GB2448662A

GB2448662A - Sliding bearing made from a silver-copper alloy

Info

Publication number: GB2448662A
Application number: GB0806934A
Authority: GB
Inventors: Kouji Zusi; Masayuki Niwa; Kentaro Tujimoto; Hiroyuki Asakura; Moritaka Fukuda
Original assignee: Daido Metal Co Ltd
Current assignee: Daido Metal Co Ltd
Priority date: 2007-04-24
Filing date: 2008-04-16
Publication date: 2008-10-29
Anticipated expiration: 2028-04-16
Also published as: JP5036390B2; JP2008266756A; DE102008019631A1; GB0806934D0; GB2448662B; DE102008019631B4; DE102008019631B8

Abstract

A sliding member which comprises a sintered layer of a copper containing silver alloy formed on a steel backing. The sintered layer comprises multiple solid solution phases. The alloy comprises (by weight): 0.35-28.5 % copper and a total of not more than 20 % of aluminium, indium, tin, titanium and/or zinc. The sintered layer may also comprise lead, bismuth, a carbide, nitride, oxide, boride or silicide.

Description

SLIDING MEMBER

Background of the invention

The present invention relates to a sliding member comprising a back metal of steel, and a sliding layer being made of a sintered alloy powder and formed on the back metal.

Conventionally, a multi-layered sliding member consisting of a Cu-Pb bearing alloy and a back metal of steel, which is disclosed in JP Patent No. 2,595,386 (see claim 1 and paragraph [0005]), has been conventionally used as a high peripheral speed bearing of internal combustion engines, and which is improved in thermal conductivity in order to enhance seizure resistance property thereof.

Brief summary of the invention

Sliding members have been required to have improved cavitation resistance as well as seizure resistance property as a countermeasure to recent high speed and high rotational engines. The assignee of the present invention tried to use a sliding material of pure Ag, which is highly thermally conductive like the conventional Cu-Pb alloy disclosed in JP Patent No. 2,595,386, and stronger than the Cu-Pb bearing alloy.

However, the sliding member made of pure Ag exhibited unsatisfactory cavitation resistance property.

Further, it was found that such a property can not be fully improved by making Ag to be an alloy only for strengthening purpose.

The present invention has been made under the

above background.

An object of the present invention is to provide a sliding member having a sliding layer made of a silver alloy excellent in the thermal conductivity and the cavitation resistance property.

According to one aspect of the invention as defined in claim 1, there is provided a sliding member comprising a back metal of steel, and a sliding layer being made of a sintered alloy powder and formed on the back metal, wherein the alloy powder is of a silver alloy comprising Ag and Cu, and wherein the sliding layer made of the sintered alloy powder has a multi-phase structure consisting of multiple solid-solution phases.

According to a first embodiment of the invention, the silver alloy comprises 0.35 to 28.5 mass% Cu.

According to a second embodiment of the invention, the silver alloy further comprises a total amount of not more than 20 mass% of any one element selected from the group consisting of Al, In, Sn, Ti and Zn.

In the invention according to the primary aspect, although the silver alloy containing copper is inferior in thermal Conductivity than pure Ag, since both Ag and Cu have highest thermal conductivity among a various metals, it is possible to provide the sliding member with the sliding layer having a thermal conductivity identical to or higher than that of a sliding material made of a conventional Cu-Pb alloy.

One of the common methods for improving strength of Ag is to add a different type metal(s) into Ag thereby precipitating or crystallizing an inter-metallic compound phase consisting of Ag and the additive metal(s) in a solid solution consisting of Ag and the additive metal (s). When a load is applied to the alloy consisting of the solid solution phase and the intermetallic compound phase(s), a deformation caused by a load propagates through the solid solution phase(s) until it reaches grain boundaries between the inter-metallic compound phase(s) and the solid solution phase, whereafter the inter-metallic compound phase(s), which is very hard and lacks ductility, will resist the deformation. Thus it was found that although the alloy consisting of the solid solution and the inter-metallic compound phases has a high strength, a damage due to cavitation is liable to occur in the alloy material.

It is considered that when a periodic load with a supersonic wave frequency, such as cavitation, is applied to the alloy, the deformation movement is blocked at the grain boundaries between the solid solution phase(s) and the inter-metallic compound phase(s) whereby occurring stress concentration resulting in a material damage originating from the grain boundaries.

