DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

260 °C for 8 hours, the residual carbon content is estimated to be lower than 0.4% by
total weight. TGA analysis indicates partial weight loss for all polymer additives.
Residual carbonaceous content is expected to further degrade in the subsequent sintering
process and result in carbon content in the sintered Ni.
The chemical kinetics of nickel oxidation has been investigated 133, 134, 145-147 and
may be described as chemisorption followed by oxide nucleation and subsequent growth.
Below 400°C, the oxidation reaction is limited by diffusion of oxygen through the oxide
film. 145, 148, 149 At higher temperatures, the oxidation of Ni is dominated by diffusion of Ni
vacancies along grain boundaries in NiO, 145, 150-156 and associated with grain growth
within the NiO film. 145 The weight gain as the result of oxidation may be summarized as
follows: 1) below 270 °C, the surface oxide layer remains stable, 2) between 270 °C and
300 °C, the surface oxide has a less than 1% mass increase over prolonged exposure to
oxygen, 3) above 300 °C Ni oxidizes at an increasing rate proportional to temperature.
Hence, the binder removal temperature for green nickel parts is chosen at 260 °C and a
prolonged isothermal hold of 8 hours is used. Excessive Ni oxidation is to be avoided
since it will either require removal in a subsequent reduction step or hamper sintering and
compromise mechanical properties in the sintering operation.
Measured weight loss profiles by TGA indicate only partial weight loss for the
polymer additives, as shown in Figure 4.5. After heat treatment at 260 °C for 8 hours, the
weight loss is 51.8%, 52.3%, and 77.5% for Darvan 821A, PEI-25K, and HMPC,
respectively. Assuming the thermal degradation kinetics for these polymers is the same in
the presence of Ni, an estimated 0.214% by weight carbonaceous material will remain
after thermal degradation, as shown in Table 4.1. Assuming the initial 0.15% carbon
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