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.

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.

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.