FB treatment 16 h
long. This time, significant changes in surface morphology can be noted with respect to asground substrate (Figure 2). Etching with Caro’s acid removed surface Co, as exhibited in Figure
3a. As a result, the etching produced a slightly more roughened morphology (Sa = 0.14 m) than
as-ground samples. When cobalt etching was performed prior to FB treatment, the surface binder
removal led to the embrittlement of the WC-Co substrate external layer, as confirmed by
experimental evidences reported in literature [14]. FB machining of such embrittled surfaces
contributed to remove WC grains and to establish more corrugated substrate morphology as
shown in Figure 3b. The duration of the FB treatment significantly affected the final surface
roughness of PF samples. In fact, Sa values of 0.17 µm, 0.25 µm and 0.27 µm were measured for
samples submitted to 3, 8 and 16 h FB treatment, respectively. This was because longer FB
treatments had much more time to complete the removal of embrittled external layers of carbide
grains, thereby establishing a rougher surface morphology. Figure 4 shows the EDS spectra of asground WC-Co and of substrates submitted to etching with Caro’s acid and then to FB treatment
9
for 3, 8 and 16 h. The data clearly showed that, for PF samples, binder concentration was below
the detection limit of the EDS technique.
Figure 5 reports the WC-Co substrate after being exposed to MPS pretreatment. As expected,
a significant roughening of the exposed surface is produced by MPS. An average roughness of
0.35 µm was measured.
Figure 6 reports the change in the full width at half maximum (FWHM) of the WC(100)
diffraction peak. As can be seen, the as-ground WC-Co substrates exhibited the widest
broadening of diffraction peak (0.35-0.36°). This is attributable to residual stresses in the
outermost layers of WC grains in accordance with several results reported in the relevant
literature [15,16]. In particular, the broadening of WC diffraction peaks can be explained by
stress induced defects, namely cracking and plastic deformation in WC grains caused by the
grinding process [15]. On the other hand, a thickness of few micrometers is expected as
deformed layer from grinding process [14,15], so the decrease of the FWHM (from 0.35-0.36° to
0.22°) for PF samples can be explained as a result of partial removal of deformed WC surface
layer. It is worth noting that PF sample subjected to a 3 h long FB treatment after etching with
Caro’s acid exhibited a 0.27° FWHM value, being the FB treatment time too short to produce
enough material removal from the hardmetal substrate. A different behavior is exhibited by FP
samples. In such case, FB treatment, acting straight on as-ground surface, was found not so able
to modify the substrate as shown in Figure 2. Consequently, just slight decreasing in FWHM was
detected even after 16 h as FB treatment time (0.27°). On the other hand, the etching with
Murakami’s reagent was found to reduce to a minimum the stress on the WC-Co substrate, as
confirmed by the narrowest FWHM of WC (100) diffraction peak (0.16°). It stands to reason that
being 15 min etching with Murakami’s reagent able to remove massive amount of material from
10
WC-Co substrate in short order, it caused a more complete removal of the deformed surface
layer, hence reducing the measured stress [14].
3.2 HFCVD diamond film
Figure 7 reports the diamond films achieved for different pretreatments as summarized in
Table 1. As can be seen examining the PF samples, the surface morphology of coated samples
becomes progressively rougher (Fig. 7, panels a to c) according to the FB treatment time. Spatial
roughness parameter Sa moved from 0.19 µm for PF sample 3 h FB treated to 0.25 and 0.26 m,
respectively, for PF samples 8 h and 16 h FB treated. An even rougher diamond film was
achieved on MPS sample (Sa = 0.35 µm). However, a total coverage and an even distribution of
the diamond films all over the substrates can be remarked under all process conditions. A
comparison among the diamond films deposited onto PF and onto MPS pretreated WC-Co
substrates shows that FB treatment allowed attaining diamond coatings with finer grain size
(Figure 8). Therefore, being the grains size related to nucleation density according to the model
of ‘evolutionary selection’ [17], it is quite evident that FB treatment caused an enhancement of
diamond nucleation density. Such effect can be ascribed to defects at the WC-Co surface induced
by the grinding process and still present onto PF samples as confirmed by FWHM values
reported in Figure 7, and/or by in situ seeding during FB process. The values of diamond grains
density are collected in Figure 9. Grains densities up to four times larger than those attainable by
seeding with diamond suspension could be achieved onto PF pretreated WC-Co substrates when
16 h long fluidized bed treatment was performed.
Figure 10 displays the Raman spectra of diamond films deposited onto PF and MPS
pretreated WC-Co substrates. In all the spectra, the first-order diamond Raman line was present
11
at 1336 cm-1. The frequency of the peak was blue-shifted of about 4 cm-1 with respect to the
value of natural diamond (1332.4 cm-1 at atmospheric pressure and 25°C). This implies that a
residual compressive stress of 1.9 GPa was present in all deposited diamond films [18]. In
addition to the first-order diamond Raman line, the broad G peak of sp² amorphous carbon
centered at about 1520 cm-1 was also present. The intensity ratio of diamond line and G band was
practically independent of substrate pretreatment. Therefore, it can be inferred that the
performance of the different pretreatments did affect neither the phase purity nor the average
stress state of the deposited diamond films.
