Pulsed Laser Deposition of Functionally Graded Diamond-Like Carbon Films with

In the first part, FGDLC films are deposited by pulsed laser deposition (PLD) technique with two different target materials, 316L stainless steel and graphite. With DLC and stainless steel film deposition data, power profiles are obtained from given target content profiles. In this thesis, we compare the adhesion strengths of FGDLC films with the relationship between crack radius and indentation loads.

In the last section, the sp3 content, residual stresses and adhesion characteristics of DLC films deposited with different deposition parameters are examined. The optimum point for higher sp3 content of DLC is found at higher laser power and lower substrate temperature. After deposition, (a) bending pattern or (b) delamination can be observed on the surface of DLC films.

Schematic diagram of an FGDLC film showing content profiles (a) based on the number of particles and (b) based on the film thickness of each material. Designed content profiles are shown in solid lines and actual content profiles are shown in data points.

Introduction

Pulsed Laser Deposition (PLD)

Diamond-Like Carbon (DLC)

Pulsed Laser Deposition (PLD) of Functionally Graded Diamond-like Carbon (FGDLC)

Objectives

Experimental Setup

Substrate and Target Material
Experimental Setup
Characterization of DLC

In this thesis, Talisker 355-4 (Coherent, USA) was used as an energy source to remove target materials. Repetition rate of the laser is 200 kHz and we choose a 355 nm laser beam for deposition of DLC films. Both target materials rotate at a speed of 80 rpm to avoid constant drilling of material.

Also, the substrate holder rotates at a speed of 30 rpm for uniform deposition of film on the substrate. Plasma plumes ablated from both targets are observed with an intensified charge-coupled device (ICCD) camera as shown in Figure 6. The gradient layer thickness is set to 450 nm, and on the gradient layer, an additional 150 nm pure DLC layer is deposited—FGDLC film total thickness is set to 600 nm regardless of the content profile.

This EDS system is attached to a scanning electron microscope (SEM, S-4800 Cold Field Emission SEM, Hitachi, Japan.) Since we want to measure the actual content profile along the thickness of the deposited films, EDS arrays are taken from films as shown in figure 7; unless films are delaminated from their substrates, EDSs are measured at 16 equally spaced points on the side of a film. The thickness of a deposited film is measured by alpha-step surface profiler (KLA Tencor, USA) by averaging 12 measurements per FGDLC film.

Figure 4.. Schematic of PLD system for deposition of FGDLC films. Plasma plumes from target materials are observed by an ICCD camera.

Design of Content Profiles and Power Curves

Deposition of Functionally Graded Films
Deposition Data
Content Profiles and Power Curves

For designed content profiles, force profiles required for deposition of FGDLC films can be obtained. From calculated effective film thickness, the deposition rate of each material at a given film thickness can be found. There are four known values of precipitation rate for stainless steel and five known values of precipitation rate for DLC at a certain thickness of each material.

Since there is some information about content function with respect to the total thickness of FGDLC film, d. In this section, the goal is to design the content profile within the gradient layer of FGDLC film. As mentioned before, the total target thickness of the film is 600 nm, which has 450 nm gradient layer and 150 nm pure DLC layer.

Before designing content profiles, deposition curves are obtained for target materials, 316L stainless steel and graphite, in the same conditions of pressure and temperature: 10-. During deposition procedure, we find that a DLC film starts to deposit with a power greater than 0.29 W and stainless steel film has a threshold for deposition than 0.2 W. As shown in the deposition data, the thickness of DLC and stainless takes steel films then slightly linear at the short time of deposition time, then films start to have smaller deposition rate as deposition time increases.

Using Origin, both deposition data are entered into the Nelder model - the Nelder model has the inverse quadratic relationship between deposition time and thickness of film. Films with polynomial content profile are designed to have a total thickness of 600 nm, which has 450 nm gradient layer and 150 nm pure DLC layer. Since their molar volumes (cm mol3/ ) of DLC and stainless steel film are different from each other, they should be determined first.

These values are found experimentally and are measured to be 6.17 for DLC films and 6.57 cm mol3/ for stainless steel films. Let us say that c%DLC( )x and c%SUS( )x are defined as content function of DLC and stainless steel based on the number of each particle. They are not based on chemical elements - carbon and iron - but film materials, DLC and stainless steel themselves.

Figure 8. Power curve and atomic percentage results from previous study [5].

Results and Analysis

Thickness
Content Profiles

For all six profiles, they are all in good agreement with target content profiles. Deposit errors are calculated based on the difference between target content function and actual results. Therefore, from their deposition errors, it is concluded that the measured content profiles match well with pre-designed content profiles.

