Plasma-enhanced, magnetron-sputtered deposition (PMD) of materials

Surface and Coatings Technology 108–109 (1998) 496–506

Plasma-enhanced, magnetron-sputtered deposition (PMD) of materials a, a a c b c Jesse Matossian *, Ronghua Wei , John Vajo , Gordon Hunt , Michael Gardos , Gary Chambers , c c c d c e Leo Soucy , Dennis Oliver , Larry Jay , C. Michael Taylor , Gary Alderson , Ranga Komanduri , Anthony Perry f a

Hughes Research Laboratories Inc. 1 , Malibu, CA, USA b Hughes Aircraft Co., El Segundo, CA, USA c Hughes Aircraft Co., Troy, MI, USA d General Motors Powertrain Group, Pontiac, MI, USA e Oklahoma State University, Stillwater, OK, USA f AIMS Marketing, San Diego, CA, USA

Abstract PMD combines conventional, balanced magnetron sputtering with an independently generated, arc-discharge plasma to deposit primarily TiN hard coatings for extending the wear life of gear cutting tools as used by General Motors Powertrain (GMPT). The work was begun in 1990 using end-mills coated in an 0.46-m diameter (18-inch diameter) chamber. This showed that the two tool-life controlling processes are deposition temperature and interfacial oxide concentration, control of which resulted in a 2–33 tool life over current commercial coating technologies. In 1994 scaling studies were begun using a 1.22-m diameter32.44-m long (4-ft diameter38-ft long) chamber to achieve this same level of performance for gear manufacturing tools such as hobs and shaper / cutters. Analytical modeling was used to correlate quantitatively the target poisoning and system stability with pumping speed, target power, gas throughout and fixture size for both chambers. The modeling was then extended to upscale the process to a 1.37-m diameter31.52-m high (4.5-ft diameter35-ft high) production prototype chamber with a 900-kg (2000-lb) capacity. This unit was incorporated into a production prototype facility in Michigan featuring tool degreasing and abrasive blast installations for surface preparation. The operating characteristics of the PMD process and the resulting cutting tool performance are presented.  1998 Elsevier Science S.A. All rights reserved. Keywords: Plasma-enhanced magnetron-sputtering; TiN coating

1. Introduction There are three main PVD coating technologies for depositing hard, tribological coatings, such as TiN, onto cutting tools for wear resistance. These are electron-beam (EB) evaporation [1–4], arc evaporation, [3,5–8] and unbalanced magnetron sputtering [3,9–12]. Each of these techniques has been developed for industrial usage and service facilities are available to provide excellent coatings of TiN, CrN, TiAlN, etc., for a variety of tool-wear applications. *Corresponding author. Tel.: 11-310-3175121; fax: 11-310-3175483; e-mail: [email protected] 1 Hughes Research Laboratories Inc., is now known as HRL Laboratories, LLC following the 1997 merger of Hughes Aircraft Company with Raytheon Systems. Hughes Aircraft is now known as Raytheon Systems following the 1997 merger of Hughes Aircraft Company with Raytheon Systems. Work under this project conducted at Hughes Research Laboratories, Malibu, CA, and Hughes Aircraft Company, El Segundo, CA, and Troy, MI, prior to Raytheon merger.

In this paper, we describe an alternative tool-coating technology called plasma-enhanced, magnetron-sputtered deposition (PMD), that was developed at Hughes Research Laboratories [13]. The PMD coating process was developed for extending the wear-life of cutting tools used by General Motors Powertrain (GMPT) in gear fabrication with the intent to provide a competitive edge in cuttingtool performance over conventional coating technologies. PMD is a sputter-based PVD coating technology. It combines conventional, balanced-magnetron sputtering with an independently generated, arc-discharge plasma to deposit hard coatings. The use of a balanced magnetron eliminates the need for magnetic coupling of targets and there is no penetration of the target magnetic field into the coating chamber volume. Scaling of the sputtering process to large-size chambers is therefore relatively simple and straightforward. The use of an arc-discharge plasma allows for large-scale and intense plasma production at low gas pressure and without the need for magnetic fields to be present in the chamber volume.

