Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology

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Nanocomposite Coatings for Severe Applications 681

arc-bond coating and high-power impulse magnetron sputtering (HIPIMS), as well as the use

of special bond or transition layers that effectively reduce internal stress build-up at the

interface and hence the possibility of coating delamination.

Owing to their very small grain sizes (ranging from a few nanometers to 100 nm), the surface

ﬁnish of nanocomposite or nanostructured coatings is much smoother than that of conventional

coatings. Unlike their monolithic counterparts with large columnar grains and typically highly

textured crystalline orientation, nanocomposite or nanostructured coatings do not contain

crystalline facets or uneven growth patterns that can cause roughening of the coating surface.

Nanocomposite and nanostructured coatings maintain their properties uniformly when applied

to complex geometry substrates (drills, mills, turbine blades, gear teeth, biomedical implants,

etc.), since there is no preferential growth directions in their nanocrystalline constituents and,

thus, no associated anisotropy of the coating properties. Furthermore, they are free of large pits,

pinholes, protrusions, and other surface and structural defects. Because of their nanostructured

and composite nature, these coatings can be produced with a wide range of physical, chemical,

thermal, mechanical, and tribological properties; hence, when used in industrial applications,

they can provide multiple functionalities. For example, they can dramatically increase

hardness and hence reduce wear of underlying substrates under harsh sliding or machining

conditions while at the same time providing low friction and excellent protection against

corrosion or chemical degradations. They can also provide superior thermal, optical, magnetic,

biomedical, electronic, catalytic, and surface acoustic properties [7, 10, 11, 16–18].

Because of their unique properties and increasing popularity, nanostructured and composite

coatings have been the subject of numerous books, journal articles, reports, research papers,

etc. This chapter is intended to provide an overview of recent developments in these coatings.

They are already used in large volumes for machining and manufacturing applications, and

new applications are emerging in the aerospace, transportation, and biomedical ﬁelds. The

main emphasis of the chapter will be on the fundamental tribological mechanisms that control

their superior friction and wear properties at severe operation conditions (high surface loads

and temperatures, lack of lubricants, and aggressive oxidizing environments). It is obvious that

future tribosystems will be subjected to much more stringent operation conditions than before,

mainly because of the increased power density dissipated (e.g. cutting tools) or transmitted

(e.g. gears and bearings) at mechanical interfaces, and because of the trend toward reduced

size and much higher mechanical and thermal loadings at the contact area. To overcome these

challenges, new coatings are urgently needed with a capacity to further improve durability and

performance and to adapt to the much harsher and rapidly changing operation conditions of

future mechanical systems. Accordingly, in the following sections, we provide an overview of

recent developments in the design and deposition of nanostructured and composite coatings.

We also review the structural and mechanical properties of such coatings that are important for

their performance in various applications.

682 Chapter 14

14.2 Advances in Deposition Processes for Nanocomposite

Coatings

Over the years, PVD and CVD processes, such as sputtering, ion plating, cathodic arc, pulsed

laser deposition (PLD), ion-beam assisted deposition (IBAD), and plasma-enhanced chemical

vapor deposition (PECVD), have undergone signiﬁcant modiﬁcations in their size,

conﬁguration, and power source requirements to become very robust and reliable and to allow

large throughput. As a result, the cost of making nanostructured and composite coatings with

such processes has decreased substantially and made them more affordable. With recent

modiﬁcations in their internal design and conﬁguration (such as sample holders, multiple

modes of rotation, strategically positioned cathodes or sputtering targets, and additional

ionization and/or evaporation sources), it has become much easier to achieve high deposition

rates and more uniform and complete coverage on uneven or odd-shaped workpieces. The use

of multiple targets or evaporation sources has enabled the production of novel nanocomposite

coatings with complex microstructures and chemical compositions that provide superhardness

and other impressive mechanical properties [4, 6, 7, 19, 20]. At present, reactive magnetron

sputtering with pulse-direct current (DC) and HIPIMS capabilities and standard and/or ﬁltered

cathodic-arc PVD techniques are used extensively by industry for the deposition of

conventional and nanocomposite coatings [9, 21–24]. Other methods, such as PLD and IBAD,

are also used but at much smaller scales for the production of nanocomposite coatings. With

the introduction of femtosecond laser sources, the PLD method has gained new interest mainly

because of the better control of the ablation processes, resulting in fewer droplets and other

irregularities on the coating surface [25]. Figure 14.2 shows schematics for the three advanced

methods currently used to produce most of the nanostructured or composite ﬁlms.

