U.S. patent application number 16/088998 was filed with the patent office on 2020-04-30 for oxidation resistant coating and methods of manufacturing thereof.
The applicant listed for this patent is Imperial Innovations Limited. Invention is credited to Samuel A. Humphry-Baker, William E. Lee.
Application Number | 20200131616 16/088998 |
Document ID | / |
Family ID | 56027607 |
Filed Date | 2020-04-30 |
![](/patent/app/20200131616/US20200131616A1-20200430-D00000.png)
![](/patent/app/20200131616/US20200131616A1-20200430-D00001.png)
![](/patent/app/20200131616/US20200131616A1-20200430-D00002.png)
![](/patent/app/20200131616/US20200131616A1-20200430-D00003.png)
![](/patent/app/20200131616/US20200131616A1-20200430-D00004.png)
![](/patent/app/20200131616/US20200131616A1-20200430-D00005.png)
United States Patent
Application |
20200131616 |
Kind Code |
A1 |
Humphry-Baker; Samuel A. ;
et al. |
April 30, 2020 |
OXIDATION RESISTANT COATING AND METHODS OF MANUFACTURING
THEREOF
Abstract
There is described a method of forming an oxidation resistant
coating on a cermet comprising tungsten carbide, tungsten boride,
or boron carbide and a metallic binder material. The method
comprises exposing the cermet to silicon in the presence of an
activator to form a mixture, exposing the mixture to an inert gas,
and heating the mixture to a temperature T for a time t, thereby
forming a coating on the cermet.
Inventors: |
Humphry-Baker; Samuel A.;
(London, GB) ; Lee; William E.; (London,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Imperial Innovations Limited |
London |
|
GB |
|
|
Family ID: |
56027607 |
Appl. No.: |
16/088998 |
Filed: |
March 29, 2017 |
PCT Filed: |
March 29, 2017 |
PCT NO: |
PCT/GB2017/050879 |
371 Date: |
September 27, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 10/06 20130101;
C22C 29/062 20130101; C23C 10/34 20130101; C22C 29/14 20130101;
G21F 1/08 20130101; C23C 10/44 20130101; C23C 10/08 20130101; C22C
29/08 20130101 |
International
Class: |
C23C 10/44 20060101
C23C010/44; C23C 10/08 20060101 C23C010/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2016 |
GB |
1605361.3 |
Claims
1. A method of forming an oxidation resistant coating on a cermet
comprising tungsten carbide, tungsten boride, or boron carbide and
a metallic binder material, the method comprising the steps of: (a)
exposing the cermet to silicon in the presence of an activator to
form a mixture; (b) exposing the mixture to an inert gas; and (c)
heating the mixture to a temperature T for a time t, thereby
forming a coating on the cermet.
2. The method according to claim 1, wherein the cermet comprises 10
wt. % of the metallic binder.
3. The method according to claim 1, wherein the metallic binder
material comprises iron, cobalt, nickel, chromium or mixtures
thereof.
4. The method according to claim 1, wherein the metallic binder is
in the form of a matrix.
5. The method according to claim 1, wherein the activator comprises
a halide salt.
6. The method according to claim 5, wherein the halide salt
comprises sodium fluoride, sodium chloride, ammonium chloride or
potassium tetrafluoroborate.
7. The method according to claim 1, wherein T is in the range from
700 to 1200.degree. C.
8. The method according to claim 1, wherein T is 1000.degree.
C.
9. The method according to claim 1, wherein t is from 0.1 to 10
hours.
10. The method according to claim 1, wherein t is from 1 to 4
hours.
11. The method according to claim 1, wherein the inert gas
comprises argon and 5 wt % hydrogen.
12. The method according to claim 1, wherein the thickness of the
coating formed is from 5 to 500 .mu.m.
13. The method according to claim 1, wherein the thickness of the
coating formed is from 40 to 70 .mu.m.
14. The method according to claim 1, further comprising a cooling
step (d) to cool the coating and the cermet from temperature T.
