Kutz M. Handbook of materials selection

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1290 DIAMOND FILMS

Fig. 2 Research activity in CVD diamond during the last 30 years.

material. Many of the physical, optical, chemical, and electrical properties of

CVD diamond ﬁlms will be controlled or modiﬁed by their degree of deviation

from perfection. Imperfections may take on many forms. They can be structural,

such as grain boundaries or polytypes; chemical, such as variations in stoichi-

ometry; or optical, such as variations in refractive index or optical handedness.

There are also less obvious imperfections such as disordered ferroelectric do-

mains, irregularly distributed isotopes, or interfaces between discrete uniform

regions of the same or different materials (Davies, 1994).

Diamond is the hardest naturally occurring material (Field, 1991). The great

hardness of diamond makes it appropriate to consider diamond as a low-wear

material. If it is very difﬁcult to break a bond then the chances of breaking

bonds (wear) will be low. Several other materials have higher bond strengths

than does diamond, but those strengths are anisotropic. The hardest material that

is known is a synthetically prepared type of diamond. Analysis of elastic constant

data for

C diamond has shown that its bond strength is greater than that of

ordinary natural diamond or commonly made CVD diamond. The reason for

this is the slightly shorter bond length that results from having an extra neutron

in the carbon atom’s nucleus. Actual hardness measurements have not been made

on the material to date, but the strength should scale with the elastic constants

(Ramdas et al., 1993). Additionally, diamond has a low coefﬁcient of friction

(0.01) in the presence of air or hydrogen (Field, 1991). This makes the material

as slippery as Teﬂon but far more wear resistant. The slippery nature of the

hydrogen-saturated surface would reduce both frictional horsepower losses and

decrease surface erosion when used in wear applications (Phelps, 1993).

The thermal conductivity of diamond is a good example of the structure–

property relationship in CVD diamond. Annealed copper is the standard for

thermal conduction in the area of high-power electronic components because it

has a relatively high thermal conductivity of 4 W/cm K. Diamond is the best

thermal conductor that exists. Diamond is very attractive material for use in

3 DIAMOND FILM DEPOSITION 1291

electronic devices because a defect-free natural single-crystal diamond has a

thermal conductivity of around 20 W/cm K (Seal, 1970; Seal and van Enckevort,

1988). An average polycrystalline CVD diamond ﬁlm made under typical lab-

oratory conditions has a thermal conductivity of 6–10 W/cm K (Plano and Adar,

1987). The values for polycrystalline CVD diamond are impressive until they

are compared to isotopically pure single crystal diamond. Isotopically pure sin-

gle-crystal diamond is made by taking polycrystalline isotopically pure diamond

made by CVD and using it as the feedstock in single-diamond high-pressure

synthesis. Recent measurements on that material have failed to ﬁnd the upper

limit on the thermal conductivity of the samples. The value currently being

quoted is 36 W/cm K, nearly twice that of the best natural single crystal (Ban-

holzer and Anthony, 1992). Thermal stress gradients caused by rubbing are not

supported by diamond. Heat transfer in diamond is so fast that the temperature

throughout the diamond ﬁlm is always uniform and the ﬁlm does not suffer from

differential thermal contraction.

Diamond ﬁlms are commonly considered to be excellent electrical insulators

because of the 10

⍀/cm resistivity measured for many natural diamonds. How-

ever, the resistivity of CVD diamond has been found to change with ﬁlm com-

position. The electrical conductivity of CVD diamond has been shown to vary

with the amount of hydrogen absorbed in the structure of the ﬁlm. Ravi and

Landstrass (1989) report that the resistivity of CVD diamond ﬁlms can be cor-

related more with the amount of hydrogen they contain than with their Raman

spectra. An ‘‘as-grown’’ diamond ﬁlm had a resistivity of about 10

⍀/cm. This

same ﬁlm was soaked for 30 min in a 600

⬚C ﬂowing nitrogen atmosphere, and

the resistivity was found to increase to about 10

⍀/cm. The process was found

to be reversible, and the hydrogen could be put back into the sample, and the

resistivity would decrease to near the as-grown value. Diamond ﬁlms with sig-

niﬁcant amounts of graphitic structural bonding have been found to be electri-

cally conductive and have physical properties that are a fraction of the natural

properties, such as thermal conductivity. Figure 3 shows how many of the special

properties of diamond lend themselves to speciﬁc applications. Several proper-

ties of diamond suggest an application in some cases such as with microwave

electronic devices.

