U.S. patent application number 10/513129 was filed with the patent office on 2005-11-03 for surface treatment of co-cr based alloys using plasma carburization.
Invention is credited to Bell, Thomas, Dong, Hanshan, Li, Chenxi.
Application Number | 20050241736 10/513129 |
Document ID | / |
Family ID | 9935730 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050241736 |
Kind Code |
A1 |
Bell, Thomas ; et
al. |
November 3, 2005 |
Surface treatment of co-cr based alloys using plasma
carburization
Abstract
The present invention relates to a method of modifying a surface
characteristic (eg. wear resistance and/or corrosion resistance) of
a cobalt-chromium based alloy article. The method comprises plasma
treating the article at a temperature in the range of from 300 to
700.degree. C. and at a pressure of from 100 to 1500 Pa for 1 to 50
hours in an atmosphere comprising at least one carbon-containing
gas, whereby to introduce carbon into a surface region of said
article. The present invention also resides in a surface-hardened
cobalt-chromium based article producible by the method of the
invention. The article is characterised by having a surface region
comprising a supersaturated solid solution of carbon in cobalt or a
surface region comprising a supersaturated solid solution of carbon
in cobalt and chromium carbides. Surface hardened articles
producible by the method of the invention include medical implants
and engineering components.
Inventors: |
Bell, Thomas; (Merseyside,
GB) ; Dong, Hanshan; (Birmingham, GB) ; Li,
Chenxi; (Birmingham, GB) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH, LLP
100 E WISCONSIN AVENUE
MILWAUKEE
WI
53202
US
|
Family ID: |
9935730 |
Appl. No.: |
10/513129 |
Filed: |
June 3, 2005 |
PCT Filed: |
April 23, 2003 |
PCT NO: |
PCT/GB03/01702 |
Current U.S.
Class: |
148/565 |
Current CPC
Class: |
A61F 2002/30922
20130101; A61F 2/38 20130101; A61F 2/30767 20130101; C23C 8/36
20130101; A61F 2002/30934 20130101; A61F 2/32 20130101; A61F 2/3094
20130101; A61F 2002/30685 20130101; A61L 27/50 20130101; A61F
2310/00029 20130101; A61L 27/045 20130101; A61L 27/303
20130101 |
Class at
Publication: |
148/565 |
International
Class: |
C22F 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2002 |
GB |
0209797.0 |
Claims
1. A method of modifying a surface characteristic of a
cobalt-chromium based alloy article, comprising plasma treating the
article at a temperature in the range of from 300 to 700.degree. C.
and at a pressure of from 100 to 1500 Pa for 1 to 50 hours in an
atmosphere comprising at least one carbon-containing gas, whereby
to introduce carbon into a surface region of said article.
2. The method of claim 1, wherein the surface characteristic to be
modified is one or more of hardness, wear resistance, corrosion
resistance and fatigue strength.
3. The method of claim 1, wherein the article whose surface
characteristic is to be modified is a medical implant, such as a
joint or knee prosthesis.
4. The method of claim 3, wherein the plasma treating is carried
out at a temperature in the range of from 350 to 550.degree. C.
5. The method of claim 4, wherein the plasma treating is carried
out at a temperature in the range of from 400 to 500.degree. C.
6. The method of claim 1, wherein the article whose surface
characteristic is to be modified is an engineering component, such
as a knife, valve, blade or shaft.
7. The method of claim 6, wherein the plasma treating is carried
out at a temperature in the range of from 450 to 700.degree. C.
8. The method of claim 7, wherein the plasma treating is carried
out at a temperature in the range of from 600 to 650.degree. C.
9. The method of claim 1, wherein said treatment pressure is in the
range of from 400 to 600 Pa.
10. The method of claim 1, wherein the duration of said treatment
is in the range of from 1 to 50 hours.
11. The method of claim 10, wherein the duration of said treatment
is in the range of from 5 to 30 hours.
12. The method of claim 1, wherein the or each carbon-containing
gas is selected from a hydrocarbon, carbon dioxide and carbon
monoxide.
