Process for reducing particle defects in arc vapor deposition coatings

A process for reducing particle defects in an arc vapor deposition coating on a substrate comprises the steps of providing a metallic wire mesh, providing an arc source adapted to impart a positive charge on coating macroparticles produced during arc vapor deposition, positioning the wire mesh in between the arc source and the substrate, applying a negative bias voltage to the wire mesh and entrapping positively charged macroparticles on the negatively charged wire mesh during coating deposition.

TECHNICAL FIELD 
The present invention relates generally to arc vapor deposition coating 
techniques, and more particularly to a process for reducing particle 
defects in arc vapor deposition coatings. 
BACKGROUND ART 
Coatings deposited by arc vapor deposition techniques are widely used to 
coat metal and ceramic substrates for improving their wear resistance. 
One of the drawbacks of arc vapor deposition coatings is that the arc vapor 
deposition process inherently results in the production of macroparticles 
of the deposited material. These macroparticles usually range from 0.1 
.mu.m to 5 .mu.m in size and get deposited on the coating surfaces. Most 
of the macroparticles are loosely bonded to the coating surfaces, 
detrimentally affecting the mechanical integrity of the coating. When 
these macroparticles fall off the coating surfaces, they leave voids in 
the coating surface and these voids cause the coated substrate to be 
exposed to the elements. 
Various methods have been developed by the coatings industry to eliminate 
these macroparticles. One such method is the use of electromagnetic 
tunnels during the arc vapor deposition process. This method usually 
requires expensive equipment and imposes an undesirable limitation on the 
surface geometry of the coated article and is not suitable for the 
production of low volume parts. Most other methods greatly reduce the 
deposition rate and the coating area 
It has been desirable to provide a system for arc vapor deposition that has 
improved manufacturing capabilities and avoids the waste of time, labor, 
equipment and natural resources. It has been further desirable to provide 
a process that allows the flexibility of coating articles with various 
surface geometries without reducing the deposition rate or the coating 
area. It is the purpose of this invention to provide a process for 
reducing by weight percent, the amount of macroparticles having a particle 
size of at least 0.1 .mu.m, deposited on a substrate during arc vapor 
deposition process. 
The present invention is directed to overcome one or more problems of 
heretofore utilized systems. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention, a process for reducing particle 
defects in an arc vapor deposition coating on a substrate is disclosed. In 
the process of this invention, a metallic wire mesh having an opening size 
is provided. An arc source, which is adapted to impart a positive charge 
on coating macroparticles produced during arc vapor deposition, is 
provided. A wire mesh is positioned in between the arc source and the 
substrate. A negative bias voltage is applied to the wire mesh. An arc 
vapor deposition coating is then deposited on the substrate, and the 
positively charged coating macroparticles are entrapped on the negatively 
charged wire mesh.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, an apparatus 10 is shown for carrying out the process 
for reducing particle defects in an arc vapor deposition coating on a 
substrate 20. The process comprises the following steps: A metallic wire 
mesh 30 having an opening size 32,32',32" (referred to hereinafter by one 
numeral 32, for purposes of brevity) is provided. An arc source 40 is 
provided. The arc source 40 is adapted to impart a positive charge on 
coating macroparticles produced during arc vapor deposition by an 
energizing source. The wire mesh 30 is positioned in between the arc 
source 40 and the substrate 20. A negative bias voltage is applied to the 
wire mesh 30 by a voltage source 50. An arc vapor deposition coating 60 is 
deposited on the substrate 20. The positively charged coating 
macroparticles are entrapped on the negatively charged wire mesh 30. 
In the preferred embodiment of the present invention, the metallic wire 
mesh 30 is made from one of copper, aluminum, steel and stainless steel, 
and preferably, the wire mesh is made of stainless steel. 
In the preferred embodiment of the present invention, the wire mesh opening 
32 is desirably in the range of about 0.2235 mm (0.0088 inch) to about 
0.864 mm (0.034 inch), and preferably, of about 0.467 mm (0.0185 inch). An 
opening size less than about 0.2235 mm is undesirable because the opening 
of the wire mesh becomes too small and detrimentally results in a 
reduction of the arc vapors passing through the wire mesh and depositing 
on the metal substrate, thus detrimentally affecting the coating quality, 
and undesirably reducing the coating deposition rate. An opening size 
greater than about 0.864 mm is undesirable because the opening of the wire 
mesh becomes too large and detrimentally allows an undesirable amount of 
macroparticles to pass through the wire mesh and reduce the coating 
quality. 