14.2.1 Hybrid Deposition Processes for Nanocomposite Coatings

For the production of nanostructured or composite coatings, it may be necessary to combine

two or more of the deposition methods mentioned earlier in one system. Furthermore, the use

of high-ionization power sources like HIPIMS and cathodic arc deposition is important for

producing coatings with strong bonding, dense structure, and superior mechanical properties.

For example, in such systems, one can do sputtering and cathodic arc PVD individually,

sequentially, or both together. One can also switch between the standard and HIPIMS modes

to achieve unique structural features. With the use of HIPIMS or cathodic arc methods, one

can also achieve robust interface mixing and, hence, strong ﬁlm-to-substrate bonding, which is

extremely important for load-carrying capacity and durability. Subsequently, the sputtering

mode may be used to achieve a dense and uniform coating morphology with smooth surface

ﬁnish. The use of ﬁltered cathodic arc or HIPIMS sources may further help in achieving a

smooth surface ﬁnish on the coated parts. Alternatively, one can use both sputtering and

arc-PVD at the same time to take advantage of both methods in not only achieving a strong

Nanocomposite Coatings for Severe Applications 683

Figure 14.2: Schematic representation of (a) hybrid magnetron sputtering and cathodic arc PVD

system (courtesy of R. Dielis, Hauzer Techno Coating), (b) closed-ﬁeld magnetron sputter ion

plating system (courtesy of D. Teer, Teer Coatings, Ltd), and (c) ﬁltered cathodic arc PVD system

(courtesy of P. Siemroth, Arc Precision GmbH).

bonding but also the rapid deposition of a dense and uniform coating. These methods are

commonly referred to as ‘hybrid’ deposition processes, and they are ideal for the production of

nanocomposite and nanolayered coatings with duplex, multiplex, or superlattice structures.

Again, in these more advanced processes, the physical size or interior architecture of the

deposition systems is slightly altered to accommodate additional targets and appropriate power

sources (cathodic arc, pulse-DC, HIPIMS, etc.) in order to permit the ﬂexibility of producing

coatings by one of the methods alone, two or three of them in sequence, or all of them together.

Most nanocomposite or nanolayered coatings that are in use today are produced by sputtering

and arc-PVD techniques, with a smaller fraction being produced by electron beam or pulsed

laser evaporation. As mentioned above, either of these techniques may be used alone or

simultaneously to achieve a graded, multilayered, or nanocomposite coating architecture. For

example, W/C or Ti/C multilayer coatings are produced by sputtering or evaporation of W and

C or Ti and C targets in sequence at short durations during deposition (Figure 14.3a) [26, 27].

For the deposition of nanocomposite W–C or Ti–C ﬁlms, the sputtering or evaporation rates of

684 Chapter 14

Figure 14.3: TEM images of (a) nanolayered Ti/C [27] and (b) nanocomposite W/C:H ﬁlms

(courtesy of Hauzer Techno Coating).

targets are adjusted to result in a discrete phase of each ingredient or their reaction products

(i.e. WC or TiC phases) embedded in a matrix of diamond-like carbon, as shown in the

transmission electron microscopy (TEM) image in Figure 14.3b. Hybrid methods have opened

interesting possibilities to embed temperature-sensitive materials inside coatings, which before

had required high growth temperatures. One example is a uniform dispersion of MoS

nanograins in an alumina oxide layer [28].

The successful design, synthesis, and production of nanocomposite ﬁlms for large-scale

applications require proper selection and control of several deposition parameters [20, 29, 30].

