6. Chemical Vapor Deposition

The coating of various types of materials, i.e., metals, ceramics, polymers, and composites, with ceramic layers and films is one of the key technologies employed for prolonging the lifetime of infrastructural materials by protecting them from harsh environments. This process also enhances properties and adds multi-functionalities to materials. Thermal barrier coatings (TBCs) on turbine blades, hard coatings on cutting tools, and corrosion-protective coatings on steel pipes and containers are applied on an industrial scale, while biocompatible coatings on artificial bones, growth of semiconductor films, and depositions of nano-layers and particles for catalysts have been developed to produce high-performance functional materials. Vapor-phase deposition techniques, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), are advantageous, as they create finely controlled microstructure in films and deposits, compared to liquid- and solid-state processes. In TBCs fabricated by the electron beam evaporation method, the growth of columnar crystals with nanostructures, such as nano-pores and gaps, contributes to the relaxation of thermal stress at the interface between coatings and basal materials, owing to the difference in thermal expansion coefficients. CVD, an industrially important vapor-phase deposition technique, has been employed for the growth of silicon semiconductors, thin film processing of integrated circuits, insulating coatings, and hard coatings on cutting tools. In CVD, regulation of the process parameters, such as deposition temperatures, pressures, and instrumental systems of heating and supplying precursor vapors, has enabled us to produce various types of deposits with well-controlled microstructures and unique textures. Single crystal-like highly pure films can be grown, while coatings with thick layers can be deposited. CVD is also employed for the synthesis of nanomaterials, such as graphene and carbon nanotubes (CNTs) and for the infiltration in manufacturing of fiber-reinforced ceramic composites (ceramic matrix composite: CMC). CVD techniques combined with high-power laser irradiation have enabled the fabrication of thick layers with discriminating microstructure at significantly high deposition rates. Although the typical applications of CVD techniques are for the deposition of films on flatter surfaces of plate-shaped substrates like wafers, CVD can also deposit coating layers and nanoparticles on particulate and porous substrates with complex surface configurations because of its excellent step coverage. These CVD techniques utilizing laser processing and powder technologies are expected to contribute to the further development of additive manufacturing. In this chapter, the basis of conventional CVD processing is briefly outlined; then high-speed deposition and refinements of microstructure by CVD using the auxiliary energy of high-powered lasers and the deposition of coatings and nanoparticles on particulate substrates by a rotary CVD technique are introduced.