Method of depositing a diamond layer on a titanium substrate

A method of manufacturing a metal part coated with a layer of polycrystalline diamond includes fabricating a metal substrate having a surface to be coated, the metal substrate containing titanium, vanadium or alloys thereof; depositing a layer of at least 10% graphitic plus amorphous carbon and designed to diffuse fully through the titanium; and depositing a diamond coating on the carbon layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of manufacturing a metal part 
coated with a layer of polycrystalline diamond as well as a metal part 
obtained by such a manufacturing process. 
The invention relates in particular to a method for manufacturing a part 
made of titanium or titanium alloy covered with a polycrystalline diamond 
layer as well as a titanium part obtained by this manufacturing process. 
Titanium and titanium alloy parts designate metal parts whose principal 
component is titanium, which may or may not be alloyed with other elements 
in a lower concentration. 
An alloy very commonly used for manufacturing such parts is the alloy known 
as "TA6V" alloy. 
It is known that titanium alloys, particularly the TA6V alloy, have good 
mechanical characteristics, particularly good tensile strength, good 
resistance to fatigue, corrosion, and creep, and a relatively low density. 
These alloys are hence widely used in the aeronautical and space 
industries. 
It is also known that titanium has very good biological immunity and is 
hence widely used in the biomedical field. 
However, titanium alloys have poor wear resistance, particularly when they 
are in contact with a metal or a metal alloy and a high friction 
coefficient with a large number of materials. 
In addition, titanium alloys can have a certain toxicity due to the 
presence of other alloy elements. 
To remedy these drawbacks, a carbon layer for example is deposited on the 
titanium alloys, for example in the form of a polycrystalline diamond 
layer, which, depending on the utilization of the alloy, allows wear 
resistance to be increased and biological immunity to be improved. 
At the present time, the polycrystalline diamond layer is deposited on the 
metal part at a high temperature, in the range from 800 to 850.degree. C. 
which, upon return to room temperature, triggers the appearance of very 
substantial residual stresses, on the order of 7 GPa, in the diamond 
layer. Present-day techniques of coating a part with a diamond layer hence 
do not allow diamond layers of greater thickness than 1 micron to be 
deposited without their flaking off upon cooling. 
Moreover, diamond deposition at high temperatures brings about relatively 
high diffusion of carbon into the underlying metal, which causes deep 
changes in the intrinsic mechanical properties of the underlying metal. 
The attempt has been made to remedy these drawbacks by depositing a 
polycrystalline diamond layer on a metal substrate at a lower temperature, 
for example less than approximately 700.degree. C. Deposition at such a 
temperature allows a thicker layer to be obtained, but adhesion of the 
diamond to the metal is relatively poor. 
SUMMARY OF THE INVENTION 
A goal of the present invention is to remedy the stated drawbacks by 
proposing a method for manufacturing a metal part coated with a 
polycrystalline diamond layer of relatively substantial thickness wherein 
the residual stresses at room temperature are less than approximately 6 
GPa and bring about no significant changes in the mechanical properties of 
the metal part. 
The method of manufacturing a metal part coated with a polycrystalline 
diamond layer comprising the steps of: 
making a metal substrate having a surface to be coated that contains 
titanium or vanadium; 
in a first step, depositing a layer of carbon that clings to the diamond, 
having at least 10% graphitic or amorphous carbon and designed to diffuse 
fully through the titanium during a second step; and 
in the second step, depositing the diamond coating over the cling layer. 