15. The method according to claim 14, wherein the coating and
cermet are cooled at a rate of from 5 to 10.degree. C. per
minute.
16. The method of claim 1, which method comprises a pack
cementation process.
17. The method of claim 1, wherein the mixture in step (a) is
formed by packing silicon and the activator around the cermet.
18. A tungsten carbide, tungsten boride, or boron carbide cermet
comprising a coating formed in accordance with the method of claim
1.
19. A cermet comprising tungsten carbide, tungsten boride, or boron
carbide, a metallic binder material, and an oxidation resistant
silicide coating, wherein the surface of the coating substantially
consists of silicides of the metallic binder material.
20. A method of forming an oxidation resistant coating on a cermet
comprising tungsten carbide, tungsten boride, or boron carbide and
a metallic binder material, the method comprising the steps of: a)
exposing the cermet to silicon; and b) heating the mixture to a
temperature T for a time t, thereby forming a coating on the
cermet.
21. A method according to claim 20, wherein exposing the cermet to
silicon comprises exposing the cermet to a vapour including a
precursor containing silicon.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to, and the benefit of, PCT
Application No. PCT/GB/2017/050879, filed on Mar. 29, 2017 and
entitled "Oxidation Resistant Coating and Methods of Manufacturing
Thereof," which claims priority to, and the benefit of, United
Kingdom Patent Application No. 1605361.3, filed on Mar. 30, 2016
and entitled "Oxidation Resistant Coating and Methods of
Manufacturing Thereof."
TECHNICAL FIELD
[0002] The present invention relates to a method for forming
oxidation resistant coatings for tungsten carbide, tungsten boride,
and boron carbide cermets (composites).
BACKGROUND
[0003] Nuclear fusion could provide unlimited energy that is
carbon-zero and free from long-lived nuclear waste and threat of
accident. With this in mind, industry is focusing on development of
fusion reactors, for example tokamak reactors, stellarators or
inertially confined fusion reactors. Development of such fusion
reactors promises a route to fusion energy that is cheaper and
faster than conventional technologies. One aspect of industrial
focus is the development of advanced materials in some key
areas.
[0004] A particularly pressing materials challenge for compact
spherical tokamak reactors is in developing more efficient neutron
shielding materials than in conventional reactors (e.g. in the
central column or the divertor of tokamak reactors). It has been
found that tungsten carbide, tungsten boride, and boron carbide
composites could be excellent shields for high-energy neutrons.
[0005] Oxidation resistance of shielding materials is an important
property of reactor materials for accident safety. In particular,
if a reactor were to break-open, its walls would be exposed to a
rapid influx of air, thus inducing surface oxidation and production
of tungsten or boron oxides.
[0006] At very high reactor temperatures, tungsten and boron oxides
become volatile, which could release hazardous transmutation
products into the atmosphere.
[0007] Coatings on tungsten carbide composites (tungsten carbide
cermets, also known as "hardmetal") have been developed in other
technological areas besides fusion reactors, where oxidation
resistance is important, particularly in the machine tool industry.
Generally, oxidation resistance can be improved by two basic means:
(i) blending small additions of oxidation resistant powders, such
as cubic carbides, into the bulk of the component during initial
processing; or (ii) by coating the surface of the material with an
oxidation resistant layer after the component has been made.
[0008] Oxidation resistant tungsten carbide, tungsten boride, or
boron carbide composite coatings for use in fusion reactors should
be capable of suppressing oxidation of the composite at high
temperatures up to 1000.degree. C., which is a reasonable maximum
operating temperature for a fusion reactor.
[0009] Typically, oxidation resistant coatings on tungsten carbide
composites that are suitable for use at high temperatures are
formed by diffusing boron into the surface. By doing so, boron-rich
compounds are formed on the surface on the tungsten carbide
composite. This process is known as boronisation. Boronised
coatings are additionally advantageous because they provide
improved surface hardness. However, the actual reduction in rate of
oxidation of boronised coatings at high temperatures is quite
limited. For this reason, more effective oxidation resistant
coatings are needed.
