Abrasive Wear Behavior of Different Thermal Spray Coatings and Hard Chromium Electroplating on A 286 Super Alloy *

In cases of decorative and functional applications, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. However, pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics has increased in recent years, related to the reduction in the fatigue strength of the base material and to environmental requirements (1). In the present study plasma sprayed coatings (aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide, chrome carbide) and electrolytic hard chrome coatings abrasive wear properties have been compared. The wear tests were conducted with a Taber abraser, at room temperature.


Introduction
Chromium has been widely used in surface finishing of metals because of the favorable properties it imparts to substrates and because the processes used are relatively mature, well understood, widely specified, and cost effective (2).This coating is produced from a wet chemical bath containing hexavalent chromium ions (Cr +6 ).In all environmental regulations, Cr +6 is classified as a confirmed human carcinogen.Hard chromium plating produces large volumes of chromium containing toxic waste, air pollution and water contamination (3).Potential process substitutions for hard chromium plating are electroless nickel in certain applications, several nickel-tungsten composite plating, and spray applications such as plasma spray coatings (2).Plasma spraying is a process widely used in industry for depositing protective and functional coatings for a large variety of applications.Industrial sectors such as aerospace, automotive, energy, mining, biomedical, etc. take advantage of the unique properties of the sprayed coatings (4).The applications of thermal spray coatings are extremely varied, but the largest categories of use are to enhance the wear and/or corrosion resistance of a surface (2).The deposition methods for the wear protective coatings are atmospheric plasma spraying (APS) and high velocity oxygen fuel (HVOF) flame spray processes.Both of these methods have their own characteristics, e.g.different spray particle velocities and flame temperatures.As a result, the coatings have different microstructures and properties (5).

Experimental Coatings
As aforementioned, electroplated chrome and four kinds of plasma spray were involved in the study.Their characteristics are listed in Table 1.The A286 super alloy was used as substrate materials for all five coatings.The thicknesses of coating layers were controlled in the range of 100-150 µm.The substrates were sand blasted prior to spraying using 36 grit alumina sand.Sulzer Metco 9MB plasma gun and GH / 732 nozzles were used.The spraying parameters were given in Table 2. Electroplated chrome coatings were produced in an industrial plant, using the industrial deposition parameters listed in Table 3.The de-hydrogenation thermal treatment (200 0 C for 3 h) has also been performed on the coating.

Characterization
Roughness was measured with Diavite DH-5, also hardness off each coating were measured by the Vickers microhardness tester (Wilson/Tucon) and given in Table 4.
The thicknesses of the coatings were determined by micro hardness tester (Wilson/Tucon).Scanning electron microscopy technique (SEM) was used to observe two different parts of the test coupons which performed abrasive wear test (right) and which didn't (left).

Abrasive Wear Tests
For abrasive wear tests, samples were prepared from A286 with 4 mm thickness and 100 mm square, according to FED-STD-141C.AMS 5525 (A286 plate form), electrolytic hard chrome and aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide, chrome carbide plasma spray coated test panels were subjected to abrasive wear test.The wear tests were conducted with a Taber abraser, at room temperature, using a 1000 g load and CS-17 abrading wheel.The results were analyzed by wear index (mg/1000 cycles) and total wear (mg/10000 cycles) data.Cycles to mg/1000 weight loss is shown in Table 5 and Figure 1, cycles to total mg weight loss is given in Table 6 and Figure 2.
The initial peak which is typical for plasma spray coatings (Figure 1) was due to the higher surface roughness.Table 4 figures out that the surface roughness values of all other coating materials are higher than those of electrolytic hard chrome.Plasma sprayed materials show rough surface properties, involving many pores, oxides and inclusions.It is possible to compare the tested coating materials with electrolytic hard chrome coatings on the SEM micrographs which were given in Figure 3.It can clearly be observed that the electrolytic hard chrome coatings show dense and smooth surface properties.On the other hand, the plasma spray coatings have porous coating structure.
Experimental data from abrasive wear tests were conclusive, indicating better results from the hard chrome coating.The abrasive wear resistance of plasma spray coatings and hard chromium plating was evaluated and the results in terms of wear In terms of hardness values, as it can be seen on Table 4, in comparison with the electrolytic hard chrome coated test coupons, similar or higher hardness values were reached by plasma spray coated test coupons.
As it can be seen in Table 5, Table 6 and Figure 1, Figure 2, aluminum oxide coatings show better abrasive wear resistance among all plasma spray coupons.This is due to high oxide content of the coating material.Coating of high oxide content is usually harder and is more wear resistant [6].

Figure 1 .
Figure 1.Abrasive wear weight loss vs. number of cycles.