Method for coating a substrate to provide a titanium or zirconium nitride or carbide deposit having a hardness gradient which increases outwardly from the substrate

Abstract

A method for coating a substrate with titanium nitride or titanium carbide or zirconium nitride or zirconium carbide is described wherein physical vapor deposition in a vacuum is used, and wherein the substrate is biased by an electrical potential and the composition of the deposit is changed by introducing a gas during the deposition to produce a hardness gradient in the deposit which increases outwardly from the substrate.

Description

This invention relates to coated substrates and, more particularly, to the coating of a substrate to produce a high hardness coating.

The production of high hardness coatings on substrates may be desired in a wide variety of applications. For example, it is well known that the production of a titanium carbide coating on the surface of tungsten carbide cutting tools or wear parts produces significantly longer cutting or wear life than is possible with the uncoated tools or parts. Another example is the coating of wear surfaces in the combustion chambers of internal combustion engines, particularly those of the rotary type, with high hardness coatings of titanium nitride or titanium carbide.

In order to accomplish this deposition, much work has been done in the area of physical vapor deposition processes, hollow cathode sputtering processes, chemical vapor deposition processes, and reactive physical vapor deposition processes. Prior art applications of such processes, however, have failed to produce hardness values as high as may be required in many instances. Moreover, in many instances of prior art deposition processes, the rate of deposition has been lower than that desirable for high production techniques. In other cases of prior art deposition techniques, mechanical failure of the coating at the interface with the substrate has occurred.

It is an object of the present invention to provide an improved method for coating a substrate.

Another object of the invention is to provide a method for coating a substrate in which very high hardness of the coating is obtained.

It is another object of the invention to provide a method for coating a substrate in which relatively higher deposition rates may be achieved than in many prior art methods.

A further object of the invention is to provide a method for coating a substrate in which a very strong mechanical bond between a high hardness coating and a relatively softer substrate is achieved.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying illustration which is a photomicrograph enlarged 325 times of the cross section of a substrate coated with titanium nitride in accordance with the invention,

Very generally, the method of the invention comprises placing the substrate in an evacuated environment and evaporating titanium or zirconium metal in the evacuated environment to produce a vapor and causing the vapor to deposit on the substrate. The composition of the vapor is changed by reacting it with a gas during deposition to produce a hardness gradient in the deposit which increases outwardly from the substrate. To improve deposition rate and hardness, an electrical potential is applied to the substrate during the deposition process sufficient to produce a voltage difference of at least 200 volts between the substrate and the crucible.

Referring now in greater detail to the method of the invention, the method is applicable to coating a variety of substrates, including metals such as aluminum, magnesium, iron, and alloys thereof, and to coating metal composites such as tungsten carbide. The particular materials used depends upon the use of the product being manufactured. For example, a coating of titanium nitride on a titanium carbide substrate will produce a very hard product suitable for use in cutting tools and wear parts.

As was the case in connection with the substrates, the nature of the deposit put down will also depend upon the end use for the product being manufactured. Coatings deposited in accordance with the invention may comprise titanium nitride, titanium carbide, zirconium nitride or zirconium carbide for wear parts, cutting tools, or corrosion resistance, etc. Generally speaking, the invention is applicable to any situation wherein it is desirable to produce a hard coating on a relatively softer substrate.

The equipment utilized to carry out the method of the invention may be of any suitable type in which a vapor of the material to be deposited is produced in a vacuum. Heating of the coating material in order to vaporize same may be accomplished by such means as resistance heating, induction heating, or preferably electron beam heating. Equipment for accomplishing high vacuum vapor deposition is known in the art and is commercially available on the market from the Airco Temescal Division of Airco, Inc., Berkeley, Calif. The particular structure of the equipment is not critical to the invention and therefore will not be further described herein.

The substrate to be coated is placed in the deposition chamber either through a suitable vacuum lock, or directly at atmospheric pressure with subsequent evacuation of the chamber. The manner in which the substrate is supported is dependent upon the substrate shape and the evaporation conditions in the chamber. Preferably, the initial pressure in the chamber is of the order of 1 millitorr or less.

In order to enhance the deposition rate, the quality of the deposit, and the hardness of the deposit, an electrical potential is applied to the substrate during deposition. The electrical potential is sufficient to produce a glow discharge. Typically, an electrical bias of negative 200 volts or greater will produce the desired glow discharge. The electrical biasing will also tend to heat the substrate, which is desirable in most cases to enhance the quality of the deposit.

The titanium or zirconium of which a nitride or carbide is to be deposited is evaporated within the evacuated environment. As used herein, the term "titanium" is meant to include pure titanium and titanium base alloys as well. The term "zirconium" is meant to include pure zirconium and zirconium base alloys as well. As is known in the art, the metal thus evaporated will pass in the vapor form within the evacuated environment and when vapor particles thereof strike the substrate, they will condense on the substrate and thereby form a deposit on the surface thereof. The particular conditions under which this occurs are well known to those skilled in the art and therefore will not be further detailed herein.

If the hard coating deposited on the substrate has a substantially higher strength than the substrate material, the respective dimensional responses of the substrate and the coating to thermal or mechanical forces may be substantially different. As a result, very high shear forces may concentrate at the interface between the coating and the substrate. If this interface is weak for any reason, such as poor adhesion or the presence of discontinuities or foreign substances, failure can result.