13. The method of claim 1, wherein the plasma treatment is carried
out in the presence of at least one unreactive gas selected from
hydrogen, helium, argon or other noble gas.
14. The method of claim 1, wherein the plasma treatment is carried
out in the presence of at least one reactive gas.
15. The method of claim 14, wherein the reactive gas is a nitrogen
containing gas.
16. The method of claim 15, wherein the plasma treatment is
effected at a temperature in the range of from 300 to 500.degree.
C.
17. The method of claim 14, wherein said reactive gas constitutes
from 0.5 to 10% by volume of the total atmosphere.
18. The method of claim 13, wherein the unreactive gas is hydrogen
or a mixture of hydrogen and argon, and the carbon-containing gas
is methane.
19. The method of claim 1, wherein the or each carbon-containing
gas constitutes from 0.5 to 20% by volume of the total
atmosphere.
20. The method of claim 1, wherein said plasma treatment is
effected in the absence of oxygen.
21. The method of claim 1 which includes an article cleaning step
prior to the plasma treatment to remove oxide scale.
22. The method of claim 21, wherein cleaning is effected by sputter
cleaning.
23. The method of claim 21, wherein said cleaning step is effected
at or below the subsequent plasma treatment temperature in an
atmosphere of one or more of gases selected from hydrogen, helium,
argon or other noble gas.
24. The method of claim 1, wherein the article is cooled after the
plasma treatment.
25. The method of claim 24, wherein the rate of cooling is from
0.1.degree. C./min up to 1000.degree. C./min.
26. The method of claim 24, wherein the cooling is achieved by
relatively slow cooling in the plasma treating atmosphere or by
relatively fast cooling by quenching in a liquid.
27. The method of claim 1, wherein a passivation and/or polishing
step is effected after completion of the plasma treatment.
28. A surface-hardened cobalt-chromium based article producible by
the method of claim 1, said article characterised by having: (i) a
surface region comprising a supersaturated solid solution of carbon
in cobalt or, (ii) a surface region comprising a supersaturated
solid solution of carbon in cobalt and chromium carbides.
29. The article of claim 28, wherein said surface region has a
thickness in the range of from 3 to 50 .mu.m.
30. The method of claim 1, wherein the Co--Cr based alloy includes
one or more other alloying ingredients selected from molybdenum,
nickel, tungsten, titanium and carbon.
31. The article of claim 30, wherein carbon is present in an amount
of from 0.04 to 1.6 wt %.
32. The article of claim 31, wherein the article is a medical
implant and carbon is present in the range of from 0.04 to 0.4 wt
%.
33. The article of claim 31, wherein the article is an engineering
component and carbon is present in the range of from 0.4 to 1.6 wt
%.
34. The article of claim 28 which is a conventional hip or knee
joint prostheses, a metal-on-metal advanced bone conservation
prostheses, a dental implant or other implant device, a knife,
blade for chemical or food processing industrials, a turbine blade,
a valve or pump in the chemical and power industries, a bushing,
steel mill equipment, a die, punch or mould.
Description
[0001] The present invention relates to a process for producing a
superior wear-resistant surface on cobalt-chromium (Co--Cr) based
alloy articles. Particularly but not exclusively, it relates to a
surface hardening process applicable to cobalt-chromium (Co--Cr)
based metal-on-metal orthopaedic implant devices (prostheses),
wherein surface hardness and wear resistant properties of the
prostheses are enhanced without loss of corrosion resistance.
[0002] Cobalt-chromium based alloys have typically been used for
orthopaedic applications because of their strength, resistance to
wear and corrosion and biocompatibility. However, under conditions
of sliding wear or articulation of the Co--Cr alloy against other
bearing surfaces (particularly, ceramics or Co--Cr alloy
counterfaces), the cobalt-chromium alloy produces wear debris from
the counterface surfaces in relative motion. This raises a major
concern over the carcinogenic effect of such Co--Cr wear debris and
the release of such metal ions as Co and Cr. Therefore, the surface
of the Co--Cr alloys must be hardened in order to minimise wear,
thus leading to long-life Co--Cr orthopaedic prostheses. Several
methods for improving wear resistance of Co--Cr based alloys and
such articles made from these alloys have been attempted.
