Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-09-30
2001-02-06
Beck, Shrive (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S249800
Reexamination Certificate
active
06183818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the fields of physics and chemistry. More specifically, the present invention relates to a new process which can deposit well adhered ultra smooth diamond films on metals with film roughness of 14 nanometers and hardness as high as 90% of that of a natural diamond crystal.
2. Description of the Related Art
The field of nanocrystalline diamond and tetrahedral amorphous carbon films has been the focus of intense experimental activity in the last few years for applications in field emission display devices, optical windows, and tribological coatings. Nanocrystalline chemical vapor deposited (CVD) diamond films have been synthesized from a variety of plasma feedgases including fullerenes or methane in argon (with and without hydrogen), as well as from methane
itrogen plasmas
1-3
.
It is generally agreed that the diamond grain size in these materials ranges from 3 to 30 nm and that the deposited films are smooth with a typical surface roughness of around 40 nm. Another class of carbon-based films called tetrahedral amorphous carbon can be grown by pulsed laser deposition
4
, cathodic arc deposition
5
, and ion deposition methods
6
. The sp
3
content in the tetrahedral amorphous carbon films can be as high as 85% and they can be made stress-free by thermal annealing
4
.
The choice of substrate used in the studies mentioned above has been silicon typically. For metals, however, the thermal expansion mismatch between the diamond film and substrate gives rise to thermal stress which often results in delamination of the film
7
. To avoid this problem in conventional chemical vapor deposited diamond, low substrate temperatures (<700° C.) have been used, often with the incorporation of oxygen or carbon monoxide to the feedgas mixture
8
. Conventionally grown chemical vapor deposited diamond films are also rough and would require post-deposition polishing for most applications.
While nanocrystalline chemical vapor deposited diamond films have been well-characterized with respect to their structure and field-emission properties, surprisingly little information is published on the deposition of nano-structured diamond coatings on metals in which the issues of interfacial adhesion and film toughness are relevant for tribological applications.
The prior art lacks an effective means for deposition of well-adhered, smooth nano-structured diamond films on metals for various tribological applications. The present invention fulfills this long-standing need and desire in the art.
SUMMARY OF THE INVENTION
The present invention demonstrates successful synthesis of nano-structured diamond films on Ti-6Al-4V alloy (90% Ti, 6% Al, and 4% V by weight) and on molybdenum using a high density CH
4
/H
2
plasma process. Also disclosed is that adding small amounts of nitrogen gas to such plasma process produces ultra smooth diamond films with surface roughness as low as 14 nanometers. The smooth diamond films are useful in tribological/wear resistant applications in bio-implants and machine tools, such as diamond coating of titanium implants, cobalt chrome femoral head and knee implants, and a magnetic video or audio tape and a recording head in a magnetic storage media.
The present invention further characterizes mechanical properties for nano-structured diamond films grown on a titanium alloy using a feedgas mixture involving an unconventionally high methane fraction (15% by volume) in nitrogen and hydrogen. It was found that the low root-mean-square (rms) surface roughness of 14 nm along with excellent interfacial adhesion, film toughness, and hardness makes these nano-structured coatings prime candidates for tribological applications.
In one embodiment of the present invention, there is provided a method of producing an ultra smooth diamond film, comprising the step of adding nitrogen gas to methane/hydrogen plasma created by a microwave discharge, wherein said plasma provides chemical vapor deposition for the production of said diamond film. Generally, the diamond film produced by the technique disclosed herein has a surface roughness of less than 100 nanometers and can be as low as 14 nanometers and a hardness as high as 90% of a single crystal diamond value. Such diamond film adheres well to metals. Preferably, the nitrogen gas is added to the process in the concentration range from about 2% to about 20% of the methane (CH
4
) used in the plasma. The plasma process is run under a pressure range from about 100 Torr to about 150 Torr and a constant substrate temperature between 700° C. to 850° C. The preferred CH
4
fraction used in the plasma process is 5 to 15% CH
4
in a balance of H
2
. The smoothest diamond films on metals are obtained when CH
4
to H
2
ratio is 15% and nitrogen to CH
4
ratio is 10%.
In another embodiment of the present invention, there is provided a method of depositing an ultra smooth diamond film on metals, comprising the step of adding nitrogen gas to methane/hydrogen plasma created by a microwave discharge, wherein said plasma provides chemical vapor deposition for depositing said diamond film on said metal. Preferably, nitrogen gas is added to the process in the concentration range from about 2% to about 20% of the methane (CH
4
) used in the plasma. Generally, the metal can be titanium implant, cobalt chrome femoral head, knee implant, and a magnetic video or audio tape and a recording head in a magnetic storage media.
Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of the invention given for the purpose of disclosure.
REFERENCES:
patent: 5055318 (1991-10-01), Deutchman et al.
patent: 5523121 (1996-06-01), Anthony et al.
Catledge Shane A.
Vohra Yogesh K.
Adler Benjamin Aaron
Beck Shrive
Chen Bret
UAB Research Foundation
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