In order to increase the power density of tribologically stressed drivetrain components, different approaches are being pursued in material and production technology. In addition to the development of efficient base materials, the optimization of surface finishing processes and the application of coating systems are promising. By combining mechanically highly stressable substrate materials and tribologically effective, extremely thin coatings, the components show modified wear and friction properties, which often lead to an increase of tooth flank load carrying capacity. A major advantage of this approach is that the highly accurate component geometry is only slightly changed by the coating.
The influence of physical vapor deposition (PVD) or plasma-enhanced chemical vapor deposition (PECVD) hard coatings on the load carrying capacity of cylindrical gears made of alloy steel has been the subject of scientific research since the 1990s. Several reports show that diamond-like carbon (DLC) coating systems reduce the occurrence of specific forms of gear damages, such as pitting or scuffing, and optimize the frictional behavior of gears. Despite the good results, PVD/PECVD coating technology has not been established in gear transmission technology yet. The use of a PVD/PECVD coating leads to higher component costs and longer manufacturing time.
The potential positive impact of applied coatings depends, among other considerations, on the previous surface finishing process. In existing works, a positive effect of a blast cleaning process between the grinding and coating process on the tooth flank load capacity of coated spur gears can be observed. This effect is explained by an improved coating adhesion for the blast-cleaned surfaces. The influence of an isotropic superfinishing between grinding and coating on the load capacity has not been investigated until now. Also, extensive research comparing the impact of different surface finishing processes for the case of uncoated gears, coated pinions, and coated gear sets has not been covered. This is necessary in order to understand the mechanisms of action for possible combinations and to consider such effects already present during the gear design phase.
State of the Art
In existing papers, a high potential of hard coatings applied by PVD or PECVD regarding an increase of tooth flank load capacity can be observed , , , and . However, the increase of load capacity achieved by the coatings is dependent on several external influencing quantities. Besides the geometry of the gears, the pretreatment of the coated surfaces has a strong impact on the resulting tooth flank load capacity . Furthermore, for the coating layer adhesion, the coatings compound of an adhesion layer and a hard wear-resistant layer is important . In this section, the state of the art concerning the influence of PVD/PECVD hard coatings and different surface finishing processes on the tooth flank load capacity is summarized.
The most common damages restricting tooth flank load capacity are scuffing, micropittings, and pittings. By the application of PVD, the load capacity of the tooth flank can be increased concerning each of these damages. For scuffing, the applicable load stage for ester oils without additives can be increased from stage 8 to the highest possible stage 12 . During the test, a higher loss of mass can be observed for coated gears, which is explained by a coating wear without exposure of the substrate material . These results are confirmed in further works  and .
For the evaluation of micropitting load capacity, the test procedure according to Emmert  is used by Grossl . The comparison of the test results of uncoated and coated spur gears shows that a coating can prevent micropittings for the complete test duration. However, the coated gears fail due to pitting damage in the tooth flank area. The pattern of damage indicates a premature tooth meshing of the gears, which have been designed without tip relief .
The pitting resistance of coated spur gears has been evaluated in the works of Hurasky-Schönwerth  and Bugiel . Especially for the use of oils without additives, a high increase of the pitting load capacity can be observed. The reached number of load cycles (LC) could be raised from 12 million LC to the maximum reachable number of 50 million LC  and . In contrast to the tests concerning the micropitting load capacity, a FZG-Cmod test gear set has been used, which has a tip relief applied.
The same test gear geometry has been tested by Grossl, concerning the pitting fatigue strength . The failure causing Hertzian pressure could be increased by 30 percent by the application of a PVD coating. The reached number of load cycles could be improved by a factor of 3 to 4 for the short-time fatigue strength. All test gears have been cleaned by a blast cleaning process before coating. Furthermore, uncoated, blast-cleaned test gears have been analyzed compared to the ground reference. By the blast cleaning process, no increase of pitting strength could be observed for uncoated gears. For the coated gears, the blast cleaning process leads to better adhesion strength of the coating, which results in lower coating wear. In addition, the surface roughness is decreased during the test more slowly by the application of a coating. According to Grossl, this leads to less micro-Hertzian contacts and because of this, to a more homogenous pressure distribution and less pressure peaks . In Bagh’s works, he develops a simulation model for the coating wear and shows that tooth flank damages only occur in the areas of exposed substrate material .
