ICMCTF1998 Session G7: Vapor Deposition and Electroplating: Competing Processes?

In spite of the fact that the underlying physical phenomena and macroscopic process variables of ECD and PVD are obviously very different, technically interesting crystalline coatings formed by both techniques have a series of similar features as, for instance,

- significant internal stress and preferred orientation,

- microcrystallinity and/or high content of lattice defects, and

condiderable thickness dependence of internal stress, texture, grain size etc.

Accordingly, similar microstructure - property relationships should often be realized. In order to test and to explain them, both reliable quantitative microstructure analysis and application of physically realistic structure-related models of coating behaviour is necessary. Starting with some comments concerning the techniques available for the quantitative microstructure parameters of thin layers and coatings, the paper gives a survey of important microstructure types occurring in ECD and PVD and presents some background especially for the description of the mechanical behaviour of the deposits in terms of microstructure characteristics. Experimental results illustrating e.g. the connections between hardness, the defect content, and grain or particle size, respectively, are mainly given for electrodeposited Cu and NI layers, and nitride and boride hard coatings obtained by magnetron sputtering.

Chrome electro-plating is widely used in the aerospace industry to provide protective cotaings for engine and landing gear components that ar exposed to corrosion and wear. However, the electro-plating process results in the release of hexavalent chromium, either in mists dispersed in the atmosphere, or in sludge that are a by-product of the plating process. Hexavalent chromium has been identified as carcinogenic, and permissible rexposure limits have recently been reduced by the US Occupational Safety and Health Administration from 0.1 mg/m3 to 0.005 mg/m3. The cost of meeting these environmental standards has led to a search for alternative coating methods. A leading contender to replace hard chrome electroplating is the High Velocity Oxy-Fuel (HVOF) spary deposition since it yields high quality coatings and emit no hazardous by-products. All current HVOF designs operate on similar principles: fuel and oxygen are injected into a high pressure combustion chamber and the combustion products accelerated through a constricting nozzle. Powder is injected either into the combustion chamber or downstream of the chamber. Powder particles are heated and accelerated by the rapidly expanding gas and ejected from the gun towards the substrate.

In this paper, we will present a modification to the HVOF gun and how it affects the coating quality through changes in particles state at the point of impact onto the substrate. When an unconfined HVOF jet issues from a nozzle into the atmosphere it contains cold ambient air. This is detrimental to the quality of coatings produced since gas temperatures are decreased, producing lower heat transfer to the injected powders, and metal particles are oxidized, reducing the integrity of the coating. Oxidation negatively affects the inter-lamellar bonding and, thus, the coating quality. Air entertainment can be reduced significantly by attaching a gas shroud nozzle to the gun which produces a high pressure, helical flowing, non-oxidizing gas stream around the HVOF jet. We compare the performance of a HVOF gun with and without a gas shroud attachment by measuring velocities of particles in the jet, particle temperatures, and gas temperatures and velocities. Coatings produced using a gas shroud showed considerable reduction in oxygen content (up to 80% less). The shroud also affected particle impact velocities and temperatures, which determine the final splat shapes and therby affect coating microsturcture.

We will also present the dynamics of particle impact on the substrate with and without a gas shroud attachment and show how the attachment of a gas shroud affects the final shape of the individual splats. We note that the splats' shape have a significant effect on the microstructure and mechanical properties of thermal spray coatings.

The conventional pretreatment of polymers for metallization by electroplating is based on hazardous and pollutive agents like chromic acid and various organic solvents. We report on the metallization of glass fiber reinforced polybutylenterephthalate (PBT with 20% glass fiber, Crastin SK 603) and polytetrafluoroethylene (PTFE) by electroplating without using such chemicals as pretreatment. The chemical pretreatment is substituted by a low pressure plasma treatment of the polymer.

The plasma pretreatment of Crastin SK 603 was performed in a parallel plate reactor operating at 13.56 MHz. As plasma gas oxygen or argon/oxygen mixture was used. A plasma pretreatment time of 5-10 minutes leads to excellent adhesion strength up to 2 N/mm as measured by peel tests of the electroplated metal coatings. The roughening and activation of the polymer surface is essential for the adhesion. Pretreatment times above 10 minutes lead to a damage of the polymer surface and low adhesion.

A combination of a selective plasma activation through a metal mask followed by a maskless palladium seeding process lead to the deposition of patterned metal lines on the polymer. Thus, this is a promising method for the direct deposition of conductor lines by electroplating as needed for the 3D Moulded Interconnect Device (3D-MID) technology to replace printed curcuit boards. The plasma pretreatment was also successfully applied for the electroplating of complicated threedimensional moulded Crastin parts by connecting both electrode to the rf generator.

The conventional metallization of PTFE by electroplating needs very aggressive and dangerous chemicals. After plasma activation and subsequent deposition of a thin (30-50 nm) titanium nitride (TiN) layer by plasma enhanced chemical vapor deposition copper could be deposited on PTFE by conventional electroplating. The thin TiN layer on PTFE acts as adhesion promotor and conducting interlayer for electroplated copper. The adhesion strength of electroplated copper using this pretreatment was 15 N/mm² as measured by a z-axis pull test.

Thus, the combination of low pressure plasma processes and electroplating takes advantage of the specific merits of both processes. The plasma activation of the polymers substitutes hazardous and pollutive chemicals ensuring a safe and fast pretreatment whereas electroplating guarantees an economic metallization of polymers.

There is a great deal of research and development work under way on chrome (and other electroplating) alternatives. Indeed, almost any wear resistant or lubricious coating is likely to be put forward as a chrome alternative. However, when we look at the industrial drivers, both for change and for the status quo, and when we examine what is happening in many industries, we have to ask where electroplating technologies are really being (or are likely to be) replaced in the production environment. Clearly, there are some places where chrome and other electroplates can be (and are being) readily replaced, while there are others where electroplating remains the technology of choice even in the face of apparently superior alternatives.

The reasons for the continued use of electroplates or the adoption of alternatives are usually a combination of technical considerations and non-technical concerns such as cost, risk, fit with the manufacturing system, and other considerations.

This paper will discuss several markets for chrome plating and its alternatives, evaluating why chrome has or has not been replaced, and what, if anything, it will take to drive adoption of a new technology.