Plasma-sprayed surfaces

Heimann, R.B. (2011) On the self-affine fractal geometry of plasma-sprayed surfaces. /. Therm. Spray Technol, 20 (4), 898-908. 

Figure 7.53 Determination of the waviness of a plasma-sprayed surface through Abbot s curve (see text, Kuhn, Stitz and Letzner, 1990).

Fig. 1 Photograph of (a) femoral hip stem with (b) inset of the microbeaded surface, (c) Photomicrograph of a plasma-deposited calcium phosphate coating on a hip stem (image kindly provided by Smith Nephew, Memphis, TN, USA), (d) Dental implant with (e) inset showing a titanium plasma-sprayed surface coating...

The ProDisc-C, produced by Synthes, is a cervical version of the lumbar ProDisc II and employs CoCr-on-UHMWPE articulation (Figure 12.10). In contrast to the lumbar design, in the ProDisc-C, the UHMWPE component is provided already inserted into the inferior endplate by the manufacturer. Bone-implant fixation is achieved by a combination of a keel and titanium plasma sprayed surface, similar to the ProDisc II. Naturally, the size and proportion of the keel and articulations are much smaller in the ProDisc-C as compared with the ProDisc-L, but the technology underlying both designs is identical. Synthes completed a prospective randomized clinical trial with the ProDisc-C for the FDA and received clearance to market the device in the United States in 2007. 

At high magnification, the microroughness of the plasma-sprayed surface is more random than that produced by any chemical process. The pores are quite deep, and many knob-like protrusions can be seen. These protrusions result from the splat cooling and rapid solidification of the Ti-6A1-4V droplets as they strike the substrate. At lower magnifications, some macroscopic inclusions are evident. 

Figure 3 Picture of the inspected part (plasma-sprayed chromium cast iron on ferritic steel). The surface presents several cracks I to 15 pm wide.

The seemingly simple question of the relation the characteristics of a mechanically prepared metal surface and adhesion to that surface has sporadically occupied attention for many decades without any very general conclusion being reached [69]. In some recent work, Amada et al. [70,71] grit-blasted a steel substrate, varying the angle between the gun and the specimen surface, and measured the adhesion of a plasma-sprayed alumina coating. They examined profiles of the... 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

Applications Plasma spraying is used to apply coatings for protection against wear and corrosion, to prevent erosion or cavitations, and to provide electrical insulation or conductivity. It can also be employed to produce bearing surfaces, abrasive properties or resistance to wetting by molten metals. The coatings can also be applied to facilitate the joining of different materials. 

Thermal Spray Surface Coating and Free-Standing Components - Plasma Spray - High Velocity Oxy-Fuel (HVOF)... 

Figure 1.8. Schematic showing (a) plasma spray deposition process and (b) droplet formation, acceleration, surface impact, and coating formation.

Early bubbling FBC units were designed to burn coal, and the heat released was removed by heat transfer to in-bed tubes and/or to the water-wall tubes used to enclose the furnace. These surfaces experienced high rates of metal loss through the combined effects of erosion and abrasion. Protective measures such as plasma-sprayed coatings and metal fins to disrupt the solids flow pattern were used. These were effective for only short periods before requiring replacement, and so maintenance requirements were high. 

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