[0010] It is therefore an object of the present invention to
provide an improved oxidation resistant coating for tungsten
carbide, tungsten boride, and boron carbide composites, capable of
providing improved oxidation resistance at high temperatures, and a
method of manufacturing said coatings.
[0011] The present invention relates to an improved oxidation
resistant silicon-rich coating for tungsten carbide, tungsten
boride, and boron carbide composites, capable of suppressing the
formation and release of toxic oxides when exposed to oxidative
conditions (e.g. in the event of the rupture of a nuclear reactor).
It has been found that the coating according to the present
invention oxidises at a rate which is approximately 3-4 orders of
magnitude slower than non-coated materials and about 2-3 orders of
magnitude slower than boronised materials, at temperatures up to
1200.degree. C. The present invention also relates to a method for
producing the improved oxidation resistant silicon-rich
coating.
SUMMARY OF INVENTION
[0012] The present invention relates to a method of forming an
oxidation resistant coating on a cermet comprising tungsten
carbide, tungsten boride, or boron carbide comprising a metallic
binder material. The method comprises the steps of: (a) exposing
the cermet to silicon in the presence of an activator to form a
mixture; (b) exposing the mixture to an inert gas; and (c) heating
the mixture to a temperature T for time t, thereby forming a
coating on the cermet.
[0013] Exposing the mixture to an inert gas helps to prevent any
unwanted chemical reactions from occurring during the formation of
the oxidation resistant coating.
[0014] The metallic binder material may be in the form of a matrix.
The metallic binder is homogenously dispersed and at high volume
fractions forms a continuous network around the ceramic particles,
while at low volume fractions it is semi-continuous or
discontinuous. The metallic binder material of the cermet may
comprise iron, cobalt, nickel, chromium or mixtures thereof. For
example, the metallic binder material could be alloys comprising
the aforementioned binder materials, such as iron-chromium, or
nickel-cobalt alloys. It has been found that using iron or alloys
thereof (e.g. iron-chromium) as the metallic binder material is
preferable due to the resistance to becoming excessively
radioactive under neutron exposure. In contrast, cobalt and nickel
based metallic binders become strongly radioactive after only a
short neutron exposure. It has been found that during the coating
process the silicon reacts with iron to form iron silicide. The
silicon reacts with the tungsten carbide, tungsten boride, or boron
carbide to form tungsten silicide, tungsten silicide and boron
silicide respectively.
[0015] As discussed above, the metallic binder is not limited to
iron, and preferable metallic binder materials may comprise, for
example, chromium, cobalt or nickel, which react with the silicon
to form chromium silicide, cobalt silicide and nickel silicide
respectively.
[0016] The metallic binder material may alternatively comprise any
other period 4 transition metals, for example scandium, titanium,
vanadium, manganese, copper, zinc, or mixtures thereof.
[0017] The cermet may comprise from 1 to 30 wt. % of the metallic
binder material, preferably 5 to 30 wt. %, more preferably 5 to 15
wt. % and most preferably 10 wt. % of the metallic binder.
[0018] Improving oxidation resistance by siliconising a cermet in
accordance with the method described herein is unexpected. Based on
known coating methods for similar composites (e.g. boronisation of
tungsten carbide composites), it would be expected that
siliconising, for example, a tungsten carbide cermet comprising a
metallic binder material, would provide a coating comprising
tungsten silicide and the reaction product of the metallic binder
material and silicon. It would also be expected that the tungsten
silicide and reaction product of the metallic binder material and
silicon would be present in amounts relative to the amount of
tungsten carbide and metallic binder material respectively within
the tungsten carbide cermet. Tungsten silicide has low oxidation
resistance at the operating temperatures of a fusion reactor
(typically between 400 to 1000.degree. C.), and therefore it would
be an undesirable component on a coating designed for oxidation
resistance.
