COATINGS FOR CUTTING TOOLS

INTRODUCTION

To reduce tool wear in metal cutting, more than 80% of commercial cutting inserts used in the industry are coated with various coatings. Coatings reduce friction, lower cutting temperatures, increase wear resistance, and thus increase tool life. Most of the research on the effect of coatings on cutting tools is done through experimental studies. The application of FEM simulation for coated tools is still limited due to requirements of large amount of computational time, availability of coating material properties, and the complexity of the analysis when considering the wear of coatings The objective of this research is to develop a FEM-based cutting model for coated tools to evaluate the effects of coating(s) on chip flow and cutting temperature. The process simulation model for cutting with coated tools requires 1) computational efficiency without sacrificing the result accuracy, 2) sensitivity to temperature changes of the tool face due to coatings, and 3) capability of enabling at least qualitative trends.

Cemented Carbide

The Cemented carbide is the complex alloy by sintering the carbide particle of 2a, 3a, 4a tribe metal (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) of the periodic law table, using the iron tribe metal (Fe, Co, Ni) as the binder. These carbide are excellent in oxidization resistance with the high fusion point and high hardness.

It is able to obtain many kinds of cemented carbide by combing 9 kinds of carbide and iron tribe metals, in which it is claiming to be WC-Co sysytem alloy with an usually Cemented Carbide.

At present, the alloy such as the WC-Co system, WC-TiC-Co system, WC-TiC-Ta (Nb) the C-Co system are used as the materials for the cutting tools.

Furthermore, the coating tool that covered with the ceramics such as Ti (C, N), Al2O3 on the suraface of Cementd carbide cutting tool is increasing, as a result that the cutting condition has sped up in recent years, and the use proportion exceeds 50% in the cutting tools.

Diamond

The diamond has the highest hardness, heat conductivity among the substances and excellent in chemical stability, From such excellent characteristic, the diamond is used to various usage.

For the application as the cutting tool, the simple crystal of the diamond is effective to the ultra precision machining of nonferrous system materials, because the extremely sharp edge of a blade is obtained.

The artificial diamond simple crystal is commercialized to the tool for the ultra precise cutting processing such as machining the mold for the laser mirror and magneto-optical disc, optic lens and be supporting it the base of the top industrial field such as electronic industry and optical technology.

The sintered polycrystalline diamond is obtained with some binders such as Co powders by sintering under ultra-high pressure and high temperature(about 5000 atm, several thousand degree of Centigrade) under which condition the diamond is chemicaly safe.

This diamond sintered body shows excellent abrasion resistance with the cutting of the nonferrous metals and unmetallic material and be taking an active part as the high accuracy, high efficiency cutting tool of the difficult processing thing such as the Al-Si alloy and cemented carbide.

cBN(Cubic Boron Nitride)

The cubic Boron Nitride which has the hardness and thermal conductivity after the diamond has small reactivity with the iron system materials and is more excellent in the chemical and thermal stability than those of the diamond.

Because of these properties, the cubic Boron Nitride has been paid attention as the tool material that is useful for machining of iron system materials for which the diamond was unavailable.

cBN is synthesized artificially with the condition of the high pressure and at the high temperature as the same as the diamond.

At present, cBN is manufactured under an ultra high pressure high temperature.

The characteristic of mechanical and themal properties of cBN are influenced largely by the kinds and addition quantities of the binders that are used in the case.

Polycrystalline type cBN has an excellent characteristic for cutting tools to machine the cast iron and heat resisting alloy, and the composite type cBN has one to machine the quenched steel. Making the most of such a characteristic cBN is used to the high efficiency cutting tool of the iron system hard material such as automobile parts such as the engine block and gear, shaft, bearing.

Also the binder-less cBN sintered body which does not include any binder was developed recently.. The binder-less cBN sintered body which is synthesized by controlling the condition of the cBN particle to be minute particle has high thermal conductivity, excellent thermal stability,high hardness and strength even under high temperature.

Because of above reason, the binder-less cBN cutting tool is paid attention as the cutting tool material of the next generation.

Ceramic Coating

The ceramics has many excellent properties such as the wear resistance, thermal resistance, corrosion resistance and they come to be applied widely in these fields. However, the form of the work to coat was restricted, because the machining after coating is very difficult, while ceramic coating has many advantages.

Because of this reason, it becomes possible to make the unique composite material that combines the excellent advantage of the base material and ceramics by Coating of the ceramics.

It becomes possible to improve tool performance, friction performance and life more substantially by the reduction of wear resistance, friction coefficient in comparison with the conventional Coating materials by applying such ceramic coating to the tools, metallic molds and machine parts.

TiN, TiCN, TiAlN Coatings 

We offer Titanium Nitride (TiN), Titanium Carbonitride (TiCN), and Titanium Aluminum Nitride (TiAlN) Coatings on all our cutting tools. 

