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PHYSICAL VAPOUR DEPOSITION PROCESS FOR DEPOSITING
EROSION RESISTANT COATINGS ON A SUBSTRATE
FIELD OF THE INVENTION
This invention relates to processes that improve service life and durability
of machine
components; to repair of components and reconstitution of their properties;
and particularly to
gas turbine blades and vanes; and primarily to coatings applied to metal
surfaces of aircraft
engine compressor blades and vanes.
BACKGROUND OF THE INVENTION
Ingested airborne solid particles, such as sand, grit and the like, can cause
severe
erosion damage to machine components, particularly, aircraft engine compressor
blades and
other components which could lead to engine structural deterioration and
failure. W.
Tabakoff, "Surface and Coating Technology", 39-40, (1989) 371 and D. Garg, P.
N. Dyer,
"Wear" 162-164 (1993) 552 disclose that solid particles ingested by turbine
engines hit the
compressor blades at various impact angles and velocities. It is known that a
high coating
hardness and fracture toughness is required to resist particle erosion at over
a wide range of
impact angles, and that an excellent erosion performance is achieved by a good
combination
of high hardness and good fracture toughness.
Nitride coatings have been applied for wear protection of cutting tools and
solid
particle erosion protection of metal parts, such as for above turbine
compressor blades and
vanes. It is known, for example, from U.S. Patent No. 6,797,335, issued
September 28, 2004
to Paderov et al and U.S. Patent No. 4,904,542, issued February 27, 2000 to
Mroczkowski
that an improvement in particle erosion resistance at high impact angles can
be achieved by
the deposition of alternating layers of metallic and ceramic materials. While
most of the
research and patents on nitride coating have been focused on multilayer wear
and solid
particle erosion resistance coating systems, limited efforts have been devoted
to the
manufacture of single layer multi-nitride coatings, although the manufacture
of single layer
coatings is simpler and, accordingly, more cost-effective.
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Modern turbine compressors spin at very high angular velocities of 40,000 rpm
and
higher. At that high rotational speed, the ingested airborne solid particles
impinge at the
airfoil leading edge at very high velocities and transfer significant part of
their momentum to
the airfoil. Under these conditions, the geometry of the leading edge is
distorted, which is
termed the Leading Edge Curl Effect. This effect leads to the deterioration in
engine
performance, increase in fuel consumption and compressor overhaul frequency.
Modern turbine compressors also work at elevated temperatures. For example,
H.A.
Jehn et al. in Thin Solid Films Vol. 153 (1987) pp. 45 report that TiAIN
coatings have a very
good high temperature oxidation resistance due to the formation of protective
A1203 layer on
the coating's surface which suppresses oxygen diffusion. W.-D. Minz, Journal
of Vacuum
Science and Technology, A4 (6) (1986) pp. 2117 reports that the oxidation
temperature of
TiAIN coatings can reach 700-800 C. X.Z Ding et al. in Surface and Coatings
Technology
Vol. 200 (2005) pp. 1372 and O. Banakh et al. in Surface and Coatings
Technology Vol. 163
(2003) pp. 57 report that CrAIN exibits even higher oxidation resistance than
TiA1N.
United States Patent No. 4,904,528 - United Technologies Corporation, issued
February 27, 1990, describes coated gas turbine engine hardware comprising a
titanium alloy
substrate having a coating thereon consisting essentially of titanium nitride
wherein the ratio
of nitrogen to titanium is greater than one. Such coatings have a residual
compressive stress
state which aids in minimizing the fatigue debit which would otherwise result
from the use of
a hard coating on a titanium substrate. Coatings are applied by the use of a
vacuum arc
deposition process.
United States Patent No. 4,904,542 - Midwest Research Technologies, Inc.,
issued
February 27, 1990, describes an erosion and corrosion resistant coating formed
of a plurality
of alternating layers of metallic and ceramic materials. The two materials
selected for the
layers have complementary wear resistant characteristics, such that one is
relatively ductile
and the other is relatively brittle. The concentration of the two materials at
the interface
between adjacent layers is graded to improve the adhesion of the layers and to
provide a more
unified coating. Preferably radio-frequency sputtering is employed to deposit
the coating.
