4.
DEPOSITION AND APPLICATION OF HARD PROTECTIVE
COATINGS
Miha Čekada
JOŽEF STEFAN INSTITUTE, DEPARTMENT OF THIN FILMS AND SURFACES, JAMOVA 39, 1000 LJUBLJANA,
SLOVENIA
Abstract
The subject of this chapter are hard protective coatings, deposited by physical vapor
deposition (PVD). These are predominantly transition-metal nitrides with a coating thickness
of a few micrometers. Two topics are emphasised. The first is the deposition of hard coatings,
with a short description of physical background of evaporation and sputtering, options for
alloy and compound deposition and metods for enhancing deposition rate and adhesion. The
second part, on application of hard protective coating, first considers the common wear
mechanisms and coating properties used to address them. An overview of established coatings
is given, concluded by general suggestions on proper coating choice for different machining
procedures.
Keywords: hard coatings, PVD, machining, hardness, wear
Povzetek
Tematika tega poglavja obsega trde zaščitne prevleke, nanesene s fizikalnimi postopki
nanašanja iz parne faze (PVD). V glavnem gre za nitride prehodnih kovin z debelino prevleke
nekaj mikrometrov. Poudarek je na dveh temah. Prva je nanašanje trdih prevlek, ki vključuje
kratek opis fizikalnega ozadja naparevanja in naprševanja, variante nanašanja zlitin in spojin
ter metode za izboljšanje hitrosti nanašanja in adhezije. Drugi del, o uporabi trdih zaščitnih
prevlek, najprej obsega pregled glavnih mehanizmov obrabe in katere lastnosti prevlek so
potrebne za rešitev teh problemov. Podan je pregled uveljavljenih prevlek, ki ga zaključimo s
splošnim pregledom, katere prevleke se svetuje za zaščito orodij pri različnih postopkih
obdelave.
Ključne besede: trde prevleke, PVD, obdelovalni postopki, trdota, obraba
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
4.1 INTRODUCTION
Tools are essential in modern industry. They present a sizeable part in the product price, and
in the case of tool failure, major bottlenecks in the production occur. Therefore, in any
technology, there is a need for the tools to last long, operate smoothly and ensure the
requested quality of the product. Due to high competitiveness, tool users are struggling to
push these goals further: longer lifetime, enhanced productivity, less failures and higher
product quality.
These goals can be partly achieved by an improved tool geometry, operation dynamics, or
tool material improvements. The latter option has quite strong limitations. For instance, the
tool has to be ductile to prevent fracture; on the other hand, we ask for high hardness to
decrease tool wear. High ductility and high hardness are contradictory demands – a material is
either hard at the expense of poor ductility, or vice versa.
An optimal answer to this problem is to construct a simple composite. The bulk of the tool is
made of a standard, ductile material, say, tool steel, while it is coated by a hard thin film. The
former ensures resistance to fracture, the latter resistance to wear. In this chapter, we will
discuss a class of thin films, which is distinguished by very high hardness, good resistance to
wear and oxidation. They are called hard protective coatings.
4.1.1 Basic features of hard protective coatings
From materials point of view, we require the hard protective coatings to be inert materials,
while the desired properties can essentially be fulfilled only by ceramics. In practice, the
choice is limited to transition metal nitrides and partly carbides. Such materials can be reliably
deposited as thin films by a group of procedures called »physical vapour deposition«, better
known by the acronym PVD. In contrast to the chemical methods, the starting material is a
solid (say, titanium), while the volatile elements are usually added as a gas (say, nitrogen). A
typical thickness of PVD-coatings is a few micrometers.
In its basic concept, there is a homogeneous coating (called a single layer coating) deposited
on the substrate (Figure 1a). In a more severe environment, there is no coating available to
fulfil all the requirements. In this case we might opt for a double layer coating, for instance
the bottom layer having a high hardness and the top layer a low friction coefficient (Figure
1b). Alternatively, a gradient coating can be used instead of two discrete layers (Figure 1c).
The double layer concept can be repeated several times to achieve a multilayer coating
(Figure 1d). The individual layer thickness can be lowered down to only a few nanometers
(while increasing the number of layers to several hundred). Such a structure is called a
nanolayer coating. Yet another concept is possible, where the process dynamics (e.g. spinodal
decomposition) causes spontaneous formation of a two-phase structure. This is called a
composite coating; or a nanocomposite coating in the case of nanometer-sized grains.