The method according to the invention can in addition comprise one or more 
of the following characteristics: 
During the metal substrate manufacturing step, a coating layer including 
titanium is deposited on a metal mass; 
Prior to deposition of the coating layer, a refractory layer designed to 
form a barrier to diffusion of carbon through the metal substrate is 
deposited on the metal mass; 
The metal mass is comprised of titanium or titanium alloy; 
The metal mass is comprised of steel or cemented carbide; 
The metal mass is comprised of cemented carbide and coated with a layer of 
titanium carbide prior to deposition of the refractory layer to improve 
its adhesion to the metal mass; 
The refractory layer is comprised of a material chosen from the group 
comprised of titanium nitride, vanadium nitride, niobium, tantalum, 
tungsten, and molybdenum, or an alloy including an element chosen from 
this group; 
The refractory layer is comprised of titanium nitride, coated with a second 
cling layer including nitrogen and titanium with a decreasing nitrogen 
concentration in the direction of the coating layer to improve adhesion of 
the latter on the refractory layer; 
The stages of depositing the cling layer and the diamond coating are 
carried out at a temperature between 400.degree. C. and 700.degree.C.; 
The metal mass being comprised of cemented carbide, the stages of 
depositing the cling layer and the diamond coating are carried out at 
temperatures between 400.degree. C. and 700.degree. C. and between 
700.degree. C. and 850.degree. C., respectively. 
The invention also relates to a metal part made of steel or including 
titanium and comprising a metal substrate having a surface to be coated 
including titanium, covered with polycrystalline diamond, characterized by 
including a carbide layer interposed between the diamond coating and the 
surface to be coated with titanium, coming from diffusion of carbon in the 
surface to be coated, the diamond coating having a suitable thickness, for 
example, between about 1 and 10 microns. 
The metal part according to the invention can in addition have one or more 
of the following characteristics: 
The metal substrate also has a refractory layer disposed under the surface 
to be coated and designed to form a barrier to diffusion of carbon in the 
metal substrate; 
The metal substrate is comprised of titanium or titanium alloy; 
The metal substrate is comprised of a steel metal mass covered with a layer 
including titanium or vanadium; 
The refractory layer is composed of a material chosen from the group 
constituted by titanium nitride, vanadium nitride, niobium, tantalum, 
tungsten, and molybdenum, or an alloy including a material chosen from 
this group; 
When the refractory layer is composed of titanium nitride, the metal 
substrate also has a second cling layer including nitrogen and titanium 
interposed between the surface to be coated and the refractory layer and 
having a decreasing nitrogen concentration in the direction of the surface 
to be coated with a view to improving adhesion thereof to the refractory 
layer. 
Other characteristics and advantages will emerge from the description 
hereinbelow provided as an example having regard to the attached drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows a plasma-assisted chemical deposition device known as a PAVD 
device and designated by reference numeral 10. 
PAVD device 10 is designed to deposit a diamond layer on a metal part 
designated by reference numeral 12, disposed on a support 13. 
"Metal part" is understood to be a part made of metal, a metal alloy, or a 
compound. 
PAVD device 10 has a silica enclosure 14 provided with an opening 16 for 
supplying gas to a deposition area 17 in which a plasma in produced by 
means of a waveguide 18 connected with a microwave wave generator 20. 
The gases whose path is illustrated by arrows emerge through an opening 21. 
In addition, silica enclosure 14 is connected in known fashion to a vacuum 
pump not shown. 
In addition, PAVD device 10 has adaptation means allowing the power of the 
microwave waves reflected on part 12 to be limited and the position of the 
plasma to be adjusted in front of the part to be coated, consisting of a 
piston 22. 
PAVD device 10 is supplemented by adaptation means, a system for adjusting 
the gas flowrate and controlling and regulating pressure, of a known type, 
which means and system are not shown. 
Metal part 12 is composed of a titanium alloy of the TA6V type. 
PAVD device 10 allows deposition on metal part 12, possibly prepared by ion 
treatment, of a layer of polycrystalline diamond at temperatures between 
400.degree. C. and 700.degree. C. To accomplish this, a cling carbon layer 
for the diamond having at least 10% graphitic or amorphous carbon and 
designed to diffuse fully through the titanium is deposited on the surface 
to be coated of the titanium alloy part, and the cling layer is coated 
with diamond in the course of total diffusion of the cling carbon through 
the titanium. 
Once the diamond has been deposited on the metal part, a fine layer of 
titanium carbide is formed under the diamond layer, whose thickness 
depends on the deposition, duration, and temperature conditions. 