Such a practice is essential for maintaining the chemical, structural, and mechanical integrity

of the workpieces that are being coated. One of the major issues in most modern deposition

technologies is the unintentional heating of substrate materials. This may cause degradation of

the mechanical properties of such materials, especially upon exposure to temperatures that are

much higher than the tempering temperatures [31]. In the case of arc-PVD and HIPIMS,

overheating is always a possibility, mainly because of the high energy and high power density

of these processes, but this problem can be effectively controlled by adjustments to process

parameters (e.g. use of pulse regimes, independently of the substrate heating and cooling

arrangements). For some tools and bearing steels like AISI 52100 and 440C, the tempering

temperatures are less than 200

◦

C. If these materials are heated above this temperature during

coating, their hardness values decrease considerably, and they may not be able to support the

hard coating on their surfaces and fail quickly during actual use. In the past, low deposition

temperatures had a large negative effect on the adhesion and microstructural density of the

deposited coatings. Poor adhesion to the substrate, poorly connected columnar structures

perpendicular to the substrate, grain-boundary voids, and defect networks were typical of the

coatings produced at low deposition temperatures by conventional PVD deposition. Such

coatings were not able to withstand the high cyclic loads of many manufacturing or

Nanocomposite Coatings for Severe Applications 685

transportation applications. As mentioned earlier, with the recent use of highly energetic

plasma technologies like HIPIMS and cathodic arc deposition, such adhesive and cohesive

problems have been effectively overcome, and the current coatings possess strong adhesion to

substrate materials as well as strong cohesion between grains.

In the case of CVD processes, because the temperatures are typically much higher, CVD may

not be a good choice for heat-sensitive substrates. If a low temperature CVD is used, a

postdeposition hardening heat treatment is often a must, but depending on the coating type,

major problems may occur on the hard coating itself. Chief among them is severe oxidation or

partial delamination of coatings from the substrate surface due to chemical reactions and

thermal distortions. High-speed steels, certain intermetallics, and cemented carbides can safely

be exposed to deposition temperatures as high as 450

◦

C without major structural, chemical, or

mechanical degradations. In contrast, for the deposition of nanocomposite coatings on

titanium-, aluminum-, and magnesium-based engineering materials, one has to use relatively

low temperatures; otherwise, major degradations or distortions may take place in such

substrates. Because of their light weights, these materials are much desired for structural

components in all kinds of transportation applications. There is also interest in using these

materials in tribological and mechanical applications for aerospace and biomedical needs.

However, mainly because of their high sensitivity to heat, extra precautions must be taken to

avoid degradation in mechanical properties or loss of structural integrity of light alloy

substrates.

As already mentioned, recent advances in power supplies have led to much higher ionization

efﬁciencies and plasma current densities in PVD systems. Cathodic arc PVD, closed-ﬁeld

unbalanced magnetron sputter ion plating, asymmetric polarity pulsed-DC sputtering, and

HIPIMS processes are good examples of techniques yielding high ionization efﬁciencies and

plasma densities. With these advances, it is now possible to maintain substrate temperatures at

acceptable levels and yet attain dense microstructures and strong adhesion in between many

coatings and their substrates. Among others, HIPIMS and droplet-emission-restricted cathodic

arc technologies represent a most recent trend and are gaining increased acceptance and

applications in many ﬁelds. All in all, the high ionization efﬁciency of these novel methods

provides high mobility and, hence, surface and bulk diffusivity of impinging ions and/or

atoms. These conditions can positively inﬂuence nucleation and growth processes to produce

strong ﬁlm-to-substrate adhesion, dense ﬁlm morphology, smooth surface ﬁnish, and nearly

perfect chemical stoichiometry, even at relatively low deposition temperatures.

14.2.2 Modern Coating Practices for the Synthesis of Nanocomposite Coatings

In today’s world, PVD and CVD methods are used extensively for a variety of industrial

applications and in large volumes to improve the performance and durability of machine tools

and other critical components [32–34]. In particular, most tools and dies used in manufacturing

686 Chapter 14

applications now have a high-performance coating. As already highlighted above, modern

coating techniques provide the means for producing high-performance nanocomposite and

nanostructured coatings with excellent adhesion and surface ﬁnish at reasonable cost and at

reduced substrate temperatures. At the moment, several coatings methods are available for the

deposition of thin hard coatings as mentioned brieﬂy earlier and covered in other chapters in

this handbook. Some of these are well established and used routinely by industry, while others

are still being developed, especially the ones involving the use of HIPIMS. The most popular

traditional coating methods include cathodic arc evaporation, magnetron sputtering, CVD, and

PLD. The modern magnetron sputtering systems can be conﬁgured to have pulsed DC or

HIPIMS power sources. These methods are described in detail in some of the other chapters in

this handbook, but a brief description is provided here for a few of the most popular methods.

