1. Microstructure and Mechanical Properties of Hydrogenated WC-C Nanocomposite
S. J. Parka),b)
K.-R. Leea), D.-H. Kob), J. H. Hanc), K. Y. Eun a)
a) Thin Film Research Center, Korea Institute of Science and Technology
b) Department of Ceramic Engineering, Yonsei University
c) Korea Research Institute of Standard and Science
Thank you Mr. Chairman.
My name is Sejun Park from KIST, Korea.
I will talk about Microstructure and mechanical properties of Hydrogenated WC-C nanocomposite

2. Applications of DLC Film
Diamond-like carbon films has high hardness, low friction and low wear. It has been successfully used for protection of materials and has enhanced the life of machinery. So it has many application with its good properties such as head-drum of VCR, heads and disks of hard disk, video tapes, artificial joint, shaving blade, forming die, and spacer tool for electron gun assembly of CRT

3. Disadvantages of DLC Film
Thermal instability
degradation beyond 500°C
High residual compressive stress
Max 10 GPa
Poor adhesion
Steel, Oxide, Sulfide, etc.
But DLC film has some disadvantages. Firstly, DLC film has thermal instalbility. Beyond 500℃, its properties are rapidly degraded. Secondly DLC film has high residual stress of maximum 10 Gpa. High residual stress of DLC film cause weak film-support bonding with substrate. And it has poor adhesion with steel, oxide, sulfide substrate. These disadvantages limit the applications of pure DLC films.

4. Trends of Hard Coatings
Functionally
gradient coatings
Nano-multilayer
films
Single layer films
Diamond, DLC, CNx,
Nitride, Carbide etc.
A.A.Voevodin et al.,
Tribology International 29 (1996) 559
Compound single
layer films
Nanocomposites
Recently, research of hard and tribological coating has two trend, to overcome the limitation of single layer films such as diamond, DLC, carbides, nitrides. One is the research of nano-multilayer films, which are obtained by functionally gradient coating or by hard superlattice coating. The other is the research of nanocomposite films, which are composed of crystalline and amorphous materials.
S. Veprek et al., Surface Coat. Technol.
(2000) 152

5. How to Overcome the Disadvantages of DLC Film
Transition metal carbide + DLC
TiC and WC nanograins embeded in DLC matrix
Nanocrystalline/DLC nanocomposite
The desirable combination of hardness, toughness, low friction and wear in ambient environment
To overcome the disadvantages of DLC films without losing its favorable properties, there has been the attempt to combine transition metal carbide and DLC. And TiC and WC nanograins have been successfully embedded in DLC matrix.
Voevodin reported nanocomposite of nanocrystalline and amorphous carbon films has the desirable combination of hardness, toughness, low friction and wear in ambient environment. But the structure and properties of nanocomposite are not well known.
A. A. Voevodin et al. Thin Solid Films, 342 (1999) 194

6. Purpose of the Present Work
The synthesis of WC-C nanocomposite thin film using a hybrid DC magnetron sputtering with RF PACVD system
Relationship between the mechanical properties and the microstructure
So, purpose of the present work is to synthesize the WC-C nanocomposite thin films using a hybrid DC magnetron sputtering with rf PACVD system and to find the relationship between mechanical properties and microstructures of WC-C nanocomposite.

7. Experimental Conditions
Roughing Pump
TMP
Matching Box RF
13.56MHz
Gas
Hybrid DC magnetron sputtering
CH4+ Ar (CH4/(Ar+CH4 ) : 0.33 – 0.58)
Deposition pressure : 1.33 Pa
Cathode target current : 300 mA
Substrate bias : -150 Vb RF biasing
Substrate : P-type (100) Si wafer
100 ㎛ Si wafer
for stress measurement W
Target
Substrate
We synthesized WC-C nanocomposite films using hybrid DC magnetron sputtering system. Because this sputtering method can easily be scaled up for industrial use. The left figure is schematic diagram of our deposition system. Sputtering gas is the mixture of Ar and CH4, and we control the ratio of CH4 in gas mixture from 0.33 to 0.58 to synthesize films with various W concentration. Working pressure of chamber is 1.33 Pa. Current of sputtering target is 300 mA. Substrate bias is MHz r.f bias of – 150V. The substrates of all the samples are P-type (100) Si-wafer. And we use 100 micro-m thickness wafer for residual stress measurement. We controlled the thickness of films as 300 nm.
W concentration was measured by RBS method. Hardness, residual stress, and resistivity of film are measured for property characterization, and Raman, XRD and TEM analysis were done for analysis of microstructure.