FIG. 2 shows that the metal part thus obtained has a layer of 
polycrystalline diamond 24 adhering to the TA6V alloy 26 by means of a 
titanium carbide layer 28. 
As an alternative, as shown in FIG. 3, a metal substrate composed of a mass 
of TA6V alloy 30 coated with a layer of refractory material 32 on which a 
titanium coating 34 is deposited, is used. 
As before, a polycrystalline diamond layer 36 is then deposited on titanium 
coating layer 34, previously coated with a cling carbon layer intended to 
form a titanium carbide layer 38. 
The refractory layer is preferably composed of a material chosen from the 
group comprised of titanium nitride, vanadium nitride, niobium, tantalum, 
tungsten, and molybdenum, or an alloy having an element chosen from this 
group. 
It is designed to form a barrier to diffusion of carbon to prevent the 
latter penetrating the TA6V alloy 30 so that the mechanical 
characteristics within the alloy become modified while the diamond is 
being deposited. 
According to another alternative shown in FIG. 4, in the case where 
refractory layer 40 is composed of titanium nitride, this layer can be 
deposited on TA6V alloy 42 with interposition of a titanium layer 44 
designed to improve adherence of refractory layer 40 to alloy 42. 
A second cling layer containing nitrogen and titanium, 46, designed to 
cling to titanium layer 48 on refractory layer 40 made of titanium 
nitride, can also be interposed between this refractory layer 40 and 
titanium layer 48. The second cling layer 46 has a decreasing nitrogen 
concentration in refractory layer 40 in the direction of layer 48 to be 
coated to improve adhesion of the latter layer to refractory layer 40. 
As in the previous example, titanium layer 48 is then coated with a diamond 
layer 50 with prior interposition of cling carbon to form a titanium 
carbide layer 52. 
In the various alternatives of the polycrystalline diamond layer deposition 
method described, the metal part is comprised of titanium alloy of the 
TA6V type. 
However, it is possible by means of this method to deposit a 
polycrystalline diamond layer on a metal part comprised of a titanium-free 
metal mass covered with a coating layer including titanium or a titanium 
alloy, vanadium, or a vanadium alloy. 
Thus, the metal mass can be comprised of steel or cemented carbide. 
In the latter case, where the metal mass is composed of cemented carbide, 
it is coated with a titanium carbide layer with a temperature of between 
400.degree. C. and 700.degree. C. prior to deposition of the refractory 
layer in order to improve adhesion of this refractory layer to the metal 
mass. The diamond layer can then be deposited at a temperature of between 
700.degree. C. and 850.degree. C. 
Examples of implementation of the method according to the invention will 
now be described. 
In all these examples, the surface on which the diamond is deposited is 
prepared by classical polishing followed by diamond polishing so that the 
surface of the metal part to be coated can be seeded. 
EXAMPLE 1 
Diamond is deposited directly on a TA6V type alloy by means of the device 
shown in FIG. 1 in two successive stages, which may be separated by a 
return to room temperature. 
In the first stage, a carbon cling layer with a graphitic or amorphous 
phase proportion of over approximately 30% is deposited on a TA6V alloy 
part under the following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 3 hours. 
In a second stage, a polycrystalline diamond coating is deposited on the 
metal part coated with the cling layer under the following conditions: 
hydrogen flowrate equal to 400 cm.sup.3 /min; 
methane flowrate equal to 4 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 30 hours. 
Thus, a part made of TA6V alloy shown in FIG. 2 is obtained, the alloy 
structure of which has not been modified, and which is coated with a 
5-micron thick diamond layer. 
The stresses measured in the diamond layer are essentially 5 GPa. These 
lower stresses hence allow a layer greater than 1 micron thick to be 
deposited without the layer peeling on return to room temperature. 
In addition, the friction coefficient of this diamond-coated alloy measured 
under classical conditions after cooling is substantially less than 0.1. 
