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 691

Table 14.1: Examples of superhard coatings and their grain size and mechanical properties

Coating Average grain

size (nm)

% Cu (at.) Hardness,

H (GPa)

E*(E/(1 − v

))

(GPa)

Refs

Zr–Cu–N 38 1–2 55 395 [59–61]

Zr–Ni–N 6 4.1 (Ni) 57 430 [62]

CrCu–N 70–90 1 35 [63]

Al–Cu–N 9, 5 8, 1 47 313 [61]

Zr–Cu–N 10–15 23 23 250 [59–61]

ZrN – 0 16 [60]

nc-TiN/a-Si

40–60 > 500 [6, 64]

nc-TiN/a-Si

/a-TiSi

> 80 [30]

Considering both models, with the selection of the right kinds of grain and grain-boundary

phases, one can produce hard and tough nanocomposite coatings. For example, if the grain

boundaries in a coating are made of a strong phase, then the dislocation motion will become

very restricted, or the dislocations will be effectively conﬁned to the grains themselves; hence,

the coating will endure high loads before there is a major deformation or fracture. Using such

an approach, many research groups have recently developed nanocomposite coatings and

demonstrated their superhard nature. Table 14.1 presents some of these coatings and their

compositions. Those coatings with hardness values of 40 GPa or more are considered

superhard. Their grains are in nano domains (3–5 nm), and the grain boundaries are decorated

by very thin (one atomic layer) and immiscible grain-boundary phases [7]. Again, the selection

of the phases that make up the grain boundaries is important. The phases must be immiscible

yet strongly bonded at the grain boundaries so that they do not easily yield to dislocation

motions from one grain to the next.

The mechanical hardness of nanocomposite coatings is among the highest, and many of them

are designated as superhard. A simple rule of mixtures cannot be used to estimate their

hardness. Typical hardness values for nanocomposite coatings vary between 40 and over

90 GPa. For example, TiN/Si

, Ni/ZrN, and Cu/MoN exhibit hardness values greater than

40 GPa [4, 5, 7, 29, 53]. If a rule of mixture had been used to estimate their hardness, their

composite hardness values should have been much lower. In reality, they exhibit hardness

values two to three times higher than the phases they are composed of. At higher

concentrations or volumes of metallic phases (over 50%), nanocomposite coatings may exhibit

hardness values less than 10 GPa, while at very low concentrations (1–4%), the hardness goes

up, and values greater than 50 GPa are feasible. In short, the hardness of nanocomposite

coatings can be tuned by varying the concentration of individual phases that make up the bulk

coating. It is important to note that the hardness of all coatings may be inﬂuenced by the levels

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of internal stresses within these coatings. Therefore, one has to determine the extent of internal

stresses in relation to the hardness of a given coating. A high level of internal stress may be

beneﬁcial to the hardness of a coating, but it is also detrimental to the fatigue and wear

resistance of such coatings.

Another impressive property that makes nanocomposite coatings unique is their superior

fracture toughness, which is important for durability of hard coatings in most mechanical

applications where cyclic shear and normal forces are applied [65]. Hardness is important for

load-bearing capacity of a coating, but fracture toughness is equally important for resisting

crack initiation and growth. Recent experimental studies have conﬁrmed that most

nanostructured and composite coatings possess relatively high fracture toughness [53, 66]. The

high fracture toughness of nanocomposite coatings has been attributed largely to their

nano-sized grain morphology. Speciﬁcally, nano-sized grains with a large-grain boundary

network are believed to provide an ideal condition for resisting crack initiation in superhard

coatings [55]. Even if a crack initiates within a grain, strong and thin (order of one monolayer)

grain boundaries can deﬂect and branch them, reducing the stress concentration factor and

arresting further crack growth. Grain-boundary sliding was also suggested as a mechanism for

crack energy dissipation at highly localized strains in nanocomposite tribological coatings

[14]. Overall, compared to most traditional hard coatings, nanocomposite coatings are ideal

for demanding mechanical applications, such as high-speed cutting tools or load-bearing

aerospace materials.