[0003] One approach to enhancing the wear performance of metallic
femoral components (hip and knee prostheses) is to coat their
surfaces with such ceramic coatings as TiN (J. A. Davidson,
Ceramics in substitutive and reconstructive surgery (P. Vincenzini
ed), Elsevier, Amsterdam, 1991 pp 157-166) and more recently
diamond-like carbon (DLC) coating (M. Allen, J. Biomed. Mater.
Res., 58(2001), pp 319-328). Potential drawbacks to this approach
are concerns over spallation of coatings (M. T. Raimondi and R.
Pietrabissa, Biomaterials, 21(2000), pp 907-913) due to the
non-metallurgical bonding and galvanic corrosion (caused by the
large galvanic potential difference and pinholes in coatings) (R.
S. Lillard et. al., Surface Engineering 15 (1999), pp 221-224).
Although a TiN coating has been explored for use as a bearing
surface against UHMWPE since the late 1980's, laboratory testing
and clinical results have been limited. Indeed, it was reported in
1998 that 5,000 hip operations might have to be repeated in the UK,
the problem being attributed to the TiN-coated surface (R. Ellis
et. al., The Times, 19 Feb. 1998, p 1.). Although simple
configuration laboratory tests indicated that the DLC coatings had
exceptional friction and wear characteristics, tests on hip
simulators produced early failures of the coatings (A. H. S. Jones
and D. Teer `Friction and wear testing of DLC type coatings on
total hip replacement prostheses` Seminar on The Friction
Lubrication and Wear of Artificial Joints--Tribology Meets Medical,
Institute of Mechanical Engineers, 30 Nov. 2000, Leeds). Clearly
there is cause for concern in using coatings for metallic femoral
components. The problem is mainly due to the formation of an oxide
scale (Cr.sub.2O.sub.3) on the alloy surface due to the strong
affinity of chromium, which is a major alloying element of Cr--Co
alloys, with oxygen in air. This oxide scale frequently cause a
poor adhesion between a coating and the Co--Cr alloy surface.
Surface treatment of Co--Cr alloys usually has to overcome this
major problem, and consequently, such coating techniques as PVD
coating, electroplating, and electrolysis plating have limitations
for Co--Cr alloys, as compared with coating and plating for most
ferrous alloys.
[0004] Another approach to improving the wear performance of
femoral components is to modify the metallic surface. In this
respect, ion implantation with nitrogen has been employed since the
mid 1980's to improve wear resistance of metallic bearing surfaces
made of Ti-6Al-4V, 316L and Co--Cr--Mo (J. I. Onate, Surface and
Coatings Technology, 142-144 (2001), pp 1056-1062). However, it
should be noted that the effectiveness of ion implantation in
enhancing wear resistance of Co--Cr--Mo alloys for metal-on-metal
articulation is limited by the inherent line-of-sight nature of the
ion beam and the very thin modified layer which is produced. It is
difficult, if not impossible, to produce a homogeneous surface
modified layer on 3-D complexly shaped prostheses using the
line-of-sight ion beams. In addition, the thickness of the modified
surface layer (normally in the range of 0.01 to 0.2 .mu.m) is far
less than the average annual linear wear rate of Co--Cr--Mo
prostheses.
[0005] In addition to ion implantation with nitrogen, known
surface-hardening methods include gas nitriding or plasma
nitriding. A nitriding process is disclosed in detail in U.S. Pat.
No. 5,912,323 issued on May 3, 1994 entitled "METHOD OF SURFACE
HARDENING COBALT-CHROMIUM ALLOYS FOR ORTHOPAEDIC IMPLANT DEVICES",
which disclosure is hereby incorporated by reference. To produce a
measurable hardened layer, a treatment duration as long as 48 hours
needs to be used. The thickness of the effectively hardened layer
is very small as evidenced by the statement that "the peak nitrogen
concentration occurs at a depth between 10 and 100 nm". This is
largely due to the strong affinity between nitrogen and chromium.