Magnetron sputtering is perhaps the oldest and most widely used technique. It can produce

very thin or thick composite coatings consisting of nanoscale to microscale hard and/or soft

layers and phases. The ingredients that make up each phase are sputtered from a metallic,

ceramic, or composite target in sequence or all together to build the nanolayered or

nanocomposite ﬁlms by means of Ar ion sputtering. Argon is introduced into a PVD sputtering

chamber and ionized by collisions with electrons trapped by a magnetic ﬁeld in the vicinity of

targets. These ions are then accelerated toward target surfaces by strong electrical ﬁelds, for

which high-voltage/low-current DC power supplies are used with a negative polarity on the

targets. In pulsed DC sputtering, reverse polarity is used to enable sputtering of insulating

materials. Sputtering occurs during negative voltage pulses of high amplitude (order of

400–600 V), which are intercepted with low-voltage positive pulses to attract electrons to the

target surface and remove any accumulated positive charge. A recent practice of pulsed DC

sputtering is to use asymmetric positive and negative voltage pulses of various lengths with an

order of 100 kHz repetition rate. The relative duration of the negative and positive pulses

deﬁnes a duty cycle, which is often adjusted to achieve an optimum sputtering efﬁciency

[35, 36].

In the case of cathodic arc PVD, high-current/low-voltage power sources are used. The range

of currents may vary from as low as 30 A to as high as 1000 A. The voltage range is from 20 to

100 V. During deposition, high electrical current is localized in arc spots rapidly moving on the

surface of the targets in intersected electrical and magnetic ﬁelds. This condition leads to

extreme heating followed by the evaporation of atoms and/or clusters from these spots in the

form of highly ionized species with typical ion energies of 20 to 150 eV. Occasionally, larger

nanoscale to microscale particles or droplets are also detached from the surface of the target

material, and some of these may also reach the substrate and become incorporated into the

growing ﬁlms. The droplets are often in the range of 0.01–20 ␮m, and as a rough

generalization, materials with higher melting points generate smaller and fewer droplets than

those with lower melting points. The droplets are generated from a portion of the target

material that is heated sufﬁciently to melt but not evaporate. Reduction of droplets is possible

Nanocomposite Coatings for Severe Applications 687

with the increase of the velocity of arc spot travel on an evaporated cathode surface under the

guidance of an external magnetic ﬁeld. Removal of droplets from the deposition ﬂuxes is

achieved with ‘droplet ﬁlters’ that typically guide the plasma stream away from the path of the

droplet emission trajectories and allow placement of an evaporated cathode out of sight from

the substrate, as illustrated in Figure 14.2(c). Filtered cathodic arc evaporation techniques

provide droplet-free and nearly 100%-ionized ﬂuxes of the evaporated materials arriving at the

substrates. This condition gives additional ﬂexibility for deposition control by substrate

biasing and ionized ﬂux steering [37–40]. All ﬁltering techniques undergo a loss of the

evaporated target material, and many industrial coaters select to use not-ﬁltered cathodic arc

evaporation methods. At the same time, large-area plasma ﬁltering was reported for industrial

scale-up use [41], which may be the next trend in cathodic vacuum arc deposition.

Both magnetron sputtering and arc-PVD methods have advantages and disadvantages. In the

case of magnetron sputtering, the current densities are not high enough to cause sufﬁcient

mixing at the ﬁlm–substrate interface and to reﬁne the grain morphology to provide a dense

structure. Relatively poor adhesion and columnar morphology may have adverse effects on

coating durability in demanding machining applications, where strong ﬁlm-to-substrate

adhesion as well as high structural density is needed. In the case of the cathodic arc PVD

technique, the coating microstructure is much denser, and the bonding between coating and

substrate material is stronger owing to intense metal ion bombardment in the beginning as well

as later during the ﬁlm growth. However, microdroplets coming from the cathode surface

increase the roughness of the resultant ﬁlms. Increased roughness, especially in hard coating

surfaces, is unacceptable for most applications where the primary objective is friction

reduction and wear resistance of the complete system. Droplet ﬁltration is an acceptable

remedy but comes with a price of inefﬁcient cathode-material utilization. Nevertheless, these

techniques allow for high process throughput at reasonable costs and are being used by most

industrial coaters.