While high hardness and toughness are critical for the proper functioning of a coating in

intended applications, for such coatings to last long, they must also have strong adhesion or

bonding to the substrate materials. In most tribological applications, adhesion determines a

ﬁlm’s endurance life, load-bearing capacity, and effectiveness in such applications. In fact,

strong adhesion between a coating and its substrate is perhaps the most important prerequisite

for most mechanical and tribological applications [67, 68]. Even if the coating is superhard

and tough, with poor adhesion, it fails quickly under the inﬂuence of high normal and shear

forces in most mechanical applications. As mentioned earlier, high levels of internal stresses

may cause adhesive as well as cohesive delamination in a coating. The effect of internal

stresses is the greatest at the ﬁlm–substrate interface. If the adhesion of the coating to the

substrate is poor, then the coating delaminates from the surface. High levels of physical

intermixing and chemical reactions between the constituents of coating and substrate materials

can signiﬁcantly improve the adhesion of coatings. Also, poor mechanical interlocking at the

coating–substrate interface may increase the likelihood for delamination.

With the use of modern deposition processes, achieving strong bonding between hard coatings

and their substrates is easy. Most of the advanced techniques such as arc-PVD and HIPIMS are

able to promote high levels of interdiffusion or mixing between coating and substrate atoms at

the interface and thus very strong bonding.

Nanocomposite Coatings for Severe Applications 693

14.4 Nanostructured and Composite Coatings

for Tribological Applications

During the 1980s, transition metal carbides, nitrides, borides, and oxides attracted the greatest

attention for tribological applications. However, during the last decade or so, attention has

shifted to multifunctional nanocomposite coatings with gradient, duplex, multilayered, and

nanocomposite architectures, mainly because of their superior tribological properties and

diverse application possibilities in transportation, aerospace, and manufacturing ﬁelds [8, 15,

32]. Owing to their nanocomposite nature, these coatings can provide high fracture toughness

in addition to high mechanical hardness and resilience under severe loading conditions for

many tribological applications. If the phases are selected strategically, they can achieve

self-lubricating capacity and provide low-friction and low-wear coefﬁcients over a broad range

of test conditions, temperatures, and environments. Some of the nanocomposite coatings may

also enable machining or manufacturing under dry or marginally lubricated conditions [5, 7].

This is a much desired goal in many engineering applications but not easy to realize. In such

coating architectures, the ingredients of each layer are strategically selected so that they may

become lubricious through tribochemical interactions with the chemical species in operating

environments under the inﬂuence of ambient or frictional heating, thus enabling tools to

operate even in the absence of additional liquid or other lubricants [69].

In most tribological applications, nanostructured and composite coatings are expected to

provide superior friction and wear. Some of the other important requirements are strong

adhesion to the substrate material, high mechanical hardness and toughness, high

thermal/chemical stability, uniform and adequate thickness, dense morphology, smooth

surface ﬁnish, and self-lubrication. Recent experimental work which was referenced in the

above paragraph has shown that when all of these requirements are met, the tribological

coatings can improve the performance and durability of tools and components. For example, in

metal-forming and metal-cutting operations, where the wear mechanism is dominated by

microfracture or plastic deformation, nanostructured and composite coatings can resist crack

initiation because of their high fracture toughness. Because of their hardness, under heavy

loading in vertical directions, they can resist deformation on the surface as well and thus

increase the load-bearing capacity of base materials.

Besides the manufacturing and machining applications, nanocomposite coatings are attractive

for use in transportation, biomedical, and aerospace ﬁelds [70, 71]. In the ﬁeld of

transportation, the use of liquid or grease lubricants (engine oils, greases, etc.) with all kinds of

chemical additives is essential for the functionality and durability of most sliding, rolling, or

rotating components. One of the key additives is zinc dialkyl dithiophosphate (ZDDP), which

is used in almost all types of engine and other industrial lubricants owing to its ability to

withstand extreme pressures and its anti-friction, anti-wear, and anti-oxidation characteristics,