0257-8972 / 98 / $ – see front matter  1998 Elsevier Science S.A. All rights reserved. PII: S0257-8972( 98 )00632-X

J. Matossian et al. / Surface and Coatings Technology 108 – 109 (1998) 496 – 506

By de-coupling the plasma-production and sputter-deposition processes, ion current densities of 1–10 mA / cm 2 (or higher) for ion bombardment at the substrate, can be achieved independent of the sputter deposition rate selected. The ion-to-atom ratio, linking ion bombardment and deposition rate, can be varied over a wide range (0.5–20 ions / atom or higher) to tailor the coating microstructure for specific applications. The limitation on high deposition rate is overheating of the substrate due to the high ion bombardment current density that is required to match the high deposition rate [14,15]. The PMD cutting-tool coating program began in 1990 and consisted of an infrastructure having three main elements illustrated in Fig. 1; process development and scale up, tool testing, and production implementation. Process development and scale up was conducted at the a-site PMD coating facility at Hughes Research Laboratories in California, while production implementation was conducted at the b-site PMD coating facility in

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Michigan. Tool testing provided the link in the program between the operation of these two facilities. The program infrastructure in Fig. 1 shows how its three main elements are further broken up into various scientificbased, laboratory-based, and factory-based disciplines and where those disciplines resided. For example, expertise from Hughes Research Laboratories and Hughes Aircraft Company (collectively referred to as Hughes in Fig. 1) is in plasma science, tribology, vacuum technology, and thinfilm deposition and characterization. This was combined and linked to outside consultant expertise in hard coatings technology, University partnerships in tool testing and plasma modeling, and factory linkages at GMPT for production implementation. It illustrates a program infrastructure with the required depth and breadth to provide for a systematic flow of technology development from the a-site facility to production implementation at the b-site facility. This established a template for initially developing PMD-based TiN for cutting-tool performance, and then

Fig. 1. PMD program infrastructure for gear cutting tools.

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advancing to other coatings such as TiAlN, multilayer coatings, and etc. The a-site PMD coating facility consists of two coating chambers, a hand-operated aqueous-based clean line for tool de-greasing, and a hand-operated, abrasive-blast unit for tool surface preparation. The two coating chambers included a small-scale, 0.46-m diameter (18-inch diameter) coater for feasibility studies, and a large-scale, 1.22-m diameter32.44-m long (4-ft diameter38-ft long) coater with computer data acquisition and control capability to study scaling and stability criteria, and automated control of the PMD process. Using the 0.46-m diameter PMD coater, the two dominant process parameters affecting tool wear life were identified; deposition temperature and interfacial oxide concentration. Controlling these parameters resulted in a 2–33 improvement in wear life beyond that achieved using conventional coating technologies, such as unbalanced magnetron sputtering, arc evaporation, and EB evaporation. Using both the 0.46-m diameter and 1.22-m diameter chambers, detailed scaling studies of the PMD coating process were conducted, as well as analytical modeling to correlate quantitatively target-poisoning and system stability with pumping speed, gas throughput, target power, fixture size, etc., for any size PMD coater. This modeling was extended to design and fabricate a prototype production coater having dimensions of 1.37-m diameter31.52-m high (4.5-ft diameter35-ft high) and with a 900-kg (2000-lb) load capacity. Actual operation of the coater matched the model-performance predictions within 20% accuracy. A 740-m 2 (8000-ft 2 ) b-site PMD coating facility was set up in Michigan to house the prototype coater and its required ancillary equipment; a fully automated aqueousbased tool clean line, and a semi-automated, abrasive-blast assembly. To review all of the aspects of the PMD program infrastructure presented in Fig. 1 goes beyond the scope of this paper. The main emphasis will be on a description of the scale-up of the PMD coating process from laboratory coaters to the prototype production facility. The science and technology of PMD will only be provided for improved understanding of the scaling process. A detailed review of PMD science and technology issues is in preparation.

