A METHOD OF BONDING MEMBERS HAVING DIFFERENT COEFFICIENTS OF THERMAL EXPANSION

Abstract: A method of bonding at least two members (1,3) having different coefficients of thermal expansion is disclosed, wherein an edge of the bonding interface between the members is located at a tapered portion (4) of at least one of the member which narrows from the member (1) having a smaller coefficient of thermal expansion towards the member (32) having a larger coefficient of thermal expansion. By this, stress arising at the interface due to the difference of thermal expansion coefficients is reduced.

Description: The present invention relates to a method of bonding members having different coefficients of thermal expansion. Especially, the invention relates to a method suitable for bonding a ceramic member and a metallic member. To join members having different coefficients of thermal expansion, there is known a bonding method in which bonding is carried out with a separate intermediate layer provided at the interface between members. For instance, Japanese Utility Model Application Laid-open No. 59-160,533 discloses a method in which bonding is carried out while a cladding material made of W or Mo is interposed between a ceramic member and a metallic member as a bonding structure. However, since in this method a residual stress is developed at the bonding interface due to difference in thermal expansion, sufficient bonding strength cannot be obtained. This is explained by way of example for bonding of a metallic member and a ceramic member with reference to Fig. 9 of the accompanying drawings. When a metallic member having a larger coefficient of thermal expansion and a ceramic member having a smaller coefficient of thermal expansion which are bonded in a normal configuration prior to cooling are cooled for the first time, the metallic member undergoes a greater shrinkage, and as shown in Fig. 9, a tensile stress is imposed upon both the metallic member and the ceramic member at an end portion A of the interface. If this state persists, peeling or cracking is likely to take place from the interface end portion A. It is an object of the present invention to overcome this problem, at least partly, and to provide a method of bonding members having difference coefficients of thermal expansion so that in the product the risk of peeling or cracking at the end portion of the interface is reduced or avoided. The present invention is set out in claims 1, 5 and 11. The invention also relates to the bonded composite article produced by the method. For a better understanding of the invention, reference is made to the attached drawings which show embodiments thereof, wherein: Figs. 1(a), (b) and (c) are diagrammatical views of embodiments of tapered portions in the bonding method according to the present invention; Figs. 2(a) and (b) are diagrammatical views showing embodiments in which the bonding method according to the present invention is embodied in actual articles; Figs. 3(a) and (b) are diagrammatical views of members which have undergone a stress analysis; Figs. 4 and 5 are graphs showing respective results of the stress analysis; Figs. 6(a) to (e) are diagrammatical views of embodiments with an intermediate layer embodying the bonding method according to the present invention; Fig. 7 is a diagrammatical view illustrating an embodiment in which the bonding method according to the present invention is embodied in a piston head; Fig. 8 is a schematical view showing results of the stress analysis of the embodiment illustrated in Fig. 7; and Fig. 9 is a diagrammatical view illustrating the drawbacks possessed by the prior art bonding method discussed above. According to the present invention, since a tapered portion is provided at a bonding interface between the members having different coefficients of thermal expansion, tensile stress developed at an end portion of the interface due to difference in thermal expansion can be effectively avoided or reduced, thereby successully preventing or reducing peeling and cracking at the interface end portion. Some shapes of the tapered portion are shown by way of example in Figs. 1(a), (b) and (c). As illustrated by these figures, the tapered portion may take any shape so long as a taper is provided from the joining member A (member having a smaller coefficient of thermal expansion) toward the joining member B (member having larger coefficient of thermal expansion). The shape shown in Fig. 1(a) is preferably. The "tapered portion" used throughout the specification and claims is defined to mean that an area of a section obtained by cutting the tapered portion in parallel with the bonding interface becomes smaller from the member A to the member B. The section of the taper may be curved instead of "straight line" as shown in Fig. 1. Further, the shape of the joining surface of the member need not be circular, but a rectangular shape or a polygonal shape may also be employed. The bonding method according to the present invention will be explained more in detail with reference to the following specific examples which are given in illustration thereof, but are not to be interpreted to limit the scope of the invention. Figs. 2(a) and (b) are diagrammatical views showing embodiments in which the bonding method according