United States Patent No. 6,033,768 - Hauzer Industries BV, issued March 7,
2000,
describes ternary hard material layers to which a small proportion of yttrium
is added to
increase the resistance to wear at elevated temperatures, are manufactured by
means of one of
cathodic arc evaporation, sputtering, combination processes of
sputtering/cathodic arc
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evaporation, sputtering/low voltage electron beam evaporation, or low voltage
evaporation/cathodic arc evaporation. The hard material coatings consist
substantially of: a
hard material layer of a binary, ternary or quaternary TiAI based
multicomponent hard
material layer comprising nitride or carbonitride with an Al-content of 10 to
70 at %, wherein
the layer contains about 0.1 to 4 at % yttrium unevenly distributed over the
entire hard
material layer in a growth direction of the coating.
United States Patent No. 6,309,738 B 1- OSG Corporation, issued October 30,
2001,
describes a hard multilayer coated tool including: (a) a substrate; and (b) a
multilayer coating
covering the substrate, the multilayer coating comprising first and second
coating layers
which are alternately laminated on the substrate, each of the first coating
layers has an
average thickness of 0.10 - 0.50 m and coatings titanium therein, each of the
second coating
layers has an average thickness of 0.10 - 0.50 pm and contains aluminum
therein, the
multilayer coating having an average thickness of 0.50 - 10.0 m.
United States Patent No. 6,797,335 B1 - Paderov, A. N. et al., issued
September 28,
2004, describes a method for depositing wear-resistant coatings on metal
surfaces of machine
components and articles to improve service life of parts and to reshape
geometrical size of
parts during repair, the method comprises (i) providing an ion-plasma
deposition chamber;
(ii) locating as an anode said machine components or articles being treated
inside said ion-
plasma deposition chamber; (iii) locating in said chamber cathodes made from
the Group
IVB-VIB metals and/or alloys thereof; (iv) establishing in said chamber a gas
atmosphere
wherein the gas is selected from the group consisting of inert or non-inert
gases and mixtures
thereof; (v) effecting, whenever necessary, ion cleaning of surfaces of said
machine
components or articles; (vi) effecting selective ion-plasma deposition of at
least three layers
of a coating, wherein: at least one layer (a) consists of said metals,
mixtures thereof or
substitution alloys, said at least one layer having a thickness of 0.02 - 5
microns, a second
layer (b) consists of interstitial solid solutions of nonmetallic atoms of
nitrogen, carbon, and
boron in said Group IVB-VIB metals, said second layer having a thickness of
0.4 - 10
microns, and a third layer (c) consists of chemical compounds of interstitial
chases of said
Group IVB-Group VIB metals with nonmetals in the form of nitrides, carbides,
borides and
mixtures thereof, said third layer having a thickness of 0.1 - 12.5 microns,
wherein said first
second and third layers have thickness ratios of about 1:2:2.5 respectively;
(vii) subjecting
one or more of said layers to treatments by implanting thereinto non-metallic
ions
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simultaneously with the step of effecting ion-plasma deposition, said non-
metallic ions
selected from the group consisting of argon, nitrogen, carbon or boron ions;
and (viii) cooling
and unloading said machine components or articles from said chamber.
United States Patent No. 7,160,635 B2 - Sheffield Hallam University (GB),
issued
January 9, 2007, describes coatings for the protection of substrates operating
at moderately
elevated temperatures, and, more particularly, for the protection of titanium-
alloy aircraft and
stationary gas turbine components as well as engine components for automotive
applications,
articles having such coatings and a method for their production.
United States Patent No. 7,211,138 B2 - Kobe Steel, Ltd., issued May 1, 2007,
describes a hard film formed of a material having composition indicated by a
chemical
formula: (TiaAlbV SidBf) (C1,Ne), in which subscripts a, b, c, d, f and e
indicate atomic ratios
of Ti, Al, V, Si, B and N, respectively, and meet relational expressions: 0.02
<a<0.5,
0.4<b_<0.8, 0.05<c, 0<d<0.5, 0<f<0.1, 0.01<d+f<0.5, 0.5<e_<l and a+b+c+d=1.
The hard
film is harder than and more excellent in wear resistance than TiAIN films and
conventional
(TiAIV) (CN) films.
