Ceramic and Metal Joining Structure

1. Field of the Invention

The present invention relates to a ceramic and metal joining structure, as a structure for joining a metal shaft to a ceramic turbine wheel of a turbocharger by the use of a metal sleeve.

2. Description of the Prior Art

Ceramic materials have a very high hot hardness but are deficient in strength such as fracture toughness. For this reason, in case, for example, where a turbine wheel of a turbocharger for an internal combustion engine is made of ceramic, it needs to be joined with a metal shaft. An example of this kind of ceramic and metal joining structure is shown in FIG. 9. In the joining structure, a ceramic shaft 101 integral with a ceramic turbine wheel is joined at its end with a corresponding end of a metal shaft 102 by interposing therebetween a buffer member 103, while being received within a metal sleeve 104 together with the metal shaft 102 at their matched ends. The metal sleeve 104 is attached to the ceramic shaft 101 and the metal shaft 102 by brazing or shrink fit.

A disadvantage of the foregoing joining structure is that the ceramic shaft 101 is liable to fracture in the place adjacent a joint end 101a and therefore poor in durability as well as unreliable in operation. This is because high stress concentration is liable to occur in the ceramic shaft 101 in the place adjacent the joint end 101a due to the fact that stresses are produced in the ceramic shaft 101 by a compressive force applied thereto from the brazing metal or the shrink-fitted metal sleeve 104 and to the fact that thermal stresses are liable to be produced in the ceramic shaft 101 in the place adjacent the joint end 101a since the joint end 101a is exposed to high temperature gases when in use.

In accordance with the present invention, there is provided a novel and improved ceramic and metal joining structure which comprises a ceramic shaft, a metal sleeve installed on the ceramic shaft and having inner and outer circumferential walls, and means for securely joining part of the inner circumferential wall of the metal sleeve to the ceramic shaft in such a manner that an axial end of the joint where the inner circumferential wall of the metal sleeve is securedly joined to the ceramic shaft is positioned between opposite axial ends of the metal sleeve and at the same time between opposite axial ends of the ceramic shaft, in which the outer circumferential wall of the metal sleeve is formed with an annular groove in the place corresponding to the axial end of the joint with respect to the axial direction of the metal sleeve.

The above structure is quite effective for overcoming the above noted disadvantages and shortcoming inherent in the prior art structures.

It is accordingly an object of the present invention to provide a novel and improved ceramic and metal joining structure which is excellent in durability, particularly durable in repeated heating, and highly reliable in operation.

It is another object of the present invention to provide a novel and improved ceramic and metal joining structure of the above described character which has an excellent heat resisting property.

It is a further object of the present invention to provide a novel and improved ceramic and metal joining structure of the above described character which can prevent or at least mitigate stress concentration in its ceramic shaft.

It is a still further object of the present invention to provide a novel and improved ceramic and metal joining structure of the above described character which can reduce thermal stresses to be produced in the ceramic shaft.

FIG. 1 is a longitudinal sectional view of a ceramic and metal joining structure according to an embodiment of the present invention;

FIG. 2 is a longitudinal sectional view of another embodiment of the present invention which is applied to a turbine rotor of a turbocharger for use in an automotive vehicle engine;

FIG. 2B is an enlarged perspective view of a metal sleeve employed in the joining structure of FIG. 2A;

FIG. 3 is a view similar to FIG. 2A but showing a less preferred example for comparison with the embodiment of FIG. 2A;

FIG. 4 is a view similar to FIG. 2A but showing a modification of the present invention;

FIG. 5 is a longitudinal sectional view of a further embodiment of the present invention;

FIG. 6 is a view similar to FIG. 2A but showing a further modification of the present invention;

FIGS. 7 and 8 are longitudinal sectional views of further embodiments of the present invention; and

FIG. 9 is a longitudinal sectional view of a prior art ceramic and metal joining structure.

