Coatings are used to fulfill functional requirements or to reduce friction and wear on components and tools. One of the main aims of using coatings is to improve durability and thus increase the sustainability of processes and products. In view of climate change and the associated increased importance of sustainability, the use of coatings, such as DLC coatings, is essential
Ionbond is a coating service provider that deposits hard, low-friction and wear-resistant coatings on its customers' products using physical vapor deposition (PVD), plasma-assisted chemical vapor deposition (PACVD) and chemical vapor deposition (CVD), thereby significantly improving their quality, performance and sustainability. These vacuum-based coating processes are used to deposit thin films that are only a few micrometers thick. The standard or customer-specific coating solutions are supplied to companies in the aerospace, medical technology, food, automotive, decoration and toolmaking industries. With 35 coating centers in Europe, North America and Asia, Ionbond's customers benefit from the global expertise and local accessibility of one of the world's largest coating networks. Ionbond is part of the IHI Group, a Japanese industrial conglomerate with significant R&D resources. The IHI Group is active in various fields, including energy and resources, social infrastructure, industrial machinery and aircraft engines.
The automotive industry is currently undergoing a transformation. For some years now, car manufacturers have been striving to reduce theCO2 emissions of their vehicles by reducing fuel consumption. They are driven by political measures and regulations. In the past, reduced fuel consumption was achieved primarily through more efficient combustion engines. Although engine downsizing reduces this, the load on the components increases due to the higher power density. In the passenger car sector, there is currently a shift towards electromobility. Such vehicles also have frictional contacts, for example in the drivetrain, albeit in smaller numbers. However, due to the higher weight of the vehicles and the higher torque of the engines, the differential gear is subjected to greater stress in the low speed range than in lighter vehicles with combustion engines. Due to the increasing demands of the automotive industry on components in tribological contact, new impetus is being given to the development of high-performance coatings that can withstand the increasing stresses and improve the durability of the components.
In the field of thin-film technology, a distinction can be made between different coating variants. Crystalline hard coatings are often used to protect components and tools against wear. These can be present as intercalated solid solutions in which non-metal atoms occupy the interstitial lattice sites of a carrier lattice structure of metal atoms. This special structure of ceramic materials gives them particularly high hardness and temperature resistance. A typical representative that is frequently used in the field of component coatings is chromium nitride (CrN).
The class of so-called diamond-like carbon (DLC) coatings, on the other hand, has an amorphous structure and is characterized in particular by its friction-reducing properties. The main component of the coatings is carbon, which is mainly sp2- or sp3-hybridized[1]. The sp2 hybridization has three σ bonds in the trigonal planar plane and one π bond orthogonal to this [2]. Due to the weaker π bonds, DLC layers with a high sp2 content are more "graphite-like". They have a low layer hardness of < 1,800 HV0.005 and a low coefficient of friction of µ < 0.1 compared to steel. In contrast, the sp3 hybridization shows four equivalent σ bonds that are arranged tetrahedrally [2]. Due to the higher bond energy, DLC coatings with high sp3 are more "diamond-like", i.e. very hard and wear-resistant with > 4,000 HV0.005. In diamond, the hardest naturally occurring material, the carbon is exclusively sp3-bonded, but in crystalline form [3].
In addition to carbon, DLC coatings can also contain hydrogen as a result of the process. a-C:H coatings can be produced using PACVD [4]. So-called precusors are used for this purpose. Precursors are the gaseous reactants from which the coating grows, e.g. acetylene (C2H2). Hydrogen reduces the cross-linking within the amorphous structure, causing the coatings to lose hardness and wear resistance. Metals or semi-metals, such as tungsten or silicon, can also be added in order to specifically adjust the friction behavior, temperature resistance or other properties. Hydrogen-free DLC coatings are referred to as a-C and ta-C. These coatings can only be deposited using PVD, as a solid carbon source without a hydrogen-containing precursor is used [4]. For the deposition of ta-C, coating processes are used with which a high degree of ionization of the plasma can be generated during coating deposition. Only with sufficiently high energy of the impinging ions can sp3 bonds be generated [5]. In an industrial environment, cathodic arc evaporation is often used for this purpose.
Due to their different structure and chemical composition, each coating variant has a specific range of properties, as described above. Technical applications sometimes place contradictory material requirements on the coatings due to their stress spectrum, such as high hardness but high toughness at the same time. In order to overcome these contradictory material requirements, coating systems can be designed in a multi-layer structure and deposited using different coating processes. Ionbond has developed a multi-layer coating system that can be used especially in highly stressed tribological contacts. This can be used, for example, on finger followers(Fig. 1a), which are a component of the valve train of a combustion engine. One application in which the coating system has already been successfully tested is differential bolts in differential gears for electric vehicles, in which high power and torques are transmitted to the drive wheels, see Figure 1b.
