ICMCTF2012 Session G5-1: Coatings, Pre-Treatment, Post-Treatment and Duplex Technology

The gaseous plasma of low pressure arc discharge has been used extensively for various surface treatment applications including heat treatment, ionitriding, ion implantation, PECVD and duplex processes. A highly ionized low pressure arc plasma with electron density up to ~10¹²cm^-3 can be generated by a shielded vacuum arc cathode, a hollow cathode or by a thermionic cathode. In this paper, the plasma properties are characterized by electrostatic probes and optical emission spectroscopy. A range of a different species can be produced in low pressure arc plasma immersion processes via decomposition of precursor molecules by electron collisions. Surface treatment of different steels and metal alloys in such a dense plasma environment can substantially affect the surface profile. The ionitriding of different types of steel in low pressure arc plasma environment is investigated. The rate of ionitriding as a function of plasma parameters, such as ion current density, pressure and gas composition are established for several types of steel and reach up to 0.5um/min for HSS in a pure nitrogen arc plasma with electron densities ranging from 10¹⁰ to 10¹² cm^-3. The ionitriding layers can be produced in arc plasma immersion processes at substrate bias as low as 30 volts and substrate temperature as low as 200 deg C, depending on type of steel. Alternatively, ion implantation of nitrogen can be produced at bias voltage exceeding 500 volts at substrate temperatures less than 100 deg C. The distribution of plasma density and uniformity of ionitriding layers in industrial scale vacuum processing chambers are investigated. The duplex coatings were also produced by ionitriding in an arc plasma immersion environment followed by TiN coatings. The ionitriding and duplex coating layers are characterized by structure, thickness, microhardness depth profile, surface roughness and coating adhesion. Surface treatment in conventional glow discharge compared to low pressure arc plasma immersion processes is presented. The results of processing complex shape components are discussed.

Among all the engineering compounds, titanium has been becoming distinct and cost-competitive in many applications by reason of its exceptional strength-to-weight ratio, high toughness, and excellent biocompatibility. However, titanium and its alloys are having many difficulties in tribological functions in consequence of their poor friction and wear resistance. Boriding seems to be a perfect candidate surface hardening process considering the positive contributions on the working efficiency and improvements in service life. In this study, the electrochemical boriding was applied to titanium in a systematic manner and the effects of boriding process parameters on the morphological and chemical state of boride layers were explored. Moreover, kinetic approach was conducted for the boriding of titanium in order to have growth equation of TiB₂ layer and to determine a practical diagram for potential applications. The presence of both TiB and TiB₂ phases were confirmed by the X-ray diffraction method. The cross-sectional examination of borided titanium verified the boride layers consisted of a homogeneously TiB₂ phase on the top and TiB whiskers toward the substrate. The morphology of TiB phase was found to have a strong dependence on boriding temperature and it varied with process temperatures in following order: whiskers structure at around the beta transus temperature (≈1173 K), dendritic branches at the temperature ranging from 1273 K to 1373 K, lateral islands at the temperature of 1473 K and above. Methodical studies over a wide range of boriding time (5 min≤ t ≤ 120 min) and temperature (1173 K ≤ T ≤ 1373 K) confirmed that the rate of the TiB₂ layer formation has a parabolic character. The activation energy (Q) and the pre-exponential factor (K_O) of TiB₂ layer were determined as 189.9kJ/mol and 4.66x10^-7m²s^-1, respectively. The specific empirical equations that can be used to estimate the thickness of the TiB₂ layers (d_TiB2) was obtained:

d_TiB2= 682.67√(exp(-22833/T)t) 1173 K ≤ T ≤ 1373 K; 200 mA/cm²

TFTs (Thin film transistor) are important for electronic devices such as display and other electronic items. The TFT performance strongly depends on the interface properties and the impurities of gate insulator films. Low temperature and inexpensive technology is required for the formation of gate insulator using silicon oxide films.

The generated O from ozone gas attacks the chains of silicone oil and precursor is formed by displacing Si-CH₃bond into Si-OH bond. Then the silicon oxide film is formed by the dehydration reaction of Si-OH bond. However, remaining Si-OH bond and impurities in the film is observed in the samples formed at low temperature.

In our previous work, we have reported an original method to reduce Si–OH bond from the silicon oxide films by dipping in ethanol before the post annealing in methanol gas. As a result, the dielectric property was improved by removal of Si-OH bond.

In this study, to modify the quality of silicon oxide formed at low temperature, UV light was irradiated to the sample before the organic gas assisted annealing.

Dimethyl-silicone-oil ([SiO(CH₃)₂]_n[Si(CH₃)₃]₂), was coated on p-type Si (100) substrates by a spin coater. This sample was set on the hot plate at 250^oC and 8% ozone gas was introduced on the sample at 15 min. The film thickness was 150 nm.

In order to modify the quality of silicon oxide thin film, the sample was irradiated UV (254 nm, 60W) for 2 hr (UV annealing) and was dipped in ethanol at room temperature for 15 min before annealed on the hot plate in methanol gas at 250 ^oC for 15 min (organic gas assisted annealing).