However, when the silver alloy has a multi-phase structure consisting of multiple solid-solution phases, as defined in present claim 1, even in the SOl1d-S1utjn phases, grain boundaries among different types of solid-solution phases having different crystal structures from one another work as deformation resistance whereby the alloy is strengthened, while the deformation propagates among different solid-solution phases through the grain boundaries whereby the stress concentration is relaxed at the grain boundaries resulting in improved cavitation resistance of the alloy.

In the first embodiment of the invention, when the Ag and Cu eutectic alloy contains not less than 0.35 mass% Cu, the alloy has a multi-phase structure Consisting of a soft and ductile a solid solution phase and a t3 solid solution phase which is harder and less ductile than the a phase. On the other hand, when the Ag and Cu eutectic alloy contains not more than 28.5 mass's Cu, the rate of the a solid solution phase, having a higher ductility than the 3 solid solution phase, increases whereby much more advantageously realizing relaxation of stress concentration at grain boundaries between the a and j3 solid solution phases resulting in further improvement of the cavitation resistance property of the alloy.

In the second embodiment of the invention, at least one element selected from the group consisting of Al, In, Sn, Ti and Zn is added into the alloy within an additive total amount range according to which inter-metallic compound phases are not formed, thereby further improving the strength of the solid solution phase resulting in further improvement of the cavitation resistance property of the alloy. However, it should be noted that if the additive total amount of one or more of the alloying elements exceeds 20 mass%, an inter-metallic compound phase may be formed in the alloy where by deteriorating cavitation resistance property of the alloy.

It is noted that soft components such as Pb and Bi can be added into the alloy in order to improve anti-seizure property within an additive amount range in which cavitation resistance property is not deteriorated. Further, it is possible to add hard particles of carbide, nitride, oxide, boride, suicide, etc. into the alloy in order to improve wear resistance property of the alloy within an additive amount range in which cavitation resistance property is not deteriorated. Furthermore, the alloy may contain unavoidable impurities, such as phosphor, which are derived from the powder production process for the alloy. Optionally it is possible to form an overlay layer, which is made of metal or resin for example, on the sliding surface of the sliding member in order to improve anti-seizure property of the alloy.

Brief description of the drawing

Fig. 1A is a schematic drawing of a metal structure of pure Ag; Fig. lB is a schematic drawing of a metal structure of an alloy including inter-metallic compounds; and Fig. 1C is a schematic drawing of a metal structure of an alloy including two types of Ag-Cu solid solution phases.

Detailed description of the invention

Herein below there will be provided a description of embodiments of the present invention.

While the metal structure of pure Ag shown in Fig. 1A is excellent in ductility because of a single phase of Ag, it is inferior in fracture toughness because of low strength of Ag. When Th is added into Ag, an alloy structure having a multi-phases consisting of an Ag-Th solid solution and an inter-metallic compound 1 of ThAg3 can be obtained as shown in Fig. i.B.

In this alloy structure, there is precipitated the inter-metallic compound in the single phase of the Ag-Th solid solution. There is a notable difference between the solid solution and the inter-metallic compound such that the solid solution has ductility whereas the inter-metallic compound has high hardness and lacks ductility. Thus, there is no continuity in material properties at the grain boundaries among the inter-metallic compound and the solid solution resulting in deterioration of fracture toughness. As will be appreciated, the single phase of pure Ag has low fracture toughness because of low strength, and it is impossible to improve the fracture toughness property of Ag merely by adding a metal element into Ag to cause an inter-metallic compound to precipitate whereby strengthening Ag, so that a fracture of such materials of Ag and an Ag alloy is liable to occur due to cavitation.

Contrasting, the eutectic Ag-Cu alloy structure shown in Fig. 1C has a mixture structure of two types of the a and 13 solid solution phases 2 and 3 wherein the 13 solid solution is harder than the a solid solution. However, since both the a and 13 solid solutions are ductile, there is a continuous material property at the grain boundaries between the a and 13 solid solutions whereby enabling improvement of fracture toughness of the alloy. In other words, in the case where the silver alloy has a multi-phase structure consisting of multiple solid solution phases, even if it consists of the solid solution, it has an improved strength since the grain boundaries among the different type solid solution phases work as resistance to deformation of the alloy, while the deformation propagates among the different type solid solution phases through the grain boundaries whereby enabling relaxation of stress concentration at the grain boundaries resulting in excellent cavitation resistance property of the alloy.