United States Patent No. 7,217,466 B2 - Joerg Guehring, issued May 15, 2007,
describes a wear-resistant coating on rotary metal-cutting tools such as drill
bits,
countersinks, milling cutters, screw taps, reamers, etc. The coating consists
essentially of
nitrides of Cr, Ti and Al with an unusually high share of Cr atoms, namely 30
to 60% referred
to the totality of metal atoms. In multilayer coatings and even more in
coatings made of
homogeneous mixed phases, this high Cr share results in particularly large
tool life distances
for the tools hardened with these coatings.
United States Patent No. 7,226,659 B2 - Mitsubishi Heavy Industries, Ltd.,
issued
June 5, 2007, describes a high wear resistant hard film having a coating layer
consisting of a
metal nitride, which is formed on the outside surface of an object to be
treated; a substrate
layer consisting of a nitride of Ti or Cr, which is provided between the
coating layer and the
object to be treated; and an intermediate layer containing compositions of the
coating layer in
contact with the intermediate layer and the substrate layer, which is provided
at an interface
between the coating layer and the substrate layer. A tool is provided with the
film. The hard
film has excellent oxidation resistance even at 1000 C. or higher and also has
very high wear
resistance.
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United States Patent No. 7,226,670 - OC Oerlikon Balzers AG, issued June 5,
2007,
describes a work piece or structural component coated with a system of film
layers at least
one of which is composed of AlyCr, _yX , where X = N, C, B, CN, BN, CBN, NO,
CO, BO,
CNO, BNO or CBNO, with the composition within the film being either
essentially constant
or varying over the thickness of the film, continually, or in steps, as well
as a process for
producing it.
However, there is still a need to provide coatings protective to particle
impact at
different velocities and impinging angles and that offer an adequate
protection, particularly,
to the leading and trailing edges of an airfoil and, at the same time,
possessing high
temperature oxidation resistance.
SUMMARY OF THE INVENTION -
It is an object of the present invention to provide a Physical Vapour
Deposition
process that produces metallic coated substrates having improved particle
erosion resistance,
particularly at relatively high impact angles of the erodent to the surface of
the substrate.
It is a further object to provide apparatus for carrying out said process.
It is a further object to provide a coated substrate made by said process.
Accordingly, in one aspect the invention provides an improved process of
coating a
substrate by physical vapour deposition with a coating selected from the group
consisting of
MAIN and MN, wherein M is selected from the group consisting of Ti, Cr and Zr,
said
process comprising
(i) loading said substrate in a vacuum chamber
(ii) locating at least one metallic cathode selected from the grot7p
consisting of M
and MAI within said vacuum chamber;
(iii) providing said vacuum chamber with a nitrogen gas atmosphere; and
(iv) vapourizing said cathode in said nitrogen atmosphere; the improvement
comprising;
(v) setting the total cathode current ratios to a desired level; and
(vi) applying a negative bias voltage selected from -75V to -I OV to said
substrate
during said vapourizing step (iii).
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Preferably, in one aspect the invention provides a process wherein said
negative bias
voltage is selected from -30V to -IOV.
Thus, the present invention provides a process that provides a single layer or
a
plurality of nanolayered particle erosion resistant coatings which
provides a single layer or nanolayered coating that significantly improves the
particle
erosion resistance at relatively high angles;
provides a single layer or nanolayered particle erosion resistant coatings
that resist
coating chipping and flaking from the sharp edges of an airfoil;
provides a particle erosion resistant coating having a relatively high
temperature
oxidation resistance;
produces said coatings by Physical Vapour Deposition (PVD) on heat-treated
metal
parts, without affecting the substrate mechanical properties.
To achieve the above mentioned objectives a single layer of a MAIN or a
nanolayered
coating structure containing MN and MAIN, wherein M is selected from Ti, Cr or
Zr, is
deposited on a metal substrate by PVD techniques, such as Cathodic Arc, DC or
AC
magnetron sputtering, Ion Beam Sputtering and the like.
The invention provides methods for producing the particle erosion coatings
comprising the following steps:
= Ex-situ substrate preparation;
= Loading the substrate in a vacuum chamber containing M and/or MAI (where M
is Ti,
Cr or Zr) cathodes, turntable with turning satellites and inlets for a
nitrogen containing
gas mixture;
= Pumping down the chamber to a required base pressure;
= In-situ surface cleaning by energetic particle bombardment;
= Vaporization of MAI or co-vaporization of M and MAI (where M is Ti, Cr or
Zr) in
nitrogen containing gas atmosphere to form a single or nanolayered coating
structure;
= Control of the coating microstructure by applying a negative Bias Voltage to
the
substrate.