[0019] Surprisingly, it has been found that even when the cermet
comprises relatively low amounts of the metallic binder material
(e.g. less than 40 wt. % of the metallic binder material), the
coating formed on the surface of the cermet is substantially all
the reaction product of the metallic binder and silicon, despite
there being less than 30 wt. % of the metallic binder within the
cermet at the start. Even more surprisingly, it has been found that
siliconising a tungsten carbide cermet comprising even a 10 wt. %
iron and 90 wt. % tungsten carbide in the cermet, in accordance
with the method discussed herein, results in a coating where the
surface comprises approximately 1 wt. % tungsten silicide and
approximately 99% wt. % iron silicide (i.e. the coating surface
comprises substantially all iron silicide). In other words, it has
been found that there is a preferential segregation of iron
silicide to the surface of the coating. This is likely to be the
result of the silicon diffusing more rapidly through the binder
metal than the ceramic particles.
[0020] A substantially pure iron silicide coating is advantageous
because of the surprising and effective oxidation resistance it
offers, even at temperatures as high as 1200.degree. C. The
preferential segregation (de-mixing) of the iron silicide causing
almost all the surface being occupied by iron silicide and almost
no tungsten silicide presence is advantageous because tungsten
silicide has a relatively low oxidation resistance. It is expected
that preferential segregation (de-mixing) effects are observed when
other metallic binders such as cobalt, nickel or chromium are
used.
[0021] The activator used may be a halide salt. Preferably, the
halide salt is sodium fluoride. The halide salt may alternatively
be any other halide salt, for example sodium chloride, ammonium
chloride, and potassium tetrafluoroborate.
[0022] The activator may be present in an amount within the range
from 5 to 50 wt. % of the substrate. Most preferably, 20 wt. % of
activator is used.
[0023] The mixture may optionally include an inert filler powder,
e.g. aluminium oxide, silicon dioxide, and/or silicon carbide.
Preferably, the inert filler powder is added during step (a).
[0024] The temperature of step (c) may be any temperature in the
range from 700 to 1200.degree. C., preferably 800 to 1100.degree.
C., more preferably 900 to 1000.degree. C. and most preferably
1000.degree. C.
[0025] The time t given in step (c) may be any time within the
range from 0.1 to 10 hours, preferably from 1 to 8 hours, more
preferably 2 to 6 hours, and most preferably 4 hours.
[0026] The inert gas may be an inert or reducing atmosphere. The
inert gas may comprise hydrogen, nitrogen, helium, neon, argon,
krypton, xenon, radon, or mixtures thereof. Preferably, the inert
gas is a mixture of argon and hydrogen comprising an amount of
argon within the range from 90 wt. % to 99 wt. % and an amount of
hydrogen within the range from 1 to 10 wt. %. For example, the
mixture may comprise 99 wt. % argon and 1 wt. % hydrogen, 98 wt. %
argon and 2 wt. % hydrogen, etc. Most preferably, the concentration
of the argon and hydrogen mixture is 95 wt % argon and 5 wt %
hydrogen. It has been found that too much hydrogen (more than 10
wt. %) in the gas mixture can cause embrittlement of the cermet and
coating if iron is used as the metallic binder.
[0027] The coating formed using the method described herein may
have a thickness of from 5 to 500 .mu.m. It has been found that the
lower limit of the thickness of the coating formed is set by the
size of the particles within the cermet. The coating thickness
should be at least several times the thickness of the size of the
particles within the cermet. For example, where particle size is 1
to 2 .mu.m a coating thickness of at least 10 .mu.m should be
obtained. A thicker coating may be needed if the cermet comprises
larger particles, e.g. if the particle size is 10 .mu.m then a
coating should have a minimum thickness of 100 .mu.m. Preferably
the coating thickness is 10 to 100 .mu.m, more preferably 25 to 75
.mu.m, even more preferably 35 to 65 .mu.m. Most preferably, the
coating formed using the method described herein has a thickness of
50 .mu.m. Different thicknesses may be obtained by varying time t
and/or temperature T.
