An electron bombardment argon ion source was used to sputter deposit 17 different metal and metal oxide films ranging in thickness from 1 to 8 micrometers on H-13 steel substrates. The film adherence to the substrate surface was measured using a tensile test apparatus. Comparisons in bond strength were made between ion beam, ion plating, and RF deposited films. A protective coating to prevent heat checking in H-13 steel dies used for aluminum die casting was studied. The results of exposing the coated substrates to temperatures up to 700 degrees are presented

Mirtich, M. J.

English

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(NASA-TM-81585) ADHERENCE OF
SP,'Jf T8H DEPOS.IrED
(N AS A)	 17 p HC A,
IUN JEAN	 N80- :31527	 !
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NASA Technical Memorandum 81585
. r .
. DIIERENCE OF ION BEAM
SPUTTER DEPOSITED METAL
F'iI.jMS ON H-13 STEEL
Michael J. Mirtich
Lewis Research Center
Cleveland, Ohio
Prepared for the
Twenty-seventh National American Vacuum Society Symposia' ^n'{^a,.:
Detroit, Michigan, October 14-17, 1980
NASA,
ADHERENCE OF ION REAM SPUTTER DEPOSITED METAL FILMS ON H-13 STEEL
by Michael J. Mirtich
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
INTRODUCTION
Dies used to make aluminum castings have a limited lifetime depending
upon materials properties and casting conditions. A typical die can usually
be used to make between 30 to 50 thousand castings. At a cost of 50 to 70
thousanc dollars per die, doubling or tripling the us,ful lifetime of a die
could result in significant financial, manpower and energy savings. Shown
in Fig. 1 is an H-13 die steel surface after numerous castings. The surface
cracks appear after many cycles of exposure to molten aluminum and cold
water, used to cool the die. This cracking phenomenon is known as heat
checkingl and is multifaceted - ranging from thermal stress to attack or
soldering to the die by molten aluminum. It has been suggested by die manu-
facturers that a protective coating of either Mo or Cr or a precious metal
might negate this problem. With this in mind, a program was developed at
NASA Lewis Research Center in cooperation with the Die Casting Research
Foundation to improve die lifetime by sputter depositing coatings on d die
surface using ion beam developed technology. Although this program consists
of three overlapping phases only the results of the first phase will be re-
presented in this paper.
In phase 1, coatings of various candidate metal and metal oxides were
sputter deposited on flat surfaces of H-13 steel (die materiel) for a pre-
liminary evaluation of the adherence o:' films of various thicknesses.
During this phase the sputter technique, sputter rate, deposition energy
level and maximum thickness were determined for each candidate Coating mate-
1
Rrial. Some of these coated substrates were also exposed to elevated tem-
perature levels, and the adherence of the coating was again evaluated. A
few substrates were coated with films using RF and ion plating techniques.
Comparisons in bond strength of the films using these techniques and ion
beam deposition were also made and presented herein.
APPARATUS AND PRUCEDURE
A 30 cm diameter argon ion beam source2
 with its ion optics masked
down to an ion beam diameter of 1U cm, was used to sputter deposit metal and
metal oxides onto H-13 steel substrates. The source, developed from elec-
tric propulsion technology, uses a hollow cathode to provide the discharge
for the ion beam source. Ion beam extraction is accomplished by a dished,
two—grid ion optics system. Neutralization of the beam is achieved by the
use of a plasma—bridge neutralizer, 2 using an argon gas cathode. The ion
source has a capability of operating at beam energies between 100 and 1500
eV. For these studies the source was operated at an energy level of 1000 eV.
The vacuum facility, 1.5 m in diameter and 7.3 m long, is sufficiently
large to minimize backsputtered facility material from contaminating the
deposited films. To further prevent contamination, a liquid nitrogen cooled
baffle was used in the vacuum tank. While the ion source was operating, the
pressure in the region of the experiment is about 3x10-4 torr
(3.9x!0-2
 Pa). This pressure was maintained for most of the depositions.
The partial pressure of N 2
 or air in processing nitride or oxide compounds
was about 1x7.0—' torr. It should be noted that at 10 -3 torr about 50
percent of the ions become neutralized through charge exchange 3
 and a
majority of the sputtering is done by 1000 eV neutrals rather than ions.
