This work examines how vapor-deposited coating of DLC (partially diamond) on stainless steel 304 substrate is affected by the sound vibration. For this, a specially designed chemical vapor deposition (thermal CVD and hot filament) apparatus having facility of generating sound vibration at different frequency is fabricated. A coating of DLC (partially diamond) has been deposited on the substrate, and the characterization of the coating has been done by SEM, EDX, and XRD. The coating of carbon is identified by EDX, and the allotropic forms of graphite and diamond peaks of carbon are found by XRD analysis. By SEM analysis, it is found that the microstructures of deposited coatings are more compact and smoother under vibration than those in absence of vibration. The experiments were conducted under different ranges of vibration including sonic and ultrasonic range. Studies have shown that the growth rate of deposited coating on a unit area is higher under vibration than that in absence of vibration. It is found that deposition rate varies with the distance between substrate and activation heater and frequency of vibration. The deposition rate does not vary significantly with the change of frequency in the sonic range. The amount of deposition under ultrasonic vibration increases significantly with the frequency of vibration upto 5-6 mm distance between substrate and activation heater. Within this distance, the difference of deposition rate under vibration and without vibration conditions increases almost linearly with the increase of frequency of vibration. Beyond this distance, the effect of frequency on deposition rate becomes almost constant. In addition, the higher the distance, the less is the effectiveness of frequency of vibration on the deposition rate in that range. The deposition rate increases due to the extra vibration of sound added to the system which may enhance the activation energy by increasing its kinetic energy. The experimental results are compared with those available in the literature, and physical explanations are provided.
Chemical vapor deposition (CVD) is a process in which a solid material formed from a vapor phase by chemical reaction is deposited on a heated substrate. The deposited material is obtained as a coating of multicrystal layer. The controlling parameters in CVD process are surface kinetics, mass transport in the vapor, thermodynamics of the system, chemistry of the reaction and processing parameters like temperature and pressure. The deposition rate which is the prime limiting factor in a CVD process is mainly controlled by the formation of required species to be deposited and its transportation in the vapor and surface kinetics [
The effect of sound vibration increases with the increase of density of media, through which it travels. Therefore, the CVD process has been selected, as CVD does not usually require very low pressure, which is necessary for PVD system. Consequently, the vacuum system in CVD is simpler and less costly. Comparing with other CVD process, thermal CVD (hot filament) is relatively inexpensive, and experiments can be readily carried out. Therefore, in this study an attempt is made to investigate the effect of sound vibration, in particular, the frequency of vibration on the deposition rate. In addition to deposition rate, the quality of deposited coating is also investigated. Deposition in absence of vibration was investigated first, and then the results were compared with the results obtained under different frequency of vibrations. Some parameters that affect the deposition were also inspected.
A thermal chemical vapor deposition (hot filament) setup (Figure
Schematic diagram of chemical vapor deposition (hot filament) setup.
A separate arrangement is designed and fabricated for generating sound shown, in Figure
Schematic diagram of sound generation system.
The deposition rates of the coating per unit area per unit time were calculated from the weight difference of substrate before and after deposition. The surface morphologies of the deposited coatings were analyzed by SEM attached with energy dispersive X-ray spectrometry (EDX). X-ray diffraction (XRD), with target of Mo (Zr), 30 kV/20 mA, and an incident angle of 1°, is used to study the composition of coated material.
Experimental conditions are shown in Table
Experimental variables.
S. number | Parameters | Range |
---|---|---|
(1) | Pressure | 20–30 Torr |
(2) | Substrate (nicrom) heater temperature | 800–1000°C |
(3) | Activation (tungsten) heater temperature | 1800–2000°C |
(4) | Substrate (nicrom) heater power | 1000 watt |
(5) | Substrate (nicrom) heater voltage | 80–100 V |
(6) | Substrate (nicrom) heater current | 7–10 amp |
(7) | Activation (tungsten) heater power | 200 watt |
(8) | Activation (tungsten) heater voltage | 5–7 V |
(9) | Activation (tungsten) heater current | 25–35 amp |
(10) | Flow rate (CH4 gas) | 0.1–1.5 L/min |
(11) | Gap between substrate and Tungsten heater | 2.5–8.0 mm |
(12) | Sound frequency | 0–110 kHz |
(13) | Deposition duration | 3–10 minutes |
(14) | Substrate size | 22 mm × 14 mm |
Figure
Deposition rate as a function of frequency of vibration (
From this figure, it is observed that deposition rate increases from 0 at no to approximately 6 KHz. From 6 KHz, the deposition rate increases linearly up to approximately 20 KHz, and after that the steepness of the curve shows higher deposition rate, and finally increment rate reduces to almost negligible amount up to the observed range. The higher deposition rate under vibration might be due to the fact that mechanical and pressure wave of propagated sound towards the substrate enhances the mass transfer rate of depositing carbon species. The variation of the rate of increment of deposition at different frequency ranges might be due to the change of resultant vibration of carbon particles which depends on the wave length of the sound at different frequency and the particle size and the mass of the species. It is observed that deposition rate under sonic range of sound vibration does not vary significantly with frequency of vibration. But the rate of deposition under ultrasonic vibration increase significantly with frequency of vibration.
Figure
Effect of vibration on deposition rate with respect to distance between substrate and activation heater
Figure
Microstructure (under SEM) of deposited coating under vibration (left-side view) and without vibration (right-side view) condition at different resolutions.
