Porous polyimide composite cage materials were prepared by free sintering and constant volume sintering

Abstract: porous polyimide composite cage materials were prepared by free sintering and constant volume sintering respectively. The ring tensile strength, shore hardness, friction and wear properties and water absorption of the two materials were studied and compared. The results show that the material prepared by constant volume sintering has better comprehensive properties. Compared with the material prepared by free sintering, its ring tensile strength is increased by 82.0%; The wear amount is reduced by 22.8%, but the friction coefficient is slightly larger; With the same porosity, there is no significant difference in moisture absorption, which is close to 1%.

Key words: rolling bearing; Plastic cage; Porous polyimide; Mechanical properties; Friction and wear resistance; Water absorption, high precision motor bearing

Porous polyimide cage is a new type of cage developed in the 1970s. It has been widely used in various high-precision gyro motor bearings. The oil-bearing porous polyimide cage obtained after vacuum oil immersion is not only an important part of the bearing, but also the carrier of lubricating oil, which has a great impact on the rotation accuracy and service life of the gyro motor bearing. At present, free sintering and constant volume sintering are the main methods for preparing porous polyimide composite cage materials (hereinafter referred to as "polyimide composites"). Their process is simple, low cost, and can control microporous parameters by adjusting process parameters. They have been widely used in various fields of long-life bearings.

With the same porosity, the mechanical properties and Tribological Properties of polyimide composites prepared by different processes, especially the pore size, are quite different. In the following, polyimide composites were prepared by free sintering and constant volume sintering respectively, and their mechanical properties and tribological properties were compared, so as to provide reference for the selection of appropriate forming process.

1. Test

1.1 Material and Sample Preparation

The raw materials used to prepare polyimide composites are polyimide plastic powder and polytetrafluoroethylene plastic powder, in which the density of polyimide molding powder is 1.40 g / cm3, the glass transition temperature is 265 ℃, and the particle size is 150 mesh; The particle size of polytetrafluoroethylene plastic powder is 150 mesh.

Take an appropriate amount of polyimide molding powder and polytetrafluoroethylene molding powder and put them into a high-speed mixer for full mixing and standby. Polyimide composites were prepared by free sintering and constant volume sintering respectively.

1.1.1 Free Sintering Forming Process

Weigh a certain amount of raw materials into the mold, then press them on the press, demould them after holding the pressure for several minutes, and finally place them in the temperature controlled sintering furnace for sintering, with the porosity controlled at 22% ~ 23%; The micropore radius is 1.0 ~ 1.2 μ m.

1.1.2 Constant Volume Sintering Forming Process

Weigh a certain amount of raw materials into the mold, press them into shape through the limiting device, then place the whole device in the temperature controlled sintering furnace for sintering and forming, and finally demould. The porosity is controlled at 22% ~ 23% and the micropore radius is 0.6 ~ 0.8 μ m.

1.2 Material Performance Test

DNS20 electronic universal testing machine is used to test the ring tensile strength, and the tensile speed is 5 mm / min; The tribological properties were measured by CFT-1 surface property measuring instrument. The reciprocating dry friction was adopted, and the frequency was 20 Hz, Ф 4 mm steel ball for 30 min; The micropore parameters of polyimide composites were measured by auto Pore IV 9500 mercury porosimeter; The hardness of the material is measured by shore hardness tester; The moisture absorption test is carried out with EL-04KA high and low temperature circulation test chamber.

2. Results and Discussion

2.1 Effect of Forming Process on Mechanical Properties of Materials

Tensile strength is one of the important indexes of bearing cage. The properties of polyimide composites prepared by different forming processes are shown in Table 1.

