High hardness hafnium nitride

A high hardness hafnium nitride formed by a reactive sputter coating of a substrate in a closed chamber with a substantially pure hafnium target, and an atmosphere of argon and nitrogen. The hafnium nitride has a face centered cubic structure with a lattice spacing of 4.55 to 4.58 .ANG., and a hardness in the range of 2700 to 5500 kilograms per square millimeter utilizing a Vickers microindentation hardness test.

BACKGROUND OF THE INVENTION 
The present invention relates to the rapid rate reactive sputtering of 
hafnium nitride onto a substrate or workpiece and to the resulting high 
hardness coating material. Methods for the coating of a metallic layer or 
a metallic compound onto a substrate include vapor deposition, plasma 
spray processes and cathode sputtering. Vapor deposition processes to 
provide a metallic thin film on a workpiece utilize the material for 
plating in a suitable atmosphere, such as a vacuum or an inert shielding 
gas, wherein the material is heated until it evaporates and is deposited 
as a film on a substrate. 
Plasma spray processes provide the material to be deposited in the form of 
a fine-grained powder which is brought into a plasma arc so that the 
particles melt and are deposited on the substrate. 
Cathodic magnetron sputtering or radio-frequency ionic sputtering involves 
a metallic target formed of the material to provide the desired coating on 
the substrate or workpiece in a gas discharge wherein the material is 
sputtered by ion bombardment; the metallic particles removed from the 
target being deposited on the substrate. The target becomes the cathode 
and the anode is located beneath the substrate. Where a thin film of 
metallic compounds, such as the oxides, nitrides, carbides or the like, 
are to be deposited on various substrates, reactive sputtering is utilized 
wherein the target consists of the metal for the plating compound and a 
neutral or inert gas, such as argon, is mixed with a reactive gas such as 
oxygen, nitrogen or methane. The particles dislodged from the target 
combine with the reactive gas to produce the desired compound which is 
deposited on the substrate or workpiece. 
In the use of carbide or high speed steel tool bits for cutting or similar 
operations, wear becomes a substantial problem along with the ability to 
be resharpened or reground. To enhance the wear life of a cutting tool 
edge, the cutting surfaces are coated with a thin film of a wear resistant 
material, such as titanium carbide or titanium nitride. The present 
invention relates to an improved high hardness coating that will 
substantially increase the wear life of tools and other functional 
articles that require protection from wear. 
SUMMARY OF THE INVENTION 
The present invention comprehends the provision of an improved high 
hardness metallic compound that can be deposited on the surface of an 
article to substantially enhance the wear properties of the article. 
The present invention also comprehends the provision of a high hardness 
coating of hafnium nitride on a substrate or workpiece. The hafnium 
nitride coating has an approximately 1.0% larger lattice spacing than that 
for bulk hafnium nitride with the hardness of the coating in the range of 
65 to 69% greater than the hardness of the bulk material. 
The present invention further comprehends the provision of a high hardness 
coating of hafnium nitride formed during a magnetron cathodic reactive 
sputtering process utilizing a hafnium target, an inert atmosphere of 
argon and a controlled quantity of nitrogen which reacts with the hafnium 
to deposit the high hardness coating on the substrate or workpiece.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring more particularly to the disclosure in the drawing wherein is 
shown an illustrative embodiment of reactive sputtering apparatus 10 for 
the plating of a thin film of hafnium nitride (HfN) of a suitable 
substrate. The apparatus includes an elongated chamber 11 having an 
elevator at one end 12 communicating with a domed loading/unloading 
chamber 13, a pallet which is carried by the elevator to receive the 
substrate to be coated, a pallet carrier receiving the pallet from the 
elevator and moving the pallet into the chamber 11 for sputtering in the 
central portion and/or etching on a platform at the end 14 of the chamber 
opposite to the elevator. 
A magnetron cathode 15 in the chamber carries a target formed of hafnium 
metal and is located in the upper wall of the chamber 11. A substantially 
rectangular ring of tubing having a plurality of openings therein directed 
downwardly and inwardly for a purpose to be later described is positioned 
encompassing the periphery of the target. The tubing ring is connected to 
an inlet 16 in the upper wall for communication with a source 25 of 
nitrogen gas. The gas source is connected to the inlet through a mass flow 
meter 26 and a control valve 27. 
A source 28 of argon gas communicates with an inlet 32 in the chamber end 
14 through a second mass flow meter 29 and a control valve 31. Also, an 
outlet 33 in the floor of the chamber 11 is connected to a vacuum system 
for evacuation of the chamber to draw all air and other contaminants out 
prior to etching and sputtering. A horizontally moving shutter plate (not 
shown) having an opening or shutter therein may be utilized below the 
target to prevent any substantial scattering of material onto other 
portions of the chamber, thus directing the material onto the substrate 
below the opening. 
Vacuum is continuously applied to the chamber 11 to draw all air and 
contaminants out through the outlet 33, and a substrate is loaded onto a 
carrier pallet in the domed chamber 13; which chamber is then sealed and 
evacuated. The elevator lowers the pallet onto a carrier which is shifted 
to the opposite end 14 of the chamber 11, and the elevator is raised to 
seal the domed chamber 13 from the main processing chamber 11. A throttle 
(not shown) is operated to close the outlet 33, and argon gas is admitted 
through the flow meter 29, control valve 31 and inlet 32 to backfill the 
chamber 11 with a partial pressure of argon. A pressure gauge 43 measures 
the pressure of the argon and provides a signal to a flow controller 44, 
which in turn signals the control valve 31 to maintain a constant pressure 
of 8 millitorr in the chamber. 