Indeed, it was found that after plasma nitriding at 550.degree. C.
for 8 h in a 75% N.sub.2-25% H.sub.2 gas mixture, only a compound
layer of CrN was observed in the nitrided case of Stellite 6B, a
Co-30Cr alloy without the formation of an appreciable diffusion
zone (P. H. Howill, Ion nitriding stellite in T. Spaluins and W. L.
Kovaes (eds): Proceedings of 2.sup.nd International Conference of
Ion Nitriding/Carburising. ASM International 1990, 175-176).
[0006] It is an object of the present invention to provide an
improved method of treatment of Co--Cr based alloy articles, which
can enable the above-mentioned disadvantages to be obviated or
mitigated.
[0007] In general, the present invention provides an improved
surface hardening process which is relatively cost-effective and
which is capable of producing, at a relatively low temperature,
combined improvement in wear and corrosion resistance of Co--Cr
based alloy joint prostheses (such as hip and knee joints) and
producing highly wear-resistant Co--Cr based alloy engineering
components (such as valves and tools) without undue loss in
corrosion resistance.
[0008] According to the present invention, there is provided a
method of modifying a surface characteristic of a cobalt-chromium
based alloy article, comprising plasma treating the article at a
temperature in the range of from 300 to 700.degree. C. and at a
pressure of from 100 to 1500 Pa for 1 to 50 hours in an atmosphere
comprising at least one carbon-containing gas, whereby to introduce
carbon into a surface region of said article.
[0009] The surface characteristic to be modified by the method of
the present invention may be any one or more of hardness, wear
resistance, corrosion resistance and fatigue strength.
[0010] Preferably, said article is a medical implant, such as a
joint or knee prosthesis, in which case said plasma treating is
preferably carried out at a temperature in the range of from 350 to
550.degree. C., and more preferably 400 to 500.degree. C. At these
temperatures, the method generally increases wear resistance and
corrosion resistance.
[0011] Alternatively, said article may be an engineering component,
such as a knife, valve, blades or shaft, in which case said plasma
treating is preferably carried out at a temperature in the range of
from 450 to 700.degree. C., more preferably 600 to 650.degree. C.
At these temperatures, the method generally increase wear
resistance, but not necessarily corrosion resistance.
[0012] Preferably, said treatment pressure is in the range of from
400 to 600 Pa and is more preferably about 500 Pa.
[0013] Preferably, the duration of said treatment is in the range
of from 2 to 50 hours and more preferably 5 to 30 hours.
[0014] Preferably, the or each carbon-containing gas is selected
from a hydrocarbon (eg. methane), carbon dioxide and carbon
monoxide.
[0015] Preferably, the plasma treatment is carried out in the
presence of at least one unreactive gas, for example selected from
hydrogen, helium, argon or other noble gas. As used herein
"unreactive" relates to a gas which does not become incorporated
into the article to any significant extent.
[0016] Preferably, the plasma treatment is carried out in the
presence of at least one reactive gas, such as a nitrogen
containing gas (eg. N.sub.2 or ammonia). As used herein "reactive"
relates to a gas which (or a part of which) does become
incorporated into the article to a certain extent. Where a
nitrogen-containing gas is used, the plasma treating step is
preferably effected at a temperature of from 300 to 500.degree.
C.
[0017] Particularly preferred gas mixtures are hydrogen and
methane, and hydrogen, argon and methane.
[0018] Preferably, the or each carbon-containing gas constitutes
from 0.5 to 20% by volume of the total atmosphere. Preferably, said
reactive gas (when present) constitutes from 0.5 to 10% by volume
of the total atmosphere.
[0019] Preferably, said plasma treatment is effected in the absence
of oxygen.