to the present invention is applied to actual articles. Fig. 2(a) is an embodiment in which the bonding method is embodied in a piston cap. The piston cap 1 made of partially stabilized zirconia is bonded to a piston body 3 made of nodular graphite cast iron through an intermediate layer 2 made of Ti. In this embodiment, a tapered portion 4 is provided bridging piston cap 1 and the intermediate layer 2 to liberate stress. Fig. 2(b) is an embodiment in which the bonding method according to the present invention is embodied in a rotary shaft of a turbo charger rotor, a gas turbine rotor, etc. A shaft portion 8 of a rotary vane portion made of silicon nitride is bonded to a rotary shaft 9 made of a metal through an intermediate layer 10 made of Mo having a tapering peripheral face 11. The taper 11 curves concavely, as seen in section. The bonding method according to the present invention can be applied to any members so long as their coefficients of thermal expansion differ from each other. In particular, the bonding method according to the present invention is effective for a metallic member and a ceramic member as mentioned above. As the ceramic member used for this purpose, use may be made of silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina, beryllia, etc. As the metallic member, use may be made of stainless steel, nodular graphite cast iron, nickel-chromium-molybdenum steel, chromium-molybdenum steel, aluminum-chromium-molybdenum steel, maraging steel, precipitation-hardenable type super alloy, etc. Further, it is preferably that a part or the whole of the metallic member is hardened by precipitation hardening, nitriding, a high frequency quenching, or the like. Next, results obtained in the stress analysis made to confirm the effects of the tapered portion will be explained. Adopting a straight edge shown in Fig. 3(a) and a tapered edge according to the present invention shown in Fig. 3(b) as an edge of an interface, the following two cases (1) and (2) were subjected to the stress analysis while the thickness "d" of the intermediate layer was varied. (1) member A : PSZ (partially stabilized zirconia) member B : FCD (nodular graphite cast iron) member C : Ti (titanium) In the above materials, the coefficient of thermal expansion increases in the order of Ti, PSZ, and FCD (Ti<PSZ<FCD). The coefficient of thermal expansion of Ti is very near to that of PSZ. The sample is a cylindrical shape of 40 mm in diameter in which the thickness of the portion A and the portion B is 10 mm. (2) A : SN (silicon nitride) B : SACM (aluminum-chromium-molybdenum steel) C : Mo (molybdenum) In this case, the coefficient of thermal expansion becomes larger in the order of SN, Mo and SACM (SN<SO<SACM). Results are shown in Figs. 4 and 5. As obvious from the results in Figs. 4 and 5, it was clear in both cases that the taper edge sample according to the present invention was smaller in terms of the maximum stress as the bonding portion than the conventional straight edge sample and that stress at the edge of the bonding portion is favorably absorbed in the taper portion. Further, as compared with the samples in which the thickness of the intermediate layer is zero, that is, direct bonding is carried out through no intermediate layer, the sample provided with the intermediate layer effectively absorbed the stress at the bonding portion. As mentioned above, the effect of the tapered portion is larger when the intermediate layer is interposed. When the intermediate layer C is intended to be interposed in the present invention, either one of the interface between A and C or the interface between B and C has only to be positioned on the taper. Figs. 6(a) to (e) show embodiments in such a case. The upper portion of a piston made of nodular graphite cast iron having a diameter of 120 mm shown in Fig. 7 was constituted by a piston cap made of partially stabilized zirconia (PSZ). Between the piston body and the piston cap was provided an intermediate layer made of Ti of thickness d of 2 mm and 80 mm in diameter. A conventional piston in which an edge of an intermediate layer was straight and a piston in which a tapered portion was provided in an edge of an intermediate layer were prepared. A tensile test jig was attached to each of the thus obtained pistons and a tensile test was carried out. Consequently, the piston cap of the conventional pistons having the straight edge intermediate layer was peeled or cracked in a range of P=500 to 2,000 kgf, while neither peeling nor cracking where produced at P=2,000 kgf in the case of the piston according to the present invention with the tapered edge intermediate layer. In Fig. 8 is shown results obtained in the stress analysis with respect to the straight edge type piston and the taper edge type piston having the configuration shown in Fig. 7, with varying thickness d of the intermediate Ti layer. As well understood, the present invention is not limited to the above-mentioned embodiments, but various modifications, variations and changes are possible. For instance, although explanation has been made with respect to the bonding between the ceramic member and the metallic member, the bonding member according to the present invention may be applied to any materials having different coefficients of thermal expansion.