2. Process development and scale up The PMD coating process is shown schematically in Fig. 2. It is comprised of a vacuum chamber, vacuum pump, balanced-magnetron target, biased substrate, and an arc-discharge plasma (represented by the coiled filament) that is produced and operated independent of the magnetron target. There are a variety of techniques for producing the arc-discharge plasma (i.e., hot-filament discharge, hollow-cathode discharge, or remote plasma source dis-

Fig. 2. Schematic of the PMD coating process.

charge). All three approaches have been practiced with the PMD coating process to produce plasma, however main emphasis was placed on the use of a hot-filament arc discharge approach for both coaters in the a-site PMD facility, as well as the large-scale production coater in the b-site PMD facility. Process development and scale up of the PMD process shown schematically in Fig. 2 was conducted using the a-site PMD coating facility at Hughes Research Laboratories. Before we describe these facilities and the scale-up from the laboratory to the prototype production facility, we present a short description of the PMD coating process.

2.1. PMD coating process In the PMD coating process shown schematically in Fig. 2, argon gas is used to operate the balanced magnetron target at a selected power level to deposit Ti onto the substrate. Nitrogen gas is introduced to form stoichiometric TiN by operating the magnetron target at the ‘knee’ of its target-poisoning curve [16,17]. An arc-discharge plasma is produced to provide ion bombardment of the substrate that heats the substrate to the required deposition temperature as well as control the coating microstructure. A negative bias (not shown) applied to the substrate controls the energy of the bombarding ion to control microstructure as well as substrate temperature. A partially ionized (about 10% ion-to-total neutral fraction) is produced by applying a discharge voltage between the hot tungsten filament and the vacuumchamber walls. This plasma diffuses to fill the chamber and envelop the substrate. The plasma that bombards the substrate is comprised of a combination of neutrals and ions as shown in Fig. 1. For depositing TiN at the ‘knee’ of the target-poisoning curve, the ion fraction incident onto the substrate is comprised of singly charged atomic nitrogen (20%) and argon ions (60%) as well as doubly charged argon ions

J. Matossian et al. / Surface and Coatings Technology 108 – 109 (1998) 496 – 506

Fig. 3. Photograph of the a-site PMD coating facility.

(20%); there being little or no ionized molecular nitrogen ions. The plasma density, and therefore the ion bombardment current density at the substrate, can be adjusted independent of the sputter-deposition of TiN by adjusting the temperature of the filament (increasing the discharge current), varying the discharge voltage (increasing the ionization efficiency), or varying the total neutral gas pressure (increasing the ion-production rate). Current densities up to 1–10 mA / cm 2 (or higher) can be achieved independent of the deposition rate.

2.2. a -Site PMD coating facility The PMD coating process described above in Fig. 2 was implemented into an a-site PMD coating facility comprised of three elements; two TiN-coating chambers shown in Fig. 3 (the 0.46-m diameter coater on the left and the 1.22-m diameter coater on the right), a hand-operated aqueous-based clean line for tool degreasing, and a handoperated abrasive blast unit used for tool preparation (neither shown). The 0.46-m diameter coater is pumped by a 0.188-m diameter (7-inch diameter) cryopump and has four, 0.08-m diameter magnetron sputter targets located at four quad-

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rants of the perimeter of the chamber. Each target can be operated up to a maximum power of 5 kW. A bifilar-wound 2% Th / W filament is suspended from the top-center of the chamber to produce plasma independent of the magnetron targets operation. A vertically oriented, single-rod rotating fixture is mounted from the bottom flange of the chamber to support tools to be coated. The 1.22-m diameter PMD coater is pumped by two, 0.254-m diameter (10-inch diameter) cryopumps. The vacuum chamber is horizontally oriented and consists of four, 0.127-m wide31.22-m long (5-inch wide34-ft long) magnetron targets oriented at four quadrants of the perimeter of the chamber. Each target can be operated up to a power of 30 kW. A single-strand 2% Th / W filament is suspended along the length of each magnetron target to produce a uniform plasma along the length of the fixture, independent of the operation of the magnetron targets. A horizontally oriented, single-rod rotating fixture is mounted along the center axis of the coater to support tools be coated. Data acquisition and control of the 1.22-m diameter PMD coater is accomplished using a computer system with the NEXTSTEP operating system. Table 1 lists the operating ranges, as well as the nominal values used for the operation of the 0.46-m diameter and 1.22-m diameter coaters.

