# On the Effect of DLC and WCC Coatings on the Efficiency of Manual Transmission Gear Pairs
## 1 Introduction  
In order to improve the efficiency of transmission gear pairs, the use of diamond-like-carbon (DLC) coatings has been proposed in the literature. The application of DLC coatings reduces the coefficient of friction between the teeth surfaces, hence reducing frictional losses [1]. The different types of DLC coatings for automotive applications have variable effects on the generated frictional losses and wear [2]. DLC-type coatings possess advantageous properties, such as extreme hardness, high elastic modulus, excellent wear and corrosion resistance, high thermal and chemical stability, and low friction characteristics [3]. Additionally, they have the ability to perform reasonably well under dry sliding conditions. It should be noted that lubrication is essential for DLC coated components not only for better cooling and tribological performance, but also for the sensitivity that these coatings show to atmospheric conditions when lubricant is not present [3]. Additionally, it has been shown that the use of tungsten carbide carbon (WCC) coatings of gears with the use of the proper ester lubricant can increase the load carrying capacity up to 40% [1]. On the other hand, the use of coatings can also lead to adverse behaviour. Due to their extreme hardness, the local contact pressure could increase significantly considering more concentrated contact. Thus, the coating material could be transferred between the two contacting surfaces [2]. This not only minimises the effect of the coating on friction but also increases wear and, consequently, introduces further adverse effects on friction reduction. As a result, the hardness is one of the most important features when a coating is selected for an application [3]. Another factor affecting the coatingperformance for gear applications is the complex gear geometry, which may limit the performance of the coating since it is not easy to manufacture a perfectly coated gear [4]. Research on the effect of coatings on friction has shown that they can reduce it up to 50% [5]. The type of lubricant used in combination with the coating, especially if additives are present, can alter the effect intensity [6]. It should be noted that the concentration of the additive in the lubricant has a major effect on the performance of the coating [7]. Finally, another property that should be carefully considered is the coating thickness, especially when used for stainless-steel gears. It has been observed that thicker coatings are more effective in reducing friction (up to 41%), compared to thinner films, which may reduce friction only up to 29% [8]. Despite complications related to the properties of DLC coatings and sensitivity to specific operating conditions, their advantages can undoubtedly contribute to the reduction of gearbox power losses. Consequently, their use has been tested experimentally by various researchers [2; 9; 10]. However, there is a dearth of reported work on modelling the tribo-dynamics of DLC coated transmission gears compared to thorough research of helical gear performance [11] and dynamic contacts [12] in general. For most cases the models are based on computationally expensive numerical methods [13; 14] which do not take into account the system dynamics [15] and only consider contact level behaviour. In this work, a novel tribo-dynamic model of a manual transmission (sub-system level) has been developed, which incorporates the effect of using DLC coatings on gear teeth pairs. The model can predict the effect of coated teeth on contact mechanics and the generated friction analytically, which is a computationally economic way compared to the numerical models that can be found in the relevant literature. The prediction of boundary friction is based on the Greenwood and Tripp method [16], enriched with experimentally measured coefficients of the asperity boundary shear strength on DLC coated surfaces using AFM.