Field of the Invention

The present invention pertains to thin, durable surface coatings and processes for providing such coatings on the surfaces of machine tools and the like.

Background of the Invention

Machine tools are required to perform a variety of forming and material removal processes. For example, such tools are commonly used to repetitively perform cutting, punching, chipping, milling, reaming, drilling, and other functions. Dies are used commonly in forming and punching operations.

Tools wear constantly upon use and must be repaired or replaced often depending upon the severity and frequency of usage. In order to improve hardness, wear resistance, and toughness properties, these tools may be surface coated with a hard TiC, AI2O ₃ , TiN, etc. layer. Coating methods vary from Chemical Vapor Deposition (CVD) methods to arc plasma spraying to various Physical Vapor Deposition Methods (PVD) including sputtering and electron beam evaporation.

Recently TiN coatings on high speed steel tooling have proven effective in improving tool hardness and wear properties. TiN coatings are now commonly used on the surfaces of gear cutters, gear-shaper cutters, drills,

reamers, taps, chasers, spade-drill blades, broaches, saw blades and for other tools. These coatings are provided by evaporative and sputtering methods in which atoms, ions and/or neutral or charged clusters from a titanium source (i.e., target) in a vacuum chamber are ejected onto the tools that are positioned in the chamber proximate an anode. Inert gas such as argon is admitted to the evacuated chamber and, upon creation of an electrical field in the chamber, serves as a source of excited plasma ions which bombard the target to dislodge the target (i.e., coating) material. Nitrogen is also admitted into the evacuated chamber and serves as a reactive gas which, along with the Ti materials ejected from the target, forms the desired TiN coating on the tool. Typically, the process is a line-of-sight type operation in which the ejected Ti particles traverse the chamber in a straight line, requiring rotation of the tools in the chamber to provide a uniform coating thereover.

Although TiN surface coatings have provided significant improvement in machine tool life, there remains a need to provide machine tool surface coatings which outperform TiN coatings in wear resistance and in heat resistance.

Of further importance is the need to provide improved coatings in a process in which the formation of undesirable impurities such as carbon particles, sodium, potassium and calcium, in the coating is inhibited.

Additionally, there is a need to provide a simplified coating process in which the need to rotate the substrates during the process to attain coating uniformity is eliminated.

Summary of the Invention

In accordance with the invention, hard, durable titanium containing coatings are provided on the desired substrate by a cathodic sputter coating process that is operated under controlled process parameters. Briefly, the desired substrates, such as tools, airfoils, turbine blades, etc. are loaded onto a stationary jig that is placed within the sputter chamber.

Three electrode cathodic sputtering is conducted in the chamber with a biasing electrode provided as the third electrode. A titanium alloy target, as explained in more detail hereinafter, is positioned in the chamber and serves as the cathode. The target substantially surrounds the substrates in the chamber. In accordance with conventional cathodic sputtering techniques, the stationary jig and associated substrates are charged as the anode, with a potential difference of from about 500-1000V existing between target cathode and anodic substrate. The chamber is evacuated to vacuum pressure of between about 10 ^_1 Torr. to 10 ^~3 Torr.

An inert gas, preferably argon or xenon, is backfilled into the evacuated chamber to serve as a source for formation of excited plasma ions upon creation of an electrical field in the chamber. These ions, in accord with conventional sputtering techniques, strike the target to dislodge atomic or molecular coating species therefrom.

In an especially unique aspect of the invention, both nitrogen and oxygen are admitted into the chamber substantially concurrently to serve as reactive gases for formation of the desired coating on the anodic substrates, this being done in a non-magnatron DC sputter system. In contrast, conventional coating processes use only one or the other of these elements to form a coating on the desired substrate (ref. U.S. Pat. Nos. 4,714,660; 4,871,434 and 4,973,338) .

Improved sputtering also results from a biasing electrode whose potential is maintained at from -22 to -23V relative to the anodic substrates. Although applicant is not to be bound to any particular theory of operation, it is thought that the electrical field between the anode and less positively charged biasing electrode provides for a second or "mini" sputtering in the assembly in which impurities, such as carbon particles, sodium, potassium and calcium which would otherwise incorporate into the coating, will eject from the anode and migrate to the bias electrode.

