ICMCTF2001 Session B1-3: Arc Technologies

¹I. I. Aksenov, V. A. Belous, V. G. Padalka, V. M. Khoroshikh, Sov. J. Plasma Phys. 4(4) (1978) 425.

Novel ceramics films has been tried to synthesize by the arc ion plating method by changing raw materials, so called targets. In this method, atoms or clusters evaporated from surface of targets by arc-discharge were ionized, reacted with process gases and reach substrates as films. However, effects of microstructures of targets on properties of synthesized films have not been fully understood so far. Keeping this fact in mind, we sintered Ti targets with different microstructures to synthesize TiN films and investigate the effects of microstructures of targets.

Ti powders with different particle size of ~ 40, ~ 80 and ~120 µm were hot-pressed with 15 MPa in vacuum at 1100 °C, respectively. Three Ti targets with density of all ~ 4.5 g/cm³ were arc-discharged with 100 A under nitrogen plasma circumstance at 3.3 Pa. The TiN films were deposited on cemented carbide and Si substrates which were biased at -20 V and ~ 500 °C. Both Ti targets and TiN films were analyzed on particle size, hardness and composition by optical microscopy, scanning electron microscopy, micro-Vickers hardness test and X-ray diffraction method. Further, microstructures of TiN films were observed in detail through transmission electron microscopy.

A filtered vacuum arc plasma source with an adjustable cathode-anode gap was used to produce a carbon plasma for deposition of coatings on various substrates. The deposition apparatus consisted of a plasma gun, a toroidal plasma duct, a deposition chamber, and a cooled substrate holder. The plasma gun consisted of a cylindrical graphite cathode, an annular graphite anode, and a mechanism providing axial movement of the cathode to the anode. The arc was ignited in vacuum of 3-8 mPa by momentarily contacting the cathode with the anode, while applying a d.c. current of 50-250 A between the cathode and the anode, and then withdrawing the cathode away from the anode in the axial direction, forming a cathode-anode gap of 2-18 mm. A carbon plasma jet passed through the anode into the toroidal duct and then to the substrate. A d.c. focusing magnetic field was applied in the vicinity of the cathode-anode gap, and a d.c. guiding field was generally parallel to the duct axis, while an a.c. magnetic field was applied to sweep the beam across the substrate. The substrates were stainless steel and polycarbonate coupons, and silicon wafers.

The adhesion and the structure of the coatings deposited on stainless steel substrates depended on the negative bias voltage (V_b) applied to the substrate relative to the grounded anode. With V_b=0, the coatings were not adherent, at V_b=-10 V the coatings were porous, but the pore density decreased with increasing negative V_b. At V_b=-15-30 V the adhesion of the coating was good, and dense, hard (HV~50 GPa) coatings were formed. At V_b=-15-30 V, the coating deposition rate measured for an arc current of 100 A, and an inter-electrode gap length of 12 mm was 100-250 nm/min, depending on the guiding magnetic field strength. Coatings deposited on polycarbonate surfaces were adherent without applying bias. However, the substrate surface was damaged after a deposition duration which depended on the magnetic field strengths. Coating microstructure, scratch adhesion, microhardness, electrical resistivity and transparency will be presented and discussed.

Higher substrate temperatures (>400°C ) are typically required to prepare a clean substrate surface prior to physical vapor deposition (PVD) processes, which is essential to achieve good film adhesion. Nevertheless, heating implies at least two major drawbacks: Firstly, heating of the substrates means additional processing time, which can take up to 50% of the entire coating process, subsequently resulting in higher costs. Secondly, in many cases such high temperatures limit the application of PVD processes. For example, previously heat-treated parts and components should not experience a temperature rise in a coating process exceeding the prior heat-treatment temperature to avoid critical material changes that could lead to unexpected failures. Temperature limits of 180°C are common. However, to deposit thin films at substrate temperatures below 180°C is a challenge to overcome utilizing cathodic arc physical vapor deposition (PVD). A typical process introduces heat to the substrates during several phases: infrared radiation substrate heating, in-situ glow-discharge cleaning, and the actual deposition process.

The paper analyzes each one of these steps regarding the introduced substrate heat loads and derives conclusions to obtain better temperature control by modifying parameters such as residual gas pressure, processing step time, substrate bias voltage, and arc current. The results were then applied to develop a low temperature deposition process on sample parts, achieving good film properties in terms of adhesion and wear performance.