The substrate disks 2.54 cm in diameter ana 1.5 mm thick were made of
the same material useu to make dies, that is H-13 steel. The disks were
2
F;
5
vacuum heat treated, in a similar manner as performed on functional dies to
achieve a Rockwell C hardness of 47. After heat treatment the substrates
were polished to either a lb or 32 microinch RMS finish.
The target materials were commercially purchased, most of which were
15 cm diameter disks, except for Au and Pt which were strips 2.54 cal wide.
These strips were pieced together and overlapped to form a square target
12.7 cm by 12.7 cm. The strips were then mounted on a 12.7 cm by 12.7 cm
steel substrate for support.
Thg H-13 disks were cleaned prior to insertion into the vacuum facil-
ity. The cleaning procedure included cleaning the disks with a 2 percent
Liquinox@ soap solution, rinsing in distilled water and drying with nitrogen
gas. This procedure was performed three times on each disk.
The substrates were mounted on a push-pull rod that could be retracted
through a vacuum chamber gate valve. Prior to deposition, first the target
and then the substrates were sputter cleaned by the ion beam for 1/2 hour at
a beam energy of 1000 eV and a current density of 2 mA/cm^. During the
sputter cleaning of the substrate, the target is located in a stored posi-
tion. During deposition the target and substrate surfaces are 19 cm apart
and normal to each other; the target is at a 45 0 with respect to the inci-
dent ion bieam. The distance from the ion source to the target was 19 cm.
The deposition rate of target material at the location of the suustrate
was measured by placing a piece of fused silica glass, masked to allow a
step to form during deposition. A surface profiler (Alpha-Step) was used to
measure the thickness of the film and hence a deposition rate could be
derived.
Presented in Table 1 are the deposition rates at the substrate location
for various target materials. The ion beam was larger in diameter than the
area of the target, and totally filled the target material. The deposition
3
.n
rates are presented in Table x for a current density at the target of
1 mA/cm2
 and an ion energy of 1UUU eV.
Film bond strength tests were also performed on H-13 specimens that
were ion beam sputter coated with eleven different materials by Commonwealth
Scienific Corporation. At Commonwealth, the 1U cm diameter source4
 used
grids dished such that the focal point was 45 cm downstream of the grid
plane. The target et a 45 0 with respect to the ion sources was located
30 cm from the 10 cm ion source. The substrates were located normal to and
5 cm from the target. Because of the grid focusing and spacial variation in
current density the ion beam aid not totally illuminate the target.
Adherence measurements, of the film to 'the substrate, were maae using a
Sebastian Series Coating Adherence Tester. Measurements are made by first
baking an epoxy coated bonding stud (0.127 cm long by 0.16 cm in diam) to
the deposited film surface at 125° C for 2 hours in an oven. This ele-
vated temperature allows for a thermal cure of the epoxy coated stud to the
film. After the stud is bonded to the film, the test stud is inserted into
the instrument and a load is applied normal to the surface at a pre-
determined rate until failure of the bond ensues. This break in the bond is
recorded in lb/in 2 , and is considered a measure of the adherence, if the
break occurs at the fil m-substrate interface. The maximum adherence from
this test is the strength of the epoxy bond (approx. 6500-9000 lb/in2).
For most of the films tested in these experiments, the bond strength of the
film to the substrate exceeded the bona strength of the stud (epoxy) to the
film. The location of the break in the bond was determined by using an
optical microscope or a scanning electron microscope.
Pal
4
.-.s
k IT a
RESULTS AND DISCUSSION
A film used to prottict a die surface must exhibit good adherence if it
is to remain in function Qn the die surface. Ion beam sputtering enables
atomic cleaning of the surface thus permiting intimate contact between the
deposited film and the actual substrate surface. Shown in Fig. 2 is the
adherence in lb/in 2
 of a 1 micrometer thick Cu film sputter deposited on
three H-13 steel disks. In one case, the H-13 steel was not sputter cleaned
by the ion beam before Cu deposition. The resulting Cu coating exhibited
rather poor adherence of only HO lb/in k . The second and third H-13 disks
shown in Fig. 2 were sputter cleaned by the ion beam at 1 keV and 2 mA/cm2
for 20 and 30 minutes, respectively. The resulting 1 micrometer Cu film had
an adherence of 6200 lb/ins . The necessity of precleaning the substrate
b y ion beam sputtering before deposition to obtain adherent coatings has
been shown by others in the literature, 5,6 and was a prerequisite to all
deposition techniques used in these studies.