3 DIAMOND FILM DEPOSITION

Diamond ﬁlms cannot be deposited on all materials and surfaces. Substrates that

present problems to diamond deposition are the ferrous materials including iron,

nickel, cobalt, in addition to copper, zinc, tin, and the precious metals. Diamond

will grow on a substrate successfully at high temperature only if the material

has a low solubility for carbon and forms a carbide. There is nearly always an

interface that keeps diamond separated from its deposition substrate, and there

is typically no direct diamond-to-metal contact. The interface usually contains

at least a few atom layers of carbide (Srivatsa et al., 1989; Williams et al., 1990).

As an example, titanium could support diamond growth through the formation

of a titanium carbide interlayer. If, however, the carbide were too thick, then the

difference in thermal expansion between the carbide and the diamond could

fracture the interface. A diamond-like carbon ﬁlm deposited on top of a substrate

can also act as an interface instead of a carbide. Figure 4 outlines the elements

1292 DIAMOND FILMS

Fig. 3 Applications of diamond materials based on property criteria

as adapted from Spitsyn (1991).

Fig. 4 Regions of the periodic table that show diamond deposition compatibility where the half

boxes indicate partial compatibility.

of the periodic table that are typically compatible with diamond growth either

singly or in multiples. Phosphorus and molybdenum may only have a partial

afﬁnity for diamond growth. The other elements are more or less compatible

with diamond growth.

Low-pressure CVD of diamond may be thought of as a process that is based

on rate variations. The deposition and etch rates of carbon in the gas phase are

3 DIAMOND FILM DEPOSITION 1293

the keystones in CVD diamond deposition. Carbon produced from the gas phase

can take the form of many carbon materials. These carbon materials include

diamond, graphite, diamondlike carbon (DLC), carbynes, fullerenes, soot, naph-

thalene, and so on. However, the rate at which each of these materials is formed

depends on the total pressure of the system, substrate composition and temper-

ature, gas temperature, ﬂowrates, and feedstock. Most low-pressure carbon-

producing systems produce nondiamond carbons in far greater abundance than

they produce diamond. It is necessary to shift deposition parameters in a direc-

tion that favors diamond carbon growth at the expense of the growth of the other

forms of carbon. However, this needs to be done while removing nondiamond

carbon from the system. The presence of nondiamond carbons tend to poison

diamond growth and interfere with the formation of good-quality diamond crys-

tals.

The etch rate of diamond in hydrogen and oxygen appears to be far less than

all other nondiamond carbons formed. This results in a diamond-rich carbon

deposit. If diamond is being formed at 10% of the rate of the other carbon

materials but is being etched at 1% of the rate of the other materials, then after

a period of time a net deposit of diamond will form. Diamondlike carbon ﬁlms

or graphite will form if the substrate temperature is below 800

⬚C. The substrate

temperature must usually be somewhere between 800 and 1000

⬚C during CVD

diamond deposition. Diamond tends not to form if the substrate temperature

rises above 1100

⬚C. CVD diamond, single-crystal natural, and high-pressure

synthetic diamond graphitize and etch above this temperature. Typically, CVD

diamond forms in a very narrow temperature window. Deposition occurs when

the methane plus hydrogen mixture is activated. Energy to activate the process

may be in the form of radio frequency (RF), thermal (hot ﬁlament), microwave

plasma, high-voltage direct current (DC), and others.

The selection of an appropriate diamond deposition process depends on the

needs of the particular development program. Concerns about deposition rate,

hydrogen concentration, substrate compatibility, and the ability to coat three-

dimensional surfaces all should be taken into account. Three-dimensional CVD

diamond ﬁlms have been made by hot ﬁlament assisted CVD. Obata and Mor-

imoto (1989) demonstrated the possibility of three-dimensional coatings by

forming diamond hemispheres and sleeves. Their work used a ﬁlament-type

reactor to produce these forms. Bachmann and Lydtin (1991) present a rudi-

mentary chart to help in the selection of a deposition process presented here as

Table 1. This type of chart should be viewed as a starting point from which the

ﬁne points of the assorted deposition techniques can then be compared for such

things as cost, scalability, and ease of use as well as being capable of producing

the particular type of diamond that is desired.