Referring now to the drawings in which like reference numerals designate like parts throughout the several views thereof, there is shown in FIG. 3 a ceramic and metal joining structure according to an embodiment of the present invention. In this embodiment, a round ceramic shaft 1 of 10 mm in diameter and a metal shaft 2 of the same diameter as the ceramic shaft 1 are abuttingly joined by interposing therebetween a buffer member 3 and united together by brazing. More specifically, the ceramic shaft 1 is made of silicon nitride while the metal shaft 2 is made of martensitic stainless steel. The buffer member 3 is three-layered to have two Ni-layers and one W-Ni layer therebetween. The joined ends of the ceramic and metal shafts 1 and 2 are then inserted into a metal sleeve 4 of 15 mm in outer diameter and 10 mm in length. The metal sleeve 4 is made of stainless steel and formed with a stepped bore including a smaller diameter bore portion 4a of 10.2 mm in diameter and 7 mm in length and a larger diameter bore portion 4 b of 11 mm in diameter. In this connection, the joined ends of the ceramic and metal shafts 1 and 2 are inserted into the metal sleeve 4 in such a manner that the buffer member 3 is placed axially in the middle of the first bore portion 4a. The metal sleeve 4 is united at the first bore portion 4a to the ceramic and metal shafts 1 and 2 by brazing with a brazing metal 5. The metal sleeve 4 is formed with an annular groove 6 in the outer periphery thereof. The annular groove 6 is semi-circular (1.5 mm in radius) in cross section and arranged in the position corresponding to an end 7 of the joint where the ceramic shaft 1 is securely joined with the metal sleeve 4, or in other words, in the place corresponding to a shoulder 8 between the smaller and larger bore portions 4a and 4b with respect to the axial direction of the metal sleeve 4. The following test will prove that the above embodiment of this invention is highly effective for preventing stress concentration and production of thermal stresses in the ceramic shaft 1.

(1) Stress Concentration Test

The length of the ceramic shaft 1 is 50 mm, and the stress concentration test is made in such a manner that a load is applied upon the free end of the ceramic shaft 1 at right angles, with the metal sleeve 4 being held fixedly in the place adjacent the joined end of the ceramic shaft 1. The result of the test is shown in Table (1) in which an example for comparison differs from the embodiment of this invention in that it is not formed with the groove 6.

(2) Thermal Stress Test

The length of the ceramic shaft 1 is 50 mm, and the thermal stress test is made by cyclically heating and cooling test articles in such a manner that on the condition that the free end of the metal shaft 2 is cooled down to 50.degree. C., the free end of the ceramic shaft 1 is first heated for one minute in the flame or gases of 900.degree. C. and then cooled down to 100.degree. C. The result of the test is shown in Table (2) in which an example for comparison differs from the embodiment of this invention in that it is not formed with the groove 6.

From the foregoing, it is to be understood that the ceramic and metal joining structure of this invention is effective for preventing or mitigating production of thermal stresses and stress concentration in the ceramic shaft 1. This is partly because the metal sleeve 4 is provided with a sleeve portion 4c out of contact from the ceramic shaft 1, which sleeve portion 4c maintains the temperature of the metal sleeve 4 below a desired value and thereby prevents production of thermal stresses since the sleeve portion 4c is not directly heated by the ceramic shaft 1 and has a heat radiating function and partly because the groove 6 reduces the compressive force applied from the metal sleeve 4 to the portion of the ceramic shaft 1 adjacent the joint end 7.

FIGS. 2A and 2B show a turbocharger rotor according to another embodiment of the present invention. This embodiment includes a ceramic shaft 11a integral with a ceramic turbine wheel 11, a metal shaft 12 adapted to install at the free end thereof a compressor wheel (not shown), and a metal sleeve 14 formed with an oil seal groove 16 for installation of a sealing member (not shown) and a slinger groove 17 for installation of a slinger or oil thrower (not shown). The oil seal groove 16 is rectangular in cross section and arranged in a position corresponding, with respect to the axial direction of the metal sleeve 14, to the axial end 7 of the joint where the ceramic shaft 11a is securely joined with the metal shaft 12. By this, the oil seal groove 16 is given an additional function similar to the groove 6 in the previous embodiment. The slinger groove 17 is semi-circular in cross section. Except for the above, the structure for joining the metal shaft 12 to the ceramic shaft 11a is substantially similar to the previous embodiment.

This embodiment is subjected to a stress concentration test similar to the previous embodiment and a thermal stress test. In the tests, this embodiment is compared with such an example that is shown in FIG. 3. The example for comparison differs from this embodiment in that a metal sleeve 19 is not provided with a stepped bore but an inner circumferential wall of a uniform cross section so that an axial end 20a of the joint where the metal sleeve 19 is securely joined with the ceramic and metal shafts 11a and 12 by brazing with a brazing metal 20 is not arranged in the position corresponding to an oil seal groove 19a with respect to the axial direction of the metal sleeve 19 and that the entire inner circumferential wall of the metal sleeve 19 is covered with the brazing metal 20 to serve for the joining. From the stress concentration test, it was found that the bending strength of this embodiment is 1.4 times as large as that of the example for comparison. From the thermal stress test, it was found that in the case of this embodiment which is installed in an internal combustion engine and put into use under the full-throttle condition while being subjected to heat of exhaust gases of 900.degree. C., nothing wrong or abnormal is produced in the ceramic shaft 11a after the lapse of 500 hours of usage. In contrast to this, the example for comparison was broken after the lapse of 100 hours of usage under the same condition as this embodiment.

