Note: Descriptions are shown in the official language in which they were submitted.
CA 02361354 2001-11-15
DIAMOND-LIKE COATING, METHOD OF ITS PLATING
AND DENTAL BUR WITH THE SAID DIAMOND-LIKE COATING
The invention is related to the field of producing highly rigid wear-
resistant coatings on metal and non-metal goods, in particular on medical
cutting tools, for example, on dental burs.
Background of the invention
Diamond-like coatings (DLC) are widely used in various branches of
science and technology. Main advantages of DLCs are their hardness, wear-
resistance, purity, biological inertness, etc. Due to such properties DLCs are
plated on medical tools, medical implants, cutting tools and are used in other
branches.
There is known a DLC plated by vacuum arc method disclosed in the
USSR Certificate of Authorship No. 1070949 comprising a DLC layer on a
substrate, DLC being plated by a pulse stream of compensated no-current
carbon plasma with the density of 101$-1019 cm 2c-1.
However, the disclosed coating does not have a necessary thickness due
to poor DLC adhesion to the substrate.
The closest prior art for the coating according to the invention claimed is
a coating disclosed in the USSR Certificate of Authorship No. 1494554
published on March 15, 1994, Bulletin No. 5. The coating has an increased
strength of the sub-layer adhesion to the substrate and to the following layer
of
DLC. Yet strength provided by this method is not sufficient for producing a
DLC with high wear resistance due to high concentration of contaminants in
DLC.
There is known a method for producing highly rigid DLCs on metal and
dielectric substrates by way of cathode spraying of graphite in a magnetic
field
at low pressure of the inert gas - krypton (10-5-10-2 Pa) on the substrate
being
cooled and having the temperature lower than 100 K (the USSR Certificate of
Authorship No. 411037 published in 1974, Bulletin No. 29). A drawback of
i
CA 02361354 2001-11-15
this method is poor quality of the coating, low productiveness and high cost
of
the products.
Main efforts of the researchers and manufacturers of DLCs are aimed at
increasing their wear resistance defined both by adhesion to the substrate and
the structure of the coating itself. One of the most important parameters of
the
DLC deposition process is the substrate (product) temperature that is not to
exceed the critical temperature 135-150° C during the process, since in
case it
is higher coatings start losing their physical and mechanical properties and
adhesion to the substrate due to thermal graphitization. Furthermore,
necessary
is high purity of the source material, high density of the carbon flow, high
purity of the substrate surface, etc. As a result, a pulse ion-plasma vacuum
arc
(further - vacuum arc) method of plating DLC was developed, the essence of
which lies in forming an intensive pulse flow of carbon ions in an electric
arc
m a vacuum.
One of the known vacuum arc methods of DLC plating is a method
disclosed in the USSR Certificate of Authorship No. 1070949 that involves
DLC condensation on the substrate produced by a pulse flow of compensated
no-current carbon plasma with density Of lOl8-1019 Cm 2c-~.
However, the said method does not also provide for necessary thickness
of the coating due to poor DLC adhesion to the substrate.
The closest prior art for the method according to the claimed invention is
a method disclosed also in the USSR Certificate of Authorship No. 1494554
published on March 15, 1994, Bulletin No. 5, the essence of which lies in the
fact that before vacuum arc DLC plating on the substrate surface the said
surface is coated by a sub-layer of metal, e.g. titanium having increased
strength of adhesion to the substrate and the following layer of DLC, the
product being under floating voltage during the process of plating the sub-
layer
of metal and DLC.
However, adhesion strength obtained by this method is still insufficient
for producing a DLC with high wear resistance. Besides, it is reasonable to
2
CA 02361354 2001-11-15
plate the titanium sub-layer only on metal substrates whereas DLC may be
plated on non-metal substrates as well.
One of the applications of strengthening coatings is their plating on
medical cutting tools, in particular on dental burs. Known are dental burs
made
of steel or hard-alloyed materials (for example, see the State Standard of
Ukraine - 22090-89). Yet these burs have a short durability due to
insufficient
hardness and wear resistance of cutting edges.
The closest prior art for dental burs claimed according to this invention is
dental burs of Romidan Dental Ltd. presented on the Internet at the address:
http:/www.romidan.com/fgburs.htm. The heads of these burs made of stainless
steel are coated by electrostatic gluing with a coating of diamond powder made
of natural diamonds. Drawbacks of these burs are their high cost and
relatively
short durability due to the fact that strength of the bond between the powder
and metal is determined by the glue adhesion to the powder and metal.
Summary of the invention
As far as the DLC and method of its plating are concerned, this invention
is based on the objectives of improving wear resistance of the product due to
the increase in the level of the coating adhesion to the substrate and
increasing
the coating thickness due to the change of its structure.
As far as dental burs are concerned, this invention is based on the
objective of increasing their wear resistance by introducing new materials and
elements.