Properties that make CVD diamond ﬁlms excellent for one particular appli-

cation may be completely unsuitable for other uses. For example, the highest

thermally conductive CVD diamond ﬁlm has the least desirable physical prop-

erties for use as a hydrostatic bearing coating. Good thermal conductivity results

from the material having a high degree of compositional purity and no nondia-

mond carbon in its structure. Nondiamond carbon has been found to help tough-

ness and resiliency in CVD ﬁlms. A low defect density is also necessary for a

high thermal conductivity. Defects in the form of grain boundaries need to be

1294

Table 1 Diamond Deposition Process Selection

Rate (

␮

m/h) Area (cm

)

Quality

(Raman) Substrates Advantages Drawbacks

Flame 30–100 ⬍1 ⫹⫹⫹ Si, Mo, TiN Simple Area, stability

Heated ﬁlament 0.3–2 100

⫹⫹⫹ Si, Mo, silica, etc. Simple, large area Contamination, stability, no O

DC discharge (low pres-

sure)

⬍0.1 70 ⫹ Si, Mo, silica, etc. Simple, large area Quality, rate

DC discharge (medium

pressure)

20–250

⬍2 ⫹⫹⫹ Si, Mo alumina Rate, quality Area

DC plasma jet 930

⬍2 ⫹⫹⫹ Mo, Si Highest rate, quality Area, stability homogeneity

RF (low pressure)

⬍0.1 ? ⫺ / ⫹ Si, Mo, silica BN, Ni Scale-up Quality, rate, contamination

RF (thermal, 1 atm) 180 3

⫹⫹⫹ Mo Rate Area, stability, homogeneity

Microwave 1 (low pressure)

30 (high pressure)

⫹⫹⫹ Si, Mo, silica WC, etc. Quality, stability Rate, area

ECR Microwave 0.1

⬍40 ⫺ / ⫹ Si Area, low pressure Quality, rate, contamination

Source: From Bachmann and Lydtin (1991).

5 DIAMOND FILM ROUGHNESS 1295

reduced for thermal conductivity, and so grain sizes must be very large. Large

grains tend to have very high relief and produce rough ﬁlms, also cracks are

easier to run through large crystals than smaller polycrystalline masses. Adherent

diamond ﬁlms suited to hydrostatic bearings will need to be low in stress, tough,

and probably not be as thermally conductive as single crystals.

4 MODIFYING CVD DIAMOND

The physical properties of CVD diamond can be modiﬁed and controlled by

altering deposition parameters, using additives, changing substrate selection, and

varying the seeding and nucleation method. For example, a carbon-rich environ-

ment will produce a diamond ﬁlm with both a larger conductivity than natural

diamond and one with a greater degree of compliance (Windischmann and Epps,

1990; Windischmann et al., 1991). Phelps and Koba (1989) showed that the

electrical conductivity and the quality of their Raman spectra of boron-doped

diamond ﬁlms were found to increase and improve with increased diborane

concentration in the deposition environment. Bernholc et al. (1988, 1992) pre-

dicted that with boron as a dopant ‘‘the reduced self-diffusion activation energy

should lead to better quality material.’’ Phelps (1990) suggested that features

observed in the Raman spectra of diamond ﬁlms are the result of defects in the

diamond structure, and boron doping of the material appears to substantially

affect its Raman spectra. Defects in boron-doped CVD diamond have been

shown many times to scale with the amount of boron doping in the material. A

TEM study by Wang et al. (1992) showed cross-sectional views of several doped

diamond at various dopant levels, and it is clear that the number of dislocations

in the diamond structure show a marked decrease. The oxidation resistance of

boron-doped CVD diamond is shown to increase with boron content by the

temperature measured at the onset of oxidation (Loparev et al., 1984).

5 DIAMOND FILM ROUGHNESS

The as-grown surface of CVD diamond ﬁlms is typically very rough. This degree

of roughness (

⬎5

␮

m peak to valley) would prevent the use of these materials

as bearings if there were no way of making the surface smooth. This degree of

roughness makes some diamond ﬁlms very good at sanding and polishing rough

areas on other diamond ﬁlms. In order for diamond ﬁlms to be used for bearings,

they need to have less than a 0.4-

␮

m peak-to-valley roughness. Surfaces with

roughness greater than this have been found to be unacceptable for bearings

made of other materials. Smooth CVD diamond ﬁlms may be made by several

different methods. These include polishing, brazing the rough side down, grow-

ing the ﬁlms very thin, or simply growing them very smooth at ﬁrst.

It is critical that a mechanical polishing procedure be avoided for smoothing

diamond ﬁlms. The reasons include the high per-part ﬁnishing cost. The only

material that is known to successfully mechanically polish diamond well is di-

amond itself (Watermeyer, 1991). This results in an expensive raw material cost

before any polishing activity even takes place. Also, a very thick ﬁlm of diamond

would have to be deposited so that some ﬁlm would be left after the polishing

procedure. Thickness is comparable to cost because of the slow deposition rates

at which this material is deposited and the slow rate at which diamond is re-

moved. A mechanical polishing step would also be the agent of ﬂaw introduc-

1296 DIAMOND FILMS

tion. The diamond ﬁlms made and likely to be used for bearing applications will

only be a few microns thick. A crack could very easily run through the thickness

of a diamond ﬁlm during the polishing process because of diamond’s brittle

nature. Lastly, there is yet no method by which a surface coated with a diamond

ﬁlm with three-dimensional reentrant surfaces can be polished to a satisfactory

smoothness while retaining size, shape, and surface quality speciﬁcations.