The present invention uses the Ag-Cu eutectic alloy, having high thermal conductivity, as a sliding layer of the sliding member, which alloy has a multi-phase structure consisting of only two types of the a and 13 solid solutions. The sliding member can be produced by the process comprising the steps of: spreading a silver alloy powder, produced by the atomizing method for example, on a back metal of steel so as to have a predetermined thickness; sintering the spread silver alloy powder at a predetermined temperature in a reducing atmosphere; cooling the sintered layer with the back metal; subjecting the sintered layer to rolling to compact it; and further sintering the compacted layer as a sliding layer.

Referring to Table 1, herein below there will be described invention specimens and comparative specimens. Each of the specimen powders of Nos. 1 to having chemical compositions shown in Table 1 was processed as follows: (1) spreading the powder on a steel strip having a thickness of 1.5 mm so as to form a powder layer; (2) continuously passing the strip carrying the spread powder layer through a sintering furnace, having a hydrogen gas atmosphere and kept at a temperature of 700t to 900t, in order to sinter the spread powder layer; (3) cooling the strip with the sintered powder layer after sintering; (4) subjecting the sintered powder layer to rolling so as to compact the sintered powder layer; and (5) further sintering the compacted powder layer under the same conditions as those of the first sintering at above Item (2).

Thus, sliding members corresponding to Invention Specimen Nos. 1 to 7 and Comparative Specimen Nos. 8 to 10 were prepared (hereafter, these sliding members are referred to as Invention Sliding Member Nos. 1 to 7 and Comparative Sliding Member Nos. 8 to 10) Each of Invention and Comparative Sliding Member Nos. 1 to 10 was analyzed by X-ray diffractometry to observe its structure in order to confirm the number of solid solution phase types. As a result, it was confirmed that each of Invention Sliding Member Nos. 1 to 7 had two types of solid solution phases as shown in Fig. 1C. Contrasting, it was confirmed that Comparative Sliding Member No. 8, of which sliding layer consists of a conventional Cu-Pb alloy, had a metal structure consisting of a single -10 -solid solution phase, Comparative Sliding Member No. 9 had a metal structure consisting of a single phase of pure Ag as shown in Fig. 1A, and Comparative Sliding Member No. 10, of which sliding layer consists of a Ag-Th alloy, had a metal structure consisting of a solid solution and a precipitated inter-metallic compound as shown in Fig. 1A.

Table 1

Cu alloy, Ag Number of Volume loss in No and Ag alloy Solid Solution Cavitation ____________ (mass) Phase test (mm3) 1 Ag-0.35Cu (2) 4.0 2 Ag-2OCu (2) 2.5 3 Ag-28.5Cu (2) 2.0 Invention 4 Ag-7OCu (2) 5.5 Specimen -_______________ ________________ ________________ S Ag-2OCu-5Sn (2) 0.6 6 Ag-20Cu-201n (2) 0.5 Ag-2OCu- 7 0.5Tj-0.25Zn-(2) 1.8 _____________ 0.2 SAl 8 Cu-lSn-22Pb (1) 10.5 Comparative Specimen 9 Pure Ag (1) 8.5 Ag-2OTh (1) 8.2 The cavitation test, which result is shown in Table 1, was conducted for Invention and Comparative Sliding Member Nos. 1 to 10. In the cavitation test, each of the specimen members was held in water, a horn was positioned so as to face to a sliding layer of the -11 -specimen with a predetermined distance (0.5 mm, for example) between the horn and the sliding layer, and a sonic wave having a frequency of 19,000 Hz was emitted from the horn for 30 minutes. Thereafter, a volume loss of the sliding layer was determined. The test result is shown in table 1.

With regard to each of Invention Sliding Member Nos. 1 to 7, a cavitation loss of the sliding layer was smaller than those of Comparative Sliding Member Nos. 8 to 10. Especially, with regard to the most of the Invention Sliding Members except for No. 4 member, of which sliding layers (i.e. Nos. 1-3 and 5-7) containing 0.35 to 28.5 mass% Cu, the cavitation losses of the sliding layers are less than one half of those of Comparative Sliding Member Nos. 8 to 10.