The recently introduced HIPIMS method seems to overcome the deﬁciencies of magnetron

sputtering and arc-PVD in such a way that it can enable strong ﬁlm-to-substrate bonding as

well as avoid any droplet formation on the surface of growing ﬁlms. This method can have a

huge positive impact on many tribological applications. HIPIMS is an ionized PVD method

based on conventional direct-current magnetron sputtering (DC-MS). In DC-MS very little of

the sputtered material is ionized since the plasma power density is not high enough. Increasing

the power density will increase the plasma density and ionize more of the sputtered material,

but it will eventually melt the target and lead to arcing, which of course is not a desirable

situation. Instead, by applying high power in repeated short pulses, the average power is kept

low enough for the cooling system to maintain the target temperature below the melting point.

The power supplies used in HIPIMS are usually based on an artiﬁcial pulse-forming network

and operate in a repetitive manner. They became available only recently as a result of

688 Chapter 14

Figure 14.4: Structural morphology of (Cr,Al)N ﬁlms deposited by (a) conventional magnetron

sputtering and (b) HIPIMS techniques [45].

breakthroughs in high-power electronic circuits that handle short but high-current and

high-voltage pulses with good reliability and at reasonable equipment costs. A HIPIMS power

source can be attached to an existing magnetron sputtering system. In principle, all that needs

to change is the power supply. In recent years, the HIPIMS method has yielded high plasma

densities (of the order of 10

−3

) consisting of both metallic and gaseous ions. The many

advantages inherent to this method have been discussed in the previous literature [22, 42–44].

The fraction of ionized species may vary between 30 and 90%, depending on the material

type. As a result, the coating becomes very dense. Figure 14.4 shows the structural

morphology of two ﬁlms: one deposited by magnetron sputtering, the other by HIPIMS [45].

Such a structural change in ﬁlm morphology leads to much better mechanical properties such

as higher hardness, greater toughness, and better structural cohesion. In summary, HIPIMS

combines many of the advantages of cathodic arc and magnetron sputtering in one technique

and holds great promise for a variety of demanding industrial applications.

In addition to the modern PVD techniques mentioned above, CVD techniques are available for

the deposition of nanocomposite or nanostructured coatings for use in various engineering

applications. Among others, plasma-activated or -enhanced CVD processes have become

popular in recent years and are used to deposit nanocomposite diamond-like carbon (DLC)

ﬁlms. Other CVD methods (such as hot-ﬁlament or microwave CVD processes) are also

available for the deposition of microcrystalline and nanocrystalline diamond ﬁlms. Compared

to DLC, diamond coatings require high deposition temperatures (as high as 1000

◦

C).

Microwave CVD, which uses a hydrogen–methane plasma, is used for the synthesis of

diamond ﬁlms at very high temperatures. The same method is also used to produce

nanocrystalline diamond ﬁlms with grain sizes in the range of 2–6 nm; in this case,

hydrogen is replaced with argon in the plasma [46]. In the case of nanocomposite DLC ﬁlms,

small nanocrystalline phases of transition metal carbides are usually dispersed within an

amorphous matrix [26, 47]. Such ﬁlms can also be prepared in a nanolayered fashion,

Nanocomposite Coatings for Severe Applications 689

consisting of a few nanometers of a metallic or compound phase followed by amorphous

carbon.

The modern PVD and CVD methods mentioned above are capable of industrial-scale

production of nanocomposite coatings and hence are used extensively by industry in the

manufacturing and transportation sectors. The deposition rates of these methods have been

adjusted to reduce the production cycles and improve the reproducibility of coating

quality and reliability from one run to another. In particular, with recent advances in hybrid

deposition processes, much greater ﬂexibility has been achieved in the design and production

of novel nanocomposite coatings in large scale. Some of these systems are based on cathodic

arc and HIPIMS techniques, which are used not only to effectively clean substrate surfaces but

also to form a graded interface that can lead to strong bonding [21]. These systems consist of

multiple targets that are strategically located within the deposition systems so that the

production of nanolayered or multilayered coatings is much easier and effective. These

designer coatings with far superior structural, mechanical, and tribological properties are able

to provide longer durability when used in machining or metal-forming applications

[11, 13, 45, 48].