694 Chapter 14

especially under severe contact conditions. Without it, most current lubricants become

essentially useless. Another important additive is molybdenum dialkyl dithiocarbamate

(MoDTC), which is used primarily to control friction between sliding surfaces. Both of these

key anti-friction and anti-wear additives were recently found to be environmentally unsafe

and/or potential carcinogens. In particular, ZDDP has been determined to adversely impact the

emission control and after-treatment devices of modern engines by poisoning their catalyst

systems (especially the NO

catalyst in diesels). Both ZDDP and MoDTC contain heavy

elements besides sulfur and phosphorus, and hence, they contribute to the particulate

emissions. Furthermore, the sulfur in these additives reacts with moisture in the combustion

chamber and forms acids, which can trigger corrosion and additional wear problems on sliding

surfaces.

At the moment, the uses of these lubricants in transportation and other industrial sectors are

being curtailed mainly because of their harmful effects on the environment. The most desirable

scenario will be to engineer both the coatings and future lubricants in such a way that they

become compatible or complementary to one another’s performance characteristics, and as a

whole, they represent a better lubrication possibility than what can be provided by either of

them. For example, in a specially designed nanocomposite coating, while one phase provides

extreme resistance to wear and deformation, a second or third phase may preferentially react

with the additives in oils to provide high lubricity and protection against wear. It is also

possible to design new nanocomposite coatings around the existing liquid lubricants or

additives to achieve further improvements in lubrication performance [8, 72]. These coatings

can be regarded as smart or adaptive tribological coatings because of their excellent

compatibility with, and favorable response to, the lubricants and test environments.

In addition to the lubricated contacts, nanostructured and composite coatings can be useful in

other tribological applications involving dry sliding at elevated temperatures or high vacuum

and corrosive environments. Despite being chemically stable, under very aggressive or

oxidative conditions, these coatings may undergo minor oxidation. In most tribological

applications, in addition to the temperature of the operating environment, signiﬁcant frictional

heating may also exist and trigger chemical and/or oxidative interactions with the solid, liquid,

or gaseous media in the test environments. Some of the reaction products are self-lubricating,

while others might be hard and abrasive; hence, they are detrimental to the lifetime of the

coated component. At present, high-speed machining or sliding is greatly desired in numerous

industrial applications, which call for much higher chemical and thermal stability for the

nanocomposite coatings. In addition to the traditional oxidation-retarding elements, such as

Cr, Ti, and Al, more exotic elements like Y and Ce are being incorporated in many composite

coatings to further enhance their resistance to oxidation [73]. Some of the tools and dies used

in many manufacturing operations are routinely coated by such means to achieve superior

resistance to oxidation as well as wear and scufﬁng.

Nanocomposite Coatings for Severe Applications 695

Conventional PVD coatings can be anisotropic, and most are made of a columnar

microstructure. Under heavy loading, deformation or fracture can occur through the column

boundaries. However, in the case of nanostructured and composite coatings, the grains are

extremely small, and the morphology is not necessarily columnar; hence, they cannot be easily

deformed or fractured under loading. The average grain size of nanocomposite ﬁlm is in a

range that is not well suited for crack initiation and propagation. In another words, Grifﬁth’s

criteria for crack growth are difﬁcult to meet in nanostructured and composite coatings [74].

Therefore, one of the main reasons why nanostructured coatings exhibit superior wear

properties is that they are better able to prevent crack initiation and growth.

Another unique feature of the nanostructured and composite coatings is that, mainly because

of their superhardness and stiffness, the real contact areas are very small during a tribological

sliding situation. When combined with high chemical inertness or stability, smaller contact

areas on a sliding surface may often lead to lower friction (especially if one of the constituent

phases is self-lubricating). For most tribological applications, low friction in addition to low

wear and high resistance to galling or scufﬁng are major requirements. Nanocomposite

coatings can be designed to include a self-lubricating phase, such as MoS

, to provide low

friction, while the hard phase can provide low wear. In lubricated sliding applications, one of

the phases may be selected from those metals that can easily form self-lubricating secondary

ﬁlms on the sliding surfaces and thus provide much lower friction. Figure 14.5 shows the

frictional behavior of a nanocomposite MoN–Sn ﬁlm under boundary-lubricated sliding

Figure 14.5: Friction coefﬁcients of steel sliding against steel and steel sliding against Sn–MoN

nanocomposite ﬁlm under boundary-lubricated sliding conditions [72] (Courtesy of CRC

Publications).

