In part 1, functionally gradient diamond-like carbon (FGDLC) films are fabricated using a novel pulsed laser deposition technique to enhance adhesion strength. Two beams are irradiated on graphite and 316L stainless steel targets, respectively, in a vacuum chamber, and the different plasmas produced are mixed in space before being deposited on a 316L stainless steel substrate. Using this method, we have built ~510 nm thick FGDLC films where the composition changes gradually from stainless to DLC in the deposition direction.
We confirmed that FGDLC films show much higher adhesion strength than normal DLC films. And also we found many properties for each material and sorted elements, which have similar properties except critical point. Keywords: Functional gradient film, Pulsed laser deposition, Diamond-like carbon (DLC), Picosecond laser, Adhesion strength.
INTRODUCTION
In this study, we produced conventional (pure) DLC films and FGDLC films and compared their adhesion properties. The result shows that FGDLC films have much better adhesion properties than conventional DLC films. To the best of the authors' knowledge, this is the first attempt to fabricate a functionally gradient DCL film, which is completely different from a multilayer DLC film with sharp interfaces.
Unlike previous multiplayer techniques, the presented method is capable of producing a DLC film with a continuously changing composition profile, which is highly effective in damping internal stresses at the interface of different materials. Moreover, this method is not limited to stainless steel-DLC functional gradient films and can be easily applied to other materials and applications.
EXPERIMENT
- Experimental Setup
- Targets and substrate
- Objective
- Plasma monitoring
- Analysis
- Deposition conditions
When the laser shutter is opened, the two suppressors operate simultaneously so that the laser power for the 316L stainless steel target decreases while the graphite target power increases linearly to build a functionally gradient DLC layer on which an additional pure DLC layer is deposited. In medicine, 316L stainless steel is commonly used for joint prostheses (e.g. shoulder, hip and knee) [8]. The 316L stainless steel blades are mechanically polished with 400, 800 and 1200 grit emery papers before being finally polished with a 0.05 μm alumina slurry.
By gradually changing the composition from stainless steel to DLC, we believe that the adhesion strength can be significantly improved and, therefore, much thicker films and coatings with better mechanical stability can be obtained. To make good quality films, two different plasma plumes from graphite and stainless steel targets must be mixed uniformly before being deposited on the substrate. The composition of the FGDLC film is studied using energy dispersive spectroscopy (EDS), and the adhesion strength is measured using a scratch tester (CSM Instruments RVT).
RESULTS AND DISCUSSION
- Deposition rates
- Plasma study
- Power design
- Thickness of FGDLC
- Composition profile
- Raman spectrum
- Surface morphology
- Buckling and delamination of pure DLC
- sp 3 fractions
- Scratch test for adhesion strength
In all cases, the carbon plasma appears much brighter and larger than that of stainless steel 316L. In fact, the stainless steel plasma becomes smaller as the process proceeds and almost disappears in Figure 1.10 (c) and (d). Therefore, we believe that the stainless steel plume is mostly neutral (especially far from the ablation point) and just looks much smaller than it really is.
Based on the obtained deposition rate curves for DLC and 316L stainless steel (Figure 1.8 and Figure 1.9), we designed temporal laser power profiles for both graphite and stainless steel targets (Figure 1.11) for producing a film FGDLC. In contrast, the laser power for stainless steel decreases from 0.85 W to 0 for 60 minutes and remains at 0 W while a clear DLC layer is deposited for 15 minutes. This pattern is very similar to the individual DLC and stainless steel deposition patterns given in Figure 1.8 and Figure 1.9, and we believe it is due to the effects of target ablation.
To investigate how the composition changes in the thickness direction, we performed an EDS analysis on the 316 L stainless steel target, the DLC film, and the FGDLC film after and 75 min of deposition, respectively (Figure 1.13). For comparison purposes, a normal DLC film with a thickness of 400 nm was produced separately using a laser power of 4 W for 60 minutes and its composition is given in Figure 1.13 (g). To understand the change in composition more clearly, Figure 1.14 shows the changes in atomic percentage of carbon and iron in the thickness direction starting from the stainless steel target (corresponding to Figure 1.13 (a)~(f)).
In Figure 1.14, the at% values at 0 nm are obtained for stainless steel 316L, and the two dashed horizontal lines are the at% values of carbon and iron for the normal DLC film, which are used as reference values. As clearly shown in Figure 1.14, as designed, the atomic percentage of carbon for FGDLC (black solid line) increases almost linearly up to 383 nm (45 minutes of deposition) and it saturates to 67.61 at%, which is very close to the value. for the normal DLC film (black dotted line). As shown in Figure 1.15, the Raman spectrum of stainless steel 316L shows a broad band centered around 500 cm-1.
In the Raman spectrum of the FGDLC film, there is a broad band at 1530 cm-1 as well as a weak disorder peak at 1350 cm-1 and a weak broad band centered around 500 cm-1, the latter of which is clearly from the stainless steel substrate. Judging from Figure 1.16, as expected, tetrahedral amorphous carbon (ta-C) is the main constituent of the FGDLC film, and there is also some stainless steel 316L, which seems to have diffused from the substrate. As shown in Figure 1.17, the surface quality in both cases is very good with no signs of spatter.
CONCLUSIONS
INTRODUCTION
EXPERIMENT
- Experimental Setup
- Collection of properties
- Filtering of gaseous and liquid elements at room temperature
- Conversion of units
- Selection of properties for grouping
- Rank order
- Grouping
Surface tension was (dynes/cm), and was converted to (N/m), solid and liquid density (g/ml) were converted to (kg m-3). Because we can expect different phenomena where different points of homogeneous boiling temperature occur. In particular, five properties are selected for comparison - boiling temperature ( ), laser absorption (A), latent heat of vaporization ( ), surface tension (γ), liquid diffusivity.
