Modern Solutions for Functional Coatings in CVD Processes

Today, many technologies for the deposition of various functional coatings using volatile compounds are united under the general name chemical vapor deposition processes from the gas phase (CDV, MOCVD, ALD, CVI, PECVD, etc.). The fundamentals of MOCVD are identical and almost independent of the experimental process parameters. The common ideological platform that unites these processes is the transport of vapors of the initial volatile compound into the zone of deposition from the gas phase using metal complexes with organic ligands as precursors (metal–organic chemical vapor deposition). Coating layers are formed during the decomposition of an “isolated” volatile compound molecule on a heated substrate, which allows for the implementation of almost any options from a system of unbound nanoparticles to continuous “thick” micron layers by varying the process parameters. The main reason for the development of a large number of different versions of CVD technology is that there is no possibility to directly control the process of precursor decay on the surface. Another important circumstance is presented by significant differences in the basic thermal parameters of a huge number of currently known precursors. The MOCVD processes find numerous applications in the production of various types of materials: powders, fibers, thin and thick films, film heterostructures, single crystals, glasses, as well as their structural varieties (amorphous materials, polycrystalline materials with different microstructures, etc.).

One of the advantages of the processes under consideration is the possibility of forming compact coatings from the most high-temperature oxides and refractory metals (iridium and platinum) at temperatures well below their melting point. Thermal barrier coatings (TBC) have undergone significant progress since the initial testing and development of thermal spray coatings. Currently, thermal barrier coatings are used in various fields of technology, including internal combustion engines, gas turbine blades of jet engines, units of pyrochemical processing, and many others [1]. According to estimates of [2], industrial gas turbines occupy a market share of 25%; the total contribution of the aerospace industry is up to 35%. The global TBC market was valued at USD 12.4 billion in 2015 and is expected to reach USD 22.4 billion by the end of 2024. Global market growth from 2016 to 2024 is measured at a compound annual growth rate of 6.6%.

Thermal barrier coatings based on yttria-stabilized zirconia (YSZ) have reached a stage of development when a further increase in the service life and/or maximum operating temperature is becoming more and more difficult [3]. In aviation, the temperature of the gas flow around a blade can rise to 1350 °C–1425 °C. In this temperature range, the existing engines operate at optimal efficiency. It is known that yttria-stabilized zirconia 7YSZ is unsuitable at temperatures exceeding 1200 °C due to its phase change and sintering, which leads to rapid wear of the top ceramic coating. In this regard, the search for new materials with low thermal conductivity, relatively low thermal expansion coefficient, and phase stability at temperatures well above 1200 °C began in efforts to replace YSZ. As an alternative, A&A Coatings offer coatings based on LaTi₂Al₉O₁₉. Combinations of two layers of 4YSZ (4% YSZ) and LaTi₂Al₉O₁₉ allow for an increase in the coating operating temperature up to 1500 °C, while providing excellent hardness and a lower coefficient of thermal expansion.

Pyrochlore Re₂Zr₂O₇ is considered another promising class of materials [4]. Gadolinium zirconate (Gd₂Zr₂O₇, GZO) and lanthanum zirconate (La₂Zr₂O₇, LZO) can be used to form TBC [5,6]. These materials have low thermal conductivity, but their crack resistance is much lower than that of YSZ, which leads to lower resistance to deformation and premature failure of single-layer coatings. Therefore, it is advantageous to create two-layer coatings, where the top layer of GZO or LZO is sprayed onto a layer of standard YSZ material. As a result, the lower part of the TBC remains the most modern TBC system and can benefit from the accumulated experience with YSZ–TBC. The mechanical aspects of such two-layer ceramic structures have not yet been studied in detail. The maximum allowable load or corresponding fracture strain of the heat-shielding coating is of particular interest.

