Open Access. © 2020 S. K. Mishra, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License Rev. Adv. Mater. Sci. 2020; 59:553–585 Review Article Suman Kumari Mishra* Toughening of nanocomposite hard coatings https://doi.org/10.1515/rams-2020-0049 Received Dec 10, 2019; accepted May 05, 2020 Abstract: For engineering applications, hardness must be complimented with high toughness for applications where high contact loads are there. A good combination of hard- ness, toughness and low coefficient of friction can be achieved, by suitable tailoring of microstructures of coat- ing in hard nanocomposite coatings. Tribologocal appli- cations require hard coatings with tailored functionalities for different applications; hard nanocomposite coatings are potential materials for such applications. Ti and amor- phous carbon based systems have shown more promising material. The present review discusses the nanocomposite hard coatings, mechanism of enhancement of toughness, multilayer hard nanocomposite coatings. Here, mainly Ti and Si based nanocomposite has been discussed as carbon based reviews are available in plenty in literature and well documented. Ti-B-N, Ti-Si-B-C, Ti-Si-B-C-N, Si-C-N, Ti-Al- N, Ti-Al-Si-N, Al-Si-N, Ti-Cr-Al-N, Zr-Si-N and some other similar system nanocomposite hard coatings are impor- tant where the gradual and intelligent additions of differ- ent elements in hard single component phase provides the combination of hardness, toughness and low coefficient of friction. Some of these systems are discussed. In the end, the future directions of research, Technology„ which are re- quired to achieve tough nanocomposite hard coatings for actual applications are also highlighted. Keywords: nanocomposite coatings; toughness 1 Nanocomposite hard coatings Nanoscale dispersion of phases, particles or controlled nanostructure in the composite can introduce new phys- ical properties and can change significantly the properties of the original matrix, which may not be possible in sin- gle conventional microstructure or material [1–4]. It can result into very effective change in thermal, optical, elec- *Corresponding Author: Suman Kumari Mishra: CSIR-National Metallurgical Laboratory, Jamshedpur, India-831007; Email: suman@nmlindia.org; skm_smp@yahoo.co.in; Fax:91-6572345213 trical and mechanical behavior [5]. In literature, nanocrys- talline materials are composed of grains, crystallites, lay- ers of the order of less than 100 nm at least in one direc- tion, but it is not only a theoretical number but should lead to physical importance. The nanocomposite coatings have either two nanocrystalline phases or a combination a nanocrystalline and an amorphous phase. The applica- tion of thin films and coatings are the alternative methods to get the required surface characteristics. Corrosion re- sistance, wear resistance, higher hardness, lower friction, controlled optical properties, aesthetics coatings, and dif- ferent functional coatings are important. Among them hard coatings are very important. Newer hard coatings are being researched by different methods of deposition tech- niques to obtain [6, 7] simple monolayer, multilayer or gra- dient coatings. The limitation is the selection of the coat- ing material. It is impossible to get numerous properties from an ideal coating simultaneously. The application of the nanostructure coatings is a potential solution to meet the complex demand. Hard nanocomposite coatings with tailored function- ality such as low coefficient of friction, wear and oxida- tion resistance, aesthetic, biocompatibility has many tri- bological applications [5, 8]. Nanocomposite coatings are isotropic hence, can be applied to three dimensional ob- jects having similar properties [9]. The high density of boundaries between nanometer size grains embedded in the polycrystalline or amorphous matrix results in an in- creased mechanical resistance. In nanocomposite hard coatings mechanical and tri- bological properties may not follow volume mixture rules rather they depend on the synergy of the constituents and grain boundary effect [10]. Along with the hardness, tough- ness also needs to be good for actual applications. Hard- ness enhancement apart from toughness has been the pri- mary goal of material scientist all over the world work- ing in the area of nanocomposite hard coatings. Hardness is the resistance to plastic deformation, which predom- inantly caused by dislocation movement under applied load in a crystalline material. Hard materials usually clas- sified as hardness around 20 GPA. Superhard materials have hardness in the range of 40 GPa, whereas materials having around or more than 60 GPa are called ultrahard materials. Diamond is the hardest material (70-100 GPa)