[0020] The method may include an article cleaning step prior to the
plasma treatment step to remove any oxide scale. Preferably, said
cleaning is effected by sputter cleaning (i.e. bombardment of the
article surface with positive ions). Said cleaning step may be
effected at or below the plasma treatment temperature in an
atmosphere of one or more of gases selected from hydrogen, helium,
argon or other noble gas.
[0021] It will be understood that after plasma treating, the
article will be cooled. The rate of cooling may be anything from
0.1.degree. C./min up to 1000.degree. C./min. Cooling may be
achieved by slow cooling in the plasma treating atmosphere or by
fast cooling by quenching in a liquid. In order to prevent
dimension distortion and oxidation, slow cooling in the plasma
treating atmosphere is preferred.
[0022] Particularly for medical implants, a passivation and/or
polishing step may be desirable after completion of the plasma
treatment.
[0023] The present invention also resides in a surface-hardened
cobalt-chromium based article producible by the method of the
present invention, said article characterised by having:
[0024] (i) a surface region comprising a supersaturated solid
solution of carbon in cobalt or,
[0025] (ii) a surface region comprising a supersaturated solid
solution of carbon in cobalt and chromium carbides.
[0026] Preferably, said surface region has a thickness in the range
of from 3 to 50 .mu.m.
[0027] The nature of the Co--Cr based alloy is not particularly
limited, and for example, any other alloying ingredients such as
molybdenum, nickel, tungsten and titanium may be included in the
alloy composition. Carbon may also be included, preferably in the
range of from 0.04 to 1.6 wt %, more preferably in the range of
0.04 to 0.4 wt % in the case of a medical implant, and 0.4 to 1.6
wt % in the case of an engineering component.
[0028] Among the Co--Cr based alloys which are useful for joint
prostheses are ASTM F75 (ISO5832/4), ASTM F799 (ISO5832/12), ASTM
F90 (ISO5832/5) and ASTM F562 (ISO5832/6) or their equivalents with
different trade names. Articles formed of these alloys which can be
surface treated at a relatively low temperature (300-500.degree.
C.) in accordance with the present invention include conventional
hip and knee joint prostheses, metal-on-metal advanced bone
conservation prostheses, dental implants and other implant devices.
Among the Co--Cr based alloys which are useful for wear and/or
corrosion resistant engineering components are Stellite 6B,
Stellite 6K, MP35N and Ultimet. Wear-resistant engineering
components made of these Co--Cr based alloys which can be surface
treated at a relatively high temperature (600-700.degree. C.) in
accordance with the present invention include knifes and blades for
chemical and food processing industrials, valves and pumps in the
chemical and power industrials, bushings and steel mill equipment.
Among the Co--Cr based alloys which are useful for hardfacing
deposits are the Stellite family (more than 20 alloys) and ERCoCr
alloys (ERCoCr-A, -B, -C or -E). Articles deposited with these
hardfacing alloys which are preferably surface treated at a
relatively high temperature (300-700.degree. C.) in accordance with
the present invention include valves, dies, punches, moulds,
turbine blades and knifes.
[0029] Embodiments of the present invention will now be described
by way of example only, with reference to the accompanying drawings
in which:
[0030] FIG. 1 is a schematic view of a dc plasma unit in which the
treatment described in the preferred embodiments below was
effected,
[0031] FIG. 2 is a micrograph of the cross-sectional microstructure
of a Co--Cr--Mo test piece treated in accordance with the present
invention,
[0032] FIG. 3 shows XRD patterns for untreated test pieces and test
pieces surface hardened in accordance with the present invention,
and
[0033] FIGS. 4-6 are graphs showing the properties of untreated
test pieces and test pieces surface hardened in accordance with the
present invention.
[0034] Typical examples of suitable Co--Cr based alloys which are
susceptible to the process of the present invention are summarised
in Table 1. The Co--Cr based alloys of which the article is formed
may be in the wrought, cast, hardfacing deposit or PM/HIP form
before the article is subjected to the process to the present
invention.