[0028] The method may further comprise a cooling step (d), wherein
the coating and the cermet are cooled from temperature T.
Preferably, the cooling step (d) cools the coating and the cermet
at a rate of 1 to 15.degree. C. per minute, more preferably the
cooling step (d) cools the coating and the cermet at a rate of 5 to
10.degree. C. per minute. Preferably, the cooling step (d) cools
the coating and the cermet to an ambient temperature. It has been
found that cooling the coating and the cermet in this way avoids
breakage/cracking of the coating and/or the cermet during the
cooling step.
[0029] The present invention also relates to a cermet comprising a
coating formed in accordance with the method described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The method according to the present invention may be carried
out in various ways and a preferred embodiment of a method of
forming an oxidation resistant coating on a tungsten carbide,
tungsten boride, or boron carbide cermet comprising a metallic
binder in accordance with the present invention will now be
described by way of example with reference to the accompanying
figures, in which:
[0031] FIG. 1--Is an X-Ray Diffraction (XRD) pattern showing the
phase composition of a tungsten carbide cermet comprising an iron
binder material;
[0032] FIG. 2--Is a Scanning Electron Microscope (SEM) micrograph
showing a typical morphology of a tungsten carbide cermet
comprising an iron binder material;
[0033] FIG. 3--Is an SEM image of a coating produced in accordance
with the present invention;
[0034] FIG. 4--Is an XRD pattern showing composition of a coating
produced in accordance with the present invention on the surface of
the coating and at 30 .mu.m deep in the coating;
[0035] FIG. 5--Is a chart comparing the oxidisation rate of
untreated tungsten carbide cermets with tungsten carbide cermet
with a coating produced in accordance with the present invention at
varying temperatures;
[0036] FIG. 6--Is a chart comparing the oxidisation rate of
coatings produced in accordance with the present invention and
untreated tungsten carbide cermets;
[0037] FIG. 7--A chart comparing oxidation resistance of a coating
produced in accordance with the present invention with an untreated
tungsten carbide cermet and a boronised tungsten carbide
cermet;
[0038] FIG. 8--Is a set of XRD patterns that compare coatings
produced in accordance with the present invention at temperatures
varying between 700 and 1100.degree. C.;
[0039] FIG. 9--Is a set of SEM micrographs showing cross-sections
of a coating produced in accordance with the present invention
after being exposed to various temperatures between 700 and
1150.degree. C.; and
[0040] FIG. 10--Is a flow chart showing steps forming a
coating.
DETAILED DESCRIPTION
[0041] The following discussion relates to a method for producing
an oxidation resistant coating via the siliconising of a tungsten
carbide composite (cermet) of the type used as a shielding material
inside a nuclear reactor, and more specifically, of the type used
inside a compact spherical tokamak reactor. The advantages of the
use of tungsten carbide composites in nuclear reactors, and the
advantages of an oxidation resistant coating on said composites are
discussed above. It should be understood that siliconised coatings
of the type discussed herein are not limited for use inside a
nuclear reactor, and may be useful for any other applications where
oxidation resistant coatings are required, e.g. the machine tool
industry.
[0042] The term "cermet" is used to indicate a structure that
combines a metal with a ceramic, where the ceramic is in the form
of particles, and the metal may form a continuous, semi-continuous
or discontinuous network around the particles, thereby forming a
matrix.
[0043] To fabricate siliconised coatings a "pack cementation"
process is employed and described below. However, it should be
noted that other routine techniques for depositing silicon could be
used. For example, an alternative chemical vapour deposition (CVD)
process such as a fluidised bed reactor (CVD-FBR) may be used, or a
fused-slurry technique may be used.
General Method
[0044] An oxidation resistant silicon coating is produced on the
surface of a tungsten carbide composite comprising a metallic
binder material using the following method: [0045] (i) the tungsten
carbide composite ("the part") is packed into a crucible containing
silicon powder and an "activator" ("the pack"). The activator is
typically a halide salt; [0046] (ii) inert gas is flowed over the
crucible; [0047] (iii) the crucible is then heated to a set-point
temperature and held for a few hours for coating growth to occur.