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a) single layer
coating
b) double layer
coating
c) gradient
coating
d) multilayer
coating
e) composite
coating
Figure 1. Basic design of hard protective coatings
4.2 DEPOSITION OF HARD PROTECTIVE COATINGS
As stated in the introduction, the basic idea of physical vapour deposition is to transfer the
material from a source (called a target) onto the surface of a substrate (a tool). One of the
basic requirements is a vacuum environment; to be more precise, the main free-path should be
comparable to the target–substrate distance. This ensures that the atoms released from the
target are able to travel towards the tools. At the same time, we are sure that the coating
contains predominantly the target material, with as few as possible impurities from the
ambient atmosphere.
4.2.1 Basics of evaporation and sputtering
substrate
coating
anode
(substrate)
plasma
evaporated
material
sputtered
material
argon
ions
– +
coating
argon
inlet
+
–
plasma generator
There are many methods, how to release the atoms from the solid target. The easiest to
understand (though of limited practical value) is to heat up the target using electric current
(Figure 2, left). It is not necessary to reach the target material boiling point. Instead, the target
material vapor pressure should be high enough to enable a reasonable deposition rate.
Typically, this means in the range of 1 Pa, which is in standard metals mostly achieved below
the melting point. The process is called evaporation. The coating is deposited throughout the
deposition chamber, not only on our tools. PVD is a line-of-sight process, thus only the
surfaces directly exposed towards the target are coated. Shaded areas are generally not coated.
cathode
(target)
material for
evaporation
current generator
Figure 2. The two basic types of PVD: evaporation (left, resistive heating is shown) and sputtering (right, diode
sputtering is shown)
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
As stated above, the resistive heating does not yield a commercially viable deposition rate.
Far better alternatives include using an electron beam, a laser or a cathodic arc. The cathodic
arc deposition is one of the most common techniques for deposition of hard protective
coatings. In all these alternatives, the main physical principle remains the same, but there are
major technical differences, therefore these methods are often considered separately.
By evaporation, the atoms are released from the target by heating. Alternatively, they can be
ejected from the target by an elastic collision. First, we have to ignite a plasma (Figure 2,
right). This is done by supplying a suitable voltage between the cathode and the anode (about
1 kV) at a suitable pressure (about 1 Pa) of a working gas (typically argon). Under these
conditions, argon is ionized and these ions bombard the cathode (i.e. target) surface with an
energy of up to 1 keV in our case. Each collision ensures that a few target atoms are ejected
from the surface; they reach the substrate surface where they grow as a coating. This process
is called sputtering.
This described principle, diode sputtering, is simple but not technologically useful due to low
deposition rate. There are several methods to enhance the ionization of the plasma and thus
increase the deposition rate. The most common option is the magnetron sputtering. A
magnetron is a device with a circularly arranged magnets, which form a closed loop of
magnetic field, a kind of a magnetic bottle. If a magnetron is placed as a cathode behind the
target, this configuration traps the electrons within the closed loop, just above the target. This
ensures a strongly ionized plasma, which causes heavy bombardment of the target, which in
turn provides a high deposition rate.
4.2.2 Advanced options of evaporation and sputtering
In the previous section, the coating composition was the same as the target composition,
indeed containing a single metal (Figure 3a). If we wish to deposit an alloy, there are several
options: we may use an alloy target of the correct composition (Figure 3b), the target may be
composed of two segments (a segmental target, Figure 3c), or we may use two separate
elemental targets (Figure 3d). The latter technique, called a co-deposition, enables the
deposition of different compositions, which is determined by the power ratio applied on the
two targets.
a) single
target
b) alloy
target
c) segmental
target
d) separate
targets
e) reactive
deposition
Figure 3: Different types of target arrangement
Similar options are available for deposition of compounds. However, compound targets are
often not well suitable, as they may be too brittle, poorly conductive, too expensive, etc. The
alternative way is the reactive deposition. The volatile element is fed as a reactive gas directly
into the chamber, while the metallic element is supplied from the elemental target (Figure 3e).
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For deposition of titanium nitride (TiN), the following reaction takes place on the substrate
surface
2 Ti + N2 → 2 TiN
(1)
The adhesion of compounds, deposited by standard PVD, is often not adequate. Nevertheless,
this can be enhanced a lot by using an additional negative electric potential on the substrates,
called a bias. A part of the atoms exiting the target is ionized. Using a bias voltage, these ions
are additionally accelerated and they impinge on the surface with a relatively high energy
(about 100 eV). By forming a pseudo-diffusion layer, this principle increases the adhesion
substantially. This technique is called ion plating. By changing the bias voltage, the coating
microstructure can be manipulated to a great extent.
Typically, a PVD-coating grows in a columnar microstructure. This can easiest be observed
on a cross-section (Figure 4, left). An even better way to observe this behavior is in a
multilayer coating, where the individual layers act as contours (Figure 4, right).