2.3. Process development and tool testing As described in the program infrastructure presented in Fig. 1, process development is intimately linked to tool testing. To develop the PMD coating process for cutting tools, we identified all the coating parameters that can affect the wear performance of a TiN-coated tool; deposition temperature, ion bombardment current density at the tool surface, TiN deposition rate, ion-to-atom ratio, nitrogen partial pressure, bulk oxide concentration in the TiN film and interfacial oxide concentration (i.e., the oxygen concentration at the interface between the cutting tool surface and the TiN coating deposited onto the surface). We designed a series of experiments for the 0.46-m diameter coater chamber to isolate the effects of these

Table 1 Listing of the operating ranges and nominal values for coating parameters in the 0.46-m diameter (18-inch) and 1.22-m diameter (4-ft) PMD coaters Coating parameter

PMD Coater 0.46-m diameter

TiN deposition rate (mm / h) Ti target power (kW) Arc-plasma discharge power (kW) Ion bombardment current density (mA / cm 2 ) Coating bias voltage (V) Deposition temperature (8C) Coating thickness (mm)

1.22-m diameter

Min / max

Nominal

Min / max

Nominal

0.2 / 10 0.2 / 5 0.1 / 2 0.5 / 10 0 / 1250 400 / 500 0.5 / 5

2 0.60 1 2 40 450 3

0.2 / 10 0.2 / 30 1 / 20 0.5 / 10 0 / 1250 400 / 500 0.5 / 5

2 7.5 10 2 40 450 3

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coating parameters on the wear performance of end-mill cutting-tools and to identify which of them dominate cutting-tool performance. This was accomplished by selecting one of the coating parameters, such as deposition rate, interfacial oxide concentration, etc., and then coating a series of tools for several values of the selected coating parameter while all other coating parameters were maintained constant. By conducting tool-wear tests of the coated tools (to be described below), the effect on tool wear life could be determined for each isolated coating parameter and the dominant coating parameters affecting tool wear life could be identified. For the PMD coating process described in Fig. 2, and the series of coating experiments described above, deposition temperature and interfacial oxide concentration were identified as the dominant parameters affecting tool life. The reason for focusing on the correlation of tool wear life with process parameters rather than microstructural parameters in this study was that the primary objective of the project was to scale the process parameters from the laboratory coating units to the prototype-production unit. By identifying the dominant process parameters affecting tool life, the focus of the scaling issues was then defined and implemented. Once completed, a study of the microstructural parameters of the PMD coating with tool life was begun and will be the subject of a separate publication. Returning to the tool wear tests for the above coating experiments, testing was conducted using controlled, laboratory cutting tests of uncoated, PMD-coated, and commercially coated (EB evaporation, arc evaporation, and unbalanced magnetron sputtering) two-flute end mills having a diameter of 9.53 mm (3 / 8-inch diameter). A batch of 100 end mills was obtained from a single vendor to conduct all uncoated and coated, end-mill wear tests. These wear tests were conducted at Oklahoma State University (OSU) (see Fig. 1) to provide for an independent, expert assessment of tool-wear performance [18]. The cutting conditions (depth of cut, cutting speeds, etc.) were selected to simulate as closely as possible the wear conditions experienced by cutting tools used in gear fabrication at GMPT. The material cut by the end mills was type AISI 5150 steel plates (the same material used to manufacture gears at GMPT) and the cutting fluid was the same as that used at GMPT for gear fabrication. To conduct the wear tests of commercial coatings, four end mills were sent out to each of three identified commercial companies for evaluating EB evaporation, arc evaporation, and unbalanced-magnetron coated end mills. Witness coupons were also sent for analysis of stoichiometry, oxide content, etc. To conduct wear tests of PMDcoated end mills, a total of 40 end mills were PMD coated; four end mills being coated per run in the 0.46-m diameter PMD chamber. Measurements of flank wear versus length-of-cut were used to assess tool-coating wear life. For either PMD-