This temperature is usually 1000.degree. C., but could be
different; and [0048] (iv) the crucible and contents are cooled and
the part removed from the pack. The cooling rate is typically 5 to
10.degree. C. per minute. Cooling the contents too rapidly may lead
to breakage of the part due to an inhomogeneous temperature
distribution.
[0049] Increasing the length of time the part is held in the pack
at the set temperature, increases the thickness of the coating
produced, and vice versa. Increasing the temperature also increases
the thickness of the coating produced, and vice versa.
A. Method of Manufacture
[0050] 1. Coating Fabrication
[0051] Tungsten carbide (WC) composites (hereafter referred to as
"the substrate" or "the cermet") were supplied by Sandvik Hard
Materials Ltd. and had a nominal composition of 90 wt. % WC and 10
wt. % ferritic binder (hereinafter referred to as "WC-Fe").
[0052] A micrograph of a typical WC-Fe composite is shown in FIG.
2. In this particular material, the WC particles, shown in white,
make up 90 wt. % of the structure, which is quite typical for these
materials. The Fe makes up only 10 wt. % and appears black.
[0053] For pack cementation coating of the substrate, the powder
pack consisted of two components: Silicon (Si) and Sodium Fluoride
(NaF) powders (supplied by Alpha Aesar), of 99% and 99.5% purity,
of mesh size 50 and 90, respectively. Powders were weighed in the
weight ratio 80 Si:20 NaF, mixed in a mortar with a pestle and
loaded into a lid-topped alumina crucible and packed around a
pellet of dimensions 7.times.4.times.4 mm. The pack was heated to
1000.degree. C. in a tube furnace in flowing Ar-5% H.sub.2 gas and
held isothermally for 4 hours. The average mass gain of the pellets
was 15.4.+-.0.5 mg/cm.sup.2, and the coating thickness was 65.+-.9
.mu.m, as determined with a mass balance and digital micrometer
with accuracies of .+-.0.1 mg and .+-.2 .mu.m respectively.
[0054] 2. Characterisation
[0055] The substrate material, as well as coated samples, were
characterized by X-ray diffraction (XRD), using a PANalytical
X'Pert powder diffractometer with a Copper (Cu) radiation source
operated at 40 kV and 40 mA. Patterns were collected at a scan rate
of 2.degree./min over a scan range of 20.degree.-90.degree.
2.theta.. The patterns were matched to ICDD Powder Diffraction
Files (PDFs) and analyzed using the Rietveld method to determine
the relative phase fractions and their lattice parameters,
employing a pseudo-Voigt profile function. Scanning electron
microscopy images were collected using a JSM 6010 SEM, operated in
secondary electron imaging mode. To determine the chemical
composition at points in the microstructure an Energy Dispersive
X-ray (EDX) system was used.
[0056] 3. Oxidation Tests
[0057] For oxidation tests, samples were loaded into an alumina
crucible inside a STA 449 F5 Jupiter Thermogravimetric Analyser
(TGA). In each experiment, the sample was heated to the set-point
at a rate of 20.degree. C./min in high purity argon, and held
isothermally. Once the temperature stabilised, synthetic air (80%
N.sub.2; 20% O.sub.2) was flowed over the sample at 100 ml/min for
a set time interval of at least 30 minutes, after which the flow
gas was switched back to Ar and cooled. Details of a similar
procedure are given in a previous study: S. A. Humphry-Baker, W. E.
Lee, Tungsten carbide is more oxidation resistant than tungsten
when processed to full density, Scr. Mater. (2016).