14.3 Mechanical and Tribological Properties

of Nanocomposite Coatings

Demands for greater power density, reliability, productivity, and energy efﬁciency in

advanced mechanical systems require new materials and coatings with far superior property

and performance characteristics. The modern deposition techniques mentioned above are

capable of producing the kinds of nanostructured and composite coatings that are needed for

such demanding applications. In fact, most of the current coatings used in high-speed

machining and other demanding applications are nanostructured or made of nanolayers or

nanophases. Some of these coating architectures are illustrated in Figure 14.1. Nanolayered

coatings consist of discrete layers of two or more metals and/or ceramics in an alternating

fashion, and the thickness of each layer may vary from a few atoms to several angstroms

and/or nanometers [7, 27, 48–50]. The ones that are in the range of a few atomic layers are

often referred to as superlattice coatings [20, 51]. From a mechanical and tribological

properties point of view, this class of layered coatings may achieve superhardness and

toughness as well as superior thermal and tribological properties [52]. By varying the

thickness of alternating layers, one can achieve nanoscale to microscale grading between the

layers and thus better control the physical, mechanical, and tribological properties. Achieving

a high-precision layer build-up (especially at subnanometer scales) is not an easy task. It often

requires the strategic selection and close control of several deposition parameters, including

temperature, deposition rate, speed of rotation, target masking, bias voltage, ionization

efﬁciency, and the average energy of the ionized and neutral species [20]. All of these

690 Chapter 14

parameters have a strong effect on surface and bulk diffusion of impinging atoms and on

adatom mobility, nucleation, and growth processes, and hence, layer uniformity or coverage of

substrate materials.

Most of the nanostructured and composite coatings used by industry are currently produced by

hybrid PVD or CVD systems. They can be produced in numerous forms. In a typical

nanocomposite coating architecture, one can have a smaller nanocrystalline phase embedded

in a larger amorphous matrix or a minor amorphous phase surrounded by a predominantly

crystalline phase [5, 7, 53, 54]. With the uses of multiple targets, one can also produce

composite coatings consisting of more than two or three phases. Strong bonding is important

between the phases so that they do not create debonding or interface cracking under

mechanical and/or tribological loadings.

The mechanical properties of nanolayered or composited coatings are much better than those

measured in single-phase ﬁlms or their bulk composites consisting of the same metals, alloys,

or ceramic phases. Several factors can inﬂuence the mechanical and tribological properties of

nanocomposite coatings. Among others, the size, shape, and orientation of nanograins that

make up the composite ﬁlm are most inﬂuential. Furthermore, several factors (the surface area

and thickness of the grain-boundary phases, the extent of residual stresses, and the presence or

absence of voids, pinholes, cracks or discontinuities within or across the ﬁlms) may also have

a dramatic effect on their mechanical and tribological properties. Veprek et al. [55] showed on

an example of superhard TiN/Si

coatings, that even the smallest impurities of oxygen in

one monolayer of amorphous Si

tissue separating TiN nanocrystalline grains dramatically

affect the mechanical properties of such materials, removing any superhardness. The literature

typically reports a broad range of mechanical properties of the produced nanocomposite and

nanostructured coatings, depending not only on the coating architecture, but also on the

deposition methods and the quality of the process control.

For the far superior mechanical properties of nanostructured and nanocomposite coatings,

several theoretical models have been proposed by researchers. In one of these theories,

Koehler [56] has proposed that the superior mechanical hardness of nanostructured coatings

stems from the fact that dislocations (even if they are generated within the grains) are

effectively stopped or blocked by grain boundaries, and gross plastic deformation becomes

very difﬁcult. In another theory, the Hall–Petch relationship of grain size versus hardness was

thought to account for the superhard nature of nanocomposite coatings [57]. In the Hall–Petch

model, the yield strength or hardness H(d) of a polycrystalline solid is related to its average

grain diameter d as in H(d)=H

+ Kd

−1/2

and K are material-speciﬁc constants).

According to this equation, a composite material with smaller grains would have much higher

hardness. In bulk materials or composites, this relationship holds quite well up to a grain size

of 0.01 ␮m. Below this size, the hardness often decreases mainly because of the increased

effect of grain-boundary sliding [58].