coated or commercially coated end mills evaluated for wear, the deviation of tool wear within a batch of four end mills coated by a single process was less than 10%, while for wear tests of end mills coated by different processes varied by factors of 2–3 times. For every end mill evaluated for wear, the flank wear was measured every 0.254 m (every 10 inches) until end of life. End-of-life (EOL) was defined after an end mill had experienced 3.8310 24 m (0.015 inches) of flank wear. For all commercially coated end mills, EOL averaged around 1.2760.127 m (5065 inches) of length-of-cut of the 5150 steel plates. This commercial performance provided the basis from which a comparison was made with PMDcoated end mills. For PMD-coated end mills, the deposition temperature was measured during a coating run by spot welding a thermocouple to the cutting edge of a static (non-rotating) end mill that was coated at the same time as the rotating end mills later tested for wear at OSU. The interfacial oxide concentration for the PMD-coated end mills was quantitatively measured using dynamic SIMS to depth profile through the TiN that was deposited onto polished stainless steel coupons coated at the same time as the end mills. The oxygen SIMS signal was calibrated relative to the nitrogen SIMS signal in the TiN using a TiN sample that was ion implanted with oxygen to a known fluence and applying the relative sensitivity factor (RSF) method [19]. The RSF determined in the TiN was used to calibrate the oxygen concentration at the interface. This procedure ignores any variation in the RSF as the matrix changes from the TiN coating to the stainless steel substrate. This may contribute to an inaccuracy in the absolute oxygen concentrations, however, relative changes in the oxygen concentrations can be measured accurately within the experimental repeatability of the measurement technique, which we estimate to be better than 10%. A final note is that Auger depth profiling was used for coupon analysis of both PMD and commercial TiN coatings to measure the stoichiometry. Within the experimental error of the Auger analysis itself, all coatings were stoichiometric. Deposition temperature was controlled by the amount of ion bombardment current density used during a coating run, while interfacial oxide content was controlled by sputter cleaning of the tool surface prior to the deposition of a coating. Prior to coating an end mill in the PMD chamber, they are cleaned in an aqueous-based, ultrasonic clean line to remove the rust inhibitor that resides on the tool after coming from a vendor. We used SIMS analysis to verify that the tool surface had only the characteristic native oxide and carbon surface layers present. This degreasing process had a reproducibility of about 10%, based on SIMS analysis of several tools. When placed in the chamber for coating, the end mills were subjected to an intense ion sputtering to remove the native oxide and carbon contamination layers from the tool surface prior to the deposition of the coating. The interfacial oxide con-

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centration was varied by varying the time of this sputter cleaning at fixed sputter power conditions. Fig. 4a shows the variation of PMD-coated end mill cutting distance at EOL versus deposition temperature. On the right-hand side of the graph, the cutting distance axis has been re-normalized with respect to the EOL cutting distance for commercially coated end mills (i.e., 1.27 m of length-of-cut). By re-normalizing in this manner, the righthand axis becomes a measure of the wear-improvement factor achieved by PMD-coated end mills compared to commercially coated ones. The bell-shaped data in Fig. 4a shows that there is a deposition temperature range centered near 4508C that can result in a 23 improvement in wear life of PMD-coated end mills. For deposition temperatures less that 4008C or greater than 5008C, the wear performance of a PMDcoated end mill equals that of a commercially coated end mill. We are not familiar with other studies demonstrating the bell-shaped dependence of coating wear performance with deposition temperature, as shown in Fig. 4a, however other researchers have found a similar functional dependence of deposition temperature on critical load and hardness of TiN coated metals [20,21].