Processes based on CVD are potentially more flexible in terms of controlling the coating properties: the possibility of creating coatings with different combinations of properties across the layer thickness is being considered. Gaseous precursors decompose on the surface with the use of various activation methods to form a coating. There is a positive experience in controlling the microstructure of coatings using various physical and chemical mechanisms such as UV radiation, activation of coating growth using an electric discharge, etc., which allow for obtaining a new class of promising functional materials focused on application in aviation and space technology.

The process of laser-induced chemical vapor deposition (LCVD) is a universal method for the local deposition of thin films, which allows for obtaining a wide range of materials and for producing parts with complex geometries [7]. The essence of the method consists of the formation of a coating during the photothermal or pyrolytic decomposition of precursor vapors on the surface of the product when interacting with a focused laser beam. Depending on the wavelength of the laser radiation and the chemical composition of the precursor, it is possible to obtain metallic, oxide, fluoride, and other structures. An important advantage of laser-induced coating deposition technologies is the ability to move from the deposition of oxide layers to metallic ones in a single approach and to synthesize alloys, composites, and various 2D and 3D structures on the surface. The LCVD has been used to fabricate microlenses in situ with the precise control of film properties [8]. The process was implemented in an evacuated reaction chamber containing precursor vapors and a substrate. The spatial resolution, when deposited on a substrate, is determined by several parameters: the size of the laser beam spot, the laser energy density, the laser wavelength, and thermal properties of the substrate. The LCVD can have a resolution of 500 nm, a deposition rate of up to 80–100 µm/s [9], and a write rate of 0.1–5 mm/s [10]. Today, the LCVD is mainly used for the production of thin films and as a repair agent for metal interconnects. Microstructures that have been fabricated using this method include carbon microcoil springs [11], microsolenoids [12], 3D photonic alumina microparts [9], and microcells [13]. The growth of carbon nanotubes was also demonstrated using this method [14]. A noticeable advantage of LCVD is the possibility of forming a microrelief of an arbitrary shape, for instance, for manufacturing the MEMS devices [15].

We suppose that the Pulse Pressure MOCVD is an underestimated method for depositing a wide range of functional layers [16]. In this version, the processes of heat and mass transfer in the reactor, which determine the kinetics of the process on the surface and, ultimately, the functional properties of the film are much more predictable and controllable. A low-pressure cyclic MOCVD reactor in which the multilayer structure can be grown in several thousand “growth cycles” was developed in [17]. In each cycle, with a typical duration of ~1 s, the growth proceeds from a low-pressure mixture (1 mbar), which is immobile with respect to the substrates during the reaction. From 1 to 30 atomic layers can be deposited in one cycle. The expected benefits include (a) an improved layer thickness and composition uniformity, (b) the highly efficient use of precursors, (c) perfectly sharp transitions, and (d) the possibility of direct scaling up to a large reactor volume.

Two-dimensional semiconductor materials with a layered crystal structure are of great interest as promising candidates for application in electronics, optoelectronics, and sensors due to their unique and excellent characteristics. Today, there are some problems in synthesizing these materials over a large area. A pulsed version of MOCVD is proposed for growing thin crystalline MoS₂ films. Crystalline MoS₂ was successfully synthesized at the lowest temperature (350 °C) in a very short process time of 550 s [18]. High-temperature casting molds wear out quickly due to thermal stresses, and it is expensive to manufacture them from high-temperature alloys. A possible solution would be the application of a thermal barrier coating based on zirconia to the mold. The TBC film should have a columnar microstructure and be at least 50 µm thick. The coating should be periodically reapplied as it is used [16]. The recent global COVID-19 pandemic has highlighted the urgent need for the practical application of antimicrobial coatings on touch surfaces. Nanostructured TiO₂ is a promising candidate for passive transmission reduction when applied to knobs, pressure plates, and switches in hospitals. In [19], the TiO₂ coatings with a thickness in the range of 1.3–16 µm were obtained by the PP-MOCVD method by changing the number of precursor vapor pulses.