The adhesion of diamond films upon FB treated substrates was compared with results
achieved on Murakami’s pre-treated substrates. Rockwell indentation tests confirmed that all
diamond films were well-adhered, with no noteworthy differences in film adhesion arising on all
the substrates investigated and under all indentation loads. Figure 11 shows the adhesion test
results in terms of crack radius vs. indentation load for all the diamond films onto differently
pretreated substrates. The (dX/dP)-1 parameter averaged on PF pretreated WC-Co substrates was
around 0.216 ± 0.03 kg/µm and very close to the value of 0.213 kg/µm achieved on MPS
pretreated WC-Co substrate, and in accordance with results one of the authors measured for films
of similar thickness and grown onto MP-pretreated hardmetal slabs [0.22 ± 0.01 kg/µm, ref. 19].
This aspect points out that PF-pretreatment was as effective as the widely recognized
Murakami’s pretreatment insofar as the adhesion level of diamond coatings on hardmetal
substrates is concerned. These findings confirm several results reported in the relevant literature
[20], i.e. that proper substrate surface roughening represents an essential factor affecting diamond
film adhesion.
Although a clear correlation between diamond nucleation density and adhesive toughness has
not been established yet, data from our laboratory [19] have shown that nucleation densities in
12
the range 0.02-0.3 µm-2 did not significantly affect the adhesion of diamond films deposited
using same CVD conditions onto WC-Co substrates previously submitted to the same
pretreatment. Nevertheless, the larger nucleation densities caused by FB treatment with diamond
powders allowed growing fine-grained smooth diamond films, which should allow better chip
evacuation in dry cutting operations and exhibit lower friction coefficient and better wear
resistance as well.
13
4
Conclusions
The effect of fluidized bed (FB) treatment in samples preparation for next CVD of
polycrystalline diamond films onto WC-Co substrates was investigated. FB treatment after
chemical etching with Caro’s reagent caused i) a perceptible modification of the surface
morphology of the substrate with the 16 h long FB-pretreatment producing the more roughened
WC-Co substrate, ii) to deposit by HFCVD a continuous and well adhered diamond film all over
the substrates with a very high nucleation density without any further nucleation enhancement
pretreatment, and, iii) consequently, to obtain smooth and fine-grained polycrystalline diamond
coatings. If the FB treatment of hardmetal substrates was performed prior to binder etching, i.e.
directly on as-ground WC-Co substrates, a smoother surface morphology was attained, thus
confirming the versatility of fluidized bed micro-machining for finishing of mechanical
components.
Chemical etching of the WC-Co phase performed by Murakami’s reagent, followed by
surface Co dissolution and seeding process by ultrasonic bath in diamond suspension led to a
roughening of the surface but lower nucleation density. Consequently, a coarser grained diamond
coating was achieved.
Rockwell indentation tests showed that the adhesion of diamond films deposited on FB pretreated substrates was as good as the one obtained for Murakami pre-treated substrates.
The large nucleation densities caused by FB treatment with diamond powders allowed
growing smoother diamond films, with promising properties in terms of chip evacuation in dry
cutting operations, friction coefficient and wear resistance.
14
Acknowledgements
The Authors wish to thank Mr. Giuseppe Piciacchia (ISC-CNR, Rome, Italy) for the
performance of the Raman spectra.
15
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16
Table 1. Scheduling of samples’ preparation
Sample 1: PF
Sample 2: PF Sample 3: PF
10 s Caro’s acid
(H2O2+H2SO4)
10 s Caro’s
acid
10 s Caro’s
acid
FB 3 h
FB 8 h
FB 16 h
CVD 8 h
CVD 8 h
CVD 8 h
Sample 4:
FP
Sample 5: MPS
FB 16 h
15 min
Murakami’s reagent
10 s Caro’s
acid
-
10 s Caro’s acid
Seeding + CVD 8 h
17
Figure captions
Fig. 1
Fluidized bed apparatus
Fig. 2
Surface morphology of WC-Co substrate after 16 h long FB treatment in two different
zones: FB treated and masked
Fig. 3
Surface morphology of WC-Co substrate after etching with Caro’s acid and 16 h long
FB treatment: (a) zone masked only during FB treatment; (b) FB treated zone
Fig. 4
EDS after etching with Caro’s acid and 3 h long FB treatment
Fig. 5
Surface morphology of WC-Co substrate after etching with Murakami’s reagent and
Caro’s acid and seeding process with diamond suspension
Fig. 6
Half-Width diffraction peak broadening of WC-Co substrate after the different
pretreatments
Fig. 7
Diamond film of sample 1 after 8h HFCVD (a), sample 2 after 8h HFCVD (b), sample
3 after 8h HFCVD (c), and sample 5 after 8h HFCVD (d) (see Table 1).
Fig. 8
SEM photographs for calculation of nucleation density of the diamond films: sample 1
after 8h HFCVD (a), sample 2 after 8h HFCVD (b), sample 3 after 8h HFCVD (c), and
sample 5 after 8h HFCVD (d) (see Table 1).
Fig. 9
Grain densities of the diamond films: sample 1 after 8h HFCVD (a), sample 2 after 8h
HFCVD (b), sample 3 after 8h HFCVD (c), and sample 5 after 8h HFCVD (d) (see
Table 1).
Fig. 10 Raman spectra of the diamond films: sample 1 after 8h HFCVD (a), sample 2 after 8h
HFCVD (b), sample 3 after 8h HFCVD (c), and sample 5 after 8h HFCVD (d) (see
Table 1).
Fig. 11 Crack radius vs. indentation load function for Rockwell indentation of the diamond
films: sample 1 after 8h HFCVD (a), sample 2 after 8h HFCVD (b), sample 3 after 8h
HFCVD (c), and sample 5 after 8h HFCVD (d) (see Table 1).
18
Figure 1
Figure 2
19
Figure 3
Figure 4
20
Figure 5
Figure 6
21
Figure 7
Figure 8
23
Figure 9
Figure 10
Figure 11
25