Fig. 4. Variation of cutting distance with deposition temperature (a) and interfacial oxide concentration (b) for PMD-coated end mills.

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Fig. 4b shows the variation of PMD-coated end mill cutting distance versus interfacial oxide concentration. The cutting distance axis has been re-normalized on the right in the same manner as described for Fig. 4a. For the data in Fig. 4b, upper and lower limits have been drawn in using solid lines. For an interfacial oxide concentration greater than a critical value of about 2310 21 oxygen atoms / cm 3 , PMD-coated end mills achieve a 2–33 improved wear performance compared to commercially coated end mills. Below this critical value, a PMD-coated end mill exhibits the same wear performance as a commercially coated tool. The critical oxide concentration of 2310 21 cm 23 , approximately corresponds to a 4% oxide concentration at the interface given an atomic density of 5310 22 atoms cm 23 . While it is known that sputter cleaning of a tool surface is important for good adhesion of a coating [22], we are not aware of any published quantitative correlation of cutting tool wear performance with interfacial oxide content, as shown in Fig. 4b. For the performance data presented in Fig. 4a, all of the PMD-coated end mill data had an interfacial oxide concentration level below the critical value to eliminate any tool-performance effects due to a non-optimized interface. Similarly, for the data in Fig. 4b, the deposition temperature was maintained in the range of 440–4708C to eliminate any tool-performance effects due to non-optimized deposition temperature. A final note for discussion is appropriate at this time. It is important to understand that for the PMD coating process, interfacial oxide concentration and deposition temperature are the dominant process parameters, control of which results in a 2–33 improved tool life over commercially coated tools. Both criteria of deposition temperature and interfacial oxide concentration must be simultaneously satisfied, as shown in Fig. 4a,b. A logical question is do the same criteria hold for commercially coated tools? We did measure the oxide content of all commercially coated end mills by conducting the same SIMS analysis on coupons coated with the end mills and found oxide levels above and below the critical value in Fig. 4b and their performance did not correlate with oxide content. Returning to Fig. 4a,b, a provocative question is whether the deposition temperature and interfacial oxide concentration values obtained for laboratory-validated end mill tool performance apply to actual gear cutting tools? To answer this question, we compared the wear performance of PMD- and commercially coated shaper / cutters and hobs used by GMPT in gear fabrication. PMD coatings were conducted in the 1.22-m diameter coater chamber at the optimized temperature range of 440–4708C, and an interfacial oxide concentration of 0.5310 21 oxygen atoms cm 23 ; about 43 lower than the critical value. For comparison, shaper / cutters were also sent out to commercial companies for reactive and are evaporation. For the shaper / cutters and hobs, flank wear was used as

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a measure of tool wear, as with end mills, until EOL was reached. However, in contrast to end mills, EOL was defined by the number of qualified gears that are produced by a tool in the factory. Fig. 5 shows the amount of flank wear experienced for three different tooth locations of a PMD-coated and a commercially coated (arc evaporation) shaper / cutter at EOL, after the same number of qualified gears were produced. The PMD-coated tool experienced an average of 2–33 lower flank wear compared to the commercially coated tool which experienced a combination flank wear and coating peel back. The same type of low flank wear behavior shown in Fig. 5 was demonstrated for all the types of shaper / cutters and hobs that were PMD-coated. A total of 30 shaper / cutters and 15 hobs

were coated using the defined PMD coating process and compared to the same number of commercially coated tools. For all PMD-coated tools, the factory wear evaluation demonstrated the same type of reduced tool wear illustrated in Fig. 5. On the basis of these results, the interfacial-oxide and deposition-temperature criteria data for the PMD coating process illustrated in Fig. 4 are applicable to generic cutting tools when coated using PMD.

2.4. Scale-up The two main elements that comprise the PMD coating process include conventional balanced magnetron sputter-

Fig. 5. Comparison of flank wear for a PMD-coated and commercially coated shaper / cutter evaluated under factory gear cutting conditions.

