Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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512

Fig.

Chapter

4.8:

Aluminum-Based Vacuum Systems

Residual gas spectrum

an aluminum

UHV

chamber.

= 5.6 X id"'^

Torr

by modulated point collector gouge

Vertical

x 10

Horizontol

t 2

"ji'M^M-^i

-H-^

45-

CH4H20

•N2~

CI Ar

CO2

[Courtesy of: H. Ishimaru, "Developments and Applications for All-Aluminum Alloy Vacuum

SystQmsr

MRS

Bulletith

15(7) (July 1990).]

ilarity of hydrogen outgassing rates reported for various surface treatments of

aluminum [13] suggests that it is determined by the basic characteristics of alu-

minum rather than any particular surface oxide formation or morphology. Pure

aluminum is reported to have a somewhat lower outgassing rate than its alloys

[14,15];

however, it must be clad to a high-strength alloy for structural integrity.

In the past, voids and vias formed during the manufacturing process sometimes

created outgassing problems. However modern, commercially available alloys

rarely exhibit these problems.

Hydrogen diffuses through metals in its elemental form [16] which is chemi-

cally active and believed to react with carbon and oxides on the surface to pro-

duce secondary background gases such as CO and hydrocarbons. In aluminum

chambers, these gasses are much less prevalent (Figure 2) [17], probably because

there is much less carbon in common alloys of aluminum than in even low-carbon

316L stainless steel.

4.8.2

DEMOUNTABLE SEALS

Elastomer sealing techniques for aluminum chambers are identical to those for

stainless steel and are widely used and understood. Metal seals have been more

4.8.2 Demountable Seals 513

difficult to implement because of the relative softness of aluminum and its high

rate of thermal expansion. However, today a number of commercially available

products have practically alleviated these problems. Metal seals are, of course,

highly desirable, because 0-rings tend to dominate the gas load in a chamber.

For example, the outgassing from a single 100 mm X 5.3 mm Viton® 0-ring

is approximately equal to that from 3 ft ^ of metal chamber surface after 1 hour

of pumping and 7 ft^ after 10 hours. Pressures below

10 "'^

torr with elas-

tomer seals are very costly to achieve, in terms of pumping speed required, un-

less special measures are taken such as differential pumping or baking of the

0-rings [18].

4.8.2.1 Theoretical Model of Seal Effectiveness

The basic seaUng mechanism for a metal seal involves forcing soft gasket mate-

rial into the microscopic grooves on a hard sealing surface through which gas can

flow (see Figure 3). As sealing pressure is increased, gasket material is pressed

deeper into the grooves reducing the cross section (hence the conductance) of the

leak paths. The pressure required to achieve a desired leak rate may be estimated

as follows [19]:

0.34

A/7

Lr'

(1)

where P = mechanical pressure between the gasket and sealing surface

R = gasket metal sealing factor: 3Kgf/mm^ for soft aluminum,

5Kgf/nmi^ for soft copper

A/7 = gas pressure across the seal (typically atmospheric)

L = length of the seal

r = peak to peak roughness of the hard seal surface

Q = desired leak rate

w = width of the seal

Using this formula, a

10"^-^

torr-L/sec leak rate can be achieved in a 100-mm-

diameter by

1-mm-wide

seal with 1/xm (0.001 nmi) surface roughness using a soft

aluminum gasket under 27.4 Kgf/mm^ sealing pressure. This serves as a conve-

nient benchmark since a factor of 10 variation in any of these parameters changes

the required sealing pressure by only 2.3 Kgf/nmi^. This pressure is much higher

than the ultimate strength of most soft aluminum alloys but is in the range of

their Brinell hardness (such as 26 Kgf/nmi^ for 1060-H14)

[20].

Therefore, work-

hardening caused by substantial plastic deformation of an aluminum gasket will

provide the necessary sealing pressure (see Figure 4).

514

Fig.

Chapter 4.8: Aluminum-Based Vacuum Systems

Sealing mechanism

demountable seals.

Depth of fill

depends on

sealing

pressure

(not total force)

Leak rate is net conductance of microscopic grooves left unfilled by flow of gasket material.

Rg.4.

Sealing pressure depends

hardness

work-hardened gasket material.

CF FLANGE WIRE SEAL

Substantial

Plastic

Deformation

This is relatively uniform since plastic deformation is substantial.

