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Superhard Amorphous Carbon Coating

May 1, 2020.

 

       Hard coating has been used in many applications, for example, cutting and machining tools, molds, dies, hard disk and other wear-resistant applications. Hardness is an important properties of a material which determines its ability to resist wear when subjected to frictional forces. The coating hardness can be classified as [1]

 

       1. Hard coating: materials with hardness greater than 20 GPa,

 

       2. Superhard coating: materials with hardness above 40 GPa, and

 

       3. Ultrahard coating: materials with hardness above 80 GPa.

 

       Diamond-like carbon (DLC) films can be defined as amorphous carbon film consisting of the mixture carbon atoms form of sp3 bonds (diamond structure) and sp2 bond (graphite structure). The properties of amorphous carbon films depend on the ratio of sp3/sp2 bonds.

 

       Amorphous carbon films can be classified as

 

       1. Hydrogenated amorphous carbon (a-C:H): films consist of the sp3/sp2 ratio of 20 - 60 %, and

 

       2. Non-hydrogenated amorphous carbon: films consist of the sp3/sp2 ratio greater than 60 %. They are termed as tetrahedral amorphous carbon (ta-C).

 

       Amorphous carbon coatings can be hard, superhard or even ultrahard, depending on deposition methods (with differences of the ratio of sp3 and sp2 carbon bonds). Plasma deposition method can form both of a-C:H and ta-C films. Hydrogenated amorphous carbon usually is prepared by plasma enhanced chemical vapor deposition (PECVD) and the hardest films is tetrahedral amorphous carbon (ta-C) prepared by non-hydrogenated, highly ionized plasmas in filtered cathodic vacuum arc deposition (FCVA). J. Robertson [2] has proposed a ternary phase diagram of an amorphous carbon films and shown that the optimum ion energy for deposition of the highest sp3 content is about 100 eV (Figure 1).

 

 

Fig. 1 Variation of the sp3 fraction as a function of carbon ion energy [2].

 

       At the Technological Plasma Research Unit of Mahasarakham University, the hydrogenated amorphous carbon (a-C:H) films were deposited using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, as shown in Figure 2. The RF plasma was generated with the frequency of 13.56 MHz. The mixture gas of Ar+C2H2 was used as a processing gas. The results show that the color of a-C:H films changes with the deposition time that corresponds to the increase of film thickness and having hardness of 15.16 GPa.

 

 

Fig. 2 Schematic diagram of RF-PECVD system.

 

       The FCVA used to prepare superhard amorphous carbon (ta-C) in this project is supported by Thailand Center of Excellence in Physics (ThEP Center). In FCVA, the high density plasma produce at the cathode spot at cathode surface expands rapidly into the vacuum with high ion velocity (Figure 3). FCVA generates highly ionized material for depositing dense, adhesive carbon films. A carbon ion energy obtain from the FCVA with graphite cathode about 20 eV [3].

 

 

Fig. 3 Schematic diagram of the FCVA deposition system.

 

       Biasing technique are used to increase carbon ion energy for plasma deposition. There are two biasing techniques to increase ion energy during the plasma deposition of the ta-C films.

 

       1. Substrate biasing: the negative voltage is applied to substrate. This technique suitable for a conducting substrate or a conducting films.

 

       2. Plasma biasing: the positive voltage is applied to plasma source (anode). This technique suitable for a non-conducting substrate or an insulating films such as amorphous carbon films [4].

 

The development of PC control system for super hard coating

 

       The thin-film coating system consists of several devices and equipment to perform the low-vacuum deposition process. The main parts of the coating system include a vacuum chamber, vacuum pumps, pressure gauges, high vacuum gate valves, mass flow controllers, a chiller, and vapor sources as well as necessary power supplies. The devices and measurement equipment need to be operated and controlled in sequence. Therefore, the computer control approach has been employed to control, measure, and display the status of the related devices in the coating system.

