Visual evaluation of Al coated brass samples show a white, temperature dependant surface appearance. Only samples treated with low arc current during ion etching, low bias voltage and low coil current resulting in a process temperature less than 138 °C have a shiny and brilliant coating. Samples treated with high arc current during ion etching, high bias voltage and high coil current resulting in a temperature higher than
138 °C show a milky and matt surface. GDOES
GDOES investigations were carried out on Al coated brass specimens with various substrate cleaning and coating processes. A typical GDOES plot is given in figure GDOES 4. The Al coating shows an Al film with an oxygen concentration of around 2.5 at-%. The interface between the Al coating and the brass substrate shows diffusion of Zn and Cu from the brass substrate into the coating and Al diffusion from the coating into the substrate. At the interface there is an accumulation of oxygen.
Roughness measurement
Roughness measurements of Al coated brass samples showed no systematic characteristic concerning the substrate cleaning and coating processes. The matt surfaces showed an increase by a factor of 4 for the roughness values of Ra=0.0474 pm, Rq=0.0646 pm, Rt=0.6179 pm and Rz=0.5038 pm.
Vickers adhesion test
Vickers adhesion tests of Al coated brass show the tendancy o f better adhesion for matt films with a process end temperature of more than 138 °C ("Good") and worse adhesion for shiny films, which were produced with a process end temperature of less than 138 °C ("Good" to "Bad"). The samples with the intermittent temperature profile, leading to a process temperature between 100 and 130 °C, showed good adhesion.
Coating thickness
The coating thicknesses are around 1.1 pm as determined by calotests. Hardness measurements
A typical hardness-depth profile is given in figure 6.24. The increase of the hardness can be correlated to the appearance of Al.
Figure 6.31 shows the hardness o f Al coatings as a function of the applied coil current, produced with a process start temperature between 70 °C and 100 °C. It shows a decreasing hardness with increasing coil current and a maximum in hardness at 2 A coil current. Figure 6.32 shows the hardness as a function of the process start temperature Ts and the end temperature Tf at a coil current of 4 x 6 A. With increasing ratio Tf / Ts, the hardness increases.
Corrosion tests
Figure 6.41 a-c shows the results of the ammonia porosity test. The samples show fine pores and an overall uniform attack. The sample produced at a temperature between
112 and 138 ° C shows by far the best result (Figure 6.41 c).
The results of sulphur dioxide corrosion tests carried out on Al coated brass specimens from process 7 shows an attack of the surface area of 30 % after 24 h, 70 % after 48 h and 100 % after 72 h. Figure 46a shows the sample after 72 h with partly dissolved Al coating.
Diffusion tests
The result of diffusion tests carried out on samples from 3 different Al processes with different process temperatures showed barrier behaviour against Zn (figure 6.23 a). The barrier coating is penetrated by Zn after roughly 15 min to 50 % and after 30 min to
100 % at an annealing temperature o f250 °C. Temperature profile during the coating process
Figure 6.40a shows the temperature profiles of Al processes with various start temperatures and process parameters.
Images from the SEM / TEM
Figure 6.49a shows the topography o f a shiny Al coating. The fracture in figure 6.49b shows a dense structured coating and cohesive failure in the brass substrate. Figure 6.49c shows the topography of a matt Al surface. The faults on the surface are probably droplets; it is also possible to see some grain boundaries.
Figure 6.49d shows a TEM image o f an Al coating at a magnification of 10 OOOx. The Al coating is fine grained (around 0.25 pm). With a higher magnification of 100 OOOx the Al grains show a high dislocation density (Figure 6.49e).
The TEM images are produced parallel to the coating surface, therefore no information on texture or equiaxed structure can be gained.
Scratch adhesion test
Scratch adhesion tests on samples with a matt appearance resulting from a high process temperature showed at 100 N a buckled coating as described in section 3.5.4, reflecting a relatively good adhesion. Scratch adhesion tests carried out on samples produced at temperatures up to 138 ° C resulting in a shiny surface showed delaminations at 100 N as described in section 3.5.4, but there is a trend to better adhesion with lower coil current.
Scratch adhesion tests were also performed with a modified scratch tester as described in chapter 2.5.2., which is especially suitable for ductile coatings on ductile substrates. The result was obtained using test angle tga o f M0~2°5 an initial vertical displacement of 0.3 pm, a horizontal displacement o f 2 mm in 2000 steps. Figure 6.22a shows the displacement in pm vs the tangential an normal force in N. Figure 6.22b shows the normal force vs the tangential force and figure 6.22c shows the distance vs tangential/normal force (friction coefficient). The transitions are not easy to find. However, a transition point can be correlated with the appearance of wrinkles on the micrograph at about 300 pm of the beginning o f the scratch.
The scratch test method being a comparative method, the value of the force at the transition does not mean anything on its own, but has to be compared with other values measured on the same system.
Bending test
Al coated brass sheet samples showed good adhesion in the bending test. They have an orange peel appearance for the bending test at 90 ° and show in part delamination for the 180 ° test.
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