Sealing

Pressure

4,8.2.2 CF Flanges

Commercially available aluminum CF flanges (see Figure 5) produce this plastic

deformation by pressing the knife-edge deep into a soft aluminum gasket. Plastic

deformation begins to occur after only 3

jmm

of penetration and produces enough

work-hardening to achieve a UHV seal long before the flange faces actually touch.

Deeper penetration merely widens the area of the seal reacting against the gasket

hardness but does not significantly increase the effective sealing pressure. Con-

finement of the gasket within its flange well stores approximately 5 /xm (6

/mm

for

4.8.2 Demountable Seals

515

Rg.5.

Aluminum

flanges and chamber.

(Courtesy of

SMC

Corporation.)

copper) of elastic strain that can compensate for slight changes in seal geometry

due to thermal gradients or mechanically induced strains.

4.8.2.3 Wire Seals

Wire seals, reported in the literature, achieve approximately the same sealing pres-

sure by substantially flattening the wire [21,22]. These require less bolting force

than CF seals because sealing pressure is concentrated over a much narrower seal

width.

4.8.2.4 Metal C-rings and Diamond-Shaped Seals

Conunercially available metal C-rings have relatively wide seal widths. Although

the average pressure over the contact area is approximately one-third that required

above, there is probably a strain concentration near the center that does produce

this.

The use of a collapsible knife-edge on the C-ring, made of soft gasket mate-

rial,

has also been used to produce high sealing pressure by concentrating sealing

force [23]. When practical considerations limit the amount of sealing pressure

supportable by a C-ring, the sealing surface can be either polished to reduce the

"r" term in Equation (1) or made rougher to create ridges that produce areas of

516

Chapter

4.8:

Aluminum-Based Vacuum Systems

local

high sealing

pressure.

Experimental data suggests

that reliable

sealing occurs

for surface roughness below l/^m and above

/mm

but not within this range [24].

4.8.Z5

Materials Considerations

For

long

life, the yield strength of the sealing surface must

substantially higher

than the hardness of the gasket material. Commercially available aluminum CF

flanges have a yield strength of 40 Kgf/mm^, which is well above the 26 Kgf/

mm^ gasket hardness. However, the

6061

alloy widely used as a chamber mate-

rial has a yield strength of only 28 Kgf/mm ^, which may be insufficient for long

life

[25].

An inert hard coating (such as ion-plated TiC) on the

flange

face can be

useful in preventing cold welding to the gasket. Without this, provisions may be

necessary for prying

flanges

apart when opening. Coatings also prevent the

for-

mation of

porous

oxides that can cause diffusion leak paths across the seal.

4.8.2.6

Potential for Seal Disturbance During Bakeout

Because of diiBferences in thermal expansion between room temperature and

150°C, unconstrained aluminum and stainless steel flanges develop a

0.1%

di-

mensional difference. This represents a

/mm

radial seal displacement per milli-

meter of radius. Also, because of changes in bolt length and flange thickness,

clamping force falls by 15% when using aluminum bolts or rises by over 30%

when using stainless steel bolts (for initial tension at half tensile strength). These

effects

are

further amplified

temperature gradients that

can

easily occur if heat-

ing or cooling is too rapid because of

the

large difference in thermal conductivity

of the two materials. Aluminum-to-stainless steel transition pieces, made by ex-

plosion bonding or special brazing, can be used to avoid these problems but are

expensive.

4.8.2.7

Constrained Flanges

To minimize the effects of thermal expansion on seal integrity, demountable dis-

similar metal flanges are normally constrained to prevent motion near the seal.

This is accomplished by tightly clamping them face to face so that static friction

between the bolt circles prevents slippage. Differential expansion produces radial

tensile stress in the stainless steel

flange

and compressive stress in the aluminum

flange, which result in small elastic strains but leave the seal geometry near the

constrained bolt circle effectively undisturbed (Figure

6).

High bolt

torque,

on the

order of 300 in-lb for -^-inch bolts, is used with CF

flanges.

Seal integrity has

been demonstrated through 150° bakeouts for standard CF sizes up to 12 inches

4.8.2 Demountable Seals

517

Fig.

Constrained flanges.

ROOM TEMPERATURE

„..L.

S.S.

{

ELEVATED TEMPERATURE

Expansion

Static friction

prevents slipping

here

Geometry in

seal area is

stable

Relative

Contraction

Elastic strains induced by differential thermal expansion are small especially in the seal area

near the bolts

[26].

