Mechatronic systems are plagued by nonlinearities and contain uncertainties due to, amongst others, interactions with their environment. Models exhibiting accurate multistep predictive capabilities can be valuable in the context of motion control and design of servo controlled systems. Neural Network Augmented Physics (NNAP) models are presented in this paper to comprehend the behavior of servo systems that contain partially unknown dynamics. By means of a hybrid modeling formalism, neural network models are closely merged with physics-based state-space derivative functions. The methodology is applied on an experimental slider-crank system consisting of a servo drive and mechanical links of which the physical behavior is only partially known. Its interaction with the environment due to a compression spring load and friction phenomena, however, remains unknown. Accurate prediction capabilities of the presented NNAP models are demonstrated on the slider-crank system. Compared to the feedforward modeling formalism, recurrent NNAP models had the ability to even further reduce prediction errors up to 33.4% on validation data. From the converged dynamic NNAP model we extracted the neural network and identified the unknown phenomena, being the spring characteristic and the friction forces, within the mechatronic system.