696 Chapter 14

conditions. When compared with an uncoated steel surface, the Sn–MoN coating provided a

much lower friction coefﬁcient. It is possible that such a reduction in friction is due to the

low-shear-strength Sn phase in the nanocomposite structure. Tin may have also reacted with

additives in oil to produce a low-shear boundary ﬁlm on the sliding surface.

During the last two decades the cost of PVD and CVD coatings has decreased signiﬁcantly. As

a result, these coatings are now used for a wide range of mainstream applications. In particular,

in the ﬁeld of machining and transportation, vacuum-based hard coatings are replacing or

supplementing the traditional surface treatments, such as carburizing and nitriding. Obviously,

different applications require different coatings with different thickness, hardness, and

chemical composition. For example, the requirements for coatings in cutting and forming

operations are different from those in engine applications. High-temperature oxidation

stability, adhesion, thermal conductivity, afﬁnity of the coating material for the counterface

material, and hardness are the most important factors in cutting and forming operations;

surface roughness, fatigue resistance, better cohesion and adhesion, wear resistance, low

frictional behavior, and responsiveness to additives in engine oils are the most important

requirements from coatings in transportation applications. In addition, some other issues need

close attention. For cutting and forming tools, highly alloyed and expensive steel substrates

(such as high speed steel and M2 tool steel) are often used, and they can endure deposition

temperatures of up to 450–500

◦

C without signiﬁcantly compromising their high hardness;

however, low alloyed and cheap steel substrates are more common in engine components

because they keep the cost down, and usually maximum deposition temperatures cannot

exceed 150–200

◦

The remainder of this section individually treats the different types of coatings developed for

tribological applications.

14.4.1 Self-Lubricating Nanocomposite Coatings

For many years, researchers and practitioners have prepared and used many types of

composite materials to combat friction and wear in mechanical applications [75, 76].At

present, several kinds of metal-, ceramic-, and polymer-matrix composites are impregnated

with graphite, molybdenum disulﬁde, polytetraﬂuoroethylene (PTFE), and hexagonal boron

nitride, which can provide low friction in sliding bearing applications. These and a few other

composite materials, which are available as fairly thick (0.1–0.2 mm) coatings, are used for

combating friction and wear, especially at elevated temperatures [77].

Most self-lubricating composite materials can also be applied on a surface by plasma spraying

or electroplating methods [78]. Electroless nickel, chromium, and nickel–phosphorus coatings

that contain small amounts of nanoscale graphite, MoS

, PTFE, and diamond particles can be

produced and used to achieve relatively thick metal-matrix ﬁlms with self-lubricating

Nanocomposite Coatings for Severe Applications 697

properties [79]. In addition to these traditional practices, the modern PVD methods mentioned

above can be used to deposit self-lubricating materials as thin (a few micrometers)

nanocomposite coatings. Conceptually, the approach is similar to the conventional methods

used in the production of the bulk self-lubricating composite materials and/or coatings

mentioned above. However, structurally, the individual grains that make up such coatings are

on the nanoscale or mixed homogeneously within the produced layers. For example, self-

lubricating MoS

ﬁlms have been used extensively by the aerospace industry to achieve and

maintain low friction for many years. However, as a monolithic ﬁlm, MoS

may not provide

very long wear life. Extensive research into alloying MoS

and WS

with certain elements or

compounds, such as Ni, Pb, Ag, Au, C, Ti, Sb

TiN, and PbO, resulted in much improved

wear life and reduced environmental sensitivity [80–83]. During deposition of alloyed MoS

or WS

ﬁlms, deposition temperatures and other parameters can be controlled to achieve dense

structure or morphology, and the texture or orientation of the ﬁlms can be manipulated to

achieve additional life extension and superior frictional behavior. Under certain deposition

conditions, the nanocomposite or alloyed ﬁlms become amorphous, but during tribological

testing, they revert to a crystalline structure, which is essential for low friction [80, 84].

