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Rolling bearing surface treatment technology means to change the surface composition, structure and performance of the treated parts by certain means to improve the service life or working performance of the parts. The surface treatment technology of rolling bearing mainly includes surface coating, surface heat treatment, surface chemical heat treatment, surface mechanical strengthening, etc.
Surface coating technologies include physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spraying (PSC), chemical plating, etc. The wear resistance, contact fatigue resistance and surface friction coefficient of bearing parts can be improved by surface coating. The following describes the surface coating technology of bearing steel.
1. Diamond-like coating
The diamond-like carbon (DLC) coating is composed of carbon with graphite structure and diamond structure. It has both the lubricating and low friction properties of graphite and the hardness of diamond (above 1200HV). It has been applied in many industrial technologies, such as automobile fuel injection system, valve system, gear and rolling bearing, etc. These coatings can generally show better wear resistance under sliding conditions, However, under the condition of high load rolling and mixed contact of bearings, the performance is poor, mainly because the cracks are generated and expanded in the columnar structure of the coating when the coating is applied under the condition of high load rolling, and then the coating flakes off. In order to avoid this fracture-type wear mode, the columnar structure must be eliminated. The optimization of process parameters and coating design can eliminate the submicron columnar structure of the coating, greatly improve the durability of the coating under high load rolling and mixed contact conditions. Metal doping of the coating, such as adding W, Ti, Cr, etc., can also improve the strength and wear resistance of the coating by forming fine carbides in the coating. DLC coating represents a development direction of bearing hard coating. It will be more and more applied to various bearing products in reducing friction and wear, reducing surface damage and improving contact fatigue life. In recent years, the design and practical application of coating modification has become a hot research field of surface coating.
ECKELS. M, KOTZALAS. M. N, DOLL. G. L studied the performance of roller bearing coated with nano-composite diamond-like carbon. By unbalanced magnetron sputtering, 1 μ M-thick W-aC: H coating is deposited on the roller surface. The thickness between W-aC: H coating and steel substrate is less than 1 μ M Cr transition layer to increase the adhesion between the coating and the substrate. W-aC: H coating is a kind of coating with a thickness of 3~5 μ M layer structure composed of alternating distribution of tungsten rich and carbon rich, and the amorphous hydrocarbon matrix in each layer contains β- The precipitation of W2C is about 17%~20%. The indentation hardness of W-aC: H coating is 12, and the elastic modulus is 130 GPa. The test shows that the W-aC: H coating has high wear resistance and hardness, and the bearing coated with W-aC: H roller can significantly increase the fatigue life when working at low Λ value, because the rolling contact area of the raceway is grinded into a mirror by the coated roller, and the indentation formed by the contaminated particles in the lubricant can be smoothed. In addition, W-aC: H coating can prevent scratches and short-term lubrication failure, and can also prevent pseudo-indentation wear failure.
MUTYALA. K.C., SINGH. H, EVANS. R.D and others studied the effect of DLC coating on the performance of ball bearings under normal, depleted oil and debris damage conditions. There are three types of DLC coating: WC/a-C: H, TiC/a-C and Ti/a-C: H. During normal lubrication operation, TiC/a-C coated steel balls have greater torque drop, followed by TiC/a-C: H, WC/a-C: H coated steel balls; In the test, compared with TiC/a-C: H, TiC/a-C and uncoated steel balls, WC/a-C: H coated steel balls can repair raceway damage and improve the operating efficiency after bearing debris damage. The factors of WC/a-C: H, TiC/a-C: H and TiC/a-C coating improving the anti-wear ability of bearings were 3.5, 1.7 and 1.3, respectively, which exceeded the factors of assembling uncoated steel ball bearings under the condition of oil starvation. WC/a-C: H coated steel ball showed the best protection effect under debris damage and oil starvation conditions, which was attributed to its favorable membrane conversion ability.
SKF has been conducting research and application of DLC coating on the surface of bearing rings and rolling bodies using PVD in recent years. The surface hardness of the DLC coating is 40%~80% higher than that of hardened bearing steel, the friction coefficient is similar to PTFE or MoS2, and it has self-lubricating characteristics, and has good adhesion with the substrate without peeling, and the bearing life and wear resistance are greatly improved. It can still work normally in the case of oil cut-off, DLC coated bearings are known as "NoWear bearing". This technology has already been applied to compressor bearings, paper machinery bearings, hydraulic motor bearings. It is also planned to use the low friction performance of DLC coating to apply it to some energy-saving and consumption-reducing occasions, such as new energy vehicles. TIMKEN uses W-aC: H to coat the roller to avoid the adhesive wear (coating) caused by the sliding between the rolling contact surfaces of the roller bearing.