In the preferred embodiment of the present invention, the arc source 40 is 
energized by an energizing source in order to vaporize the deposition 
material. The energizing imparts a positive charge on the macroparticles 
produced. The arc source 40 is energized desirably with a current in the 
range of about 50 Amps to about 250 Amps, and preferably, with a current 
of about 75 Amps. A current less than about 50 Amps is undesirable because 
it does not provide enough energy for stable vapor formation. A current 
greater than about 250 Amps is undesirable because it produces excessive 
macroparticles at a high velocity, thus detrimentally causing too many 
macroparticles to pass through the wire mesh. 
In the preferred embodiment of the present invention, the wire mesh 30 is 
desirably positioned at a distance of at least 10 mm from the substrate 20 
and preferably, at a distance of at least 15 mm. A distance of at least 10 
mm is desirable to eliminate the macroparticles that were not trapped by 
the wire mesh but were only slowed down. 
In the preferred embodiment of the present invention, the wire mesh 30 is 
desirably positioned at a distance in the range of about 10 mm to about 
400 mm from the arc source 40, and preferably, at a distance of about 50 
mm. A distance less than about 10 mm is undesirable because the 
macroparticle energy would overcome the entrapping effect of the wire 
mesh, thereby allowing excessive macroparticles to pass through it. A 
distance greater than about 400 mm is undesirable because the efficiency 
of macroparticle entrapment would diminish, since most macroparticles 
would not reach the wire mesh. 
In the preferred embodiment of the present invention, the negative bias 
voltage is desirably in the range of about 20 volts to about 200 volts, 
and preferably, about 50 volts. A negative bias voltage less than about 20 
volts is undesirable because it would not provide sufficient negative bias 
to entrap the macroparticles. A negative bias voltage greater than about 
200 volts is undesirable because the high negative bias it would attract 
some of the vapor phase coating material, thus detrimentally reducing the 
coating deposition rate. 
In the preferred embodiment of the present invention, desirably at least 
50% by weight, and preferably, at least about 90% of the macroparticles 
produced are entrapped in the wire mesh 30. It is desirable to entrap at 
least about 50% of the macroparticles in order to significantly improve 
the coating integrity and quality. 
In the preferred embodiment of the present invention, the coating is 
titanium nitride and the substrate is steel. Alternatively, any suitable 
coating materials such SiN, Al.sub.2 O.sub.3, SiC, TaO for example, can be 
deposited on various substrates such as various metals and ceramics, for 
example. Such alternatively used coating and substrate materials are well 
known to those skilled in the art of arc vapor deposition techniques. 
In the preferred embodiment of the present invention, the wire mesh 30 has 
a flat planar configuration. However, in another embodiment of this 
invention, the wire mesh is of a cylindrical configuration, with the arc 
source disposed within the wire mesh. Other alternate geometrical shapes 
are possible without departing from this invention. 
EXAMPLE A 
A titanium nitride coating was deposited on a steel substrate according to 
the process of the present invention. The wire mesh was a stainless steel 
wire mesh having an opening size of 0.0185 inches. It was positioned at a 
distance of 50 mm from an arc source. The arc source was positioned at a 
distance of 100 mm from the steel substrate. The arc source was energized 
with a current of 75 Amps. A negative bias voltage of 50 volts was applied 
to the wire mesh. A titanium nitride coating having a thickness of about 
0.002 mm (2 microns) was deposited on the steel substrate. 
Photomicrographs were taken of the surface of this coating. These 
photomicrographs are shown in FIGS. 3, 5, and 7. 
A titanium nitride coating was also deposited on a similar steel surface by 
conventional (proir art) process, without the negatively biased wire mesh. 
Photomicrographs were taken of the surface of this coating. These 
photomicrographs are shown in FIGS. 2, 4, and 6. 
Referring to FIGS. 2-7, and comparing FIGS. FIGS. 2, 4, and 6 with FIGS. 3, 
5, and 7, it is observed that the coating deposited by the proces of the 
present invention has a significant reduction in the deposition of 
macroparticles, thus improving the integrity of the coating. 
INDUSTRIAL APPLICABILITY 
The process of the present invention is particularly useful for depositing 
arc vapor deposition coatings with very few macroparticles. This process 
allows the flexibility to coat articles having various surface geometries 
without detrimentally affecting the deposition rate and without requiring 
expensive equipment. This invention thus represents a savings of time, 
labor, resources and equipment for the coating industry. 
The process of the present invention is particularly useful for depositing 
wear resistant coatings of very high mechanical integrity on various wear 
surfaces. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the drawings, the disclosure and the appended claims.