 

 

Fig. 4 Diagram of the coating system developed at the Technological Plasma Research Unit, Mahasarakham University.

 

       The diagram of the coating system developed at the Technological Plasma Research Unit, Mahasarakham University, is shown in Figure 4. A stainless steel cylindrical shape vacuum chamber, with a diameter of 550 mm and a height of 360 mm, was fabricated at Synchrotron Light Research Institute at Nakhon Ratchasima province. The chamber (19) is designed to support both academic research and industrial collaboration. A base pressure of better than 5x10-6 Torr can be achieved using a turbo-molecular pump (2, TMH1001, Pfeiffer) backed by a rotary vane pump (1, Trivac D 40B, Leybold). In order to minimize the coating cycle time, the backing and roughing process can be performed with a control valve (3), a backing valve (4), and a roughing valve.

 

       A wide-range gauge can measure the vacuum pressure from 103 Torr down to 10-7 Torr, while a capacitance gauge is used to monitor the operating pressure in the range of 1 mTorr to 100 mTorr. In order to prolong the life-time, these gauges were isolated from the chamber using high vacuum valves (6, 8). An inert gas, as well as a reactive gas, is independently introduced to the chamber through a mass flow controller (10, 11). Argon is needed for the sputtering process, while N2, O2, and hydrocarbon gases are used in reactive magnetron sputtering for nitride, oxide and carbide coatings, respectively. A vapor source (16), located inside the coating chamber, can generate the vapor flux (17) to form a thin solid film on the substrate surface (18) using a suitable power supply (12).

 

       As mentioned above, the coating system is operated using computer control. The essential part is, therefore, a computer (13) specially designed for measurement and control tasks. A PXI computer from National Instrument is a suitable platform to control individual devices, measure essential parameters, and display the status of the coating system. 

 

 

Fig. 5 The box diagram presenting the PXI computer control concept.

 

       The box diagram presenting the PXI computer control concept is depicted in Figure 5. The PXI, as a computer, consists of hardware and software. Apart from the primary interface and communication ports, e.g., USB, RS232, and IGBT, several types of measurement and control modules can be installed in the PXI. The modules chosen for the coating system include a source meter unit (SMU) module, a relay module, an analog input/output module, and a digital multimeter module. These selected modules could directly control and measure the necessary parameters of the connected devices in the coating system. For example, the SMU card supplies a voltage or a current level to set the gas flow. The relay card can switch the gate valves on the “close” position or “open” position. The analog IO card can measure the voltage signal of the gauges to display the vacuum pressure or operating pressure.

 

       In order to fully control all devices and manipulate the measured signal, the executable program named Vacuum Control, as shown in Figure 6, was developed using LabView graphical programming.

 

 

Fig. 6 The user interface of Vacuum Control for the semi-auto operation of the coating system.

 

References

 

[1] S. Zhang, D. Sun, Y. Fu and H. Du, “Recent advances of superhard nanocomposite coatings: a review”, Surface and Coatings Technology 167 (2003) 113–119.

[2] J. Robertson, “Plasma deposition of diamond-like carbon”, Japanese Journal of Applied Physics 50 (2011) 01AF01.

[3] A. Anders and G. Y. Yushkov, “Ion flux from vacuum arc cathode spots in the absence and
presence of a magnetic field
”, J. Appl. Phys. 91 (2002) 4824.

[4] A. Anders, N. Pasaja, S. H. N. Lim, T. C. Petersen and V. J. Keast, “Plasma biasing to control the growth conditions of diamond-like carbon”, Surface & Coatings Technology 201 (2007) 4628–4632.

 

Reported by

 

Dr. Nitisak Pasaja, Assist. Prof. Dr. Artit Chingsungnoen and Assist. Prof. Dr. Phitsanu Poolcharuansin

The Technological Plasma Research Unit, Department of Physics, Faculty of Science, Mahasarakham University, Mahasarakham 44150, Thailand

Email: nitisak.p@msu.ac.th