These coatings, when applied to the appropriate tool for a given application, will accomplish some or all of the following:  1) increase tool life, 2) reduce down time, 3) allow increased feed and speed rates, and 4) reduce tooling costs per job/piece.  In general, coated cutting tools offer substantial cost reductions.

The above noted coating benefits may be diminished due to:  1) an incorrect tool choice, 2) an incorrect coating choice, 3) the parts material, 4) the machine tools rigidity, 5) the spindle and/or tool holder(s) run out, and 6) the type and feed of coolant, if any.

 The following gives more specific information on TiN, TiCN, and TiAlN:

“TiN” Titanium Nitride  

“TiN”: 

The Titanium Nitride Coating is bright gold in color with a surface hardness reaching 81 Rc and a .4 coefficient of friction.  TiN has very good corrosion resistance, heat transmission and excellent wear resistance with a wide range of materials, including iron based materials, hardened steels and stainless.  Forming operations can expect a decrease in galling and welding of work piece material with a corresponding improvement in surface finish of the formed part.  Increased tool life can run 3 to 8 times greater with increased feeds and speeds (finished parts per hour); a serious consideration.  Longer tool life or greater feeds and speeds depend upon the application, coolant, and other conditions.

     --TiN Coated Hardness:  2800 HV

     --Coating Thickness:  2-4 Microns

     --Thermal Stability:  550 Deg. C(1000 Deg. F)

  

“TiCN” Titanium Carbonitride 

“TiCN”:

 The Titanium Carbonitride Coating is bronze in color with a hardness reaching 90 Rc and a .3 coefficient of friction.  TiCN Coating offers improved wear resistance to abrasive, adhesive or difficult-to-machine materials such as cast iron, aluminum alloys, tool steels, copper, inconel and titanium alloys.  As with TiN, feeds and speeds can be increased and tool life can improve as much as 800% depending on the application, coolant, and other conditions. 

     TiCN Coated Hardness:  3000 HV

     Coating Thickness:  2-4 Microns

     Thermal Stability:  400 Deg. C(750 Deg. F)

 

“TiAlN” Titanium Aluminum Nitride 

“TiAlN”:

 The Titanium Aluminum Nitride Coating is purple/black in color with a surface hardness in the upper 80 Rc range with a .3 coefficient of friction.  TiAlN Coating is a high performance coating which excels at machining of abrasive and difficult-to-machine materials such as cast iron, aluminum alloys, tool steels, and nickel alloys.  TiAlN’s improved ductility makes it an excellent choice for interrupted cuffing operations.  Its superior oxidation resistance provides unparalleled performance in high temperature machining.  TiAlN does not exhibit edge brittleness and can be used for interrupted cuts without chipping.  TiAlN Coating should be targeted at applications that generate the highest heat level at the tools cutting edge. 

     TiAlN Coated Hardness:  2800 HV

     Coating Thickness:  2-4 Microns

     Thermal Stability:  750 Deg. C(1350 Deg. F)

PHYSICS AND CHEMISTRY OF COATINGS

Methods of x-ray analysis, mass-spectrometry and probing diagnostics have been used to investigate physical parameters and processes occurring in a mixed gas-metal low temperature nonequilibrium plasma of different composition. It is shown that under given conditions the maximum achievable ionization of metallic vapors (up to 100 %) and high plasma-energy parameters provide for a defect-free strong cohesion of coatings with base materials and result in the formation of assigned structure and properties of coatings with coating thickness reaching 10 to 100 microns.

It is found that, in conditions of slowed adatomic two-dimensional migration and in the absence of kinetic slow-down of metal-metalloid reactions in localized regions of the surface layer, the condensate forms the structures of amorphous and microcrystal types. This process is also accompanied by the formation of strongly supersaturated solid solutions and metastable phases, absent in equilibrium diagrams.

It is shown that the polyenergetic multiphase ion flow, generated by a plasma source, allows to carry out multipurpose technologies of ion implantation at significant depths in order to modify the material surfaces. It has found application to increase the corrosion resistance of materials.

As a result of researches, a number of ecologically clean technologies of coating deposition at low temperatures have been introduced into certain engineering and consumer domains (strengthening of cutting tools and machine components, anticorrosive and protective-decorative coatings, thick-coated molding-type products, high-temperature protective coatings, etc.). For realization of these technologies special installations BULAT, AIR, YANTAR’, POTOK, BAZAL'T and others are designed and constructed.

COATING DESIGN

In many instances, the material which is structurally required for a component does not yield ideal surface properties for the application. Coatings can optimize these surface properties. For example, coatings can improve the wear resistance, biocompatibility, or electrical properties of a surface. A wide variety of coating methods are available, depending upon your specifications. These techniques include plasma spraying, sputtering, reactive evaporation, etc. Coating materials and methods are evaluated for your application. Thin film design services are available; devices include various types of sensors and optical components.

Client Solutions:

Selection of the optimum method of applying titanium nitride coating increased the life of cutting tools.

