2023 March the Fourth Week KYOCM Technical Knowledge: "Surface Coating Technology" for Rolling Bearing Materials and Heat Treatment

Rolling bearing surface treatment technology refers to the use of certain means to change the surface composition, structure, and performance of the treated parts, in order to improve the service life of the parts or improve their working performance. Surface treatment technologies for rolling bearings mainly include 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), and electroless plating. Surface coating can improve the wear resistance, contact fatigue resistance, and reduce the surface friction factor of bearing parts. The following describes the surface coating technology of bearing steel.


1. Diamond-like coating

Diamond-like carbon (DLC) coatings, composed of graphite and diamond structured carbon, have both the lubricating and low friction properties of graphite and the hardness of diamond (above 1200HV), and have been used in many industrial technologies, such as automotive fuel injection systems, valve systems, gears, and rolling bearings. These coatings typically exhibit better wear resistance under sliding conditions, However, under high load rolling and mixed contact conditions such as bearings, the performance is poor, mainly due to the formation and propagation of cracks in the columnar structure of the coating when the coating is applied under high load rolling conditions, resulting in flake peeling of the coating. To avoid this fracture type wear pattern, it is necessary to eliminate this columnar structure. Optimizing process parameters and coating design can eliminate submicron columnar structures in the coating, greatly improving 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 hard coating for bearings. It will be increasingly 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 have become a hot research field of surface coating.


ECKELS. M, KOTZALAS. M. N, DOLL. G. L studied the performance of roller bearings coated with nanocomposite diamond-like carbon. By unbalanced magnetron sputtering, 1 μ M-thick W-aC: H coating precipitates onto the roller surface. The thickness between W-aC: H coating and steel substrate is less than 1 μ M of 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 A lamellar structure composed of alternating distribution of tungsten and carbon rich layers of m, and the amorphous hydrocarbon matrix in each layer contains β- The precipitate of W2C has a content of about 17% to 20%. The indentation hardness of the W-aC: H coating is 12 and the elastic modulus is 130 GPa. The test shows that W-aC: H coating has high wear resistance and hardness, and roller coated W-aC: H bearings can significantly increase their fatigue life when operating at low λ values, because the rolling contact area of the raceway is ground to a mirror surface by coated rollers, and can smooth out the indentation formed by contaminated particles in the lubricant on the raceway. In addition, the W-aC: H coating can prevent scratches and short-term lubrication failures, as well as pseudo indentation type wear failures.


MUTYALA. K. C, SINGH. H, EVANS. R. D, et al. 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 coatings applied: 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 tests, compared to TiC/a-C: H, TiC/a-C, and uncoated steel balls, WC/a-C: H coated steel balls can repair raceway damage and improve 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 Company 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% to 80% higher than that of hardened bearing steel, and the friction coefficient is similar to PTFE or MoS2. SKF Company has self-lubricating characteristics, and has good adhesion with the substrate without peeling, greatly improving the bearing life and wear resistance, and can still work normally even when oil is cut off, DLC coated bearings are known as "NoWear bearing". This technology has already been applied to compressor bearings, paper machine bearings, and hydraulic motor bearings. It is also planned to utilize 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 rollers, avoiding adhesive wear (coating) of roller bearings caused by sliding between rolling contact surfaces.


In recent years, there has been a lot of research on DLC coatings in China. In addition to DLC, Xi'an University of Technology and others have developed a Cr doped Graphite like Carbon (GLC) coating (Figure), which improves the adhesion between the coating and the substrate through the use of a Cr transition layer, and improves the coating strength through the C/Cr multilayer 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 conducted on bearings. Tests on steel ball coated GLC bearings have shown that at high speeds, bearing vibration and temperature rise are significantly reduced, and DLC can be used as a solid lubricating material to ensure that the bearings can still operate for a certain period of time after a short oil cut. This characteristic is particularly important for aviation engine bearings.



Cr doped GLC coating


2. Solid lubricating coating

Solid lubrication coatings are mainly applied? Soft materials or laminated structural materials with good lubrication performance are generally used in vacuum and other occasions where oil or grease lubrication is not easy to use.


Soft metal coatings include Au, Ag, Pb, etc., which are generally deposited by ion plating or sputtering, and are used for high vacuum rolling bearings, or to improve the lubrication state between the cage and the rolling body, such as silver plating on aircraft engine bearing cages.