The invention will be further described in conjunction with the appended drawings and detailed description.

Description of the Drawings

Fig. 1 is a schematic sectional diagram of a three-electrode sputtering chamber used in accordance with the invention;

Fig. 2 is a schematic depiction of the anode and biasing electrode of the sputtering chamber; and

Fig. 3 is a cross-sectional view of a substrate coated in accordance with the invention.

Detailed Description of the Preferred Embodiment

Turning now to the drawings and specifically to Fig. 1 thereof, there is shown a sputter ion plating assembly 2. Assembly 2 is preferably Model GLO-TINE 24X36 Abar-Ibsen, from Abar-Ibsen Company but could also be any other commercially available ionic sputter chamber assembly provided it is capable of achieving or can be modified to attain the sputter coating process parameters that are detailed herein. Assembly 2 comprises a vacuum vessel 4 and lid 6 which together provide a sealed environment within for provision of vacuum conditions necessary for the sputtering operation. A Cathodic target 8a, 8b, 8c is provided in the assembly and, as shown, is shaped to substantially surround

substrates 22 that are disposed on stationary mount 20 fitted through lid 6 of the assembly. The cathodic target comprises relatively spaced apart inner and outer cylindrical wall liners 8a and 8b, respectively, and a circular bottom wall liner 8c, all formed from titanium alloy, as described below. The inner cylinder 8a is placed on the bottom wall liner 8c in the center of the outer cylinder liner 8b which is closely surrounded by the vacuum vessel 4. The spacing of the cylinders from each other is dependent on the size of the substrate to be deposited and is arranged to form a circular opposing cathode and to maintain the substrate in the middle of the negative glow region of the glow discharge and not in the cathode-fall region.

Heating elements 10 are provided for heating the gaseous environment, thereby suppressing homogeneous nucleation of particles of the material in the gaseous environment. Homogeneous nucleation occurs readily at high concentrations of vapor species within an ambient gaseous environment and is to be avoided, as disclosed in U.S. Pat. No. 4,236,994. Its occurrence gives the deposited material a porous, friable and columnar structure characteristic. Cooling fans (not shown) or other equivalent means are used to cool the vessel. Biasing electrode 24 is positioned above mount 20.

Vacuum connection 12 communicates with the interior of assembly 2 to provide the vacuum conditions required for

the coating operation. Gas lines 16 and 18 provide a source for entry of the desired inert gas, preferably argon, that is necessary to form the excited ion plasma cloud necessary for sputter coating. Gas lines 16 and 18 also serve as entry ports for the reactive gases, oxygen and nitrogen, respectively. Alternatively, a common entry port for all three gases could readily be provided.

The assembly is operated as a three-electrode sputter coating apparatus in which the target 2 serves as the cathode, with the mount 20 and associated substrate 22 charged with a positive D.C. current of from about +500 - +1000V to serve as the anode. I have found that maintenance of biasing electrode 24 at from -22 to -23V less than the voltage of anode mount 20 provides enhanced coating in that impurities which may otherwise coat onto the substrate are not normally formed thereon. A potential difference of -22.6V is most preferred. Further, use of the biasing electrode tends to provide increased toughness micronodular coating structures compared to unbiased coatings. The target 8 is preferably maintained at ground but a negative voltage potential could also be imparted thereto.

In accordance with the invention, target 8 is composed of either Ti-6 A1-4V alloy or Ti-8 Al-1 Mo-lV alloy. The former alloy is presently preferred based on preliminary testing. Both of these alloys can be purchased

from a variety of sources, including Titanium Alloys, Pittsburgh, PA. The target materials are rolled and machined into the desired shape for use as targets 8 in the assembly. orkpieces or substrates 22 are preferably cleaned prior to operation of the sputtering coating process. In this regard, any one or more conventional cleaning processes, including alkaline cleaning, electrolytic cleaning, emulsion cleaning, solvent cleaning, acid cleaning, pickling, abrasive blast cleaning and salt bath descaling can be employed.

The cleaned substrates 22 are then loaded onto the stationary mount 20 that serves as an anode in the process. One surprising aspect of the invention is that adequate throw or carry of the coating material is provided such that the coating will substantially uniformly coat over the substrate without requirement of rotating the substrates 22.