As in bulk self-lubricating composite materials, the soft solid lubricant phases in

self-lubricating nanocomposite ﬁlms are often selected from graphite, hexagonal boron nitride,

and transition metal dichalcogenides such as molybdenum, tungsten, and niobium disulﬁde or

diselenide. Their self-lubricating characteristic is related to a layered crystal structure.

Speciﬁcally, the crystal structures of these solids are made of atomic layers in a stack. The

atoms lying in the same layer are closely packed and strongly bonded to each other, while the

layers are relatively far apart, and they are held together by weak van der Waals’ forces. When

present between sliding surfaces, these layers can align themselves parallel to the direction of

relative motion and slide over one another with relative ease to provide low friction. Among

the many types of nanocomposite coatings, the ones that contain WS

and MoS

have become

very popular owing to their very low friction and wear coefﬁcients. They are well-suited for

many types of aerospace applications [71, 83]. Typical friction coefﬁcients available through

the use of such nanocomposite coatings are in the range of 0.01–0.05 in vacuum or inert test

environments.

With the addition of certain metals (like Ti, Cr, and Ni), relatively hard versions of MoS

and

other ﬁlms have been produced, and as a result much longer wear life has been achieved in

these coatings. The friction coefﬁcients of such coatings were also shown to be much lower in

humid air compared to those of the monolithic MoS

ﬁlms. For example, the friction

coefﬁcient of a MoS

–Ti ﬁlm against steel or ceramic balls is below 0.1 in humid air and less

than 0.05 in dry N

; when compared with conventional MoS

ﬁlms, its Vickers hardness is

much higher and hence it lasts longer in tribological applications [85]. Furthermore, the

frictional properties of this coating are not greatly affected by moisture in the test environment,

and it is proposed for use in various dry-sliding and machining applications (e.g. milling,

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drilling, tapping, cold-forming dies and punches, stamping, bearings, and gears for aerospace

and vacuum applications) [85, 86].

Various research groups have attempted to co-sputter MoS

with hard nitrides, borides, and

oxides to achieve much longer durability in sliding bearing applications. As an example, this

method can be used with either TiN or TiB

targets or a single target that consists of both TiN

and MoS

. As a result, the coatings are made of distinct nanolayers or phases of hard ceramic

and soft MoS

phases. Depending on the amount of the hard phase, the hardness of the

composite coating can be as high as 20 GPa, while their friction coefﬁcients are in the range of

0.1–0.3, even in humid environments. These coatings can also perform well in machining [87].

When softer metallic or ceramic phases are used, the resultant nanocomposite ﬁlms are also

soft. For example, sputter deposition of multiple layers of MoS

and Pb results in a relatively

soft ﬁlm, but the wear life is much longer than that of either the MoS

or Pb ﬁlms [88].

Using sputtering, pulsed laser, or arc-PVD methods, more complex multicomponent

TiC/DLC, MoS

/TiC/DLC, or WS

/WC/DLC coatings have also been introduced, and as a

result much improved tribological properties have been achieved over broad ranges of

temperature and environmental humidity [15, 89]. These are also called adaptive or chameleon

coatings owing to their reversible tribological properties, permitting stable operation and

endurance in the cycled humidity and space-ambient environments needed for aerospace

applications. Figure 14.6(a) shows a schematic of such a coating architecture, while Figure

14.6(b) shows how an actual nanocomposite coating produced according to such architecture

behaves during testing in humid and dry environments [15]. The hard TiC and DLC phases in

these coatings provide high strength and resistance to wear, while the solid lubricants DLC and

MoS

ensure low friction in humid and dry environments, respectively.