                       Table 1 Properties of Polyimide Composites (average value)

It can be seen from table 1 that the density, ring tensile strength and shore hardness of polyimide composites prepared by constant volume sintering process are higher than those of materials prepared by free sintering process, in which the ring tensile strength reaches 73.0 MPa, an increase of 82.0%. The difference is mainly due to the different sintering forming processes: in the free sintering forming process, when the temperature rises above the glass transition temperature, the composite enters the viscoelastic state. At this time, the free expansion phenomenon is significant because there is no external force on the material; Constant volume sintering process is to fix the composite in a limited space and apply a certain pressure on the material to keep the volume of the composite unchanged and reduce the free expansion. The micropore radius of the composites prepared by constant volume sintering is smaller. Under the condition of the same porosity, the composites with denser micropore distribution and stronger structure can be prepared by constant volume sintering, so the ring tensile strength and shore hardness are also larger.

2.2 Effect of Forming Process on Friction and Wear Properties of Materials

The friction and wear test results of polyimide composites are shown in Figure 1 and table 2. It can be seen from the figure that the friction coefficient of the composite prepared by constant volume sintering process is greater than that of free sintering material; However, its wear resistance is better, the wear mark depth is reduced by 43.7% and the wear amount is reduced by 22.8% (Table 2).

Figure 1 friction coefficient of two Polyimide Composites

The reason for the above phenomenon may be that the polyimide composite prepared by free sintering process has a large amount of wear, and the composite itself contains PTFE lubricating material. A layer of lubricating film is formed on the contact surface during the friction process. With the increase of wear, the larger the lubricating film area is, the smaller the friction coefficient is; The polyimide composite prepared by constant volume sintering process has small micropore radius, good compactness and adhesion between material particles. In addition, it has high hardness and good wear resistance, but it is not easy to form lubricating film and make the friction coefficient larger.

     Table 2 Effect of forming process on wear resistance (average value) of materials

2.3 Effect of forming process on water absorption of materials

Polyimide material can absorb a small amount of moisture in the air, cause the size of the cage to change, and also have a certain impact on the oil storage performance of the cage.

The cage made by free sintering and constant volume sintering forming process is placed in Soxhlet extractor for cleaning for 24 hours (petroleum ether cleaning agent, 82 ℃), then the cage is placed in vacuum drying oven for drying for 30 minutes, weighed on precision balance, and finally placed in high and low temperature circulating test chamber for test. The test temperature is 25 ℃ and the humidity is 60%. Weigh it every 1H.

The moisture absorption rate of polyimide composite cage prepared by free sintering forming process slows down after 1 h under the environmental conditions of 25 ℃ and 60% humidity, and the quality does not change after 4 h; Under the same temperature and humidity, the moisture absorption rate of polyimide composite cage prepared by constant volume sintering process slows down after 2 hours, and the quality does not change after 4 hours, reaching the adsorption saturation state.

The moisture absorption of porous materials is a microporous fluid flow mechanism. The penetration of liquid in isotropic and interconnected pores can be regarded as a diffusion process. The moisture absorption process of porous polyimide cage can also be regarded as the diffusion of moisture in the air. Polyimide composites with the same formula have basically the same moisture absorption capacity with the same porosity, which is about 1%, but the time to reach the moisture absorption equilibrium is slightly different.

3. Conclusion

(1) the mechanical properties of polyimide composites prepared by constant volume sintering process are better than those of free sintering.

(2) the polyimide composites prepared by free sintering process have low friction coefficient and poor wear resistance.

(3) when the porosity is the same, the moisture absorption capacity of polyimide composites prepared by the two processes is basically the same.

(4) in practical application, due to the different forming processes of constant volume sintering and free sintering, their applications are also different. The mechanical properties of polyimide composites prepared by constant volume sintering process are better than that of free sintering process, but the self-lubricating performance is worse than that of free sintering process. Generally, when selecting cage materials for bearings, load and self-lubricating performance are the main factors to be considered. The polyimide composite prepared by constant volume sintering process has better mechanical properties, which makes its load in bearing application larger than that of free sintering process, but its self-lubricating performance is worse than that of free sintering process, and the demand for lubricating oil is also large when the bearing works. Therefore, the design and material selection of bearings should be determined according to the actual working conditions.

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