8. W Concentration in the Film
This figure is the results of W concentration for various Ch4 fraction in sputtering gas by RBS measurement. We could make WC-C films whose W concentration is from 5.2 at.% to 42 at.%. As CH4 ratio in gas mixture increases, W concentration in the film linearly decreases. Below 5.2 at.% of W concentration, we couldn’t make uniform WC-C films because of W target poisoning

9. Residual Stress of the Film
W Sputtering at –150Vb
W Sputtering of Poisoned Target with Carbon
PA-CVD with CH4
Now you will see the mechanical properties of WC-C nanocomposite films.
These results are the changes in residual stress of WC-C thin film for various W concentration. The DLC films deposited by PA-CVD with CH4 has very high stress, 2.4 Gpa, but the stress of DLC films deposited by sputtering of poisoned target with carbon was very low value, 0.4 Gpa.
As shown in this figure, by addition of W, the stress of WC-C has much lower value of about 0.5 Gpa than that of DLC films made by PA-CVD and shows almost no change up to 13 at.%. But the stress increases linearly with the increase of W concentration when W concentration is more than 13 at.%. At 42 at.% of W concentration, Stress of WC-C film is 2.75 Gpa.

10. Hardness of the Film
W Sputtering at –150Vb
PA-CVD with CH4
W Sputtering of Poisoned Target with Carbon
This figure is the results of hardness measured by nano-indentation method
The hardness of DLC film deposited by PA-CVD is 17 Gpa. And the DLC films deposited by sputtering of poisoned W target has low value of hardness, 10 Gpa. The hardness of WC is 22.3 Gpa in literature. But the WC-C nanocomposite film with 6.9 at% of W concentration has minimum value of hardness, 8 Gpa. The hardness of films increases with the increase of W concentration in films. Hardness and stress value of WC-C are similar to those of DLC films by sputtering of poisoned target rather than those of DLC films by PA-CVD. The results of hardness and stress of WC-C shows mechanical properties are not proportional to W concentration. Thus the decrease of hardness at the initial stage of W concentration may be more closely related to the structural change of WC-C film than W concentration.

11. XRD Spectrum of the Film
So we observed microstructure of WC-C to investigate the relationship between structure and mechanical properties of WC-C films.
Firstly We observed XRD
This figure is result of XRD. This figure shows only the peak of cubic WC at 37 degree, and the intensity of peak increases as W concentration increases. The peak of cubic WC is very broad. This broadness of the peak means that WC crystalline is almost nm-sized. So this shows that nano nm-sized WC crystallines in the film are formed by W addition and these WC crystallines exist in amorphous carbon matrix and no other phase is formed.

12. Diffraction Pattern of the Film
(200)
(220)
(111)
(311)
These micrographs are selected area diffraction pattern of WC-C films. The ring patterns of graph are indexed as Cubic WC whose lattice parameter is nm. Ring pattern of Cubic WC is more sharp and show some speckles as W concentraion increases. This informs that the more WC crystalline exist in films with increase of W contents. This results agrees well with the result of XRD.
6. 9 at.%
14.5 at.%
26 at.%

13. TEM Micrograph of the Film
W : 6.9 at.%
This micrograph is plan-view dark field image of WC-C thin film with 6.9 at.% W concentration. Bright spheres in micrograph are WC crystalline. The size of WC crystalline is 3 to 6 nm. As shown in this micrograph, WC crystallines are uniformly distributed in amorphous carbon matrix and they are respectively isolated.
40 nm
Diffraction Pattern
Plan-View Image