From the foregoing, it is to be understood that the ceramic and metal joining structure of this invention is effective for mitigating the stress concentration and the production of thermal stresses and can increase the durability.

FIG. 4 shows a further embodiment of the present invention. This embodiment differs from the previous embodiment of FIGS. 2A and 2B in that the smaller diameter bore portion 4a is formed to be shorter in axial length than the larger diameter bore portion 4b so that the shoulder 8 is arranged in the position corresponding not to the oil seal groove 16 but to the slinger groove 17. By this, when the metal sleeve 34 is joined at the smaller diameter bore portion 4a with the ceramic and metal shafts 11a and 12, the end 7 of the joint where the metal sleeve 34 is securely joined with the ceramic and metal shafts 11a and 12 by brazing with the brazing metal 5 is positioned so as to correspond to the slinger groove 17 with respect to the axial direction of the metal sleeve 34.

This embodiment was subjected to a stress concentration test and a thermal stress test substantially similar to the previous embodiment of FIGS. 2A and 2B. From the tests, it was found that the bending strength of this embodiment is 1.3 times as large as that of the example for comparison and nothing wrong or abnormal is produced in the ceramic shaft after the lapse of 500 hours of usage. It is therefore to be understood that the ceramic and metal joining structure according to this embodiment is effective for mitigating the stress concentration and the production of thermal stress.

FIG. 5 shows a further embodiment of the present invention. This embodiment differs from the previous embodiment of FIG. 1 in that the metal sleeve 44 is adapted to be installed only on the ceramic shaft 1 and that the metal sleeve 44 is not formed with a stepped bore but an inner circumferential wall 44a of a uniform cross section. The inner circumferential wall 44a of the metal sleeve 44 is not joined at its entirety with the ceramic shaft 1 but joined in part with same by brazing with a brazing metal 5 in such a manner that the end 7 of the joint where the metal sleeve 44 is securely joined with the ceramic shaft 1 by brazing with the brazing metal 5 is positioned so as to correspond to the groove 6. This embodiment can produce substantially the same effect when compared with an example which is not provided with the groove 6. In this embodiment, the brazing procedure is performed in such a way that a filler metal 5 in a state of paste is applied on the ceramic shaft 1 and the metal sleeve 44 is such a manner as to be held in place therebetween and then heated in a reducing or protective atmosphere (e.g. in hydroagent of 820.degree. C.) to be molten.

FIG. 6 shows a still further embodiment of the present invention. In this embodiment, the brazing metal 5 is applied on the metal sleeve 54 and the ceramic and metal shafts 11a and 12 in such a way that the brazing or joining area extends, with respect to the axial direction of the metal sleeve 54, from the place corresponding to the groove 16 to the position coinciding with an axial end of the metal sleeve 54 on the metal shaft side. In this embodiment, the metal sleeve 54 is made of a high strength material of Fe-Ni-Co alloy while an Ag-Ni-Cu alloy having a good heat resisting property is used for the brazing filler metal 5. The buffer member 3 is tow-layered to include a layer made of at least one kind of low expansion metal and a layer made of at least one kind of low Young's modulus alloy. The above mentioned low expansion metal is desirable to be a W-Ni alloy, W-Cu alloy, W-Cu-Ni alloy, W-Ni-Fe alloy, W-Ni-Fe-Cr alloy or the like metal that mainly contains W (tungsten) and Mo (molybdenum). The low Young's modulus alloy is desirable to be Cu, Ag, Ni, Fe-Ni alloy (42% Ni), SUS403, etc. The overall thickness of the buffer member 3 is designed to be 0.04-0.4 times as large as the largest width of the joining surfaces (i.e. the diameter of the ceramic or metal shaft in this embodiment). This embodiment can produce substantially the same effect as the embodiment of FIGS. 2A and 2B.

While the present invention has been described and shown in the above, it is not limitative. For example, as shown in FIG. 7, the ceramic shaft 21 may be formed with an increased diameter end portion 21a while the metal sleeve 44 is of the kind having the inner circumferential wall 44a of a uniform cross section. The ceramic shaft 21 is joined at the increased diameter end portion 21a with the inner circumferential wall 44a of the metal sleeve 44. The groove 6 is formed in the outer periphery of the metal sleeve 44 and arranged in the place corresponding to a shoulder 21b at the inner termination of the increased diameter end portion 21a. Further, as shown in FIG. 8, the metal sleeve 4 may be attached to the ceramic shaft 1 by shrink fit.