Presumably, this will be because the stress relaxation at grain boundaries was enhanced since the multi-phase structure of the respective invention sliding layer contained a much more amount of the cx solid solution phase than the f3 solid solution phase, wherein the a solid solution phase has a higher ductility than that of the 3 solid solution phase. On the other hand, with regard to Invention Sliding Member No. 4, while the multi-phase structure of its sliding layer contained a much more amount of the 13 solid solution phase, having less ductility, than the a solid solution phase, the cavitation resistance property was improved as compared with Comparative Sliding Member No. 8 which sliding -12 -layer consists of a conventional Cu-Pb alloy, although Invention Sliding Member No. 4 is somewhat inferior in the cavitation loss than the other Invention Sliding Members.

With regard to Invention Sliding Member No. 5 in which sliding layer Sn is dissolved in a Ag-2OCu alloy (by mass's), and Invention Sliding Member No. 6 in which sliding layer In is dissolved in a Ag-2OCu alloy (by mass%), those Cu alloys consist of two type phases wherein Sn or In is dissolved in the a or 13 solid solution phase, so that the load stress concentration due to cavitation is hard to occur at grain boundaries among the different solid solution phases in the Cu alloys. Further, the solid solution phases per se are strengthened by additive Sn or In. Therefore, Invention Sliding Member Nos. 5 and 6 had higher cavitation resistance property than Invention Sliding Member No. 2 which sliding layer consists of Ag and 20 mass% Cu.

With regard to Invention Sliding Member No. 7 in which sliding layer small amounts of Ti, Zn and Al are added into a Ag-2OCu alloy (by mass%), the Cu alloy consists of two type phases wherein Ti, Zn and Al are dissolved in the a and J3 solid solution phases, so that the load stress concentration due to cavitation is hard to occur at grain boundaries among the different solid solution phases in the Cu alloys. Further, the solid solution phases per se are strengthened by additive Ti, -13 -Zn and Al. Therefore, Invention Sliding Member No. 7 had higher cavitation resistance property than Invention Sliding Member No. 2 which sliding layer consists of Ag and 20 mass% Cu.

On the other hand, with regard to Comparative Sliding Member No. 10 in which sliding layer inter-metallic compounds is precipitated in a solid solution phase, there will occur load stress concentration at grain boundaries among the solid solution phase and the inter-metallic compounds, so that Comparative Sliding Member No. 10 is inferior in the cavitation resistance property than those of Invention Sliding Member Nos. 1 to 7. This will be because the load stress concentration is liable to occur at grain boundaries among the solid solution phase and the inter-metallic compounds both of which physical properties are notably different to each other since the solid solution phase has high ductility as metal and the inter-metallic compounds have no such a high ductility not like as metal, and because the inter-metallic compounds have no stress relaxation capacity since they lack ductility.

The silver alloy of the invention which forms the sliding layer may optionally contain a soft component, such as Pb or Bi, in order to improve anti-seizure property of the alloy within an additive amount range in which cavitation resistance property is not deteriorated. Further, it is possible to add hard particles of carbide, nitride, oxide, boride, suicide, etc. into the alloy in order to improve wear resistance property of the alloy within an additive amount range in which cavitation resistance property is not deteriorated. Optionally it is possible to form an overlay layer, which is made of metal or resin for example, on the sliding surface of the sliding member in order to improve anti-seizure property of the alloy.

Claims

Claims: 1. A sliding member comprising a back metal of steel, and a

sliding layer being made of a sintered * alloy powder and formed on the back metal, wherein the alloy powder is of a silver alloy comprising Ag and Cu, and wherein the sliding layer made of the sintered alloy powder has a multi-phase structure consisting of multiple solid-solution phases.
2. The sliding member according to claim 1, wherein the silver alloy comprises 0.35 to 28.5 mass% Cu.
3. The sliding member according to claim 1 or 2, wherein the silver alloy further comprises a total amount of not more than 20 mass% of any one element selected from the group consisting of Al, In, Sn, Ti and Zn.
4. A sliding member comprising a sliding layer made of sintered alloy powder having a metal structure substantially as hereinbefore described with reference to and as shown in Figure 1(b) and 1(c).