A Method of Using Continuous Surface Magnetization and Friction Coefficient Variations for Monitoring the Tribological Properties of CrO2/SS400 Steel for the Flexible Gas Seal

Abstract The experimental results demonstrate that the dynamic variations in surface magnetization and friction coefficient showed great potential for determining the onset of CrO2 film fracture. It is also shown that the CrO2 film resulted in low friction and low surface magnetization during the friction process, and is wear-resistant. Therefore, this study is novel and practical for the industry. Graphical Abstract CrO2/SS400 sample demonstrates no rutting wear, high wear resistance, low friction coefficient, and low magnetization.


Foreword
The rapid growth of contemporary industry has increased the demand for longer service life and better precision and reliability for all the parts of any piece of equipment. This is reflected in the large volume of research into wear damage. Dry friction, and wear due to friction which is the primary type of damage encountered in the drive elements of precision machinery, wind turbines, aerospace mechanics, the roller bearings. A load, vertical to the disk surface, is applied to the pin specimen using the level rule. This setup enables the accurate measurement of the friction coefficient between the specimens.

Test specimens
The thickness of the CrO 2 coating is approximately 1 mm. The disk specimen is made of SS400 steel. They are shown in Figure 2 and their material properties are given in Table  1. The pin and disk specimens were sequentially polished by 600-2000 grade of emery papers to a surface roughness, Ra, in the range 0.05-0.1 µm before each surface finishing.

Experimental procedure
The experimental parameters are shown in Table 2. Prior to each friction test, the specimens are cleaned with acetone in an ultrasonic cleaner and securely locked in position in the tester. As the output electric potential from the Gauss meter during the rotating friction process is in the order of mV, a DC isolated amplifier is used at a gain of 50. The response time of the measuring system is less than 1 ms with the accuracy of 0.1% full scale. Figure 3 shows the typical responses of surface magnetization and friction coefficient with sliding distance produced by CrO 2 /SS400 steel under a normal load of 60 N and a sliding speed of 200 mm/s. It is seen from this figure that the average friction coefficient was between 0 and 0.12 and the surface magnetic field was close to 0 G. A closer look suggests the following: the friction coefficient was practically constant between 0 and 0.1, coatings [14][15][16][17][18][19] are all widely used approaches to the problem. Hard ceramic surface coatings have been used [20][21][22][23] and reduce component friction and wear. However, with its greater hardness, very good chemical stability, and lower friction coefficient, a CrO 2 layer [24,25] beats its ceramic material rivals in protecting drive elements from damage by friction. In most application CrO 2 is employed in grinding silicon nitride or chip for its chemical stability. This is not the case with railway, bridge, pylon, and scores of electric facilities as steel (e.g. SS400) are most likely the ones against CrO 2 . In addition, the surface magnetization of steel needs to be considered as this may lead to electromagnetic interference with electronic equipment. [26][27][28] This paper concentrates on the friction between a CrO 2 layer and SS400 steel to measure changes in friction coefficient and surface magnetism under conditions of dry friction and severe wear. Scanning electron microscopy (SEM) was used for qualitative evaluation of wear in a study of the antifriction properties of CrO 2 film on a flexible air seal sheet. The results of this study may be valuable in the design of film coatings for products subject to dry friction.