To control the coating microstructure that allows improved erosion resistance
at high
angles and good sharp edge coverage the range of the applied negative bias
voltage varies
from -75V to -IOV, preferably between -30V and -IOV.
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The working nitrogen partial pressure during deposition can be between 8.Oxl0-
3Torr
to 2.5x10-2 Torr, preferably between 1.1x10-2 Torr and 2.0x10-2 Torr.
The coatings applied by the process according to the invention are most
valuable
when the substrate is selected from titanium, iron, nickel, cobalt, manganese,
copper,
aluminum and molybdenum. Examples of commercially available composite
substrates
include stainless steel special alloys, such as, AM350TM and 17-4PHTM, nickel-
chromium
alloys, such as, Inconel 718TM, and titanium alloys, such as, 6A1-4V.
Preferably, the invention provides a process as hereinabove defined wherein
said
coating comprises MAIN or MN having a thickness selected from 10-100 microns.
Further, the invention provides a process as hereinabove defined wherein said
coating
comprises a plurality of alternating nanolayers of MN and MAIN, wherein each
of said
nanolayers has a thickness selected from 3 to 50 nanometers.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, preferred embodiments
will now
be described, by way of example, only, with reference to the accompanying
drawings
wherein
Fig. 1 is a diagrammatic cross-section of a coated substrate made by a process
according to the present invention;
Fig. 2 represents an X-ray diffraction pattern of a coated substrate according
to the
invention;
Figs. 3, 4 and 5 are block diagrams of comparative erosion-testing results
represented
as the mass loss versus the mass of erodent blasted;
Fig. 6 is a diagrammatic horizontal cross-section of a Steered CA PVD
apparatus of
use in the practice of the present invention;
Fig. 7 is a diagrammatic horizontal cross-section of an alternative Steered CA
PVD
apparatus of use in the practice of the invention; and wherein the same
numerals denote like
parts.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. I shows generally as 10, a coated stainless steel (AM350) substrate 12
having a
single layer of TiA1N 14 bonded to substrate 12 through a bonding layer 16
initially formed
for providing better adhesion between layers 12 and 14.
Fig. 2 represents an XRD pattern of a TiA1N layer and shows it to have mainly
a face-
centred cubic TiN solid solution phase with grain sizes of about l Onm. XRD
also shows that
wurtzite A1N is present.
It is known that a high coating hardness and fracture toughness is required to
resist
particle erosion at a wide range of impact angles. An excellent erosion
performance is
achieved by a good combination of high hardness and good fracture toughness.
The latter can
be achieved by a careful selection of the coating composition and control of
the coating
microstructure by choosing optimal deposition parameters. In the practice of
the present
invention, it has been found that the layer coating structures deposited at
low bias voltage
exhibits increased fracture toughness and at the same time retains a high
hardness value. The
unique structure also acts as a crack inhibitor by retaining the crack
propagation and results in
a higher erosion resistance at large impact angles and an increased leading
and trailing edge
protection of a substrate.
Figs. 3, 4 and 5 illustrate comparative erosion-testing results with TiA1N
where the
mass loss is plotted versus the mass of A1203 erodent blasted. The A12O3
powder had an
average particle size of 50 m and a A12O3 particle velocity of 180-200 m/s was
used. The
erodent - air ambient flux was directed towards the testing substrate part at
an impact angle
of 20 or 60 relative to the substrate surface plane (Fig. 5). The weight
lost of the tested
material (TiA1N or AM350) was measured after every 250g and 50g of erodent
blasted at an
erodent flux angle 20 or 60 . A total of 1 Kg for 20 and 150g of erodent for
60 was
blasted during each test. The erosion improvement is defined as the mass loss
of the coating
divided by the mass loss of the uncoated stainless steel substrate AM350
substrate at one and
the same amount of erodent blasted.
Thus, Fig. 3 shows a mass loss of a TiAIN coating at an impact angle of 60
and Fig.
4 at an impact angle of 20 . Fig. 5 shows the erosion improvement over bare
stainless steel
AM350.
