U.S. patent number 3,928,112 [Application Number 05/430,825] was granted by the patent office on 1975-12-23 for process for producing stabilized anatase titanium dioxide surfaces for durable adhesive bonding.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Army. Invention is credited to Willard C. Hamilton, Raymond F. Wegman.
United States Patent |
3,928,112 |
Hamilton , et al. |
December 23, 1975 |
Process for producing stabilized anatase titanium dioxide surfaces
for durable adhesive bonding
Abstract
The conventional Phosphate-Fluoride process for treating
titanium parts to produce anatase titanium dioxide surfaces for
adhesive bonding, which includes (1) an acid pickling bath step,
(2) a phosphate-fluoride bath step, and (3) a warm bath step, is
modified by adding about 2%, by weight, of an alkali metal sulfate,
such as a neutral sulfate of sodium or lithium, to the bath in any
of steps (1), (2) and (3), to stabilize the anatase surface and
thereby prevent transformation thereof to the undesirable rutile
structure.
Inventors: |
Hamilton; Willard C. (Webster,
NY), Wegman; Raymond F. (Ledgewood, NJ) |
Assignee: |
The United States of America as
represented by the Secretary of the Army (Washington,
DC)
|
Family
ID: |
23709209 |
Appl.
No.: |
05/430,825 |
Filed: |
January 4, 1974 |
Current U.S.
Class: |
156/319;
148/256 |
Current CPC
Class: |
C23C
22/83 (20130101); C23C 22/36 (20130101) |
Current International
Class: |
C23C
22/05 (20060101); C23C 22/83 (20060101); C23C
22/82 (20060101); C23C 22/36 (20060101); C23F
007/06 (); C09J 005/04 () |
Field of
Search: |
;148/6.14R,6.15R,6.17
;117/127,49 ;156/319 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
729,148 |
|
Mar 1966 |
|
CA |
|
754,514 |
|
Mar 1967 |
|
CA |
|
Other References
Sastry, R.L.N., Effect of Some Impurities on the Anatase-Rutile
Transformon, Indian Journal of Chemistry, Vol. 3, Sept. 1965, pp.
414-415..
|
Primary Examiner: Kendall; Ralph S.
Assistant Examiner: Wolfe, Jr.; Charles R.
Attorney, Agent or Firm: Edelberg; Nathan Gibson; Robert P.
Webb; Thomas R.
Claims
We claim:
1. In the method of adhesive bonding of titanium parts to other
parts wherein said titanium parts are subjected, prior to adhesive
bonding, to a phosphate-fluoride process to produce anatase
titanium dioxide surfaces for adhesive bonding, which process
includes (1) an acid pickle bath step, (2) a phosphate-fluoride
bath step, and (3) a hot water bath step, and said anatase titanium
dioxide surfaces, after being produced, are adhesively bonded to
other parts,
the improvement consisting of adding about 2% of a neutral alkali
metal sulfate to the bath in any of said three steps for
stabilizing the anatase surfaces and thereby preventing
transformation thereof to the undesired rutile structure.
2. The method as in claim 1, wherein said phosphate-fluoride
process comprises the following steps, in the order named:
A. cleaning said parts;
B. rinsing said parts with water at room temperature to
80.degree.C.;
C. immersing said parts, for about 2 minutes at room temperature,
in an aqueous pickling solution containing:
about 2%, by volume, 70% HF,
about 35%, by volume, 70% HNO.sub.3, and the rest water;
D. rinsing said parts with water at room temperature;
E. soaking said parts, for about 2 minutes at room temperature, in
an aqueous solution containing:
about 2%, by volume, 70% HF,
about 5%, by weight, Na.sub.3 PO.sub.4,
about 2%, by weight, KF, and the rest water;
F. rinsing said parts with water at room temperature;
G. soaking said parts in water, for about 15 minutes at about
65.degree.C.;
H. rinsing said parts with water at room temperature for about 45
seconds; and
I. drying said parts in air at about 65.degree.C;
and any of the steps C, E and G is modified by adding to the bath
about 2%, by weight, of a sulfate of sodium lithium, for
stabilizing said anatase surfaces as stated.
3. The method as in claim 2, wherein deionized water is used in
each of steps B through H.
4. The method as in claim 2, wherein the sulfate is added to the
pickling solution in step C.
5. The process as in claim 4, wherein the sulfate is Na.sub.2
SO.sub.4.
6. The method as in claim 2, wherein the sulfate is added to the
solution in step E.
7. The method as in claim 6, wherein the sulfate is Li.sub.2
SO.sub.4.
8. The method as in claim 2, wherein the sulfate is added to the
water in step G.
9. The method as in claim 8, wherein the sulfate is Na.sub.2
SO.sub.4.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to the treatment of titanium parts,
either pure titanium or alloys of titanium with other metals such
as aluminum, to produce anatose titanium dioxide surfaces that can
be bonded to other parts, e.g. by an epoxy type adhesive, to
produce durable, long-life bonds.