1TABLE 1 Examples of useful Co--Cr based alloys Alloy Cr Mo Ni W C
Ti Co Wear-corrosion resistant biomedical alloys F75 27-30 5-7
<1 -- <0.35 -- bal (ISO5832/4) F799 26-30 5-7 <1 --
<0.35 -- bal (ISO5832/12) F90 19-21 -- 9-11 14-16 <0.15 --
bal (ISO5832/5) F562 19-21 9-11 33-37 -- <0.15 <1 bal
(ISO5832/6) Wear-resistant alloys Alloy 6B 28-32 <1.5 <2.5
3-5 0.8-1.2 -- bal Alloy 6K 28-32 <1.5 <2.5 3.5-5.5 1.5-1.7
-- bal Corrosion-resistant alloys MP35N 19-21 9-11 34-36 -- -- --
bal Ultimet 25-27 4-6 8-10 1-3 <0.06 -- bal Hardfacing alloys
ERCoCr-A .about.28 -- -- .about.5 .about.1.2 -- bal ERCoCr-B
.about.29 -- -- .about.8 .about.1.5 -- bal ERCoCr-C .about.31 -- --
.about.13 .about.2.5 -- bal ERCoCr-E .about.27 .about.6 -- .about.8
.about.0.2 -- bal
[0035] The surface-treatment-process can be applied as a final
procedure without causing deterioration of the properties of the
substrate or dimensional distortion of the article. Articles for
which the process of the present invention is suitable include
articles as ferrules, valves, gears and shafts. There is no
particular limit in the size of articles that can be treated using
the process of the present invention.
[0036] In order to demonstrate the advantages of the present
invention, a series of Co--Cr based alloys (Table 2) were treated
in accordance with the present invention.
[0037] In the Examples, surface treatment was carried out using a
dc plasma nitriding apparatus shown in FIG. 1. The apparatus
comprises a sealable vessel 10, a vacuum system 12 with a rotary
pump (not shown), a dc power supply and control unit 14, a gas
supply system 16, a temperature measurement and control system 18
including a thermocouple 24, and a work table 20 for supporting
articles 22 to be treated.
2TABLE 2 Alloy composition of the Examples Sample designation*
Condition Composition (% by wt) A Wrought Co-27.6Cr-5.5Mo-0.06C B
Wrought Co-37.4Cr-6.1Mo-0.19C C Cast (MMT) Co-29.2Cr-6.1Mo-0.21C D
Cast (DEP) Co-27.2Cr-0.17C E Cast (DRILL) Co-29.6Cr-5.9Mo-0.05C F
PM/HIP Co-28.2Cr-5.8Mo-0.04C *For each sample designation, a
different sample was treated at 400, 500 and 600.degree. C.
(designated A400, A500, A600 etc.)
[0038] The articles 22 to be treated were Co--Cr based alloy discs
25 mm in diameter and 8 mm in thickness. The discs were placed on
the table 20 inside the vessel 10. The table 20 was connected as a
cathode to the power supply and control unit 14, and the wall of
the vessel 10 was connected to the dc source as the anode. The
temperature of the discs 22 was measured by the thermocouple 24
inserted into a hole of 3 mm diameter drilled in one of the discs
22 or a dummy sample. After the sealable vessel 10 was tightly
closed, the rotary pump was used to remove the residual air
(oxygen) and thus reduce the pressure in the vessel. When the
reduction in pressure reached 10 Pa (0.1 mbar) or less, a glow
discharge was introduced between the article 22 (cathode) and the
vessel wall (anode) by applying a voltage of 400 volts to 900 volts
between these two electrodes. A heating gas of hydrogen was at the
same time introduced into the vessel 10. The pressure of the
hydrogen gas in the vessel 10 was increased gradually as the
temperature of the articles 22 increased. No external or auxiliary
heating was employed, and the articles 22 were heated by the glow
discharge only.
[0039] In other embodiments (not shown), an external heater
attached to the vessel may be employed, or a combination of
external heating and electrical glow discharge heating may be
employed. Direct current (dc) discharge, pulsed dc discharge or
alternating current (ac) discharge may be used.
