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.
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+
|
|
|
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
|
|
|
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
|