The boiling temperature is used as reference point to compare starting point of homogeneous boiling temperature depending on critical point. Latent Heat of Vaporization ( ) – Latent heat of vaporization is proportional to the amount of energy absorbed during vaporization. During mass removal by evaporation, latent heat of vaporization determines the amount of mass removed.
But actually when the laser interacts with the material, it will melt as the temperature goes beyond the melting temperature, even beyond the rapid boiling temperature. The reason why the liquid state is chosen is that evaporation occurs above the boiling temperature. Set each property and filter with respect to the boiling point of each material.
Initially, five groups are selected, but a group containing Dysprosium (Dy), Erbium (Er) is excluded as those elements do not have the difference between boiling temperature and critical temperature compared to other elements (Dy: 5721.5 K, Er: 6573.25 K ). For the boiling temperature they are within ± 2% of 4965 K, for the latent heat of vaporization they are within ± 8%. After filtering chromium and cobalt, deviation process is performed again, ± 8% deviation for reference boiling temperature (3051 K), ± 18% deviation for reference latent heat of vaporization (5749 kJ/kg), ± 21% deviation for reference surface tension (1.67 N/ m), and at 355nm they are inside.
Tantalum and Osmium are excluded due to their difficulty to purchase and lack of information on properties.
![Figure 2.1 Boiling temperature, melting temperature, and critical temperature [20].](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10529747.0/49.892.123.773.130.577/figure-boiling-temperature-melting-temperature-critical-temperature-20.webp)
RESULTS AND DISCUSSION
- Group 1: Titanium, Vanadium
- Group 2: Iron, Nickel and Copper
- Group 3: Niobium, Molybdenum
- Group 4: Tungsten, Rhenium
- Group 1: Titanium, Vanadium
- Group 2: Iron, Nickel and Copper
- Group 3: Niobium, Molybdenum
- Group 4: Tungsten, Rhenium
Among iron, nickel and copper, the difference between the critical point and the boiling point of copper (2288 K) is the smallest and copper has a completely different structure. The iron surface appears to be melted through each power, but the nickel begins to phase shift from its maximum absorbed power. But the two materials show similar morphology, but molybdenum has more bubbles on the lower surface.
Ablated area of tungsten is larger than rhenium and there are more bubbles with tungsten than rhenium. Latent heat of vaporization in this group is the lowest (tungsten: 4388.1 kJ/kg, rhenium: 3780.7 kJ/kg), therefore the amount of mass removed during ablation is greater than other groups. Two materials are similar in pattern, but titanium has more bubbles and fine particles at the bottom.
And the lowest power two materials show the difference, titanium shows the evidence of boiling while vanadium looks molten. Moreover, the debris size of titanium is smaller than that of vanadium and the average debris size decreases as the intensity increases. All surface morphologies of iron are melted, nickel is close to iron surface, but nickel shows more bubbles on the surface.
From this 532 nm wavelength, the pattern of each material is close to the results of 355 nm wavelength. Like 355 nm wavelength, molybdenum is close to melting, but niobium is close to boiling, although they have similar melting and boiling points. As seen in Figure 2.17 and Figure 2.18, melting is dominant for molybdenum, and molybdenum has a smoother surface than niobium.
At this 532 nm wavelength, tungsten and rhenium are less ablated than 355 nm wavelength, since the photon energy at 532 nm is lower than 355 nm and the spot size is slightly larger than 355 nm.
CONCLUSIONS
FUTURE WORK
Efeoglu, The effect of a TiC transient layer on a DLC-based functional gradient coating prepared by an unbalanced closed-field magnetron sputter plating system. Eason, R., Pulsed laser deposition of thin films: application-led growth of functional materials 2007, Hoboken, NJ: Wiley-Interscience. Bell, B., et al., Pulsed laser deposition of multilayer hydroxyapatite-diamond-like carbon films and their adhesion aspects.
Merel, P., et al., Direct evaluation of the sp(3) content in diamond-like carbon films by XPS. Tabbal, M., et al., Effect of laser intensity on the microstructural and mechanical properties of pulsed laser-deposited diamond-like carbon thin films. Yamamoto, K., et al., The fraction of sp(3) bonding in carbon thin film prepared using pulsed laser deposition.
Yaws, CL, Transport properties of chemicals and hydrocarbons: viscosity, thermal conductivity and diffusivity of C1 to C100 organics and Ac to Zr inorganics 2009, Norwich, NY: William Andrew. Yaws, CL, The Yaws Handbook of Thermodynamic Properties for Hydrocarbons and Chemicals, 2006, Houston, Texas: Gulf Pub. Hongrae Cho*, Hyungson Ki “Pulsed Laser Deposition of Functionally Gradient DLC Films”, Korean Society of Laser Processing, June Gyeongju, Korea.
Pulsed laser deposition of functional gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser. I would like to thank my family - father and mother, Seungje Cho, Sunhak Park, and cute sister, Suhyun Cho. Much gratitude goes to my colleagues from Laser Processing and Multiphysics Systems Laboratory, Jaehoon Kim, Yoojai Won, Sangwoo So, Chun Deng.
Thank you to all graduate students in School of Mechanical Engineering at the Ulsan National Institute of Science and Technology (UNIST).