An early method of preparing titanium surfaces for bonding, used in
the production of UH-1 aircraft by Bell Helicopter Company in 1963,
involved cleaning the metal with an alkaline cleaner. This method
produced a surface containing the rutile structure of titanium
dioxide. Although this surface will accept the adhesive and produce
very good initial bond strengths, it will not produce a
satisfactory durable bond. The surface has been found to have a
preference for moisture, rather than the adhesive, and when exposed
to a moist atmosphere will become unbonded from the adhesive.
In 1966, a "Phosphate-Fluoride" process for treating titanium was
introduced by Bell Helicopter Company in the manufacture of all of
its aircraft. This process includes the following steps:
A. immersing clean titanium parts for several minutes at room
temperature in an aqueous pickling solution containing hydrofluoric
and nitric acids;
B. soaking the parts for several minutes at room temperature in an
aqueous solution containing hydrofluoric acid, trisodium phosphate
and potassium fluoride;
C. soaking the parts in hot water for at least 15 minutes; and
D. various rinsing and drying steps.
In accelerated tests performed on epoxy-bonded joints incorporating
titanium parts processed by the above methods, we have found that
the change in processing from the alkaline method to the
Phosphate-Fluoride process resulted in an increase in the
durability of the joints from about 10 hours for the alkaline
method to about 65-75 hours for the Phosphate-Fluoride method.
These accelerated tests involved subjecting the bonded joint to a
sustained load of 880 psi (pounds per square inch) at 60.degree.C.
and 95% relative humidity. We also found that the surface produced
by the Phosphate-Fluoride process was the anatase structure of
titanium dioxide, and that the anatase structure has a preference
for the adhesive rather than water. However, we also found that
this anatase structure undergoes a change to the rutile structure
when the joint is subjected to elevated temperature, high humidity
and stress. This change in structure has been shown to occur in
test specimens and in aircraft components returned from service
after bond failures were found. The crystalline rearrangement is
accompanied by about an 8% change in volume.
Based upon the above findings of crystalline structure and
rearrangement we embarked on a program of investigation, the intent
of which was to produce a stable surface layer containing the
anatase structure of titanium dioxide. The surface layers had to be
produced in thin layers by a practical production method without
affecting the properties of the titanium or titanium alloy
itself.
Investigations made, at the Indian Institute for Science, and
elsewhere, on the effects of the presence of various impurities on
the transformation of the anatase to the rutile structure of
titanium dioxide indicated that certain impurities tend to inhibit
this transformation. For example, in a paper entitled "Effect on
Some Impurities on the Anatase-Rutile Transformation," by R. L. N.
Sastry, Indian Journal of Chemistry, Vol. 3, September, 1965, pp.
414-415, the author reported that the alkali metal ions, ammonium,
barium, sodium and potassium hinder the transformation, whereas the
lithium ion favors the transformation. He stated that both
SO.sub.4.sup.2 .sup.- and Na.sup.+ (ions) inhibit the
transformation, and observed that the effect was additive in the
case of sodium sulphate. Some of the various investigations of the
anatase-rutile transformation related to titanium dioxide used as a
whitener in paints. We know of no suggestion for purposely adding
certain impurities to processes for preparing titanium for adhesive
bonding to improve the durability of the resulting bonds.
An object of the present invention is to provide stable anatase
titanium dioxide surfaces on titanium parts for durable adhesive
bonding to other parts.
In accordance with the present invention, the Phosphate-Fluoride
process for treating titanium parts to produce anatase titanium
dioxide surfaces for adhesive bonding is modified by adding about
2%, by weight, of a neutral alkali metal sulfate to the solution or
water in any of (1) the acid pickle step, (2) the
phosphate-fluoride soak step, or (3) the warm water soak step, to
stabilize the anatase structure and thus prevent transformation
thereof to the undesired rutile structure. Accelerated tests, under
the conditions given above, on bonded joints made with titanium
dioxide surfaces prepared by such modifications of the
Phosphate-Fluoride process have shown that the addition of the
alkali metal sulfate in the basic process increased the durability
of the joint, at 880 psi., from about 65-75 hours, by 480 to 700%.
The highest durability, 528 hours average, was produced by adding
2%, by weight, of sodium sulfate to the acid pickle step (1), while
the lowest durability, 361 hours average, was produced by adding
2%, by weight, of lithium sulfate to the phosphate-fluoride step
(3).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The complete flow chart of the conventional Phosphate-Fluoride
process for treating titanium parts is as follows:
Step 1. Rinse the parts with a solvent, e.g. acetone, at room
temperature.
Step 2. Immerse in an alkaline cleaner, e.g. aqueous Oakite HD 126,
for 5 to 15 minutes at 60.degree. to 80.degree.C.
Step 3. Rinse with running water at room temperature to
80.degree.C.
Step 4. Immerse for about 2 minutes at room temperature in an
aqueous pickle solution containing 1.8%, by volume, of 70% HF, 35%,
by volume, of 70% HNO.sub.3, and the rest water.
Step 5. Rinse with water at room temperature.
Step 6. Soak for about 2 minutes at room temperature in an aqueous
solution containing 2%, by volume, of 70% HF, 5.1%, by volume, of
Na.sub.3 PO.sub.4, 2.1%, by weight, of KF, and the rest water.