The EDX analysis shows that the coating on the substrate has considerable amount of carbon particles under vibration (13%) and without vibration (11%) condition as shown in Figure
EDX analysis of coating on substrate with vibration at top and without vibration at bottom (
XRD analysis on specimens is shown in Figure
Comparison of the d-spacings of XRD spectrum of the deposited crystal with the actual d-spacings for graphite, diamond, and Fe (
Peak number |
2 |
Measured | Diamond | Graphite | Fe ( | ||||
( | ( | ( | |||||||
(1) | 19.5 | 2.0982 | 89 | 111 | 2.095 | ||||
(2) | 21.2 | 1.9317 | 64 | 105 | 1.930 | 103 | 1.9200 | ||
(3) | 22.5 | 1.8214 | 86 | 200 | 1.8214 | ||||
(4) | 23.3 | 1.7597 | 53 | 108 | 1.6650 | 104 | 1.7950 | ||
(5) | 26.0 | 1.5796 | 14 | 109 | 1.5800 | 106 | 1.5400 | ||
(6) | 32.6 | 1.2660 | 100 | 220 | 1.283 | ||||
(7) | 33.4 | 1.2365 | 14 | 101 | 1.2200 | 110 | 1.2280 |
Comparison of the
Peak number |
2 |
Measured | Diamond | Graphite | Fe ( | ||||
( | ( | ( | |||||||
(1) | 19.5 | 2.0982 | 92 | 111 | 2.095 | ||||
(2) | 21.2 | 1.9317 | 42 | 105 | 1.930 | 103 | 1.9200 | ||
(3) | 22.5 | 1.8214 | 100 | 200 | 1.8214 | ||||
(4) | 23.3 | 1.7597 | 83 | 108 | 1.6650 | 104 | 1.7950 | ||
(5) | 26.0 | 1.5796 | 17 | 109 | 1.5800 | 106 | 1.5400 | ||
(6) | 32.6 | 1.2660 | 75 | 220 | 1.283 | ||||
(7) | 33.4 | 1.2365 | 21 | 101 | 1.2200 | 110 | 1.2280 |
XRD analysis of coated surface without sound vibration at top, with sound vibration at middle, and without coating on stainless steel substrate at bottom (
For comparison of data of area under the significant peaks of Figure
Comparison of intensity among substrate without coating, coating without sound vibration and coating with sound vibration.
Serial number | |||||||
1 | 2 | 3 | 4 | 5 | 6 | 7 | |
Conditions | 19.5 | 21.2 | 22.5 | 23.3 | 26.0 | 32.6 | 33.4 |
Fe ( | Diamond/ gaphite | Fe ( | Graphite/ diamond | Diamond/ graphite | Fe ( | Graphite/ diamond | |
intensity (I) | |||||||
Substrate without coating | 38 | 16 | 13 | ||||
Substrate with coating without sound vibration | 30 | 12 | 17 | 8 | 3.5 | 11 | 4 |
Substrate with coating with sound vibration | 17 | 9 | 12 | 11 | 5 | 10 | 5 |
Possible causes of higher deposition rate, compactness and smoothness of the deposited coating under sound vibration condition can be explained as follows.
The complex chemical and physical processes, which occur during diamond CVD, comprise several different but interrelated features. The process gases of the chamber before diffusing toward the substrate surface pass through an activation region (a hot filament), which provide energy to the gaseous species. This activation causes molecules to fragment into reactive radicals and atoms, creates ions and electrons, and heats the gas up to temperatures approaching a few thousand Kelvin. Beyond the activation region, these reactive fragments continue to mix and undergo a complex set of chemical reactions until they strike the substrate surface. At this point, the species is adsorbed and entrapped within the surface, some portions are desorbed again back into the gas phase, or diffuse around close to the surface until an appropriate reaction site is found. If a surface reaction occurs, one possible outcome, if all the conditions are suitable, is diamond. During this process, the addition of sound vibration might increase the energy level of the depositing species. The increase of deposition rate and the surface quality of the deposited coating might be due to elimination or reduction of the potential barrier [ due to the sound media, particles vibrate back and forth and for equilibrium condition, some extra energy remains in the process [ at constant temperature, the amount of adsorption depends on pressure [ as movement of the particles increase, the concentration of diffusing carbon elements increases [ extra vibration of sound may increase the momentum difference of carbon and hydrogen due to their atomic mass difference in methane (CH4) molecule [
The results obtained under this study shows that sound vibration increases deposition rate with more compact and smoother surface finish. Similar study is conducted to observe the effects of ultrasonic vibrations on the localized electrochemical deposition (LECD) process by Yeo et al. [
The following can be concluded from this study: deposition rate increases significantly (about 18% higher) under sound vibration condition than that of no sound vibration condition; the deposition rate under sonic vibration increases slightly with the frequency of vibration; the deposition rate under ultrasonic vibration increases significantly with the frequency of vibration up to a certain value, and after that value, the deposition rate remains almost constant; for a particular frequency of vibration, the deposition rate decreases with the distance up to a certain value, and after that the deposition rate remains almost constant (up to observed distance); percentage of diamond/graphite in the deposited coating increases about 10% with the addition of sound vibration; the surface morphology under SEM analysis of the deposited coatings under sound vibration condition is observed as more compact and smoother surface finish than that of without vibration condition.
Therefore, by maintaining an appropriate level of frequency of vibration and the distance between substrate and activation heater deposition rate of Carbon (diamond/graphite) may be maintained to higher value.
Substrate heater temperature
Activation heater temperature
Distance between substrate and activation heater
Pressure of the reactor chamber.