Table 4. Calculated deposition errors of FGDLC films from initially designed content profiles

Conclusion

Properties and Adhesion Behaviors of Functionally Graded Diamond-like (FGDLC) Films

Objectives
Preparation of FGDLC Films
Results and Analysis

Residual Stress
Adhesion Strength
Effective Hardness

Conclusion

Using the same method as in section I, EDS technique is also used to measure the content of deposited FGDLC films. Also, for each sample, film thickness is measured at 12 different positions using the scanning electron microscope (SEM,) and thicknesses of FGDLC films range from 587 to 591 nm, which is close to the target thickness, 600 nm. In general, the poor adhesion of DLC films is known due to the high compressive residual stress after their deposition, regardless of the deposition technique.

In this section, we want to study how the gradient layer affects the residual stress of FGDLC and DLC films. An alpha-step surface profiler (KLA Tencor, USA) is used to obtain surface profiles of samples before and after deposition of DLC or FGDLC films. In Figure 17, calculated compressive residual stresses are plotted against the content profile of FGDLC and two types of DLC films.

Comparing the residual stress in FGDLC films with that of pure 600 nm DLC, which has the same thickness as 600 nm, the residual stress mainly decreases as the content of stainless steel increases in the gradient layer of FGDLC films. Due to the thinness of the samples (~600 nm), the relatively small size of the substrate, and the brittleness of the DLC, indentation is the best choice to test the adhesion of FGDLC films. Adhesion strengths of deposited FGDLC films are compared with interfacial cracks induced by Rockwell indentation—the interfacial crack is induced by indentation as (a) in Figure 18 .

When calculating dP/dc to test the adhesion strength of DLC and FGDLC films, two methods are used. The effective hardness of DLC and FGDLC films is measured using the nano-indentation technique with Nano Indenter XP (MTS, USA.) The maximum indentation depth is set as 50 nm — the maximum indentation depth for hardness should be less than 10% of the thickness total of a thin film to exclude the effect of substrate deformation from indentation [28]. As shown in the figure, the measured effective hardness values of the FGDLC films range from 17.0 GPa to 59.8 GPa.

In this section, the ultimate goal is to improve the adhesion strength of DLC film by introducing gradient layer into the film. Also, the effect of content profile on compressive residual stress and effective hardness is investigated. Through the successful production of FGDLC films, adhesion strength of DLC films is greatly improved and their residual stress is greatly reduced.

Figure 14. SEM images of (a) the side of FGDLC films and (b) surface of film.

Properties of Diamond-like Carbon (DLC) films Deposited by Pulsed Laser Deposition

Objectives

Preparation of DLC Films

Results and Analysis

sp3 Content
Residual Stress
Adhesion Strength

For each experimental parameter set, three DLC films are deposited and for each film the sp3 content is measured three times. From nine measurements, the two largest and two smallest values are excluded, so five data points per each deposition condition are used to observe the behavior of the sp3 content along deposition parameters. It is clear from the graph that the highest sp3 content value is placed in the upper left corner, which has the highest laser power and the lowest substrate temperature.

To test the significance of the factors—laser power and substrate temperature—an analysis of variance (ANOVA) is used to analyze the sp3 content results. For ANOVA, the significance level is set at 0.05; if the P-value of a particular experimental factor is less than the significance level, it is considered a significant factor on the response, which in this section is the sp3 content. Strictly speaking, since the P-value from the substrate temperature is much smaller than that from the laser power, it means that the substrate temperature affects the experimental factor more than the laser power on the sp3 content of DLC films.

As shown in Figure 25, compared with sp3 content of DLC films shown in Figure 24, the residual stress of DLC films is the largest when it has the largest sp3 content. There may be more significant factors on this behavior of residual stress of a DLC film, such as deposition mechanisms of the film; however, in this section it can only be compared with the behavior of sp3 content of the film. To obtain the highest value of sp3 content of DLC films deposited by PLD technique, higher laser power and lower substrate temperature are required experimental conditions.

Because only a 355 nm beam is used in this thesis, the wavelength effect on the sp3 content is currently unknown. The compressive residual stresses behave similarly with the sp3 content, but the adhesion properties behave oppositely with the sp3 content. When the sp3 content is high, the bond strength is weak and the compressive residual stress is large.

From the results, higher sp3 content causes greater compressive residual stress within DLC films and this will be one of the main reasons for bad adhesion of DLC films. Ki, “Pulsed laser deposition of functional gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser,” Acta Materialia , vol. Kapat, “Characterization of pulsed laser-deposited diamond-like carbon films,” Surface & Coatings Technology, vol.

Hong, “Interfacial studies for improving the adhesion of diamond-like carbon films on steel,” Applied Surface Science , vol. Leu, “Reduced roughness and improved mechanical properties of multilayer diamond-like carbon films by periodic arc deposition,” Journal of the Electrochemical Society , vol.