Presented in Table 11 are the adherence: of various thickness ion beam
deposited metal and metal oxide films on H-13 steel. The adherence measure-
ments are the average of 5 measurements on each sample. Some of the deposi-
tions were done at NASA—Lewis Research Center and some under contract at
Commonwealth Scienific Corp. Although both facilities used different size
ion sources, the experimental procedures were the same. The use of differ-
ent facilities and sources does not seem to affect the results, for MO de-
posited at both LeRC and Commonwealth exhibit the same good adherence for
films <4 mm thick. The symbol * used in Table 11 indicates a break in the
bond at the epoxy-film interface, meaning the film—substrate bond strength
was greater than the value indicated in the table. For all the metallic
films <4 um, the bond strengths between the film and the substrate were
5
..
greater than the epoxy-film bond. The only exception was B 4C, which
formed a poor bond on H-13 for all thicknesses. Films of Au, Ng, Mo and
Ta5Si 3 , 8 um thick, exhibit as good an adherence as a 1 um coating of
the same material. For the other materials, film-substrate bonds could be
broken at 8000 lb/in2 or less. It was also found that the adherence of
the films )) ,.al e independent of the substrate surface finish (16 or U pin.
finish).
To obtain proper stoichiurw, try it was necessary at times to bleed N2
or air into the vacuum system when depositing a nitride or oxide curnNound.
introduction of air into a vacuum chamber during deposition can lead to
limited lifetime of the hollow or filament cathodes used in the ion source
and hence a M90 film of only 2.4 um maximum thickness was deposited. From
the data presented in Table 11, it is clear that depositions of metals and
metallic compounds on H-13 steel using an ion source have a bond strength
much greater than the upper limit adherence measuring capabilities of the
instrument used to make the measurement.
Film bond strengths were also measured on some materials that were de-
posited on H-13 using other deposition systems. Shown in Fig. 3 is a plot
of the film adherence as a function of thickness for Mo, Gr, Ni and ZrO2
using ion beam, radio frequency (rf) or ion plating deposition systems. Ion
plating of Mo was done on contract at Hohlman plating. The upen symbols
indicate that the bond broke at the epoxy-film interface and the closed sym-
bols a break at the film-substrate interface. For Mo, ZrO 2
 and Cr (<3 um
thick) deposited with either an ion beam or rf sputter system, the adherence
of the films were better than the epoxy bond strength. Ni film adherence on
H-13 steel was greater than 8000 lb/in 2 when an ion beam source was used
for film deposition. Adherence of rf sputter deposited films ranged from
6
about 7000 to 280 lb/in2
 depending on film thicknesG, Tne use of an ion
beam or rf source appears to give bettor Mo film adherence than from deposi-
tion using a commercial ion plating system.
In the second phase of this program the best candidate coatings from
phase I are to be coated on a thermal fatigue test specimen- G)
 This
coated test specimen will then be emerseu in liquid .aluminum and then cooled
by water fur 15 000 cycles, to simulate part of the environment of a die
casting die, To further evaluate the adherence of the coatings in phase i
at elevated temperatures some of the coated H-13 disks were exposed in an
oven for 15 minutes at temperature levels of QU O and 7UU° C and allowed to
cool to room temperature. This procedure was performed three times before
another adherence measurement. Shown in Fig, 4 is film adherence as a func-
tion of exposure to elevated temperatures. Also listed in the figure are
the films deposited by an ion source, whose adherence remained unchanged at
4(0° C and even after exposure to 700 0 C. For these listed materials the
film-substrate bond strength was still greater than the epoxy-film bond
strength. The 1 um Cu film deposited by rf, the 8 um Mo
 film deposited by
ion plating and the 8 um A1N film deposited by the ion beam show decreases
in adherence after exposure to elevated teri ►perature. Not all films were
tested at elevated temperatures.
To qualitatively check the mechanical integrity of the film—substrate
bond, a center punch was used to indent three coated surfaces. Shown in
Fig. 5 are (1 um, 4 um) Mo and a G um Au films that were subjected to in-
aenting by an automatic center punch. Although there were tears in all the
films at the edge of the crater, the ion beam deposited Mo ana Au films
experienced only a slight crack in the film at the crater edge, whereas the
ion plated Mo was torn from its H-13 substrate.