Experimental apparatus
The experiments were conducted on a pin/disk friction tester with the measuring systems as shown in Figure 1 to investigate the tribological properties of CrO 2 /SS400 steel. The disk surface is set vertical to the ground to simplify wear mechanisms. The stationary pin specimen is placed on a rest, connected to the load cell and supported by     The results of CrO 2 /SS400 steel friction experiments under a normal load of 100 N and a sliding speed of 200 mm/s are shown in Figure 5. It can be seen from this figure that the average friction coefficient lies around 0.33 and the surface magnetic field varies over a range of from 0 to 10 G. A closer look suggests the following: to start with the friction coefficient rises fast from 0 to around 0.35 over a distance of from 60 to 170 m during which is remains stable. After 170 m, the coefficient shows a considerable degree of fluctuation. The surface magnetic field is very low and hardly changes up to 170 m after which it rises rapidly and may reach 10 G by 240 m after which it remains very high.
Under a load of 150 N the typical results for the same pair of samples, CrO 2 /SS400 steel, at a sliding speed of 200 mm/s are shown in Figure 6. The average friction coefficient is steady at about 0.25, while the surface magnetic field varies over a range of from 0 to 40 G. A closer look suggests the following: to start with the friction coefficient rises fast from 0 to around 0.25 and remains steady at this level throughout the experiment. The surface magnetic field varies within a range from 0 to 40 G during the friction interval wavelength was very long and showed no significant changes throughout the period of the experiment. The strength of the surface magnetic field was zero all the way and magnetization was therefore negligible.
The typical responses of surface magnetization and the friction coefficient with sliding distance produced by CrO 2 /SS400 steel under a normal load of 80 N and a sliding speed of 200 mm/s are shown in Figure 4. It is seen from this figure that the average friction coefficient lies between 0.35 and 0.45, and the surface magnetic field varies between 0 and 10 G. A closer look suggests the following: in the initial stages, the friction coefficient rises fast from 0 to around 0.3 over a distance of from 10 to 30 m, it then edges up slowly to around 0.4 when friction distance reaches about 30 m. After a rapid initial rise the friction coefficient then remains constant at around 0.4. On the other hand, the strength of the surface magnetic field stays under 10 G for some time, but when the friction distance goes above 80 m it starts to rise and by the time the distance reaches 300 m the field may have reached a strength of 10 G.  lie within a range 0-0.12, while the surface magnetic field remains at 0 G. A closer look suggests the friction coefficient remains flat within a range of 0-0.1 throughout the experiment period while the surface magnetic field stays at around 0 G, with almost no fluctuation whatsoever. With a dynamic friction coefficient that varies over a range of only 0.01-0.12, CrO 2 /SS400 outperforms the friction test. It then soars to around 18 G when the friction distance hit 30 m and may reach as high as 40 G at a distance of 260-280 m. Figure 7 shows the comparisons of surface magnetization and friction coefficient responses for four different material pairs of at 60 N and 200 mm/s. It can be seen that the average friction coefficients of CrO 2 /SS400 steel CrO2 X500 X1000 60N X2000

SEM images of friction wear
SEM images of CrO 2 /SS400 steel samples under the different loads and a sliding speed of 200 mm/s are shown in Figures 8-11. It is abundantly clear from the images that the other three specimens in this regard by 10-to 100fold against Fe/Fe, five-fold against Fe/Sn-film/Fe, and is more than three-fold better than Fe/Sn-Al 2 O 3 film/Fe and also surpasses all three cases with respect to surface magnetic field.

Conclusions
The feasibility of using continuous surface magnetization and friction coefficient variations for the evaluation of the tribological properties of CrO 2 /SS400 steel was investigated under dry friction condition. From the experimental results and SEM observations of the extent and nature of the wear, the following conclusions have been drawn: (1) The dynamic variations in surface magnetization and friction coefficient showed great potential for determining the onset of CrO 2 film fracture. the greater the load, the more the damage to the CrO 2 layer surface. At a load of 80 N, there is not much wear, but at 150 N there are visible breaks in the surface accompanied by a tremendous increase in the strength of the magnetic field, see Figure 6. Figure 12 shows SEM images of the four material pairs taken of samples subjected to 60 N and a sliding speed of 200 mm/s. In Figure 12(a) CrO 2 /SS400 wear particles within a range of 100-250 μm in size form a neat pattern of flakes rather than deep ruts. Wear particles of Fe/Fe, shown in Figure 12(b), are larger and in longer ruts which suggest strong adhesion between friction surfaces and more wear. Wear particles in the tincoated iron substrate sample, as seen in Figure 12(c), suggest the surface treatment has lessened the wear as compared to that between Fe/Fe, there are fewer ruts which are also narrower. On the Fe/Sn-film/Fe specimen the wear particles look like pressed flakes in long narrow ruts. This shows tiny peaks and small ruts with pressed flakes on the friction surfaces of Fe/Sn-film/Fe which were found only in this specimen. Wear particles from Sn-Al 2 O 3 film-coated iron substrates, shown in Figure 12