The set objective as to the coating is solved as follows: the known coating
comprises a diamond-like substance layer plated on the substrate by carbon
deposition in the gas phase under reduced pressure conditions, said layer
being
an aggregate of successive sub-layers of diamond-like coating with the reduced
concentration of contaminants in the areas of the sub-layers interface.
The decrease in the concentration of contaminants is obtained as follows:
during the process of plating the coating on the product surface by deposition
3
CA 02361354 2001-11-15
of a carbon plasma flow from a pulse arc source in a sealed chamber at
pressures below atmosphere pressure, before turning on the carbon plasma
source the plasma flow is blocked with a shutter, then the source is turned
on,
and then in the predetermined period of time the shutter is made open. Then a
relatively thin DLC sub-layer is plated, the source is turned off, the shutter
is
closed, and the process is repeated.
The improvement claimed in this invention consists in the creation of the
coating structure out of a number of successive sub-layers with a sharp
decrease in concentration of contaminants both at the interface of the
successive sub-layers and within one sub-layer. This increases adhesion of sub-
layers to each other and to the substrate and so allows to increase the DLC
quality and thickness that in turn increases the coating strength and wear
resistance.
A further improvement of the coating according to the invention is an
additional titanium sub-layer plated by titanium deposition in the gas phase
before plating the diamond-like coating on the steel substrate. Adhesion of
the
whole coating to the substrate is increased due to the fact that titanium has
increased adhesion to metal, especially to steel, as a metal. A particular
effect is
obtained when before plating titanium the titanium plasma flow is blocked with
a shutter for a certain period of time. This also leads to decrease in amount
of
contaminants at the interface and to increase in the DLC adhesion to the
substrate and wear resistance of the coating.
In specific embodiments of the invention as a result of the research
performed the following parameters of the coating were optimised:
thickness of the diamond-like coating sub-layer is not less than 0.1 ~.m;
amount of the diamond-like coating sub-layers is within the range of 1 -
20.
The objective set as to the method is solved by adding new operations to
the known method of DLC plating on the product surface comprising plating a
4
CA 02361354 2001-11-15
diamond-like coating on the product surface by deposition of a carbon plasma
flow from a pulse arc source, the said process taking place in a sealed
chamber
under pressures lower than atmosphere pressure, the temperature of the product
being controlled during the deposition process, and the product being under
floating voltage, said new operations being as follows: before turning on the
carbon plasma source this source is completely blocked with a shutter, then
the
source is turned on, and in the predetermined period of time the shutter is
made
open.
The blocking of the carbon plasma source with a shutter before turning it
on does not allow the carbon plasma flow to be deposited on the products, and
it is these initial periods when a great amount of contaminants, which
impregnate the cathode during free air entering into the chamber and during
the
process of pumping out by oil pumps, are penetrating into the plasma when
leaving the surface of the carbon plasma source cathode made of graphite and
being a porous material with a well-developed surface. Contaminants present in
initial layers of coatings reduce significantly the adhesion of one layer to
another one and impair the coating quality. Therefore, the improvement
claimed in this invention reduces sharply the quantity of contaminants getting
on the product surface and hereby increases the metal adhesion to the product
surface and to the DLC, which in turn increases wear resistance of the DLC.
A further improvement of the method according to the invention is that
before plating a diamond-like coating on metal products a sub-layer of metal
is
plated on the product surface from a separate stationary source of metal
plasma, the said source being blocked with a shutter before turning it on,
then
this source is turned on, and in the predetermined period of time the shutter
is
made open.
As it is known, a sub-layer of metal, e.g. titanium has an enhanced
adhesion to the substrate metal, and on the other hand, a sub-layer of metal
plated by the aforesaid method has an increased purity and therefore has an
enhanced adhesion to the following sub-layer of DLC.
CA 02361354 2001-11-15
Besides, similar to the case with the carbon plasma source, the shutter
blocking the metal plasma flow being formed after turning on the source does
not allow this metal plasma flow to be deposited on the substrate during
initial
periods of the source operation when the amount of contaminants in the flow is
the highest.
Thus, this improvement further increases the DLC adhesion to the
substrate.
In a further improvement of the method according to the invention before
plating a metal sub-layer on the product surface a separate stationary ion
source
of argon is turned on, and ion bombardment of products with argon ions is
performed during the predetermined period of time. Such a bombardment heats
up at least the surface layer of the products and contributes to cleaning the
surface from contaminants entering the external space. This further increases
the metal adhesion to the product surface.
In a specific embodiment of the bombardment of products with argon ions
the energy of argon ions is established within the range of 5 - 6 keV, and a
positive potential is applied to the products within the range of 800 - 1,500
V.
In a specific embodiment of the method, which comprises plating a metal
sub-layer, after turning on the metal plasma source with the closed shutter
the
arc current is maintained within the range of 90 - 100 A, and the duration of
the predetermined period of time, in which the shutter of the metal plasma
source is made open, is established within the range of 50 - 60 seconds. These
ranges were determined experimentally to be the most appropriate ones for the
maximum removal of contaminants from the metal plasma source cathode.