0.5 µm
1 µm
Figure 4. SEM micrograph of a typical single layer coating (left) and a multilayer coating (right)
The multilayer coatings can be easily deposited by alternatively switching two targets on and
off (cf. Figure 3d). In industrial deposition systems, this »switching« is provided by rotation
of the tools. In this way, a particular spot on the surface is exposed first to one target, then to
another. To ensure maximum homogeneity of the coating, tools are typically rotated in a
complex planetary-type rotation (Figure 5).
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
Figure 5. A typical batch for industrial deposition on cutting tools
Yet another parameter to be varied is the power. In its basic form, it is supplied by DC, at a
constant voltage, which is optimal for metallic targets. For insulators, an AC power supply is
necessary, usually in the radio-frequency regime. For thermally sensitive materials, a useful
compromise is a pulsed-DC mode. It enables a relatively high ionization during the pulse-on
phase, and a relatively long pulse-off time allowing the material not to overheat.
4.2.3 A standard deposition sequence
A commercial deposition is essentially a recipe, i.e. a set of parameters being applied along
the process. The parameters are voltages (applied on the cathode(s), anode, substrates), gas
flows, possible ramps, etc. In old machines this used to be done manually, nowadays it is
common to write a program instead. A typical deposition contains the following phases:
1.)
2.)
3.)
4.)
5.)
Evacuating the chamber to the desired starting pressure
Heating the substrates for degassing and to reach the desired substrate temperature
Ion etching the substrates to ensure a technologically clean surface
Deposition of the coating (in the narrow meaning of the word)
Cooling followed by venting
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4.3 APPLICATION OF HARD PROTECTIVE COATINGS
There is no all-purpose coating, applicable to every machining process or workpiece material.
In contrast, modern coatings are specific, i.e. adapted to a certain technological process to
maximize its impact. Before choosing the correct coating, we must pinpoint which is the most
critical wear mechanism. Afterwards, we find the one coating which best addresses that kind
of wear.
4.3.1 Wear mechanisms and coating properties
Some coating properties are required for any application. A trivial example is adhesion – the
coating should adhere well to the substrate, even in cases of severe mechanical or other loads.
Long-term stability and lateral homogeniety are among those properties too. Other properties,
on the other hand, are application-specific.
By abrasive wear, the main mechanism of tool degradation is the mechanical removal of
chips of the tool by the workpiece. Though the workpiece as a whole might be relatively soft
(e.g. plastics), there may be additives in the material (oxide fillers for instance), which can be
much harder than the tool steel typically used in machining. Chips of the tool itself can act to
abrade the tool further. To prevent abrasive wear, high hardness is the main property of
choice. Hard workpiece materials (and hard additives if present) require an even harder
coating. Often, though not always, the hardness is the principle property used to decide the
correct coating. An alternative approach of abrasive wear reduction is by lowering the friction
coefficient. However, low friction coefficient is accompanied by a low hardness.
By adhesive wear, the main tool degradation mechanism is sticking of the workpiece material
onto the tool surface. This may reduce the workpiece quality (e.g. in extrusion), reduce the
cutting performance (e.g. in milling) or even cause whole parts of the tool to spale off
together with the built-up workpiece material. The coating properties we are looking in this
case are chemical inertness, low surface energy and low roughness.
By chemical wear, there is either oxidation (mainly at elevated temperatures) or corrosion (in
the presence of corrosive media). Chemical inertness is one of the main properties too, but
also electronegativity (for corrosion) and thermal conductivity (for high-temperature
applications).
4.3.2 Common types of hard coatings
While talking about hard coatings, it is inevitable to say a few words about hardness as a
property. It is not a physical quantity per se, but a property defined experimentally, as the
resistance against local plastic deformation. It is measured by the indentation. Table 1
presents hardness of some basic materials and common hard coatings
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
Table 1. Hardness of materials
material
hardness (HV)
stainless steel
250
high-speed steel
900
cemented carbide
1500
TiN
2300
CrN
1800
TiAlN
3000
diamond
10,000
Most hard coatings are transition metal nitrides. There is a limited choice of carbides (e.g.
TiC), carbonitrides (e.g. TiCN, CrCN) or oxides (e.g. Al2O3). Among the metallic elements,
there are predominantely titanium, chromium and vanadium, and as an important secondary
element aluminium. The compounds can be binary (e.g. TiN), ternary (e.g. TiAlN) or a higher
order.
The first hard coating, available since late 1970's is the TiN. From today's perspective it has
not particulary outstanding properties, however, due to its reliability and well-known features
it is still relatively common, though its use is slowly decreasing. The other binary compound
is the CrN, also available for decades. Its main advantage is chemical inertness and low
affinity to sticking. Therefore, it is still used in niche applications.
The first commercially available ternary coating is TiAlN, sometimes written as (Ti,Al)N or
TixAl1–xN to emphasise its variable stoichiometry. The properties depend a lot on the Ti:Al
ratio. Today, this is the standard coating for cutting applications, and a baseline for novel
coatings developed in recent years.