[0040] After the articles 22 were heated to the prescribed
temperature, a gas mixture of hydrogen (98.5%) and methane (1.5%)
was introduced into the vessel 10 and the plasma treatment started.
Treatment temperatures from 400.degree. C. to 600.degree. C. were
employed for a treatment time of 10 hours. The working pressure in
the treatment step was 500 Pa (5.0 mbar) for all the Examples.
[0041] During the plasma heat treatment, the methane is ionised,
activated and dissociated to produce carbon ions and activated
carbon atoms and neutral molecules, which then diffuse into the
surface of the disc forming a carbon diffusion layer. When the
plasma treatment is carried out at a relatively low temperature
ranging from 300 to 550.degree. C., the carbon atoms mainly reside
in the cobalt lattices, forming a supersaturated solid solution
with a possible nanocrystalline structure due to the relatively low
temperatures employed in the treatment. The resultant layer has a
high hardness, good fatigue strength and excellent wear and
corrosion resistance (see below). When the plasma treatment is
carried out at a relatively high temperature ranging from 600 to
700.degree. C., the carbon atoms partially reside in the cobalt
lattices forming a supersaturated solid solution and partially
combined with carbon forming chromium carbides. The resultant layer
has a high hardness, fatigue strength and excellent wear
resistance.
[0042] After the completion of the plasma treatment, the glow
discharge was turned off and the articles 22 were allowed to cool
in the vessel 10 in the treatment atmosphere down to room
temperature before they were removed from the vessel.
[0043] The articles 22 were then subjected to X-ray diffraction
analysis for phase identification, glow discharge spectrometry
(GDS) analysis for chemical composition determination, surface
hardness measurements, metallography analysis of the cross section,
electrochemical corrosion tests and wear tests.
[0044] A typical micrograph showing the cross-section of the
carburised sample is given in FIG. 2. The sample was
electrolytically etched in a 10% H.sub.2SO.sub.4 water solution. It
can be seen that the carburised specimen is characterised by an
"un-etched layer" on the surface followed by a carbon diffusion
layer ("case") beneath and the heavily etched substrate ("core").
The un-etched layer is dense, no details can be revealed even under
high resolution FEG-SEM. The thickness of the total carburised case
depth increases with increasing carburising temperature. In
conjunction with the chemical composition analysis, the case depth
was determined to be 3.1, 9.3 and 20.2 .mu.m respectively for the
A-sample treated at 400.degree. C., 500.degree. C. and 600.degree.
C. for 10 hours.
[0045] FIG. 3 shows the XRD patterns for the untreated and
carburised Co-27.6Cr-5.5Mo-0.06C alloy (Sample A). As can be seen,
the untreated alloy consists of a mixture of f.c.c. structured
.gamma.-Co and h.c.p. structured .epsilon.-Co. It can be deduced
from the height of the XRD peaks that the untreated alloy contains
mainly .gamma. with a small amount of .epsilon.. Plasma carburising
has changed the phase constituent in the alloy surface. As shown in
FIG. 3, only two main diffraction peaks at lower angles and some
minor peaks at higher angles are detected. These peaks could not be
matched to either .gamma.-Co, .epsilon.-Co or any other phases
given in the existing Powder Diffraction Database. However, they
exhibit the same characteristics as those generated by the S-phase
in plasma nitrided or carburised stainless steel (Y. Sun X. Li and
T. Bell Surface Engineering, 1999, Vol 15, No. 1, pp 49-54). It
thus follows that S-phase was indeed produced in the plasma
carburised Co--Cr--Mo alloy surface. The two main S-phase peaks,
indicated as S(111) and S(200), correspond to the .gamma.(111) and
.gamma.(200) of the substrate but are at lower angles. It is
thought that the peak shift is caused by the solution of carbon
which expands the f.c.c lattice structure of the substrate.
