Coating stress analysis under temperature increase for diamond and silicon CVD coatings

The CVD–coated parts are found in a wide range of applications and must meet high quality requirements. The stress state due to temperature changes on diamond and silicon dioxide coated parts is studied. For single–layer coatings, the stress state under tensile load is analysed. In addition, the strength of the diamond–coated part under bending stress is also investigated.


Introduction
Deposition techniques emerged in the 1950s and have since experienced continuous development.Surface engineering, specifically the field of planned and controlled surface design, has grown into a significant discipline, satisfying the demands of various scientific and industrial sectors.The majority of industrially applied hard coatings are deposited using either Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) methods.CVD and PVD processes offer noteworthy advantages in terms of the versatility and control over the composition and structure of coatings.These processes have proven successful in coating a wide range of mechanical components, as depicted in Figure 1 [1].

Figure 1.
A comparison between the heat resistance limitations of materials and the temperature requirements of various applications and coating processes [1].

2
CVD is a collective term for all techniques that involve the use of gaseous precursor materials introduced into a reaction chamber to produce a coating [2].The modification and transformation of surfaces according to specific requirements remain in a rising trend, whether it involves optical [3] or aesthetic surface modifications [4], surface treatments with electrically conductive coatings [5], wearresistant [6] or corrosion-resistant layers [7], or other property-enhancing coatings [8].The principles of CVD involve the controlled reaction of precursor gases in a reactor chamber to produce a chemical reaction at the substrate surface.A wide range of chemical reactions are used, including thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitridation.These reactions can be employed individually or in combination to achieve the desired deposition process [9].The precursor gases, which may be in liquid or gaseous form, contain the desired elements or compounds that will form the coating material.These gases are introduced into the reactor chamber along with a carrier gas, which helps transport the precursors to the substrate.The deposition mechanism in CVD can be categorized into two main types: thermal CVD and plasma-enhanced CVD (PECVD) [10].In thermal CVD, the chemical reactions are primarily driven by the thermal energy provided by heating the precursor gases.The high temperature allows the precursors to decompose or react with each other, forming the desired coating material.The reaction may involve the release of byproducts, which are carried away by the carrier gas.The deposited film grows layer by layer on the substrate surface.In PECVD, plasma is introduced into the reactor chamber to enhance the deposition process.The plasma is generated by applying a high-frequency electric field or by other means such as microwaves and the plasma consists of energetic species, including ions, electrons, and radicals, which increase the reactivity of the precursors and facilitate the deposition process.PECVD can operate at lower temperatures compared to thermal CVD, making it suitable for temperature-sensitive substrates [11].The choice of precursor gases, their concentrations, reaction temperatures, and deposition conditions can be carefully controlled to achieve specific film properties such as thickness, composition, crystallinity, and surface morphology.The versatility of CVD allows for the deposition of a wide range of materials, including metals, metal oxides, semiconductors, and organic films.Overall, CVD offers precise control over film deposition, enabling the production of high-quality coatings with tailored properties.Its applications span across various industries, including microelectronics, optoelectronics, energy storage, catalysis, and corrosion protection, among others.
Thin film growth is a fascinating area of research with immense technological implications.Experimental observations have revealed three basic nucleation modes in thin film growth: (i) Island or Volmer-Weber Growth, where nucleation sites form randomly on the substrate, leading to the creation of isolated islands of material that continue to grow independently until they cover the entire substrate, resulting in a rough and non-uniform surface morphology.(ii) Layer or Frank-van der Merwe Growth, where nucleation and growth occur in a layer-by-layer fashion, forming a smooth and well-ordered film structure without islanding due to strong film-substrate adhesion.(iii) Island-Layer or Stranski-Krastonov Growth, involving a combination of island growth and layer-by-layer growth, where initially isolated islands coalesce into a continuous layer on the substrate surface, and the subsequent growth proceeds in a layer-by-layer fashion, indicating an intermediate adhesion between the film and the substrate, leading to a two-stage growth process.These nucleation modes are fundamental to understanding the thin film growth process and play a crucial role in determining the resulting film structure, surface morphology, and properties [12].Hard coatings typically exhibit island growth [13].
The properties of thin films, such as thickness, composition, crystallinity, and surface morphology, can be finely tuned by adjusting deposition parameters and growth conditions.Understanding the impact of these parameters is crucial for tailoring thin films to meet the specific requirements of diverse applications.

Calculation of normal stresses caused by temperature change in CVD coatings
Coated parts play an important role in technical practice.In this case, as a result of the temperature change, a significant stress can arise in the material with a lower coefficient of thermal expansion, since the two different materials work together.In the next slot, we will examine the stresses arising in diamond and CVD coatings.

Diamond CVD coating
In the following, we will determine how much stresses develops in the coating of the CVD diamondcoated carbide wafer of length l0 and width b shown in Figure 2, if the temperature rises by 50 °C (Δt = 50 °C).
The change in length of CVD coating due to temperature change: The difference between the changes in length: Since the change in length of the CVD coating is smaller, the change in length of the carbide creates stresses in it, which is calculated in the following.The extensional strain can be calculated as follows: We determine the stresses arising in the CVD coating as a result of the temperature change: In the following, we will determine how much stresses develops in the SiO2 coated carbide shown in Figure 3, if the temperature rises by 50 °C (Δt = 50 °C).
The area A 0 =aa of the S i O 2 coating increases to A 1S due to the increase of temperature (a=20 mm) ) 400 (1 2 4.5 10 50 ) 400.18 In terms of simplifying the model, the relationship used is: The calculation shows that a temperature change of 50 °C generates a stress of 88.5 MPa in the rigid material.

Strength of diamond coated element
Next, we present the strength calculation of the structural element used for bending shown in Figure 4.The top layer is made of carbide, which has been coated with diamond CVD [15].
During our calculations, we take into account the following material characteristics and geometric dimensions:  the thickness of the diamond CVD coating h1 = 10 μm = 0,01mm,  the width of element b = 20 mm;  the thickness of the carbide h2 = 25 mm,  the bending moment M = 0,5 kNm.

Area of cross sections
The reduced static moment calculated for the z 0 axis The reduced centre of gravity The stress arising in the lower thread of the "1" layer The stress rising in the upper thread of the "2" layer

Conclusions
Based on the results of these calculations, the following conclusions can be drawn: -stresses are induced in coated structural elements is different from the base metal due to temperature changes, which should be considered in the mechanical design; -the stress state in the coated structural elements is different from the base metal under stress.
-the different combinations of materials result in significant stress jumps which must be taken into account in the design; -this method can also be used effectively for the design of laminated structures (e.g.sandwich structures).The literature review section shows the importance of CVD technology in the industry.Therefore, it is important to be able to estimate the achievable properties in advance, and for this purpose we have shown some calculation methods in the article.

Figure 4 .
Figure 4. shows the stress distribution of the two-layer structural element along the height.

Figure 4 .
Figure 4. Stress distribution along the height of the two-layer structural element International Conference on Applied Sciences (ICAS 2023) Journal of Physics: Conference Series 2714 (2024) 012023 11th