Thin film temperature sensors were manufactured with sputtered platinum coatings. Plasma sprayed coatings were used to bring worn parts back into mechanical tolerance.

Erosion Resistant Coating

A titanium-based multilayered coating was developed to improve cavitation and particle erosion resistance of steel components. During testing, it was superior to other processes on the market. The coating was also shown to increase the fatigue life of stainless steel components. This coating is covered under patents owned by Material Interface, Inc. and is available for commercialization.

Thin Film Devices

Multilayered thin film devices can be designed for a variety of applications, including semiconductor sensors, protective coatings and biomedical implants.

Lanthanum Hexaboride

Thin film lathanum hexaboride can be applied through a patented coating process to reduce the work function of surfaces and provide high electron emission currents at relatively low temperatures. Applications include flat panel displays and electron optic devices.

ULTRA HARD COATINGS

EXAMPLE:DIAMOND AND DIAMOND LIKE COATINGS

The Potential of Diamond Coatings

Diamond is not only the hardest material known to man it also has a number of other remarkable properties including a very high thermal conductivity

(approximately four times that of copper), high chemical innertness, very low electrical conductivity and, when properly prepared, very low surface friction comparable to TEFLON. Most remarkable of all, however, is the fact that diamond can be created as a thin or thick coating at low pressure and high temperature using plasma enhanced chemical vapor deposition (PECVD) methods. With these properties diamond coatings can achieve exceptional levels of performance in a wide variety of applications including:

ü  High performance cutting tools

ü  Low wear low friction machine parts

ü  Lubricant free bearings for use in harsh chemical environments

ü  High performance heat sinks for electrical components

ü  Prosthetic devices requiring compatibility with blood

ü  Highly scratch resistant coatings for lenses

The potential uses for diamond coatings are virtually unlimited yet as of this time relatively few applications have come to fruition mainly due to technical issues associated with achieving stable coatings on commercially useful surfaces.

Technical Barriers to Commercial Use of Diamond Coatings

With all of its remarkable properties and outstanding applications one has to wonder why diamond coatings are not already in wide use throughout industry. The primary reason for this is the difficulty in achieving stable coatings on surfaces of commercial interest. At present diamond coatings have been successfully applied to a limited range of materials known as the "refractories" which already have physical properties approaching that of diamond. These materials include single crystal silicon, silicon carbide, silicon nitride and similar compounds of tungsten. However, when one attempts to coat diamond onto common surfaces such as ferrous metals severe problems arise due to physical-chemical incompatibilities.

Without careful surface preparation, nucleation of the diamond growth process will fail and no coating will be achieved. Also, even if this problem can be overcome the resulting coating will have very poor adhesion and will quickly delaminate. Finally, even assuming that the interface incompatibility problem can be solved, the resulting coating is still compromised by high levels of intrinsic stress. All of these difficulties result in coatings of marginal quality which must be kept very thin.

Overcoming The Barriers to Diamond Coatings

The staff of the C4 Technologies Company have spent their careers in the microelectronics industry solving the very problems which prevent the successful implementation of diamond coatings. Problems of adhesion and interface compatibility commonly arise when trying to build multilevel wiring structures using insulators, metals and and semiconductor materials. C4 has carried its expertise from the field of microelectronics to that of diamond coatings and has determined effective processes for achieving compatibility of diamond with ferrous metal substrates. In addition, C4 has developed methods for controlling the level of stress buildup in diamond coatings which has led to a patented process for coating diamond onto ferrous metals. Details on the C4 Technologies approach to controlling stress driven failure modes in coatings is outline in the following case study.

CUTTING TOOLS APPLICATIONS

ü  High Speed Steel tools life increased 3 to 30 times.

ü  Carbide tools life increased 2 to 5 times. Deposition temperatures as low as 480-840F (250-450C) protect carbide's binder from deterioration, by comparison with the CVD process applied at more than 1,850F (1,000C).

ü  Isolates the tool from the part, avoids edge buildup and tool cratering.

ü  Reduced friction against workpiece and chips, reduced spindle torque, less vibrations, better finish. 

ü  Speed and Feed increased 10 to 50%.

ü  Reduces or eliminates cooling (with specific coatings).

ü  Repeatable, stable performance of the coatings between batches.

ü  When required, our stripping procedures remove completely old coatings without attacking base steel.

ü  Regrinding and recoating saves 25 to 75% on tooling costs.

ü  1-3 days batch turnaround.