The molecular structure of the silicon oxide was measured by Fourier transform infrared spectroscopy (FT-IR). The metal–oxide–semiconductor (MOS) structures were fabricated depositing Aluminum. Then, the current density–electric field (J–E) characteristics were measured.

The sample with UV and organic gas assisted annealings was evaluated by FT-IR. A peak of Si-O cage structure was decrease with the annealings. The Si-O cage structure (incomplete structure) was changed to the Si-O stretch structure by irradiating UV and annealing in organic gas. The decrease of the incomplete structure is good for the improvement of electrical properties.

The dielectric property of organic gas assisted annealing sample was poor, and the leakage current was more than 10^-4 A/cm² even at 5 MV / cm. On the other hand, the leakage current of the UV and organic gas assisted annealings sample was 10^-6 A / cm² at 5 MV / cm. The dielectric property was improved by reduction of Si-O cage structure. The J-E characteristic was considerably improved by the UV annealing before organic gas assisted annealing.

Polypropylene (PP) materials are widely used as food packaging due to their lightness, low cost, and optical transparency. However, most of the polymer materials have low gas barrier property which may cause a great damage on the quality of food products. Thus the improvement of the low gas barrier property is widely desired.

Recently, thin solid coatings by diamond-like carbon (DLC) and silicon oxide (SiOx) using plasma have become prominent methods. Several researches have been reported on the gas barrier properties of polymers coated with DLC, eventually improving the gas barrier properties of certain types of polymers such as polyethylene terephthalate (PET) used for bottles. However, DLC-coated PP could hardly show high gas barrier property. In order to solve this problem, surface modification prior to the DLC deposition was introduced.

For the surface modification, silane coupling treatment was investigated. A silane coupling agent is a common adhesion promoter which is widely used in polymeric composites with glass or silicate substrates. In addition, the silane coupling agent possesses high optical transparency, and it can easily spread over polymer films while coating since it is in a liquid state. Furthermore, the silane coupling agent can solidify itself through hydrolysis and condensation processes which can be an easy and fast way to modify the surface of PP.

According to the results of the gas permeation test, it was found that the polymers modified with a variety of silane coupling agents prior to DLC coating showed high gas barrier properties. Especially the silane coupling agents with an amino group showed very low oxygen transmission rate, which was comparable to that of DLC-coated PET with extremely high gas barrier property. The barrier improved factor (BIF) became approximately 300 times higher than that of the untreated PP. This method can be applicable to several types of polymers including polyethylene (PE) .

In this talk, the motivation and principles behind “Cathodic Arc Plasma Treatment” (CAPT), a method involving application of high and low bias voltages to the substrate in a cyclic-pulsing manner during cathodic arc evaporation process will be introduced. This method differs from other relevant techniques since it does not aim to modify coating structure or surfaces with ion bombardment or ion implantation. It aims to benefit from the diffusion enhancing atomic - bulk heating effects induced through substrate - ion collisions during high bias voltage applications for alloying the substrate material with the condensing cathode material. The role of substrate temperature, magnitude, duration and type of high bias voltage and cathode current on surface alloying and intermetallic formation will be discussed based on studies conducted on binary Al-Cu¹ Cu-Al^{2, 3}, and Co-Cr⁴ and Al-Fe-Cu¹ ternary systems.

References:

1. Corlu, B., Ürgen, M. (2009). Surf. Coat. Tech., 204(6-7), 872-877
2. Corlu, B., Urgen, M. (2010) Surf. Coat. Tech. 205(2), 540-544.
3. Arpat E., Urgen M., Intermetallics 19 (2011) 1817-1822

4. Akkaya, S.S, Sireli, E., Alkan B., Urgen M., Surf. Coat. Tech., doi:10.1016/j.surfcoat.2011.09.046

Inconel alloys are very common nickel based super-alloys which are used extensively for a variety of high-temperature and aggressive environment applications such as metal seat ball valves, high temperature fasteners, nuclear reactors etc. They are extremely resistant to high temperature corrosion and boriding them would further enhance their corrosion resistance and improve their tribological properties. In this study, we performed electrochemical molten salt boriding which was carried out in a borax bath at 950°C for just 15 minutes to produce a very homogeneous, boride layer of 81µm in thickness. Different boride phases were identified by X-ray diffraction and the morphology of the borided surfaces was characterized by both optical and scanning electron microscopy (SEM). Microhardness measurements for different layers of nickel borides were measured and found to be in the range of 1500-1900 HV. Pin on disk wear tests were performed under dry and lubricated conditions and much superior wear performance of borided surfaces with a wear depth of 1-1.5 µm was confirmed compared to the base Inconel alloys with a wear depth of 13-15 µm.. The aim of this study is to show that electrochemical boriding is applicable to Inconel alloys and this method can be considered as an alternative way of producing hard boride layers on Inconel and hence make them more resistant to mechanical and environmental degradations.