[0058] To calculate the oxidation rate constant, the mass gain
signal was normalised by the instantaneous sample surface area. The
initial area was measured using a micrometer of accuracy.+-.0.002
mm--and for coated samples this was assumed constant, since the
amount of oxide up-take was small. However, for uncoated samples
the area reduction during oxidation was significant and calculated
by assuming that the substrate (of density 14.1 g/cm.sup.3) recedes
isotropically in all directions and that the mass gain upon
formation of the oxide film is about 19.4%. This mass increase
factor was calculated using the following equation:
f-WC+(1-f)-Fe+(2f+1/2)O.sub.2.fwdarw.(2f-1)-WO.sub.3+(1-f)-FeWO.sub.4+f--
CO.sub.2,
where f is the molar fraction of WC, which, is about f=0.72 for our
WC-Fe samples (based on a nominal mass fraction of 0.9).
B. Results
[0059] 1. Microstructure of Coatings
[0060] FIGS. 1 and 2 show the phases and microstructure of the
substrate before coating application. FIG. 1 shows an XRD pattern,
which indicates there are three phases present: hexagonal WC (PDF
101-3982), ferritic .alpha.-Fe (1-1267) and a small quantity of
cubic Fe.sub.3W.sub.3C phase (170-7470). FIG. 2 shows a
representative SEM cross-sectional image, showing mainly WC grains
in light grey and Fe binder in black. A small quantity of
Fe.sub.3W.sub.3C, sometimes known as eta phase, is shown in dark
grey, which is a reaction product from sintering under a
sub-stoichiometric carbon content, due to narrow and carbon-rich
two-phase region in WC-Fe cermets. Stereographic analysis reveals
the WC grain size is 0.8.+-.0.12 .mu.m.
[0061] FIG. 3 shows an SEM cross-sectional image of the siliconised
coating. The lower part of the FIG. 3 shows the substrate, which
contains some small pores of a few .mu.m in diameter, which were
typically observed in the region between 20 and 200 .mu.m from the
substrate-coating interface. Above the substrate is the siliconised
coating, which had a typical thickness of 65 .mu.m.+-.9 .mu.m--as
measured using a micrometer on three nominally identical samples.
The coating structure contains an outer crust, which appears in
dark contrast. The inset of FIG. 3 shows a higher magnification
image of the crust, revealing that the outer crust is made up of
two layers of differing chemical composition.
[0062] FIG. 4 shows the phases present in the coating, via XRD
patterns taken from the top surface of the coating (i.e. the
specimen crust), and a pattern taken after the top 30 .mu.m of
material was removed by mechanical polishing (i.e. bulk of the
coating). The top surface material is made up almost exclusively of
iron silicides, namely: FeSi, FeSi.sub.2, Fe.sub.3Si.sub.7, with a
small amount of WSi.sub.2 (PDFs 192-3808, 20-0532, 35-0822 and
192-4516 respectively). Rietveld analysis revealed that their
relative volume fractions are about 0.1 FeSi, 0.31 FeSi.sub.2, 0.58
Fe.sub.3Si.sub.7, and 0.01 WSi.sub.2. Thus the crust is
predominantly iron-rich, with an iron-to-tungsten ratio about 500
times higher than in the substrate. By contrast, the bulk of the
coating, as shown in the XRD pattern from a depth of 30 .mu.m, is
made up of predominantly WSi.sub.2, with a smaller amount of
FeSi.
[0063] 2. Oxidation Kinetics
[0064] The oxidation mass gain kinetics of the siliconised and
substrate material are compared in FIG. 5, between 800 and
1,000.degree. C. Each thermogravimetry trace starts at the moment
air is injected into the furnace, i.e. once the sample is already
settled at the isothermal set-point. For the substrate, a large and
monotonic mass gain begins at the instant that air is introduced.
The oxidation rate increases with increasing temperature, reaching
a total mass gain of 21, 30 and 42 mg after 20 mins at 800, 900,
and 1,000.degree. C. respectively. By contrast, the siliconised
specimens show mass gains of between about 0.01 and 0.02 mg over
the same temperature interval, and thus the curves appear flat and
superimposed on one another.
[0065] To allow more quantitative comparison, FIG. 6 compares the
oxidation rate constants of the substrate and siliconised samples.