[0046] The surface mechanical properties of the plasma carburised
samples were assessed using a Mitutoyo MVK-H tester under various
loads, ranging from 0.025 kg to 1 kg. FIG. 4 shows micro hardness
measured on the sample A surface with various load. It can be seen
that the hardness on the untreated surface (A000) is fairly stable
under a testing load of above 0.05 kg, with an average value of HV
486. After carburising, the surface hardness of the Co--Cr--Mo
alloy was increased under all testing loads. Under indentation
loads below 0.1 kg, surface hardness values of more than 1100 HV
were obtained for the 500.degree. C. and 600.degree. C. treated
samples (A500 and A600 respectively). The surface hardness values
for the 400.degree. C. treated sample (A400) were not as high as
those for the 500.degree. C. and 600.degree. C. treated samples,
but they are still much higher than those for the untreated alloy.
As the testing load increased, all the measured hardness values
decreased, showing a hardness gradient with testing load and this
with the indentation depth. Such a diffusion type of hardness
distribution is essential in ensuring optimum performance of
surface treated system, since a sudden structural, compositional,
and property change at the layer/core interface may lead to
catastrophic interfacial failure of the layer during service. It
was also found that the edges of the Vickers indentation
impressions on the carburised samples were sharp and clear. No
cracks were observed around or inside the indentation impression,
and there was no evidence of interfacial failure between the
carburised layer and the substrate even at a higher load of 1 kg.
These observations suggest that the carburised layer possesses good
ductility, high toughness and high load bearing capacity.
[0047] Plasma carburised and untreated test pieces were subjected
to pin (WC ball)-on-disc wear tests sliding against 8 mm WC balls
at a speed of 0.03 m/s with a initial maximum Hertzian contact
stress of 1500 MP. Wear rate, in terms of volume loss per meter
sliding distance per Newton load (mm.sup.3m.sup.-1N.sup.-1), was
calculated and is shown in FIG. 5. As can be seen from FIG. 5, the
wear rate of all the carburised test pieces was dramatically
reduced by more than one order of magnitude as compared with the
untreated test pieces.
[0048] Electrochemical potentiondynamic sweep corrosion tests were
conducted at room temperature in a flat cell with Ringer's solution
which contained 9 g/l sodium chloride, 0.42 g/l potassium chloride,
0.48 g/l calcium chloride and 0.2 g/l sodium bicarbonate in
distilled water. The polarisation curves are shown in FIG. 6. As
compared with the untreated test piece, the corrosion potentials of
all plasma carburised test pieces were moved to more passive
values, indicating improved corrosion resistance. The current
densities of both the untreated and the A400 and A500 test pieces
are practically the same. The A600 test piece showed a higher
current density scan potential was over -0.2V.
[0049] The applicability of the present invention is demonstrated
in Table 3. Treatment was carried out at 500.degree. C. for 10 hrs
in a gas mixture of methane and hydrogen. Microhardness was
measured on the treated sample with a Vicker's indenter at a load
of 0.1 kgf, indicated as HV0.1. As can be seen, all five types of
Co--Cr based alloys in wrought, cast or PM/HIP form can be
effectively surface hardened by applying the method of the present
invention.
3TABLE 3 Surface hardness of samples A to F. Sample Number
Condition Surface Hardness HV0.1 Wrought 1200 B Wrought 1150 C Cast
MMT) 1200 D Cast (DEP) 1000 E Cast (DRILL) 1240 F PM/HIP 960
[0050] In a modification of the above dc plasma method not shown),
an advanced active screen plasma technology can be used to treat
articles made of Co--Cr based alloys with improved surface quality.
The articles to be plasma treated are placed inside a metal screen
which is connected to the cathodic potential. The worktable and the
articles to be treated are placed in a floating potential or
subject to a relatively lower bias voltage (e.g. -100.about.-200V).
As an example, a casting material (material D in Table 3) has been
plasma treated in a active screen plasma unit and the surface
hardness increased from <400HV0.1 (untreated material) to
.about.1050HV0.1 (active screen plasma treated).
* * * * *