In recent years, many studies have been carried out on DLC coatings in China. In addition to DLC, Xi'an University of Technology and others developed a Cr-doped graphite-like carbon (GLC) coating (Fig.), which improves the adhesion between the coating and the substrate through the use of Cr transition layer, and improves the coating strength through the C/Cr multi-layer composite film forming technology. In addition to low friction and wear, it has no catalyst effect, lower brittleness, and better bonding strength. It has been successfully applied to cutting tools, greatly improving tool life. Preliminary application tests have also been carried out on bearings. The bearing tests of steel ball coated with GLC show that the vibration and temperature rise of the bearing are significantly reduced at high speed, and DLC can be used as a solid lubricating material to make the bearing still operate for a certain time after a short time of oil cut-off. This characteristic is particularly important for aviation engine bearings.
Cr doped with GLC coating
2. Solid lubricating coating
Solid lubricating coating is mainly applied? Soft materials or laminated structural materials with good lubrication performance are generally used in vacuum and other occasions where oil and grease lubrication is not easy to use.
The soft metal coatings include Au, Ag, Pb, etc., which are generally coated by ion plating or sputtering, and are applied to rolling bearings for high vacuum, or to improve the lubrication state between the cage and the rolling body, such as silver plating on the cage of aero-engine bearings.
Typical laminated structural materials, such as MoS2 and graphite, are generally sputtered or sintered into films using organic and inorganic adhesives. MoS2 is generally used for high vacuum and graphite is generally used for high temperature.
The polymer material, represented by PTFE, has a unique belt structure, which shows low friction, is easy to form a transfer lubricating film on the mating surface, is resistant to chemicals, and is not easy to be affected by environmental media. Generally, the adhesive is used to sinter the film, and is used for bearings in clean or corrosion resistant environments.
When the nano-diamond is on the rolling contact surface, it can reduce the friction as the rolling element in the bearing. The nano-diamond coating can also be used as a lubricating coating. The CNx with a hardness that may exceed the diamond can also be used as a solid lubricating material.
3. Other coatings
Yamada and others introduced insulated bearings for railway traction motors. One is to spray aluminum oxide coating on the outer diameter surface and end face of the outer ring and seal the hole with resin; One is to spray glass fiber reinforced PPS resin and add non-conductive high thermal conductivity filler to improve thermal conductivity. Both coatings have high insulation performance, can avoid electric corrosion and the temperature rise is the same as that of uncoated bearings, which can effectively prolong the bearing life.
Р.И.ДИ This paper introduces the use of polymer materials to improve the service life of machine tool spindle bearings. Friction, corrosion and wear will occur on the mating surface of the bearing, bearing seat and shaft, increasing the mating clearance and increasing the maximum contact stress in the bearing. After the polymer coating is applied on the outer ring, the mating surface of the outer ring and the shaft is guaranteed to be in elastic contact state. When the coating is loaded, the elastic deformation occurs, which increases the effective bearing area in the bearing, reduces the maximum contact stress, and improves the bearing life. When the outer ring is loaded circularly and locally, the outer ring is coated with 6 Φ Sealant (thickness 0.125 mm) and ВК The bearing life of polymer (thickness 0.09 mm) is more than 3-4 times of the calculated life. In order to improve the load distribution coefficient in the bearing, the thickness of the coating should be as thick as possible, and the material with low elastic modulus should be selected.
SKF has studied the effect of blackening on bearing performance. Laboratory research, bearing test and field experience show that blackening provides certain anti-friction chemical corrosion protection, can reduce hydrogen permeation, and can also enhance the ability to resist moisture corrosion damage (such as static corrosion). In addition, the blackened bearing steel surface improves the safety factor when the bearing has adhesive wear, coating damage or surface fatigue and other failure forms under boundary or mixed lubrication conditions. In addition to using blackened bearings for new equipment, SKF also uses them as replacements for conventional bearings in routine maintenance of wind power plants. In order to achieve the best performance of the bearing, SKF recommends that the inner ring, outer ring and rolling body be blackened.
Low temperature ion sulfurizing is a surface modification technology that appeared in the late 1980s. Its basic principle is similar to that of ion nitriding. Under a certain vacuum, the sulfur-containing gas is ionized by high-voltage direct current, and the generated sulfur ion bombards the surface of the workpiece, and reacts with iron on the surface of the workpiece to produce 10% FeS μ M thick sulfide layer. Sulfide is a good solid lubricant, which can effectively reduce the friction coefficient of the contact surface of steel parts, and its friction coefficient will further decrease with the increase of load. Therefore, it can greatly improve the wear resistance of the bearing under heavy load, and increase the life of the bearing by about 3 times.
The effect of low-temperature phosphating is similar to that of sulfurizing. 0.05~0.25 can be obtained on the surface of the workpiece by immersing the workpiece in TAP solution (tridecanoic phosphate) at 40 ℃ for 4 hours μ The surface layer of Fe2O3 and Fe4 (P2O7) 3 with thickness of m can reduce the friction coefficient and improve the wear resistance. The phosphatized M50 steel bearing does not get stuck in the case of short-term oil cut-off, which improves the reliability of the bearing.
By plating nickel on the rolling surface to inhibit the penetration of hydrogen, the hydrogen content in the steel is greatly reduced, and the abnormal white structure can be prevented from peeling off.
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2023-03-15