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Fig. 6. Comparison of experimentally measured and model-predicted target poisoning curves for the 0.46-m diameter (18-inch) (a) and (b) 1.22-m diameter (4-ft) PMD coaters.

ing combined with plasma enhancement. We therefore studied the scaling relationships of reactive magnetron sputter deposition of TiN, with and without an arc-discharge plasma present. First, we conducted a detailed literature search of existing modeling studies for conventional, reactive sputter deposition using balanced magnetron sputtering [23–29]. The most comprehensive study that we found was conducted by Berg and co-workers [16,24–26], which links pumping speed, target power operation, surface coverage of TiN on the substrate, etc., with target poisoning. We adapted these modeling equations to the PMD coating process and applied them to the 0.46-m diameter and the 1.22-m diameter PMD coater operation in the absence of an arc-discharge plasma. The modeling involves the solution of seven equations with seven unknowns, requiring as input, factors such as: the chamber pumping speed in the absence of Ti, the sticking coefficient of nitrogen to Ti, the substrate area, target operating power, chamber area, etc. The output is the target-poisoning curve and its stability to changes in system operating parameters. Fig. 6 shows a comparison of the adapted-model predictions and the measured target-poisoning curves for both the 0.46-m diameter and the 1.22-m diameter PMD coaters operated at two different target power levels. The excellent quantitative and qualitative agreement between the experimental performance and the model predictions allowed for understanding and physical insight into the

scaling behavior of the PMD coating process in the absence of an arc-discharge plasma. It also provided for studying system stability and the sensitivity of target poisoning on pumping speed, gas throughput, and substrate area that would otherwise not have been revealed by experiments alone. To understand the effects of the arc-discharge plasma on target poisoning, we operated the 1.22-m diameter coater at selected target power levels and measured the changes in target-poisoning behavior. Fig. 7 shows that for both low and high target power, the presence of an arc-discharge plasma gives additional stability at the knee of the target-poisoning curve. There is only a slight shift in the location of the knee. The results presented in Fig. 6 demonstrate that the adapted models of Berg can be used to very accurately predict the performance of two extremely different size PMD coater systems in the absence of an arc-discharge plasma. With the presence of an arc-discharge plasma being mainly to add additional stability to the knee of the target-poisoning curve, the model equations were then adapted for designing a PMD prototype production coater.

3. Production implementation and tool testing We designed a prototype production coater that could handle large 900-kg (2000-lb) batch loads of cutting tools. Support equipment included a fully automated aqueous-

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Fig. 7. Effect of arc-discharge plasma on target poisoning behavior for two target power levels in the 1.22-m diameter PMD coater.

based clean line for tool de-greasing, and semi-automated abrasive blast unit for tool preparation. A 740-m 2 (8000ft 2 ) b-site PMD coater plant containing this equipment was set up in Ypsilanti, MI. Fig. 8 shows two views of the coater (with and without the mezzanine). A scale size is provided by the human standing beneath the mezzanine in Fig. 8b. The chamber consists of a bottom load design to provide for 3608 access of a retracted load, safety of personnel in handling of large hot loads of tools, as well as to provide for simultaneous, front and rear loading and unloading of parts during coating operation. Because of the bottom load design, a mezzanine assembly was constructed around the chamber for easy access to the top and bottom of the coater for maintenance. A six-man team of technologists from Hughes Research Laboratories and Hughes Aircraft Company, supported by GMPT, integrated the entire facility. This team conducted detailed documentation of its three subsystems prior to conducting coatings of tools. These subsystems included the pumping subsystem, the plasma-production subsystem, and the TiN sputter-deposition subsystem. The pumping subsystem is comprised of two, 0.51-m diameter (20-inch diameter) oil diffusion pumps that are symmetrically located on opposite sides of the chamber, as shown in Fig. 8a. The pumping speed in the coating chamber is fully throttleable from 0 to 5000 l / s, with a