LOW-PRESSURE DIAMOND COATINGS ON CUTTING TOOLS

Research on low pressure diamond  synthesis  started around the same time as the HP-HT synthesis.  Eversole of Union Carbide successfully synthesized diamond using  thermal  pyrolysis  in 1962.  Since  graphite  is the more stable form of carbon under these conditions,  co-deposition of graphite along with diamond and consequent low growth rates  resulted ( a few A/hour).  Also the early  attempts were  fraught  with  difficulties  in  etching  graphite   simultaneously  while depositing  diamond.  Since the growth rates were extremely low, the  commercial viability  was hardly  considered.  Not much  progress  was made till Angus Case Western  recognized  that graphite can be etched  simultaneous as the diamond is deposited if hydrogen in present in the atomic  state.  This clue  prompted  the Russian  researchers  Deryagin and his  associates to carry  vigorously  the low pressure diamond synthesis using hydrocarbon and hydrogen mixture with means for converting   molecular   hydrogen  to  its  atomic   state.  While   significant publication  appeared in the English literature, the developments were not taken seriously in the western world.

Low pressure CVD diamond  synthesis offers an unique  oppertunity for cutting

tool applications in that such thin coatings can be developed more economically. They have been increasing  considered for cutting tool  applications  ever since the  successful  synthesis of diamond at practical  rates (1 mm/hr or higher) by Matsumoto  of Japan in  1982.  Since  then  many  researchers  from  university, industry, and national laboratories all over the world have been addressing this application  with  different  degrees of success.  At OSU we are working on four techniques, namely, combustion  synthesis,  microwave assisted CVD, hot-filament CVD, and laser assisted,  microwave CVD using a range of cutting tool materials, cemented  tungsten  carbide with different  cobalt  content and silicon  nitride.

INCREMENTAL IMPROVEMENTS IN COATED CARBIDE CUTTING TOOLS ADD UP TO IMPORTANT GAINS IN CONSISTENCY AND RELIABILITY

Today, the most dramatic advances in coated-carbide cutting tools for turning can be expected from developments in PVD (Physical Vapor Deposition) coatings. Leading cutting-tool companies, including Carboloy, are now exploring the possibilities of such new PVD coating materials as titanium aluminum nitride (TiAlN), titanium nitride (TiN), and chromium-based and amorphous compositions.

In the well established, and mainstream, area of CVD (Chemical Vapor Deposition) coated carbides for turning, progress continues to be made, but less dramatically. The most impressive gains in this area occurred during the 1970s and '80s, when major advances in carbide, coating and chipbreaker technologies led to quantum leaps in tool life and metal-removal capabilities. These advances were then optimized and organized in the late 1980s in ways that allowed cutting-tool manufacturers to offer families of coated-carbide inserts for turning. One of the first such families was Carboloy's Secolor program for ferrous alloys, which covers virtually any application over a full range of work materials with a relatively small number of carbide grades and grooves. In many cases, such insert families are complemented by coded selection systems which make it easy for customers to choose exactly the right insert grade and groove for a given work material and type of operation.

Today, except for the remaining frontiers of diamond and cubic-boron-nitride coatings, the advances being made in CVD-coated carbides are incremental, rather than fundamental. That is not, however, to minimize their significance. In combination, these smaller improvements are bringing significant gains in cutting-tool consistency, reliability -- and even productivity.

STRENGTHENING THE CUTTING EDGE

New processing capabilities are also being used to increase the performance reliability of coated carbides. An example is a new oxide-coated carbide recently introduced by our company (Carboloy grade TP100) for medium to high-speed machining of steels and cast iron. The first of a planned new generation of turning grades, this oxide-coated grade combines all the best characteristics of two existing Carboloy grades in the P05 to P15 application area, including a multi-layer coating with a thick oxide layer for high cutting speeds and excellent wear resistance. At the same time, the grade marks a significant improvement in the performance reliability and life of oxide-coated inserts.

A problem with most oxide-coated inserts, particularly when machining irons, is that micro-chipping along the cutting edge generates rapid early wear that is both excessive and ragged. If this chippage could be eliminated, the wear generated during the early break-in period would be minimized and both the reliability and useful performance life of the insert increased. This result has in fact been achieved with Carboloy's new oxide-coated grade, which is manufactured with a proprietary process using a new edge-strengthening technology that provides increased chippage resistance without sacrificing wear resistance. The benefit of this advancement -- both for tool life and performance reliability -- is clear from the following example.

Let's assume that a customer is machining a common cast iron where surface finish and part size are monitored carefully. Let's also assume that the customer knows he has to index the insert when it reaches a certain level of wear, or he will lose part size. That level of wear is indicated "A" in the graph (Fig. 1). As the graph shows, with a conventional oxide-coated grade you get rapid early wear as iron build-up on the cutting edge results in micro-chipping. With the strengthened edge on Carboloy's new-generation insert, this chippage is eliminated. Early wear occurs by normal abrasion, rather than chipping, so tool life and consistency are both increased. (See Fig. 2.)

The availability of a tougher cutting edge also increases the insert's range of applications. With the proper chipbreaker, the new grade can be used, for example, for semi-roughing or interrupted cuts, where an earlier grade/groove combination for the P05 to P15 application area ran the risk of chipping. At the same time, the new grade can take these cuts at higher cutting speeds than were previously possible with a tougher alternate grade in the same application area.