In the last 10–15 years, interest in the application of MOCVD processes for the deposition of functional coatings on medical devices has increased dramatically. As a rule, these products have a complex geometry and/or porosity, and this imposes a rather strict set of requirements on the applied technology of coating deposition. First, it has a high scattering power, uniformity, continuity, and purity with the ability to control the composition of the coating material. An additional argument in favor of choosing this technology is the potential use of products made of polymeric materials in medical practice, for example, implants. This circumstance almost completely excludes the possibility of using electrochemical and “paste” technologies for the deposition of functional layers. In particular, noble metal coatings eliminate the problem of visualization due to radiopacity (UCM and polymer implants) and the problem of insufficient biocompatibility (metal implants) due to their exceptional chemical inertness. To obtain metal-containing coatings based on noble metals, both ALD and CVD processes are used [20,21,22,23,24]. Ir- and Pt-containing films (several tens of nanometers) on articles of various shapes, up to trench coats and nanotubes, were obtained in [22,25,26,27,28,29]. Examples of noble metal coatings on carbon materials are known [30,31]. Only some publications are devoted to the deposition of these materials on medical devices [32].Test planar objects made of Ag–Pt bimetallic alloys (up to 7 wt.% Pt) were studied in [24]. In [33], iridium layers were deposited onto endocardial electrodes using the MO CVD method. In general, in many cases, chemical vapor deposition is the best technological choice, since it produces functional layers that meet the most stringent medical requirements.

CVD processes require attention to multi-scale phenomena, and ideally, for modeling such systems, a multi-scale computational complex is required, including analyses of the quantum effects, molecular dynamics, or kinetic Monte Carlo method (CMC), as well as CFD in the framework of macroscopic continuum models [34]. Different coating deposition methods require different modeling methods. The simulation of chemical vapor deposition at unsteady pulse pressure (PP-CVD) is carried out either using molecular dynamics or using direct Monte Carlo simulation (DSMC) [35]. The results showed that in the PP-CVD process, the arrival time of precursor particles to the deposition surface is much shorter than the reactor downtime, which leads to the high conversion efficiency of the precursor. Macroscopic CFD modeling of heat, substance, and momentum transfer processes in a CVD reactor is based on databases of the thermodynamic, kinetic, and thermophysical properties of substances. Additionally, as noted in [36], without high-quality data, even the most advanced model may have no value.

CVD processes are characterized by low precursor concentrations of the order of 1% or less, flows and heat transfer under such conditions, and the flow in the reactor being often modeled independently of the transfer phenomena and chemical kinetics. A distinctive feature of CVD processes in flow reactors is the competition of free and forced convection mechanisms, which leads to nonlinear phenomena manifested in a multiplicity of states with fixed flow and thermal parameters or to the emergence of non-stationary scenarios of coating deposition [37,38], which significantly complicates the modeling of the CVD process. CFD in combination with modern machine learning algorithms makes it possible to develop simplified models of reduced order [38]. Such solutions can be used both for predicting the CVD system and for operational control during the production cycle, allowing for the control of nonlinear effects. Another approach that makes it possible to significantly simplify the modeling of the problem and, in some cases, make it possible to obtain analytical estimates is based on the use of the approximation of similarity of transfer processes [39]. When analyzing the reacting near-wall flow, it is possible to use correlations characterizing convective heat and mass transfer in non-reacting systems. To achieve this, it is necessary to switch to variables (generalized concentrations) so that the governing equations and boundary conditions of such a problem, written in dimensionless form, would remain identical to the equations and boundary conditions of the non-reacting system, also presented in dimensionless form. Within the framework of the model of reacting boundary layer, the coating deposition process may be considered the process of independent global reactions of diffusion combustion of precursors (for example, Zr(dpm)₄ and Y(dpm)₃) under convection conditions on a permeable surface. The analysis of generalized patterns of transfer processes allows us to assess the conditions that ensure the effectiveness of the use of the precursor substance. In addition, estimates show that the atomic composition of metals in the precursor mixture and in the coating may differ.

The great potential of CVD technologies allows us to confidently predict the growth of interest in methods that control the morphology and properties of coatings.