The existing works show a high potential increase of tooth flank load capacity by PVD/PECVD hard coatings. It can be shown that such coatings can take over the tasks of extreme-pressure (EP) and anti-wear (AW) additives of oils. Furthermore, the surface finishing process before coating can influence the resulting tooth flank load capacity, and in some studies, a reduction of tooth root strength by the application of a coating can be observed. The effectiveness of an adapted coating process, avoiding tempering of the substrate material, has not been tested until now. Extensive research concerning the influence of different surface finishing processes on the tooth flank load capacity of uncoated and coated gears has not been the focus in existing works. Also, the existing works focus on the coating of both gears in contact and not on the coating of just one gear combined with optimized surface finishing processes.
Objective and Approach
The goal of this paper, as presented in Figure 1, is the investigation and determination of the influence of surface finishing processes on the impact of PVD/PECVD coatings concerning the pitting load capacity of gears. To do this, the short-time pitting strength of coated and uncoated test gears with different surface finishing is evaluated. Also, both cases of coating one and two gears are investigated. By coating only one gear, a possible reduction of coating costs and a simultaneous increase of the pitting resistance is targeted. As a diamond-like carbon (DLC) coating, a modified tungsten carbide coating (a-C:H:W (WC/C)) will be applied. Due to an optimized coating process, consistent coating adhesion without tempering of the substrate material will be achieved. The result of this optimization will additionally be proven by the investigation of tooth root strength by means of pulsator testing.
Test Specimen, Test Rigs, and Test Conditions
For test gear geometry, a FZG-Cmod type gear set is chosen (see Figure 2), which has been used in former works  and . For the coating, a modified tungsten carbide coating (a-C:H:W (WC/C)) is applied. Three different coating cases are investigated:
- Both gears are coated
- Only the pinion is coated
- Both gears are uncoated
For the uncoated case in which both gears are coated, three different surface finishing processes are applied:
- Profile grinding (REF)
- Profile grinding followed by blast cleaning (MS)
- Profile grinding followed by isotropic superfinishing (ISF®)
For the case in which only the pinion is coated, only the blast cleaning and the isotropic superfinishing process are the focus, as the state of the art shows no promising results for one-sided coating without a previous finishing process .
In total, eight different test variants are evaluated concerning tooth flank and tooth root fatigue. The different variants are manufactured in one batch as far as possible. For the evaluation of tooth flank fatigue, three tests per variant are performed on a short-time fatigue load level (load torque M1 = 450 Nm) at a back-to-back test rig, according to ISO 14635. The tests with a completely coated gear set are based on the investigations of Bagh . The tests are run with a rotational speed for the pinion of n1 = 2250 min-1 until a limiting number of load cycles (LC) of Nlimit = 40·106 LC. For the lubrication system, a sump lubrication with an oil (viscosity of SAE 75W) is used, regulated at a temperature of TOil = 90 ± 3°C. For the evaluation of tooth root short-time fatigue, three tests are performed on each of two load levels for the variant’s uncoated reference (REF) and blast-cleaned coated gear set (MS-WCC2). The tests are run at a hydraulic pulsator with a test frequency of fpuls = 30 Hz until a limiting number of load cycles of Nlimit = 3·106 LC.
Analysis of Surface Structure
The result of surface roughness measurements of the different variants is shown in Figure 3. In addition to the traditional surface parameters Ra and Rz, the Abbott-curve parameters Rk, Rpk, and Rvk are analyzed. The results for the different surface finishing processes without a coating meet the expectations derived from the model concept of the process technologies. As the isotropic superfinishing process removes the surface peaks, this surface variant shows the lowest values for Ra, Rz, Rk, and Rpk. The value of Rvk is the highest for the isotropic superfinished variant, as the amount of valleys is rising relatively by the removal of the peaks. Furthermore, the coating does not influence the surface roughness parameters, except for the values of Rvk. Concerning an isotropic superfinished surface, the Rvk is reduced by the application of a coating; concerning a blast-cleaned surface, Rvk is increased. A possible explanation for this can be the relation of coating thickness to the height of the surface peaks and thereby the ability of the coating to follow the surface structure geometry. A thorough investigation concerning the observed phenomena may be part of further investigations.