IMPROVING SEATING AND HEAT TRANSFER

Advances in grinding technology help to further increase the reliability of the new-generation oxide-coated turning grade. By means of an internally-developed process for which a patent has been sought, the oxide coating is ground off the insert island. This results in three advantages to the tool user. First, insert indexability is enhanced, since the operator can more accurately determine tip location. Second, the insert seats more firmly in the toolholder, eliminating any chance of chipping caused by improper seating. And third, with removal of the insulating effect of the ceramic coating, heat transfer between the insert and toolholder seat is improved, reducing the chance of deformation and cratering.

These advantages of the oxide-removal process are admittedly small, especially when compared to past performance gains made possible by major advances in carbide metallurgy, coating technology and chipbreaker design. Taken together, however, the improvements represent a significant further step toward the more consistent, reliable, and user-friendly tooling that is now a principal focus of cutting-tool product development. Carboloy plans to use the same technique for removing island coatings in the manufacture of all subsequent grades in its new generation of coated carbides for turning.

CONTINUING PROGRESS TOWARD FEWER GRADES AND GROOVES

For the past decade, one important trend in the cutting-tool industry has been the "rationalization" of coated-carbide grades and chipbreaker geometries: i.e. a reduction in the number of grades and grooves required to effectively machine an entire common group of work materials. Thus, for example, all of the major cutting-tool companies have by now introduced a family of coated carbides and chipbreaker geometries to effectively machine nearly all ferrous alloys.

Today, incremental product improvements are helping to further this trend. A case in point is the strengthened cutting edge on Carboloy's new oxide-coated carbide. This enhancement allows extended use of the grade for semi-roughing and interrupted cuts which previously required use of a tougher (though slower) alternate grade designed for the same application area. At the same time, because of the synergistic relationship between grade and chipbreaker performance, the added edge toughness permits use of a freer-cutting (thus more brittle) chipbreaker geometry capable of covering a broader range of machining conditions. The result is that a single grade and groove can now effectively cover the same range of applications that would previously have required two or more grades and grooves.

With additional developments of this kind, it can be expected that future generations of coated carbides for turning will effectively machine the full range of ferrous alloys with even fewer grades and grooves than are currently offered. The payoff for customers will be further reductions in stocking requirements and even greater ease and reliability of insert selection.

NEW GRADES AND GROOVES FOR SPECIALIZED APPLICATIONS

This is not the whole story of progress in coated carbides for turning, however. While the trend continues to further reductions of grades and grooves in families of coated carbides, new coated carbides for special-purpose applications are also being introduced.

One area already well served by special-purpose PVD-coated carbides and chipbreakers is high-temperature alloys for aerospace. Carboloy and other manufacturers have developed complete families of grades and grooves for these applications. Our company has also made it easy for customers to choose the right insert for the job by developing a grade/groove selection chart based on the machinability of the aerospace material and the type of machining to be performed.

Besides aerospace alloys, powder metallurgy materials represent another niche application area targeted by today's coated-carbide product development.

SUMMING UP: QUALITY IMPROVEMENTS ARE KEY

In developing new coated-carbide grades and grooves, cutting-tool manufacturers are making use of the latest advances in coating, sintering, grinding and inspection technologies to achieve a number of small, but significant, quality improvements. They include:

ü  Better sintering control.

ü  Better coating uniformity.

ü  Controlled coating structure.

ü  Tighter honing tolerances.

Along with special advanced processes such as the edge strengthening and island-coating-removal grinding technique used by Carboloy in manufacturing its new oxide-coated carbide, these improvements are helping to make today's coated-carbide inserts more consistent and reliable than ever before. The gains involved are for the most part incremental, not fundamental. But in a manufacturing world dominated by automated operations, just-in-time production, and rigorous quality requirements, they are extremely important.

THE NEXT GENERATION OF PVD COATINGS FOR CUTTING TOOLS

Introduction

Thin films of titanium nitride (TiN) deposited by Physical Vapour Deposition ( PVD ) technology have a wide range of commercial application in such areas as hard coatings for wear resistance, as diffusion barriers and for optical coatings. Claims for improved performance of PVD TiN cutting tools were first publicised in the early 1980’s (1). Over the last decade there has been a growing acceptance of this performance benefit by manufacturing industry, although many cutting tool manufacturers have been less enthusiastic to promote a technology which might be seen as eroding their production volumes. The acceptance has been driven by the need for greater productivity from expensive high-performance machine tools and the demand that the cutting tools perform at optimum efficiency giving minimum machine down-time. In addition, demands for improved quality of machined components can be met with coated tools which, in many cases, can deliver better surface finish and less sub-surface deformation and, hence, better properties.