One technique that produces smooth diamond ﬁlms is a ﬂip-chip brazing

technique. A rough diamond ﬁlm is grown on a polished substrate and removed

after deposition. The ﬁlm may simply be turned over and brazed rough side

down onto a second working substrate. The smooth interfacial surface is then

the one that is outermost.

Smooth diamond ﬁlms can be made by controlling the nucleation density and

deposition parameters. Making ﬁlms smooth to begin with reduces the chances

of ﬂaw introduction and makes the entire process much less expensive. Smooth

diamond ﬁlms can be deposited directly on the substrate growth surface with no

postdeposition treatments. It was learned several years ago that thin, very smooth

diamond ﬁlms could be grown on polished tungsten through the use of dia-

mondlike carbon ﬁlms as an interlayer between the diamond and the substrate

(Phelps and Koba, 1989). The diamondlike carbon deposition method will be

covered in a later section on diamond ﬁlm seeding techniques. The average size

of a CVD diamond crystallite will be small if there is no room for it grow

laterally. If crystallites are densely packed, then a thinner ﬁlm can be grown that

is pinhole free. A ﬁlm will be smooth if it is very thin and densely packed.

Large rough facets will not develop if crystallites can be kept small throughout

the ﬁlm thickness. This can be done by poisoning the diamond growth environ-

ment with excess carbon.

6 DIAMOND FILM THICKNESS

There are several aspects to deciding what thickness of diamond ﬁlm is accept-

able for a bearing surface. The working deﬁnition proposed by A. C. Gonzalez

is that for a particular system ‘‘a thick ﬁlm is one that will interact with the

system as if it were bulk diamond’’ (Phelps et al., 1992).

In that same system, ‘‘a thin ﬁlm will allow the properties of the substrate

material to be sampled.’’ The scale of thickness is completely dependent on the

scale of the system. A thin ﬁlm might be 0.5

␮

m thick if the maximum foreign

particle size is 0.25

␮

m in diameter where a thick ﬁlm would be 5

␮

m thick.

Similarly, a thin ﬁlm might be 70

␮

m thick if the maximum foreign particle

size is 20

␮

m in diameter where a thick ﬁlm would be 200

␮

m thick. A central

concern is how thick must a ﬁlm be to be thick enough. The size and hardness

of foreign particulate matter arriving at the bearing surface will determine the

thickness of the diamond ﬁlm. Figure 5 shows schematically how the deﬁnitions

of thick and thin apply to a sharp foreign object applied to the ﬁlms under the

same load.

A second concern dealing with thickness takes the deposition process and the

mechanical system where the ﬁlm will be used taken into account. Figure 6

shows the trades between having thick and thin ﬁlms in a wear system. Currently,

there is no correct answer to ‘‘what is the proper thickness?’’

7 DIAMOND FILM ADHESION 1297

Fig. 5 Representing the difference between thick and thin coatings where a probe samples

only the bulk ﬁlm properties in a thick coating and a probe samples both the coating and the

substrate in a thin coating.

Fig. 6 Decision box showing relationship between thick and thin coatings.

7 DIAMOND FILM ADHESION

Adhesion of CVD diamond ﬁlms to working substrates is one of the most im-

portant aspects of diamond wear coating technology. Diamond has the particular

disadvantage that little is known about how to best attach a diamond ﬁlm to a

substrate surface. Good adhesion must be considered at the system design level

in order to realize superior state-of-the-art bearings. Adequate adhesion between

the diamond ﬁlm and the substrate must be present in order to use the remarkable

mechanical properties of diamond. There are many reasons why a diamond ﬁlm

would not be adherent. Identifying these reasons can lend direction to solving

the adhesion problem. The ﬁrst and foremost reason that diamond ﬁlms delam-

inate from substrate surfaces is that the bond strength between the ﬁlm and its

substrate is not sufﬁcient to hold onto the ﬁlm. The strength necessary to hold

1298 DIAMOND FILMS

onto a ﬁlm will vary with the type of substrate, the ﬁlm deposition conditions,

the substrate preparation, etc. Means of increasing the bond strength between

the diamond ﬁlm and the substrate are chemical, physical, and mechanical. A

chemical means of enhancing adhesion is through the use of carbide forming

substrates. Molybdenum is frequently used as a substrate material for diamond

deposition, however, diamond does not form a good strong carbide with this

metal. A metal that is more suited to diamond growth is tungsten. Tungsten

forms a strong, stoichiometric carbide that supports diamond nucleation and

growth.