Newly developed coatings namely seldom include new elements. Their average chemical
composition tends not to change much (e.g. around Ti0.5 Al0.5N), however, the main difference
is in the microstructure: multilayers, composites, different grain sizes, etc. The TiN/TiAlN is a
common multilayer, which is distinguished by improved toughness and/or hardness compared
to a single-layer TiAlN.
Different coating producers share the most common coatings, while they may offer many
patented or confidential specifics. These details are often hidden by non-intuitive trademarks,
which makes even a specialist wonder what is the background.
There is a completely different class of hard coatings, called diamond-like carbon (DLC).
Carbon is present in the nature in two very dissimilar forms: the extremely hard diamond
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(containing the sp3 chemical bonds) and the very soft graphite (with sp 2 bonds). If we add
hydrogen, a ternary diagram can be used to show the possible »phases« (Figure 6).
Figure 6: Ternary diagram C(sp2)–C(sp3)–H [J. Robertson, Diamond-like amorphous carbon, Materials Science
and Engineering: R, 37 (2002) 4–6, 129–281]
In principle, using advanced deposition techniques (both PVD and CVD – chemical vapor
deposition), we can deposit most types of coatings within the ternary diagram. By properly
choosing the process parameters, we can get a higher hardness or better lubricating properties.
The prefix »a« stands for amorphous and »ta« for tetrahedral amorphous.
4.3.3 Choice of hard coatings for different machining types
Any job coater has a selection of coatings of his own. If several of them are asked to suggest a
coating for a specific application, they might give different answers, favorizing their specific
coatings. Inevitably, the price plays a role too. Nevertheless, there are some general
suggestions based on scientific research and experiences in the industry.
For cutting applications (milling, drilling, turning) the first coating of choice is TiAlN. For
less demanding operations such as low workpiece hardness (carbon steel) or low cutting speed
(tapping), TiN works fine as well. With increasing workpiece hardness, coating with a higher
hardness should be used: TiAlN with a larger percentage of aluminium (often called simply
AlTiN), multilayer TiAlN-based coatings, nanocomposite silicon-containing coatings. The
thresholds of workpiece hardness to choose a particular coating depends on the coating
producer.
For non-ferrous metals, the solutions mainly depend on the metal (or alloy type). CrN is
usually suggested for machining of copper, DLC for machining of aluminium and diamond
for machining of graphite and Al-Si alloys.
In the case of forming applications, on the other hand, there are less guidelines as the
conditions differ a lot from case to case. A starting point would be TiAlN, but where a low
friction coefficient is asked for, a DLC may be favorable. For hot working of light alloys
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Trajnostne tehnologije kovinskih materialov – Sustainable Technologies of Metallic Materials
(extrusion, die casting), CrN is preferred. In extrusion of plastics, the primary coating is TiN.
If corrosive agents are present, CrN is a better choice, while in the case of hard additives (e.g.
glass fibres), TiAlN is usually taken. In specific cases, different variants of DLC are used too.
4.4 CONCLUSIONS
A user of hard protective coatings is primarily interested in the applicability of the coating for
a specific application, with far less interest in the background. However, such a »black-box«
understanding may substantially limit the user's ability to pinpoint problems and find
solutions. Therefore, a basic knowledge is advisable, how hard protective coatings are made,
what is the basic physical and chemical background of their properties and finally why a
certain coating is useful for a certain machining application. In today's undergraduate studies
of mechanical engineering, materials sciences and similar courses, it is common that the
topics of hard protective coatings has little emphasis, if any. I hope that this short resume will
overcome this disadvantage.
4.5 REFERENCES
For a short tutorial overview as this one it is difficult, and indeed less meaningful to cite
specific papers. There are several review books, which better fit this purpose. A short list of
most authoritative titles on coating deposition and its background is given below. For
industrial practice on the other hand, the experiences depend a lot on the supplier of these
services. While many of them offer their own recommendations, in my opinion one is worth
specifically mentioning here due to its extensive scope; the link is given at the bottom.
D. M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, Second Edition, Elsevier, 2010
M. Ohring, Materials Science of Thin Films, Second Edition, Academic press, 2002
R. F. Bunshah (Ed.), Handbook of Hard Coatings: Deposition Technologies, Properties and Applications, Noyes
Publications, 2001
B. G. Mellor (Ed.), Surface Coatings for Protection Against Wear, Woodhead Publishing Series in Metals and
Surface Engineering, 2006
D. Smith, Thin-Film Deposition: Principles and Practice, McGraw Hill, 1995
Oerlikon Balzers Coating Guide, http://coating-guide.balzers.com/web/en
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