Similar constraints

are

used

other demountable flange systems such

those used in CERN's LEP particle accelerator [27] and the metal C-ring seal pro-

posed

KEK [28].

518 Chapter 4.8: Aluminum-Based Vacuum Systems

4.8.2.8 Unconstrained Flanges

It is sometimes desirable to leave flange faces unconstrained. For example, CF

flanges can be assembled with a small gap between the faces to allow for addi-

tional tightening in case of future leaks. In this case, differential expansion causes

the gasket to roll so that the inner edge tilts slightly toward the aluminum flange.

Experimental data have shown that reliable CF seals can be maintained for up to

about 2.1 degrees of roll [29]. Thus, standard CF sizes of up to 6 inches can be

baked as high as 150°C without measurable leaks. Since the aluminum gasket

material softens at elevated temperatures, bolts should be made of aluminum so

that thermal expansion reduces clamping force by a like amount, preventing the

knife-edges from biting deeper into the gasket. Wire seals in unconstrained flanges

appear to allow seal surface displacement up to at least half the original thickness

of the wire [30]. Metal C-rings also allow substantial displacement.

4.8.3

CLEANING AND SURFACE FINISHING

Outgassing from interior chamber surfaces can be minimized by creating a thin,

dense, stable passivation layer. This both reduces the amount of gas that can be

entrapped on the surface and provides a barrier against gas diffusion from the

bulk. The AI2O3 that naturally forms on aluminum can be made to have excellent

vacuum characteristics and has been widely used. This oxide should be as pure as

possible, because hydroxides and impurities, such as magnesium oxide, tend to

disrupt the lattice structure, leading to lower density and higher porosity. Suitable

finishes are produced by completely removing the scale as received from the mill

and growing a new layer under controlled conditions. Scales can be removed ei-

ther by mechanical cutting (e.g., extrusion or machining) or by chemical etching.

High-performance scales can be grown on all standard alloys commonly used

in vacuum chambers. However, pure aluminum (1000 series) tends to produce

slightly better results, probably because it does not contain alloying elements that

disrupt the oxide structure.

4.8.3.1 Mechanical Finishes

Mechanical finishing methods have been designated EX and EL processes [31].

The former involves extruding or machining the surface, without cutting fluid, in

an oxygen and argon atmosphere at standard pressure that immediately reoxidizes

die newly exposed bare metal (Figures 7 and 8 respectively). Argon makes up

4.8.3 Cleaning and Surface Finishing

519

Fig.

Schematic diagram of the EX special extrusion process.

1) Al Pipe, 2) Vacuum tight end, 3) Argon and oxygen mixture, 4) Port-hole die, and 5) Gas

Tank. (Courtesy of H. Ishimaru, "Developments and Applications for All-Aluminum Alloys

Vacuum Systems," MRS Bulletin, Vol. XV, No. 7, July 1990, p. 24.)

about 90% of the atmosphere to dilute the oxygen and prevent explosive "burn-

ing" of machining chips and particles. Moisture content must be no more than a

few ppm to minimize the formation of hydroxides. The EL process involves ma-

chining in a normal air atmosphere but using alcohol (normally ethanol) as a cut-

ting fluid

[32].

This results in a similar thin, dense oxide scale; however, the actual

chemical mechanism that produces it is unclear. It has been reported that scratches

can be repaired by manually scraping with Scotch-Brite® soaked in ethanol.

Fig.

Schematic diagram of EX macliining process.

©-

^^&

.^S^W^-.SS'^SS.SSSSWV^^^SS'

•^^^^uY>^•>Y>^^^^^^^'>^^^•>^>^^^^^a

Pump

f?i-6

—t>k3 Ar^Oi

(1) vacuum chamber, 2) axis of the lathe, (3) axis of the tool, (4) tool, (5) work, (6) vacuum

seal, and (7) vacuum gauge. (Courtesy of H. Ishimaru, "Developments and Applications for

All-Aluminum Alloys Vacuum Systems," MRS Bulletin, Vol. XV, No. 7, July 1990, p. 24.)

520 Chapter 4.8: Aluminum-Based Vacuum Systems

4.8.3.2 Chemical Finishing—A Well-Documented Recipe

Chemical finishing methods are much easier than mechanical cleaning and offer

vacuum performance that is often comparable. Although many successful pro-

cedures have been demonstrated, the cleaning procedure developed for the Ad-

vanced Photon Source at Argonne National Laboratory is among the simplest and

most carefully studied and documented

[33,341.