To avoid such failure, the method of the invention creates a hardness gradient in the coating. Ideally, this is accomplished by making the mechanical and thermal properties of the coating and the substrate at the interface identical. Transition from the mechanical and thermal properties at the interface to the higher strength properties on the outer surface of the deposit are made to occur gradually from the interface to the outer surface. The elimination of thermal and mechanical property discontinuities distributes shear stresses over a volume, rather than concentrating them at a surface plane at the interface, thus rendering the resultant product more resistant to thermal or mechanical cycling.

To this end, the composition of the vapor produced in the evacuated environment is changed gradually during the deposition process to produce a hardness gradient in the deposit which increases outwardly from the substrate. This is accomplished by the introduction of a reactant gas to the vapor gradually increasing the gas pressure, such as from about one micron to about 50 microns. Typically, all that is necessary is to turn on a bleed source and allow a gradual build-up of the partial pressure of the bleed gas in the vapor. The reactant gas may such as to produce a nitride or carbide, and may be acetylene, nitrogen, methane, etc., for example. A reduction of damaging stress concentrations at the interface between the coating and the substrate results.

Referring now to the illustration, the photomicrograph enlarged 325 times illustrates the cross section of a substrate coated in accordance with the invention and illustrating the results of Vickers Pneumatic hardness tests in kilograms per square millimeter taken across the cross section. The substrate is indicated at the very bottom of the illustration and five regions of the coating are indicated at A, B, C, D and E in the photograph. These regions represent different partial pressures of nitrogen present during the evaporation process.

Region A, in which no nitrogen bleed was used, is pure titanium. Region B was deposited at 3 to 4 microns partial pressure nitrogen, region C at 4 to 6.5 microns partial pressure of nitrogen, region D at 10 microns partial pressure of nitrogen, and region E at 12 microns partial pressure of nitrogen. Deposition rates for all regions was 0.0008 inch per minute except for region E, in which the rate was 0.0003 inch.

Evaporation occurred from a pure titanium ingot with a 25 kilowatt electron beam at a starting pressure of 0.01 micron in the vacuum chamber. The temperature of the substrate was 1600.degree.F. Pressure of the nitrogen was increased to the 12 micron maximum in a time period of 40 minutes and then was held constant in the region E. As may be seen, pure titanium deposited at the substrate surface in the region A offered relatively low hardness values, and the Vickers hardness values increased substantially in the regions B through D. Finally, the outer surface offered the maximum Vickers hardness of 2,450 kilograms per square millimeter under a 100 gram load.

The illustration indicates that the surface hardness is substantially increased by depositing titanium nitride in accordance with the invention.

The following table set forth below illustrates titanium nitride hardness as a function of electrical substrate bias and nitrogen pressure. As ma be seen, there is a general tendency in the range set out for hardness to increase as a function of gas pressure and as a function of a negative bias voltage.

TITANIUM-NITRIDE HARDNESS AS A FUNCTION OF BIAS AND NITROGEN PRESSURE Vickers Hardness Substrate N.sub.2 Pressure Number - kg/mm.sup.2 Bias-Volts Microns ______________________________________ 300 0 3 1120 -200 3 1220 -400 3 1290 -800 3 1380 -1600 3 1250 -2500 3 1380 -5000 3 235 0 0.3 770 -200 0.3 840 -400 0.3 800 -800 0.3 840 -1600 0.3 810 -2500 0.3 915 -5000 0.3 ______________________________________

The substrate temperature was 1,600.degree.F and the deposition rate was 1 .+-. 0.3 mils per minute in all cases.

Other titanium base materials which have been successfully deposited include Ti-3Al-2.5V on aluminum substrates with a nitrogen bleed to form titanium nitride. The same material has been coated with the use of a methane and an acetylene bleed to form titanium carbide in each case.

Zirconium has been deposited in accordance with the invention and similar results have been achieved. Hardness values of zirconium nitride deposited by evaporating zirconium with a N.sub.2 bleed were as follows:

Vickers Hardness N.sub.2 Pressure Number-kg/mm.sup.2 Microns ______________________________________ 494 .015 750 .03 494 .05 1625 .3 1950 1 1000 1.3 2325 1.6 2375 1.9 ______________________________________

As may be seen, in the range shown, a general tendency toward increased hardness with increasing partial pressure of N.sub.2 exists. Zirconium carbide may be produced in a similar manner.

It may therefore be seen that the invention provides an improved method for coating a substrate in which very high hardness of the coating is obtained. Relatively high deposition rates may also be achieved in comparison with many prior art methods. A very strong mechanical bond is obtained between the high hardness coating and the relatively softer substrate.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying illustration. Such modifications are intended to fall within the scope of the appended claims.

Claims

What is claimed is:

1. A method for coating a substrate with a nitride or carbide of titanium or zirconium, comprising, placing the substrate in an evacuated environment, evaporating titanium or zirconium from a crucible in the evacuated environment to produce a vapor and causing the vapor to deposit on the substrate initially as titanium or zirconium, applying an electrical potential during deposition sufficient to produce a voltage difference of at least 200 volts between the substrate and the crucible, and introducing a reactant gas with a gradually increasing partial pressure to the vapor to change the composition of the deposit from its initial composition to increasing nitride or carbide in the direction outwardly from the substrate to produce a hardness gradient in the deposit which increases outwardly from the substrate.

2. A method according to claim 1 wherein the reactant gas is nitrogen in order to produce a nitride coating.

3. A method according to claim 1 wherein the reactant gas is methane in order to produce a carbide coating.

4. A method according to claim 1 wherein the reactant gas is acetylene in order to produce a carbide coating.

5. A method according to claim 1 wherein the substrate is biased negative with respect to ground.

6. A method according to claim 1 wherein the substrate is heated during deposition.