EXAMPLE 2 
A metal part made of TA6V alloy is coated with a polycrystalline diamond 
layer by means of the device in FIG. 1 in two successive stages which may 
be separated by a return to room temperature. 
In the first stage, a carbon cling layer is deposited on the alloy part 
under the following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 3 hours. 
In the second stage, the part is coated with polycrystalline diamond under 
the following conditions: 
hydrogen flowrate equal to 180 cm.sup.3 /min; 
carbon monoxide flowrate equal to 15 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 25 hours. 
The TA6V alloy part thus obtained is coated with a diamond layer 8 microns 
thick and its characteristics remain substantially unchanged. In addition, 
the stresses measured in the diamond layer after cooling are approximately 
5 GPa. These lower stresses allow a layer greater than approximately 1 
micron in thickness to be deposited without the layer peeling upon return 
to room temperature. The friction coefficient measured in classical 
fashion is less than approximately 0.1. 
EXAMPLE 3 
Diamond deposition is accomplished on a TA6V alloy previously coated by a 
physical vapor deposition (PVD) technique with a titanium nitride 
refractory layer 3 microns thick, with a second titanium nitride layer 
having a nitrogen concentration that decreases in the outward direction 
and is 1 micron thick to increase adhesion of the refractory layer, and a 
titanium layer 3 microns thick. 
The diamond is deposited by means of the device of FIG. 1 in two successive 
stages which may be separated by a return to room temperature. 
In a first stage, the cling carbon for the diamond is deposited under the 
following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 3 hours. 
In the second stage, the diamond is deposited under the following 
conditions: 
hydrogen flowrate equal to 180 cm.sup.3 /min; 
carbon monoxide flowrate equal to 15 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 7 hours. 
This produces a TA6V alloy part whose structure has remained unchanged, 
coated first with a titanium nitride refractory layer comprising a 
diffusion barrier for the carbon and with intermediate sublayers including 
an outer diamond layer 1.5 microns thick adhering to the titanium by means 
of a titanium carbide layer. 
Such a metal part is shown schematically in FIG. 4 described above. 
The stresses measured in the diamond layer after cooling are substantially 
5 GPa and the friction coefficient is substantially less than 0.1. 
EXAMPLE 4 
Diamond deposition is accomplished on a TA6V alloy previously coated by PVD 
with a titanium nitride refractory layer 3 microns thick, a second 
titanium nitride layer intended to improve adhesion of the refractory 
layer, having a decreasing nitrogen concentration gradient in the outward 
direction and a thickness of 1 micron, plus a titanium layer 0.5 micron 
thick. 
The diamond is deposited by means of the device according to FIG. 1 in two 
successive stages which may be separated by a return to room temperature. 
In the first stage, a cling carbon layer for the diamond is deposited on 
the metal part under the following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 3 hours. 
In the second stage, the alloy is coated with a polycrystalline diamond 
layer under the following conditions: 
hydrogen flowrate equal to 180 cm.sup.3 /min; 
carbon monoxide flowrate equal to 15 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 7 hours. 
Thus, one obtains a TA6V alloy part whose characteristics have not been 
changed, coated with intermediate sublayers including a titanium nitride 
refractory layer forming a carbon diffusion barrier and an outer diamond 
layer 1.5 microns thick adhering to the titanium nitride through a 
titanium carbide layer and an underlying titanium carbonitride layer. 
Moreover, the stresses measured in the diamond layer after cooling are 
substantially 5 GPa and the friction coefficient is substantially less 
than 0.1. 
EXAMPLE 5 
Diamond is deposited on a TA6V alloy previously coated by PVD with a 
niobium refractory layer 0.5 micron thick then a titanium layer 3 microns 
thick. 
The diamond is deposited by means of the device according to FIG. 1 in two 
successive stages which may be separated by a return to room temperature. 
In the first stage, a cling carbon layer for the diamond is deposited on 
the metal part under the following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 3 hours. 