Results of the Pitting Tests
For the evaluation of pitting fatigue for each variant, the number of load cycles for a failure probability of 50 percent is derived from the test results of three tests at one load level. The result of this approach is shown in Figure 4 for the case of uncoated gears and for the case that only the pinion is coated. For the uncoated, profile ground reference (REF), a number of load cycles of NL,50% = 4.351·106 LC is derived according to the probability distribution of Weibull. The test results for the ground reference show a very low scatter, which indicates a good stability of testing conditions. By the application of an isotropic superfinishing (ISF®) process, the number of load cycles can be increased significantly to NL,50% = 21.71·106 LC. A possible explanation for this can be the very smooth surface of these gear sets, which leads to less micro-Hertzian contacts. In addition, the load is carried by a higher material volume, and the mean stress can be reduced.
The application of a blast cleaning (MS) process leads to a comparable result; the number of load cycles is NL,50% = 22.05·106 LC. The residual stress state has not been measured within the project, but it is known from the state of the art that the blast cleaning process induces compressive residual stress in the near-surface zone of the material. This can be an explanation for the increase of load capacity by blast cleaning. The positive behavior of the blast-cleaned gears cannot be further improved by the additional application of the WC/C-coating (MS-WCC). The reached number of load cycles is only NL,50% = 8.68·106 LC. The highest pitting fatigue is estimated for the isotropic superfinished, coated variant (ISF®-WCC). Two of three tests reached the maximum number of load cycles of Nlimit = 40·106 LC without damage. Hence, for this variant, the chosen load level is no longer in the short-time fatigue area, and the number of load cycles for a failure probability of 50 percent cannot be calculated based on the described method. In all surface finishing variants, coated and uncoated, the tests show a higher scatter compared to the ground reference (REF), although all variants have been manufactured in the same batch. Consequently, there is an influence of a further finishing process additional to grinding on the scatter of pitting strength.
In Figure 5, the numbers of load cycles of the uncoated gears are compared to the results of a coated gear set. For the variant REF-WCC, the number of load cycles is with NL,50% = 8.14·106 LC, which is twice as big as the reference (NL,50% = 4.51·106 LC). The high number of load cycles reached by the application of an isotropic superfinishing process is not further increased by the additional application of a coating for both gears. The number of load cycles for the ISF®-WCC2 variant is only NL,50% = 3.23·106 LC. A possible explanation for the very low load capacity of the ISF®-WCC2 variant is the low value of Rvk that was observed during the surface roughness measurements. Having two gears with a low Rvk in contact leads to a low volume for possible oil storage, which can have an effect on the lubrication. However, the potential of increasing the pitting fatigue strength by a blast cleaning process is further increased by adding a coating. All three tests of the variant MS-WCC2 reached the limiting number of load cycles without damage.
The damage patterns of the performed tests are presented in Figure 6. The pitting damages occur mostly in the tooth flank area at double tooth contact. In some cases, damages appear at a higher diameter in the region of single tooth contact. Overall, the damage patterns of the different variants are comparable. Only the blast-cleaned gears show larger, more irregular material outbreaks. A reason for this can be the more inhomogeneous surface structure caused by the blast cleaning process. For the coated gears, all damages occur in tooth flank areas, where the coating is worn and the substrate material is in contact. Comparing the coated surface finishing variants, the ISF® pinions show the most coating wear.
Summarizing the pitting tests, it can be stated that an isotropic superfinishing process as well as a blast cleaning process increase the short-time pitting fatigue of the uncoated test gears. For the case in which only one gear (pinion) is coated, the isotropic superfinished surface shows the highest potential concerning pitting fatigue. A reason for this can be the lower roughness of isotropic superfinished surfaces, which prevents coated, sharp roughness peaks provoking an extensive wear of the counter gear. For the case of both gears coated, the blast-cleaned gears show the most promising behavior. A reason for this can be the improved coating adhesion, which leads to less coating wear. The ISF® variants show more extensive coating wear, which may be explained by the low Rvk value. According to Bagh, the coating wear is critical for the load capacity of coated gear sets .