Fig. 2 a): Schematic representation and b) SEM image of the multilayer coating system in cross-section and c) light microscope image of a spherical section
Figure 2a shows the schematic structure of the newly developed coating concept. The different individual layers are partially visible in the scanning electron microscope (SEM) image (Fig. 2b) as well as in the light microscope image of a calotte section(Fig. 2c). First, a chromium adhesion promoter was applied to the steel material to improve the bonding of the coating. A CrN support layer was then applied. This crystalline hard material layer exhibits both high hardness and good fracture toughness and restricts the deformation of the overlying amorphous ta-C and a-C:X layers (X = H, Si, W, WC, Ti, Al, Zr, N, O). This is advantageous because amorphous materials such as DLC could otherwise be overloaded due to their structurally reduced fracture toughness. There is another Cr or CrC layer between the CrN and ta-C layers for better bonding. The Cr-CrN-Cr intermediate layers cannot be separated from each other on the images and have a total thickness of sCr-CrN-Cr = 2.2 µm. A sufficiently high thickness is required to ensure a good supporting effect. The ta-C intermediate layer is the actual functional layer. It has a very high nano-hardness of 4,000 - 8,000 HV0.005 and is particularly wear-resistant. Therefore, an individual layer thickness of onlysta C = 0.8 - 1.2 µm of the ta-C functional layer is already sufficiently high. The a C:X top layer fulfills the function of a running-in layer. The roughness peaks of the layer surface and the mating body of the tribological pairing are initially smoothed by this. The individual layer thickness of the a C:X running-in layer issa C:X = 0.8 - 1.0 µm. This layer thickness was selected in relation to the roughness shown below in order to ensure sufficiently good running-in.
Fig. 3 (a): SEM image of a lapped steel surface after coating, (b) SEM image of a ground steel surface after coating and mechanical post-treatment to remove the droplets and (c) light microscope image of a Rockwell C indentation.
The ta-C functional layer was produced by means of cathodic arc evaporation. While the arc burns on the carbon target, molten droplets of carbon (graphite) are emitted in addition to the vaporization of the carbon and are deposited on the components to be coated. Figure 3a shows the SEM image of a lapped steel surface after coating. These defects are clearly visible on the surface. These are the so-called droplets, which are a few micrometers in size. If the droplets are not firmly embedded in the coating, they can detach from the coating surface during tribological contact and accelerate wear as abrasive particles. They must therefore be removed in a mechanical post-treatment process after coating. Figure 3b shows the SEM image of a ground steel surface after coating and after such a mechanical post-treatment to remove loosely attached droplets.
The surface appears much smoother after the mechanical post-treatment and the loosely attached droplets have been removed. The smooth surface is also reflected in the roughness values of the post-treated coating (see Table 1). Based on the light microscope image of a Rockwell C impression in Figure 3c, it can be concluded that the coating is well bonded to the substrate. The adhesion class according to DIN 4856 corresponds to HF1 [6]. Furthermore, no cohesive flaking can be observed, which would indicate a cohesive coating failure.
|
Rz [µm] |
Ra [µm] |
Rp [µm] |
Rv [µm] |
Rk [µm] |
Rpk [µm] |
Rvk [µm] |
|
0,43 |
0,04 |
0,14 |
0,29 |
0,13 |
0,05 |
0,10 |
Tab. 1: Roughness parameters of the coating after mechanical post-treatment
In order to investigate the performance of the multi-layer coating concept in a tribological application, differential bolts were coated, installed in a differential gear and tested on a gearbox test bench. For this purpose, a high-load test was carried out in which higher differential speeds than could ever occur in reality were simulated. In addition, the load was deliberately distributed unevenly and bevel gears with extremely high roughness values of Rz = 8 µm were used as mating parts. This test regularly led to system failure as a result of scuffing damage in uncoated differential bolts and in differential bolts coated with conventional DLC coatings from other manufacturers.
Fig. 4 a): Photographic, b) light microscope and c) SEM image of a differential bolt in the contact area with the running surface of the bevel gear after use in a transmission test bench: high load test
Thanks to the newly developed, multi-layer coating concept, it was finally possible to carry out the high-load test without scuffing damage ending the test prematurely. The photographic image in Figure 4a shows the contact area with bevel gear after use. The oil grooves are located at the top and bottom of the image. In the area of the running surface, the surface appears to be polished due to the run-in. These areas were examined under a light microscope using SEM, see Figure 4b. Vertically aligned grinding grooves are visible, which were created by machining the bolt before coating. The horizontal scratches, on the other hand, originate from the disassembly of the gearbox. Finally, black dots are visible. These are holes in the coating. These were formed by the removal of droplets during mechanical reworking. More important, however, is something that is not visible: cracks, fractures and cohesive flaking, which would indicate a coating overload. The SEM image in Figure 4c confirms the conclusion that the coating was able to withstand the load. The remaining droplets firmly embedded in the coating, as well as the coating surface between the grinding grooves, are smoothly polished and there are no traces of overload. The analysis of the coating thickness by means of calotte grinding showed no change in the coating thickness in the area of the running surfaces. This suggests that the coating system is not only highly robust but also highly resistant to wear. Scuffing can therefore be effectively prevented and the service life, robustness and stability of the gearbox is increased as a result.
Literature
[1 ]A. Grill, Diamond and Related Materials 8, (1999), 428-434
[2] J. Robertson, Materials Science and Engineering R, Reports 37, 4-6, (2002), 129-281
[3] Y. Lifshitz, Diamond and Related Materials 8 (1999) 1659-1676
[4] VDI Guideline 2840, Fundamentals, Coating Types and Properties, (2012)
[5] C. Donnet, A. Erdemir, Tribology of diamond-like carbon films, Springer Science and Business Media, (2008)
[6] DIN 4856:2018-02, Carbon coatings and other hard coatings - Rockwell indentation test for the evaluation of adhesion