Typical laminated structural materials such as MoS2 and graphite are typically sputtered or sintered into films using organic or inorganic adhesives. MoS2 is generally used for high vacuum applications, and graphite is generally used for high temperatures.


Polymeric materials, represented by PTFE, have a unique belt structure that exhibits low friction, is easy to form a transfer lubricant film on the mating surface, is resistant to chemicals, and is not susceptible to environmental media. They are generally sintered into a film using adhesives, and are used for bearings in clean or corrosion resistant environments.


When nano diamond is on the rolling contact surface, it can reduce friction like a rolling element in a bearing. Nano diamond coatings can also be used as lubrication coatings. CNx, which may have a hardness higher than diamond, can also be used as a solid lubrication material.


3. Other coatings

Yamada and others introduced insulated bearings for railway traction motors. One is to spray an 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 a non-conductive high thermal conductivity filler to improve thermal conductivity. Both coatings have high insulation properties, which can avoid electrical corrosion and have the same temperature rise as non coated bearings, effectively extending bearing life.


Р.И.ДИ The use of polymer materials to improve the life of machine tool spindle bearings is introduced. Friction, corrosion, and wear can occur on the mating surfaces of bearings, bearing seats, and shafts, increasing the mating clearance, and increasing the maximum contact stress in the bearings. After coating the outer ring with a polymer coating, it ensures that the mating surface of the outer ring and shaft is in elastic contact. When loaded, the coating undergoes elastic deformation, increasing the effective bearing area in the bearing, reducing the maximum contact stress, and improving the bearing life. During cyclic and local loading of the outer ring, the outer ring is coated with 6 Φ Sealant (thickness 0.125 mm) and ВК The bearing life of polymers (thickness 0.09 mm) is more than 3-4 times the calculated life, respectively. In order to improve the load distribution coefficient within the bearing, the thickness of the coating should be as thick as possible, and materials with low elastic modulus should be selected.


SKF has studied the impact of blackening on bearing performance. Laboratory research, bearing tests, and on-site experience have shown that blackening provides a certain degree of friction chemical corrosion protection, can reduce hydrogen penetration, and can also enhance the ability to resist moisture corrosion damage (such as static corrosion). In addition, blackened bearing steel surfaces increase safety factors when bearings exhibit failure forms such as adhesive wear, coating damage, or surface fatigue 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. To achieve optimal performance for bearings, SKF recommends blackening of the inner, outer, and rolling elements.


Low temperature ion sulfurizing is a surface modification technology that emerged in the late 1980s. Its basic principle is similar to that of ion nitriding. Under a certain degree of vacuum, high-voltage direct current is used to ionize the sulfur containing gas, and the generated sulfur ions bombard the surface of the workpiece, where they react with iron to form a 10% FeS based alloy μ A sulfide layer about m thick. Sulfide is a good solid lubricant that can effectively reduce the friction coefficient of the contact surface of steel parts, and its friction coefficient further decreases with the increase of load. Therefore, it can greatly improve the wear resistance of bearings under heavy loads, increasing the bearing life by about three times.


The effect of low-temperature phosphating is similar to that of sulfurizing. 0.05 to 0.25% can be obtained on the surface of the workpiece by immersing the workpiece in a TAP solution (tridecanoic phosphate) at 40 ℃ for 4 hours μ M thick surface layers of Fe2O3 and Fe4 (P2O7) 3 reduce friction coefficient and improve wear resistance. The phosphated M50 steel bearing does not get stuck in the event of a short-term oil outage, improving the reliability of the bearing.


By plating nickel on the rolling surface to inhibit the penetration of hydrogen, Junichiro Ino significantly reduces the hydrogen content in steel and can prevent the peeling of abnormal white structures.


More about KYOCM Linear motion rolling bearing:

Linear motion bearing or linear slide rail is a bearing designed to provide free motion in one direction. There are many different types of linear motion bearings. Mobile linear slideways such as mechanical slideways, XY worktables, roller worktables and some dovetail slideways are bearings moved by the drive mechanism. Not all linear guideways are motorized, and non-motorized dovetail slideways, ball bearing slideways and roller slideways provide low-friction linear motion for inertia or manually driven equipment. All linear guideways provide bearing-based linear motion, whether it is ball bearing, dovetail bearing, linear roller bearing, magnetic bearing or fluid bearing. XY table, linear platform, machine slide and other advanced slide rails use linear motion bearings to provide multi-axis motion along X and Y.