Results of the Tooth Root Bending Tests
The reduction of tooth root fatigue strength by the application of a coating is explained by a loss of hardness in the surface zone due to temperatures during the coating process that are in the range or slightly below annealing temperatures of the substrate material. For this paper’s work, the coating process has been optimized in order to avoid this effect. The optimization is validated by pulsator tests for the uncoated, not shot peened, reference variant (REF) and a blast-cleaned, coated gear (MS-WCC). For this, the short-time fatigue behavior is evaluated by two to four tests on two load levels for each variant. The failure probability is estimated according to the probability distribution of Rossow. The results of the tests are shown in Figure 7.
For the lower load level with a normal force of FN = 24 kN, the number of load cycles for a failure probability of 50 percent is estimated to NL,50% = 172.62·103 LC for the ground reference. For the blast-cleaned, coated pinion, two out of three tests reached the limiting number of load cycles of Nlimit = 3·106 LC without damage. Hence, the lower load level is raised from FN = 24 kN to FN = 25 kN for the coated pinion. For this new lower load level, the number of load cycles is NL,50% = 91.39·103 LC. For the upper load level with a normal force of FN = 26 kN, the estimated numbers of load cycles are comparable for the two variants. The reference variant reaches NL,50% = 98.15·103 LC, and the MS-WCC variant reaches NL,50% = 87.55·103 LC.
In Figure 8, microscopic images of the fractured surfaces are shown. The visual appearance and the structure are comparable between the ground reference and the blast-cleaned coated pinion. Near the surface, a fine fracture structure is noticeable, resulting from the case hardening heat treatment. Summarizing the pulsator test results and the fractured surfaces, it can be stated that the applied coating process does not influence the tooth root bending strength in a negative way. The results for the ground reference and the blast-cleaned coated variants are comparable concerning scale and appearance.
Today, surface finishing processes as well as coating processes are in focus more in order to increase the tooth flank load capacity of gears. Existing works show a high potential of PVD/PECVD coatings in order to avoid scuffing and micropitting damages. An increase of the pitting load capacity is also noticed. The potential positive impact of such coatings concerning the pitting strength is dependent on many factors, especially the previous surface finishing processes. Therefore, this paper has focused on the investigation and determination of the influence of surface finishing processes on the impact of PVD/PECVD coatings concerning the pitting load capacity of gears. A possible negative influence of the coating process on the tooth root bending strength is avoided by an optimized coating process strategy and validated by additional pulsator tests.
For the evaluation of the influence of different surface finishing processes for uncoated gears, back-to-back tests are performed. In comparison to the ground reference, the isotropic superfinished as well as the blast-cleaned gear sets show a high increase of pitting strength (with a reached number of load cycles: factor 4-5). Through the combination of these surface finishing processes with a WC/C coating, the influence on the pitting load capacity is dependent on the coating strategy for the gear set. If only the pinion is coated, the low roughness of isotropic superfinished surfaces avoids extensive wear of the uncoated flank. Therefore, the pitting load capacity can be increased by the application of a coating. For the case that both gears are coated, it is more promising to blast clean the gears before coating. This variant shows the highest pitting load capacity of all investigated gear variants. A possible explanation for the low load capacity of isotropic superfinished, coated gears is the low Rvk value of this variant, which can influence the lubrication. This aspect should be further investigated in future works.
The performed pulsator tests confirm the thermal stability of the coating process. A tempering of the substrate material during the coating process is successfully avoided. Comparing the test results of ground reference gears and blast-cleaned coated gears, as well as the resulting fractured surfaces, it is shown that the optimized coating process has no negative influence on the tooth root bending fatigue.
In future works, the observed effects need to be further investigated concerning reasons and mechanisms of action in order to predict the influence of surface finishing and coating processes already during the design phase of gears. Furthermore, a coating-adapted design of the gear macro- and microgeometry should be the focus. For instance, effects like a premature tooth meshing increase the coating wear and shorten the lifetime of coated gears. In addition, the derived knowledge could be transferred to an application with other types of gears, such as helical or bevel gears.
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* Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the authors and may not represent the position or opinion of the American Gear Manufacturers Association (AGMA). This paper was presented October 2015 at the AGMA Fall Technical Meeting in Detroit, Michigan. 15FTM01.