maximum nitrogen-gas throughput capability of 1800 sccm. A Polycold refrigerator coil was used with each diffusion pump to provide for an additional pumping speed of 25 000 l / s for water vapor removal from the chamber. A fully loaded chamber can be pumped from atmospheric pressure to 2310 25 Torr in about 10 min and a base pressure of 5310 26 Torr in about 1 h. The plasma subsystem is comprised of the same tungsten filament design used in the a-site 1.22-m diameter coater; single-strand, 2% Th / W filaments were suspended in front of the sputter targets. An arc-discharge plasma is created with the filaments operated as cathodes and the vacuum chamber operated as an anode. The discharge power supply has a maximum power rating of 60 kW and nominal operating power levels near 10 kW were used in coatings. Ion current densities of 0.5–10 mA / cm 2 at the substrate fixture surface were verified using an array of Faraday cup current probes placed radially and azimuthally at 15 different locations of the substrate fixture for static current density measurements under various discharge power conditions. Plasma uniformity measurements of less than 20% variation were achieved both axially and azimuthally at the surface of the fixture. The TiN sputter-deposition subsystem is comprised of four, vertically oriented, 0.127-m wide31.22-m long (5inch wide34-ft long) magnetron sputter targets (the same targets used in the a-site 1.22-m diameter coater) mounted

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Fig. 8. Photograph of the b-site prototype PMD coater in Michigan without (a) and (b) with the mezzanine. The scale size is illustrated by the human standing below the mezzanine in (b).

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external to the chamber. Two additional targets can be mounted on the chamber if required. The targets were designed for 60 kW maximum operating power. The substrate fixture used in the coater for TiN deposition has a triple-rotating design (one primary planet, four secondary planets and five tertiary poles) for maximizing loading of parts in a fixed volume. Deposition rates of 1–10 mm / h. are achieved. We first duplicated the same coating quality, with low interfacial oxide concentration, for stainless steel coupons placed on the substrate fixture, and then coated actual shaper / cutters. In this phase of the PMD coating program described in this paper, the prototype coater facility was advanced to demonstrate production feasibility. 4. Conclusion We have described the evolution of plasma-enhanced, magnetron-sputtered, deposition (PMD) from laboratory to prototype-production facility. PMD combines conventional, balanced-magnetron sputtering with an independently generated, arc-discharge plasma to deposit hard coatings. The main advantage offered by PMD compared to arc evaporation, EB evaporation, and unbalanced magnetron sputtering is the de-coupling of the ion-production and sputter-deposition processes. This capability allows for ion current densities of 1–10 mA / cm 2 or higher for ion bombardment at the substrate, independent of the sputterdeposition rate. Ion-to-atom ratios of 0.5–20 ions / atom or higher are achieved to tailor the coating microstructure for specific applications. Hughes Research Laboratories developed PMD technology with primary emphasis on the deposition of TiN as a hard coating for extending the wear life of gear cutting tools used by General Motors Powertrain (GMPT). Deposition temperature and interfacial oxide concentration are the two dominant PMD process parameters affecting tool wear life. Control of them during coating results in a 2–33 improvement in tool wear life compared to conventional coating technologies, such as unbalanced magnetron sputtering, arc evaporation, and reactive evaporation. Analytical modeling of the PMD coating process was conducted to quantitatively correlate target-poisoning and system stability to pumping speed, target power, gas throughput, and fixture size. A 1.37-m diameter31.52-m high prototype PMD production coater was designed and built with a 900-kg tool-load capacity for coating tools at a 740-m 2 facility set up in Michigan. Acknowledgements The authors wish to thank Mr John Elverum of Hughes Research Laboratories for technical support in fabricating experimental hardware. We also express appreciation to the business team unit led by Mr Jerry Frost of GMPT and Mr Steve Arrowood of Hughes Aircraft Company. The work

performed under the PMD coating program was conducted at Hughes Research Laboratories and Hughes Aircraft Company prior to the merger of Hughes Aircraft Company with Raytheon Systems in 1997. Partial funding for the program was provided from Hughes Research Laboratories and GMPT.

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