Figure 14.6: (a) Schematic illustration of a nanocomposite coating with chameleon-like surf ace

adaptive behavior and (b) frictional behavior; a YSZ/Au/MoS

/DLC coating during a test with dry

nitrogen and humid air environments. Note that frictional response is reversible and dominated

by lubrication provided by MoS

in dry nitrogen and by DLC in humid air [15].

Nanocomposite Coatings for Severe Applications 699

Figure 14.7: (a) High-resolution TEM image of TiN/Si

nanocomposite coating, and

(b) dependence of hardness on the thickness of the grain-boundary Si

phase [90].

14.4.2 Superhard Nanocomposite Films

Nanocomposite coatings are made of either alternating layers of two or more chemically

distinct phases or one of the phases dispersed or embedded into another phase. In the case of

layered nanocomposite coatings, layers can be made of metallic and/or ceramic-based

materials, and the thickness of each layer can be controlled down to 1 nm or less or up to

several tens of nanometers. Mixed nanocomposite coatings can have a minor and a major

phase; often, the minor phase is in the crystalline form while the matrix or major phase is

crystalline or amorphous. Figure 14.7 shows a high-resolution TEM image of the TiN/Si

system, together with its hardness proﬁle with respect to the thickness of the grain-boundary

phase [90]. The TiN/Si

system is one of the most studied, mainly because of its

reported superhardness. While TiN represents the major or crystalline phase, Si

makes up

the grain-boundary phase and is reported to be amorphous. The other major phases that have

been used in the synthesis of similar superhard coatings include VN, NbN, ZrN, and (TiAl)N,

again, embedded in an amorphous Si

matrix [7, 30, 74, 91, 92].

Owing to their superhardness, these coatings are resistant to wear, abrasion, and erosion. Their

thermal stability has also been reported to be excellent; hence, they are well suited for

high-speed machining or high-temperature metal-forming operations. Their sliding friction

coefﬁcients are not very low under dry-sliding conditions. A liquid and/or solid lubricant must

be used to achieve reasonably low friction coefﬁcients in sliding contacts. The superhardness

and high thermal stability of these coatings are thought to be the main reasons for their

protection against wear. Under lubricated sliding conditions, they can also provide low

friction, which is important for saving energy in many sliding contacts. Besides the system

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Figure 14.8: Cutting performance of various hard coatings while milling a CK45 steel. (Cutting

conditions: feed rate, 0.23 mm/tooth; depth of cut, 2 mm; cutting speed, 179 m/minute). S26:

indexable uncoated insert; S26 + TiN: TiN-coated; S26 + TiAlN: (TiAl)N-coated; 826-1 and -2:

same inserts coated with (Ti

1−x

)N/a-Si

nanocomposite coatings; 8026:

nc-(Ti

1−x

)N/a-Si

multilayer nanocomposite coating [91] (Courtesy of Pure and Applied

Chemistry).

mentioned, numerous other multilayer superhard coatings have been developed in recent years

and studied extensively under various machining and sliding conditions. For example, a series

of composite systems consisting of CrN/NbN and TiAlN/VN has been developed and studied

extensively for their friction- and wear-reducing abilities [29, 73, 93].

The nanocomposite coatings mentioned above have been fully developed and offered for

industrial uses in recent years. Compared to their single-component predecessors, such as TiN,

TiC and (TiAl)N, they work extremely well. For example, in a series of machining trials,

nanocomposite TiN/Si

coatings exhibited signiﬁcantly higher durability, as shown in

Figure 14.8 [91]. As can be imagined, during such cutting operations, the cutting edges of

tools become very hot. Therefore, the nanocomposite coatings must have the ability to resist

oxidation and maintain their superhardness and structural integrity; otherwise, they lose their

nanocomposite natures and then their superhardness, which is essential for their impressive

performance in such severe applications [94].

14.4.3 High-Temperature Adaptive Nanocomposite Coatings

High-temperature friction and wear reduction in oxidizing environments is one of the most

difﬁcult challenges for which nanocomposite coating architectures provide a considerable

breakthrough, offering surface adaptation capability in various temperature regimes, resisting

oxidation, and sustaining high contact loads. Oxides are most typically used. Many oxides