A potential is applied to the substrate at the etching station, and the 
argon gas ionizes to provide argon ions which bombard the substrate to 
clean the surface to be coated. Once cleaned, the potential is interrupted 
and the substrate is shifted to a position beneath the hafnium target and 
cathode 15, and nitrogen gas is then admitted through the mass flow meter 
26, control valve 27 and inlet 16 to be directed downward by the tubing 
ring to the substrate. A potential of a negative 450 to 500 volts is 
applied to the cathode 15 from a target power supply 34 and a bias voltage 
of minus 80 to 100 volts is applied to the substrate, whereupon the 
resulting argon ions bombard the hafnium target. The hafnium atoms 
displaced by the bombardment pass downward with the nitrogen gas to react 
and form hafnium nitride onto the cleaned surface of the substrate. Once 
the desired thickness of hafnium nitride has been achieved, the potential 
is interrupted and the substrate removed through the domed chamber 13. 
A sampling tube 35 at the chamber end 14 provides a mass spectrometer 36 
with a gas sample indicating the amount of excess (unreacted) nitrogen gas 
remaining in the chamber during sputtering. The nitrogen peak on the mass 
spectrometer screen is measured and a signal is generated to a flow 
controller 42 which in turn provides a signal to the nitrogen control 
valve 27 to control the partial pressure of the nitrogen in the chamber 
substantially continuously during the sputtering cycle. A more complete 
description of the sputtering operation and control circuits therefor is 
found in the application Case No. 082014-BWL filed concurrently herewith 
and reference is made thereto. 
The sputtering reaction was carried out in a Materials Research Corporation 
(MRC) 902M In-Line sputtering system with both DC and RF magnetron, RF 
etch, and DC bias capabilities. Targets of hafnium and titanium and 
zirconium, for comparison, were MRC VP grade materials. Hafnium nitride 
was reactively sputtered from both a bonded planar hafnium target and from 
a Research S-Gun adapted to the MRC machine. Titanium nitride was 
reactively sputtered from both Inset and bonded planar titanium targets, 
and zirconium nitride from a zirconium Inset target. The target to 
substrate distance was three inches. 
Argon was bled into the machine at a pressure in the range of 4.0 to 14.0 
millitorr, although a preferred pressure is 8.0 millitorr. Nitrogen gas is 
admitted at an optimum flow rate based on the power applied to the target 
as shown in the following table. 
TABLE I 
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Target Power - kW 
N.sub.2 Flow, sccm* 
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1.0 5.3 
2.0 10.6 
3.0 15.9 
4.0 20.2 
5.0 24.6 
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*standard cubic centimeters per minute. 
The nitrogen and argon gas flows were controlled with MKS 260 flow 
controllers, and the chamber gas composition during sputtering was 
monitored with a UTi (Uthe Technology International) Model 100C mass 
spectrometer. The temperature of samples immediately emerging from the 
sputtering reaction never exceeded 315.degree. C. (600.degree. F.). 
The hafnium nitride coating was deposited on several substrates with 
similar results. The substrates included Corning 7059 borosilicate glass 
slides, pieces of M-2 tool steel hardened to Rc65, and Valemite VC-2 
carbide cutting tool inserts. Also, wear rings made from SAE 4620 
carburized steel were coated with hafnium nitride; the coating thickness 
in all instances being in the range of 4.0 to 5.0 um. As examples of the 
deposition rates for hafnium nitride, to provide a coating thickness of 
approximately 3500 angstroms (A.degree.), a scanning rate of the substrate 
under the shutter for the target was 10.0 inches per minute for a target 
power of 6.0 kilowatts (kw). For a stationary substrate, the deposition 
rate of hafnium nitride was 2950 A.degree. per minute at 5.0 kw target 
power. 
The sputter coated hafnium nitride was found to have a higher hardness 
compared with the bulk material. Utilizing a standard Vickers 
microindentation hardness test, the following table discloses the hardness 
of the hafnium nitride coating compared to other known group IVb metallic 
nitrides in bulk and sputter coated forms. 
TABLE II 
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Vickers Hardness Sputter 
(optical) Bulk Coated 
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TiN 2000 kg/mm.sup.2 
2650 kg/mm.sup.2 
ZrN 1500 2350 
HfN 1600 4200 
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It was found that there is a variation in hardness readings for optical 
versus scanning electron microscope (SEM) readings of the Vickers Hardness 
at 100 gms. and 200 gms. load for the three materials as shown in the 
following table. 
TABLE III 
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Material Optical SEM 
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TiN (100 gms.) 3200 kg/mm.sup.2 
2350 kg/mm.sup.2 
TiN (200 gms.) 2650 2400 
ZrN (100 gms.) 3350 2625 
ZrN (200 gms.) 2350 2500 
HfN (100 gms.) 5510 3110 
HfN (200 gms.) 4200 2700 
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With respect to the hafnium nitride coating from the sputtering process, 
not only is the hardness 68% greater over the bulk material from values 
found in the literature, but the cell size has an increase in the lattice 
spacing for the face centered cubic structure of approximately 1% as 
follows: 
TABLE IV 
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Property Bulk HfN CVD HfN Sputtered HfN 
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Cell size-A.degree. 
4.52 4.51 4.55-4.58 
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Also, conventionally as the amount of nitrogen in bulk hafnium nitride 
increases, the cell size decreases. However, in the sputtered material, as 
nitrogen increases, the cell size also increases. Thus, the sputtered 
hafnium nitride provides a final coating of higher hardness and increased 
cell size compared to titanium nitride and zirconium nitride which, 
although having an increased hardness, do not have any change in the cell 
size of the sputter coated material over the bulk material. All three 
nitrides have face centered cubic structures. Obviously, the Vickers 
hardness values are an average of several readings at each of the testing 
loads for both the optical and scanning electron microscope readings.