7
CONCLUDING REMARKS
Two different Argon ion beam sources were used to sputter deposit
adherent metallic films up to 8 um thick on H-13 steel. The adherence of
the coatings was in most cases greater than the upper limit of the adherence
measuring device. To obtain adherent thin films, it was necessar,, *o pre-
clean the substrate in the ion beam before deposition. It was necessary, at
times, to introduce N 2
 or air to obtain the proper stoichiometry in N2
or 02
 metallic compounds. For all pure metallic films 4 pm thick or less
the bond strength of the film to the substrate were greater than the epoxy
to film bond (-8000 psi). Films of Au, Ag, M  and Ta 5Si 3 8 um thick
exhibit as good a bond strength on H-13 as do 1 um thick films of these
materials. Elevation to 700° C of certain ion beam sputter deposited
coatings did not affect the film—substrate bond strength.
M  and Cu films deposited by ion plating and rf sources showed de-
creased adherence after exposure to elevated temperatures.
REFERENCES
1. J. C. benedyk, D. J. Moracz, and J. F. Wallace, Proceedings of the Sixth
Society of Die Casting Enqineers International die Casting Exposition
and Congress, Cleveland, Ohio (November 16-19, 1970), SLICE Paper
No. 111.
2. J. S. Sovey, "A 30—Cm Diameter Argon Ion Source," AIAA Paper i6-1017,
November 1976.
3. R. S. Robinson, "Physical Processes in Directed Ion Beam Sputtering,"
Colorado State Univ., Fort Collins, Colorado, NASA Cr-159667, March
1979.
4. G. R. Thompson, "Ion Beam Coating — A New Deposition Method," Solid State
Technol. 21 (12), 73-77 (1978).
8
b. J. A. Thornton, "Influunce of Suustrdte Temperature and bepu'.iteU Rate on
Structure of Thick bputterud-Loppur WdLings," o. Vac. Sci. Technul.,
12 (4), 830-835 (19/6).
6. D. M. Mattox, "Surface Cleaning in Thin Film Technology," Thin Solid
Films, 63 81-96 (1978).
1
9
10
Pf
TABLE I. — SPUTTER DEPUSITION RATES FUR V kIO U, TARGET MATERIALS
[Ion energy, 1000 eV, ion current density at the target, 1 mA/cm2;
target to substrate distance, 19 cm, ion source to target, 19 cm.j
Target Ag Au $i3N4 Cr3C2 Pt Ni Cr AIN Co Mo W Zr02 Ta5Si3
material
Deposition 120 97 96 96 48 27 26 25 24 19 16 1b 15
rate, R/min
r'
K
TABLE I1.
	 ADHERENCE OF MATERIALS ION BEAM SPUTTER
UEPOSITEU ON H-13 STEEL
Coating
	 Coating adherence for various thicknesses, lb/in2material
1 Pm 2 um	 4 um8 um
NASA—Lewis coatings
Au 7930* 7760* 8OU0* 7120*
Ag 8420* 8320* 8430*
(10 um)
Pt 777U*
(1.5 um) 7210*(2.5 um) 7690*(3.5 um) 7430*(5.5 um)
W 5400 6200
Mo 8020* 8480* 8460* 8460*
(3 um) (6 um)
Ni 834U* 8640* 8180*
(3.6 um)
Cr- 789u`* 8060* 8230*
Commonwealth Eng. coatings
Co 8060 *
(0.6	 um)
4160*
(1.2
 um)T* 7690(4.8 um)M 90 6340*
(0.3 um)
b650*
(0.6 um)
*
(1.2 um) 6250*(2.4 um)
A1N 8240* 8460* 8435* 4940
S13N4 8206* 8520* 8136* 2815
Cr3C2 71.15* 8123* 8086* 8033
Mo 78UO* 7600* 6700* 5400
Cr203 7326* 7776* 7116* 6573
Ta5S13 7625* 7918* 7743* 7847*
B 4C 3365 570 303 0
CrB 2 7486* 7838* 803U* 0
Sic 7282* 7968 477 6488
*Bond broke at epoxy—film interface.
11
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Adherence of ion beam sputter deposited metal films on H-13 steel

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