Claims: A method of bonding two members comprising a first member (A) and a second member (B) having a larger coefficient of thermal expansion than the first member (A) wherein the members are joined at a planar interface, and there is a tapered portion (4;11;etc.) provided adjacent the interface in at least one of the members and shaped so that the cross-sectional dimensions of the member or members containing the tapered portion reduce in the direction from the first member (A) towards the second member (B), characterized in that the cross-sectional dimensions of the second member adjacent the interface are not greater than the cross-sectional dimensions of the first member adjacent the interface. A method according to claim 1 wherein the tapered portion is provided on one of the members and ends at the interface. A method according to claim 1 wherein the tapered portion is on both members and extends across the interface. A method according to any one of claims 1 to 3, wherein the joining of the members is effected by one of brazing, diffusion bonding, melting bonding and adhering with an adhesive or a combination thereof. A method of bonding a first member (1,8,A) and a second member (3,9,B) having a larger coefficient of thermal expansion than said first member, wherein the bonding is carried out with an intermediate member (2;10,C) and has one of (a) a coefficient of thermal expansion between the respective coefficients of expansion of the first and second members, (b) a coefficient of thermal expansion near the coefficient of thermal expansion of the first member, and (c) a coefficient of thermal expansion near the coefficient of thermal expansion of the second member, or (d) is a metal having a low rigidity, said intermediate member being interposed between the first and second members whereby there are two planar bonding interfaces at opposite faces of the intermediate member, characterized in that a tapered portion is provided in at least one of the three members (1,2,3;8,9,10;A,B,C) adjacent one said interface, this being an interface between two of said members having different coefficients of thermal expansion, the direction of narrowing of said tapered portion being in the direction from the first member towards the second member. A method according to claim 5 wherein said tapered portion extends over at least part of the intermediate member (2;10;C). A method according to any one of claims 1 to 6, wherein the members joined at said interface adjacent said tapered portion are a ceramic member and a metallic member respectively. A method according to claim 7, wherein the ceramic member is made of silicon nitride, silicon carbide, sialon, zirconia, mullite, alumina or beryllia. A method according to claim 7 or claim 8, wherein a part or the whole of the metallic member is hardened by a precipitation hardening, nitriding and high frequency quenching or any combination thereof. A method according to any one of claims 7 to 9 wherein the metallic member is made of at least one of stainless steel, nodular graphite cast iron, nickel-chromium-molybdenum steel, chromium-molybdenum steel, aluminum-chromium-molybdenum steel, and maraging steel. A bonded composite article comprising two members bonded together at a planar interface and comprising a first member (A) and a second member (B) having a larger coefficient of thermal expansion than said first member (A), there being a tapered portion (4;11; etc.) provided adjacent the interface in at least one of the members and shaped so that the cross-sectional dimensions of the member or members containing the tapered portion reduce in the direction from the first member (A) towards the second member (B) characterized in that the cross-sectional dimensions of the second member adjacent the interface are not greater than the cross-sectional dimensions of the first member adjacent the interface.

Inventors: SOMA, TAKAO, YOSHIDA, AKIHIKO
Applicants: NGK INSULATORS, LTD.
Application year: 1986
Main classification: C04B37
IPC codes: C04B37, B23K20, F01D5, F02F7, F16D1