Current State of Commercial Development

Before any consideration of the next generation PVD coatings, it is important to establish current performance of, and expectations from, TiN since this coating is the flagship of PVD coating technology. Unfortunately, this is by no means a trivial exercise. There are many different commercial PVD systems operating in coating centres worldwide. The major variation in these systems is the method of titanium vapour production, whether electron beam evaporation, arc evaporation and sputtering. Within each of these methods there are different commercial operators using very different process parameters and configurations which must inevitably lead to variations in the final TiN coatings. Hakansson et al (2) in a study of five commercially available PVD methods found that although the TiN films had relatively similar microstructures, the substrate interfacial structures were quite different. In part, these differences were attributed to differences in the pre-treatment cleaning procedures and the use of titanium and/or argon ions during the sputter cleaning immediately prior to coating.

The effects of substrate temperature variations have also been shown to affect the stoichiometry of TiN coatings(3). Another essential feature of the PVD process is plasma assistance controlled by the bias voltage. This parameter has been shown (4) to play a major role in determining the degree of internal residual stress and hence the hardness of coating.

Benmalek et al (5) characterised TiN films deposited by different commercial suppliers using arc evaporation, electron beam evaporation and sputtering. They concluded that the PVD coating properties depend on the nature of the deposition process but more evidently on the deposition parameters used by the coating supplier. They recorded the well recognised observation that arc deposition systems give rise to a higher surface roughness because of droplet emission and that arc deposited coatings are also more textured than TiN films produced by electron beam evaporation and sputtering. They observed a very wide variation in hardness ranging from 1500 HV25 to 3300 HV25 and equally wide variations in the critical loads measured by scratch hardness testing (12.5N to 35.8N).

Given the wide variation in the characteristics and properties of commercially available TiN coatings it is not unreasonable to expect significant differences in the performance of TiN coated cutting tools. In the mid-80's a comprehensive study of Plasma assisted Coating Technology ( PACT ) was carried out at the National Centre of Tribology in the UK. In this study (6) the performance of drills in the bright, steam tempered and TiN coated condition was recorded. Tool life was determined as the number of holes generated up to a corner wear of 200 microns. A wide variation in the performance of TiN coated drills from the different coating/tool suppliers was observed. On average the TiN coated tools showed an improvement of 610% over steam tempered drills and 2,680% over bright drills. However, in the worst cases the TiN coated drills did not perform as well as steam tempered drills. Accelerated testing to screech failure was also performed at a cutting speed of 2,000 rpm and a feed rate of 0.3 mm/rev. These conditions are frequently used by coating centres to evaluate coated tools and as a quality control check. In the latter, the expectation is for a controlled batch of six drills to make 100 holes, noting that the uncoated drill can make only one or two holes. In the tests at NCT the best TiN coated drills could just about meet this criterion whereas the worst TiN could barely outperform the uncoated drills. In a recent study, the present authors carried out accelerated tests on TiN coated drills and end mills. The results of this study are presented in Table 1 and Figure 1. Table 1 shows that under accellerated conditions there is a wide variation in drill life. It should be noted that uncoated drills would be expected to fail in less than ten holes under the severe test conditions. The endmill results shown in Figure 1, which is a test of performance in interrupted cutting, suggests rather less variation in performance. Again it should be noted that the wear ratio for uncoated endmills is 250.

PVD Method		No. of holes drilled
Sputtered	Supplier 1	1,1, 1, 1
“	Supplier 2	268, 27, 33, 182
“	Supplier 3	184, 143, 127, 58, 251, 413
Cathodic Arc	Supplier 4	11, 22, 7, 5, 28, 30
“	Supplier 5	200+, 153, 850+, 300+, 300+
LV electron beam	Supplier 6	562, 419
“	Supplier 7	500+, 46, 216
“	Supplier 8	200+, 300+, 200+, 120+, 200+

Accelerated drill tests carried out on 0.25” quenched and tempered P20 material. The spindle speed was 1250RPM with a feed rate of 0.25mm /rev.

Figure 1. Accelerated endmill test results on quenched and tempered M35 HSS 1/2” diameter endmills. Each bar represents a different manufacturer. The test conditions are: feed rate - 117mm/min; depth of cut - 12.7mm; radial depth of cut - 1.9mm; spindle speed - 710RPM; pass length - 155-170mm; no coolant. The wear ratio is the gradient of a plot of [average flank wear land (mm)]2 versus [pass length x radial depth of cut x number of passes x a geometry constant].

It appears that eight years after the work was carried out at the National Centre of Tribology the performance of PVD TiN coated tools still varies significantly depending, among other things, on the coating supplier. Of course, the ‘other things’ which can affect performance are variability of the tools themselves, ie in terms of geometry and microstructure, and the potential for contamination process from other tools during the PVD. The risk of contamination can be minimised by good quality control practices but, in the commercial world, can never be eliminated.

The above results show that a definitive datum line for the performance of PVD TiN coated cutting tools cannot be unambiguously established. Consequently, one should not be too surprised to find some of the ‘next generation’ PVD coatings not necessarily outperforming some of the better TiN coatings.