A vacuum of from about 10 ^-1 to 10 ^-3 Torr. is provided in the enclosure. Preferably, a vacuum of 10 ^-2 Torr. is provided. This low pressure or "soft" vacuum is thought primarily responsible for the aforementioned excellent 'throw" or carry of the sputtered coating particles. The envelopment of the substrates by the target also aids in this phenomenon.

During sputtering, argon is fed to the assembly within a range of from 300-460 SCCM, with 425 SCCM being a preferred feed rate. Initially, nitrogen is fed to the

enclosure with oxygen feed commencing within about 1/2 hour and continuing concurrently with the nitrogen feed. The nitrogen is fed to the enclosure within a range of about 12.5 to 13.5 SCCM with oxygen feed rates being maintained at from about 10 to 14 SCCM. The molar ratio of oxygen:nitrogen fed into the enclosure may range from about 1:1 to about 1.25:1. Preferably, the two reactive gases are both fed at 13.1 SCCM resulting in a slight molar excess of oxygen relative to nitrogen. The sputtering process proceeds for about 5-8 hours.

The sputtered coatings of the invention show significant improvement over prior art TiN coatings in regard to heat and wear resistance of cutting tools coated thereby. Accordingly, at present, the processes and coatings of the invention are used to coat toolings and other parts in which heat, abrasion and shear resistance are important. The substrates 22 can be composed of a plurality of different metals ranging from a wide variety of steels, stainless steels, titaniums, carbides and inconel-type alloys and ceramics such as AI2O3, SiN and Siθ2- Although the substrates are preferably to be used ultimately as machine tooling products, including dies and gears, other substrates that may be beneficially coated in accordance with the invention include turbine blades, airfoils, etc.

SUBSTITUTE SHEET

The preferred Ti-6 A1-4V alloy target has the following composition.

N .012 (percentages given by weight)

C .01

O .128

Fe .18

Ti 90.06

Al 5.9

V 3.7

H 80 ppm

V less than 50 ppm

As to the Ti-8 Al-1 Mo-lV target material, this has the following composition:

N .015

C .01

O .125

H .125

Ti 89.175

Al 8.0

V 1.25 Mo 1.25

When the above target materials are sputtered in conjunction with the conjoint use of both N2 and O2 as reactive gases, thin, durable coatings on the order of 2.0 x 10 ^-5 to 8.0 x 10 ^~ 5 inches in thickness are formed. In fact, preliminary results in connection with one test run indicate that the coating thickness was 6.5 x 10 ^~ 5.

Coatings resulting from sputter coating with the above targets and reactive gases may be described as including the elements Ti, Al, V, Y, O and N, with Mo also being present when the Ti-8 Al-1 Mo-lV target material is used.

When the preferred Ti-6 A1-4V target is used, Ti, Al, and V are present in oxide or nitride form. The formula Ti V AIO2N is used for simplification. Similarly, when the Ti-8 Al-1 Mo-lV target is used, Ti, Al, and Mo are present in oxide or nitride form. The formula Ti V Mo AIO2 is used for simplification.

Turning now to Fig. 2, there is schematically shown substrate 22 and bias electrode 24 during sputtering. Since the bias electrode 24 is maintained at from -22 to -23V lesser potential than the substrate (e.g., anode) 22, a "mini" or second sputtering causes the excited Ar plasma to impinge upon substrate 22, dislodging impurities, such as C, Na, K, and Ca therefrom and provides a dense equiax nodular microstructure.

Fig. 3 illustrates substrate 22, here shown as a drill bit, coated on its surface, with a uniform thin, layer of coating 26 thereover. Coating 26 has a thickness of from 2.0 x 10~ ⁵ to 8.0 x 10~ ⁵ inches.

The following table is illustrative of the increased longevity of tools coated with the above-described Ti V AIO2N material over identical tools coated with a conventional TiN material.

TOOL WORK MATERIAL PIECES MACHINED

OR OPERATIONS PERFORMED BEFORE RESHARPENING

TiN Ti V A10 ₂ N COAT COAT

INSERT CAST IRON 134 402 (TNMG-432)

105 1029

35 156

39 191

100 175

150 1817

150 240

30 120

175 800

1500 10,870

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the spirit and scope of the invention.

SUBSTITUTESHEET