In the second stage, the alloy is coated with a polycrystalline diamond 
layer under the following conditions: 
hydrogen flowrate equal to 180 cm.sup.3 /min; 
carbon monoxide flowrate equal to 15 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 600.degree. C.; 
deposition duration equal to 7 hours. 
Thus, one obtains a TA6V alloy part whose structure has not been changed, 
coated with intermediate sublayers being a niobium layer forming a carbon 
diffusion barrier and an outer diamond layer 1.5 microns thick adhering to 
the titanium nitride through a titanium carbide layer. 
Moreover, the stresses measured in the diamond layer after cooling are 
substantially 5 GPa and the friction coefficient is substantially less 
than 0.1. 
EXAMPLE 6 
Diamond is deposited on a 30CD12 type steel (composition approximately 
0.30% C, 0.55% Mn, 0.25% Si, 3% Cr, 0.40% Mo) which has previously been 
coated by PVD with a refractory layer forming a titanium nitride diffusion 
barrier 3 microns thick, a titanium layer with a nitrogen gradient 
directed to the outside of the part and 1 micron thick, and a titanium 
layer 3 microns thick. 
The diamond is deposited in the device of FIG. 1 in two successive stages 
which may be separated by a return to room temperature. 
In the first stage, a cling carbon layer for the diamond is deposited on 
the alloy part under the following conditions: 
hydrogen flowrate equal to 200 cm.sup.3 /min; 
methane flowrate equal to 12 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 2.5 Torr; 
temperature equal to 580.degree. C.; 
deposition duration equal to 3 hours. 
In the second stage, the alloy part is coated with a polycrystalline 
diamond layer under the following conditions: 
hydrogen flowrate equal to 400 cm.sup.3 /min; 
methane flowrate equal to 4 cm.sup.3 /min; 
microwave power equal to 400 W; 
pressure equal to 10 Torr; 
temperature equal to 580.degree. C.; 
deposition duration equal to 9 hours. 
Thus, a steel part of the 30CD12 type is obtained, whose structure in 
unmodified, coated with intermediate sublayers one of which is a titanium 
nitride layer constituting a carbon diffusion barrier, as well as an outer 
diamond layer 1.5 microns thick adhering to the titanium through a 
titanium carbide layer. 
The stresses measured in the diamond layer after cooling are approximately 
6 GPa and the friction coefficient is less than approximately 0.1. 
The various embodiments described allow a metal part to be obtained having 
a surface containing titanium coated with a polycrystalline diamond layer 
adhering strongly to the surface of the part due to prior deposition of a 
cling carbon layer diffusing into the titanium and forming a titanium 
carbide layer. 
FIG. 5 is a Raman spectroscopy recording showing the intensity of a wave 
reflected by the metal part as a function of the wave number of the 
incident wave. This recording is obtained from the cling carbon layer for 
the diamond of the metal parts coated with diamond according to Examples 1 
to 6. 
The intensity peaks at 1340 and 1580 cm.sup.-1 show the graphitic nature of 
the carbon deposited as the lowintensity carbon peak is buried in the 
1340.sup.1 band. 
From this spectrum, and bearing in mind the Raman diffusion cross section 
ratio between the non-diamond carbon and the diamond, which is 50 to 75, 
the proportion of amorphous or graphitic carbon can be estimated to be 
over 30%. 
This cling carbon layer totally diffuses into the underlying layer 
including titanium and thus no longer appears at the end of the diamond 
layer deposition operation. It allows the metal part to be coated with a 
polycrystalline diamond layer at a temperature lower than 700.degree. C. 
Alternatively, the cling layer allows the cemented carbide parts whose 
intrinsic properties remain unchanged up to 850.degree. C. to be coated, 
achieving better cling of the diamond layer. It can thus be seen that this 
diamond layer can be deposited at temperatures of between 700.degree. C. 
and 850.degree. C. 
As stated above, the cling layer allows a diamond layer thicker than 1 
micron to be deposited while preventing it from peeling on return to 
ambient temperature and preventing a change in the alloy entering into the 
composition of the metal part to be coated with diamond.