1 Introduction
Rare earth elements have been widely used in materials science because of their special electronic structure and physical and chemical properties. In recent years, the application of rare earths in tribological materials has also been paid attention to. The addition of rare earth compounds (LaF3, CeF3, CeO2, La2O3) to polymers can significantly improve their mechanical and frictional wear properties [1, 2]. Nickel-based alloys have excellent mechanical and high-temperature oxidation resistance and have been widely used in various gas turbines. Some studies have been carried out on alloys and nickel-based self-lubricating materials using oxides [3~5], sulfides [6,7] to reduce friction and synergistic anti-friction [8-10] of oxides and sulfides. Sliney's research shows that LaF3 and CeF3 can effectively lubricate nickel alloys with a friction coefficient of 0 5 at room temperature and a friction coefficient of around 0 2 at 500 °C [11]. In literature [12], the effect of CeF3 content on the friction and wear properties of nickel alloys was tested. Ni-Cr-based alloys containing different rare earth compounds (LaF3, CeO2 and La2O3) were prepared by powder metallurgy hot pressing method, and their friction and wear properties were evaluated. .
2 test method 2 1 alloy preparation
Based on NiCr alloy powder with purity higher than 96%, a certain amount of Mo, Al, Ti, B powder was added, and different mass fractions of LaF3 and 4% CeO2, La2O3 powder were added. After thorough mixing, the film was cold-pressed into a 45 mm × 15 mm disk shape, and then these disk samples were once loaded into a graphite mold, and each of the two samples was separated by a graphite sheet. This set of samples was prepared on a FVPHPR10 (made in Japan) vacuum hot press. After vacuuming to 5-10 Pa, it was protected by argon gas and started to heat up; the heating rate was 20 ° C / min; the hot pressing pressure was 16 MPa; the temperature was 1280 ° C; the holding pressure was 20 min.
A metallographic photograph of a nickel-based alloy to which three different rare earth compounds are added. It can be seen that the nickel-based alloys with LaF3 and CeO2 have finer grains and more uniform pore distribution, while the nickel-based alloys with La2O3 are coarsened, and the pores are aggregated and spheroidized.
The three alloys were analyzed by XRD. The results show that the phase structures of these alloys are basically the same, that is, they are mainly composed of Ni-based solid solution, Ni3 (Al, Ti), and a small amount of Cr18Mo42Ni40. No elemental Mo, Al, Ti and rare earth compounds are detected. It is indicated that the material has been fully alloyed in the preparation process, and a small amount of rare earth compound added may be decomposed or solid-dissolved into the nickel-based solid solution during hot pressing. 2 2 friction and wear test
The friction and wear performance test was carried out on a MM200 type ring-block tester. The alloy was prepared as a block, and the friction coupler was a quenched 45# steel ring with a hardness of 56 HRC and a diameter of 40 mm. Before the test, the surface of the ring and the block sample were polished with 500# sandpaper, then washed with an alcohol cotton ball, and dried in the air for use. The test was carried out at room temperature, in the atmosphere and under dry friction. The test ring speeds were 200r/min (sliding speed 4 19m/s) and 400r/min (sliding speed 8 38m/s); the loads were 100N and 200N respectively, and the sliding distance was 15084m.
The mass loss before and after the sample block was measured by an electronic analytical balance with a sensitivity of 0 1 mg. The wear surface morphology of the test piece was observed by optical microscope and scanning electron microscope. Jinan Sida Testing Technology Co., Ltd. professional friction and wear testing machine manufacturers!
3 test results and discussion 3 1LaF3 content on the friction and wear properties of the alloy
The effect of LaF3 content on the friction coefficient (μ) and wear quality of alloy samples is shown. As the sliding speed and load increase, the friction coefficient of several different LaF3 alloys decreases. At low load and low speed (100N, 4 19m/s), the friction coefficient of the alloy is high, the load and speed are doubled, and the friction coefficients of several LaF3 alloys are reduced by nearly 50%. Under several test conditions, the alloy with a LaF3 content of 4% has a low coefficient of friction zui. It can be seen that the wear amount of the LaF3 nickel-based alloy which is not added increases as the test speed and load increase. At low load and low speed (100N, 4 19m/s), the wear amount of the alloy slightly increased with the increase of LaF3 content. The sample wear amount of the LaF3 content is high at high speed and high load.
Based on the results of friction and wear tests under these three conditions, the comprehensive performance of the alloy with a LaF3 content of 4% is good. 3 2 Effect of different rare earth compounds on friction and wear properties of alloys
The friction coefficient and the wear amount of the nickel-based alloys of La2O3 or LaF3 and CeO2 with a mass fraction of 4% added under three test conditions are shown. It can be seen from a that the influence of these different rare earth compounds on the friction coefficient of nickel-based alloys is not significant, but the friction coefficients of the three alloys decrease with the increase of speed and load. It can be seen that the wear amount of the CeO2 nickel-based alloy added under the three test conditions is low. The wear of the three alloys at low load and high speed is lower than that at low load and low speed. 3 3 wear scar morphology and analysis
A SEM photograph of the surface morphology of the alloy wear is shown. It can be seen that at low load and low speed (100N, 4 19m/s), the wear surface of the added LaF3 alloy is rough, the surface has pits left by the wear debris and some loose pieces of abrasive debris that will fall off. It shows that the friction surface is oxidized, and the wear mainly manifests as adhesion, furrow and oxidative wear. As the sliding speed increases, the friction surface becomes smoother and the surface sticking off pit is reduced. Shown at high load speed (200N, 8 38m/s), the friction surface heat softens or even melts as a result of frictional heat, and the friction surface of the alloy becomes smoother, but there are still scratches. No loose oxides have been observed on the friction surface, and the oxide is extruded into a film to bond with the softened and molten metal matrix to form a friction-protective "enamel layer" [13], which reduces metal and metal during friction. Sticking between.
It can be seen that the friction coefficient of the alloy decreases rapidly with the increase of load and speed, indicating the antifriction effect of the "enamel layer". Since the enamel oxide layer has high hardness, it generally has better wear resistance than the matrix [5, 9]. It can be seen from Fig. 3b that the alloys with several rare earth compounds are generally less worn at high load and high speed than at low load and low speed, and the wear under unit load and unit sliding distance is less than half at low load and low speed. (or slightly more), further confirming the anti-wear effect of the "enamel layer".
The surface wear scar of the CeO2 alloy added is similar to that of the LaF3 alloy, except that the friction surface is slightly smoother than the LaF3 alloy. There are oxide platforms that are strip-shaped in the sliding direction, which is the result of the growth of the oxidized "enamel layer" during the friction process. The formation and wear-off of the "enamel layer" during high-temperature (or high-load high-speed) friction is a process of competing with each other and then being in a dynamic equilibrium, so it can play a self-protection role for a long time.
4 Conclusion
(1) In the nickel-based alloy containing several different rare earth compounds prepared by powder metallurgy hot pressing method, the addition of rare earth compound can significantly improve the wear resistance of the alloy under high load and high speed, especially the wear amount of CeO2 alloy added. less.
(2) The friction coefficient and wear amount of the rare earth compound alloy added at high load and high speed are obviously lower than those under low load and low speed, which is related to the antifriction and wear resistant "enamel layer" formed by the friction surface. Jinan Sida Testing Technology Co., Ltd. professional friction and wear testing machine manufacturer
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