It is designed for 6063 and 2219

alloys and may be summarized as follows [35]:

Use water-based cutting fluids and avoid oils during initial manufacturing.

Preclean with a high pressure spray of 2% Almeco 18® (Henkel/Parker Am-

chem P3 Detergent).

Clean in an ultrasonic bath of 2% Almeco 18 at 65°C for 10 minutes. When

alloy 2219 is present, temperature must be reduced to 50°C to prevent exces-

sive copper buildup on the surface. Copper formed inadvertently can be re-

moved with 2% Citrinox®. The latter is also used instead of Almeco 18 when

copper components are present in the structure.

Rinse with deionized water at room temperature for 10 minutes. (Elevated

temperatures were found to cause the oxide scale to substantially thicken.)

Blow-dry with hot, dry nitrogen.

The scale that reforms in the presence of air and moisture is a heterogeneous mix

of hydroxides and oxides (y-Al(0H)3, y-AlOOH, and 7-AI2O3). When heated to

150°C, these transform to 7-AI2O3 [36] creating a relatively stable and homoge-

neous passivation layer. For this reason, at least one bake-out should immediately

follow any chemical finishing procedure.

Surfaces heavily contaminated with oil should be precleaned with organic

solvents. Other cleaning agents such as Alconox® have been successfully used

[37,38].

Ultrasonic agitation should be intense (approximately 8 mW/cm-^) but

not enough to cause pitting due to cavitation. Oxides of alloying elements, such

as magnesium, that the detergent does not attack, sometimes form a smut that can

be removed with an agent such as Wyandotte Alutone® [40]. (This has not been a

problem with 6063 cleaned with Almeco 18.) Additional cleaning can be done

with strong acids or bases such as Amklene® (principally KOH) [41] or Amchem

etchant #33 [42], however the benefits of this appear to be marginal [43]. Several

rinsing cycles are often used. Initial rinsing with tap water followed by deionized

water has been demonstrated to be effective [44].

4.8.4

MECHANICAL CONSIDERATIONS

Aluminum is commercially available in a large variety of alloys with widely vary-

ing characteristics. Most alloys suitable for vacuum systems have a yield strength

4.8.4 Mechanical Considerations 521

compai'able to stainless steel 304 and generally require wall thicknesses only

slightly greater. An important cost advantage of aluminum over stainless steel is

the relative ease with which complex chamber geometries can be fabricated by

machining from a single block or by extrusion. Alloy selection usually depends

on fabrication issues such as weldability, machineability, extrudability, availabil-

ity, and cost. Sometimes corrosion resistance and chemical compatibility with the

process must also be considered.

4.8.4.1 Alloy Types and Designations

Generally, only wrought aluminum alloys are suitable for high-vacuum chambers,

because cast alloys are too porous. Wrought alloys are designated by a four-digit

number followed by a hyphenated suffix specifying temper. The first digit speci-

fies the dominant alloying element as follows: 1, >99.00% aluminum; 2, copper;

manganese; 4, silicon; 5, magnesium; 6, magnesium and silicon; and 7, zinc.

The other three digits follow no particular pattern.

Data on manufacturability, strength, temperature affects, and chemical compo-

sitions of selected alloys are given in Tables 2, 3, 4, and 5 respectively [45]. All

alloy series have been successfully used in high-vacuum systems except the 7000

series,

which is unsuitable because of the high vapor pressure of its principal al-

loying element, zinc. Alloys in the 2000 series may require special shielding in

applications sensitive to copper. Usually, the main body of a chamber is made

from 5000 or 6000 series alloys such as

5083,

6061, or

6063.

Components requir-

ing high strength can be made from 2000 series, especially 2219, which is the

most weldable of this series.

The main challenge to machining aluminum is its tendency to form long chips

that tear rather than shear from the surface. Softer alloys tend to build up on the

cutting tool unless it is free of scratches and carefully lubricated to inhibit bond-

ing [46]. Diamond tools can be used for especially fine finishes. In some cases,

machining can be done without cutting fluid to minimize residual contamination.

However, the use of water-based cutting fluids has been shown to provide com-

parable vacuum performance [47]. Mineral oil should be avoided. The machine-

ability ratings in Table 2 are in increasing order of chip length and finish quality.

4.8.4.2 Chamber Wall Thicicness Requirements

Chambers can be considered as being composed of flat plates and cylinders for

purposes of determining the wall thickness required to withstand the differential

pressure created by an internal vacuum.