Next Generation PVD Coatings

The next generation of PVD coatings of significance after TiN, particularly for the Australian market, are TiCN and TiAlN. These coatings are fast becoming of commercial significance in European, North American and Asian markets. The claim for the superior performance of TiCN over TiN in interrupted cutting is based on lower friction characteristics and better heat transfer coefficients (7). Probably the reason for the slower emergence of TiCN as a PVD coating of significance is related to the processing difficulties, namely, controlling the stoichiometry and residual stress, and choosing the appropriate hydrocarbon carrier gas. Results for the performance of TiCN coatings on drills and end mills are shown in Table 2 and Figure 2. Again, the performance of TiCN drills shows a wide variation with different suppliers. The performance of TiCN coated end mills showed improved performance over TiN with the exception of one supplier.

PVD Method		No. of holes drilled
Sputtered	Supplier 1	107, <1, 109, 544
“	Supplier 2	345, 460+
“	Supplier 3	1, 1, 1, 1
Cathodic Arc	Supplier 4	130, 82, 32, 299, 134, 92
LV electron beam	Supplier 5	214, 466, 513, 470, 432, 302, 241

Accelerated drill tests carried out on 0.25” quenched and tempered P20 material. The spindle speed was 1250RPM with a feed rate of 0.25mm /rev.

TiAlN is growing in commercial significance at a slow rate that TiCN. This may be a reflection of the fact that the PVD system best suited to producing TiAlN is sputtering using alloy target materials and sputtering is not yet widely used by commercial coating centres. The electron beam systems are clearly at a disadvantage because of the large differences in melting points between Ti and Al. At this stage, the rationalisation for the potential of TiAlN is the possible formation of Al203 on the rake face thus giving longer tool life at higher cutting speeds than TiN and TiCN.

Looking beyond the immediate future, it would appear that PVD technology has much to offer. A wide range of binary and ternary compounds can be vapour deposited for a variety of applications. Indeed it is an example of a technological solution looking for problems or commercial opportunities.

Figure 2. Accelerated endmill test results on quenched and tempered M35 HSS 1/2” diameter endmills. Each bar represents a different manufacturer. The test conditions are: feed rate - 117mm/min; depth of cut - 12.7mm; radial depth of cut - 1.9mm; spindle speed - 710RPM; pass length - 155-170mm; no coolant. The wear ratio is the gradient of a plot of [average flank wear land (mm)]2 versus [pass length x radial depth of cut x number of passes x a geometry constant].

COATING	PVD METHOD	COMMENT
Al2O3	Unbalanced Magnetron	Non-crystalline form
CBN	High Voltage Electron Beam	Limited by internal stress
C3N4	Arc	Harder than diamond
C3N4	Sputter, Arc	Hard, low friction
MoS2	Sputtering	"soft" coating
TiN/WBN	Sputtering	Polycrystalline superlattice coatings
Multilayer TiAlN	Magnetron sputtering	Coating thickness up to 20mm

Table 3 shows just a few examples of PVD coatings published in the recent literature. In many instances, these coatings have yet to be further developed before they become commercially viable. For example, Sproul (8) points out that Al203 deposited by PVD is amorphous, which does not perform as well as crystalline Al203 deposited by CVD. Likewise CBN can only be deposited to a thickness of 0.1um before internal stress causes the coating to lift from the substrate. It may be that the problem of build-up of internal stress may be overcome by having alternating layers of very thin coatings.

Hoffmann et al (9) have reported achieving thicknesses of 20um without any problems of adhesion using TiAlN. They also reported a threefold improvement in performance of 6 mm drills over TiN cutting in a test material of 300 Brinell hardness. Sproul reported (10) that the multi-layering of TiN and NbN produced a coating more than twice as hard as either of the coatings when used alone. Indeed, the reported value of 5200 Kgf mm-2 approaches the hardness of cubic boron nitride. It is instructive to step out of the research laboratories and look at the claims of manufacturers of coating equipment. These companies have large investments in keeping their coating centres around the work at the competitive edge of hard coatings for cutting tools. Balzers of Liechtenstein are acknowledged worldwide for producing a high quality PVD coating using their low voltage electron beam technology. They have perfected a high quality and highly reproducible TiN and TiCN with the further option of CrN. Their technology would seem to have limitations for producing coatings such as TiAlN and multi-layer coatings.

Multi-Arc, with coatings based on cathodic arc technology, offer a wider range of coatings such as TiN, TiCN, TiAlN, ZrN, ZrTiN and CrN. Major sales would probably be in TiN, TiCN with anticipated growth in TiAlN. PVT in Germany and Hauser Techno Coating in Holland also offer arc technology concentrating on TiN, TiCN and CrN. The arc coating technology has always been viewed as the PVD technology with potential for development because of the inherently high ionisation levels and ease with which multi-layers can be developed.