14. Resistivity of the Film
This figure is results of resistivity of WC-C nanocomposite measured by 4-point probe. As the W concentration increases, resistivity of films decrease. The resistivity of WC are the order of 10-5 ohm-cm, which is much lower than that of pure DLC film. So the resistivity of WC-C decrease with the increase of W due to formation of WC. And there is a difference in slope of decrease of resistivity.
In results of resistivity, we can find very interesting phenomenon. There’s an abrupt drop of resistivity between 13 at.% and 15 at.%. It means that there is the abrupt increase of electrical conduction path in that range of W concentration. This increase of electrical conduction paths can be an evidence that the spherical WC grains are contacted and become worm-like shaped.

15. G-Peak Position of the Film
PA-CVD with CH4
W Sputtering of Poisoned Target with Carbon
W Sputtering at –150Vb
For analysis of the DLC matrix, we measured Raman spectroscopy. Raman spectra reflect only the bonding structure of DLC matrix because Cubic-WC is inactive to Raman excitation. This figure shows the G-peak position after deconvolution of Raman spectra by Gaussian fitting method. As W concentration increases, G-peak position shifted to higher wavenumber. And the shift of G-peak position is saturated around 10 at.% of W concentration. Raman peak cannot be deconvoluted when W contents are more than at.%, because of very weak intensity of peak. This shift of G-peak position means that the DLC matrix of WC-C nanocomposite becomes more graphitic with increasing W concentration from 0 to about 10 at.%. The reason for graphitization of the DLC matrix seems to be the increase of momentum transfer of heavy W atoms.
The graphitization of DLC matrix explains that the slope of decrease of resistivity in range of 6.9 to 10 at.% is steeper than beyond 10 at.% in the result of resistivity.

16. Structural Change with W Concentration
DLC Matrix
c-WC Crystalline
Now I’d like to summarize the structural changes of WC-C nanocomposite shown before.
Below 13 at.% of W concentration, 3-6 nm sized spherical WC crystalline are uniformly distributed in DLC matrix. But, the particles are isolated because of small content. In this concentration range, DLC matrix becomes more graphitic.
In the range of 13 to 15 at.%, WC crystallines started to contact with each other because of the increase of WC contents. In this concentration range, no more graphitization of the DLC matrix occurs.
Beyond 15 at.% of W, more WC nanocrystals are contacted to generate work-like shaped WC grain. The mechanical properties of the film can be understood in terms of this structural change.
W : Below 13 at.%
W : at.%
W : Beyond 15 at.%

17. Relationship between Structure and Mechanical Properties
This drawings is hardness results and schematic diagrams of WC-C film with W concentration to show relationship between mechanical properties and micro-structure.
In this range, even if WC is much harder than that of DLC films deposited in this work, WC-C mechanical properties are determined by mechanical properties of DLC matrix because WC nanocrystals are isolated. Hardness of the film decreases in this concentration range, because the DLC matrix is graphitized with W incorporation.
And in the range of 13 to 15 at.% of W concentration, WC nanocrystals are contacted and form channels. So mechanical properties of WC-C film are affected by WC crystalline, and hardness begins to increase.
And over the 15 at.%, physical contacts are proportional to W concentration, so hardness increases with W concentration.

18. Conclusions
Nanocomposite of WC phase and DLC matrix was produced by the addition of W into DLC films. The content of WC phase was proportional to the W concentration in the film.
The mechanical properties of WC-C nanocomposite were closely related to the physical contact of WC nanocrystals.
Below 13 at.% of W, mechanical properties of WC-C film were determined by DLC matrix because WC nanocrystals were isolated.
Beyond 13 at.% of W, WC nanocrystals began to contact. Hence, the mechanical properties of WC nanocrystals affected the mechanical properties of the nanocomposite film. Because the physical contact increased with increasing WC content, the hardness of the film increased.
Conclusions.

19. Friction Coefficient of the Film
This is the friction coefficient of WC-C films