ICMCTF1998 Session B2: CVD Hard Coatings & Technologies - Plasma & Duplex Coatings, Ion Nitriding & Surface Modifications

Deposition of the hard coatings onto steel substrate can produce a surface with specific properties such as hardness, wear resistance.

However, corrosion resistance of the coated steel substrate can decrease as a result of galvanic coupling between the substrate and the electrochemically nobler coating. Once it has started, corrosion propagates at the bottom of the structural defects in the coating generally named pinholes or at the interface between substrate and coating.

This interface between the coating and substrate is responsible for adhesion properties and partly also for the high wear resistance of the coated substrate. Besides, the interface can play an important role in corrosion prevention of the coated substrate.

Duplex coating technique performed with arc plating can result in substantial increase of the corrosion performance of the coated substrate. Duplex coatings can be divided into two groups based on the principles by which they are produced: - structure- dense coatings, that are produced by deposition of thick pure metal layer (e.g. Ti) followed by nitriding the surface, - interfacial coating, that are produced by an initial plasma treatment (e.g. nitriding or implantation) of the substrate followed by deposition of the coating. The interface between the steel substrate and the coating consists of plasma treated upper layer of the substrate and an interlayer between substrate and coating.

This paper presents a study of corrosion performance of duplex coating in different aqueous solutions. The coatings were produced in one set in an arc plating PVD- plant. Electrochemical evaluation of the coating was done by polarization methods and electrochemical impedance spectroscopy (EIS).

Plasma nitriding is now widely used in manufacturing for surface hardening of ferrous and non ferrous materials. For that purpose, plasma generation is most frequently fulfilled by a dc glow discharge with the underlying limitations associated with this technology. First, the temperature of the substrate to be treated is strongly coupled with the discharge parameters that control the plasma reactivity and secondly, the processing pressure is relatively high (100-1000 Pa). Nevertheless, more flexibility and control are required for plasma nitriding of promising non-ferrous materials such as titanium, aluminium and their alloys. In the case of aluminium, low temperature and low pressure nitriding processes associated with high plasma reactivity are necessary.

These requirements are fulfilled by the recently developed enhanced or intensified plasma nitriding processes such as : triode arrangements, electron cyclotron resonance microwave plasma or high current, low voltage thermionic arcs. The purpose of this paper is to review these new nitriding processes from both technological and fundamental point of view. Particular attention will be paid to the metallurgical results obtained on austenitic stainless steels, titanium and aluminium.

Many researcher have written that the incident ion energy during chemical vapor deposition(CVD)of diamond-like carbon (DLC) effects the resulting properties. Workers effect film properties such as hardness, density, intrinsic compressive stress, and refractive index by varying the processing parameters. However, there is little agreement as to the actual ion energy during DLC deposition in self-biasing plasma chambers. Ion energies are difficult to estimate at higher operating pressures, because the energetic ions collide with low energy neutral species in the plasma sheath resulting in a broad distribution of energies. Stated ion energies for similar films range from 60 to 1200 eV ^1,2,3. Some authors measure only bias voltage V_b, which is not an accurate measurement of ion energy E_i. Others estimate ion energy with electrical impedance measurements. In this work, the plasma and process chamber are characterized by the method of impedance analysis. The impedance analysis ¹ predicts a variation in ion energy that is linear with applied power. Alternatively, another model² predicts the bias voltage to vary with Power^1/2Pressure^-1/2. The relationship between a film property and two different processing parameters, bias versus ion energy, may well show different trends. There are other ion energy, flux and momentum measurements that are also meaningful. In this work, deposition pressure and power are varied to show dependence of ion energy and bias on processing conditions. Once the processing parameters are characterized, the processing conditions are systematically varied. Pressures are controlled from 400 to below 1 mTorr in order to achieve both collisional and non-collisional plasma sheaths respectively. Without collisions, ions strike the substrate with E_i=aV_b. The films are analyzed for stress,hardness and density. Also, Raman spectrum are presented.

¹ J. H. Lee, D. S. Kim, Y. H. Lee, B. Farouk, J. Electrochem Soc., 4,1451 (1996). ² Y. Catherine, P. Couderc, Thin Solid Films, 144 265 (1986). ³ D. R. Mckenzie et al., Diamond Relat. Material, 51(1991).