A further improvement of the method according to the invention, which
comprises plating a metal sub-layer, is that during the said predetermined
period of time a separate stationary ion source of argon is turned on, and ion
bombardment of products with argon ions is performed with the energy 5 - 6
keV, while a potential is applied to products within the range of 1,400 -
1,600
V. The effect of the bombardment on the degree of the metal sub-layer
6
CA 02361354 2001-11-15
adhesion to the product surface is described above, and performing the
bombardment during the metal plasma source preparation for operation reduces
the period of plating the coating. The aforesaid ranges of physical values are
also determined experimentally and are the most appropriate ones for the said
operation.
In a specific embodiment of the method, which comprises plating a metal
sub-layer, this sub-layer is plated with the positive potential on the
products
being within the range of 180 - 200 V. The said range is optimum for plating a
metal sub-layer and is also determined experimentally.
A further improvement of the method according to the invention is to
repeat the process of the diamond-like coating plating several times with
plating a sub-layer of the coating each time, wherein each time before turning
on the source of carbon plasma the plasma flow is blocked completed with a
shutter, then the source is turned on, and in the predetermined period of time
the shutter is made open. This allows to obtain reduced concentration of
contaminants in every sub-layer interface area, which decreases stresses in
this
area and enables to increase the coating strength and wear resistance to
further
extent.
In a specific variant of any of the aforesaid embodiments of the method
according to the invention the duration of the predetermined period of time,
in
which the shutter is made open, is within the range of 40 - 80 seconds. This
experimentally determined period of time is sufficient for contaminants to be
removed to the utmost from the surface of the carbon plasma source cathode.
By experimental research of conditions of DLC plating on products
without exceeding the critical temperatures specific values of the process
parameters were chosen, namely:
- energy of carbon ions not exceeding 0.4 keV;
- duration of the carbon source pulses being 10 sec with the pulse
period-to-pulse duration ratio not less than 30,000;
CA 02361354 2001-11-15
which may be used in the method according to the invention both separately
and in the aggregate.
A further improvement of the method according to the invention is that
before opening the pulse carbon source with a shutter it is turned off, and
the
ion source of argon is turned on for the predetermined period of time. This
allows to clean the product surface before plating the next sub-layer on
products due to the increase in their temperature and enhance the adhesion of
the following DLC layer to the preceding one.
In specific embodiments of the method, wherein a number of sub-layers
of the coating are plated, the thickness of each sub-layer of the diamond-like
coating is not less than 0.1 ~m and/or the amount of sub-layers is within the
range of 1 - 20.
A further improvement of the method according to the invention is to
introduce an additional new operation of argon ion bombardment of the
product surface from a separate ion source during the process of the DLC
deposition from a pulse source of carbon plasma.
In the known methods of producing DLC with the help of a pulse source
of carbon plasma it was impossible to produce a coating more than 1.5 pm
thick due to poor adhesion and large internal stresses in the coating, while
many practical applications require coatings up to 5 ~,m thick and more.
As it is known, the DLCs produced by the known methods have a
columnar structure having large internal stress when the coating is
sufficiently
thick, which results in the cracking of coating at the border of grains and in
its
detachment from the substrate with the great thickness of the coating.
Furthermore, one of the causes explaining the quality impairment of the
DLC plated by the said method on small-sized products with sharp cutting
edges and poor thermal conductivity is the local heating of products to
temperatures higher than critical temperature, which results in graphitization
of
metastable diamond structure of the coating and disastrous reduction of wear
s
CA 02361354 2001-11-15
resistance and strength of the coating. During the bombardment of the product
surface with argon ions obtained from a separate ion source, which takes place
simultaneously with the DLC deposition from the pulse carbon source, the
argon ions irritate the columnar structure, which contributes to more uniform
stress distribution in the coating and reduces the internal stress appearing
in the
coating during the deposition, whereby graphitization of the structure is
avoided. Thus increased quality of the coating is provided, and it is possible
to
increase its thickness.
By experimental research of conditions of DLC plating on products
without exceeding the critical temperatures the energy of the doping argon gas
ions was chosen to be within the range of 4 - 6 keV in the specific embodiment
of the invention.
The objective set in this invention as to the dental bur improvement is
solved as follows: in a dental bur comprising a wear-resistant coating layer
the
coating is a diamond-like coating comprising a number of successive sub-
layers with reduced concentration of contaminants in the sub-layers interface
areas.
The technical result obtained thereby lies in the increase in wear
resistance of burs due to the plating continuous and homogeneous coating and
to the increase in the coating adhesion to the bur material.
The improvements and specific embodiments of the dental bur according
to the invention are similar to those described above for the coating.
9