Step 7. Rinse with running water at room temperature.
Step 8. Soak in water for about 15 minutes at about
65.degree.C.
Step 9. Rinse with running water at room temperature.
Step 10. Dry with air at room temperature to 65.degree.C.
Deionized water is used in steps 2 through 9.
Average durabilities of 75, 25 and 9 hours were obtained from the
accelerated tests referred to above on epoxy-bonded joints
including titanium surfaces treated by this conventional
Phosphate-Fluoride process (without modification), at stresses of
880, 1,320 and 1,760 lbs./in..sup.2, respectively.
In accordance with the invention, any of steps 4, 6 and 8 of the
conventional Phosphate-Fluoride process is modified by the addition
of several ounces per gallon of a neutral alkali metal sulfate to
the bath, to stabilize the anatase structure of the titanium
dioxide surface. Four examples of modified processes which were
carried out and tested successfully will be described. In each
example, the titanium parts used were "Titanium 6, 4 Alloy," which
consists of 6% aluminum, 4% vanadium, and the remainder
titanium.
EXAMPLE I
Step 1. Degrease, with vapor or solvent;
Step 2. Alkaline clean: immerse parts in alkaline cleanser for 5-15
minutes at 60.degree. to 80.degree.C.;
Step 3. Rinse, with water at room temperature to 80.degree.C.;
Step 4. Pickle: immerse for 2 minutes at room temperature in an
aqueous solution of:
1.8%, by volume, of 70% hydrofluoric acid,
2%, by weight, of neutral sodium sulfate, and
35%, by volume, of 70% nitric acid, and the rest water;
Step 5. Rinse, with water at room temperature;
Step 6. Phosphate Fluoride Treat: soak parts for 1.5 to 2.5 minutes
at room temperature in an aqueous solution of:
5.1%, by volume, trisodium phosphate, of
2.1%, by weight, of potassium fluoride,
2%, by volume, of 70% hydrofluoric acid, and the rest water;
Step 7. Rinse, with water at room temperature;
Step 8. Hot Water Soak: use water at about 65.degree.C. for 15
minutes;
Step 9. Final Rinse, with water at room temperature for 30 seconds
to 1 minute; and
Step 10. Dry; with air at room temperature at 65.degree.C.
Preferably, deionized water is used in steps 2 through 9. The
process of Example I differed from the conventional
Phosphate-Fluoride process by adding 2%, by volume, of neutral
sodium sulfate in pickling step 4.
Average durabilities of 528, 105 and 61 hours, were obtained from
accelerated tests, at stresses of 880, 1,320 and 1,760
lbs./in..sup.2, respectively, conducted under the conditions
described above, on many samples of adhesive bonds made with
titanium parts treated by the process of Example I. It should be
noted that the improvement in durability produced by the new
titanium treatment of Example I over the convention treatment was
about 700%, 420% and 680%, at 880, 1,320 and 1,760 lbs./in..sup.2,
respectively.
EXAMPLE II
In Example II, the conventional Phosphate-Fluoride process was
modified solely by adding 2%, by weight, of neutral lithium sulfate
to the solution in step 6, and phosphaste-fluoride soak step.
EXAMPLE III
In Example III, the conventional Phosphate-Fluoride process was
modified solely by adding 2%, by weight, of neutral sodium sulfate
to the hot water bath in step 8.
EXAMPLE IV
In Example IV, the conventional Phosphate-Fluoride process was
modified by:
a. adding 2%, by weight, of neutral sodium sulfate to the pickling
solution in step 4; and
b. adding 2%, by weight, of neutral lithium sulfate to the
phosphate-fluoride solution in step 6.
Average durabilities obtained from accelerated tests, as described
above, on samples of adhesive bonds made with titanium parts
treated by the processes of Examples II, III and IV, respectively,
are listed in columns C, D and E of the following table, wherein
column A contains the durabilities for the conventional process and
column B contains the durabilities for Example I: Stress A B C D E
______________________________________ 880 75 528 361 394 370 1320
25 105 144 135 84 1760 9 61 32 14 29
______________________________________
The table shows that the durabilities for Examples II and III are
higher than for Example I at 1,320 lbs./in..sup.2, but lower at 880
and 1,760 lbs./in..sup.2. Column E shows that the addition of the
neutral alkali metal sulfate to two of the steps in the process of
Example IV does not produce any greater effect than the addition
thereof to only one step, as in Example I, II and III. Each of the
durabilities listed in the above table was obtained by averaging
anywhere from 5 to 16 different tests made under the same
conditions, and the individual values for these tests varied
considerably, largely due to the failure of some bonds for reasons
other than a change from anatase to rutile structure, which
accounts for some of the variations in the results of various
processes. For example, the value 14 for Example III (column D) at
1,760 lbs./in..sup.2 was obtained by averaging 18, 40, 37, 3, 3, 4,
4 and 2. If the last five values are ignored as failures resulting
from some other cause, the average value becomes 27, which is close
to the corresponding values for Examples II and IV (C and E), and
represents a minimum of about 300% improvement in durability
produced by the present invention at 1,760 lbs./in..sup.2.
* * * * *