Each data point represents the overall gradient of the mass gain
signal, as determined using a least-squares linear fit, after any
substrate surface area corrections are applied. The substrate shows
rapid increase in oxidation rate over the temperature range
600-1000.degree. C., i.e. from about 0.04 to 90 mg/cm.sup.2-h. By
comparison, the siliconised samples maintain very low oxidation
rates of between 0.02 and 0.08 mg/cm.sup.2-h. The rate constants
were 3 orders of magnitude lower than the substrate at the highest
temperatures. As indicated by the error bars in FIG. 6, the rates
of mass gain were close to the accuracy of the microbalance. The
siliconised samples were stable up to 1150.degree. C., but failed
at 1200.degree. C., which is close to the melting point of
Fe.sub.3Si.sub.7 phase, which is 1209.degree. C.
[0066] FIG. 7 shows the typical oxidation behaviour of the
siliconised material, vs. an un-coated substrate material, and a
typical boronisation treatment coating that is common to industry.
In this particular experiment all samples were heated to
1000.degree. C. In the case of the uncoated substrate, a large mass
gain is shown at the instant air is flown over the sample. By
comparison, boronised treatments show some improvement in oxidation
rate, however, the siliconised coatings produced by the method
described herein show almost complete suppression of oxidation,
where the rate of mass gain is decreased by 3-4 orders of
magnitude.
[0067] The siliconised coatings formed by the method described
herein provide effective oxidation resistance over a range of
temperatures. It has been established that at temperatures between
800.degree. C. and 1150.degree. C. the coatings are highly stable,
and effective oxidation protection is expected at lower
temperatures still. FIG. 6 shows data collected so far on the
oxidation rates of the siliconised materials as compared to the
uncoated WC-Fe substrate. There is a 3-4 order of magnitude
improvement over a range of temperatures.
[0068] 3. Siliconised Oxide Layer Structure
[0069] FIG. 8 shows the phases present in the upper crust of the
siliconised coatings, via XRD patterns of the specimen surface.
Each pattern is taken from an oxidised sample after a 30 minute TGA
test at various temperatures between 700 and 1100.degree. C.--such
as those depicted in FIG. 5. The key feature to note is the
emergence of SiO.sub.2, which was best-matched to the
crystabollite-low phase (PDF 1-76-937), as demarked by the
appearance of the (101) peak at 22 degrees 2.theta.. Further
inspection of FIG. 7 reveals further key transitions occurring with
increasing temperature in the iron silicide crust: firstly, at
about 700-800.degree. C. there is a gradual conversion from
predominantly Fe.sub.3Si.sub.7 in the as-coated condition to
predominantly FeSi.sub.2. Next, above about 900.degree. C., the
Fe.sub.3Si.sub.7 is replaced by FeSi, which is accompanied by a
marked increase in the SiO.sub.2 peak intensity. In none of the XRD
patterns was any evidence for iron oxide found.
[0070] FIG. 9 shows cross-sectional SEM micrographs of the oxidised
samples at 700, 900 and 1150.degree. C., compared to the as-coated
state. The upper-most part of the coating is shown, i.e. only the
transition from the coating bulk to the outer crust--which are
mostly WSi.sub.2+FeSi and FeSi.sub.2+Fe.sub.3Si.sub.7 respectively,
in the case of as-coated samples. In all three oxidised samples, a
thin layer of SiO.sub.2 is visible above the outermost iron
silicide layer. The SiO.sub.2 thickness is uneven, and appears to
penetrate into asperities in the outer iron silicide crust, in some
cases forming encapsulated regions of SiO.sub.2 deep into the
crust. Overall, the amount of SiO.sub.2 in the outer layer,
relative to Fe.sub.xSi, increases with increasing temperature; at
1150.degree. C. the SiO.sub.2 penetrates well over half of the
crust depth. This is in agreement with the increasing intensity of
the (101) SiO.sub.2 peak in FIG. 8, and the increasing rate of mass
gain in FIG. 7.
* * * * *