The thermal expansion of diamond is far less than all metals and many non-

metals. Most substrates that diamond is deposited onto have thermal expansion

coefﬁcients that are larger than diamond such as silicon and tungsten. Diamond

ﬁlms deposited on these substrates at 1000

⬚C are in compression at room tem-

perature. If the tensile stresses resulting from shear due to compression are

greater than the bond strength between the diamond and its substrate, then the

ﬁlm will rupture at the substrate-to-ﬁlm interface. The ﬁrst means of controlling

stress and, thus, adhesion would be to reduce the thermal expansion mismatch

between the diamond ﬁlm and the substrate. Pure silica glass has a lower degree

of thermal expansion from room temperature to 1000

⬚C than does diamond.

Diamond is found to be in tension at room temperature after it has been depos-

ited onto silica glass at 1000

⬚C. However, the addition of a small amount of

boron to the silica glass will raise its thermal expansion coefﬁcient to near that

of diamond. Interlayer materials such as diamondlike carbon or carbide formers

such as tungsten or silicon could be used to control the thermal expansion dif-

ference between diamond and the substrate of choice. Likewise, a diamondlike

carbon or carbide former deposited between the diamond ﬁlms and the substrate

could allow the use of substrates other than good carbide formers to be used for

diamond deposition.

It may be possible to control and manipulate the residual stress found in

deposited diamond ﬁlms. Diamond ﬁlms can be made more compliant by making

the ﬁlm very thin or by changing the elastic constants of the diamond ﬁlm. This

can be done very precisely by controlling the amount of nondiamond carbon

that is present in the ﬁlm. Adding diamondlike carbon or graphite to a diamond

ﬁlm reduces the stiffness of the material, and it should be possible to make a

low-stress diamond ﬁlm on tungsten. This could be done through a graded in-

terface. A poor-quality diamond ﬁlm would be deposited on top of the substrate,

and during the deposition process it could be possible to reduce the nondiamond

component and produce a good-quality diamond ﬁlm.

The last means of enhancing the adhesion between the diamond ﬁlm and its

substrate does not lower the stress at the interface but increases the rupture

strength of the interface. This is done by mechanically interlocking the ﬁlm onto

the substrate. The work of a team of researchers at the University of Minnesota

(Tsai et al., 1992, 1993) has produced a method of depositing diamond ﬁlms on

incompatible substrates. This is done by electroplating nickel over a partially

grown (island form) diamond ﬁlm on a diamond-compatible substrate surface.

CVD diamond is further deposited on top of and across the electrodeposited

ﬁlm and connecting the islands of earlier deposited diamond. The islands of

earlier diamond act as attachment anchors. Later, a continuous diamond ﬁlm is

7 DIAMOND FILM ADHESION 1299

Fig. 7 Process steps used to electrodeposit a metal interlayer

between the diamond ﬁlm and the substrate.

Film Adhesion

Controlled by:

Stress at Film Interface

Strength of Bond

Film Adhesion

Controls:

Maximum Film Thickness

Number of Restarts

Film Roughness

Controlled by:

Deposition

Nucleation Density

Film Roughness

Controls:

Friction Losses

Bearing Wear Rate

Film Thickness

Controlled by:

Deposition Time

Stress at Interface

Film Thickness

Controls:

Debris Punch Through

Service Life

Fig. 8 Relationships between ﬁlm character, physical properties, and service behavior.

grown across these islands. The substrate does not have to be compatible with

diamond because no initial adhesion is necessary between the substrate and the

diamond seeds. The substrate simply has to be compatible with the electrode-

posited ﬁlm on top and not form a eutectic composition at diamond deposition

temperatures. This method holds great promise for depositing diamond directly

on diamond-incompatible materials such as INCO 718. Adhesion is the single

most important factor currently affecting the use of diamond ﬁlms as wear and

bearing surfaces.

There are several means by which an adherent diamond ﬁlm may be produced

on a substrate surface.

1. Deposit the diamond wear ﬁlm on top of a strong carbide-forming sub-

strate. This results in a strong diamond-to-carbide chemical bond and help

hold the ﬁlm onto the substrate.

2. Deposit the diamond ﬁlm on an intermediate layer that is a strong carbide

former deposited on top of the substrate. This results in a strong diamond-

to-carbide chemical bond and a strong carbide-to-substrate chemical

bond.