The PVD technology, which has probably been least exploited but appears to offer great flexibility for advanced multi-layer coatings, is that of sputtering. Perhaps the most interesting recent development is that of combining arc and sputter (ABS) technology by Hauser Techno Coating. Large commercial systems are on the market with the possibility of coating TiAlN, CrN, TiCN, ZrN, in addition to multi-layer superlattices and metal stabilised carbon. Teer Coatings in UK have spent the last ten years developing coatings by sputtering. Their systems use multiple unbalanced magnetrons which allow dissimilar metals to be sputtered from pure metal targets simultaneously to form alloy nitrides such as (TiAl)N, (Ti,Zr)N, (Ti,Al,U)N and (Cr,Zr)N. The development of multi-layer coatings is relatively easily carried out using the multiple sources. Teer Coatings claim to have developed defect-free alumina coatings and thin films of molybdenum sulphide coatings for machining non-ferrous alloys. The attractiveness of this ‘shopping list’ of coatings should be tempered with the commercial realities that viable market volumes have yet to be identified and established for these new coatings.

Conclusion

From the viewpoint of PVD coatings for cutting tools, it would appear that a range of new coatings will emerge over the next few years. Notwithstanding this inevitable development, it is important to recognise the wide variability in performance of PVD coated cutting tools, which is related to the method of vapour deposition and the different process parameters involved. It may be that a ‘good’ TiN coating may well out perform many of the new generation coatings on offer from the research laboratories certainly in the immediate future.

COATED TOOL GUARENTEES SUPERB RESULTS IN MANUFACTURING:

IMPROVE YIELDS AND PRODUCTIVITY :

            Coated tools last for longer period time. Moreover coated tools enable use of higher parameters.

REDUCE MACHINE DOWNTIME :

            Coating acts as a wear indicator, tools change frequency can now be calculated and planned.

Result: Minimized tool breakage caused by fatigue and over use.

CUT MANUFACTURING STAGES :

            Coated tools allow higher degrees of metal deformation enabling reduction of manufacturing stages. Also cleaning effort is considerably reduced as components with finer surface and with minimal coolant are produced.

REDUCE COSTLY AND TIME CONSUMING FINISHING OPERATIONS :

            Coated tools stay in shape longer and produce top – notch ready to install components with finer surfaces, tighter manufacturing tolerances and better dimensional accuracy. This is important particularly for work – pieces that need to look perfect.

CONSERVE CONSUMABLE :

            The option for environmentally friendlier production range from smaller lubricant quantities to dry production, as coated tools reduce friction, generates less and moreover as coatings sustain high temperature encountered during process. Cleaning can even be eliminated entirely in dry processes – a must in food and pharmaceutical applications.

BE ECO – FRIENDLY :

            With coated tools you use less and work with ecologically safer lubricants. This not only reduces the effort and cost involved in re – conditioning but also makes your Eco – balance look good.

COSTS ACROSS THE BOARD :

            Less is more – particularly in context of tool wear. Coatings make a tangible contribution to cost saving in several respects.

            RESULT : You manufacture with greater cost effectiveness.

CUT TOOLING COSTS :

            With improved productivity, reduced machine down time and higher cycle frequencies.

            Fewer tool failures also reduce load on tool maintenance.

CUT COST OF QUALITY :

            With reduced inspection cost due to reliable process control coated tools invariably produce top – notch ready to install components with finer surfaces, tighter manufacturing tolerances and better dimensional accuracy.

            Also coated tools reduce rejection and generates less scrap.

CUT EXPENSE ON CONSUMABLES :

            Coated tools ensure reliable separation of tool and work piece even under adverse lubrication condition avoiding seizure.

COATING MTL.	Titanium Nitride (TiN)	Titanium Carbonitride (TiCN)	Titanium Aluminum Nitride (TiAlN)			Titanium Aluminum Nitride/Tungsten Carbide/Carbon (TiAlN/WC/C)
Coating Thickness (µm)	1-4	1-4	1-5	1-3	1-4	2-6
Coating Color	Gold	Blue-Gray	Violet-Gray	Violet-Gray	Blue-Gray	Black-Gray
Key Characteristics	Basic TiN hard coating	Enhanced hardness and wear resistance over TiN	Nanolayered coating, high oxidation resistance	High oxidation resistance, hardest nitride coating	Smooth morphology, highest oxidation resistance	Combined hard and lubricant coating layers
Primary Applications	General purpose coating for cutting, forming, plastic molding	Cutting carbon steel, alloy steel and cast iron	Broad-based coating for cutting all steels, cast iron, stainless steel; forming; die-casting; dry machining possible	Specialized for carbide end mills for hardened steel workpieces; dry machining possible	For cutting hard steels (>50 HRC) and aerospace materials incl. Titanium, Inconel; only for carbide tools	For improved chip flow and possible dry machining e.g. in drilling, tapping, avoids buildup edge