Due to the high temperature oxidation resistance, corrosion resistance, and good radar waves absorption performance, the radar absorbing coatings (RACs) composed of ceramics and its composites have attracted extensive attention. The RACs is composed of dielectric absorbent and matrix. According to the absorbing mechanism, RACs can be divided into magnetic loss RACs and dielectric loss RACs. High temperature RACs is usually dielectric loss RACs. This article describes the basic principle of radar waves absorption by dielectric loss RACs, and the influence mechanism of temperature, dielectric absorbent content, coating thickness, coating structure design, and other parameters on its radar waves absorption performance. Subsequently, this article introduces the test method of radar waves absorbing performance of RACs and the process of spraying method to prepare RACs, especially the atmospheric plasma spraying method. Finally, this article summarizes the latest research progress of high temperature RACs, mainly focusing on the RACs of doped oxide ceramics (especially alumina), carbide ceramics, and MAX phase ceramics. This review suggests the possibilities of improving RACs absorbing performance through the composition optimization and structural design.

In order to save the time and cost of friction and wear experiments, the coating composition (different contents of Al, Ti, and Cu elements), ratio of hardness and elastic modulus (H³/E²), vacuum heat treatment (VHT) temperature, and wear form were used as input variables, and the wear rates of high-entropy alloy (HEA) coatings were used as output variables. The dataset was entirely obtained by experiment. Four machine learning algorithms (classification and regression tree (CART), random forest (RF), gradient boosting decision tree (GBDT), and adaptive boosting (AdaBoost)) were used to predict the wear resistance of HEA coatings based on a small amount of data. The results show that except for the GBDT model, the other three models had good performance. Because of the small amount of data, the CART model demonstrated the best prediction performance and can provide guidance for predicting the wear resistance of AlCoCrFeNi-X (Ti, Cu) HEA coatings for drilling equipment. Furthermore, the contribution of different factors to the wear rate of AlCoCrFeNi-X (Ti, Cu) HEA coatings was obtained. Al content had the greatest influence on wear rate, followed by H³/E², wear form, and VHT temperature.

Copper (Cu)-based composite coatings have been widely applied in all kinds of important industry fields due to their outstanding comprehensive properties. The preparation temperature of a composite coating is the key factor affecting the properties, so the cold spray (CS) technology is characterized by low-temperature solid-state deposition, which ensures its emergence as the most promising technology for preparing the Cu-based composite coatings. In this paper, first, the principle of CS technology and the deposition mechanism of the coatings are introduced. On this basis, the deposition mechanism of Cu-based metal/ceramic composite coatings is further explored. Secondly, the effects of key CS process parameters (particle velocity, particle morphology, and substrate state) on the quality of the Cu-based composite coatings are summarized, and the current research status of cold-sprayed Cu-based composite coatings in the fields of corrosion resistance, wear resistance, self-lubricating properties, and electrical conductivity is reviewed. Moreover, the improvement of the performance of Cu-based composite coatings by various post-process treatments of coatings, such as heat treatment (HT) and friction stir processing (FSP), is elaborated. Finally, the future development of Cu-based composite coatings and CS technology is prospected.

Friction, wear, and contact fatigue are the main causes of energy loss, material waste, and equipment failure. The aim of remanufacturing is to repair and modify the damaged equipment surface, and the surface coating is the major material that allows the remanufactured parts to be used in a new round of operation. Thus, the design and preparation of surface coatings are very important to repair, strengthen, or modify the friction pairs, in order to ensure long-term operation of the remanufactured parts. Recently, a lot of research on designing and preparing friction pair surface modification coatings has been conducted by the National Key Laboratory for Remanufacturing (NKLR). The research conducted achieved the following goals: the mechanism of micro/nano multilayer surface modification coatings with long-term efficacy life was revealed, and the corresponding design considerations and preparation methods of nanocrystalline micro tribological coatings were innovatively developed. A series of new “two-step” processes to prepare sulfide solid lubricating coatings were developed. The competitive failure mechanism of the surface coating in simultaneous wear and fatigue conditions was revealed, and some composite coatings with dual properties of wear resistance and fatigue resistance were prepared. Based on the stress distribution of friction surface contact areas and the piezoelectric effect, a failure warning intelligent coating is designed and developed. These coatings have been successfully applied to critical friction components, such as the spindle of large centrifugal compressors, engine cylinder piston components, and driver gear pairs.