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Example 1
Fig. 6 shows generally as 20 a Steered Cathodic Arc coating apparatus having a
vacuum chamber 22 within housing 24, a high vacuum pump 26, a turntable 28
having a
plurality of turning satellites 30 for holding AM350 substrate samples 32, and
a trio of
cathodes 34. Housing 24 has a nitrogen purge source 36. Each of cathodes 34 is
formed of
an alloy of 33 atomic % Ti and 67 atomic % of Al, and arranged in a plane on
one of chamber
24 vertical sides 38.
The general process steps comprise the following steps.
(i) Preparation of the AM350 substrates and subsequent loading on satellites
30;
(ii) Evacuation of atmosphere in chamber 22 to desired base pressure level by
pump 26;
(iii) Energetic particle bombardment of surface of substrate 32;
(iv) Passing N2 gas into chamber 22 to desired pressure;
(v) Vapourizing TiAl from cathodes 34 to form a single layer of TiA1N on
substrate 32, in this embodiment, under control to provide the desired
microstructure of the
applied negative bias voltage, selected from -75V to -IOV, preferably between -
30V and -
I OV. The working nitrogen partial pressure during deposition is selected from
8.0 x 10-3 Torr
to 2.5 x 10-2 Torr, and preferably between 1.0 to 2.0 x 10-2 Torr.
The aforesaid process is used preferably to deposit a single coating layer of
TiAIN,
and had the main process conditions listed in Table 1.
Table 1
Parameter Preferred Value Units
N2 Pressure 1.1 - 2x10"Z Torr
Bias Voltage 15 V
Number of TiAI 33/67 3
at% cathodes
Total Cathode Current 300 Amps
for TiAI cathodes
Temperature 420-450 C
Coating Thickness 18-20 m
Cathodes Sizes 10 lcm
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Example 2
An alternative embodiment of the apparatus, shown generally as 50 in Fig. 7,
has a
vacuum chamber 52 with housing 54, high vacuum pump 26, turntable 28, turning
satellites
30, AM350 substrates 32, and nitrogen inlet 36.
In this embodiment for producing a plurality of alternating nanolayers
constituting the
coating on substrate 32, the cathodes consist of two TiAI (33 atomic % Ti:67
atomic % Al)
alloy cathodes on distinct wall surfaces 56 and 58 of housing 24, and a single
Ti cathode 60
on wall 62, all cathodes were arranged in a horizontal plane.
The general process of operation is analogous to that given under Example 1
and
wherein co-vaporization of Ti and TiAl is selected to control the relative
nanolayer
thicknesses by selecting appropriate table rotation speed, cathode current
ratio and nitrogen
partial pressure.
The main process parameters are given in Table II.
Table II
Parameter Preferred Value Units
N2 Pressure 1.7 - 2.0 x 10"2 Torr
Bias Voltage 25 V
Number of pure Ti cathodes I
Cathode Size 7 Cm
Number of TiA133/67 at% 2
cathodes
Cathode Current for Ti 50 A
Total Cathode Current for TiAI 200 A
Temperature 280-350 C
Coating Thickness 18-20 m
As described hereinabove, the erosion tests were performed using A1203 powder
as
erodent with an average particle size of 50 m. The mass loss and erosion
improvement for
the single and nanolayered coatings deposited in Examples I and 2,
respectively are
presented in Table 3 and compared with the particle erosion results from an
uncoated
stainless steel AM350 substrate. The mass loss, measured in mg, and erosion
improvement
were compared for impact angles 20 and 60 . The improvement was calculated by
dividing
the are AM350 substrate mass loss to the coating's mass loss before the
coating's
breakthrough at 150g and 1000g of erodent blasted for the erosion test at 60
and 20 ,
respectively.
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Tested Part 20 impact angle 60 impact angle
Mass Loss, mg Improvement Mass Loss, mg Improvement
Bare AM350 1134 1 96 1
Example 1 11.0 103.1 4.5 21.3
Example 2 12.0 94.5 4.8 20.0
Although this disclosure has described and illustrated certain preferred
embodiments
of the invention, it is to be understood that the invention is not restricted
to those particular
embodiments. Rather, the invention includes all embodiments, which are
functional or
mechanical equivalence of the specific embodiments and features that have been
described
and illustrated.
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